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Star Testing (1 Viewer)

RandyBo

Well-known member
United States
In another thread @jcwu88 posted two links by Roland Christen (Astro Physics):

Notes on Star Testing Refractors
Star Testing Complex Optical Systems

I just purchased my first spotting scope and have never attempted a star test so, this is all new to me.
Therefore I would really appreciate everyone’s comments on the following (copied from the second link above):

“In my experience, when an optic measures 1/10 wave AT THE DESIGN
WAVELENGTH, then the performance will be nothing short of stunning.
Peter's criterion was a bit less at 1/8 wave P-V. Either way, the
customer will have an excellent optic.

By the way, Ceravolo is my mentor on testing of optics. Even though I
have been making optics for a long time, without his guidance in the
testing lab, I never would accomplished what I did. And yes, I used
to use the star test in the distant past when I made longer
refractors. Some of my optics came out good and have surprised me
when retested on the interferometer. But for the fast F6 and F7
triplets I make now, the star test falls short.

The dilemma for manufacturers then is, should we do our best to
produce smooth high contrast optics, or should we please the star
test crowd and do some hand aspherizing to get a more pleasing
out-of-focus star image? I can tell you that it is easy to do some
rough compensation with quick local polishing at several zones to get
more equal inside and outside star patterns, but the result will
almost certainly be a loss of contrast. Add to that a nice big
central obstruction to get rid of the offending inner zones, and
presto! you have a nice "fast food" Mak-Cass that doesn't work any
better than a typical SCT.

In our case, we will do our utmost to produce the closest faximile to
the star patterns in Suiter's book, but they will never be exactly
equal. The overriding concern will be that the optic has a very
smooth and accurate wavefront to produce the highest contrast
possible in the final image, which I assume will be in focus.


Roland Christen ASTRO-PHYSICS”



Thanks,
Randy
 
This is high end stuff by two world class opticians.

For a beginner to star testing with the average spotting scope the simple instructions should be valid.

Personally the test of high end optics comes with planetary observations.
But it takes years of experience.
The planets are in focus.

Also I didn't use fast ED scopes but slow refractors and Dall Kirkhams and Maksutov Cassegrain.

(I had a computer malfunction with a longer answer. I'll try just this).

Regards,
B.
 
This sentence from the above article is what I am having the most trouble with: “But for the fast F6 and F7 triplets I make now, the star test falls short.”
I was under the impression that most spotting scopes are faster than F6 or F7.
 
True.
Spotting scopes seem to be f5.5 or f/6.
It also depends on the aperture.

It seems that the Kowa 99 and Swarovski 115 may have very fast front elements with extra magnification from the elements behind to make them very compact.

Optically speaking I would think it difficult to make them work well at high magnifications, say 100x plus.
But at lower magnifications they may work O.K. and with better examples, well.

It may be that in these circumstances star tests don't work well, except to show astigmatism, lack of centrality and maybe coma.

There was a test that mentions a very well performing preproduction Kowa 99, with the expectation that later production versions might be even better.

This may be unrealistic as the first ones may be hand built with great care, whereas production ones have less care.

Personally, I would star test them for the basics then put 100x, 150x and 200x eyepieces and look at Jupiter.

But as bird watching scopes at 50x or 70x even average or poor examples might work well.

The makers probably don't do star tests and use other tests.

The fine ED scopes mentioned by Christen would be tested at 300x, maybe more.
I would expect near perfection at 75x per inch of aperture.

Regards,
B.
 
True.
Spotting scopes seem to be f5.5 or f/6.
It also depends on the aperture.

It seems that the Kowa 99 and Swarovski 115 may have very fast front elements with extra magnification from the elements behind to make them very compact.

Optically speaking I would think it difficult to make them work well at high magnifications, say 100x plus.
But at lower magnifications they may work O.K. and with better examples, well.

It may be that in these circumstances star tests don't work well, except to show astigmatism, lack of centrality and maybe coma.

There was a test that mentions a very well performing preproduction Kowa 99, with the expectation that later production versions might be even better.

This may be unrealistic as the first ones may be hand built with great care, whereas production ones have less care.

Personally, I would star test them for the basics then put 100x, 150x and 200x eyepieces and look at Jupiter.

But as bird watching scopes at 50x or 70x even average or poor examples might work well.

The makers probably don't do star tests and use other tests.

The fine ED scopes mentioned by Christen would be tested at 300x, maybe more.
I would expect near perfection at 75x per inch of aperture.

Regards,
B.
Thank You.
From your reply can I assume that at normal spotting scope magnifications a poor star test may not effect your (in focus) view at all?
 
No, I am not saying that.

A poor star test will affect the quality of the in focus view.
But at the lower magnifications used by bird watchers, the effect may not significantly affect the ability to identify birds.

Also people with very fine eyesight will be more affected by a poor scope than people with less fine eyesight.

In addition, to complicate matters, spotting scopes typically have compressed optics.
The length of the scope is considerably shorter than the fine ED triplet astro scopes made by Roland Christen.

This is somewhat similar to camera lenses where telephoto lenses are shortened by the use of a negative group at the back.
Telephoto lenses in general perform less well than Symmetrical lenses or long focus lenses without compression.

I don't know how well star tests reflect the actual quality of the image in short compressed optics in spotting scopes.
Others here have much more experience testing spotting scopes than I do.

I have experience with long focus telescopes and numerous camera lenses, which are not the same as typical spotting scopes.

A star test will also show if spotting scopes optics are pinched, usually because components are too tightly held.

Spotting scopes, because they are so short, particularly those of greater than say 80mm aperture, are I think difficult to make.

For me, a long focus 80mm, 100mm, 120mm or 150mm doublet or triplet refractor would be the choice for planetary viewing.
But deep sky objects would be well catered for by a spotting scope, although the apertures are generally small by astro standards.

Regards,
B.
 
Hi,

it should be noted, that even for fast apochromatic ED triplets (and only those are affected by the effect Roland points out in the article quoted above), the star test is a useful tool to determine the optical quality. The only thing one needs to keep in mind that some mild undercorrection (a kind of spherical aberration) might be seen even in a perfect example of those - for the reasons pointed out by Roland.

See the link below for an example of this effect in a very good scope - the star test image in post 1 is taken in white light and shows mild undercorrection while the one in post 20 is take with a green filter and is close to perfect.


A spotting scope with a bad star test might be ok at 30x but soft at 60 or higher.

Joachim
 
Thank You,
@Binastro and @jring

I think I have somewhat better understanding now but no where close to where I would like it to be.

When I read the above article the first impression I got was that Mr. Christen does not put much faith in the results obtained when star testing fast triplets (and I assumed spotting scopes also) but, obviously that is not the case.

I really like the idea of being able to conduct a fairly simple test to assess one’s own optics so, this is something I will certainly pursue in the future but, only after I gain a better understanding of the theory, procedures and interpretation of the results.

Again, I would like to thank you guys for your help.

Randy
 
These are good questions, at least to me. So, thanks for posting them.

I am currently evaluating a new scope. I purchased one, returned it, and now am trying to make sure I do not jump the gun on THIS one. I MIGHT have jumped it on the first one, but I am pretty sure I didn't.

Anyway, my star test photos suck, so I cannot post any at the moment. I have some experience with astro scopes. Much more so than with spotters, although I have had a handful of them. Two or three alpha level scopes, and several classic era/Japanese made Bushnell, Swift, etc. The point is, not completely a rookie, but when it comes to star testing spotters, I would say I am.

If I can add my question to this thread, my current scope initially worried me. It seemed to show almost NO rings inside or outside of focus. This was on an artificial star. I have concluded that it might have been due to lack of thermal equilibrium, but I am open to correction on that. The images are sharp from the lowest magnification and all the way into the 100x range, during field use, cloudy day, and at a range of roughly 30 yards. But, the star test was concerning.

I tested it on an actual star last night. I wish I could have taken a photo, but could not. During this episode, I could definitely see the rings on both sides of focus. Inside and outside seemed very similar. The weird thing, to me anyway, is that the rings were not very distinct. Again, on both sides of focus they appeared this way. If I move my eye slightly off axis, the image is less discernible, which is not a shock, but seems very sensitive. Also, I think I detect roughness, due to what appear to be broken or interrupted rings, but more so spots on the image, almost like floaters but not.

Sorry, not a good description, but best I can do at the moment. Ring any bells?

Again, not to take over the thread, but it seemed appropriate to discuss here since I would rather not call out the scope company directly quite yet.
In use, I have optimism for this example. But, I do not want that optimism to influence my star test, or vice versa.
 
Hi Joker,

It might be the Seeing and temperature variations.

Was the temperature day and night similar?

My friends use the jet stream to decide whether to observe or not.
I cannot remember, but anything over 50mph is poor and also the direction.

Maybe try the scope on Jupiter at the highest magnification you have.
Try both eyes.

Regards,
B.
 
The best Seeing conditions here are at 3 a.m.
Almost as good sometimes at 2 a.m.

A star for testing should be high and say magnitude 2 to Mag 0.

Planets should be high. Unfortunately not so in northern hemisphere currently.

A spotting scope should be out for 20 to 30 minutes but not so long as to mist up.

Telescopes maybe an hour.
Big Maksutovs 2 hours.

The observer should be dressed well with anorak and scarf to prevent body heat changing results.

Autumn and spring best.
16C, 61F day and night ideal.

Regards,
B.
 
Thanks, Binastro! I have looked at defocused stars dozens of times, but never quite trust my judgement. I hope my adapter arrives in the next day or two, so I can post a photo.

I am trying to use the scope as much as possible to get a feel for it. I hope to compare my astro refractor and the spotter. My astro scope is a 4 inch ED Orion. Not perfect, but very nice quality glass. Just have not had a chance to do that. Side by side will help me.

I used it tonight. I added the extended/doubler to the mix. It was a bit more difficult to focus tonight, since the seeing was worse. Plus, I think the extender need a bit more cooling time. But, Jupiter was quite sharp, but not as sharp as last night. The swimming image made it difficult to focus, so I blame the seeing.

I did a quick star test on I star high overhead (sorry, I did not take the time to figure out which one by name). The seeing, obviously, affected this.

I defocused to 3 rings either direction. Inside focus showed distinct rings, but not overly bright. They were concentric. The outer ring had fuzzy spikes radiating out. Slight lavender color if my eye was off axis. Outside focus showed very similar rings. They were concentric. Not brighter or dimmer, but seemingly less distinct. The same fuzzy spikes radiated from the outer ring, but the inner rings seemed to swim more than inside focus. Also, off axis eye placement had slightly more color. On axis, it seemed to disappear for both.

I have new batteries, so as soon as my adapter arrives, provided it works well, I will provide some photos. Thanks, again.
 
The star test seems O.K., but affected by poor Seeing and temperature effects.

One cannot reject a scope because of imperfect weather conditions.

The proof of the pudding is in the eating.

If a scope performs well in actual use at all available magnifications, then it is good enough and fit for purpose.

Regards,
B.
 
The star test seems O.K., but affected by poor Seeing and temperature effects.































One cannot reject a scope because of imperfect weather conditions.































The proof of the pudding is in the eating.































If a scope performs well in actual use at all available magnifications, then it is good enough and fit for purpose.































Regards,















B.











































The star test seems O.K., but affected by poor Seeing and temperature effects.







One cannot reject a scope because of imperfect weather conditions.







The proof of the pudding is in the eating.







If a scope performs well in actual use at all available magnifications, then it is good enough and fit for purpose.







Regards,



B.

The star test seems O.K., but affected by poor Seeing and temperature effects.

One cannot reject a scope because of imperfect weather conditions.

The proof of the pudding is in the eating.

If a scope performs well in actual use at all available magnifications, then it is good enough and fit for purpose.

Regards,
B.
I agree, regarding the in-field performance being the deciding factor. However, since there is only a small window of time to use it before it is permanently mine, and I have limited opportunity to get use it in real circumstances, the star test is really helpful for me.

But, you are right. Too much emphasis on it alone might be a mistake.

If it isn't the weather, its is work.

My job(s) REALLY conflict with my hobbies. LOL
 
Hi,

from what you described about what you saw (concentric circles on both sides), I would guess that your Kowa is quite good... and whatever spikes you see are either a little rough roof edge (Kowa has been known for those lately - are the spikes always at top and bottom?) or just bad seeing.

Joachim
 
How bad is this one? I couldn't avoid the thermal issues. Images at the ep are round, but they're swimming. However, the outside image is clearer than the inside, no matter how I cut it. Coma seems slightly evident inside focus. Keep or send back?

Also, this was with the doubler attached, at max power, and with 2x optical zoom on my phone. I cropped these photos to make them large enough to see.
 

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If the astigmatic shape is just an artifact of unsteady air in the photos and you see circular rings at the eyepiece then you have a winner. Everything else including coma and spherical aberration looks plenty good enough. What scope is this?

Henry
 
If the astigmatic shape is just an artifact of unsteady air in the photos and you see circular rings at the eyepiece then you have a winner. Everything else including coma and spherical aberration looks plenty good enough. What scope is this?

Henry
Thank you.

It is possible that there is still a slight bit of astigmatism, but definitely nothing as obvious as the photos. The scope is sitting on the enclosed/unheated porch until morning. I get up early and it is finally supposed to be clear, but frigid. Hopefully I can make sure a true star test concurs with the artificial star in the photos.

It is somewhat clear right now, but so windy that it is nearly impossible to hold the scope still. I did take a quick peek at Jupiter at 120x. Not as impressive as my 6" Maksutov-Newt was, but still a sharp disc with 5 bands. Obviously, it is difficult to keep up with the earth's rotation at that magnification.

In real world use, I believe the image holds up well at 120X. I would say that in good environmental conditions, it performs excellently.

The star test, with the fuzzier image inside of focus vs. the clearer image outside of focus confused me a bit. However, if it looks ok to you guys, then the real world and star test results compliment each other.

It is a new Swarovski STX95.
 
The star test, with the fuzzier image inside of focus vs. the clearer image outside of focus confused me a bit.

Hi,

that is called overcorrection... but not a lot... this is not due to being a fast apo though...
If you can see only a bit of stig too if the seeing evens out, that is a better than average sample (with the average being fairly low in spotters, unfortuately).

Can you compare to the Kowa side by side?

Joachim
 
Hi,

that is called overcorrection... but not a lot... this is not due to being a fast apo though...
If you can see only a bit of stig too if the seeing evens out, that is a better than average sample (with the average being fairly low in spotters, unfortuately).

Can you compare to the Kowa side by side?

Joachim
Thank you. I appreciate it.

So, I was able to have a few minutes outside before work this morning. Under a real star, and at full power (120x with the extender) the image was far less astigmatic, but still slightly oblong in each direction. But, other than the difference in the fuzziness and the orientation of the slight ovalness (truly slight, and I almost don't trust my eye regarding this judgement, due to 15*F and lack of a tracking mount), I would say the inside and outside focus images are very similar in brightness etc.

Could the astig be due to pinched optics? Is that even something that happens with spotters?

Unfortunately, I could not justify to my wife (or, myself really) having multiple spotters (at the prices for these anyway) at the moment. So, my Kowa was sold. I could, if I ever have the time to do it that coincides with decent weather, compare to my 4" ED refractor. I just have not had a chance.

I do more digiscoping than just birding with my scopes. So, under normal use, I would say it is pretty phenomenal. But, without a direct comparison to a known quality, I am a little reluctant to fully commit quite yet. Thus, I am relying on this test.

Yes, I might seem a little anal, but buying new, which I normally have not done, adds, at least for me, a higher level of expectation. I THINK that this scope is better than average, and so far, it is better than I remember my Kowa being, and probably better than a Zeiss I had previously, but it is also a larger aperture scope than either of those.

Lastly, something that I find weird, when I tested the scope outside this morning, the "fuzzier" image was on the oustide of focus, rather than inside, as it was while using the artificial star. Outside, my scope sat for at least 45 minutes in the 15 degree temps. So, I am wondering if pinched optics ARE an issue. Could the expansion/contraction of the glass be affecting the test?

Thanks!

EDITED to add: I did try to take a photo of the real star test, but it was impossible for me. It would have been a much more distinct image than the artificial star was. I think my artificial (Hubble Optics) is far too dim to be sufficient (?) But, outside, the moving target made it too difficult.
 
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