[pvelez (Pete)]

At the moment I am dabbling in photometry - climbing the light(learning) curve. One point that its constantly stressed in the literature is the need to calibrate with flat frames.

I have been particularly unsuccesful in my efforts to date.

To give you an idea of my issue, I've attached 3 images. The first shows an uncalibrated image taken with Maxim DL. The vignetting is reasonably apparent.

The second is a copy of the flat frame I took using a t shirt at twilight on Friday.

The third is the first image calibrated in Maxim DL. As you can see, something has gone awry in the processing.

Now the flat image looks quite bright. However, when I open it in ImagesPlus and review the histogram, the curve peaks quite nicely around the 30% level. I suspect that Maxim auto-stretches the image for display.

My suspicion is that the auto-stretch is also applied when calibrating the image so that rather than removing the vignetting, the program overcompensates.

My next thought is that there is some serious reflection going on inside my scope or the extension tube of the focuser. I am using a fast schmidt-newt - f4- so I have a MPCC in front of the nosepiece to avoid coma. I am using a SBIG ST-8300 which has a big sensor.

If you look carefully at the flat, you will see that the bright patch at the centre is a narrow elipse extending almost to the top and bottom of the frame but not to the side. I suspect internal reflections are to blame.

But if that's the case, why does it only show up in the flats and not on the CCD?

Or perhaps my images are still overexposed.

I am most puzzled. Any tips would be gratefully received.

Pete