Star Test Result Question (1 Viewer)

[CMB]

I am currently star testing two spotting scopes (Kowa TSN-883 and Swarovski ATX85mm) and I have a question about the results. Photos in a post below.

The "star" is an LED flashlight covered with tin foil with a pin hole poked in it and placed 30m away (I measured).

On the Kowa:

• the inside focus is beautiful the rings are centered, even, the circle is a nice round airy disc with no distortion indicating no astigmatism.
• the outside focus is a different story. I cannot resolve rings except for the very outside ring. The star is essentially a nice round blob, which I understand to be over correction for spherical aberration. Again no distortion of the blob which I take to indicate no level of astigmatism that I can discern.
• at focus the star is a sharp point of light. No comma shape. No spikes shooting off the star.
• The yellow prism line is visible as a sharp vertical line straight up and down on both inside and outside focus.

On the Swarovski:

• the inside focus shows a nice circle with rings resolved, but it is not as sharp or crisp as on the Kowa. Nice round circle with nothing showing discernible astigmatism
• the outside focus is also a blob with the outside ring slightly more defined than the Kowa and the ability to see one inside ring. So also over correction for spherical aberration but a hair less. Again the circle is nice and round with nothing showing discernible astigmatism
• focus shows a sharp point of light with no comma shapes or spikes.
• no noticable yellow prism line

Other scopes we own and have tested also tend to show the outside focus as a blob with concentric rings not really resolvable.

So my question...

Outside focus that shows over correction for spherical aberration. On spotting scopes is it fairly normal not to be able to resolve the concentric rings on outside focus? Are these bad samples or would you consider them "average" samples?

The fact that both scopes don't show any noticeable astigmatism in the optics (or so minimal that I can't see it) I see as more desirable optics than having scopes with some astigmatism but less correction for spherical aberration. Am I right about that, or missing something?


When testing the 883 and ATX85 visually with terrestrial viewing on ducks, distant mountains, rock formations, etc., both are very clear at all magnification levels. Both have good edge to edge clarity. Both focus sharp. I am extremely sensitive to CA and I can see it in essentially all binoculars I have used. As expected the ATX does show CA around the edges but it is gone in the center. As expected the Kowa controls CA much better. Comparing each scope indoors using a dollar bill taped to a wall at a distance, both scopes resolve detail just as well. Difference in brightness is so minimal it's essentially a tie. As expected each scope shows colors a little differently.

With terrestrial viewing I don't see anything objectionable with either scope.

Thank you all for your thoughts and comments.

Chris

 

[henry link]

How many rings are you using? Between 2 and 5 is the most useful.

Very sharply defined rings on only one side of focus is not a good thing. As spherical aberration increases the rings gradually disappear on one side while they grow more sharply defined on the other side. What is wanted is identical or at least a pretty similar ring appearance on both sides, not the most sharply defined rings.

Both of these sound like they have "more than a little" spherical aberration, provided you are using no more than 2 to 5 rings of defocus. Using too many rings will make the side with weak rings look worse than it is. It's good that there's no astigmatism, but SA is not desirable either. Both degrade the image compared to the same scope with neither.

The next step is to measure the resolution to see how much it's damaged by the SA or compare the scope to one that star tests really well.

Henry

 

[CMB]

How many rings are you using? Between 2 and 5 is the most useful.

Very sharply defined rings on only one side of focus is not a good thing. As spherical aberration increases the rings gradually disappear on one side while they grow more sharply defined on the other side. What is wanted is identical or at least a pretty similar ring appearance on both sides, not the most sharply defined rings.

Both of these sound like they have "more than a little" spherical aberration, provided you are using no more than 2 to 5 rings of defocus. Using too many rings will make the side with weak rings look worse than it is. It's good that there's no astigmatism, but SA is not desirable either. Both degrade the image compared to the same scope with neither.

The next step is to measure the resolution to see how much it's damaged by the SA or compare the scope to one that star tests really well.

Henry

Henry,

Thanks for your comment.

On inside focus it was between 2-5 rings of defocus with both scopes. I could see them well enough to adjust between 2-5. On outside focus I defocused until the circle was the about the same diameter as on inside focus.

Attached are two photos hand-held digiscope with an iPhone using ProCamera to be able to adjust exposure. The first photo is Inside focus. the second photo is Outside focus.

 

[henry link]

Chris,
Those are much too far out of focus, maybe 15-20 rings or more. You really can't even see most of them because they're all blended together.

Here's an image of 3 rings inside focus that just happened to be on my desktop. It's from a binocular, stopped down from 42mm to 21mm. The spikiness and distortion of the ring shapes come from air turbulence.

Henry

 

[CMB]

Henry,

Yes, I’ll try again tomorrow. I had three rings like that earlier today. I messed up the test tonight.

Chris

 

[kabsetz]

For how far defocused those two photos are, to me it actually looks quite good. But as Henry says, you can tell much better if you go 3-5 rings

 

[CMB]

I tried to get more star test images outside today. This is being done hand holding an iPhone to the ocular. I cannot get a good full image of the Outside focus. So here is one Inside focus, and three additional attempts at Outside focus. For the artifical star I used both an LED flashlight with tin foil over it and a pin hole, and also a silver metal globe about 1" diameter. I could not get a good photo of Outside focus with either one.

Chris

 

[henry link]

Too much turbulence is the problem this time. Sometimes the air is just not steady enough for photographs. Were you able to see clearer outside focus rings visually?

 

[CMB]

I could see two, but the second was a challenge to see. I tried again this morning before work. Same results.

I have been able to compare four spotting scopes purchased at different times. Our two Pentax (65 & 80) with TeleVue fixed power eyepieces, Swarovski ATX 85mm, and Kowa 883. On all four of the scopes, to me, the outside focus looks the same. It's a challenge for me to see rings on outside focus. That puzzles me given they are different brands, made at different times, in different factories, using different lens materials.

Maybe it's my eyes?

For me star testing hasn't been as helpful as I had hoped it would be in showing me if one of the scopes we received (Swaro ATX and Kowa 883) are optically better configured/made than the other. And in star testing all four scopes, it hasn't shown me that the Swaro ATX and Kowa 883 are better than our current Pentax scopes. The results from all the scopes seem to be much more similar than different. Maybe I am missing something.

Field observations with our eyes (and working with the ergonomics in how the scopes fit and function) has been more useful for us, and has resulted in a decision between the Swaro ATX and Kowa.

 

[henry link]

I should have mentioned that in order for all those spikes and flares to be attributed to air turbulence they should be in motion, something like looking at a stream bottom through flowing water.

The 30m distance would seem to be long enough, but maybe your pinhole is too large or maybe your target and the scope are too close to the ground or you are looking across a surface like asphalt or concrete that exacerbates turbulence. You might try an indoor test in steady air at the longest distance possible using the smallest pinhole you can make.

It's also not impossible that all four scopes have a lot of spherical aberration. It is a common defect in spotting scopes.

Henry

 

[CMB]

Thank you Henry. This has been a learning process.

The outside tests were done over all grass to try and eliminate as much convection as possible. I even did one of them early in the morning when it was 22° outside before the sun had risen above the peaks.

I also tried to do an inside test for more stability. The inside test was the first two images where the rings were far too defocused. The longest distance I can get inside is about 34-35 feet.

I am thinking that all the scopes have similar amounts of spherical aberration.

Both Kowa and Swarovski offer extenders. I've read lots of reports that the images through the Swaro ATX and Kowa 883 hold up well when using the extenders. Would large amounts of spherical aberration result in images starting to break down more easily at higher magnifications?

Thanks again.

 

[CMB]

Think I finally got some useable Outside focus images.

As you suggested I redid the foil and tried to put the smallest hole in it that I could. Then I did this test inside. The distance from scope to artifical star is about 36 ft.

Attached are an Inside and three Outside focus photos done today, indoors, with the smaller pin hole.

 

[jcwu88]

Think I finally got some useable Outside focus images.

As you suggested I redid the did foil and tried to put the smallest hole I could. Then I did this test inside. The distance from scope to artifical star is about 36 ft.

Attached are both an Inside and three Outside focus done today, indoors, with the smaller pin hole.

Hi CMB,

I did star test of five scopes (from 50mm- 99mm, Nikon ED and Kowa fluorite) out of curiosity. I got exactly the same results as you observed: the inner focus pattern is clear and out focus pattern is barely visible.
Did some search online and found many discussions of refactor star test on astro forum like cloudynights. There is not clear conclusions but some in common test
1. Current ED or Apo triplets
2. F/5~F/7 (relative fast)
Above are (similar as spotting scopes) also have similar results as clear inner focus pattern with smeared outside focus pattern.
One explanation is the chromatic aberration: different wavelengths light interference outside focus that makes the diffraction patterns more complicated. Some tests showed using filters can significantly improve outside focus patterns which may support the CA theory .

Jay

 

[henry link]

Hi Chris,
I think you may be back to using a few too many rings for a clear view. Just to remind myself that spotting scopes don't have to be this bad I star tested my wife's Nikon Monarch 82 ED again. What I saw was remarkably little difference between the appearance of the inside and outside rings and essentially no other defects. If Nikon can do this in a Chinese factory with a $1600 scope why can't Swaro and Kowa do the same?

Hi Jay,
I use a very narrow band green filter to evaluate monochromatic spherical aberration. If there is much less difference between the inside and outside rings with the filter in place than there is in a white light test that indicates the presence of spherochromatism, the form of SA that varies with wavelength. I've gotten pretty good results by just using a green Christmas tree ornament reflecting a glitter point of the sun.

Henry

 

[Binastro]

Is the difference because of prism type and the speed of the system?

Regards,
B.

 

[henry link]

Is the difference because of prism type and the speed of the system?

Regards,
B.

Well, the prisms are the same type (Schmidt for angled version, Porro for straight), but the roof in the Nikon is offset so it doesn't split the objective light cone. Because of that there's no need for phase correction and no problem if the roof edge isn't perfectly made, a defect that seems to afflict more Kowa's than it should.

The focal ratio of the Kowa 99 and the Monarch 82 are just about the same (a little over f/6), but the Kowa's 600mm focal length is somehow crammed into a physically short distance, so it might not perform up to the normal expectations for an f/6 triplet.

Henry

 

[jcwu88]

Hi Chris,
I think you may be back to using a few too many rings for a clear view. Just to remind myself that spotting scopes don't have to be this bad I star tested my wife's Nikon Monarch 82 ED again. What I saw was remarkably little difference between the appearance of the inside and outside rings and essentially no other defects. If Nikon can do this in a Chinese factory with a $1600 scope why can't Swaro and Kowa do the same?

Hi Jay,
I use a very narrow band green filter to evaluate monochromatic spherical aberration. If there is much less difference between the inside and outside rings with the filter in place than there is in a white light test that indicates the presence of spherochromatism, the form of SA that varies with wavelength. I've gotten pretty good results by just using a green Christmas tree ornament reflecting a glitter point of the sun.

Henry

Hi all,
I found one good article for star test for modern ED fast scope, the link as follows:

https://www.altairastro.help/star-testing-a-modern-ed-doublet-refractor-with-altair-camera/

Jay

 

[Binastro]

99mm aperture and more so 115mm aperture spotting scopes need longer focal ratios for broadly similar performance.
So perhaps f/7 to f/7.5 and f/8.5.

In addition if the Kowa 99mm is more compact it suggests a stronger telephoto component.

Also fluorite crystal is not easy to work with and to avoid too many rejects maybe the quality control has to slip.

Regards,
B.

 

[CMB]

Jay: Thank you very much for the link to that article. It is a very intereting read and helpful.

Continuing with the star test...

Here are two more photos taken today. They are indoors (outside conditions are not good) and the artifical star is 36 ft away down a dark hallway. They are very slightly defocused even though it doesn't look like it.

Two photography issues: One issue I am having is that the artificial star image in the camera is so small that I am having to do a "picture in picture" zoom (a feature of the ProCamera app) of the artificial star to even see it in the camera and get it in the frame as a round object. A second issue is that even with an ISO of 25 and EV - 6.5, the camera is trying to lock on the flashlight body and is focusing on that. The amount of defocus is so small that the camera auto focus is overriding the scope's defocus and making the artifical star a pinpoint again. This happens even if I shift the focus point away from the artificial star point.

The photos show that both images are equally circular and even. No astigmatism. Photos of in-focus show no coma.

If anyone has any suggestions on how to capture a better image handheld I would appreciate hearing it. The image of the star's light through the ocular is so small that I have reached my limit of being able to see the image in the camera to photograph it.

 

[jring]

If anyone has any suggestions on how to capture a better image handheld I would appreciate hearing it.

Hi,

quite frankly those star test images are good. Both from an imaging point of view and also the optics.

Joachim