The Rise and Fall of Medium Format on my 10" LX200
By Chris Vedeler Copyright 1997
1- Introduction
Many of the best astrophotographers use medium format cameras with their telescopes. They do this because medium format permits a larger area of sky for a given focal lenght of telescope, much in the way a Nagler eyepiece gives you a much wider field of view than a Plossl at the same magnification. Medium format also permits a much bigger enlargment before the grain becomes a problem. The lure of medium format is clear.
My Mamiya 645 with the Lumicon Giant Easy Guider
attached to my 10" LX200.
2- Technical Issues with Medium Format on a Meade 10" LX200
Medium format cameras and accessories are expensive, and those needed to connect it to the telescope are no exception. With a 60mm wide negative you need at least 60mm wide opening into the optical tube assembly to avoid vignetting. The only vender that I could find that makes a device to connect a medium format camera to a SCT telescope is Lumicon. The Lumicon Giant Easy Guider costs $395, and the Mamiya camera adapter cost $145. That about doubled my introductory costs into medium format with the used Mamiya 645 camera I bought costing $550. The Lumicon Giant Easy Guider offers an 80mm diameter opening and a very nice connection from the camera to the telescope. With my guide scope there is no need to use the Lumicon Giant Easy Guider as a guider, but it is nice to have in case I do decide to manually guide off axis.
Using the Lumicon Giant Easy Guider without the focal reducer (i.e. at f/10) the light cone at focus is 62mm in diameter with my 10" LX200. This means I would expect only a little vignetting at the extreme edges of the negative. Using the focal reducer at f/6.5 the light cone shrinks down to 56mm in diameter. This means that significant vignetting will occur on the medium format film plane. But, as you can see in the figure below, it is still significantly larger than the area covered by a 35mm.
My LX200 with guide scope and Mamiya 645 attached.
3- Real World Experiences
After careful analysis of my first roll taken at the f/6.5 setting in the
GEG, I am now convinced that medium format gives very little advantage over
35mm with the 10" f/10 LX200. The vignetting was worse than predicted. I put a blank frame of 35mm
film over the 120 negative to get an idea of what I was really getting after
cropping all the vignetting. There was really only about 10% more area.
This is what I figured out. At f/10 there would be significantly less
vignetting using the medium format, but the area of sky would be about the
same as f/6.5 with using 35mm. What you get with medium format ultimately
is less grain for a given enlargement. However at f/10 I will need to
either expose much longer, or use a faster (i.e. grainier) film. This just
to get the same aria of sky as the 35mm at f/6.5. Clearly there is little
advantage.
What I have discovered is that the light cone coming out the back of the
scope is what it is. The available light is a constant. Using a focal
reducer just shrinks the size of the light cone thus making it brighter.
With 35mm, there is a lot of light that is wasted off the sides of the
36x24mm field at f/10 that can be brought onto the frame at f/6.5. With
medium format there is not enough "wasted" light at f/10 as you are
capturing almost the entire light cone on the film, so going to f/6.5 is
just making the f/10 image smaller and brighter on the film causing
significant vignetting.
