Learning and Using MPO PhotoRed

Copyright, 2006 © Bdw Publishing

Brian D. Warner

Palmer Divide Observatory

Course Highlights

• The Theory and the Practice - understanding the reduction process and methods

• Planning Your Observation Runs

• Working the Tutorials in the Users Guide

Photometric Reductions

• The Johnson-Cousins Standard

– Uj Bj Vj Rc Ic

– CCD filter specs from Bessell

– Landolt redefined standards for CCD using

equatorial fields

– Henden fields OK for secondary standards

if individual stars chosen carefully

Photometric Reductions

Photometric Reductions

Transforms

Learning and Using PhotoRed

Transforms First

• Transforms (approximate) required for Modified Hardie Method (FOE)

• Independent of FOE

• Initial Transform for a filter

M = (m - k’fX) + Tf(CI) + ZPv (standard CI, not inst)

• Rearrange and drop constants (shift line up/down but not slope)

M - m = Tv(CI)

Transforms First

Method• Image a field with well-known standard magnitudes (2-4

images in each filter)

• Must use at least two standard filters and have catalog values for each filter

• Measure images

– For each star measurement: standard magnitudes (for each standard filter) and instrumental magnitude.

Transforms First

Method• Find the “hidden” transforms

– Plot standard CI (Y-axis) versus instrumental CI (X-axis)

Converts a given instrumental CI to the standard CI required for the final transforms

Transforms First

Transforms First

Method• Find the final transforms

– Plot M-m (Y-axis) versus computed standard CI (X-axis) (using hidden transforms)

– The resulting slope is the transform for the filter

– The Zero Point is NOT necessarily the correct nightly ZP! Valid only for the assumed X.Modified Hardie Method self-adjusts

Transforms First

Summary

• Use a high (X~1) field with standard catalog values

• Shoot in at least two standard filters

• Use assumed FOE value

• Slope is transform. ZP may need to be changed.

First Order Extinction

Learning and Using PhotoRed

First Order Extinction

• Symbol: k’f f = filter or color index

• A function of “Air Mass” (X) = mag/X

• Different value for different colors

• Usually expressed for color index, e.g., k’bv but NOT in PhotoRed

• “Typical” values:

k’b = 0.4 - 0.5 k’v = 0.2 - 0.3 k’r = 0.15 - 0.25

First Order Extinction

Comparison Star Method

Requirements

• Follow a field throughout the night (or at least visit periodically)

• Wide range of air mass values

• Conditions MUST remain constant

First Order Extinction

Comparison Star Method

Method

• Measure instrumental magnitude and air mass for each image

• Plot data with Y = mag and X = air mass

• Solve for slope of line

First Order Extinction

First Order Extinction

Modified Hardie Method

Requirements

• Two fields with known standard magnitudes, one at high air mass and one at low air mass

• Can be same field but adds requirement that conditions do NOT change.

• MUST have at least first guess transforms

• Standard mags in each filter (V for Clear)

First Order Extinction

Modified Hardie Method

Method

• Observe first field in selected filters. Get 2-4 images in each filter

• Repeat for second field as quickly as possible

• Measure instrumental/standard mag and air mass for 5-10 stars on all images (all filters)

First Order Extinction

Modified Hardie Method

Method • Compute Average of M-m-TfCI and air mass for each field.

– M-m-TfCI should be a constant (assuming perfect color match)

– Extinction causes M-m-TfCI to be smaller

• Extinction = (AvgL - AvgH) / (XL - XH)

First Order Extinction

First Order Extinction

Modified Hardie Method (Variation)

• Shoot fields with wide range of air mass as quickly as possible. (Hipparcos and SDSS stars in Practical Guide)

• Compute as before

Allows intermediate values of X and so better control over slope solution

First Order Extinction

• “Fix” Transforms

Use derived k’ for each filter, recalculate transforms. This also finds the true nightly zero point

OR

Running Hardie Method in PhotoRed finds transforms as part of process and can apply the zero points correctly (saves going back)

The Differential Approach

Learning and Using PhotoRed

The Differential Approach

• The “Messy” FormulaMo - Mc = M

M = ((mo - k’fX) + Tf(CIo) + ZPf) - ((mc - k’fX) + Tf(CIc) + ZPf)

• Simplified (Reduced) Formula

M = (mo - mc) + Tf (CIo - CIc)

CI is standard color index

The Differential Approach

• Deriving Standard Mags for the Target

– Must know standard magnitudes for comparisons

– Must know standard color indices for comparisons and target (“hidden” transforms to the rescue)

The Differential Approach

• Deriving Standard Mags for the Target

The WRONG way:

Average the reduced standard magnitudes of the comparisons and subtract that from the reduced target magnitude

The RIGHT way :

Find the differential value for each target-comparison and then find the average (and standard deviation) of that sum

The Differential Approach

• Finding the Standard Color Index for Comps and Target

CI = ((m1 - m2) * TCI) + ZPCI (X1~X2)

TCI and ZPCI are “hidden” transforms slope and zero point

1. For each pair of filter measurements compute CI values If more than one image in each filter, find for each pair - do not average magnitudes in each filter first.

2. Find average/s.d. of sum of CIs for each star and target

The Differential Approach

• Finding the Standard Mags for the Comparisons

M = (m - k’fX) + Tf(CI) + ZPf

– For each observation of the comp, compute M

– Do include the k’fX and ZPf terms. Remember: if Xt~Xr, then assumed k’ and resulting ZP values are valid.

– Find the average /s.d. for each comparison

The Differential Approach

• Finding the Standard Mags for the Target

Mt = Mc + (mo - mc) + Tf (CIo - CIc)

– For each observation of the target, compute Mt

– Use CI values found earlier

– No k’, X, or ZP terms required

– Plot versus time (JD)

The Differential Approach

The Binzel Method

Learning and Using PhotoRed

The Binzel Method

Reasons to Use

• Approximate conversion to standard system

• Can use mostly Clear observations with a few observations of a reference field and target in a standard color

• Fairly straightforward

• Procedure included in PhotoRed

The Binzel Method

Restrictions

• Comps and target should be similar in color (color terms not included)

• Target and Reference field should be near same air mass (so that X is minimized)

• Target and Reference field must be shot at about the same time (avoid changing conditions)

• Images of target field must take minimum possible time (avoid changes in target magnitude)

The Binzel Method

Method• Shoot target field with Clear until it and reference field

well above horizon (X~1) and nearly same air mass

• Move to reference field and shoot series of images in standard filter

• Move to target field and shoot alternating filters as quickly as possible, e.g., CVCVCV. Shorten exposures to minimize error due to target variability.

• Resume shooting with Clear filter

Reductions• See

– PhotoRed documentation

– MPB Article in MPB 32-4(www.MinorPlanetObserver.com/mpb/default.htm)

– 2nd Edition of “A Practical Guide for Lightcurve Photometry and Analysis”

The Binzel Method

Observations and Reductions Plan

Learning and Using PhotoRed

Assumptions

• Most observations in Clear

• Want to Transform to V, use V/R filters and V-R CI

• Using Modified Hardie Method for First Order Extinction– Using two Henden fields

– Highest will be used for transforms

• Asteroid field >30° altitude at start of run

• Canopus used to measure most Clear observations

• PhotoRed used only for Reductions

Observations and Reductions Plan

Observations

• Shoot both Henden fields at start of night, getting 2-4 images in Clear plus two standard filters, e.g., V&R

• Shoot Target field images in Clear and two filters.

• Continue shooting in Clear

• (Optional) Shoot target field in Clear and two filters when near meridian.

• (Optional) Shoot Henden fields towards end of night. This reverses the role of high/low field.

Observations and Reductions Plan

Measuring Images - Canopus

• Create a new session in Canopus for Clear filter observations.

• Measure Clear images only

• Open PhotoRed

Observations and Reductions Plan

PhotoRed Reductions Steps

• “Transforms” using higher Henden field

• “FOE All-sky” – Modified Hardie (reset nightly zero points)

• “Color Indices Comps/Target”

• “Standard Mags - Comps”

• “Standard Mags - Target” (import/export Canopus data)

Observations and Reductions Plan

“Time to Make the Donuts”

Learning and Using PhotoRed