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Weird Ha Flats

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#1 DJL

DJL

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Posted 28 November 2020 - 01:50 PM

Hi all - Now that I have a cooled OSC camera and a filter wheel with IR cut, Ha and Dark filters I'm taking flats indoors the day after a session. I typically take flats with 2 layers of white T-shirt and an iPad on a white page. I point the scope at the ceiling with the iPad balanced on top of it. 

 

Here's a screenshot of the stretched flat fits file created in the finder by the Mac Observatory app with crazy colors. I am surprised to see that there's an inverse vignette - the corners are brighter than the center.  When I view the file inside Observatory, it is a more even gray but still lighter at the corners. When I apply this flat to the image I get dark vignetting at the corners.

 

I'm going to take the filter wheel apart and see if there are any light leaks around the filter (since it's an APS-C clip in an adapter ring). Any thoughts on what else could be causing this would be appreciated.

 

 

 

 

 

 

 

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#2 idclimber

idclimber

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Posted 28 November 2020 - 02:17 PM

You could try the flat again but with the lens cap on. Keep the light on top of the scope illuminating the cap instead of the optical train. It should look like a dark frame or a dark flat if there are no light leaks. 



#3 DJL

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Posted 28 November 2020 - 06:05 PM

You could try the flat again but with the lens cap on. Keep the light on top of the scope illuminating the cap instead of the optical train. It should look like a dark frame or a dark flat if there are no light leaks. 

I did that, the "flats" were then identical to the dark flats. Took it all apart, didn't see anything wrong, put it all together again. Time will tell - thanks for the suggestion.



#4 idclimber

idclimber

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Posted 28 November 2020 - 08:19 PM

Then I would suspect some type of internal reflection of your light source. 




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