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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.
I just measured the light cone and its about 8 1/2 inches to the film
plane... this is a short distance..do I need to be concerned about field
stops with the range being that short.
I took apart my ST 80 out of curiosity and noticed it has only 1 stop, and
that has only a double element...mines four
> 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.
Isnt Vixen only allowed to sell through Orion still, or was that contract
up?
> 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.
This might be hopefully good and true knowing that I only have 8 inches to
work with.
There is a 2 inch stop unfortunatley down the Barrel that reduces to 2
inches.
I looked hard for any other piece that would let me open it up, but with no
luck.
> 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
Thanks Frank,
I think before I glue this scope together, Im going to take a test shot here
in the city to see what my limitations are and see what I need to do to make
it better if I can even do so.
Regardless, if I cant get what I want through this scope, I will have to try
and find better optics to work with.
My expectations on this venture arent a whole lot as far as picture results,
but the expectations of me learning the how to and so on are much higher.
I Still wonder though with the FOV whether a little bit of vignette would
even something to worry about.
Thanks,
Brian
>
> -----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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