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Astrophotography and Sketching >> CCD Imaging & Processing

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Star surfer8
professor emeritus


Reged: 05/15/06

Odd gradient
      #5628570 - 01/17/13 06:01 PM Attachment (22 downloads)

Hi,

Got my first CCD camera two months ago and of course there hasn't been a clear night since. Finally had a go at first light yesterday with the QSI683wsg and got some RGB frames of IC443. Processed the data today and it looks like there's an odd gradient in the image that I've never seen with DSLR imaging. Have any of you ever had this issue? I would appreciate any help figuring this out. Thanks.

Here's a calibrated image taken with the QSI and TOA130 with flattener. Notice the magenta colored gradient in the center.


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Star surfer8
professor emeritus


Reged: 05/15/06

Re: Odd gradient new [Re: Star surfer8]
      #5628575 - 01/17/13 06:03 PM Attachment (13 downloads)

Here's the uncalibrated version.

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Star surfer8
professor emeritus


Reged: 05/15/06

Re: Odd gradient new [Re: Star surfer8]
      #5628583 - 01/17/13 06:06 PM Attachment (7 downloads)

Here's the raw red channel.

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Star surfer8
professor emeritus


Reged: 05/15/06

Re: Odd gradient new [Re: Star surfer8]
      #5628588 - 01/17/13 06:08 PM Attachment (6 downloads)

Raw green channel.

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Star surfer8
professor emeritus


Reged: 05/15/06

Re: Odd gradient new [Re: Star surfer8]
      #5628591 - 01/17/13 06:10 PM Attachment (6 downloads)

Raw blue channel.

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Star surfer8
professor emeritus


Reged: 05/15/06

Re: Odd gradient new [Re: Star surfer8]
      #5628609 - 01/17/13 06:19 PM

You can see the gradient in the separate RGB channels and the uncalibrated version. I was able to process some of it out in the calibrated version but it's still there. I've checked the equipment and it doesn't seem to be anything there that would cause this. I'm new to CCD imaging so I'm not sure what to make of this. Like I said, I've never had this issue with DSLR imaging using the same scope. Any ideas would be appreciated. Thanks.

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rigel123
Postmaster
*****

Reged: 06/29/09

Loc: SW Ohio
Re: Odd gradient new [Re: Star surfer8]
      #5628870 - 01/17/13 08:55 PM

It almost looks like some fogging in the middle of each image, maybe on the window of the CCD? Not sure, maybe someone more experienced can weigh in.

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shams42
Pooh-Bah
*****

Reged: 01/05/09

Loc: Kingsport, TN
Re: Odd gradient new [Re: rigel123]
      #5628943 - 01/17/13 09:45 PM

In what order did you shoot the colors?

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Star surfer8
professor emeritus


Reged: 05/15/06

Re: Odd gradient new [Re: shams42]
      #5630951 - 01/19/13 01:25 AM

Thanks Warren,

I duplicated the imaging setup from that night then checked the camera for fogging but the CCD window glass looks clear.


Shams,

The colors were shot in the order of RGB. How would that affect the cause of the gradient?


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Rick J
Postmaster
*****

Reged: 03/01/08

Loc: Mantrap Lake, MN
Re: Odd gradient new [Re: Star surfer8]
      #5630978 - 01/19/13 02:02 AM

The raw files which I assume means no flats or darks show a hot spot. Looks to me like your flats aren't doing the job removing them and need to be replaced with better ones for each color filter.

The red show little hint of a hot spot but blue shows a strong one. Magenta is due to too much red and blue compared to the green. So if the red flat was used on the blue image and the blue flat used to flat the red image that would create the problem as well.

Rick


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siovene
sage


Reged: 01/18/11

Loc: Espoo, Finland
Re: Odd gradient new [Re: Rick J]
      #5631510 - 01/19/13 12:49 PM

I agree with Rick. Your flats are not working. I actually had the very same problem, and solved it by correctly calibrating the flat frames too, with their dark and offset frames.

I didn't want to believe that that was the issue, but it really was.


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