Reposting ST-8300C query with fits header and image statistics - Help!

[astropixel]

Hi. I have followed the advice of a previous post about PI settings for SBIG ST-8300C cameras and produced an integrated image. However, I am not able to stretch the image. It's appearance after stretching is that of a linear image. I did notice that the image could not be stretched to any extent by the CCDOps processing tool immediately following file download - which seemed odd at the time. All files were subsequently saved as fit files.

I have attached a composite screenshot of image statistics and fits header. I am not able to tell from this whether there is an obvious problem.

I hope the information provided is adequate.

Many thanks

Rowland

[Carlos Milovic]

As far as I know, there is nothing on the FITS header that may prevent the image to be stretched (I'll assume that is not complex valued... this would be VERY odd). Use the HistogramTransformation  or the AutoHistogram processes to stretch it.
If you still canno't do it, please upload the file (or a crop of it).

[astropixel]

Thanks Carlos. I am not able to stretch the image to any useful extent, by any means. Here is a crop of the integrated file following a histogram stretch. The values of the fit file are 40 - 50 adu more than the jpeg posted.

Thanks again.

[pfile]

are the subs you stacked calibrated? i've found that sometimes things can go very wrong in calibration and lead to calibrated subs that have very little data in them.

what happens if you debayer, register and integrate the raw subs from the camera?

[astropixel]

The subs were all calibrated in PI. I noticed that the CCDOps processing tool had a similar effect on newly downloaded subs. The tif files.generated by CCDOps were similarly very dark.

[astropixel]

While I think of it, the background value of the grey subs is zero. Following debayer, background is 7.

The BP script and manual method produced the same results, when using a single dark frame or a master of 10 frames.

I used 10 bias frames and 5 flats to create master files.

[pfile]

what happens if you don't use the flats, or simply debayer, register and stack the i16 images coming off the camera?

[astropixel]

I haven't tried that because the individual subs will not stretch to any extent either. I expected a properly processed stack would improve the situation.

EDIT: added screen shots of CCDOps showing a good fit file and the bad one. The values are very different. Both are 6 minute files and present very differently following download. Is there a way to recover this?

The light frame is more like a dark. The shutter was open and there is no lens cap and I have checked that the camera is working correctly.

The only other alternative is that 6 minutes at f/6 is an inadequate exposure at a dark site - though, I doubt that the results would be as restrictive as they appear from this image set. Is it possible that the exposure time was insufficient?

[Carlos Milovic]

I think that you may have a problem with the bias or dark. It seems that the mean background value of any of them is much higher that the background of the lights, and thus, you are clipping data. Check all your calibration files and the scaling factor that PI is calculating.

[astropixel]

Hi Carlos.

I removed a set of shorter exposures and used only 6 minute light and dark frames (as well as bias and flats). Scaling factors ~1.05 on average - no change.

I have tried processing with DSS, CCDStack and Fitsliberator to no avail - same outcome.

It is significant as in my previous post, that the converted uncalibrated fits in CCDOps cannot be stretched - show no detail and the pixel values considerably lower than the other good frame.

[Carlos Milovic]

Please show the statistics for you master bias, master dark and any of the single uncalibrated frames.

[astropixel]

As requested - PI statistics

[Carlos Milovic]

There is something really wrong somewhere. I have two guesses:
1) Capture or saving process (check your capture software).
2) Bad FITS header that is cropping data (either change fits values or experiment with PI format settings).

Take a look at all of those statistics. The median value is 0.0 in all frames. That means, that at least half of the pixels in the image have a 0.0 value; they are pure black, no information, clipped. So, either you are loosing information with your capture settings, or they are stored wrongly, or there is a setting in the header that is clipping the data when loaded (try changing the pedestal value to zero).

[astropixel]

Thanks Carlos. I have come to a similar conclusion about image capture, but was not sure how to tackle the problem post acquisition.

[astropixel]

Thanks Carlos. I ran this exercise earlier. However, at second look, the values that differ significantly between a 'bad' fits to 'good' fits from the same camera are;

XORGSUBF good fits 410 - bad fits 0

CBLACK - good fits 1365 - bad fits 0

CWHITE - good fits 4115 - bad fits 526

Clearly, something wrong here. Changing the bad values to the good values for example, has no effect. The pedestal set from -100 to 0 makes no difference either. I suspect that the CCD is frosting up with cooling, although it looks very clear on inspection and it has just been serviced by SBIG, who replaced the dessicant?