A couple of months ago I wrote in thismagazine about how technology such asCCD cameras and the HyperStar systemwere making astrophotography much sim-pler. And, while I certainly praised the factthat this made the process of imaging moreenjoyable and opened up this aspect of ourhobby to many more people, I may havelamented a bit the demise of the sufferingthat used to be required to take astropho-tos. It didn’t seem right that it should be soeasy.

Of course, I’m not really complainingabout saving time and trouble. I’m finewith not going back to suffering forastrophotos. A major nuisance of deep-skyimaging was the process of focusing. TheMicroTouch Autofocuser, available in bothwired and wireless models, is a powerfuland precise motorized focuser that helpsturn focusing from a dreaded, time-con-suming, arduous task, into a simple click ofa button.

The Way it WasThinking back to my days (or nights,

rather) of film astrophotography, it neverceases to amaze me how far we’ve cometechnologically in less than a decade. My

first astrophotos were focused by just look-ing through the camera viewfinder andhoping for the best. This resulted in some,shall we say, less than successful results.Take for instance the night I drove 150miles to reach skies darker than the insideof a cow, only to take eight hours of out-of-focus astrophotos.

I decided that was unacceptable,although that may not have been the exactterm I used at the time. So I upgraded tousing a high-power loupe to magnify theimage on the groundglass in the camera’sviewfinder. This helped a bit, but was still ahit or miss proposition. Next was the onlysurefire method of focusing a film camera:the knife-edge focuser. Mine was built froman old plastic CD case, a piece of film,some tape and a couple rubber bands. Andit was about as easy to use as a piece of filmtaped to a CD case and attached to a cam-era with rubber bands – which is to say, notvery.

The jump to CCD technology alonewas a huge leap forward in the ease andconsistency of focusing. Imagine the differ-ence between, say, trying to hit a baseballwith your eyes closed versus swinging withyour eyes open. Your batting average is

going to go way up. With a CCD camerayou can actually get feedback from thecomputer on the size of a star. Make thestar as small as possible and you’re golden.While my average went from below theMendoza line to a perfect thousand, focus-ing remained tedious.

The process still necessitates focusingby hand, and there are other hassles thatmake it more time-consuming than neces-sary. Viewing the computer screen fromwhere you can reach the focus knob isn’talways easy. You have to touch the scopeand that induces vibrations. You have tomake a guess about the actual star size,since the values displayed change slightlydue to atmospheric turbulence. In the end,it typically takes at least ten minutes tofocus by hand. There’s room for improve-ment.

Impatience DrivesInnovation

Technology saves an amazing amountof time. We used to have to wait for thingslike dinner, film processing, pregnancytests. Now we get instant results, althoughthe element of suspense is lost, for better orworse.

Astronomy TECHNOLOGY TODAY 57

By Scott Tucker

The Starizona MicroTouch Autofocuser

Impatience isthe Mother ofInvention

Think of all the things that technologyhas made faster and easier: e-mail, forexample. Remember when you used tohave to walk all the way to the mailbox toget unsolicited junk? Or, consider how on-line dating has changed lives. You used tohave take time to clean up and put on niceclothes to go to a bar. But now you can surffor prospective dates on-line from the con-venience of home, while wearing a beat-upKansas Live in Tuscaloosa 1977 T-shirt andfuzzy bunny slippers. You can’t get dates ina bar in fuzzy bunny slippers (or at least it’snever worked for me). The Internet hasrevolutionized activities as varied as the artof spreading rumors, the task of dissemi-nating misinformation, and identity theft.

But now technology can do somethingyou’ll actually find useful! It can automati-cally focus your telescope. Plus it can do itbetter and faster than you. A process thatused to take 10 to 15 minutes now requiresonly 30 seconds of your precious time.That means more time for you to work ondeleting all your junk e-mail.

Focusing NumbersFocusing with a CCD camera,

whether manually or automatically,involves the analysis of values displayed bythe software controlling the camera. Thebasic idea is to determine the size of thestar, turn the focus knob, see if the star getssmaller or bigger, and continue turning theknob to make the star as small as possible.Doing this requires going past the actualpoint of best focus, because you don’t know

what the minimum size of the star will beuntil you’ve passed it and started to makethe star bigger again.

The usual value for measuring the sizeof a star is its full width at half max, orFWHM. Imagine a graph of the intensityof a star. A star is not a perfect pinpointafter passing through an optical system.Instead, it is brightest at its center andquickly drops off in brightness away fromthe center of the star image. The profile ofthis intensity has a bell shape, and a per-fectly focused star creates a very tall, skinnybell. As the star is defocused, the bellspreads out, and the peak intensity (the topof the bell) gets lower because the light isbeing spread out over a larger area. TheFWHM of a star is simply the width of thebell at a point halfway to the top of the bell– the full width of the star image at half itsmaximum intensity.

The smaller the FWHM value, thesmaller the star image. Also, the maximumpixel value, or the intensity of the brightestpixel in the star image, increases as FWHMdecreases due to more light being concen-trated in a smaller area.

So what are the typical values of theFWHM? It depends greatly on the tele-scope, the seeing conditions and the CCDcamera. This is why you have to pass thepoint of best focus to find the minimumFWHM value. Even with the same tele-scope and the same camera, it might be dif-ferent than on a previous night due tochanging atmospheric conditions.

Ideally, it would be best to focus the

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telescope every time you point it to a newobject. This allows correction for shiftingoptical components, expansion or contrac-tion of the scope due to changes in temper-ature, and changes in seeing conditions. Ifyou are manually focusing, this gets prettyold pretty quick. Technology to the rescue!

AutofocusingThe idea behind autofocusing is sim-

ple. It does the same thing you would do,only faster and more accurately. The soft-ware looks at the FWHM values (or a relat-ed value called the Half Flux Diameter,about which below) and moves the focuserto reach the minimum value. The advan-tages are many – not least of which is thatyou don’t have to touch the scope andinduce vibrations, and of course, the usualadvantage that a computer can do mathfaster than you.

The real advantage of the software-driven focusing system, though, is that the

computer program knows exactly howmuch to move the focuser to reach a givenFWHM value. It does this by initiallylearning your telescope system. It deter-mines how much to move the focuser tochange the FWHM value by a certainamount. Then, by measuring the currentfocus position and the current FWHMvalue, it knows how much to move toreach the minimum value. Plus it knowswhat the minimum value should be, basedon the characteristics of your system.

This is Where theMagic Happens

The MicroTouch Autofocuser isASCOM compatible and can be con-trolled by a number of software packages.The focuser ships with a copy ofFocusMax, software by Larry Weber andSteve Brady. This program integrates seam-lessly with camera control software such asMaxIm DL to allow easy autofocusing.

FocusMax is initially calibrated to theuser’s telescope system. It learns howquickly the telescope focus converges onthe ideal point and how much to move thefocus motor to effect a certain change infocus position. Once this is known, thesoftware simply needs to determinewhether the telescope is inside or outsideof focus and the size of the star image.FocusMax uses a measurement called HalfFlux. This measurement is similar toFWHM but is less sensitive to factors suchas seeing conditions and light pollution.After calculating the size of the star, it is a

simple matter of offsetting the focuser bythe correct amount to reach exact focus.

While an understanding of this proce-dure is useful, all you really need to knowis that it works. I prefer to think of it theway I think of my car: I don’t need to knowthe exact inner workings of an internalcombustion engine. I just need to knowthat I can put gas into it and it will get meto a Mexican restaurant. I give it fuel, itgives me burritos. The rest is just magic.You feed an autofocuser photons and itgives you perfect pinpoint stars in less thana minute. Magic!

The entire process is amazingly fast.The focuser is moved to a starting positionand a low-resolution image is taken of thefull field of view. The brightest star is auto-matically detected. A high-resolution sub-frame is then taken of just the bright star.The focus is moved and the star is imagedagain. The software then takes a sequenceof five or six more images of the star toaverage out fluctuations due to seeing con-ditions. There is a dramatic 5-second pausewhen you think maybe something hasgone terribly wrong. Then – abracapocus!– a perfectly focused star image appears onthe screen and you can breathe again.Magic!

The MicroTouchAutofocuser

The MicroTouch was designedaround the Starlight InstrumentsFeathertouch Focusers. These focusers arehigh-precision and feature 10:1 fine focusadjustments. They are available for justabout any type of telescope, includingpopular imaging systems such as refractorsand Schmidt-Cassegrains. The quality andprevalence of these focusers made themideal for a new autofocuser system.

By driving the 10:1 fine focus shaft ofthe Feathertouch Focuser, the MicroTouchprovides extremely high accuracy. Precisionis important, especially for fast-focal-ratioimaging systems such as HyperStar. A tele-scope with a fast focal ratio has a very nar-row depth of focus, so being able to move

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the focuser in tiny increments is essentialto achieving exact focus. Each pulse of theMicroTouch stepper motor equals a mere1/300th of a revolution of the focus shaft.This allows precise focusing of even themost sensitive imaging systems.

Avoiding EntanglementHaving no wires attached is a lofty

ideal – just ask Pinocchio. A typical CCDimaging setup has cables for telescope con-trol, the CCD camera, an autoguider, thefocuser, and power cords for all thoseitems, plus other accessories like a dewheater. It’s only a matter of time before youtrip over all those wires, hit your head, andsee the wrong kind of stars.

You can get wireless phones, wirelessnetworks, wireless Internet connections,wireless remotes for your TV, and evenwireless control for your telescope. Whynot a wireless focuser? Aside from the cool-ness factor (which is always the mostimportant thing, I think), there is a practi-cal side. Many imagers are starting to go

60 Astronomy TECHNOLOGY TODAY

This image of the Iris Nebula was taken by Larry Moore using a HyperStar equippedCelestron C14 and Starlight Xpress SXVF-H9C one-shot color CCD camera.

STARIZONA MICROTOUCH AUTOFOCUSER

remote. In the dead of winter, when theskies are clear and the temperatures are 30degrees below comfortable, it’s nice to sitinside while your telescope suffers out inthe elements. Having one less cord to runfrom your scope to the computer simplifiesthings. The wireless model of theMicroTouch Autofocuser has a range of upto 300 feet.

Additional FeaturesThe MicroTouch Autofocuser also

features temperature compensation. Thisallows the focuser to compensate forchanges in focus due to the expansion orcontraction of the telescope’s optics ormechanical structure. This is ideal forlong-exposure imaging.

The temperature compensation, andother features such as the motor speed anddirection, focus position and digital read-out, wireless connection, and more, can becontrolled either through the MicroTouchhand control or through the stand-alonesoftware included with the autofocuser.

Firmware updates for the hand controland motor control can also be downloadedto the autofocuser from the StarizonaWeb site.

Autofocusing is a perfect solution foravoiding one more of the challenges ofastrophotography. Wireless connectivityadds the bonus of being able to sit insidetoasty warm in the winter or out of thehumid bug-filled nights of summer, atyour computer in your fuzzy bunny slip-pers, capturing images of the universe andwondering how you ever got along withoutall this technological magic.

More details on theMicroTouch Autofocuserat www.starizona.com

For tips and tutorials on focusing, seethe Guide to CCD Imaging at www.stari-zona.com/abc/ccd/ccd.aspx. This site alsofeatures a detailed discussion on themechanics of focusing, including depth offocus, focusing for a curved field, andfocusing tolerances.

Astronomy TECHNOLOGY TODAY 61

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