Posts Made By: Mike Hosea

My 2 cents is that you'll have a lot more satisfaction with 2 quality eyepieces and a Barlow than you'll have with a case full of cheap eyepieces.
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Mike

Well it's not much of a "spot" is it? I see this as a situation where difficulties that arise at the extremes lead to the best results in a fairly broad central range. The precise range varies by target and individual. I do avoid large exit pupils for long-term observation for a variety of reasons, not the least of which would be the corneal aberrations that become apparent.
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Mike

The first version of the 6mm, as described in some detail on the web site, is available, and my understanding is that the 4mm should be in transit from the manufacturer to Burgess if things have gone according to what Bill told me a couple of weeks ago over the phone. There are a few 4mm prototypes around, one of which is in the possession of Tom Trusock, who has tried it but is probably just waiting for some decent planetary observing conditions to give it a critical test drive. I don't remember what Bill said about the 9mm availability, but my vague impression is that the announced 3 focal lengths would all be available in a matter of weeks (the 6mm now, the 4mm next, and then the 9mm) with the aside that the 6mm as it is being sold today will not be the final version. Burgess has plans for more focal lengths, but he has to fund the production of additional focal lengths with proceeds from the sale of the early focal lengths. I think maybe a 2.5mm is next in the pipeline, or maybe a 5mm. It sounds a little bit flexible. If there's a particular focal length you would like to see, you should probably let him know when you order one of the announced focal lengths.
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Mike

Larry,

It measured at about 70 degrees for me, but maybe what you really need to know is the field stop diameter. I don't remember the number off the top of my head, but I measured it with digital calipers and put the number in a table in my report on Cloudy Nights.
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Mike

This will depend on your 1.25" adapter. The "F" column in the table gives 0.29" for the Panoptic 27, while all the Parfocal B eyepieces have an "F" value of 0.25" . So if both eyepieces were 2", we'd say that you'd need 0.29 - 0.25 = 0.04" of focus travel when going from the Panoptic 27 to a Parfocal B eyepiece. That's OUT travel when going from Parfocal B to the Panoptic 27 and IN travel when going from the Panoptic 27 to the Parfocal B point. Let's say for the sake of argument that your 1.25" adapter adds 0.4" of height. Then your Parfocal B eyepieces will actually have a new "F" value of 0.25 - 0.4 = -0.15". Now going from the Panoptic 27 to the Parfocal B eyepieces will require 0.29 - (-0.15") = 0.44" of focus travel. That's IN travel when going from the Pan to the Parfocal B, OUT travel going the other way.

I'm a mathematician, not an optical designer, and that sounds like a very advanced optical design question. I guess I can conceive of such a thing, same as you. It would be fantastic.
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Mike

You're computation is for zero rectilinear distortion. For zero angular magnification distortion,

field_stop = focal_length * AFoV * pi / 180.

Wide angle astronomical eyepieces usually come within several percentage points of having zero angular magnification distortion, but rectilinear distortion is often less well-controlled (as you have discovered!).
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Mike

I use a 36mm for a "finder" eyepiece on my 10" f/5 Dob-Newt. I actually did a little report: http://www.cloudynights.com/documents/BurgessSWA.pdf .
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Mike

Well, I guess I should cast a dissenting vote. I've owned all the Radians except for the 6mm and all the Type 6 Naglers except for the 13mm. I'm a minimalist and from that list down now to just two Type 6 Naglers and no Radians. This process included a direct comparison between the 5mm Radian and 5mm Nagler Type 6. It's been a few years now, and I don't miss the Radians at all. They have more lateral color on a degree-for-degree basis than the Type 6 Naglers, and the eye relief pretty much the SAME for 60 degrees of apparent field, i.e. if your eye is about 20mm away from the lens you don't see the field stop in a Type 6 Nagler but you do see everything that you can see in a Radian, except not as much lateral color. And if you want the wider field you can have it anytime you want it. In my book that just makes the Type 6 Naglers more versatile and more useful than Radians. However, if 12mm of eye relief is not enough and you feel strongly that you should have a field stop in view at all times, then a 5mm Radian or 5mm Pentax XW might be better for you.
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Mike

I'd vote for the 18mm Radian between the two, but it only has about 75% of the TFoV of the Pentax, so these eyepieces don't quite seem interchangeable. It's the field curvature of these longer 1.25" Pentax's that puts me off in my 10" f/5 Newt. The 18mm Radian, IIRC, is flat and sharp to the edge.
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Mike