Old 30-12-2012, 03:50 PM
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PRejto (Peter)
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Join Date: Jan 2011
Location: Sydney, Australia
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Experimenting With a Flat Field Reducer


I could use some advice. I'm experimenting with a Borg 7870 and my TEC140. I'm using a special adapter made by AOK (Switzerland) that allows the 7870 to sit inside the focuser. It calls for 55 mm back focus. The adapter allows the 7870 to be positioned easily at different distances from the CCD so there is plenty of room to experiment, which I am sure will be necessary as the 7870 was designed for Borg equipment. I also probably need to take into account that there are 2 filters and the CCD window in the light path. I think I might need to add up to 2 mm, or 57 mm to get this correct at least as a starting point.

I figure that I could use CCDInspector to examine the field curvature and aspect to locate the best position, but I've run into some difficulties and need to start over when the weather permits.

My routine would be to focus on a star as close to the center of the CD as possible, then take a 15-20 sec exposure (short to minimize any star distortion due to guiding) and examine the frame with CCDInspector. The goal would be to get the flattest field and least distotion possible. Is this method likely to work? Is there something else I should consider?

I did try this already but I think I messed it up completely by not taking care to focus only on a star dead center on the CCD. The results showed an impossible amount of tilt and collimation issues which I think will be eliminated if I focus correctly while testing. I've been using FocusMax. Is there a way to ensure that FocusMax will use a star that is in the center of the CCD? Will it do so if the star in the center is the brightest in the field of view? So far I have not discovered how to make FocusMax use a particular star; it seems to make the choice automatically.

Thanks for any assistance!

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