Photometry with AIP4Win

Hi Folks:

I've a question with regard to getting photometric measurements with AIP4Win and I thought I might put it here since, I'm hoping, the forthcoming answers will also help others.

As some of you may have seen before, I'm taking images from the Bradford Robotic Telescope. Via the eye, I think the images have been quite good. Now, mind you, they're not filtered so I've been categorizing them as "unfiltered with V zeropoint."

Now, I'm at least semi-familiar with AIP4Win (v2.3) and its use with regard to photometry, but I'm not getting the photometric accuracy that I'm expecting out of the images.

I'm doing single star photometry. Obviously I know the integration time of my image (4 seconds in this case) and I figure out the Z value emperically by doing photometry on one of the comparison stars. For example, concentrating on ζ Aurigae, it's magnitude, according to our finder chart, is listed at 3.8. Assuming that's correct, I then find Z=13.325. With that set up, if I do photometry on η Aurigae I should find its magnitude close to the listed value of 3.2. I don't. In this case I get 3.453 which I consider really off!

Conversely, I concentrate on η Aurigae and determine Z=13.07 to get a magnitude of 3.2 (the listed value of this star on the chart). If I then try to get the value of ζ Aurigae, expecting something near to 3.8 I get 3.54. Again, this is unacceptally off.

Now I know the images I'm getting from Bradford are not filtered. They are "flat fielded" although I don't know the details of the process they're using. I am absolutely not expecting millimagnitude accuracy here by any means, but the errors I am getting seem to indicate I could do a better job with my eye.

What am I doing wrong?
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Doc Kinne, KQR