Bruce: Now I see what this device is all about. You don't need to come to focus with it. The defocused image is a doughnut and you collimate by centering the hole in the doughnut. I can't comment on the usefulness of the tool but a defocused view on a real star will do just as good. The book by Harold Suitter that I mentioned is a classic on the physics of the amateur telescope (by a physicist no less). And it is well worth the time spend reading it. When you mention "star test" or artificial star, many (older) amateurs think in terms of the concepts in his book. His main emphasis is on evaluating the figure of the parabolic Newtonian reflector mirror. You can do this with an artificial star but you have to move it out to a distance of several hundred meters depending to the size and speed of the mirror. Doing the test at a short distance will make the image appear as if the mirror has a nasty amount of spherical aberration. That's why warning flags go up when you mention a test star at a short distance. Many good mirrors have been called bad either by mistake, or as part of a ploy to sell you on a refiguring service. As a rule then, don't let someone "test" your telescope for you. It's a good bet that they don't know any more about the subject than you do and it's so easy to be oh so wrong. As similar warning goes for collimation. An adequately aligned telescope in the hands of a helpful but incompetent stranger can be thrown so far off that the evening's viewing is ruined. DT
________________________________ From: Bruce Hugo <bruce.hugo@yahoo.com> To: "utah-astronomy@mailman.xmission.com" <utah-astronomy@mailman.xmission.com> Sent: Monday, February 10, 2014 7:48 AM Subject: Re: [Utah-astronomy] Not enough backfocus
Hey Daniel,
Here is the verbiage and calculations on the device: http://www.hubbleoptics.com/artificial-stars.html
It recommends the telescope be placed at a distance based on two formulas. M=336*D/F^3 where D is in inches. Distance = M*D*F The 12" mead is 12" aperture, f/10. I came up with 483.8" = 40.3 ft.
I'll re-read the instructions and check the calculation. You guys know the physics of optics much better than I. I'll have to bend your ear(s) this year at the sun and star parties. I want to learn more.
I took the eyepiece out of the diagonal and moved it back until I got focus on the 57 ft distant tree. The eyepiece was a good 5"-6" off the diagonal. Hahahaa! So it is pretty far out if focus. Guess the easy collimation out the window is not possible and I'll have to disassemble the scope and set it up in the yard some night when we have a clear night. Listening to the weather report, that's not going to happen this week!
Bruce
Bruce:
40 feet doesn't sound right to me, but my only reference is Suitter and he is looking at Newtonians.? and he is doing more >than just collimation.? Perhaps, just for collimation you don't actually need to come to focus.? Just centering the blob will >suffice.
DT
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