"Star Testing" vs. "Business Card Testing" (1 Viewer)

Leif said:

Given that many BF users own scopes, we thus have the means to do an online scope sample variation survey. In other words, we all post a brief description of how our scope star tests at ~60x, not forgetting scope details. No doubt, given the previous enthusiasm for my suggestions, no one will be interested ...

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How many Zeiss owners frequent this forum? Obviously those with prior experience "star testing" and owners of multiple scopes would be the optimal contributors. I would like to know whether excellent results with a "star test" corresponds to excellent result reading a business card at 100 feet.

High end telescopes used for astronomy are generally carefully corrected for spherical aberration so that they may give "diffraction limited" performance. Such instruments tend to be used at higher powers than most birding scopes, so such care is necessary. I have yet to hear anyone marketing a birding scope - with the exception of Questar - make any claims about correction for spherical aberration. A nice resolution chart with resolution targets at varying contrast levels would probably be a better way to compare birding scopes. OTOH, most high end birding scopes work pretty darn well, and I am not sure such comparisons are really needed. If your scope "snaps" into focus at the powers you use, and gives nice detailed views, does it matter if some other scope might do a tad better?

Clear skies, Alan

AlanFrench said:

[...] A nice resolution chart with resolution targets at varying contrast levels would probably be a better way to compare birding scopes. OTOH, most high end birding scopes work pretty darn well, and I am not sure such comparisons are really needed. If your scope "snaps" into focus at the powers you use, and gives nice detailed views, does it matter if some other scope might do a tad better?

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Yes I suspect the primary purpose would be to reassure the owner of a new scope that he has not received a damaged or dud scope. A resolution chart that anyone could print on a decent laser printer would be of interest.

AlanFrench said:

OTOH, most high end birding scopes work pretty darn well, and I am not sure such comparisons are really needed. If your scope "snaps" into focus at the powers you use, and gives nice detailed views, does it matter if some other scope might do a tad better?

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You've got a point. Testing after you already own the scope may not do you that much good unless you are trying to track down some specific issue. Though it might matter if some other scope did a tad better but at half the cost.

I'm more concerned about optical quality as it pertains to digiscoping. I'm much less concerned about getting the nth degree of quality when visually observing. A nice wide and apparently sharp view makes me happy.

Jay Turberville said:

You've got a point. Testing after you already own the scope may not do you that much good unless... [...].

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...Unless you want to know if you bought a turkey. I thought this German site looked interesting. They seem to sell astronomy scopes and sell the Zeiss.

http://www.flourideoptics.com/

galt_57 said:

Yes I suspect the primary purpose would be to reassure the owner of a new scope that he has not received a damaged or dud scope. A resolution chart that anyone could print on a decent laser printer would be of interest.

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Hi,

what about this one: http://medfmt.8k.com/mf/USAF.pdf found on: http://medfmt.8k.com/mf/resolution.html

Steve

hinnark said:

what about this one: http://medfmt.8k.com/mf/USAF.pdf found on: http://medfmt.8k.com/mf/resolution.html

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Jay T. also suggested these;

http://www.normankoren.com/Tutorials/MTF5.html
http://www.bealecorner.com/trv900/respat/

I also want to try a simple page from a newspaper.

Since the diffraction limit of telescope apertures is generally expressed in arc-seconds, you should want whatever test pattern you choose to be easily convertable to that form so you can tell how close the scope is to theoretical perfection and how it compares to other scopes.

Edmund used to have a large printed poster of the USAF 1951 test with the pattern repeated in different colors and a formula to convert the results to arc-seconds.

Test patterns for photography will probably use lines per mm, which ought to be convertable to arc seconds if you know the formula. The issues in telescope resolution are a bit different from camera lens resolution, where resolution only needs to be a little better than the finest grain film. A diffraction limited telescope with the same basic specifications as the Zeiss Diascope (85mm aperture, 504 mm focal length) will, if my math is correct, have resolution at the center of the prime focus image of about 300 lines per mm, better than any camera lens and much better than film or a ccd. An excellent camera lens actually makes a pretty mediocre telescope.

Lines per mm

henry link said:

Test patterns for photography will probably use lines per mm, which ought to be convertable to arc seconds if you know the formula. The issues in telescope resolution are a bit different from camera lens resolution, where resolution only needs to be a little better than the finest grain film. A diffraction limited telescope with the same basic specifications as the Zeiss Diascope (85mm aperture, 504 mm focal length) will, if my math is correct, have resolution at the center of the prime focus image of about 300 lines per mm, better than any camera lens and much better than film or a ccd. An excellent camera lens actually makes a pretty mediocre telescope.

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Henry,

For what it's worth, Raynox claims their DCR-1540PRO 1.54x teleconverter lens has "340 lines/mm resolution power at center (MTF 30%)." http://raynox.co.jp/english/digital/fz10/index.htm

Not sure if they are talking about exactly what you mentioned, altho it sounds like it.

Regards,
Bob