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PatrickGilliland

GSO RC 10" Repeating Donut/Set Up Issue advice

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HI all,

Wondered if anyone could help my with the following - set up new RC and managed 6x600 unguided to obtain the following image.  At first glance not too bad for first attempt - however as always there is a but with this hobby

post-37169-0-87724700-1427635059_thumb.j

And now the but....if I stretch this image I can see a repeating donut which I believe is from the black circle in the scope as shown below.

post-37169-0-18363200-1427635138_thumb.j

The red arrows highlight some of the areas but it repeats all over the image.

I have collimated with a tak collimating scope and it all appears to be as it should.  The only other thing I can think to mention is that I tried a 2" field flattener (astro-tech).

Does anyone have any thoughts on what could cause this?

Thanks in advance

Paddy

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Dust bunnies on your camera. Use a flat frame to remove them. And clean the camera cover glass from time to time.

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I think it could be dust, flats would remove them, somebody will either verify or diagnose properly.......

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As others have said these dark circles are  due to dust in the optical train near to the imaging chip. You can remove them with flat frames.

If you want to clean the dust from whichever surface(s) are affected you can use the tool here http://www.ccdware.com/resources/dust.cfm to identify which elements to clean.

HTH

Derrick

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May have been the cause - however new investigation required now as to why the glass had got dirty so quickly as was all thoroughly cleaned on build a few days earlier.

Looked like condensation had dried on it but never suffered that with this camera before.

Thanks for the tool though useful to know.

Paddy

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The actual dust can be invisible to the naked eye.

Dave

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That is dust close to the sensor, probably on the glass screen in front of it. I notice you use a camera with the 8300 chip, so the camera has a shutter. Check that the dust particles stay in the same place each exposure ( stretch your individual subs). With the SX H18 I had, the coating was rubbing off the shutter and depositing dust every cycle, so it was impossible to remove the effect using flats. With a QSI camera, you probably don't have a problem but it's worth checking.

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There do seem to be an inordinately large number of doughnuts appearing in your image although that is a pretty savage stretch, of course!

Just in case you want your own local calculator on a spreadsheet, the dust-to-sensor distance can be calculated using the following:-

distance of dust from sensor = dust diameter (mm) * pixel size (microns) * focal ratio/1000

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