Old 08-02-2018, 04:08 PM
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Taking Flat Field Frames

Hi everyone,
Please excuse me if this sounds a silly question but I would like to get some clarification about taking Flat Frames.

I am on the understanding that the use of good Flats etc are very important to how the final image will appear.

My question is if I were to take a series of Flats after my initial set up can I use these files for the entire observing session and image several other objects around the sky providing the camera and same filter position on the scope are not moved?

Or, do I need to take a new set of Flats if I point the unaltered scope or camera at another object?

Thanks for your advice and recommendations in advance.
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Old 08-02-2018, 04:41 PM
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pfitzgerald (Paul)
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Hi Eris

Provided you don't change the camera's orientation between objects you can use the same flats. The idea of the flats is to remove vignetting at the corners of your image and also any 'dust bunnies' that would otherwise impair the quality of your final image.

Hope that helps.

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Old 08-02-2018, 04:43 PM
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RickS (Rick)
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You can keep using the same flat masters so long as there's no significant change in your imaging train. Slight focus changes are usually OK. Slewing to a new object is fine. Rotating the camera (and filters together) is often OK unless there's some asymmetry of the image circle. What usually stops flats being useful is when you get new dust spots or a dust spot moves on the cover glass or a filter.

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Old 08-02-2018, 08:30 PM
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Rotation of the field can indeed make a different image circle which renders your flats made for one rotation invalid for another one, even when using the same image train.
Dust is avoidable. Most cameras (Nikon, Canon) have a manual dust cleaning option which allows to blow dust away from the sensor with a blowing bulb. This option pops up the mirror and shutter so you have access to the sensor. Obviously, no lens or telescope should be mounted, use a bare body. Do this when holding the camera facing downward to be sure all dust falls off when blowing. Do this before each astrophoto session.

And when you use Photoshop for post processing and use a lens with a lens profile (so not a telescope prime focus), you can just select the 'Lens correction' option and then select the lens you used. This eliminates the need for flats.
Do this after stacking and before post processing.
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Old 08-02-2018, 08:44 PM
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Sorry everyone I omitted to say that I am using a CCD camera with both lenses and scope.
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Old 09-02-2018, 02:50 PM
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you need a different master flat for every imaging configuration: lens/scope-filter/nofilter-camera(if different ones, even of the same exact model)-component relative orientation (eg if using a nosepeice or rotating camera mount for framing), but not for every target.

The flats (which you take a few of and combine into a master in the interests of averaging) are a sample of the path taken to each pixel by "identical" (hence "flat field") photons from the end of the sky to the start of your sensor, so that you can remove the variation your equipment introduces and approximate "what the sky would look like if your optics were perfect". Any change above the bottom of the sky (your choice of target, the atmosphere etc.) or below your sensor (the camera's amplifier bias, dark current etc.) are not the responsibility of the flat field to correct.

Rotations of sensor vs lens/telescope objective do matter, more the a) dustier/damaged the glass b) greater the number of optical elements, since nothing is perfect. Think about things like the inevitable nicks and shiny spots in the felt/baffling inside the OTA, or how many not-completely-blacked lens edges there are in an SLR lens. I tell people not to reuse flats between nights at the observatory for the simple reason that even if we left the camera attached (we usually don't), we park the scope, so the dust inside has all day to settle differently.

OK, that might sound like an argument for flat fielding again each time you switch targets, but there's also a "within reason" element to our use of time...
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