Old 18-01-2012, 02:44 AM
vic4loc (Victor)
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Help with taking Flats

I'm trying to capture flats, not sure if I'm doing it correctly.

I have a LX90 8" and QSI583, I simply point the scope about 20 Dec west at sunset, put the scope out of focus, took a few shots to get the average ADU value around 25000-30000, and got the image below.

What is that blob in the middle? Is this the expected flat image from an SCT? Do I need to put the scope into focus?

I have tried putting a white cloth over the scope, still getting that blob in the middle.

Any advise would be greatly appreciated.

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Old 18-01-2012, 05:09 AM
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mill (Martin)
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First of all, never put the scope out of focus.
You have to leave everything the same as your imaging session or your flats will be useless.
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Old 18-01-2012, 06:29 AM
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Wow that is a big blob, is that a cropped image, or as taken, seems very unusual, but as Martin says, keep it focused.

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Old 18-01-2012, 10:07 AM
vic4loc (Victor)
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The image is uncrop.. I will try taking flats again with the scope focus. Thanks for the feedbacks.

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Old 18-01-2012, 10:50 AM
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Rigel003 (Graeme)
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If your scope was well out of focus the central blob is likely a shadow of the secondary. It's important for flats to be taken with exactly the same optical setup as your light exposures, including focus and rotation angle of camera, reducers, filters etc. If you're taking twilight flats the white T shirt over the end of the scope is a good idea to even out the background and stop individual stars from registering. Best solution of all is to buy a light box from Peter. Highly recommended.
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Old 18-01-2012, 12:18 PM
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DavidTrap (David)
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Definitely the secondary shadow.

I went down the light box route to get 3+ second exposures to avoid any shutter blur with shorter exposures on my QSI.

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Old 18-01-2012, 12:24 PM
swannies1983 (Dan)
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I use Al's Virtual Light Box (resources section). I'm happy with the results so far.
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