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Astro Focusing problems



 
 
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Old June 6th 07, 03:54 AM posted to alt.binaries.pictures.astro
Terry B
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Posts: 64
Default Astro Focusing problems

Sorry wrong image. This is the correct ones.
"Terry B" wrote in message
...
Dear All
I have bought a skywatcher 120mm x 600mm refractor to use mainly as a
guide
scope. However I have been trying to image through it as a test. It
certainly produces a nice purple fringe around bright objects visually but
I
expected this from an acromat.
I can image and produce sharp stars when I use a red or green filter but I
can't seem to get the image through a blue filter to focus. I realise that
the focus will be in a different position through the shorter wavelength
filter but I thought that the blue filter which is relatively close to the
violet should come to a pinpoint focus also.
The accompanying pics are little subframes of a bright star with a cross
over the front to produce diffraction spikes. These help with focus like a
hartman mask.
The red and green images produce nice diffraction spikes but the blue (the
first image) won't do it. All the filters also stop IR (allegedly) so I
think this should not be the problem. THe filters are astronomic LRGB
filters.
Any suggestions?
Is this inherent with all achromats or do I have a dud?











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File Type: jpg green mask.jpg (7.6 KB, 72 views)
 




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