383L+mono Flats Failure

[niteman1946]

I've sent an email to tech support on this and am now waiting for a response.  So while I'm waiting, here it is for the forum.
A very slight version of this happened about a month ago.
http://www.atik-cameras.com/forum/index.php?topic=401.0

Now I am getting really poor results with NGC891.  My previous capture (M27) did not exhibit this problem.
http://www.astrobin.com/full/24278/?mod=none
1.  Upper Left:  Raw sub
2.  Lower Left:  Calibrated sub without the Master Flat (but with Bias and Dark)
3.  Upper Center:  Calibrated sub with the Master Flat (and Bias and Dark)
4.  Lower Center:  Calibrated sub with uncalibrated Master Flat (and Bias and Dark)
5.  Upper Right:  Master Flat
6.  Middle Right:  Master Bias
7.  Lower Right:  Master Dark

It's evident (to me) that the Master Flat is introducing some pretty ugly stuff.  Not sure that the Bias should look like it does either.  I've shot three different sets of Flats ranging from average ADU values of 25k to 35k.  No luck.
Processing was in PixInsight and all images are auto-stretched.  I cannot process the gradients (light, dark and motes) out with the software.
I tried DSS to see if the problem was unique to PixInsight, and found the same results.  This kinda puts a hitch in my buying a carousel and filters until I find a solid solution.

Any suggestions would be greatly appreciated. 
Thanks,

Mark

[topboxman]

Hi Mark,

Did you take Bias (or darks) under dark sky or in a dark room? If you took Bias (or darks) under sunlight even with the scope or camera covered, the light can leak through the camera. Cameras are not light-leak-proof.

I do not see a problem with your flat. The vignetting looks about the same size as your raw/uncalibrated light. A bad bias with good flat can ruin the calibration. Your dark looks normal. It's normal to see bad/hot pixels if the dark is heavily stretched and Kodak CCDs are typically noisier than Sony CCDs.

I use Atik 460EX mono camera. I use Nebulosity to do calibration because it has a cool Bad Pixel Mapping feature. I never use dark subtraction thanks to low noise of Sony CCD. I have tried calibrating with PixInsight and don't find it any better than Nebulosity and calibrating with Nebulosity is faster. From then on it's PixInsight from star alignment/stacking all the way to post processing.

Peter

[niteman1946]

Hey Peter,

Both Bias and Darks were taken in the dead of night.  It never gets black around here at night, but any stray ambient light should be minimal.  Also the Bias are shot at 0.20sec.  I would expect the Dark to be more problematic since it was shot at 600sec (like the lights).

I agree the Flat Master looks OK, but it does change the calibrated sub when it is added to the calibration step (see lower left for before, and upper center for after).  I've not seen this extreme of gradient variation in the calibrated (and ultimately integrated) images up 'til now.

As mentioned, I have tried three new sets of Flats due to the problem cropping up on NGC891.  I was happy with the Flats, Darks and Bias before that.  If I could blame PixInsight I would.  But I think you had previously suggested for me to try another software, and I did (DSS).

It may be that the background conditions in this image (NGC891) are the culprit.  M27 for instance has an extremely starry and bright field of view, and was not problematic.

Mark

[topboxman]

Your Bias exposure may be a little too long. Try set Bias exposure to 0.00. If your software does not support 0.00 second, try at 0.01 second. Nebulosity supports 0.00 second.

Peter

[niteman1946]

I use Images Plus for capturing, including Bias.  IP limits the minimum exposure length to 0.2 seconds. Atik states that 0.20sec is the minimum time for the shutter to operate. But since we're talking Bias here (effectively a very short dark), this does not seem like a logical restriction by IP.
Anyway, Artemis Capture (Atik) does allow very short exposures.
I tried taking a new set of Bias at 0.001 seconds.  Capturing the Bias files at this exposure worked, but the result on the finished image did not change.
Appreciate the suggestion though, learned something new.

Mark
 

[topboxman]

I hope it's not too much trouble for you to upload one raw light, one master dark, bias and flat in FIT or TIF file formats to a server that accepts large files and provide us a link where you uploaded the files. You can zip all four files into a single *.zip file.

I hope you created the Master dark, bias and flat with any software except PixInsight. Nebulosity will not read any FIT files saved by PixInsight. I prefer to calibrate with Nebulosity than PixInsight.

Thanks,
Peter

[niteman1946]

Thanks Peter,

Yep, my masters are made in PixInsight, sorry.  And judging from my one try of DSS, that software does not make masters.
I did get a response from Jonathan at tech support.  He thinks I may be getting ice on the chip, and it may be leading to (at least) some of my problems.
I will be out of pocket for a couple of days, but will followup when I get back.

Mark

[topboxman]

That's okay. I just wanted to see your images in original FIT format to analyze your data. I can handle PixInsight processed images.

Anyway, if ice was building you would notice uneven ice buildup on the images. I believe ice starts to build at the center of the image and grow unevenly from there but I don't think I see ice on your images. This looks like true vignetting to me because the vignetting looks pretty even.

What scope are you using? Are you using focal reducer or field flattener? I am curious because I am wondering the cause of vignetting.

Peter

[niteman1946]

I had received an email earlier from Jonathan at Tech Support as a follow-up to the one I had sent. Now all emails back to him are kicked back as undeliverable.
Anyone else having this problem?

Mark

[vince]

Hi Mark,

Sorry just noticed your post! We are now aware of a problem with the support emails and are now sorting it out. Back to normal soon! Can you resend your mail to Jonathan please.

Thanks
Vince

[niteman1946]

Hi group,

All appears well now.
Jonathan with tech support walked me through the removal and reconditioning of the desiccant tablets.  This corrected the gradient problem.  Apparently, the camera had accumulated (quite) a bit of moisture on the image chip.
This is something I will watch more closely here in Texas as the temps drop.
 
Thanks for your help.

Mark