Transcript Learning and Using MPO PhotoRed

Learning and Using
MPO PhotoRed
Brian D. Warner
Palmer Divide Observatory
Copyright, 2006 © Bdw Publishing
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
Learning and Using PhotoRed
Transforms
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.
Learning and Using PhotoRed
First Order Extinction
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)
Learning and Using PhotoRed
The Differential Approach
The Differential Approach
• The “Messy” Formula
Mo - Mc = DM
DM = ((mo - k’fX) + Tf(CIo) + ZPf) - ((mc - k’fX) + Tf(CIc) + ZPf)
• Simplified (Reduced) Formula
DM = (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
Learning and Using PhotoRed
The Binzel Method
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 DX 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
The Binzel Method
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”
Learning and Using PhotoRed
Observations and
Reductions Plan
Observations and Reductions Plan
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)
Learning and Using PhotoRed
“Time to
Make the Donuts”