AlanFrench
Well-known member
Out of curiosity, does anyone know where the optics for the Zeiss spotting scopes are made now?
Clear skies, Alan
Clear skies, Alan
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Zeiss sample variation test by Jan M. (1 Viewer)
- Thread starter kabsetz
- Start date
Mike Penfold
Well-known member
Alan,
At least some are made in Czechoslovakia. See the attached photograph of a Zeiss 65A, "Made in Czech Republic."
Thanks for the link.
Mike
Attachments
Mike Penfold
Well-known member
The chart at the Edmund Industrial optics site provides resolution in line pairs per millimetre. Had a good look on BirdForum and the web, but can't find: distance from objective lens to USAF resolution chart for testing; how to convert from lp/mm to arc seconds of resolution; arc second ranges from most desirable to least desirable for a Zeiss 65A/zoom.
Mike
henry link
Well-known member
Mike,
The formulas you need are in the Zeiss subforum thread just below this one titled "Zeiss 85mm Contrast and Star Test questions", posts #5 and 7.
Sorry, for some reason I couldn't post a link.
Henry
The formulas you need are in the Zeiss subforum thread just below this one titled "Zeiss 85mm Contrast and Star Test questions", posts #5 and 7.
Sorry, for some reason I couldn't post a link.
Henry
dahyon
Well-known member
Mike
Roger Clark describes his own version here with worked examples:
http://www.clarkvision.com/imagedetail/relative-lens-sharpness/index.html
Cheers Dave
Roger Clark describes his own version here with worked examples:
http://www.clarkvision.com/imagedetail/relative-lens-sharpness/index.html
Cheers Dave
Mike Penfold
Well-known member
Thanks Henry, Dave,
From your posts, Dawes Limit for the Zeiss 65A is 120/65 = 1.85 arc seconds. Forgive the astronomically-challenged: not sure how the ranges for cherry/good/lemon are derived; how focal length for a scope is calculated, in order to obtain distance from the objective lens to the USAF 1951 resolution chart; scoured the Edmund website/telephoned them, but can't find conversion information from lp/mm to arc seconds.
Mike
From your posts, Dawes Limit for the Zeiss 65A is 120/65 = 1.85 arc seconds. Forgive the astronomically-challenged: not sure how the ranges for cherry/good/lemon are derived; how focal length for a scope is calculated, in order to obtain distance from the objective lens to the USAF 1951 resolution chart; scoured the Edmund website/telephoned them, but can't find conversion information from lp/mm to arc seconds.
Mike
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henry link
Well-known member
Mike, The formula for converting lp/mm to arc seconds of resolutuon is: Resolution= 8121/(Distance in inches X lp/mm). For instance, if you can resolve 4 lp/mm at 100' then the resolution is 1.69 arcsec. 1.69 = 8121/ (1200X4). Birding scopes are very unlikely to show resolution as good as 120/Dmm on the USAF chart. 140/Dmm is more like it for a reasonably good one. 160/D is getting pretty bad. You'll have to make the call in between. 45X may not be enough magnification to see the full resolution of a good 65mm scope.
Distance to the target is measured from the front (objective) lens of the telescope.
Henry
Distance to the target is measured from the front (objective) lens of the telescope.
Henry
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If anyone is interested, I have used a PDf version of the test chart with good results for testing systems. I buy/sell a lot of lenses, etc so use for a good quantitative result. If you know the FL of the lens, scope, etc, then you can use the chart.
I haven't used to calculate resolution on spotters per se but I do have a "standard" test I use with binoculars and scopes using this very chart. In fact, I printed instructions on back and laminated it!
Thus lamination and eyepiece will be unknown variables in my results but I find it very effective to size up gear quickly.
http://www.takinami.com/yoshihiko/photo/lens_test/pdml-procedure.html
http://www.takinami.com/yoshihiko/photo/lens_test/USAF.pdf
I haven't used to calculate resolution on spotters per se but I do have a "standard" test I use with binoculars and scopes using this very chart. In fact, I printed instructions on back and laminated it!
Thus lamination and eyepiece will be unknown variables in my results but I find it very effective to size up gear quickly.
http://www.takinami.com/yoshihiko/photo/lens_test/pdml-procedure.html
http://www.takinami.com/yoshihiko/photo/lens_test/USAF.pdf
Mike Penfold
Well-known member
Yesterday the night sky was transparent and stable, a good opportunity to star test a Zeiss 65A, which a local amateur astronomer completed. The scope is really, really good.
This answers my issue, but doesn't address the need for an inexpensive, clear means for birders to test samples of sports optics for performance factors relevant for birding before they leave the store. AFAIK, Eagle Optics is the only retailer offering a 30 day return policy.
Forget star testing for the average birder, reluctant to shell out about $300.00 USD for a Zeiss tripler, unwilling to spend many hours unraveling the mysteries of Harold Suiter's Star Testing Astronomical Telescopes, in order to complete the testing to purchase one scope or binocular.
One viable alternative is to use a reference scope or binocular with the optics being considered, along with an Edmund Industrial Optics credit card-sized USAF 1951 resolution chart, as suggested by one evaluator of sports optics. However, some birders don't have access to reference scopes or binoculars. Some stores only stock one sample of a scope. IMO, most birders aren't prepared to order multiple samples of a scope to compare.
IMO, if an inexpensive, practical, detailed means of testing sports optics were available to test single samples in a store, birders would use it. IMO, the testing process relies on an aggregate of necessary and sufficient details. The USAF 1951 resolution chart for about $3.00 is a good place to start.
The chart on the Edmund Industrial Optics website converting results on the USAF 1951 resolution chart to line pairs per millimetre is necessary.
The conversion formula from line pairs per millimetre to arc seconds is necessary.
The Dawes Limit for the scope or binocular is necessary. In other places on the web, the Dawes Limit for a scope's performance is calculated as 116/diameter of the objective lens in millimetres; i.e., with the Zeiss 65, 116/65 gives a Dawes Limit of 1.78 arc seconds. Is this the correct calculation?
Mike
This answers my issue, but doesn't address the need for an inexpensive, clear means for birders to test samples of sports optics for performance factors relevant for birding before they leave the store. AFAIK, Eagle Optics is the only retailer offering a 30 day return policy.
Forget star testing for the average birder, reluctant to shell out about $300.00 USD for a Zeiss tripler, unwilling to spend many hours unraveling the mysteries of Harold Suiter's Star Testing Astronomical Telescopes, in order to complete the testing to purchase one scope or binocular.
One viable alternative is to use a reference scope or binocular with the optics being considered, along with an Edmund Industrial Optics credit card-sized USAF 1951 resolution chart, as suggested by one evaluator of sports optics. However, some birders don't have access to reference scopes or binoculars. Some stores only stock one sample of a scope. IMO, most birders aren't prepared to order multiple samples of a scope to compare.
IMO, if an inexpensive, practical, detailed means of testing sports optics were available to test single samples in a store, birders would use it. IMO, the testing process relies on an aggregate of necessary and sufficient details. The USAF 1951 resolution chart for about $3.00 is a good place to start.
The chart on the Edmund Industrial Optics website converting results on the USAF 1951 resolution chart to line pairs per millimetre is necessary.
The conversion formula from line pairs per millimetre to arc seconds is necessary.
The Dawes Limit for the scope or binocular is necessary. In other places on the web, the Dawes Limit for a scope's performance is calculated as 116/diameter of the objective lens in millimetres; i.e., with the Zeiss 65, 116/65 gives a Dawes Limit of 1.78 arc seconds. Is this the correct calculation?
Mike
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Mike Penfold
Well-known member
Bruce,
The amateur astronomer and his wife have just installed a Meade Advanced Ritchey-Chretien telescope in their second observatory.
Tests were done for resolution (tack-sharp); collimation (perfect); and astigmatism (none). There was coma around stars at the periphery of the view at low power, a trade-off for the huge area of field that's an advantage of this scope for birding. (Some people dislike this softness at the edges at low power.) There was slight chromatic aberration, an apparent feature of a refractor telescope. The "really, really good" description is my inference.
Since all I was interested in was cherry or lemon information, and as I'm not and never will be an amateur astronomer (although the moons of Jupiter are cool), I couldn't tell you the names of the night sky features that were used, if that's what you're interested in.
The Zeiss 65A with zoom was able to resolve two stars separated by 5 arc seconds. It was mounted on a Manfrotto 055CL/Bogen 3021BN and a 128RC/3130 head (although I'm using it for birding with a Benro C-027n6 tripod and Manfrotto 700RC2 head). We were unable to test it on two stars separated by 2.4 arc seconds, because the above mounting wouldn't give a high enough angle.
To my eyes, the stars were as clean and sharp through the Zeiss as they were in the couple's 106 mm Takahashi.
Let me know if you have further questions, as you may be interested in exact details, in order that you can attempt to replicate the above performance.
Mike
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Thanks Mike. I have only done a few star tests, but I seem to have less luck with scopes than you, so I mostly have learned a few things about identifying defects. I don't really know how to interpret a star test so as to say a scope is a cherry. All I could say is that I don't see the defects I have seen before. I was interested in the details of what they saw when looking through the scope, but it would probably be over my head anyway! I'll dig through the archives some more. I'm sure Kimmo or Henry have posted a lot on this subject.
Thanks again,
Bruce
Thanks again,
Bruce
henry link
Well-known member
I think I shouldn't have mentioned Dawes at all. It's a very specific criterion derived from spliting sixth magnitude white double stars, but it's a number that is often quoted for telescope resolution, somestimes given as 116/D, sometimes 117/D, somtimes rounded off to 120/D.
The USAF 1951 bar target is very different from double stars, but I have found that the best small refractors (by which I mean astronomical APO's) can yield resolution measurements of 115-120/D on the chart in daylight. I've never measured a birding scope that good. The very best was about 130/D and the best Diascope about 135-140/D.
Henry
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Mike Penfold
Well-known member
Thanks Henry.
How is the optimum distance between the objective lens and the resolution chart calculated?
Mike
How is the optimum distance between the objective lens and the resolution chart calculated?
Mike
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henry link
Well-known member
Mike,
The printing quality of the target determines the minimum distance you'll need to use. The target needs to be far enough from the scope so that you are not forced to use bars so small that they are too fuzzy to be accurate. For a 65mm scope I would want to be able to see perhaps 1.7 arc seconds just to have a cushion below the scope's actual resolution. So, for example, if you determine that the smallest usable bars are Group 2, Element 3 (5.04 lp/mm), then the distance needs to be 1611 inches so that G2-E3 will equal 1.7 arcsec.
An addition problem is that 45x may not be enough magnification. I wouldn't be able to see any better resolution than about 2 arcsec at 45x no matter how good the telescope.
Henry
The printing quality of the target determines the minimum distance you'll need to use. The target needs to be far enough from the scope so that you are not forced to use bars so small that they are too fuzzy to be accurate. For a 65mm scope I would want to be able to see perhaps 1.7 arc seconds just to have a cushion below the scope's actual resolution. So, for example, if you determine that the smallest usable bars are Group 2, Element 3 (5.04 lp/mm), then the distance needs to be 1611 inches so that G2-E3 will equal 1.7 arcsec.
An addition problem is that 45x may not be enough magnification. I wouldn't be able to see any better resolution than about 2 arcsec at 45x no matter how good the telescope.
Henry
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landingsafe
New member
Some consideration on the test of J.A. Meijerink. Indeed I find a little qualitative difference among the 5 samples of Zeiss. I find however strange to compare scopes lens of 85 mm (Zeiss) with scopes lens of 88 mm . 3 mm probably make the difference at this extreme test.
Alberto
Alberto
Just a short note,
Jan's published tests are done visually, without the aid of a booster. He states this on top of the box that has the results for the different magnifications with the different samples. This means that the arcsecond figures even at 60x and even with his visus of 1.5 cannot meet the 120/D condition with scopes that have objectives this large. Also, since he uses the single-line formula, one needs to double the figures to get the resolution in line-pairs. However, this does not diminish the value of his tests when comparing different specimen to each other. It only means that his results do not tell us how close or far from a faultless diffraction-limited performance they are.
Kimmo
Jan's published tests are done visually, without the aid of a booster. He states this on top of the box that has the results for the different magnifications with the different samples. This means that the arcsecond figures even at 60x and even with his visus of 1.5 cannot meet the 120/D condition with scopes that have objectives this large. Also, since he uses the single-line formula, one needs to double the figures to get the resolution in line-pairs. However, this does not diminish the value of his tests when comparing different specimen to each other. It only means that his results do not tell us how close or far from a faultless diffraction-limited performance they are.
Kimmo
