P3D Re: Stopping down a mirror based lens system

[John Ohrt <johrt@xxxxxxxx>]

Bob Howard wrote:
> 
> RE: Larry Haines wonders about the mirror (Matsukov et al) tele lens
> 500mm. They are typically f/8 in the 500mm, (a Zeiss F/5.6 one cost
> an arm and a leg>) They do not stop down..so f/22 isn't a
> possibility. (I do have one with discs that sort of adjusts
> exposure). Also the f/8 is a physical f/stop and the secondary mirror
> cuts out part of the light so it is effectively f/11. Depth of field
> in very shallow so not for stereo execpt at infinity where one would
> have to use hyper to get anything. They do not hand hold well either,
> except some of the latest cameras might let you use 1/2000-1/8000 if
> you had fast enough film to use f/11 in sunlight. BobH


The FOV is consistant with the stated f/ ratio, and of course the
obstruction blocks light.  The obstruction also degrades contrast.  The
best way to stop it down is to use an off axis aperture that is circular
in shape and just large enough to impinge neither on the effective
primary or secondary mirror edges.

If you want one of good quality, check with amateur astronomers.  Tell
them you want an OTA (optical tube assembly) of catadioptic or Makutsov
design.  There are also accesories such as field wideners and field
flatteners which (I believe) are not offered to with the "camera" lens
designs.  Domestically, both Celestron and Meade offer such OTAs.

Regards,
John


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