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#11
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How good is good enough?
@hpl.hp.com () wrote in message ...
I recently got an Orion 127 mm Mak-Cas telescope, and although the images seem to be pretty good, I was wondering are there any tests for determining if a given telescope is performing as well as it should? How sharp a point should a star focus to? It seems to depend on the brightness of the star too. At high (150X) magnification, I do see little rings around some stars: are these the Airy circles? I'm new at this so any pointers would be appreciated. Don't laugh, but daytime testing can reveal glaring deficiencies in a scope. One of my "tricks" after reassembling a scope is to attempt to read the label and serial number of a microwave dish about 1/4 mile away from my back yard. You could do the same for gear mounted on distant power or telephone poles. I recently did this with my Orion Apex 90 after I flocked its baffle tube and did a rough indoors collimation and, by golly, the digits in the serial number were crystal clear and I only needed to perform a minor additional tweak later that day (actually night :-) with a star test. Another test is examining leaves on a distant tree and see if you can detect the veins in the leaves and, hopefully, the absence of any blue/purple color fringing around the leaf's edges; this is also a good test for refractors. FWIW, the Apex/StarMax 127 (I have one, too) is overall a very good performer and there are some exquisite astrophotos taken using them that people have posted to the "OSAO" Yahoo group. |
#12
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How good is good enough?
On Sat, 14 Feb 2004 9:26:54 -0600, Rod Mollise wrote
(in message ): snip Rod Mollise Author of _Choosing and Using a Schmidt Cassegrain Telescope_ BTW, Rod, I just ordered your book. Diane gave me a gift certificate at Amazon and told me to order a good book on telescopes or astronomy. It is amazing how some people react to getting a big bunch of roses for Feb. 14. ;-) Especially if they don't have to remind you. -- Harry F. Leopold aa #2076 AA/Vet #4 The Prints of Darkness The Internet is full, we can not accept any more posts until further notice. Thank you. - Kalinka Djnepropetrovska |
#13
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How good is good enough?
On Sat, 14 Feb 2004 9:26:54 -0600, Rod Mollise wrote
(in message ): snip Rod Mollise Author of _Choosing and Using a Schmidt Cassegrain Telescope_ BTW, Rod, I just ordered your book. Diane gave me a gift certificate at Amazon and told me to order a good book on telescopes or astronomy. It is amazing how some people react to getting a big bunch of roses for Feb. 14. ;-) Especially if they don't have to remind you. -- Harry F. Leopold aa #2076 AA/Vet #4 The Prints of Darkness The Internet is full, we can not accept any more posts until further notice. Thank you. - Kalinka Djnepropetrovska |
#14
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How good is good enough?
@hpl.hp.com () wrote in message ...
I recently got an Orion 127 mm Mak-Cas telescope, and although the images seem to be pretty good, I was wondering are there any tests for determining if a given telescope is performing as well as it should? How sharp a point should a star focus to? It seems to depend on the brightness of the star too. At high (150X) magnification, I do see little rings around some stars: are these the Airy circles? I'm new at this so any pointers would be appreciated. Albert Your scope is apparently in good collimation and is producing correct airy discs. You next task would be to examine Jupiter (for instance) or Saturn to see how contrast is. If at around 150x the planet is bright with black sky around it, chances are the scope is decent, the optics smooth. If the planet is surrounded by a bright glow, then some of the light is being diffused, reducing contrast. Three things can make this happen; The central obstruction contributes a tiny amount, but this won't radically effect images. If the optical surfaces are rough, this will definitely cause a loss of contrast, also, if the optics are spherically aberrated (curves are imprecise) this will diffuse and blur a planetary image because the rays hitting different parts of the optics are coming to a different focus position from one another. In order not to misdiagnose any problems, it's important to let the scope reach the same temperture as the outside air and to have good, steady seeing conditions (when stars appear not to twinkle). When the scope is properly cooled down and the seeing is good, (star images when defocused are stable, without any waves going through them or motion) you should be able to detect clearly Cassini's division in Saturn's rings, and festoons or cloud whorls in the belts of Jupiter, as examples. -Rich |
#15
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How good is good enough?
@hpl.hp.com () wrote in message ...
I recently got an Orion 127 mm Mak-Cas telescope, and although the images seem to be pretty good, I was wondering are there any tests for determining if a given telescope is performing as well as it should? How sharp a point should a star focus to? It seems to depend on the brightness of the star too. At high (150X) magnification, I do see little rings around some stars: are these the Airy circles? I'm new at this so any pointers would be appreciated. Albert Your scope is apparently in good collimation and is producing correct airy discs. You next task would be to examine Jupiter (for instance) or Saturn to see how contrast is. If at around 150x the planet is bright with black sky around it, chances are the scope is decent, the optics smooth. If the planet is surrounded by a bright glow, then some of the light is being diffused, reducing contrast. Three things can make this happen; The central obstruction contributes a tiny amount, but this won't radically effect images. If the optical surfaces are rough, this will definitely cause a loss of contrast, also, if the optics are spherically aberrated (curves are imprecise) this will diffuse and blur a planetary image because the rays hitting different parts of the optics are coming to a different focus position from one another. In order not to misdiagnose any problems, it's important to let the scope reach the same temperture as the outside air and to have good, steady seeing conditions (when stars appear not to twinkle). When the scope is properly cooled down and the seeing is good, (star images when defocused are stable, without any waves going through them or motion) you should be able to detect clearly Cassini's division in Saturn's rings, and festoons or cloud whorls in the belts of Jupiter, as examples. -Rich |
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