Re: [APML]: 2X converter

[Space Boy <astro@cu-online.com>]

The Astro-Photography Mailing List
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John Gleason wrote:
> 
> When a Pentax 1.4X TC was used with my 6" f/7.1 AP and 100 field flattener,
> I found severe astigmatism beginning around the 70% zone on 120 format
> film.  It needs to be confirmed, but I might conclude that the TC might
> yield better results without the field flattener.
> 
> jg
> 

  I just remembered something.  Back in '94, when I got the AP Traveler,
asking Roland Christen about the 1.4x: he told me tested it (in
combination with AP scopes) & found some problems.  I suppose he could
design something *custom*, but I don't think there would be enough
interest (who wants to photograph at f10, not many).

 I checked Pentax catalog, and there is a special 1.4X for the 800mm/6.7
EDIF, called the T5-1.4X (apparently designed for LONG FL).  The front
element extends _forward_ (just like the Nikon TC301, which is designed
for longer FLs, >300mm).  I have a feeling this would work better, but
it would physicallyinterfere with the field-flattener (it would have to
be removed).

  I know someone who has this lens, I should try to get him to do some
deep-sky with it (w & w/o the T-1.4X)


Space Boy


PS
What does astigmatism look like on your neg?  Stars that look like
triangles?