60x resolution problem (1 Viewer)

Lou,

I have to disagree about how much magnification is needed for a "legitimate" star test. It depends on the aperture and quality of the scope and also what you're looking for. 100x is more than enough to see the Airy disc in virtually any birding scope, since most of them will have Airy disc diameters above 2 arc seconds. 60x is plenty to see the usual defects like astigmatism, pinching, misalignment and, judging from Bruce's results, a bad roof prism. 200x will just show the same defects in more detail.

Maybe I'm overstating it. I've read elsewhere that about 10x per cm of diameter is adequate, so 85x for the Diascope 85. Lessee....I calculate the Airy disk to be ~8.2ee-6 rad (or 1.8 arcseconds) for an 85mm aperture. The average person can just resolve 5ee-4 rad (~100 arcseconds). So to just resolve the disc you need 60x. Of course, with star testing you compare the diffraction patterns at equal defocus on each side of focus and those patterns are larger than the disk itself.

My vision is reasonably acute but I find that I like about 1.5 to 2 times the suggested 85x to make out the first 2~5 rings and to resolve detail in those rings that are diagnostic of optical problems.

Anyway, as far as I can make out star-testing my Diascope at 110x, it has a very small amount of spherical aberration, and I find the views at that power to be outstanding. This jibes with reviews by others who find the view comparable to the revered Questar. Now the question is, how typical is this? I've looked through an 80HD which had a nice lush image at 60x, but lacked resolution of fine detail that was clear in the Diascope. Was that a bad example?

Sample differences in performance through heat haze is not intuitive. On a hot sunny day in June looking across the marsh northeast of Shallow Lake, you can see the vigorous shimmering without optics. A couple of years ago a world-class birder (That would not be me folks.) wished for an overcast day, using a Kowa 824 while doing a Red-necked Grebe survey in the IBA north of Dyer's Bay.

Has anyone else had difficulty understanding H.R. Suiter's Star Testing Astronomical Telescopes? It would be helpful if someone wrote a collaborative piece with Suiter, explaining in operational detail, for those without an astronomy background, the star testing of variables in sports optics that make a significant difference in birding performance.

Mike

Mike,

I fully agree that sample differences being visible in performance through heat haze is not intuitive. My own intuition strongly suggested the opposite until my consistently contrary experience in situations where I could directly and repeatedly compare two otherwise identical scopes with different aberration levels forced me to alter my opinion. Of course, low aberrations are not a miracle cure, and improved atmospheric conditions improve the view much more dramatically. However, for me it was a paradigm shift of sorts to go from "a cherry sample makes a difference in those relatively few instances when the conditions are near-perfect or the target is not very far" to "a cherry sample makes a difference whenever I'm trying to get the most detail out of a given view, almost irrespective of viewing conditions."

Kimmo

Mike,

Yes, you need a keen interest in optics to wade through Suiter's book, although just looking at the pictures will tell you a lot. The kind of star testing I've advocated for birding scopes is not really so complex. Almost everything you need to see about assembly defects is visible just from the shape of the slightly out of focus diffraction pattern at 60x. If it's not perfectly round there is astigmatism or pinching and if the bright central spot is obviously off center the optics are misaligned. If you can recognize those and the spike of a bad roof prism you are pretty well equipped to avoid a real lemon.

Aberrations are more difficult to evaluate. The danger there is that too high a standard of performance will be expected from birding scopes. A nearly perfect star test from a birding scope is extremely rare (most likely a very happy accident) because the scopes are complex which means there are many opportunities for small errors to add together and focal ratios are low so good corrections are hard to achieve. Astronomical refractors with "perfect" star tests are kept to higher focal ratios and even then require expensive hand figuring or aspherizing. That kind of thing is just not done for birding scopes and really isn't needed for small aperture scopes that are never used at really high magnifications. I think birders would benefit from using a simple star test to avoid defective scopes. What we don't need is a star test cult. There is something like that among amateur astronomers who use high end refractors and endlessly argue about which perfect scope is the most perfect.

Henry

Lou H said:

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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Thanks to everyone for the very helpful responses. I knew I wasn't going to be home during daylight hours this week to do a resolution test, so I just sent the scope back. I'll post a follow-up when the replacement arrives.

Bruce

Po'ouli said:

Thanks to everyone for the very helpful responses. I knew I wasn't going to be home during daylight hours this week to do a resolution test, so I just sent the scope back. I'll post a follow-up when the replacement arrives.

Bruce

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You've done the right thing - forget about star tests etc. There is absolutely no way that a good Diascope should produce an image which could be remotely described as "awful", no matter how picky you may be.

Pete

Lou H said:

Anyway, as far as I can make out star-testing my Diascope at 110x, it has a very small amount of spherical aberration, and I find the views at that power to be outstanding. This jibes with reviews by others who find the view comparable to the revered Questar. Now the question is, how typical is this? I've looked through an 80HD which had a nice lush image at 60x, but lacked resolution of fine detail that was clear in the Diascope. Was that a bad example?

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Sorry this a bit off topic, but what type of lens do you need to get a good 110x with a diascope?

Pete

You would need an astronomical eyepiece of about 4.56mm focal length as the scope focal length is 502mm. (502/4.56=110x). You'd need the astro adapter for the diascope and to know it would focus to infinity. Perhaps you could p.m. Lou H to see what he uses.

dipped said:

You would need an astronomical eyepiece of about 4.56mm focal length as the scope focal length is 502mm. (502/4.56=110x). You'd need the astro adapter for the diascope and to know it would focus to infinity. Perhaps you could p.m. Lou H to see what he uses.

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Thanks for that - I see Lou has since answered it in a previous thread. I've looked up the recommended lens and it's a bit pricy for my current level of interest! (£180)

Pete

mrpjdavis said:

Thanks for that - I see Lou has since answered it in a previous thread. I've looked up the recommended lens and it's a bit pricy for my current level of interest! (£180)

Pete

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But then I thought "what the hell?" and bought one. As Lou says, it is superb. However there is a downside-it has very short eye relief, so I can only see about a quarter of the huge field of view if I keep my specs on, as I normally do.

I'm now just waiting fro the wind to stop howling and the lashing rain to go away, so I can use it for its intended purposes (perched Goshawks in the Forest of Dean and Peregrines/geese at Slimbridge).

Pete

At last the rain has stopped and we've had some clear days and nights. I've used the astro eyepiece (which gives 107x) - really useful for studying the geese and peregrines at Slimbridge, provided it is reasonably bright. I've just set up the scope outside to look at the moon (first quarter). It is truly breathtaking, almost filling the field of view. I also found Saturn (by chance!) and I can see the ring and even the gap between the planet and ring.

All in all I am delighted with the eyepiece, well worth the outlay.

Pete

Po'ouli said:

Thanks to everyone for the very helpful responses. I knew I wasn't going to be home during daylight hours this week to do a resolution test, so I just sent the scope back. I'll post a follow-up when the replacement arrives.

Bruce

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So here it is Jan 27, 2007....Bruce hasn't gotten scope back yet or what???

mrpjdavis said:

At last the rain has stopped and we've had some clear days and nights. I've used the astro eyepiece (which gives 107x) - really useful for studying the geese and peregrines at Slimbridge, provided it is reasonably bright. I've just set up the scope outside to look at the moon (first quarter). It is truly breathtaking, almost filling the field of view. I also found Saturn (by chance!) and I can see the ring and even the gap between the planet and ring.

All in all I am delighted with the eyepiece, well worth the outlay.

Pete

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Is this with a Zeiss 85 FL Diascope???

imaxfli said:

Is this with a Zeiss 85 FL Diascope???

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Yup - it's a b*gger to keep steady though!

Pete