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RE: [APML] 2 inch camera adapter question
Brian,
Others have addressed this issue and suggested larger draw tubes. A very
good idea if you can afford them or have the facilities to make them.
Before you start cutting metal, it would be a VERY good idea to make a scale
drawing of the scope with your camera setup. Ray trace from the film image
plane up the OTA to the objective lens. Anything that cuts into this bundle
of rays will result in vignetting. However, don't get too concerned if you
can't fully illuminate the corners. The T-ring's 41mm aperture will prevent
that - you just don't want anything else to produce additional aperture
stops. BTW, Vixen produces a 60mm visual back and a wide T-ring (actually
it's a regular T-ring with some set screws that allow you remove the inner
threaded portion of the ring and attach it to their visual back) for their
scopes that, I'm told, results in much less vignetting on a 35mm frame. You
might consider adapting this wide T-ring to your draw tube.
However, there is another reason to consider "better as the enemy of good
enough" and that is your objective lens may not have a flat focal plane or
it may have other off-axis problems. If this is the case, then restricting
the fully illuminated circle may actually produce a better telescope for
your application. I shoot 35mm with a TV102. Most folks would consider
this scope to have a very good objective lens, but it requires a focal
reducer/field flattener to do photography and I still get some vignetting in
the corners of my frame. There is so much else to learn in this
hobby/obsession that it's sometimes best to do the best you can on your
design and then get it done and go out add catch some photons. You'll see
things you like and things you don't like. When the scope becomes the
limiting factor in your art, then build a better one with what you've
learned. Hope this $0.02 worth helps.
Frank
-----Original Message-----
From: Brian Larmay [mailto:astrobri@ameritech.net]
Sent: Tuesday, March 04, 2003 3:40 PM
To: Discussion of Film Astrophotography
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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