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Astrophotography and Sketching >> Film Astrophotography

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AstroMan0
member


Reged: 06/19/12

Loc: Dedham, Massachusetts
Focusing Through Deep Red Filter
      #5494715 - 10/29/12 02:07 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?

Edited by AstroMan0 (10/29/12 04:48 PM)


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Michal1
sage


Reged: 07/25/10

Loc: Czech Republic, Central Europe
Re: Fucusing Through Deep Red Filter new [Re: AstroMan0]
      #5494873 - 10/29/12 03:51 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.com/HTML/I_ASTROP/FOCUS/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.


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AstroMan0
member


Reged: 06/19/12

Loc: Dedham, Massachusetts
Re: Fucusing Through Deep Red Filter new [Re: Michal1]
      #5494930 - 10/29/12 04:26 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.


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Michal1
sage


Reged: 07/25/10

Loc: Czech Republic, Central Europe
Re: Fucusing Through Deep Red Filter new [Re: AstroMan0]
      #5495054 - 10/29/12 05:40 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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