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Jim Solomon's Astrophotography Cookbook 

(v0.4, Last Updated: 5/5/05)

http://home.comcast.net/~jimc8ngt/articles/cookbook/

Introduction

I believe my astrophotography technique is now producing results that are at the limit of what's possible with my rather modest equipment. I therefore offer this "How To" guide to other astrophotographers who are attempting to climb the learning curve, and who would like to get the mostout of similar equipment. I hope such folks will find this useful.

This is exclusively a "How To" for long-exposure astrophotography of Deep Space Objects (DSOs);i.e., anything requiring a very long, tracked, exposure to adequately capture. This is therefore

specifically NOT a "How To" for planetary photography, mostly because I don't consider myself verygood at it, and also because the technique is so radically different from DSO photography that it needsits own treatment elsewhere.

 Note also that this document does not delve very deeply into the theoretical underpinnings of digitalastrophotography, or even the "basics" of astrophotography. As such, it is assumed that the reader isalready familiar with and understands the following concepts:

• How to couple a camera to a specific telescope.

• The need for a mount to "track" the apparent movement of the stars over the duration of theexposure.

• The need for many entry-level mounts, and even some high-end mounts, to be "guided" tocorrect, in real time, tiny errors in those mounts' tracking ability.

• The need to polar align the mount fairly accurately.

• The basic (digital) technique of taking many relatively short sub-exposures, and then  stacking them to yield a much longer effective exposure time.

• The need to shoot in "RAW Mode" in your Digital SLR, as opposed to Large/Medium/SmallJPG mode.

See some of the excellent Introductions to digital astrophotography available on the 'net for more info.on these topics.

The sections below are broken down as follows. First, I give a brief synopsis of my equipment. Thendescribe the three phases of my astrophotography technique in detail; namely,  planning, acquisition,and processing.

Definitions

Here are a few terms used throughout this guide, which I define here to make sure we're all inagreement on what they mean:

• Frame (noun) -- synonym for exposure. E.g., if I take 15 exposures at 4min each of my target,I speak of having acquired "15 Light Frames at 4min each".

• Frame (verb) -- the process of centering the target object in the imaging camera.

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• Lights -- frames taken with the imaging camera through the imaging scope, with the dust capoff . I.e., these are the actual exposures of the target object.

• Darks -- frames taken at the same ISO, exposure time, and temperature as the Lights, but withthe camera's body cap in place. Darks and Offsets are used to mitigate the effects of variousnoise sources in the camera.

• Offsets (aka Bias) -- frames taken at the same ISO and temperature as the  Lights, but with asshort an exposure time as the camera allows (1/4000'th of a second in the case of the 300D) and

with the camera's body cap in place.• Flats -- frames taken of an evenly illuminated target, typically the sky just after sundown.

These are used to correct for vignetting in the optical path of the imaging scope, and should betaken with the lowest ISO setting the camera allows.

• Target -- the DSO you are setting out to photograph.

• Clipping (aka Saturating) -- the act of overloading a digital sensor to the point where it reportsits maximum intensity value. A clipped/saturated frame is one taken with an ISO setting that istoo high; an exposure time that is too long; or both. Clipping causes a loss of information thatcan never be recovered.

Background

Many newcomers to digital astrophotography are confused by the notion of Lights, Darks, Offsets, andFlats, so here I'll give a very brief background on these concepts. The CMOS or CCD imaging chip inmost Digital SLRS will very faithfully and linearly collect light from the object you're trying to image.Unfortunately, the "signal" collected from the target object will get degraded by thermal noise andother noise sources. Darks and Offsets are the means by which we try to characterize and mitigate theeffects of these noise sources. Also, the telescope/lens optical train may not fully illuminate theimaging chip, depending on its size, resulting in a phenomenon called vignetting . Flats are the means

 by which we try to characterize and mitigate the effects of this vignetting, and further the means bywhich we mitigate the effects of dust that might have settled on the imaging chip in the camera.

The formula that relates these physical phenomenon, and the actual frames we'll collect over a night of imaging, are as follows:

(1) Light = (Signal * Flat Signal) + Dark + Offset

where Signal is the image of the target object we wish we could collect under ideal circumstances,

and Light is the image we actually captured. Rearranging the terms, we have:

(2) Signal = (Light - Dark - Offset) / Flat Signal

But realize that the Flats we capture with the camera will, in turn, be "polluted" by Darks and Offsetsin their own right, and so we must subtract Flat Darks and Flat Offsets from the Flat Lights as follows:

(3) Flat Signal = Flat Light - Flat Dark - Flat Offset

So, plugging equation (3) into equation (2), yields the final result:

Light - Dark - Offset(4) Signal = ------------------------------------

Flat Light - Flat Dark - Flat Offset

Thus, the basic digital astrophotography technique involves capturing all of the frame types listed onthe right-hand side of Equation 4, and using them in an image-processing program to produce the"Signal" term on the left. This process of subtracting Darks and Offsets, and dividing by Flats, is called

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"calibration" of the Lights. Note, finally, that a Dark as captured in an exposure actually contains theOffset, and, so, in processing, we will often just subtract this Dark from the Light, without explicitlysubracting an Offset. But, as stated, that captured Dark actually contains the Offset, and subtractingthat captured Dark has the effect of removing the Offset as well.

Equipment

Almost all of my DSO astrophotos (from herein I'll drop the adjective "DSO", since this entiredocument deals with DSOs) are acquired with a Canon Digital Rebel at Prime Focus of my Celestron8" f/5 Newtonian. Here's the list of equipment that comes into play in this configuration:

Cameras:Imaging: Canon Digital Rebel (aka 300D) Digital SLR Guiding: Philips ToUcam Pro II 840k webcam

Mount:Celestron Advanced Series with GoTo (aka AS-GT)

Telescopes:Imaging: Celestron C8-N: 8" f/5 Newtonian Reflector, fl=1000mm=> upgraded focuser to JMI NGF DX3 low-profile model

Guiding: Orion ST80: 80mm f/5 Achromatic Refractor, fl=400mmGuide scope mounting:

Orion 07381 Guide Scope Rings (pair), 105mm I.D.Orion 07382 Guide Scope Ring Mounting Bar 

Adapters, filters, etc:T-Ring: Orion 05224 for Canon EOS camerasComa Corrector: Baader MPCCBarlow: Celestron "Kit" 2x Barlow (used with Guide Scope)

Extension: Orion 05123 1.25" Extension Tube (to reach focus with webcam)Computers:

Guiding, Focusing, and Acquisition: Toshiba TECRA 8100 laptopProcessing: Custom built PC with 3GHz P4, 2GB RAM, and WinXP Pro

Software:Guiding: GuideDog v1.0.6Focus and Acquisition: DSLRfocus v2.7.1EXIF Preview et. al.: Canon FileViewer UtilityProcessing: IRIS v4.34, Photoshop v6.0

Cables:Webcam to Laptop: USB cable that comes attached to webcam

DSLR to Laptop:  • Long exposure control: C300P-20 Parallel port to shutter control cable  • Focus and framing: USB cable that comes with 300D

Laptop to Mount: Serial port to AS-GT Hand Controller RJ-22 port, home built cablePower Supplies:

Mount: Celestron 18773 A/C Adapter (Warning! Celestron says not to use this!)DSLR: Canon ACK-E2 A/C Adapter Kit for Digital RebelLaptop: included A/C Adapter 

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Planning

As will become evidently clear, my setup and acquisition are quite involved and, therefore, timeconsuming. So I like to have as much work done "up front" as possible before going out for the night.The better the planning, the better things will go when shooting. The activities in this stage are asfollows:

1. Pick a target. Use charting software or the many available catalogs to pick a suitable target. In particular, I try to pick large objects that fill the camera's Field of View; bright objects that havea reasonably high mean surface brightness; and objects that are well placed. Pay particular attention to when the object transits and on which side of the Meridian you'll be shooting it. 

2. Choose a camera orientation. Determine if the object has greater extent in the East-Westdirection or the North-South direction, and be sure to orient the long dimension of the camera'ssensor in that direction when attaching it to the imaging scope. I prefer to orient the camera inthe "North is up" direction in all cases, unless the object begs for a "North is left" orientation.Examples of the latter include M81/M82, M42, and others. 

3. Pick a candidate Guide Star. It will speed the process of Guide Star acquisition to have arough idea of which Guide Star you're going to use and knowing how far, and in whichdirection, that Guide Star lies from the target. Also, the further the Guide Star from the center of the target, the more accurate your Polar Alignment will need to be. See the section on Polar Alignment to learn why this is the case. 

4. Devise a plan to find the target. Do you have a GoTo mount with scary-good accuracy? Doyou have Digital Setting Circles? Are you a Star-Hopping Demon? In any case, you'll need tofigure out a way of centering the target in your imaging camera without removing the camera.(Why? -- because you're not allowed to remove the camera between taking your first FlatFrame and your last Light Frame.) My favorite method is to use a home-brewed spreadsheet that serves a similar function to the Precise GoTo function in my mount. I home-brewed this

 because the AS-GT's Precise GoTo does not allow me to pick the reference object, andsometimes it, sigh, picks reference objects on the wrong side of the Meridian. Or, just as bad, it

 picks stars you've never heard of and can't locate. I prefer to use an unmistakable object as thereference object, center it, and then use the spreadsheet to compute the RA and DEC offsets tothe actual target from the reading on the Hand Controller's "Get RA/DEC" function. 

5. Take an educated guess at ISO and exposure time. This can generally be done "in the field", but if you have the time, it never hurts to research what other folks have used to shoot the sametarget. Or to look through your own portfolio and see what seems to work and what doesn't. Itend to lean toward 4min sub-exposures @ ISO 400, as that leaves a very large dynamic range(lack of clipping of bright objects), and 4min is usually long enough to capture a decent amountof detail in each sub-exposure. Also, at 4min, if an airplane flies through and ruins a frame,like, so what?, it's only 4min. Dimmer targets will require longer exposures and/or higher ISOs.You'll need to experiment with this to see what works best for you.

Acquisition

My acquisition process consists of the following distinct phases:

1. Setup2. Acquire Flats3. Polar Alignment4. Focus

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5. Acquire Target6. Acquire Guide Star 7. Take a Test Shot8. Acquire Lights9. Acquire Darks10.Acquire Offsets

Setup

The setup phase can and should be done during the daytime. Basic setup of your mount, scope, laptop,etc., is way beyond the scope of this document, so I'll focus on the astrophotography-specific aspectsof the setup:

0. Verify the collimation of the optics.

• This isn't really specific to astrophotography, but collimation is essential to good results, particularly with the fast optics of a Newtonian. I like to orient the scope in the rough direction

I'll be shooting before verifying the primary's and secondary's tilt with a laser collimator.

1. Setup and Configure the Imaging Camera and Imaging Scope:

• Attach the 300D to the T-Ring, the T-Ring to the MPCC, and pop the MPCC into the Newtonian's (2") focuser.

• Focus the 300D as accurately as possible on a distant target.

• Align the imaging scope's finder to the center focusing dot in the 300D's viewfinder, againusing a distant target.

• Align the camera North-South or East-West. The easiest way to verify the orientation is to slewthe mount in RA or DEC, and to make sure the object moves along the focusing dots in the

viewfinder in a way consistent with your chosen camera orientation. Rotate the camera untilthis is as exact as possible.

• Lock the focuser and make sure the tensioning knobs that hold the MPCC in the focuser aretight.

2. Setup and configure the Guiding Camera and Guide Scope:

• Pop the webcam into the 2x Barlow, the 2x Barlow into the 1-1/4" diameter extension tube, andthe extension tube into the ST80's focuser.

• Focus the webcam! Accurately focusing the webcam during the daytime will save you atremendous amount of grief at night, when you're trying to figure out whether a potential guide

star is really off-chip or whether the focus is so bad that it wouldn't appear on the screen even if it the star was on-chip.

• Align the ST80's finder with the center of the field of view of the webcam. Again, Guide Star acquisition will be inordinately simpler with an accurately aligned finder.

• "Roughly" align the ST80 with the Newtonian by adjusting the thumbscrews in the GuideScope rings. Not doing so can cause you to miss a potential Guide Star, or to seek a Guide Star that is really beyond the range of adjustment of your Guide Scope. It's best to start with a roughalignment. You can do this by making sure the finders of the two, respective, scopes arecentered on the same distant target.

3. Balance the scopes in the mount:

• Mount, align, and focus the cameras as per #1 and #2 above.

• Remove (and replace!) any dust covers that will be off (or on) during the actual imageacquisition. You want the weight distribution to be identical to how it will ultimately be when

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you're shooting.

• Swing/slew the mount to the approximate location of the target.

• Balance the scope in DEC, with a slight imbalance in the direction that opposes the motionresulting from pressing the Up arrow on the Hand Controller.

• Balance the scope in RA, with a slight imbalance to the East.

Here's a little more background on why I intentionally misbalance the scope ever-so-slightly in RA and

DEC. The AS-GT mount always approaches a target in a consistent direction to minimize the impactof backlash. For the Northern Hemisphere, that direction is the same as the direction the mount moveswhen pressing the "Up" and "Right" arrows. So, I intentionally misbalance in DEC ever-so-slightly,such that the misbalanced weight is acting in opposition to the action of the Up arrow. So, for example,if I'm shooting an object West of the Meridian, the scope will be on the East side of the mount, and theUp arrow will move the scope South. Therefore, I like the extra weight to be on the North side of themount in DEC, so that the Up arrow is pushing against that weight. Similar arguments apply in RA,

 but the directions are more straightforward. Since the mount is always tracking the movement of thestars in the Westward direction, the East side of the mount should be ever-so-slightly heavier in RA inorder for the gears to be pulling against that weight.

Acquire Flats

When I first started taking astrophotos, I assumed I could "blow off" Flat Fields, Dark Frames, etc. Iwas wrong. You are too. Just do it. You'll thank me. My process for acquiring flats is evolving, so I'lldescribe the process I'm currently using.

First, some background. The purpose of Flats is to characterize the optical train as accurately as possible, so that later, in processing, you can correct for things like vignetting (uneven illumination of the optics, particularly a darkening at the edges of the camera's field). A good set of Flats will alsocorrect for dust spots on the camera's sensor. The ideal Flat is taken of a (perfectly) uniformly

illuminated target. In practice, the twilight sky makes a reasonable approximation to this ideal, and, in particular, the twilight sky a few hours East of Zenith makes for an excellent approximation.

Thus, I use the twilight sky (just after civilian sundown) as my "uniformly illuminated target". Othersuse different methods and they may be getting better results than I am. As I said, my procedure isevolving. In any case, here's what I do:

• Do a Quick Align in the mount so you can slew to a desired RA/DEC.

• Slew the scope to a DEC equal to your Latitude, and an RA that is 2 Hours  East of theMeridian. I saved a User Object for this for convenience. Note that this User Object must be a"Land Object", not a "Sky Object", because, if you think about it, you want the scope to point

to the exact same part of the sky regardless of the current time or date.• Take your exposures. Definitely use ISO 100 and "RAW" Quality mode. Exposure time is

something I'm currently playing around with, but I believe exposures in the 1/8 to 1 secondrange (at f/5) are appropriate. Begin snapping right around sundown, as soon as the histogramon the 300D's LCD begins to move off of the saturation side of the graph, and just keep onsnapping until the histogram is rather far to the left. Later you can figure out which of theseframes you'll use to compute your Master Flat Frame. Right now we just want to capture a

 boatload of them to allow flexibility later on in processing.

• Important: Make sure to move the scope East between each exposure, to ensure that any starsthat are recorded can be eliminated later in processing. I move by "30 RA seconds" betweeneach frame, which is equivalent to an angular movement of roughly 6 arc-minutes at my

Latitude (remember, we set the mount's DEC to our Latitude).• When finished capturing the Lights, very carefully remove the camera from the T-ring, without

moving/rotating the T-Ring, MPCC, or focuser, put the body cap over the bayonet, and capture15 Darks at the exact same settings you used to capture the Lights (e.g., 1 second exposures at

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ISO 100).

• With the cover still in place, take 15 Offsets at the same ISO (hopefully ISO 100) and at thefastest exposure your camera allows, which is 1/4000'th of a second for the 300D.

• At this point I usually remove the flash card from the camera and copy the Flats (Lights, Darks,and Offsets) to my PC ... just to make sure I don't accidentally delete them.

• Remove the body cap from the camera and carefully pop the camera back onto the T-ring.

Polar Alignment

In a guided system, one need not worry about RA or DEC drift, especially if the guiding software isguiding the mount in both RA and DEC. However, Polar Misalignment causes Field Rotation inaddition to RA and DEC drift, and it is this rotation that is problematic. In general, field rotation getsworse as the Polar Alignment error gets larger. It also gets worse for objects close to the poles (i.e.,DECs near +90° or -90°). And it becomes more problematic the further the Guide Star is from thecenter of the imaging camera's field. The latter is so because the field will appear to rotate around theGuide Star , and the further it is away from the center of the imaging camera's field, the more that fieldwill tend to "slide off the imaging camera" over the course of the night's image acquisition.

So, how accurate must the polar alignment be? Well, accurate enough for the task at hand. Certainly itmust be accurate enough so that no discernible field rotation occurs over the duration of a  singleexposure. And it must be accurate enough to prevent the field from rotating over the course of thenight to the point where there's very little intersection among all of the night's Light frames.

Generally, I like to Drift Align to the point where I see no discernible drift over a span of 4 to 5minutes. Many people will see this to be overkill and they're probably right. But, hey, this is myastrophotography cookbook! <grin> I've drilled indentations into the pavement so I can replace themount's tripod legs in the exact same position each time I set up. On most nights, this gets me "closeenough" that I don't even bother to Drift Align at all. Of course, I did a "scary accurate" Drift Align

 before I drilled those indentations in the first place. On nights where I'm shooting close to the pole(M81, for example), or on nights that I need to wait for the target to transit, I'll spend the interveningtime honing my Polar Alignment with a Drift Alignment.

Drift Alignment isn't really as scary as most people think it is. You should learn how to do it. Mycurrent method involves using the webcam with GuideDog software running (but not guiding!) so Ican view a star on my laptop's screen. (Remember, it's illegal to remove the imaging camera from theimaging scope between the acquisition of Flats and acquisition of Lights.) I turn on GuideDog's cool"double reticle" which makes it very clear if the star is drifting. Be absolutely sure to align the webcam

 North-South and East-West if using this method. Be accurate! Slew the mount in RA and DEC and make sure the star follows the reticle; otherwise, the camera orientation is off. Also, be sure you know"which way is North" when looking at the laptop's screen. One of my favorite tutorials on DriftAlignment can be found at Andy's Shotglass (click on the Drift Alignment link).

Focus

There are wonderful tools available to DSLR users to help them focus, and DSLRfocus is one suchtool that I use for focusing. Spend some time at this stage to focus as accurately as possible. You'll beglad you did. Take an extra 5min to really nail the focus -- I assure you that N perfectly focused frameswill give you a better result than N+1 poorly focused ones! Here's my procedure:

• Shut down GuideDog if using it to Drift Align and disconnect the USB cable leading to thewebcam.

• Connect the 300D to the laptop via the supplied USB cable.

• Run DSLRfocus in "focus mode", turn the camera on, and connect to the camera.

• Center a star in the "dead center" of the field of the DSLR.

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• Use ISO 100 and an exposure in the range of 1 to 4 seconds. You want the star to be as brightas possible but without clipping! 

• Use Medium/Normal image quality at first. Then hone in and verify with Large/Fine quality.

• Use mirror lockup if your camera supports it. 300D users can use the Wasia Hacked firmwareif they're brave.

• When you think you've got the focus nailed, engage the focus lock on your scope's focuser andthen take another shot "just to make sure". If the focus isn't perfect, repeat until it is.

Acquire Target

There are many ways to find and center the target in the field of the imaging camera. For really brightobjects, I just center them using the imaging camera's viewfinder. M42 is an example of one suchtarget.

For more illusive targets, I use my Precise GoTo Spreadsheet to compute the RA and DEC offsets(deltas) between the Target Object and some bright, nearby, easy-to-find, unmistakable, ReferenceObject. The Reference Object is almost always a nearby bright star or planet. I then center the

Reference Object in the imaging camera's field and then pull up "Get RA/DEC" in the mount's HandController. Then I subtract the offsets from my spreadsheet, and slew the mount to the resultingcoordinates. In all cases, the slews must finish with the Up and Right arrow keys, in order to deal withany backlash in a consistent fashion. In my experience, this method is amazingly accurate and easy toexecute.

In any case, here's the procedure I use to acquire (and verify) my target:

• Use the "Precise GoTo" spreadsheet as described above to get the target centered.

• Still using DSLRfocus in "focus mode", take a test exposure (ISO 1600, 30 seconds,Small/Normal quality) and inspect the result. Even fairly dim targets will reveal themselves at

these exposure settings. If the target is not centered correctly, slew the mount in the appropriatedirection and repeat this process until it is.

• Using the mount's Hand Controller, store a "User Object" (Save Sky Object) at the currentcoordinates, so you can easily return to this location if you accidentally hit a slew button or youaccidentally change the RA or DEC when removing/replacing dust caps.

• In DSLRfocus, set the camera to the ISO you'll actually use for your Light frames, to RAWquality mode, and to Bulb exposure, then disconnect from the camera and shut downDSLRfocus.

• Turn off the camera, and physically remove the USB cable from the camera and the laptop.

Acquire Guide Star

Acquiring a Guide Star can either be simple and quick, or excruciatingly painful. The pain factor isinversely proportional to how well the Guide Scope's finder is aligned with the center of the webcam'sfield in the Guide Scope. If it is very well aligned, then putting the Guide Star in the finder's cross hairswill land the Guide Star on the chip every time. Otherwise, prepare to be frustrated. Here's mysequence for acquiring a Guide Star, picking up where we left off above:

• Plug in the webcam's USB cable to the laptop.

• Fire up GuideDog in "Focus" mode and crank the Gain in the webcam's control panel.

• Use the Guide Scope's finder to find a suitable Guide Star.

• Adjust the Guide Scope rings (not the mount's RA/DEC!) to center the Guide Star in the GuideScope's finder.

• If you carefully aligned your Guide Scope's finder to the Guide Scope/webcam, then the GuideStar should be visible in GuideDog at this point. If not, you'll have to "hunt" for it.

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exposures in order to maximize the signal to noise ratio of the stacked result. By dither , I mean slewthe mount ever-so-slightly in RA and/or DEC in a random direction between each exposure, in order tomove the target around on the imaging chip. This serves to mitigate the problem caused by unevensensitivity of the individual pixels on the chip, as well as the effect of hot pixels. It will allow you tostretch the final processed image a bit more than you otherwise would, without revealing ugly-looking"pattern noise" in the background, and therefore allow you to reveal more detail in your target. I highlyrecommend it, but I acknowledge that it's labor intensive and a royal pain in the you-know-what.

Controlling your laptop remotely with something like pcAnywhere can ease the pain level of dithering.

Some more comments on dithering. You must completely disable guiding while dithering; otherwise,GuideDog will return the Guide Star to exactly where it was previously, thereby negating the ditheringaction. To completely disable guiding, be sure to click (uncheck) the Guide button and the Lock button

 before slewing the mount. Then reenable guiding by clicking the Lock button, selecting the Guide Star,and clicking the Guide button again.

Here's my procedure for capturing Lights:

• Verify that the Eyepiece Cover is in place on the 300D's viewfinder.

•

Configure DSLRfocus with desired number of frames, inter-frame spacing, etc. I generallyallow for 1min "down time" between frames, which gives me time to do my dithering and for GuideDog to stabilize before the next frame begins.

• Enable guiding in GuideDog

• Let 'er rip!

• Dither: Between each frame, disable guiding completely (both the Guide and the Lock buttonsin GuideDog), slew the mount a TINY amount in a random direction, re-enable guiding, andrepeat.

• Shoot as many frames as you can. More frames are always better than less, and with all of thissetup time, and with all of the time that you will spend processing, it makes sense to shoot asmany Light frames as possible. Shoot until the object sets or the realities of your sleep

requirements dictate.

A few more notes on dithering. The "pattern noise" on these consumer DSLRs tends to be vertical andhorizontal in nature. So, a really bad dithering pattern would be to slew the mount in a linear fashion,either in RA or DEC. A much better dithering pattern is an "outward spiral" which, when you start toget too far away from the center, you can spiral back inward. Also note that a smidgeon of fieldrotation (from polar misalignment) can actually be a good thing in this regard, since it will further serve to randomize these vertical and horizontal patterns, once the frames are registered (derotated) in

 processing.

Acquire Darks

 Now that you've acquired your Lights, it's time to (immediately) begin capturing your Darks. I like tocollect at least 9 Darks, perhaps more if the Light frames had a relatively short exposure time. In anycase, collect an odd number of Darks since you will more than likely be median-combining them, andthe numerical median operator just likes to have an odd number of samples.

I let DSLRfocus time the Dark frames just like with the Lights. In fact, I like to go from capturingLights to capturing Darks "without skipping a beat" in DSLRfocus. By this I mean that if I've got 1minof inter-frame "down time" during my Lights, I like to have exactly that same interval between the lastLight and the first Dark, and use that same inter-frame spacing between the capture of all the Dark 

frames. This procedure yields an almost identical match between the Lights and the Darks in regards tothe (lack of) cool-down time of the sensor between frames. Here's my procedure:

• Disable guiding in GuideDog.

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• Pop the 300D off of the T-ring and put the Body Cap on the bayonet.

• Keep the Eyepiece Cover in place on the 300D's viewfinder.

• Have DSLRfocus time the capture of at least 9 Dark frames at the same ISO, same exposuretime, and same inter-frame "down time" as the Lights.

• Take these immediately after capturing Lights in order to make sure the temperature whentaking the Darks is as close to possible to the temperature when taking the Lights.

• Take at least 9 Dark frames. 15 is better.

While the Darks are being captured, I either: begin processing the Flat frames; begin putting the rest of my equipment away for the night; or begin doing visual astronomy of some of the "eye candy" that'swell placed at the current time. I also continue to remind myself that I'm not done when the Darks arefinished. There remains one more step ...

Acquire Offsets

You're almost done. What remains is to capture a handful of short-as-possible Offset (Bias) frames.Here's my procedure:

• Disconnect the exposure-timing cable that connects the laptop's parallel port to the 300D'sshutter control port.

• Set the exposure time on the 300D to 1/4000'th of a second.

• Keep the Body Cap in place over the bayonet.

• Keep the Eyepiece Cover in place on the 300D's viewfinder.

• Keep the ISO at the same setting you used for your Lights and Darks.

• Take at least 9 Offsets. 15 is better.

Congratulations, you've finished your acquisition! Now it's time to process the result. Ok, actually,tomorrow morning is time to process your result. <grin>

Processing

 Now that you've acquired your Flats, Lights, Darks, and Offsets, you can begin processing your framesto bring out as much detail as possible. Doing so requires some fairly sophisticated software. Myfavorite application by far for this task is the freeware package named IRIS by Christian Buil. Others

 prefer ImagesPlus by Mike Unsold, which, as of this writing, was being sold for roughly US $200.But, again, this is my cookbook, and I like IRIS, so this processing sequence will be explained in terms

of IRIS. The outline is as follows:

1. Create Master Flat2. Create Master Dark 3. Calibrate Lights4. Convert CFA to RGB5. Register 6. Stack 7. White Balance8. Crop9. Remove Gradient

10.Stretch11.Touch up in Photoshop12.Optional Optimizations

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What follows is not meant as an introductory tutorial on IRIS. For that, see Christian's various tutorialslinked from the IRIS home page. In particular, be sure to read the Illustrated tutorial on DSLR 

 processing, and the more in-depth Digital camera image processing tutorial.

For now, let's make sure IRIS is configured correctly:

• In the File->Settings... panel, set the File type to PIC and set the Working path to a

directory/path on a disk with boatloads of available storage.• Click on the Photo icon on the toolbar (the one that looks like a camera) to bring up the

Camera Settings dialog. Set the Binning to 1x1, Digital camera Model to the correct choicefor your camera (CANON (10D/300D/Rebel) in my case), and RAW interpolation method toGradient. Leave the Apply box unchecked under White balance.

• Click on the Command window icon on the toolbar just left of the Photo icon to bring up thecommand window. You'll be entering your commands into this window.

• Select the Analysis->Display data... item from the menu bar to bring up the informationwindow.

 Now you're ready to go...

Create Master Flat

1. Convert RAW Flat Lights to PIC (CFA). Select Digital Photo->Decode RAW files... which,somewhat disarmingly to first-time IRIS users, will push IRIS to the background of your Windows. It does this so you can pull up Windows Explorer and navigate to the directorywhere your RAWs are stored. Do so, select all of the Flat Lights (the .CRW files in the case of the 300D; .NEF, .PEF, etc., for other camera types), and drag them into the IRISDecode RAW files dialog. Give the sequence a name (I use fl for flat lights), and then hit the->CFA... button. IRIS will convert the selected RAW files to PIC.

 2. Convert RAW Flat Darks to PIC (CFA). With that window still open, hit Erase list and thendrag all of your Flat Darks (again, the .CRW files for the 300D; .NEF etc. for other cameratypes) into the main portion of the window. Give this sequence a different name than the oneyou used for Flat Lights (I like fd for flat darks), and then hit the ->CFA... button. IRIS willconvert the selected RAW files to PIC. Click the Done button when finished. 

3. Make the Flat Master Dark . My current process involves median-combining the individualFlat Darks to make a Flat Master Dark, without using the Flat Offsets. But IRIS' dark-creation

 procedure requires a Master Offset when creating a Master Dark. So, we must first produce a"dummy" offset file that contains zeros at every pixel location. The easiest way to do that is to

load any of the files we've already converted to PIC, fill the image with zeroes, and then save it.Here's how to accomplish this in the command window:>load fd1>fill 0>save zero

 Now select Digital Photo->Make a dark... and in the resulting dialog box, set Generic Name tofd (or whatever you called your Flat Dark sequence), Offset image to zero (the dummy offsetwe created above), Number to the number of images in your Flat Dark sequence, and Methodto Median. Hit OK and IRIS will subtract off this null (dummy) offset and then median-combine the Flat Dark images. The result is stored in IRIS' memory but not yet on disk. Sowhen this operation finishes, enter the following:>save flat-master-dark

 4. Identify hot pixels. Use the find_hot command on the flat-master-dark to identify the hot

 pixels. The trick is to determine a threshold value over which IRIS will deem a pixel to be a"hot pixel". One method that works for me is to deem any pixel that exceeds the "Mean plus

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(16 times sigma)" as a hot pixel. However, I know for sure that this does not work for a friendof mine's 300D, as it produces far too many hot pixels. In the case of Flat Darks, there should

 be very few hot pixels that reveal themselves at such short exposures. Try using differentthresholds with the find_hot command until you get somewhere in the neighborhood of 20

or so hot pixels. Here's an example:>load flat-master-dark>stat

Mean: 125.0 Median: 125Sigma: 2.1Maxi.: 274.0 Mini.: 114.0>find_hot flat-cosmetic 158.6Hot pixels number: 3

where flat-cosmetic is the name of the file into which IRIS stores the list of hot pixels, and thethreshold (158.6) equals the Mean plus (16 times the Sigma); i.e., 125 + (16 * 2.1). 

5. Select the Flat Lights. Inspect the Flat Lights one at a time by loading them into IRIS (>load 

filename in the command window; e.g., >load fl1) and looking for signs of clipping.

One way to do this is pan to the brightest portion of the image, draw a rectangle with the

mouse, right-click the mouse and select the Statistics item. IRIS will then display the max, min,and other useful statistics about the selected area. Be suspicious of maximum values that areanywhere near 4095, which is the saturation value of the 12-bit pixels in the 300D. (But also trynot to be fooled by the presence of one or more "hot pixels" that happen to lie within your rectangle. Draw a few rectangles to make sure.) I try to stay away from any Flats Light thathave pixel values greater than about 3500. Generally speaking, I'll use 9 Flat Lights to createmy Master Flat, starting with the brightest image in the sequence that doesn't clip andcontinuing through the subsequent images in the sequence until a total of 9 is reached. At this

 point, the easiest thing to do is create a new sequence with only those selected Flat Lights,starting with the first desired image in the sequence. So, as an example, let's say that of all theFlat Lights, we want to use the 6th through the 14th (14 - 6 + 1 = 9). In that case, I'd type the

following command into the command window:>reindex fl flselected 6 1 9

where the new sequence is called flselected and it consists of 9 total images, beginning with the6th image in the fl sequence. 

6. Calibrate the selected Flat Lights against the Flat Master Dark. The easiest way to do thisis to use the Preprocessing... item in the Digital Photo menu. However, this function is reallydesigned to calibrate your "actual" lights against a Master Dark, Master Flat, Master Offset, etc.So, we need to trick it by putting in a few dummy values. First, we need to create a "dummyflat", because the last thing we want to do is divide our Flat Lights by an actual Flat field. Wedo this similarly to how we created the dummy offset above, by loading any file and filling itwith a uniform, non-zero value. Here's how:>load zero>fill 20000>save 20000

 Now bring up the Digital Photo->Preprocessing... menu, and enter the following values:Input generic name = flselected, Offset = zero, Dark = flat-master-dark (Optimize = not checked), Flat-field = 20000, Cosmetic file = flat-cosmetic,Output generic name = fld (flat light with dark applied), Number = 9. IRIS will then subtractthe flat-master-dark from each selected Flat Light and also "fix" the hot pixels. 

7. Create the Master Flat. Select Digital Photo->Make a flat-field... and fill in the fields asfollows: Generic name = fld, Offset image = zero, Normalization value = 20000, and Number = 9. IRIS will subtract the (dummy) offset from our calibrated, selected Flat Lights, normalizethem so they're all the same intensity (brightness), and then median-combine them. The result isin memory but not yet on the disk, so be sure to save it:>save master-flat

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 8. (Optional) Disk cleanup. At this point, if you need to reclaim space on the disk, you can delete

all the files in your IRIS working directory except for your RAW camera files, master-flat.picand zero.pic.

Create Master Dark 

1. Convert RAW Darks to PIC (CFA). Use the same procedure as described in the CreateMaster Flat section. Call this sequence d for dark. 

2. Create the Master Dark. Select Digital Photo->Make a dark... and fill in the fields as follows:Generic name = d, Offset image = zero, Number = 9 (or however many darks you have), andMethod = Median. Hit OK and IRIS will subtract this (dummy) offset and then median-combine the Dark images. Be sure to save the result:>save master-dark

 3. Identify hot pixels. Use the same procedure as described in the Create Master Flat section to

identify hot pixels in the Master Dark. Here we're shooting for hot pixels on the order of severalhundred. My algorithm of "Mean plus 16 times Sigma" works for my 300D here too. You'llhave to experiment with yours. Here's an example:>load master-dark>statMean: 120.3 Median: 119Sigma: 9.5Maxi.: 4008.0 Mini.: 91.0>find_hot cosmetic 272.3Hot pixels number: 82

where cosmetic is the name of the file into which IRIS stores the list of hot pixels, and thethreshold (272.3) equals the Mean plus (16 times the Sigma); i.e., 120.3 + (16 * 9.5). 

4. (Optional) Disk cleanup. At this point, if you need to reclaim space on the disk, you can deleteall the files in your IRIS working directory except for your RAW camera files, master-flat.pic,zero.pic, master-dark.pic, and cosmetic.lst.

Calibrate Lights

1. Convert RAW Lights to PIC (CFA). Use the same procedure as described in the CreateMaster Flat section. Call this sequence l for light. 

2. Calibrate Lights against Master Flat, Master Dark, and Hot Pixel Map. Bring up theDigital Photo->Preprocessing... menu, and enter the following values: Input generic name = l,Offset = zero, Dark = master-dark (Optimize = not checked), Flat-field = master-flat,Cosmetic file = cosmetic, Output generic name = ldf (light with dark and flat applied), and

 Number = N , where N is the number of light frames you have. For each Light frame, IRIS willsubtract the master-dark, divide by the flat-field, fix the hot pixels, and save the result in a newfile.

Convert CFA to RGB

1. Bring up the Digital Photo->Sequence CFA conversion... menu and enter the following values:Generic input name = ldf , Generic output name = ldfrgb, Number = N (where N is the number of Lights), and Output files type = Color.

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Register

As of IRIS v4.34, use the following, automated procedure to automatically shift (translate), rotate, and,if necessary, scale each calibrated Light frame in order to line them up for stacking:

>setmatch 2>setspline 1

>setregister 1>coregister2 ldfrgb ldfrgbcoreg  N 

where N is the number of Light frames.

Stack 

As a first cut, use a simple summation to combine the registered frames. If the details get clipped(saturated) with the add2 command, then use the add_norm command which will multiply the sum bya constant value to prevent clipping:

>add2 ldfrgbcoreg  N >save sum 

where N is the number of Light frames.

Inspect the result. If there are a significant number of uncorrected hot pixels, cosmic rays, or other  bright (or dark) anomalies, then use Kappa-Sigma stacking. As of v4.34 there was a bug in IRIS that prevents this from working on color images, so one must separate the RGB files into individual red,green, and blue (grayscale) sequences before stacking. Here's how:

1. Separate the RGB sequence into a Tricolor sequence. Select Digital Photo->Sequence RGBSeparation... and enter the following values: Generic input name = ldfrgbcoreg, Generic R = r,Generic G = g, Generic B = b, and Number = N (where N is the number of Lights). 

2. Kappa-Sigma stack each color pane. Here's how:>composit r 4 1 1  N >save rtemp

>composit g 4 1 1  N >save gtemp>composit b 4 1 1  N >save btemp

>tr rtemp gtemp btemp>save sum 

(where N is the number of Lights).

Play around with the sigma parameter (4 in the example above) and the iterations parameter (1 in theexample above) to strike a healthy balance between rejection of aberrant values on the one hand, andgetting a good stack of valid data on the other. The second 1 in the composit command is a flag tellingIRIS to normalize the result to prevent clipping (overflow). If a "straight stack" with no normalizationdoes not result in clipping of the details you care about, then change that second 1 to a 0.

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White Balance

Adjust the visualization sliders to get a decent view of your stacked result. Find an area with as few bright stars in it as possible, and draw a rectangle with the mouse. Then execute the followingcommand:

> black

Then, select Digital Photo->RGB balance... to compensate for the varying sensitivity of the 300D tored, green, and blue light, respectively. I use Christian's (the IRIS author's) proposed coefficients of R = 1.96, G = 1.00, and B = 1.23, although my sense is that this gives a tad too much red and a tad toolittle blue. But some of this is personal preference, and you can always salt-n-pepper to taste inPhotoshop at a later stage. At this point, be sure to save the result:

>save sum-wb

Crop

Inspect the edges and the four corners of the image and you'll see some "registration anomalies". Thegoal at this stage is to crop the image to that area which represents the intersection of all the registeredframes. Adjust the visualization sliders to allow you to see these registration anomalies, and todetermine where to crop. By clicking in each of the four corners of the image, you can look in theOutput window to see the coordinates of those locations where you clicked. Then use the IRISwindow command to crop to the coordinates that you provide:

>window  x1 y1 x2 y2>save sum-wb-crop

where ( x1, y1) are the (x,y) coordinates of the lower-left cropping point, and ( x2, y2) are the coordinatesof the upper-right cropping point.

Remove Gradient

If there's any light pollution at your site, especially if that light pollution is uneven throughout the sky,it's likely that your image has a rather ugly-looking background gradient at this point. Also, if for somereason your Master Flat isn't correcting for vignetting, the background will also have such a gradient.IRIS has a pretty cool set of commands for attempting to remove this gradient, but you'll have toexperiment a bit to get the desired result. Repeat the following commands until you get a reasonable

result, adjusting the visualization sliders and, perhaps, shrinking the image at each iteration so thewhole thing fits on the screen (use the zoom out button on the toolbar):

>load sum-wb-crop

>setsubsky  sigma  poly_order >subsky

I usually try sigma = 1 and poly_order = 3 at first, and then cycle both parameters until I get thedesired result. When satisfied, be sure to save the result:

>save sum-wb-crop-subsky

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Stretch

I absolutely love the Hyperbolic Arc Sin (asinh) stretching function in IRIS, as I find it provides amuch more pleasing result than the Digital Development Process (DDP) available in most image-

 processing programs. Interestingly, a variant of the asinh stretch is used by JPL to process Hubble photos. Finding the right alpha parameter (the aggressiveness of the stretch), and the right intensity parameter (a post-stretching scaling factor to prevent clipping or to make the result brighter), is largely

a matter of trial and error. So, I iterate among the following commands until I get the desired result:

>load sum-wb-crop-subsky>asinh alpha intensity> visu 32767 -2000

I usually try alpha = 0.005 and intensity = 30 at first. For each value of alpha, you want to find theright intensity value to make sure the brightest feature that you don't want clipped will have anintensity value of 32767. As for the alpha parameter, changing it by tiny amounts can have a hugeeffect. So, if 0.005 is a pretty middle-of-the-road stretch, 0.010 is a very aggressive stretch, and 0.001is a fairly benign stretch. Again, play with the values until you get a result that shows as much detail as

 possible, but without bringing up the background noise to unacceptable levels. When satisfied, be sureto save the result:

>save sum-wb-crop-subsky-asinh

Touch up in Photoshop

At this point IRIS has proven to be a useful and effective soldier. It's now time to export the data toPhotoshop for final touchup. Unfortunately, IRIS uses "signed 16-bit integer math" and Photoshopuses "unsigned 16-bit integer math", and so one must jump through a couple of hoops before exporting

from IRIS to Photoshop. Specifically, we have to take the signed integer range of [-32768..32767] andmap it into the non-negative range of [0..32767] without clipping on either the positive or negativeend. Here's how:

>load sum-wb-crop-subsky-asinh> mult 0.499992370489052>offset 16383.7499961852

Then save to Photoshop:

>savepsd sum-wb-crop-subsky-asinh

In Photoshop, you'll have to use the Levels command to stretch these values back to the full, unsignedinteger space of [0..65535]. That can be done easily by pulling up the Levels command and setting the

 black point to around 60, and the white point to 128. Depending on your Photoshop setting, sometimesthe Auto levels command will take care of this automatically and give the desired result. Since thisisn't a Photoshop tutorial, I'll just mention that I sometimes adjust the white balance and overall stretcha bit to get the final result I want. I'll also rotate the canvas if the camera was "upside down" to make

 North=up (or North=left, depending on the camera orientation).

Congratulations!!! You're probably now staring at a killer astrophoto. If it looks good, or even if itdoesn't, shrink it, convert it to JPG, and send me a copy of it! Also, if you have feedback or questions

about this document, please send those comments as well. My E-mail address is solospam at comcastdot net.

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Optional Optimizations

Here are a few of my "tricks of the trade" to further improve upon the results that can be accomplishedabove:

• Bring the background level to zero before stacking to improve the effectiveness of kappa-sigmastacking and to increase dynamic range of the normalized stack. See the noffset2 command for 

more details.• If you find that dark-subtraction is producing "black holes" in your frames, then create a "real"

Master Offset and use that in the creation of your Master Dark. Then use "optimize"ed dark-subtraction when calibrating your Lights. The benefit will be better dark-subtraction, but at the

 possible expense of having more residual amp glow in the result.

• Write your own program to stack, white balance, and stretch, using double-precision floating point variables. That will give you a virtually limitless dynamic range. Or convince Christianthat he should add this capability to IRIS. <grin>

Archive Your Results

Burn a CD or DVD now with all of your RAWs (.crw and .thm) and your final, full-resolution, full-size processed result -- both the IRIS file (.pic) and the Photoshop file (.psd). If there's space, includethe Master Flat, Master Dark, and Cosmetic file as well.

Questions and Answers

My Goodness, you're anal. Do you really go through all of this for each image?

Yes. Astrophotography is one of the most difficult flavors of photography there is, and there area whole lot of things that can go wrong. So, I try to minimize the chances of blowing it , sinceeven the non-anal version of setup, acquisition, and processing takes a ton of time. A lot of thesteps I take are specifically there to correct a previous "blown night" of imaging. I think myresults speak for themselves, especially when considering that I'm using a scope/mount whichtogether cost US $950, and a camera that sells for about US $900.

Why do you align the 300D North/South or East/West?

Aesthetically, because I like my pictures to be in the de facto standard orientation of North = Up.Also, this allows a simple and direct comparison of my images with a sky chart or the DigitalSky Survey, etc. And, more practically, because if the framing happens to be off when I take mytest shot, I know immediately in which direction (RA or DEC) I need to slew the mount to fixthe framing.

Why do you Drift Align? Can't I just use this cool Polar Axis Finderscope I bought?

Good luck. I find such tools to be a waste of money, because if I'm doing visual astronomy, themount can handle a huge polar alignment error, and if I'm doing astrophotography, those toolsdon't provide nearly enough accuracy. I don't understand why so many people are terrified of Drift Aligning. Practice it on a cruddy night with cruddy transparency, when you can only seethe brightest of the bright stars and can't do anything else anyway. It becomes second nature after a little bit of practice.

Why do you use IRIS? I like <some image processing program> better.

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Because IRIS rules. I love it and it produces tremendous results. Yes, it takes a while to walk IRIS' learning curve, but once you do it makes perfect sense and, well, it is astonishingly

 powerful. The price is hard to beat too (free!).

Why do you collect Offsets but not use them in processing?

Actually, I usually don't use them, but sometimes I do, so it's real nice to have them and it only

takes a minute or two to collect them. Technically, the Darks contain the Offsets, and if youmedian-combine the Darks (that haven't had the Offset subtracted), then you're really ending upwith a "Master Dark plus Offset", and it's that very beast which we're subtracting from theLight's in the Calibrate Lights step. Thus, in effect we are subtracting an Offset from the Lights,

 just not the specific one(s) we collected.

However, sometimes I notice that the Calibrated Lights "look too noisy" to me, even after dark subtraction, etc. The cause of this is usually a large temperature difference between the timewhen (most of) the Lights were captured and the time when the Darks were captured, which can

 be several hours in time and 10 to 20 degrees Farenheit in temperature. In such a case, the Darksaren't as well matched to the Lights as I'd like. In that case, I'll use the collected Offsets to create

a Master Offset (see Digital Photo->Make an offset...), and then use that Master Offset (insteadof the dummy, zero-filled file) in the creation of the Master Dark. Then, in the Calibration stage,I'll supply the actual Master Offset (instead of the dummy, zero-filled file) and also check theOptimize box. This asks IRIS to scale the Master Dark to an optimum intensity beforesubtracting it from each Light, in order to maximize the Signal to Noise ratio of the result. Butthis function of scaling the Master Dark before subtraction only works if the Master Offset has

 been subtracted from each Dark in creation of the Master Dark.

Why do you collect Flats every night? I heard you only need to do this once per optical configuration?

You heard right. Maybe. If your optics are perfectly collimated; if that collimation never needs

adjustment; if the illumination of the imaging chip is perfectly symmetric with respect to thecenter of the chip and/or you put the camera in the exact same orientation every time; if thenumber and location of dust spots on the sensor never changes ... then yes, you only need to takeone set of Flats for each such configuration. But, in my humble opinion, those are someextremely big "if's", and so I've decided to take Flats every time I set up. It's easy to do anddoesn't take very long. Just do it.

What GuideDog/camera/mount settings do you use for guiding your mount?

With the standard caveat that Every mount is different , and These settings might not work well  for you, here are the settings I've found work well for my mount:

Parameter Value Units

Autoguide Rates 1 99.0 % of Sidereal

Radius 24.0 pixels

RA Guide Rate 15.0 "/s

DEC Guide Rate 15.0 "/s

Maximum Error 15.0 "

Minimum Error 2 1.5 "

RA Backlash 0.0 msDEC Backlash 0.0 ms

Declination Corrections yes (checked)

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Use Autostar Pulseguide no (unchecked)

Aggressiveness 3 124.0 % of computed correction

GuideScope Focal Length 4 1026.0 mm

 Notes:1  The Autoguide Rates (AZM RATE and ALT RATE) are set on the AS-GT mount's Hand

Controller, in the Scope Setup menu. These rates must match the RA and DEC Guide Ratesconfigured in GuideDog.

2  On nights of poor seeing, I increase the Minimum Error parameter to 2.0 or even 2.5 so thatGuideDog isn't chasing the seeing, and is really only correcting for errors in the mount'stracking.

3  You'll have to experiment with the Aggressiveness parameter. I've found that somewhere between 100% and 150% works well for me.

4  You might wonder why a 400mm GuideScope (the ST80) when fitted with a 2x Barlowyields an effective focal length of 1026mm, not 800mm. That's because a Barlow really only

 provides its rated focal-length multiplier nominally, and the actual factor depends on the exactspacing of the eyepiece, or in this case, the webcam's CCD from the Barlow's lens elements.

In my setup, the actual factor ends up being 2.565x, and hence that effective focal length. Theeasy way to measure the effective focal length, is to shoot the same, distant, daytime targetwith the webcam through the GuideScope, with and without the Barlow. Then figure out inPhotoshop or some other program by what factor you need to shrink the Barlowed image sothat the features in the image have the same size as the un-Barlowed image. Then take thatfactor and multiply by the GuideScope's focal length.

As for the webcam, I adjust the ToUcam's settings to the mid-point on the first page of  parameters (i.e., Brightness, Contrast, Saturation, etc.), and I use the slowest possible frame rate(5 fps). On the second page, I use the longest exposure time (1/20sec) and then adjust the Gainuntil the star is nice and bright but not clipped in the preview window. With very bright stars, I

 back off the exposure time to prevent clipping.

 I think your procedure is seriously flawed. Here's why ...

Then, by all means, please send your comments to me! I'm always looking for a good tip or a better method than the one I'm currently using. My address is solospam at comcast dot net.

Revision History

v0.4 5/5/05

• Fixed some typos.

v0.3 5/4/05

• Clarified dithering procedure: one must (un)click the Lock button in GuideDog before slewing the mount. (Thanks Andy.)

• Added a Background section on Lights, Darks, Offset, and Flats

v0.2 5/4/05

• Incorporated comments from initial reviewers. (Thanks Jared, Jim, and Rich.)

v0.1 5/3/05

• Initial release

Copyright © 2005, Jim Solomon. All Rights Reserved.