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Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: What is the issue with my flats? new [Re: Sean13]
      #5658752 - 02/02/13 07:18 PM

The essential feature of flats is you change nothing - focus, apperture, lens elements or camera orientation on scope must not be changed

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Mike Clemens
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Reged: 11/26/05

Loc: Alaska, USA
Re: What is the issue with my flats? new [Re: Tonk]
      #5658763 - 02/02/13 07:24 PM

> The essential feature of flats is you change nothing -

Which I still haven't come to terms with. If I am shooting 4 basic sets a night of a target, (2) sets 0 and 90 degrees rotated before and after the meridian, the only way to get good flats would be to have a robotic thing which flips an illuminated panel in front of and out of the way of the refractor.


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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: What is the issue with my flats? new [Re: Mike Clemens]
      #5658774 - 02/02/13 07:37 PM

Meridian flips do not change camera-to-scope orientation

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Mike Clemens
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Reged: 11/26/05

Loc: Alaska, USA
Re: What is the issue with my flats? new [Re: Tonk]
      #5658789 - 02/02/13 07:43 PM

Even on a 2 meter long 80 pound refractor with a 12 pound focuser hanging 4 feet away from the mount? My pointing correction terms sure feel it.

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pfile
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Reged: 06/14/09

Re: What is the issue with my flats? new [Re: Mike Clemens]
      #5659023 - 02/02/13 10:25 PM

well then your sensor is no longer square with the optical path, so forget about flats... that's a bigger problem.

but are you actually rotating the camera? like loosening it and rotating it and retightening it? in that case you do need new flats. there are in fact robotic flat things like you just described - the flatman is one.

if you never change the position angle of the camera then one set of flats is fine.


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avarakin
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Reged: 07/13/09

Loc: Parsippany NJ, USA
Re: What is the issue with my flats? new [Re: pfile]
      #5659176 - 02/03/13 12:12 AM

Something is horribly wrong with those flats. I see a lot of noise. Looks like they were taken at very high ISO. Another theory is that the software which was used for opening the flats was trying to do some kind of auto exposure, and given that flats are, well flat, it amplified all possible imperfections of the image. Honestly I doubt that this is issue with camera.

Alex


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pfile
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Reged: 06/14/09

Re: What is the issue with my flats? new [Re: avarakin]
      #5659204 - 02/03/13 12:38 AM

i think he stretched the flats just so you can see the vignetting and the patterns.

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avarakin
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Loc: Parsippany NJ, USA
Re: What is the issue with my flats? new [Re: pfile]
      #5659275 - 02/03/13 01:54 AM

I hope he realize that :-)

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bluedandelion
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Re: What is the issue with my flats? new [Re: avarakin]
      #5659685 - 02/03/13 10:32 AM

I have seen this before myself. Before I built an ELP based lightbox, I would use our large flat screen TV to take flats. I would take the flats the day after imaging. Now some may say that should not be done, but I did not move the camera or the focus. Only the scope did get moved from my back yard to the house. The flats worked fine.

Saw this with my C9.25. In my case it was uneven illumination. Stretching a T-shirt across the dewshield solved the problem. Although I agree that exposure is possibly also the answer here. If you stretch an unevenly lit field the effect of uneven illumination becomes pronounced.

Ajay


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hoa101
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Reged: 02/04/12

Loc: Northern Virginia
Re: What is the issue with my flats? new [Re: bluedandelion]
      #5673693 - 02/11/13 09:37 AM

UPDATE:

I wanted to update you guys, since alot of people were attempting to help.

My (brand new) Canon T3 arrived, and I tested it on Saturday. The issue I observed with the flats on the used body is completely gone. Since I did not change methodology, the sensor on the used one must have been the issue.

I can only speculate that maybe it was cleaned improperly. Or I suppose that banding could be a defect in the sensor itself, which would be worse.

In any case, I'm glad I returned it. Problem solved.


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pfile
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Reged: 06/14/09

Re: What is the issue with my flats? new [Re: hoa101]
      #5674119 - 02/11/13 01:57 PM

cool, that's good news. it's hard enough taking good flats without the camera messing things up for you!

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Alex McConahay
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Reged: 08/11/08

Loc: Moreno Valley, CA
Re: What is the issue with my flats? new [Re: Sean13]
      #5674145 - 02/11/13 02:20 PM

>>>>>I could be wrong but I'd think you want the largest aperture you can get.

You want your flats at the same aperture (and focal length) as your lights for that evening. That is the only way to assure you are getting the same level of vignetting, the same shape of the light cone, etc.

And that is not answering the question regarding the pattern in the sensor.

My question would be--how much ADU (or whatever) is there between the dark part and the light. It is possilbe, if you stretch just right, to make anything look real bad. If it is just a couple of levels off (out of 65 k) you can see it if you stretch just right.

Alex


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hoa101
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Reged: 02/04/12

Loc: Northern Virginia
Re: What is the issue with my flats? new [Re: Alex McConahay]
      #5676672 - 02/12/13 10:01 PM

Pretty sure you want flats to not only be the same aperture, but also the same focus. Since I was taking astrophotos with a camera and lens only, this is not difficult. People with cameras attached to scopes have to worry about orientation and whatnot as well.

People discussing use of the highest aperture - that is actually a common test daytime photographers use to check for dust on the sensor. You go to f/22, point at a uniformly illuminated source, ISO 100, and take a couple exposures. Any dust in there will be completely obvious.


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