Re: [APML] More fun in southern France

[Philip Perkins <philip@astrocruise.com>]

Hello Mike,

Thanks.  I'll reply on list because the AP focal reducer also works very 
well with Pentax 67 and RC 12.5".  In fact the edge of the Pentax 67 field 
is a much more severe test.  I saw no sign of any aberrations caused by the 
focal reducer.  Though I will add the caveat that I have not yet scanned 
the relevant negs (only examined them on a light box at 10x) so there may 
be some very minor aberrations not yet seen.  Certainly nothing shows up in 
the field size of the ST-8E chip.

The only thing that confused me is what back focus to use with the 
telecompressor in place.  As you know, back focus with the RC design is 
rather critical (or more accurately, primary to secondary air spacing but 
which can translate to the same thing).  Moreover, I use the fixed 
instrument adapter tubes with my particular RC (focusing is by servo 
adjustment of the secondary) so I can't easily adjust back focus on the 
fly.  I talked with Dave Rowe on the subject and it turns out to be not 
that simple (dependent on lens prescription and designed lens-to-film 
spacing).  I patched up a solution by first of all trying to get the lens 
as close as possible to the typical lens-to-film spacing if a 35mm camera 
had been attached (with the OAG and Pentax it was still *much* further away 
than that but what the heck - I did what I could).  Then secondly I 
attached a camera *without* the focal reducer such that the film plane was 
at exactly the correct back focus distance, then adjusted focus (which 
moves the secondary) for the correct focus point.  In this step I 
established the correct primary to secondary air spacing without having to 
actually measure it.  Then thirdly, while maintaining that same focus 
point, I attached the camera *plus* focal reducer and swapped adapter tubes 
(I have several of different lengths) until I got as close as possible to 
focus without actually touching the secondary focuser.  In most cases this 
point was reached by actually screwing the focal reducer directly into the 
RC rear cell - I couldn't get closer than that, but it turned out to be 
pretty close.  It seems to have worked!

Best wishes for the New Year
--Philip


At 17:54 30/12/01 -0500, you wrote:
>Wonderful image and nicely processed.  I was not aware that
>the RC could be use with the AP telecompressor.  Is there any
>off-axis astigmatism noticeable at field edges?



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


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>