Big Scopes

[birderk]

I have a Nikon Fieldscope 82 EDa which I think is really excellent in terms of optics and handling.

However, there are many times when I feel scopes in the 77-85 mm range are at their limits. So I have four questions:

1) Steve Ingram has suggested in some situations (heat shimmer, fog) a smaller scope may have better resolution than a larger scope. Has anyone found this to be true? That is, say, seen a Swarovski 65 outperform its larger brother at equivalent powers in poor conditions?

2) When we fail to resolve birds with our 77-85 mm optics, is it generally atmospheric conditions or the size of the objective lens which are the limiting factors?

3) Has anyone compared the Televue 85 to the Questar Field model? The questar field model has seemed to have fallen out of favor with birders.

4) Has anyone used the Celestron C5 or Meade 105 and 125 mm ETX for birding? I imagine the questar and televue with the lack of an image erector can outperform the top end Nikon/Zeiss/Swarovski/etc scopes but it seems that these really large aperature scopes might be the best way to get really good views of distant birds. I have not seen any in the field but their costs are reasonable and I wonder if they might be worth their cost as a "distance" scope.

Brendan

[iporali]

Hi Brendan,

1) With all respect to Steve's expertise I really don't believe this. I have not done enough side-by-side comparisons to know it quantitatively, but I can't see any optical/physical reasons, why it should be so - well, maybe less shake on similar tripods. The Swaro ATS is the only scope which has the same focal length ("power") in both 80 and 65 mm models - this means that the f-ratio of smaller body is higher and may actually have fewer aberrations, but its resolving power is lower nevertheless. With the Leica or Zeiss scopes 20x on the 77/85 body always outperforms 20x on the 62/65 body. Have you seen this test: http://www.alula.fi/gb/smalltelescopes3-2002.htm ?

2) In my experience it is mostly atmospheric conditions that limit my seeing. I often use 60x of my zoom but many times it does not help to go past 45-50x. In perfect conditions good birding scopes could go maybe up to 80-100x.

3) Henry Link uses the Astrophysics 90mm APO, which must be heads and shoulders above any birding optics (or Televues & Questars) allowing powers of something like 200x (?). It would be interesting to hear Henry's insightful views on this.

4) I have tried a cheap Chinese 102mm Maksutov-Cassegrain (SkyWatcher?), but it does not have nearly as much contrast as my Swarovski and at low powers its field-of-view is quite narrow. Maybe it would work better at higher powers?

Regards,

Ilkka

[kabsetz]

Brendan,

1) I second Ilkka here. In my experience, what matters most in heat-shimmer situations is the overall level of aberrations in a scope. Aberrations smear the image, heat shimmer smears the image, and the sum of the two is significantly worse than either alone. In otherwise identical scopes (same make and model), the ones with fewer aberrations perform significantly better when there is shimmer, and this holds even between units which are pretty good to begin with - not just good vs. poor. In shimmer, a small scope with fewer aberrations can thus outperform a larger one with more aberrations, probably even when the measured resolution of the larger scope is somewhat better in crystal-clear air. For obvious reasons, though, I have not conducted an exhaustive test with various size scopes with gradaded aberration levels, but I feel confident in saying that given equally aberration-free specimen, a larger scope would always do better even in shimmer. When it comes to fog, contrast rules. Contrast also is highly aberration-dependent, so the same arguments are partially relevant here also.

2) Much, much more often the limiting factor is the atmospheric conditions. However, don't we all wish to optimize our chances under any and all conditions? Therefore, I'm not willing to settle for a scope which only works well enough under conditions where nobody can identify anything very difficult or distant, and therefore like to have as much aperture as I feel happy to carry and handle - I would very much like to see a well-executed 90-95mm birding scope.

3) I have looked through a Questar only a couple of times, two specimen, and neither was of very recent manufacture. They were not overwhelming experiences. The Tele-vue 85 I have not seen, but have tried a Pronto. Based on that experience, I am quite ready to believe and expect that the 85 would be noticeably but not dramatically better than good specimen of standard 77-85mm ZeLeSwaNikKow birding scopes, but very likely has much more consistent quality from unit to unit. It is pretty darn big and heavy though.

4) I have once loaned a Celestron C5 but, again, was not overwhelmed. It is possible that it was not properly collimated, as back then I did not know enough to really test for that. The convenience factor alone turned me off, though.

I would also like to hear Henry's take on this, especially if he has had a chance to do proper comparisons between his A-P and a good specimen of the Zeiss 85 or a comparable birding scope.

Kimmo

[laservet]

I have the Astro-Physics 90 mm f/5 (actually 92.5mm aperture, 450mm f4.9) and the Astro-Physics 92mm 604mm f6.6, just picked up the Leica APO Televid 77. I should get a chance to compare them over the holidays and will post my impressions here.

The AP's aren't armored and aren't waterproof, so the Leica wins on that front. :)

[AlanFrench]

I have the Astro-Physics 90 mm f/5 (actually 92.5mm aperture, 450mm f4.9) and the Astro-Physics 92mm 604mm f6.6, just picked up the Leica APO Televid 77. I should get a chance to compare them over the holidays and will post my impressions here.

The AP's aren't armored and aren't waterproof, so the Leica wins on that front. :)

Paul,

That would be interesting. I have been using the AP 90 EDL "Stowaway" (f/5 version) for birding and astronomy since March, 1997, but have only had a chance to compare it to an older Questar and a Swarovski 65mm ATS HD. The AP was a clear winner in both cases, providing brighter, more detailed views, and allowing the use of higher powers. It is the scope I use when I don't need something light and portable or waterproof.

One telling comparison is views of Saturn and Jupiter at high powers. The Stowaway gives a nice, crisp detailed image. The Questar was something of a disappointment - about the equal of my trusty 60mm Unitron. The 65mm ATS did quite well, but is limited by its aperture and not at the same level of optical quality of the other two. This is no big surprise since the Stowaway is corrected extremely well for spherical aberration by aspherizing two surfaces, is a triplet, and uses true fluorite. The Questar is also well corrected. The Swarovski is quite good, but almost certainly does not get the same attention.

I have the Stowaway on an old Linhof tripod that was sold for Questars, although I replaced the annoyingly flexible head with a nice altazimuth unit I picked up at the Stellafane swap tables. The set up is a bit on the heavy side, but it does not move at all in the wind.

Clear skies, Alan

[laservet]

Alan,

I use them both at high power for astronomy on an AP 400 GTO. For wide field impromptu astro and daytime birding I keep the f5 Stowaway on a Telepod head on a Gitzo cf tripod. I can pick up the entire setup with my little finger. The scope is a superb performer. For portable astro and solar observing with a Daystar filter I keep the f/7 Stowaway on a Unistar Light Deluxe on a much larger Gitzo cf, heavier than the other setup but quite a bit more stable. The f/7 is great for observing dragonflies up close and personal on their perches from 100-200 feet away, beautiful image at 200x and above.

I like the Leica but haven't had it long enough to really put it through its paces.

Best regards,
Paul

[AlanFrench]

Alan,

I use them both at high power for astronomy on an AP 400 GTO. For wide field impromptu astro and daytime birding I keep the f5 Stowaway on a Telepod head on a Gitzo cf tripod. I can pick up the entire setup with my little finger. The scope is a superb performer. For portable astro and solar observing with a Daystar filter I keep the f/7 Stowaway on a Unistar Light Deluxe on a much larger Gitzo cf, heavier than the other setup but quite a bit more stable. The f/7 is great for observing dragonflies up close and personal on their perches from 100-200 feet away, beautiful image at 200x and above.

I like the Leica but haven't had it long enough to really put it through its paces.

Best regards,
Paul

Paul,

I have an AP 400 QMD I use with the AP 90 EDL. Often, though, I use the Linhof. I enjoy the ability to rack the scope up and down for the ideal eyepiece position.

One of my upcoming projects is putting a "finder" on the scope so I can use it with the Linhof for hawk watching. I want something that will give me a wide view of the sky with one eye while looking through the scope with the other eye.

I'd also like to make something to replace the little peep-site on the ATS 65. Again, I think it should be set up to be used with the non-viewing eye without changing position.

Clear skies, Alan

[kabsetz]

Paul,

Looking forward to your comparison between the A-P and the Leica. When you do it, could you please also provide a star-test report on the Leica so we can estimate roughly how good of a specimen it is. Since the worst birding scope specimen (not necessarily Leica, but Leica is no exception when it comes to variation) I have encountered have been quite poor - worse rather than better than half-wave - and the best have been better than 1/8 wave, an estimate of aberrations is most usefull for making the comparison meaningfull for us poor souls who do not possess an A-P-quality instrument.

As it currently stands, my belief is that the chief advantage the high-quality astro refractors have over the premium birding scopes is their much better assembly tolerances and quality control, whereby sub-1/6-wave units are rare among them while they are probably the majority among birding scopes.

Kimmo

[henry link]

Here's my take on the original questions.

1) I think this idea comes from the effect of poor "seeing" conditions in astronomy. In small aperture instruments the wave lengths of air turbulence may be longer than the diameter of the air column in front of the objective. Under those conditions you may see a slowly waving image that is relatively clear in a 60mm telescope, while the image in 200mm telescope looks more stable but blurry. I've seen a little of this in very unsteady air when comparing the image in my Tak 50mm with the image in the AP 92mm, but under these conditions the resolution of both scopes is seriously compromised. I doubt that a 65mm would ever have an advantage over an 80mm from this.

2) I completely agree with Kimmo. When the good conditions come along I want to be able to take advantage of them. I've found that there are many days when there are two windows of steady air when air and surface temperatures equalize, one early in the morning and one late in the day, and cloudy days can have very stable air. Under ideal conditions I think a first rate scope will continue to show smaller details up to a magnification at least 20% above the objective diameter in mm's.

3) I haven't seen a TV-85, but I think the optics of the Questar were surpassed long ago by medium focal length APO refractors. A mirror scope with a large central obstruction will never have the contrast and light transmission of an unobstructed refractor of the same aperture. In addition, I find the image in Mak/SCT's, even Questars, to have a certain unpleasant "look" which I would describe as gauzy. Focus never feels completely secure and relaxed compared to a high quality refractor. Perhaps this results from too much optical complexity employed to correct the very fast primary.

4) There is a birder here in North Carolina who uses a C-5 and, believe it or not, a C-8 with a bino-viewer. I've looked at birds through his C-8 at 150X. The image is impressively bright and there are glimpses of details that smaller scopes would not show, but the image quality is unpleasant to me in the ways described above, and the scope and tripod weigh 40 lbs.

I've only compared my 92mm AP Stowaway (FL=604mm) carefully to a good sample of the Zeiss 85mm and a Swarovski ATS-80 HD of uncertain quality (not star tested). The AP was considerably better than either at 60X, but it even looked sharper and higher contrast at 30X. I would add that this Diascope star tested about as well as a Tak Sky-90, so it was pretty good, perhaps 1/6 wave. If I could find a 1/8 wave sample of a Zeiss, Nikon or Swarovski I think I would happily retire the 10 lb. AP from all birding that requires a long walk.

[birderk]

Thanks for everyones very interesting comments. It seems from everyones comments that 85-100 mm refractors can give the best possible view.

Unfortunately they are to my mind specialty scopes due to their weight, size, lack of weatherproofing and reversed field of view. For a cost of well over $2000 I think I could only justify purchasing one if (a) I was interested in astronomy, (b) used it as primary birding scope or (c) the view throught the fieldscope wasn't quite good already.

I would still like to do a comparison of my fieldscope to some of the big mirror scopes. I have so far been unimpressed. However when I have seen mirror scopes in the field, they are usually old, battered, with a low quality eyepiece, and 45 degree agle image erecting prism which seems to me to be a less than ideal comparison.

I am curious, Henry, if you traded in your A-P scope for a Nikon/Zeiss/Swarovski you would not have a way to reach the ~100x range with the available eyepieces. Would you be missing anything?

Brendan

[henry link]

Brendan,

I would continue to use the AP when I wanted the very best view and didn't have to walk very far, but I think the best specimens of the current big birding scopes are approaching "good enough" compared to high end APO's to make astronomcal scopes less attractive to me for birding than they used to be.

Both Zeiss and Swarovski offer 1.25" adapters for using astronomical eyepieces. As I recall the Swarovski's focal length is 462mm and the Zeiss about 500mm, so a 4-5mm eyepiece would make about 100x. The Televue 3-6mm zoom might be a good choice. Unfortunately it would take a better specimen than I have seen to make that much magnification really useful with either of those scopes. I've seen star test photos from an exceptionally good Nikon 82mm that should be useful at 100x, but Nikon doesn't make an adapter for astronomical eyepieces and not every sample of the Nikon is going to be that good.