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Focusing Through Deep Red Filter

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#1 AstroMan0

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Posted 29 October 2012 - 01:07 PM

Does anyone have any tips on focusing though a deep red filter (Wratten #29)? I couldn't even see the brightest stars through it. All I could see to focus on was the moon. I know I can't focus without the filter and then put it on, because then the focus will change. Also, I am using a fast achromatic refractor, so the deep red may come into focus at a different point than the other colors. What should I do?

#2 Michal1

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Posted 29 October 2012 - 02:51 PM

Noah, focusing on the camera's focusing screen isn't recommended even without any filter. There are many methods that are more convenient here, see e.g.:
http://www.astropix....CUS/METHODS.HTM
I suggest making a parfocalized eyepiece (described in the link). Once you have an eyepiece parfocalized with the camera + filter, you can use it to focus on a star. I use the combination of a parfocal eyepiece and a Hartmann mask. If you found something unclear, don't hesitate to ask.

#3 AstroMan0

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Posted 29 October 2012 - 03:26 PM

Thanks for the reply,
I would use a parfocalized eyepiece, etc. But the problem is that my setup is based on 2 inch accessories, such as a 2" off axis guider, 2" adapters, 2" filters, etc. (1.25" adapters gave me severe vignetting). I don't currently have any 2" eyepieces, and from my experience, they are pretty expensive.

#4 Michal1

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Posted 29 October 2012 - 04:40 PM

My parfocal eyepiece is 0.75" ;) (image). I found a paper tube that firmly holds the eyepiece and tightly fits to a film container. I have to focus with a 2" to 1.25" adapter, then unscrew it and mount the camera instead of it.


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