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ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

60x resolution problem (1 Viewer)

Po'ouli

Well-known member
I just received a Diascope 85 with the 20-60x zoom from Eagle Optics. The resolution in the 20-30x range is excellent, but it starts losing it at 40x and by 60x it is pretty awful in normal use. I'll do a star test tonight, but I'm wondering if there is a way to tell if the problem is with the eyepiece or the scope. Any suggestions?

Thanks,

Bruce
 
Bruce,

In my experience, problems with eyepieces are very rare. In fact, I'll go as far as saying that among the top brands and models, I have not yet encountered an eyepiece that I would call a lemon, i.e. one that would severely compromise the image quality of a scope if compared with another specimen like it.

Now, I don't remember if the Zeiss eyepiece can be mounted in any other rotational orientation except for the one indicated by the dot, but in general a good way to determine whether aberrations are introduced by the eyepiece or the scope is to rotate the eyepiece relative to the scope body and to investigate whether aberrant patterns rotate with the eyepiece. This works for astigmatism and eccentric ring patterns indicative of miscollimation.

It'll be interesting to hear how your star-test goes. People use language differently, but in my vocabulary, "awful" is not a word suitable for describing the image of a good sample of the Zeiss at 60x. A lemon, on the other hand, will look awfull at high magnifications.

Kimmo
 
Po'ouli said:
I just received a Diascope 85 with the 20-60x zoom from Eagle Optics. The resolution in the 20-30x range is excellent, but it starts losing it at 40x and by 60x it is pretty awful in normal use. I'll do a star test tonight, but I'm wondering if there is a way to tell if the problem is with the eyepiece or the scope. Any suggestions?

Thanks,

Bruce
Bruce,
Are you experiencing image degradation due to heat shimmer?
John
 
John Traynor said:
Bruce,
Are you experiencing image degradation due to heat shimmer?
John

John: No, it isn't just atmospheric effect, the resolution is really bad with up close viewing also. Things look hazy and a bit like there's a double image. It makes you want to keep fiddling with the focus knob. Really disappointing...

Bruce
 
kabsetz said:
Bruce,

In my experience, problems with eyepieces are very rare. In fact, I'll go as far as saying that among the top brands and models, I have not yet encountered an eyepiece that I would call a lemon, i.e. one that would severely compromise the image quality of a scope if compared with another specimen like it.

Now, I don't remember if the Zeiss eyepiece can be mounted in any other rotational orientation except for the one indicated by the dot, but in general a good way to determine whether aberrations are introduced by the eyepiece or the scope is to rotate the eyepiece relative to the scope body and to investigate whether aberrant patterns rotate with the eyepiece. This works for astigmatism and eccentric ring patterns indicative of miscollimation.

It'll be interesting to hear how your star-test goes. People use language differently, but in my vocabulary, "awful" is not a word suitable for describing the image of a good sample of the Zeiss at 60x. A lemon, on the other hand, will look awfull at high magnifications.

Kimmo


Kimmo: Thanks for the response. I just did a star test. I'm not very experienced at this, but the scope actually seemed to perform well. At 60x one side of focus had round concentric rings, and on the other it was more of a round white ball (no discernable rings). Am I missing something?

When I say resolution was awful, I mean it was bad enough that I had a hard time identifying the fine detailed features of birds that were fairly close. When I tried to focus on the bark of a nearby stump, the bark pattern had hazy to double edges. It just wouldn't come into sharp focus. I don't consider myself to be real picky. I just want a clear view. I guess I'll try to do a resolution test with a dollar bill tomorrow.

More to come...

Bruce
 
The 60x end usually dim in any scope, so there is a clear difference to what you see at 45x and at 60x. But your up close examples seem to suggest something else.

My scope is almost useless at 60x, I just go a little over 45x. I have looked through Swarowskis and others and they were only a little better. I have never seen a really good image at 60x. With some scopes we got some useful views on sunny days at 60x.
 
Bruce,

The inside/outside focus pattern you describe is pretty typical for the Zeiss scopes, and as such not a reason for concern. I think you should concentrate more on what the star looks like in best focus, as well as defocused just a couple of diffraction rings either side. If the scope is faulty to the point of showing a soft image at 40-60x, it might not show in ring patterns with 8+ rings, but will always show as an absence of a clean point in best focus. A spiky, blobby, crosslike or comet-like "star" is typical of a poor scope in best focus. "Pinched optics" also show best as a usually triangular ring pattern just inside or outside focus, when you have defocused some three or so rings to be visible. A faulty prism in the Zeiss can show as a vertical fuzzy dividing line in the defocused ring pattern, being more easily seen in the "fuzzy ball" side of focus than in the "clear rings" side.

John's point is also relevant here. Do check the scope indoors where you can be sure there is no heat haze in the air.

Kimmo
 
Po'ouli said:
I just received a Diascope 85 with the 20-60x zoom from Eagle Optics. The resolution in the 20-30x range is excellent, but it starts losing it at 40x and by 60x it is pretty awful in normal use. I'll do a star test tonight, but I'm wondering if there is a way to tell if the problem is with the eyepiece or the scope. Any suggestions?

Thanks,

Bruce
I have this scope and yesterday was looking a a distant peregrine falcon at 60x. It was easily possible to enjoy the view of a view of the bird's sharp bill even at such a distance. The Zeiss zoom at 60x is usually regarded as being one of the sharpest eyepieces around, especially in the central viewing area. It is at 20x that the edges of the view should appear somewhat kess sharp - this is because Zeiss have produced a uniquely wide field of view and there is some trade off in sharpness in the outer edges of the view. Also, unless viewing conditions were particularly poor, then you shouldn't have noticed much fall off in brightness, eitheras the large 85mm objective is exceptionally good at gathering light.

I would, if possible, compare your scope with someone else's but I suspect that you have a faulty product that needs changing. The view through such an excellent optic at all magnifications should not dissapoint you in any way - quite the reverse, it should delight you. Bad luck - but I'm sure Eagle Optics will quickly exchange it for you.
 
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I have never seen a really good image at 60x. With some scopes we got some useful views on sunny days at 60x.

Tero

May I suggest you look through a new Kowa scope like the 883 when you get the chance. you might be surprised.
 
Bruce,

Our 85 Zeiss with the zoom eyepiece is useful at 60x to the far side of a Kincardine sewage lagoon when there's no heat haze (which gives a shimmering appearance to the image).

Sample variation is everything.

You probably don't want to let Eagle Optic's exchange/return period lapse, as Steve's comment to exchange the scope PDQ is excellent. On another thread this year, Mike McDowell at Eagle Optics has offered to check optics before they're shipped.

Mike
 
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Paul Jarvis said:
I have never seen a really good image at 60x. With some scopes we got some useful views on sunny days at 60x.

Tero

May I suggest you look through a new Kowa scope like the 883 when you get the chance. you might be surprised.
It is an odd comment - a Kowa, Leica, Nikon... all superb at maximium resolution.
 
Po'ouli said:
I just received a Diascope 85 with the 20-60x zoom from Eagle Optics. The resolution in the 20-30x range is excellent, but it starts losing it at 40x and by 60x it is pretty awful in normal use. I'll do a star test tonight, but I'm wondering if there is a way to tell if the problem is with the eyepiece or the scope. Any suggestions?

Thanks,

Bruce

Bruce,Here is an interesting review about some spotters with yours included.Just google up- 6mmbr spotter review
Steve
 
kabsetz said:
Bruce,

The inside/outside focus pattern you describe is pretty typical for the Zeiss scopes, and as such not a reason for concern. I think you should concentrate more on what the star looks like in best focus, as well as defocused just a couple of diffraction rings either side. If the scope is faulty to the point of showing a soft image at 40-60x, it might not show in ring patterns with 8+ rings, but will always show as an absence of a clean point in best focus. A spiky, blobby, crosslike or comet-like "star" is typical of a poor scope in best focus. "Pinched optics" also show best as a usually triangular ring pattern just inside or outside focus, when you have defocused some three or so rings to be visible. A faulty prism in the Zeiss can show as a vertical fuzzy dividing line in the defocused ring pattern, being more easily seen in the "fuzzy ball" side of focus than in the "clear rings" side.

John's point is also relevant here. Do check the scope indoors where you can be sure there is no heat haze in the air.

Kimmo


Kimmo: I didn't get a chance to do a resolution test today, but I re-did the star test. At sharpest focus, the star has spikes all around it and doesn't snap into sharp focus. The soft circle outside of focus has two prominent vertical lines near the center. What do you make of that?

Thanks for your help,

Bruce
 
scampo said:
I have this scope and yesterday was looking a a distant peregrine falcon at 60x. It was easily possible to enjoy the view of a view of the bird's sharp bill even at such a distance. The Zeiss zoom at 60x is usually regarded as being one of the sharpest eyepieces around, especially in the central viewing area. It is at 20x that the edges of the view should appear somewhat kess sharp - this is because Zeiss have produced a uniquely wide field of view and there is some trade off in sharpness in the outer edges of the view. Also, unless viewing conditions were particularly poor, then you shouldn't have noticed much fall off in brightness, eitheras the large 85mm objective is exceptionally good at gathering light.

I would, if possible, compare your scope with someone else's but I suspect that you have a faulty product that needs changing. The view through such an excellent optic at all magnifications should not dissapoint you in any way - quite the reverse, it should delight you. Bad luck - but I'm sure Eagle Optics will quickly exchange it for you.


Steve: Very interesting. It just so happens that I saw a peregrine perched in a distant tree yesterday when I first took the scope out, and my experience was quite the opposite of yours. I could make out enough of the head to be certain it was a peregrine, but that was it. There was nothing sharp about it, and no detail of the beak could be discerned.

Thanks for the feedback. This isn't sounding good for this scope...

Bruce
 
Po'ouli said:
Kimmo: I didn't get a chance to do a resolution test today, but I re-did the star test. At sharpest focus, the star has spikes all around it and doesn't snap into sharp focus. The soft circle outside of focus has two prominent vertical lines near the center. What do you make of that?

Thanks for your help,

Bruce


This sounds to me like a very good description of a bad sample of the Diascope. The vertical lines indicate a poorly ground roof edge in the prism. The spiking could be caused by this, but it can also be astigmatism, pinching, or a combination of both. Prominent spiking at sharpest focus and not snapping into a distinct best focus always mean that the scope will not work well at high magnifications and that contrast is severely compromised even at low magnifications. Interestingly, a scope like this also performs much worse than a good one in heat haze, although one would (at least I used to) presume that heat haze would equally spoil the image of good and bad scopes alike.

Kimmo
 
mooreorless said:
Bruce,Here is an interesting review about some spotters with yours included.Just google up- 6mmbr spotter review
Steve
What an interesting review - and from a non-birder for a change, with a different mind-set and one not seemingly swayed by branding. I'm pleased to see the Zeiss coming up trumps but that, overall, each of the top scopes has much to offer the serious birder.
 
Po'ouli said:
Steve: Very interesting. It just so happens that I saw a peregrine perched in a distant tree yesterday when I first took the scope out, and my experience was quite the opposite of yours. I could make out enough of the head to be certain it was a peregrine, but that was it. There was nothing sharp about it, and no detail of the beak could be discerned.

Thanks for the feedback. This isn't sounding good for this scope...

Bruce
Well, the light here was clear and of course, the weather is very cool, so you might have had heat haze but it's seeming more and more as if it's just gotta go back, Bruce. Bad luck on that - but it looks as if you've bought one of the best brands of scope out there. Sufficient reviews stack up to suggest that, anyhow.
 
kabsetz said:
..... Interestingly, a scope like this also performs much worse than a good one in heat haze, although one would (at least I used to) presume that heat haze would equally spoil the image of good and bad scopes alike.

Kimmo

Kimmo,

I was reminded of this observation of yours recently while reading Suiter's "Star Testing Astronomical Telescopes". In the chapter titled "Accumulated Optical Problems" there is a section called "Breaking the Camel's Back". In it there is an MTF diagram in which the MTF curve is progressively degraded by a number of small problems, no one of which would matter much by itself. There is 1/5 wave undercorrection, moderate pinching, slight misalignment and 0.07 waves RMS roughening from atmosheric turbulence. The air turbulence has a very similar effect on the curve that the other problems have. Put them all together and the curve is poor, but if a couple of the optical defects are corrected the curve is substantially improved even with the presence of the turbulence. Just like what you see.

It does seem that using a roof prism in a scope is asking for trouble. A sloppy roof edge that wouldn't be much of a problem in a 10x binocular could become intolerable at 60x. I'm surprised they work as well as they do, but I wonder if this is one more reason the angled Nikon Fieldscopes can star test so well, no roof edge in the light cone.

Henry
 
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Henry,

I have read the Suiter book and remember the section you mention (as well as the diagram he called "the wobbly stack"), although not in such detail. However, I remember thinking when I read it that the amounts of atmospheric turbulence birders encounter on sunny days are so much greater than those typical in the night sky that they would effectively render all other typical aberrations meaningless. Since then I have tested a number of scopes against an unusually aberration-free specimen, whereby I could very clearly see that when atmospheric conditions were hardest, the differences between lower aberration and higher aberration specimen stood out the clearest. This was one of my many "eye-opener" experiences.

On the Nikon prisms, yes, I believe that it is an exceptionally successful prism design. Unfortunately, though, there are many recepies for making a lemon scope that don't require a sour prism as an ingredient.

I'll reiterate my earlier suggestion that you get your hands on a Fieldscope III A and reap the prism out of it to use on your A-P and Takahashi scopes. Then we'd really know what the prism is made of.

Kimmo
 
Bad Diascope!

Po'ouli said:
I just received a Diascope 85 with the 20-60x zoom from Eagle Optics. The resolution in the 20-30x range is excellent, but it starts losing it at 40x and by 60x it is pretty awful in normal use. I'll do a star test tonight, but I'm wondering if there is a way to tell if the problem is with the eyepiece or the scope. Any suggestions?

Thanks,

Bruce

I have a Diascope 85 and it has outstanding resolution at 60 and even at 110 power (tested on stars and a resolution chart). You need to be able to see the Airy disc to do a legitimate star test so you need very steady skys and more than 60 power, even more than the 110x I used... something on the order of 200x.

Certainly the views at high power are dimmer, and over any distance during the day seeing conditions will also limit resolution, but if you are not get outstanding views at nearby objects (20-30') then send the scope back for another.

Lou H
 
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