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Astro Focusing problems
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? |
#2
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Astro Focusing problems
On Wed, 06 Jun 2007 02:53:48 GMT, "Terry B"
wrote: 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. I suspect the result through the blue filter is a combination of chromatic aberration and spherochromatism. Assuming normal glasses, the focal length is very short for a 120 mm objective and the result is less control of aberrations. Bud -- The night is just the shadow of the Earth. |
#3
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Astro Focusing problems
Terry B wrote: 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? Yes this is inherent to all low cost achromats especially at f5. William's diagnosis is accurate for this f5 Skywatcher lens. The color displacement especially in the violet is very typical. It would be interesting to see your red and green shots for comparison. If you happen to have a 300-600 line per mm diffraction grating perhaps in slide form and it produces a bright 1st order spectrum, hold the grating over a 25-30mm eyepiece focused on Vega. You should see a fairly well defined line from red into blue but very pronounced vertical splaying of the line into the violet. This is a literal depiction of what your achromat is doing, chromatically. If you have a dob around try the same test and you will see the difference. Now imagine that star line converted to a round airy disk - the very same chromatism seen in horizontal form with the grating is now being distrubuted out spherically. But the use of the grating allows you to see in graphic terms what your achromat is doing chromatically vs. say a newtonian mirror. Notice also the three rounded triangles of your star images vs a tight circular airy disk. These rounded points in the airy disk are the spherical component William is speaking of. Very likely your lens has an outer zone of undercorrection. Rack in a star until you get a ringed diffraction pattern. Very likely your pattern shows a bright thick outer ring which is the result of undercorrection at the edges of the lens. Sometimes if you lessen the tension on the retaining ring at the front of the lens you can improve slightly the affects of undercorrection. You certainly cannot do any harm.You can always put back the same retaining ring tension (and image quality) you had before. Everything you show is classic and normal for the lens you have. Best regards - Jerry |
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