variable star differential photometry
DESCRIPTION
Variable Star Differential Photometry. CCD. Photometry Is The Measurement Of A Stars Brightness. Early Measurement Systems Originated with Either Hipparchus or Ptolemy, Greek Philosophers (circa 150 BC-150AD). Assigned “1” to the Brightest Stars - PowerPoint PPT PresentationTRANSCRIPT
Variable Star
Differential Photometry
CCD
Photometry Is The Measurement Of A Stars Brightness
Early Measurement Systems Originated with Either Hipparchus or Ptolemy, Greek Philosophers (circa 150 BC-150AD)
Assigned “1” to the Brightest StarsAssigned “2” to the Next Brightest, etc., All the Way to “6”
In 1854 The English Astronomer, Pogson, Suggested that Magnitude Be Defined as a Brightness
Ratio of ~ 2.5 Between Successive Magnitudes Such that a Change in 5 Magnitudes Would Exactly Equal 100 Times Change in Brightness.
m1 – m2 = -2.5log(F1/F2)
What Are Variable Stars?Stars That Simply Vary Their Light Output Over Time
They May Be Part Of A Double Star System Or They May Be A Single Star
~ 2 % of All Stars Show a Measurable Change In Brightness
Visualization by Andy Beardmore
There Are Various Classifications forVariable Stars. A Few Examples:
Cepheids: Period ~ 1 - 70 Days, Vary ~ .1 -2.0 Magnitudes
RR Lyrae Stars: Period ~ .2 - 1 Day, Vary ~ .3 – 2.0 Magnitudes
Long Period Variables: Period ~ 80-1000 Days, Vary ~ 2.5 – 5 Magnitudes
V368 Peg Super OutburstWith Humping10/05/09
Su Ursae Majoris Types of CataclysmicVariables: Frequent & Short OutburstsLasting ~ 1 - 2 Days. Occasionally SuperOutbursts Lasting ~ 10 – 20 Days WithSmall Periodic Modulations CalledSuperhumps.
Why Observe Variable Stars?
The Data Is Very Important to Astronomers & Astrophysicists
Variable Star Data, Dependent Upon Type, Can Be Used To Determine: Luminosity, Temperature, Radius, Mass Composition, Rotational Period & Distance
There Is Also a Critical Need
For Us To Understand & Monitor The More Nasty High Energy Eruptive Variables Such As GRB’s (Gamma Ray Bursts), Supernovae & BL Lac Objects (Blazars).
Our Survival May Depend On These Observations!
The American Association Of Variable Star Observers
With It’s Paid Staff of ~12 Folks & Many Volunteers , LocatedIn Cambridge, MA, isThe Main Repository For Submitted Variable Star Data
You Do Not Have to Be a MemberTo Submit Observations.
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World Wide There are About 260 “Main” Observatories (Infrared,Optical & Radio)
The ~ 260 Big “Scopes” Are Over Whelmed With Requests For Their Limited Time
Then There Is The Humongous Size Of Our Universe With Zillions of Stars
Therefore, Amateur’s Are Very Important to The Data GatheringProcess & Play a Critical Role Observing Variable Stars.
The AAVSO Receives FrequentRequests From Professional Astronomers For Photometry Data as Well asEducators,Students,& OtherAmateurs
AAVSO Members & Observers Are Often Asked To Support Scientific Projects By
Professional Astronomers, Whether Using Land or Space Based Equipment, To Make Observations Of Specific Targets
Three Request Examples From Jan. 2010
Monitor HMXB’s In SupportOf 1.85 Meter Dominion Ob.(Victoria, BC) for Phd @ Univ.of Saskatchewan, Canada
Monitor KT Eri/Nova Eri 2009 In Support Of Swift Satellite X-Ray Campaign for Phd @ Univ of Leicester, England
Monitor Recurrent Nova UScorpii For Outburst to TriggerObservations by Hubble, Swift& Spitzer for Phd@ LSU, USA
Observing Requires A CCD Camera - Preferably OneDesigned For Photometry, i.e.NAB (non Anti-Blooming),Monochrome & Cooling Ability
ST-7/8/9/10XMEST-402ME
A Lot Of Potential Variable Star Observers Already Own AB (anti-
Blooming) CCD’s – ‘taint no thangWe Can Work With That: Either Turn AB Off, Keep Exposures to
½ Full Well Capacity Or Plot the Linearity
A[pg
CCD Linearity - NABG vs ABG
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time (Seconds)
AD
U's Series1
Series2Series3
Notice How Much More Random Noise Is In The Hotter Image
+ 25C Dark Image - 25C Dark Image
CCD’s Need To Be Cooled
While Mentioning Darks
Photometry Requires That Images Be Calibrated: Subtract Darks & Flats!
Flats Contain Light Path“Artifacts” Within The Optical “Train.”
Darks Contain The Random ElectronicNoise Inherent Within The CCD.
Different Models of CCD Chips Have Different Spectral
Responses
Each Of The Three PopularChips Shown Has A Different Spectral Response In The Region Of A Star’s Light.
If Unfiltered ObservationsWere Made With Each Then The Reported Magnitudes Would Be Wildly Different
One Of The Strengths Of CCD’s Is That When We Use The “Right” Filter We Can Equalize The Passbands Of The Various Types Of Ccd’s.
Observe With “V” Filter!
The Johnson-Cousins FilterStandard System
We Need To Find A Chart For TheFOV We Are Going To Observe: 3C 66A
WWW.AAVSO.ORG
Enter Target Name Here To Find The VSP
VariableStar ChartPlotterEntry Screen
aka VSP
Target Name
FOV Scale
FOV Scale In Arc Minutes
Magnitude Limit
FOV Orientation
Use DSS Image
Next Click OnThe “ComparisonStars” At Top Of Plotted Chart To Acquire Comp Data
Magnitudes ShownAre For Visual Use
Comp Star Data For 3C 66A
“V” Filter Values
m1 – m2 = -2.5log(F1/F2)
Instrumental Magnitudes
Mag(ins) = ZPmag-2.5log(StarFlux)
Mag Formula:
Can Be Rewritten ForInstrumental Magnitudes
Where ZP Is A Somewhat Arbitrary Zero Point Used By Software
Differential Photometry
The Differential Magnitude (Vmag) Is Computed By Taking The (difference) Between The Variable (unknown) Stars Instrumental Magnitude And A Known Stars Instrumental Magnitude Such That:
Vmag = (v-c) + Cmag
Where v & c are instrumental magnitudes
Cmag is the known Comp Star Magnitude
The Differential PhotometryEquation Can Also Be Expressed As:
Vstd = (Vins – Cins) + Cstd
Now It’s Time To Do A Real World Example
AGN
NASA Drawing
3C 66A Original Image
3C 66A
3C 66A Is A Blazer , a TypeOf “AGN”Which Is a Catch All For Galaxies That Have Bright & ConcentratedEmissionsFrom TheirCentral Regions
3C 66A Images by Tim Crawford
Comp
3C 66A
Check
Comp = 142 Star & Check = 148 Star
Compute IM’s
Above Is The AIP4WIN Log Output Showing The Instrumental Mags Of The Variable Star, The Comp Star & The Check Star.
Comp & Check Star Data For 3C 66A
“V” Filter Values
Vmag = (v-c) + CmagWhere v & c are instrumental magnitudesCmag is the known Comp Star Magnitude
Vmag = (14.321-14.525) + 14.182 = -.203 + 14.182 = 13.979 With An Error =.014
Check Star
In An Ideal World: K-C = k-cThe Difference Between the Known Check Star Mag and The Comp Star Mag = the Difference Between Their Instrumental Mags (IMO, Any Difference up to ~ .06 Is probably OK). K C k c
14.771 – 14.182 = .589 & 15.112-14.525 = .587
In this Case K-C ~ = k-c (Difference of .002)
WWW.AAVSO.ORG
You Do Not Have To Be A Member Of The AAVSO To Report Observations
However, You Do Have To Secure Observer Initials From The AAVSO To Make Reports.
This Is Easy To Do . Choose the New Observers Link From The Main Page Then Choose the FAQ’s
After Sign In To Blue & Gold
WWW.AAVSO.Org
Filled Out Observation Form With The Information From The ComparisonStars Data & What We Generated
Chart “name” & Comp Label From Comp Star Page
The Julian Date (JD) Is The Interval Of Time In Days And Fractions Of A Day Since January 1, 4713 BC Greenwich Noon
Many Of The Photometry Programs, Including AIP4WIN (above), Now Allow You To Simply Enter the Comp Star Value, Or Multiple Comp Star Values, Directly Into The Program So That You Do Not Have To Do Any Math As The Vmag Will Be Automatically Output.
This Is A Beta But Similar To Current Pgm
Above Is An Example Of AN AAVSO Up-Load Ready File AS Output By The “Measurement Magnitude Tool” Of AIP4WIN.
Note That No Calculations Are Required & The Air Mass Has Been Calculated
Best Kept Secret OFVariable
Star Observing
You Can Do CCD Observing In Spite Of Mr. Moon!
LogoContributeTo Science By ObservingVariable Stars!
