> Following an odd comment that I saw concerning the positioning of the
> JMI focuser, I had another go at re-positioning. I found that I was
> able to fit the focuser directly to the back of the LX200, and could
> still fit my f6.3 converter to the rear of the focuser. It is cloudy
> here for several more days yet so I cannot test this configuration. Can
> anyone say that this should work? I have been using the other
> configuration:
> telescope-f6.3-JMI-CCD (MX916)
> and always have a fair amount of vignetting, and wonder whether this new
> system might work better.
It's all down to distances...
The compression 'ratio', generated by a focal compressor, is not fixed. It
depends on the spacing to the CCD, and rises (quite quickly) as the distance
increases. Hence with your current setup, the ratio will be changing as you
focus. The 'design' distance, is about 90mm from the point where the thread
stops on the compressor body. If your seperation is more than 90mm from
this, you will be getting a higher compression ratio.
Imagine that the CCD surface is 120mm behind this point. The compression
ratio will then be 0.5* (the focal length of the compressor is about 235mm,
and the formula for ratio is r = (f-s)/f - 's' is the seperation, 'f' is the
focal length of the compressor, and 'r' is the ratio given). Now the CCD you
have is about 11mm diagonally. Your light 'cone', for each section of the
optical path, grows at the effective focal ratio, for the length of that
section. Assume the scope is giving f/10 (it won't be, because of the large
backfocus required, probably more like f/11 in this case, but 'close
enough'), then the focal ratio between the compressor and the CCD, is f/5,
and the cone will grow by 120/5 by the compressor. At the compressor, the
light path will then be 34mm across. From this point it'll carry on growing
at f/10 down the scope.
Now if your spacing was larger than the 'guess' above, both the distance
rises, and the focal ratio over the distance goes down, giving a massive
increase in the light path diameter...
If instead the compressor is attached at the 'design' distance from the CCD,
and then the focusser is put in front of this, the light path at the
compressor, becomes just 24mm across. The light cone then carries on growing
at f/10, and will be about the same size at the front of the JMI, as it was
at the compressor in the other configuration. If your current spacing from
the compressor to CCD, is lower than the 'guess' figure, then your existing
configuration will be better. If however it is more than this guess, then
the new configuration will win. The compressor will also give better optical
results close to it's design spacing (in fact these units are remarkably
forgiving...).
It is also worth being aware that some vignetting is pretty much
unavoidable. Software packages that 'stretch' the image for display, make
levels that are tiny, look much worse than they are. A good 'flat frame',
can correct for normal levels easily.
The 'normal' configuration, is to have the compressor after the focusser,
and use then try to get the seperation between the CCD, and the compressor
close to the design distance. There is enough space here, to allow a 'slim'
colour wheel to also be fitted at this point.

Best Wishes

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