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Good & bad samples, cherrypicking, etc (1 Viewer)

first of all - in german we value the lemon and thus call a bad example a cucumber (Gurke). There is no fruit or veggie term for a very good example though...

The cream on the cherries is the best ...(Sahneteil);)

I mostly had mechanical problems with binoculars (except for one Noctivid, the optics were suboptimal) mostly the focuser runs unevenly or jerkily, why is something not noticed during the final inspection?

Andreas
 
I can't help but feel that criticising a binocular for not performing well when boosted to twice or thrice its magnification seems a bit like criticising a Prius for not driving well at 200mph - that's just not what the vehicle in question was ever designed to do?

A high magnification star test isn't some artificially harsh stress test. It's more of a diagnostic tool that alerts you when something is worse than it should be and what exactly the problem is.

In the case of spotting scopes the result is straightforward. A star test at 60x is a very good predictor of the image quality the eye will see at 60x and provides a good explanation of why the image looks as it does.

Low power binoculars intended for daylight use pose a unique problem since they are are seldom used at their full apertures. To simulate their aberrations as the eye experiences them at various light levels star tests at reduced apertures are needed. For instance, a star test at around 20-30x of an 8x binocular stopped down to 20mm is a good predictor of that binocular's optical quality in bright sunlight. If that star test shows low aberrations you can be reasonably assured the axial image will be excellent in bright sunlight. However, if aberrations persist at such a small effective aperture the binocular is almost certainly defective enough to produce a visibly inferior image compared to a "perfect" 8x binocular, at least for observers with good acuity who look closely.

Henry
 
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What always struck me about the decision that something is a 'lemon' is that you must have the opportunity to do a side-by-side comparison with another set (or more) of exactly the same model; not something that most users have the chance to do.
Needless to say it must also be at the same time, in the same light, comparing the same properties etc. in the same test conditions.

Without being pedantic, we're also talking about a manufacturing variation, rather than an out-and-out fault.
 
What always struck me about the decision that something is a 'lemon' is that you must have the opportunity to do a side-by-side comparison with another set (or more) of exactly the same model; not something that most users have the chance to do.
Needless to say it must also be at the same time, in the same light, comparing the same properties etc. in the same test conditions.

Without being pedantic, we're also talking about a manufacturing variation, rather than an out-and-out fault.

Hi,

no, at least for spotting scopes, a star test will show very clearly what is amiss. And at least with optics dealers who cater to the astro market, an example with a documented bad star test will usually be exchanged.

They tend to guarantee diffraction limited optics aka a Strehl value of 0.8 or better (at least for the usual Made in China scopes, premium optics suppliers might state higher) - which for the low magnifications used in a spotting scope - even with extender - would be perfectly fine.

As Henry has pointed out, in binoculars the optical quality is a bit of a moot point as you would have to have very strong aberrations for a visible effect at 8 or 10x and star testing is also not so easy as you need an aberration free magnifier positioned at the correct place behind the binoculars...

Joachim
 
'Star testing complex optical systems' by Roland Christen makes interesting reading.
A short read.

It refers to Suiter, fast Apos, Maksutovs etc. and the false results given by star testing.

Personally, I do star testing, but my main criteria are actual planetary observations rather than 'theory'.

Regards,
B.
 
Hi,

no, at least for spotting scopes, a star test will show very clearly what is amiss. And at least with optics dealers who cater to the astro market, an example with a documented bad star test will usually be exchanged.

They tend to guarantee diffraction limited optics aka a Strehl value of 0.8 or better (at least for the usual Made in China scopes, premium optics suppliers might state higher) - which for the low magnifications used in a spotting scope - even with extender - would be perfectly fine.

As Henry has pointed out, in binoculars the optical quality is a bit of a moot point as you would have to have very strong aberrations for a visible effect at 8 or 10x and star testing is also not so easy as you need an aberration free magnifier positioned at the correct place behind the binoculars...

Joachim
Yup - i was referring to binoculars solely. Scopes are a different business.
 
'Star testing complex optical systems' by Roland Christen makes interesting reading.
A short read.

It refers to Suiter, fast Apos, Maksutovs etc. and the false results given by star testing.

Personally, I do star testing, but my main criteria are actual planetary observations rather than 'theory'.

Regards,
B.

Hi Binastro,

thanks for mentioning this essay, an interesting read - here is the link btw:

http://www.csun.edu/~rprovin/roland/startest2.html

It should be mentioned though, that Roland (Christen, the owner of Astro Physics, a quite legendary telescope manufacturer) does refer to comparing star tests of very high quality instruments with aberrations at the limit of what a star test can show and that in some cases a not quite perfect and a quite perfect example at defocus can be undistinguishable at perfect focus.

For demonstrating why a spotter does not perform at 60 or 100x and the quite bad aberrations involved causing this, the star test is good enough.

Joachim
 
Hi,

first of all - in german we value the lemon and thus call a bad example a cucumber (Gurke). There is no fruit or veggie term for a very good example though...


- Zones - always a problem in grinding - cannot be adjusted.

Joachim

Hi Joachim

First and most importantly, I love German Gurken.

A subsidiary question: what are zones in this context of grinding lenses?

Tom
 
For demonstrating why a spotter does not perform at 60 or 100x and the quite bad aberrations involved causing this, the star test is good enough.

Joachim

Joachim

But it is very rare that a Birdforum member posts that he/she has found their spotter does not perform well during nature observation and describes what is wrong with the images of birds or animals and then investigates this using a star test. In fact I can't recall a single example, but my memory certainly isn't perfect.

Isn't it more usual that no observing problem is mentioned but the member wants to find out if he has a cherry or a lemon and so performs a star test and only after this is it decided whether there is a problem with the scope. If no problem is detected during normal observation it seems perverse to me to go looking for problem using the star test.

Lee
 
Joachim

But it is very rare that a Birdforum member posts that he/she has found their spotter does not perform well during nature observation and describes what is wrong with the images of birds or animals and then investigates this using a star test. In fact I can't recall a single example, but my memory certainly isn't perfect.

Isn't it more usual that no observing problem is mentioned but the member wants to find out if he has a cherry or a lemon and so performs a star test and only after this is it decided whether there is a problem with the scope. If no problem is detected during normal observation it seems perverse to me to go looking for problem using the star test.

Lee
I agree 100% as you may have gathered from previous post I made. I have zero interest in star testing and doubt it's reliability.
 
It's a sad fact that many birders soldier on for years with poor quality specimens and never know it. I've seen a number of examples myself.

Why don't they notice? I think because they have no reason to know what standard should be applied to a telescope image. It's just not a piece of general knowledge. I have a friend who continued to use scope with a cracked prism for 20 years without complaint. He just assumed that no spotting scope achieves a usable image above 25-30X and his scope was bad even at 20x. I guess ignorance is bliss.
 
It's a sad fact that many birders soldier on for years with poor quality specimens and never know it. I've seen a number of examples myself.

Why don't they notice? I think because they have no reason to know what standard should be applied to a telescope image. It's just not a piece of general knowledge. I have a friend who continued to use scope with a cracked prism for 20 years without complaint. He just assumed that no spotting scope achieves a usable image above 25-30X and his scope was bad even at 20x. I guess ignorance is bliss.


I know what your saying Henry, but the view from all my bins and my Pentax scope are excellent. I don't see how they can be improved, not by much anyway. Maybe the scope, but that's mainly because of the loss of light at high mags. Ignorance is bliss, but all you need to is compare your optics against others and see were your at.
 
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