Affiliation
American Association of Variable Star Observers (AAVSO)
Fri, 09/11/2015 - 03:38

Hello Everyone,

I'm having issues using IRIS to perform some differential photometery on a faint magnitude 16 variable star I recently discovered using my 4-inch f/7 refractor.  I staked sets of 6x90second lights and darks along with flats and bias recoreded with my Canon XS DSLR using IRIS.  Following evaluation of the green channel, I obtained some very inconsistent Intensity values from IRIS on a single night of observing ranging from 4000 counts to over 6000 counts on the variable without any trend evident in the inconsistency.  Also, both comarison stars exhibited the same degree of nonprecision.  One challenge is that there are two faint stars flanking the variable on each side so to isolate the variable, I am using aperature values of 5,6,7.  It then becomes a challenge to properly center the variable and comaprisons within the 5 aperature circle.  Is there a way to make it easy using IRIS like a magnify feature?  Not sure how to handle future evaluations.  Any good pointers would be appreciated.

Thank You Kindly

Affiliation
American Association of Variable Star Observers (AAVSO)
Mag 16 and 4"

 

Hi Kevin,

First, making accurate photometry at mag 16 using a 4" and a DSLR is not an easy task ! I am not so surprised you get such variability in particular if it's a raw DSLR count of the foreground (the inner circle) ? or is it corrected from some comparison star ? Second, the problem is your far too small external circle.

If you have some nearby stars that could be a problem you should put them into the annulus (between circle 1 and circle 2), by the way its should be large enough, not 6 if the inner is 5, but much more. Then the external circle (the third) shall be large enough to provide as much as possible pixels to provide the best possible background measurment and noise averaging. With 7 only your background measurment is probably strongly affected by noise. If some small star fall into that background area, use the median option. It should eliminate them from the count. As an example your circle radiuses could be 5, 12, 30 or more...

Clear Skies !

Roger