Re: [APML] Fun in southern France

[Philip Perkins <philip@astrocruise.com>]

Hi Bobby,

Thanks - this shot is one of those 'strange ones' - didn't come out as I 
planned and so far I can't see the reason.  I'll have another go at the 
processing a little later and see if I can come up with some inspiration.

You've asked a good question - I'm looking for the same thing!  Firstly the 
FF - I have one for the RC but it was designed for use in conjunction with 
the OAG (designed by Chuck Vaughn) with a 35mm camera behind and must be at 
a prescribed distance from the film plane.  When the Pentax is attached the 
distance is much greater and off axis star images are severely 
aberrated.  Dave Rowe did some excellent ray traces which clearly showed 
the problem (I could send them to you with Dave's permission).  These 
traces showed that with the Pentax I would be better off not using this FF 
at all.  So for the Pentax a new FF would need to be designed.  Not only 
that, but a new OAG needs to be designed as well because the existing one 
(designed only for 35mm) cuts off too much of the light cone.  You can see 
the set-up here:
http://www.astrocruise.com/misc/rc_pentx.jpg

In this particular set-up I had the AP .75x focal reducer attached - you 
can see it just after the 2.7" fixed instruments adapter tube.  There is a 
special flange adapter (hidden) to which the OAG is attached by those brass 
screws.  As mentioned above this OAG is a problem for use with medium 
format.  But an even bigger problem is the adapter for the Pentax that 
attaches it to the OAG.  This adapter is made by Lumicon and has inherent 
flexure which I could not remove, and besides which, it has a restricted 
port (it narrows down to the standard 2" port that fits Lumicon OAGs).  So 
what's really needed is a new OAG design that has a proper Pentax adapter 
with no restrictions in the light path and has a removable (screw-in) FF so 
that different FF's can be used (or none at all) according to instrument 
type.  Any takers on the design of such a thing?  Seems to me that the 
design could be based on the GEG.  Dave Rowe already offered up a design 
for a FF for the RC which seemed to work well and be cost effective.  The 
only caveat to such a FF is that it will be expensive owing to limited 
quantities using big pieces of glass.

Thanks
--Philip


At 04:32 29/12/01 -0600, you wrote:
>Good to hear you had a productive outing Philip! The NGC 1977 shot is great
>(those spikes are a nice touch <g>).  Can you offer up any tips for me if I
>wanted to put in a 2.7" focuser for my 12.5 newt and have a FF made that
>would allow me to use my Pentax 67 on it? I would need an optical engineer
>to tell me what could be done in terms of a field flattner. If there was a
>good 120 film out there now this would be easy for me to consider.
>Bobby


Philip Perkins - philip@astrocruise.com
Wiltshire UK & Luberon France
Astrocruise - http://www.astrocruise.com


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