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Nebulosity, darks, temp... question

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#1 psb

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Posted 08 January 2013 - 05:30 PM

I am playing with a borrowed SBIG camera, trying my hand at a little narrowband imaging. I don't know what I'm doing, so let me put that out there straight away.

A few nights ago I tried to image the Horsehead nebula. In Nebulosity I set the TEC to -15 C. (I was working under the the assumption that Nebulosity is reporting the CCD's actual temperature, rather than a temp relative to ambient or something.)

It was late so I decided to take my darks the next morning, indoors.

The lights (Ha) look as I would have expected – reasonable data and a lot of noise.
My stack of darks look reasonable to me too. However when I pre-process my individual lights with the dark stack the result seems to show no nebulosity at all. In fact there appears to be not much appart from a couple very bright stars. Certainly no DSO detail.

What might I be doing wrong?

Thanks,
Phil

#2 agmakr

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Posted 08 January 2013 - 05:54 PM

If i understood right, what you see is the non streched image.
Apply DDP, or levels/curves and the nebulosity will be there.

#3 psb

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Posted 08 January 2013 - 07:05 PM

Thanks Angelos but no, that's not it.

The pre-stretched histogram doesn't look good at all – there's no data in the mid tones... or high lights either, for that matter. It's all clipped at the shadow end. Stretching produces no image.

If, however, I stack the the lights without applying the darks I can stretch out a fine image (although there are bad pixel stacking artifacts everywhere.)

#4 Peter in Reno

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Posted 08 January 2013 - 07:36 PM

Since you are imaging with NB filter, the ADUs will be very low and close to read noise. After auto-stretching with Nebulosity (Auto checked above histogram), then uncheck Auto box, set Black slider to 0 and try to bring the White slider closer to Black point until you see nebulosity. Don't be surprised the White slider can be as low as 500 or less.

If this works, that means Nebulosity does not auto-stretch well enough with images containing very low ADUs. I had the same issue and it worked if you follow my instructions.

Peter

#5 psb

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Posted 08 January 2013 - 07:54 PM

Peter, thanks.

First of all that didn't seem to help. I'm pretty sure there's nothing there. I suspect I've done something wrong with my darks.

Perhaps you can help me fill a gap (one of many) in my understanding. Like I said, the lights – by themselves – seem to stretch out nice. I would have thought that the application of (appropriate) darks wouldn't affect the "good" part of the image data. No?

Thanks,
Phil

#6 Peter in Reno

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Posted 08 January 2013 - 08:12 PM

Can you upload one light sub and one dark sub in original FIT file formats somewhere and provide a link. Or you can zip these two files into a single zipped file and upload it.

If you are unsure where to upload it, you can try www.sendspace.com .

Peter

#7 psb

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Posted 08 January 2013 - 08:29 PM

Thanks Peter.
Here is a link to a dropbox folder containing one light and one dark:

https://www.dropbox....veu3/qDXxPeQHfw

_
Phil

#8 Charlie B

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Posted 08 January 2013 - 08:49 PM

Both look fine to me. I suggest making a bad pixel mask and processing that way.

Charlie B

#9 Peter in Reno

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Posted 08 January 2013 - 10:17 PM

The camera appears to be quite noisy. In fact the minimum ADU for dark is actually higher than the light. I think 120 seconds or 2 minutes is not long enough for Ha. For narrow band imaging, I suggest much longer than 10 minutes. I have never seen a dark sub starting at over 7500 ADU!!!!

When I tried to calibrate the light with the dark, I got the same thing as yours. Then I calibrated using Bad Pixel Mapping from your dark, and it works and looks cleaner.

Peter

#10 psb

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Posted 09 January 2013 - 11:18 AM

I'm not sure what' going on.

I tried shooting a few darks using CCDSoft and they look much better, with a min of 91. Applying those darks to the original image data doesn't work, of course, but does produce the results I would expect. I think now I will try to capture a set of lights with CCDSoft.

Does anyone else use Nebulosity for capture with an SBIG camera?

#11 Peter in Reno

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Posted 09 January 2013 - 11:23 AM

Nebulosity should work with SBIG cameras but I never own a SBIG camera. Just make sure SBIG drivers are installed.

I use Nebulosity and it's a wonderful capture as well as processing software.

Peter






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