I
have been experimenting with a homemade adaptor to permit Medium Format
(6x7) Pentax camera lenses to be used with my Astronomical CCD cameras.
The
adaptor also features support for a color filter wheel and digital
focuser.
One of the interesting properties of the medium format lenses is their
long
back focus. That permits me to easily add a color filter wheel and an
inline
digital focuser. If I were to use a lens for a 35mm camera I would have
significantly less back focus and would probably be unable to use the
focuser.
The other interesting thing about the medium format sized lenses is
that I
am using only a small part of the "sweet spot" in the center of the
FOV. I
am thinking that may permit me to still take good images without having
the
world's most expensive optics etc.....
I completed the initial configuration of the adaptor late Sunday
evening,
Aug 1. Of course as luck would have it I was clouded out at home. The
next
two nights also featured significant cloud cover. Last night, however,
I had
a break in the clouds long enough to permit me to take a few test
exposures with the
150mm
Takumar f/2.8 lens I initially used.
Initial results show elongated stars in the lower right corner of each
image. Furthermore there is some defocusing in that part of the image.
This
all points to a tilt of the lens axis relative to the optical axis. My
next
move will be to add a tilt adjustment (push pull screws) to
hopefully permit that to be "tuned out".
In interest of documenting what I have done, I have prepared a web page
that
shows pictures of the adaptor in action, as well as test images. Some
are
terrestrial (the first ones) and the ones from last night. For indoor
shooting under normal room lighting, I was using an [OIII] filter with
30
second exposures. The indoor images are not dark subtracted or flat
fielded.
The astro images were dark subtracted but not flat fielded.
Comments welcome.
http://www.narrowbandimaging.com/pentax_6x7_lens_adaptor_page.htm
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