Re: Images with our new CCD telecompressor

Tim Khan


Firstly, Incredible image of M57!

As Adrian, I also purchased the 0.67x telecompressor.
On my 155EDF, in order to get the best images at the
edge of the ST-10, would you recommend the
telecompressor in conjunction with the field



--- chris1011@... wrote:

I don't know what the limit would be. Generally,
bringing the reducer closer
to the focal plane always makes any added
aberrations smaller because they
converge on the native aberrations of the main
instrument. Going farther away
introduces several problems. Even if the aberrations
do not increase at the
edges, the physical size of the illuminated field
will shrink, so the edges and
corners will start to vignet rather rapidly. The
reason for this is that the lens
is sized just big enough to fit inside a 2" barrel,
however the further
upstream you place it, the larger the light ray
bundles become. Thus the edge rays
from your instrument will be cut off. Secondly, this
lens sits inside the
focuser, and in some Cassegrain instruments there is
very limited distance that a
2" item can be pushed up into the light beam.

Lastly, I would bet that with a faster instrument
like your F9 RC, the edge
aberrations will really begin to show at any higher
compression ratios. This is
due to a combination of field curvature and
astigmatism of the main instrum
ent added to lateral color and other secondary
aberrations of the
telecompressor. I can just see these effects
starting with my own Mak in the very corners of
the format. There is no way to get perfect
performance over the entire field
with every conceivable instrument type and focal
ratio without designing a
specific optic for each focal length and adding more
elements. My own research
has shown that as you add more elements with air
spaces to control more off-axis
aberrations, you inevitably end up with horrid
reflection ghosts that come
out and bite you when you begin to process stretch
your images - even with the
most perfect multi-coatings. Only a cemented doublet
with unequal external
curves can totally eliminate that problem, as you
can see by the extreme
stretching exercise I did on the second M57 image.

Roland Christen

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