RE: [APML] Pentax 6X7

["ken" <kenzo@kennethambrose.com>]

Wow, even I "got it" immediately from the drawing, thanks!

Ken Ambrose (new member)

-----Original Message-----
From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
Behalf Of Chuck Vaughn
Sent: Wednesday, October 31, 2001 9:09 AM
To: astro-photo@seds.org
Subject: Re: [APML] Pentax 6X7


Dean,

I'd like to add a few comments to the other excellent ones you received.
When using a camera body as film holder with a telescope, you will
virtually always have some amount of vignetting. If for no other
reason, the opening to the camera body (the mounting flange) will act
as a hard stop for some of the rays from the objective. The farther
from the film plane the opening is, the worse the vignetting.

Even without a computer program to do it for you, it's fairly easy to
make a full or scaled drawing of your system to determine the 100%
illuminated circle where the vignetting is caused by hard stops in
front of the film plane.

I made a very simple drawing to show how a hard stop vignettes the film
plane.

ftp://ftp.aa6g.org/vignetting.gif  (8KB)

The red and blue dashed lines converge to create the 100% illuminated
field and the green dashed lines show how much of the objective is
obstructed in the very corner of the frame.

You can see how it drops from 100% to a lower value over a short distance
on the film.

In addition to hard stop vignetting, multiple mirror systems have
another type of vignetting. As you move off axis, at some point the
entire primary is no longer seen (reflected) by the secondary. As
you move past this point gradually more and more of the primary is
vignetted. This type of vignetting results in a 100% illuminated circle
of some size and then a slow drop-off in illumination towards the
corners of the frame.

Combine the slow drop-off with the fast drop-off and you could have
something that resembles this over the frame:

ftp://ftp.aa6g.org/illumination.gif  (2KB)

Chuck <aa6g@aa6g.org>

----------------------------------

> Hello All-
> I am in the process of constructing a telescope for astrophotography
> that is capable of providing a large flat field and am interested in
> feedback concerning a camera to design it around.  The Pentax 6X7 seems
> to be in popular use amoung many of you and the use of a medium format
> camera one can use for non-astronomy applications is a draw over a devoted
> system for this telescope.  I see many of you use yours at F/5 to F/6 -
> is there vignetting at these speeds caused by adaptors or the mounting
> flange?  There would certainly be some at the F/3.6 to F/4 of the system
> I'm envisioning.  I'd be interested in hearing any comments regarding
> the Pentax camera or if you would have other recommendations, either on
> list or off.  Thanks in advance.
>
> -Dean


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