Posts Made By: Mike Hosea

Well, of course AP is a quality outfit. I only asked for them to evaluate 2 of the 6 filters, and they ended up replacing both for me. It did take awhile, but I was feeling patient because I was still going to bed before Jupiter was in good position to observe. I think I like the filters, but I've only had one session observing Jupiter with them, and conditions weren't very good.

One of the filters I sent back had a coating flaw at the edge, and when clean it still displayed a fingerprint-like pattern when I fogged it with my breath. That was iffy. I probably wouldn't have pursued a return on that one if it weren't for a more obvious problem with the other. The other filter seemed to be jammed in the casing at a slight angle (as evidenced by the retaining ring being cross-threaded), and I got a series of reflections around bright objects.

As for efficacy versus standard filters, I'm not sure. I'm only using them in situations where contrast and sharpness of bright objects are critical, so while one person might ask why pay twice as much, I tend to ask why go with the Chinese filters (which have no AR coatings at all or inferior coatings) just to save $60 (or whatever it happens to be).
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Mike

The x axis is the deviation from the paraxial focal length, and the y axis is the angular distance off-axis. So, in English, the curves tell you how you how the focuser will need to move in order to switch from focusing the center at infinity and focusing a point at a given distance off-axis at infinity, assuming your telescope has no curvature or astigmatism (ha!). There are two curves. One represents the meridional (also called tangential) focal surface, and the other represents the sagittal focal surface. You can read some about this at

http://www.microscopyu.com/tutorials/java/aberrations/astigmatism/index.html

and

http://www.vanwalree.com/optics/astigmatism.html

but basically you want these two curves to coincide as much as possible, and you want them to be close to the vertical center line. When they swing to the right as they rise, you will have positive field curvature (requiring in-travel to compensate for defocus at the edge), and if they swing to the left you will have negative field curvature (requiring either out-travel or accommodation by the eye to compensate for defocus at the edge of field). The farther apart they are, the more astigmatism you will see, rather than field curvature.

This is incredible data to get from a manufacturer, but unfortunately since it is so rare to have, we don't have much to compare it to.
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Mike

Here's a thread to read.

http://www.astromart.com/forums/viewpost.asp?forum_post_id=390223

Vignetting is caused by the eyepiece, NOT the Barlow. These Barlows are all designed with sufficient clear apertures for all reasonable amateur scopes. Vignetting occurs because the Barlows introduce divergence, which some eyepieces handle with aplomb (including MOST negative-positive designs) and others cannot. A 50 degree Plossl, for example, can handle little divergence without vignetting, while a Radian, Pentax XL, or some such can usually handle quite a lot. I can tell you from experience that a 7mm T6 Nagler in a 2.8x Klee doesn't vignette at all.
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Mike

Even if you don't care about these particular eyepieces, it's definitely interesting. I hadn't imagined that the similarities would go quite that far, although frankly, I'm not surprised.
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Mike

I think that price should be left out of the comparison. Even when money is a consideration, which is not always the case, what constitutes a meaningful price difference will vary from person to person.

As for the off-axis softness, purpose matters. For planetary observation it is appropriate to weigh it less than with DSO observation. It matters some (e.g. with lunar observation) but not as much. On the other hand, for DSO observation, particularly for open clusters and such, edge sharpness matters for multiple reasons. For one thing you're using scotopic and averted vision and for another the object itself may be spread out. For example, if you're sketching, you'd probably like to have the option of not panning. Consequently edge sharpness should be factored in accordingly for DSO observation.

You win the head-to-head if you're better for the intended purpose, even if you cost 10 times as much. It's challenging to write a review that communicates performance differences in such a way that people can factor in the price differences for themselves, but this is what the author should try their best to do, IMO. It's okay for them to communicate their personal assessment of price versus performance, but it should be done in the conclusions section in a relatively isolated way rather than applied throughout.
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Mike

Jon,

From the standpoint of aberrations and such, after the eyepiece is the better way, no question. The only issue is eye relief.
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Mike

No, I don't know where you can order one, but it shouldn't be too hard to locate something on a cheaper eyepiece, trimming it down if necessary. If you just want the grip and don't care about the cosmetics, you could use 3M Temflex rubber tape. This should be available from your local home improvement store where they have electrical tape. You pull off the backing and stretch it as you wrap. Then it turns to a solid piece of rubber. It's not hard to remove--worst case you can just wedge something non-marring underneath it and cut against that, and it will peel away completely, leaving no trace that it was ever there.

It's been a long time. Both are pretty good eyepieces, IMO, but the XL is MUCH more comfortable, IMO. For me, the 12T4 required a steady head to avoid blackouts.
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Mike

Alan, thanks for noting this glare. I noted it in the 5mm XW and thought I was starting to think I was all alone. I believe this glare is from the several not-quite-knife-edge baffles.

As for Daniel Mounsey's comments, I consider him very perceptive, much moreso than myself as regards several aspects of planetary eyepiece performance. He often sees bigger differences than I do and possibly can see smaller differences than I CAN. So, philosophically, how do you react to a situation where you can't see a difference and somebody else can? My philosophy is to focus on my own satisfaction and let aficionados like Daniel tend to theirs.
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Mike