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Basic Photometry. Written by myself in year 1998.

March 2006. Please note:

With the advancement of CCD astrometry and photometry, software now enables the amateur to measure accurately at the touch of the keypad on a personal computer. Reference and comparison stars can be drawn into the software off huge databases such as The U.S.N.O. A2 for such tasks. See "Astrometrica" under my Links pages.

Photometry of AGN's Variables and Supernova/Nova magnitude fluctuations.
Using Starlight Express Windows Photometry program.
The ADU or "ANALOG to DIGITAL UNIT" is the arbitrary unit used by the computer
to add up charges on each pixel.
The computer then converts these into magnitudes !.
It is most important to use professional filters for very accurate measurements. 
Quasars are blue objects so B filter 420nm may be used.
Photometry
A chart showing the object and comparison stars is necessary.
An example of this is in The picture which shows Quasar 3C232 near the galaxy 
NGC 3037. See "CCD image library" for processed field and information on this image.
In Starlight Express CCD photometry program select a raw image of well defined brightness. 
Do not alter the raw image in any way as readings will not be correct. Do not select fit 
to frame or make any contrast stretches. Brightness only does not seem to alter things as 
all three requirements mentioned below are relavent to each other.

Toolbar. Select View
Select Photometry Of Image.
OBJECT Centre pointer on object. Click on it. 
COMPARISON Select known reference comparison star. Click on it.
BACKGROUND. Select a blank area of sky which is not at the edges of the field. Click on it. 
It is advisable to ensure that the area selected is away from any galaxy as deep exposures
often reveal extended regions which may intefere with the Background levels. But do not 
select to far from the object and comparison.
Make a note of the MAGNITUDE DIFFERENCE and move to the U B V R data on the sequence chart 
about the object and comparison stars. 
Example below of photometry for CCD frame ( + ) .73

QSO 3C232 13th February 1997 at 0135UT  by JRF at MTO UK.
Sky Background = 878662 ADUs
Reference Minus Sky = 166566 ADU's
Object Minus Sky = 85408 ADU's
Relative Magnitude of Object with Respect To Reference = .7252195
( call this .73 ) 
 
Calculation.
QSO V = 15.78
Ref Star "D"        = 15.82 in "B"
Subtract B-V        =   .62
                    = 15.20 = V Magnitude
                    ADD .73
Magnitude of object = 15.93 So 15.93 
                              -15.78 = .15 fainter on this occasion.



Useful Book.
"A Handbook of Quasistellar and BL Lacertae Objects" by Eric R.Craine. 
Steward Observatory University of Arizona 
Pachart Corporation
Pachart Publishing House
P.O. Box 6721
Tucson
Arizona 85733

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