What is wrong with my flats?

[DanielF]

Phil, I don't believe there is a problem with the darks. They seem good enough, and they do not require to be on the scope at all.

Daniel, the overcorrection is a clear indicator that there is a pedestal problem somewhere, either on the flats or the lights. The "model" behind our images is that they consist on several sources:

Img_adquired = (Img_ideal+noise)*uneven_illumination*pixel_sensitivity + bias + dark_current + noise

Since we can capture the external sources in the flat, dark and bias frames, we end with the following equation to recover the "ideal" image (of course, ignoring the noise):

img_calibrated = ( img_adquired - bias - (dark-bias) )/(flat - bias)

If we have an additional pedestal value in the lights, we end with:
img_calibrated' = img_calibrated + pedestal/(flat-bias)
(we have the same image as before, plus a gradient from the inverse of the flat... i.e. an over correction).

Also we may have these problems with a negative pedestal in the flats.

I found the value by experimentation. It kind of makes sense that the pedestal is the mean of the bias, since that is the electrical pedestal value, although I don't know why it is doubled in your images. Also I experimented with a gain factor 2x (dividing the lights by two), and then calibrating, but it seems that in that case the bias and dark frames do not match the data.

That's it. I encourage you to investigate this thing further. Try making the capture in another software, and see if the problems still appear.

Sorry for not responding for a while, but the weather has been bad so I have not been able to try to capture a new set of images.
I did however capture bias frames with another computer (I use a Mac) and other software. I don't think there's a very big difference between them but some of you might see something?

Here's a bias with Nebulosity running on a PC
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/biastestNebPC.fit

Here's a bias captured with Starlight Xpress own capture software (Had to save as tiff, the fits files were unreadable for some reason).
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/biastestsx.tif

And one more thing as well, I had to send the camera back to SX for cleaning of the CCD. It's possible that the big black spot is a paint chip from the shutter (other have had the same problem with this model). So I can't test anything until I get the camera back from England.

/Daniel

[Catanonia]

Having exactly the same issue with my F2.8 PowerNext (with ASA corrector).

I have tried everything, but some interesting things to try here.

1. Baader focuser - check I have one - check for light leaks
2. Stray light into rear cell - check, will check this too as heard this can be a probelm too.
3. Darks Bias on the scope - Will re-create my bias and darks on the scope and see if this solves it.

Currently I am having to use Maxim to manually subtract my flats at 5% scale from my lums to resolve my issue.

The problem is NOT PI, it happens in Maxim and DSS as well and it seems the scaling is off by 95% or more in my case.