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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 Assigned “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)
m1 – m2 = -2.5log(F1/F2)
Instrumental Magnitudes
Mag(ins) = ZPmag-2.5log(StarFlux)
Mag Formula:
Can Be Rewritten For Instrumental Magnitudes
Where ZP Is An 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 Photometry Equation Can Also Be Expressed As:
Vstd = (Vins – Cins) + Cstd
Now It’s Time To Do A Real World Example
AM Her Type VS aka Polars
© Russell Kightley Media
To Find A Chart For The FOV You Are Going To Observe
WWW.AAVSO.ORG
Enter Target Name Here To Find The VSP
Then Select Variable Star Charts Then Variable Star Plotter (VSP)
Can Also Enter Target’s Coordinates
AM Her
Enter Target Name Here To Find The VSP
Remember, Chart Values Are For Visual Use
BUT You Need The Chart To Pick Out Your Comparison Stars & To Locate VS
AM Her Original Image AM Her Is The Namesake Of The Class of “AM Her Stars” Known As Polars
In Which The Highly Magnetic Field Of A White Dwarf Accretes Material From A Red Dwarf Star
AM Her Image by Tim Crawford
AM Her
V Filter
To Accomplish The Photometry We Need Software Designed For This Purpose
Lots Of Options
AIP4WIN MIRA
IRAF (free) MPO Canopus/Photo Red
MAXIM VPHOT (free to AAVSO Members-Web Based)
AIP4WIN Software
We Need To Determine First What The Radius Of The Annulus Ring Will Be
Aperture Radius Determination
Rule Of Thumb: 2*FWHM
In this Case: 2*3.57 = 7.14 Pixels
Inner Annulus Radius = 1.5*Aperture Radius
Outer Annulus Radius = 2.5* Aperture Radius
However, There Is Another Alternative To The Aperture Rule Of Thumb of 2*FWHM
Note The Insert Showing The Previous Star Up Close With A 7 Pixel Aperture Radius---It Is A Bit Larger Than The Star Appears To Be
“When The Aperture Is To Large, it degrades The Signal To Noise Ratio.” Richard Berry
When The Aperture Is to Small You Do Not Get All The Signal Included
We Want The Curve To Flatten Out Before The Aperture Bar
Here We Want To See It Flatten Between the Aperture And Inner Annulus
Six Pixel Aperture Will Work
Aperture Analysis
Change From 7 Pixels To 6
Enter Target Name Here To Find The VSP
Remember, Chart Values Are For Visual Use
BUT You Need The Chart To Pick Out Your Comparison Stars & To Locate VS
Check Star
Comp Star
AM Her
O
.
Insert AAVSO Chart Top Showing Photometry Option
Comparison Star Data For AM Her When Using A CCD
aka A Sequence
AM Her
Comp Star
Check Star
Comp Star = 13.109 --- Check Star = 14.312
Above Is The AIP4WIN Log Output Showing The Instrumental Mags Of The Variable Star, The Comp Star & The Check Star.
Variable Comp Check
Instrumental Mags
Vmag = (v-c) + Cmag Where v & c are instrumental magnitudes Cmag is the known Comp Star Magnitude
Vmag = (13.564-13.209) + 13.109 = .355 + 13.109 = 13.464 With An Error =.01
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 14.312 – 13.109 = 1.203 & 14.422-13.209 = 1.213
In this Case K-C ~ = k-c (Difference of .01)
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.
WWW.AAVSO.ORG
Choose Observing Tab Then Observing At A Glance
WWW.AAVSO.ORG
Filled Out Submission Form
Now That You Have An Understanding Of The Basics There Is A More Convenient
Way: Use The Magnitude Measurement Tool of AIP4WIN
Select The Image(s)
Other Software Pgms – Have Convenience Options, Also
Then Select The Aperture Tab
(you will then need To select a star on the Image before the next Step)
Next Select The Stars Tab & A Saved Stars File
This Is The Heart Of the Photometry Processing PGM With The MMT
Circle: Target (V) Comp Star (C1) Check Star (C2)
Then Select The Report Tab & AAVSO Format
After Selecting The Execute Tab A File Will Be Created With All The Required Data For Direct Uploading To The AAVSO
Execute Tab
Save The Output File & It Will Upload Directly To The AAVSO
WWW.AAVSO.ORG
Browse for Your Saved File, Click The Upload File Box & It Will Ask You To Verify The Data.
