1. Variable Star
Differential Photometry
CCD

2. 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
Assigned “2” to the Next Brightest, etc., All the Way to “6”

3. 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)

4. 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

5. ~ 2 % of All Stars Show a Measurable Change In Brightness
Visualization by Andy Beardmore

6. There Are Various Classifications for
Variable Stars. A Few Examples:
Cepheids: Period ~ Days, Vary ~ Magnitudes
RR Lyrae Stars: Period ~ Day, Vary ~ .3 – 2.0 Magnitudes
Long Period Variables: Period ~ Days, Vary ~ 2.5 – 5 Magnitudes

7. V368 Peg Super Outburst With Humping 10/05/09
Su Ursae Majoris Types of Cataclysmic
Variables: Frequent & Short Outbursts
Lasting ~ Days. Occasionally Super
Outbursts Lasting ~ 10 – 20 Days With
Small Periodic Modulations Called
Superhumps.

8. The Data Is Very Important to Astronomers & Astrophysicists
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

9. 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!

10. The American Association Of Variable Star Observers
With It’s Paid Staff of ~12 Folks & Many Volunteers , Located
In Cambridge, MA, is
The Main Repository For Submitted Variable Star Data
You Do Not Have to Be a Member
To Submit Observations.

11. There are About 260 “Main” Observatories (Infrared,Optical & Radio)
1.55 Meter USNO
World Wide
There are About 260 “Main” Observatories (Infrared,Optical & Radio)

12. 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

13. Therefore, Amateur’s Are Very Important to The Data Gathering
Process & Play a Critical Role
Observing Variable Stars.

14. The AAVSO Receives Frequent
Requests From Professional
Astronomers For Photometry
Data as
Well as
Educators,
Students,
& Other
Amateurs

15. 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

16. Three Request Examples From Jan. 2010
Monitor HMXB’s In Support
Of 1.85 Meter Dominion Ob.
(Victoria, BC) for Univ.
of Saskatchewan, Canada
Monitor KT Eri/Nova Eri 2009
In Support Of Swift Satellite
X-Ray Campaign for
Univ of Leicester, England
Monitor Recurrent Nova U
Scorpii For Outburst to Trigger
Observations by Hubble, Swift
& Spitzer for LSU, USA

17. NAB (non Anti-Blooming), Monochrome & Cooling Ability
Observing Requires A CCD Camera - Preferably One Designed For Photometry, i.e.
NAB (non Anti-Blooming),
Monochrome & Cooling Ability
ST-402ME
ST-7/8/9/10XME

18. A Lot Of Potential Variable Star Observers Already Own AB (anti-
Blooming) CCD’s – ‘taint no thang
We Can Work With That: Either Turn AB Off, Keep
Exposures to
½ Full Well
Capacity Or
Plot the
Linearity
A[pg

19. CCD’s Need To Be Cooled + 25C Dark Image - 25C Dark Image
Notice How Much More Random Noise Is In The Hotter Image

20. While Mentioning Darks
Photometry Requires That Images Be Calibrated: Subtract Darks & Flats!
Darks Contain The Random Electronic
Noise Inherent Within The CCD.
Flats Contain Light Path“Artifacts” Within The Optical “Train.”

21. Different Models of CCD Chips Have Different Spectral Responses
Each Of The Three Popular
Chips Shown Has A Different
Spectral Response In The
Region Of A Star’s Light.
If Unfiltered Observations
Were Made With Each Then
The Reported Magnitudes
Would Be Wildly Different

22. 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
Filter Standard System

23. We Need To Find A Chart For The FOV We Are Going To Observe: 3C 66A
Enter Target Name Here To Find The VSP

24. Variable Star Chart Plotter Entry Screen aka VSP Target Name FOV Scale
FOV Scale In Arc Minutes
Magnitude Limit
FOV Orientation
Use DSS Image

25. Stars” At Top Of Plotted Chart To Acquire Comp Data
Next Click On
The “Comparison
Stars” At Top Of Plotted Chart To Acquire Comp Data
Magnitudes Shown
Are For Visual Use

26. Comp Star Data For 3C 66A
“V” Filter Values

27. Instrumental Magnitudes
m1 – m2 = -2.5log(F1/F2)
Mag Formula:
Can Be Rewritten For
Instrumental Magnitudes
Mag(ins) = ZPmag-2.5log(StarFlux)
Where ZP Is A Somewhat Arbitrary Zero Point Used By Software

28. 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

29. The Differential Photometry Equation Can Also Be Expressed As:
Vstd = (Vins – Cins) + Cstd
AGN
Now It’s Time To Do A Real World Example
NASA Drawing

30. 3C 66A Original Image 3C 66A Is A Blazer , a Type Of “AGN” Which Is a
Catch All For Galaxies That
Have Bright &
Concentrated
Emissions
From Their
Central Regions
3C 66A
3C 66A Images by Tim Crawford

32. 3C 66A
Comp
Check
Compute IM’s
Comp = 142 Star & Check = 148 Star

33. Above Is The AIP4WIN Log Output Showing The Instrumental Mags Of The Variable Star, The Comp Star & The Check Star.

34. Comp & Check Star Data For 3C 66A
“V” Filter Values

35. Vmag = (v-c) + Cmag Vmag = (14.321-14.525) + 14.182 = -.203 + 14.182
Where v & c are instrumental magnitudes
Cmag is the known Comp Star Magnitude
Vmag = ( ) =
= With An Error =.014

36. Check Star In An Ideal World: K-C = k-c
The 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
– = .589 & = .587
In this Case K-C ~ = k-c (Difference of .002)

37. 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

38. After Sign In To Blue & Gold

39. Filled Out Observation Form With The Information From The Comparison
The Julian Date (JD) Is The Interval Of Time In Days And
Fractions Of A Day Since January 1, 4713 BC Greenwich Noon
Chart “name” & Comp Label From Comp Star Page
Filled Out Observation Form With
The Information From The Comparison
Stars Data & What We Generated

40. Many Of The Photometry Programs, Including
This Is A Beta
But Similar
To Current
Pgm
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.

41. 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

42. Best Kept Secret OF Variable Star Observing
You Can Do CCD Observing In
Spite Of Mr. Moon!

43. To Science By Observing Variable Stars!
Logo
Contribute
To Science By Observing
Variable
Stars!

44. Arch Cape Presentation Observatory tcarchcape@yahoo.com Oregon
Tim R Crawford - CTX