Re: ATM Help with relay lens design for large Maksutov

[Andy Saulietis <iss@pvtnetworks.net>]

Marcus writes,

>We attempted to build a relay lens system using f2.6 achromats
<to move the image plane back out through the mirror to the rear
>of the telescope, a <distance of about 20 inches.
>We experienced excessive coma.
  
>My question is----How can be build an effective relay lens assembly
>to move a small portion of the f3.7 image plane at about a 1 to 1
>size ratio? We have to translate the image about 20-22 inches back
>through the 4 inch hole in the primary/support with good star images.
>The CCD chip size is about 14X10 MM.

Simple achromats will have too much distortion & chromatic
aberration to use as a relay system at these f-ratios.

I have used high quality (Angenieux, Nikon etc) 35mm lenses
to relay images at 1:1 - 1:3 ratios to couple image intensifier
tubes to video cameras. The f-ratio of the relay lenses for your
f3.7 will need to be ~2.8 or faster, these are readily available in
the 50mm focal lenght of 'standard' 35mm cameras. You can also
use 'longer' lenses (80-200mm) but the required f-ratio
has to be maintained to prevent vignetting. Longer 'fast'
lenses get *expensive* quickly.

The lenses are used facing each other, with the scope focus
at the lens focus of the 1st relay, & the CCD focus at that of
the 2nd relay. The light between the relay lenses is parallel
so the distance is variable; the unvignetted field size of the relay
system will limit how far the lenses can be separated. Using a
faster lens for the 2nd (CCD) relay will allow the relay distance
to be increased. Focusing can use one or both of the relay
lenses' built in focus, or just move the CCD cam in a separate focuser.

The main problem with this method is the *large* number
of lens elements, resulting in increased scatter and reduced
transmission. These effects can be reduced by using high
quality lenses with good AR coatings & sealing up the
relay lens assembly to keep dust & contaminants out.

A 22" Mak? WOW...






Andy Saulietis
ISS Enterprises
39 Silver Fox Trail
Mayhill NM 88339
505-687-3067 voice
505-687-3021 fax, call voice or e-mail to set up
e-mail: iss@pvtnetworks.net