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Re: [APML] 2 inch camera adapter question

Brian,

The length of the focus tube can hav an effect on vignetting and exposure IF
it blocks part of the objective (forming an effective apature mask).  When I
built my 4" f6 refractor I wanted it to focus both a camera and my range of
eyepieces.  This dictated a rather long draw tube.  I used a 2" focuser for
a while but I eventually built a 3" crayford focuser.  In this I have a
drawtube with a large inside diameter.  I bring this to 2" at the end only.
This has worked out well.  I have some pictures of the focuser parts and a
description of the build process here:
http://www.gatecom.com/~ckessler/ATM%20Focuser.htm

The refractor kit is described here:
http://www.gatecom.com/~ckessler/bw_optik_4.htm

I hope this helps

Clay Kessler

----- Original Message -----
From: "Brian Larmay" <astrobri@ameritech.net>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Tuesday, March 04, 2003 3:39 PM
Subject: Re: [APML] 2 inch camera adapter question


> Thanks Frank,
>
> Does the length of focus, such as the focusing barrel/tube have impact on
> vignette or how long to expose for also?
>
> or....
>
> Should I make the main barrel of the refractor tube as close as possibe to
> prime focus, so I dont have to have a long focusing barrel?
>
> This is hard to explain because Im making this refractor from scratch and
> Ive never built one before.
> I should really be going to ATM, but being that this scope is photographic
> in nature, I will ask here.
>
> Thanks,
> Brian
>
>
>
> ----- Original Message -----
> From: "Frank Parrotta (Navy Yard)" <FParrotta@TSC.COM>
> To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
> Sent: Tuesday, March 04, 2003 10:09 AM
> Subject: RE: [APML] 2 inch camera adapter question
>
>
> > Brian,
> >
> > As John has said, you really can't use the 1.25" if you want to
illuminate
> > as much of the 35mm frame as you can.  However, you're still not home
> free.
> > The camera T-ring will also vignette the frame as well - the amount
> > depending on the focal ratio.  I find that the corners get pretty dark
but
> > the central core of the frame is usable with some vignetting removal
> > processing.  If you want to get an idea of just how much light falloff
> > you'll have to content with, set up your scope with the camera attached
> and
> > aim it at an evenly illuminated source (e.g., blue sky, white painted
> wall,
> > etc.).  Set the shutter to "B" and open the camera back.  Fire the
shutter
> > and look through the camera back opening (where the film would be) and
> > notice the illuminated circle produced by the objective.  Move your eye
> > around to the frame's edges and corners - the amount of the illuminated
> > circle that is obscured at each location will be proportional to the
> amount
> > of vignetting you will get at those locations.  Hope this helps.
> >
> > Frank
> >
> > -----Original Message-----
> > From: Brian Larmay [mailto:astrobri@ameritech.net]
> > Sent: Sunday, March 02, 2003 3:40 PM
> > To: astro-photo@seds.org
> > Subject: [APML] 2 inch camera adapter question
> >
> >
> > Hi!
> >
> > What are the advantages photographically of a 2 inch camera adapter over
a
> > 1.25.
> >
> > Thanks,
> > Brian
> >
> >
> > My Photography
> > Nature: http://www.astrobri.com/nature_photography.htm
> > Astrophotography: http://www.astrobri.com/astrophotography_ind.htm
> > Storm chasing photography:
> > http://www.astrobri.com/weather_pictures_2002_season.htm
> > Northern Lights: http://www.astrobri.com/northern_lights.htm
> > Miscellaneous: http://www.astrobri.com/miscellaneous.htm
> >
> >
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