Variable Star CCD Differential Photometry Photometry Is The
Variable Star CCD Differential Photometry
Photometry Is The Measurement Of A Stars Brightness Early Measurement Systems Originated with Either Hipparchus or Ptolemy, Greek Philosophers (circa 150 BC-150 AD) 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. m 1 – m 2 = -2. 5 log(F 1/F 2)
Instrumental Magnitudes Mag Formula: m 1 – m 2 = -2. 5 log(F 1/F 2) Can Be Rewritten For Instrumental Magnitudes Mag(ins) = ZPmag-2. 5 log(Star. Flux) 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 AM Her Type VS aka Polars Now It’s Time To Do A Real World Example © Russell Kightley Media
To Find A Chart For The FOV You Are Going To Observe WWW. AAVSO. ORG Then Select Variable Star Charts Then Variable Target(VSP) Name Here Star. Enter Plotter To Find The VSP
AM Her Can Also Enter Target’s Coordinates
Remember, Chart Values Are For Visual Use BUT You Need The Chart To Enter Target Name Here Pick Out Your To Find The VSP Comparison Stars & To Locate VS
AM Her Original Image AM Her Is The Namesake Of The Class of “AM Her Stars” Known As Polars V Filter AM Her In Which The Highly Magnetic Field Of A White Dwarf Accretes Material From A Red Dwarf Star AM Her Image by Tim Crawford
To Accomplish The Photometry We Need Software Designed For This Purpose Lots Of Options AIP 4 WIN MIRA IRAF (free) MPO Canopus/Photo Red MAXIM VPHOT (free to AAVSO Members-Web Based)
AIP 4 WIN 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
Aperture We Want The Curve. Analysis To Flatten Out Before The Aperture Bar Six Pixel Aperture Here We Want To See It Flatten Between the Aperture Will Work And Inner Annulus Change From 7 Pixels To 6
Remember, Chart Values Are For Visual Use BUT You Need The Chart To Enter Target Name Here Pick Out Your To Find The VSP Comparison Stars & To Locate VS Check Star Comp Star
Insert AAVSO Chart Top Showing Photometry Option AM Her O .
Comparison Star Data For AM Her When Using A CCD aka A Sequence
Check Star AM Her Comp Star = 14. 312 --- Check Star = 13. 109
Instrumental Mags Variable Comp Check Above Is The AIP 4 WIN Log Output Showing The Instrumental Mags Of The Variable Star, The Comp Star & The Check Star.
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 AIP 4 WIN Other Software Pgms – Have Convenience Options, Also Select The Image(s)
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 (C 1) Check Star (C 2) 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
Oregon Observatory Presentation Tim R Crawford - CTX www. arch-cape. com tcarchcape@yahoo. com
- Slides: 30