Posts Made By: Mike Hosea

If the 9.7mm Super Plossl is a good one, I don't think the 19mm Panoptic with the Barlow is going to outperform it in an f/10 SCT. On the other hand, Barlowing the 19mm Panoptic is a very practical thing to do. Just make sure to use a quality 2x Barlow (I prefer a high-quality doublet like the Tele Vue 2x).

Frankly, I might go either way without fear of missing anything.
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Mike

Well, on the one-upped business, the 24.5mm Meade SWA was a contemporary of the 24mm Tele Vue Wide Field. Some say it was a derived product, but at the least I think you could say that the Meade SWA's were "inspired" by the Wide Fields. Tele Vue's decision to introduce 22mm and 27mm Panoptics to begin with may have had something to do with the Panoptic line topping out at 35mm instead of 40mm like the Wide Fields. I'm sure that all the reasons, whatever they may have been, were deemed good at the time, especially with the big Dob revolution in full swing and generating, no doubt, a lot of demand for high-end eyepieces with 2" barrels. But for the long haul at least, Tele Vue had discounted too much the value of generating the widest possible true field in a 1.25" focuser, and it had to end up costing them sales. I'm sure they realized this soon enough, but perhaps only after Al had successfully shrunk the 9mm and formerly huge 13mm Naglers to their Type 6 versions did a 24mm Panoptic for 1.25" focusers seem like a logical next project.
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Mike

They don't. There are two measurements of "field", TRUE field of view (TFoV) and APPARENT field of view (AFoV). Think of a little circle projected onto the sky that you look at with the naked eye and suppose that what you can see inside that circle is what you can see in the eyepiece. If you've ever seen a Telrad or Rigel Quickfinder this is particularly easy to imagine. The TFoV is the angle your eye sweeps out as you look (without telescope) from one edge of this little circle to the opposite edge along a diameter. The AFoV is the angle your eye sweeps out as you look from one edge of the field in the eyepiece to the opposite edge. Your telescope+eyepiece projects the the TFoV into the AFoV (that's magnification).

All these super- and ultra-wide angle eyepieces are doing differently in one sense is to project the same TFoV into a larger AFoV. In other words, they magnify the same TFoV a little more versus another eyepiece with a smaller AFoV and the same TFoV. For example, the 20mm Tele Vue Plossl and 12mm Nagler Type 4 both have 17.1mm field stops and hence the same true field of view. The 12mm Nagler Type 4 just presents that same view to your eye at a greater magnification. Look from one edge of your computer monitor to the opposite edge. Now move your face 50% closer and repeat. Same TFoV, bigger AFoV. No magic there.
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Mike

Chris,

We don't generally hear about Al's plans before he has something to show at a star party. Then it's usually just a few months before being formally announced. I know a 16mm Radian has been suggested before, but I haven't heard of anybody seeing one.
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Mike

Coatings don't have that much room to improve in terms of reflectance. Light gathering goes with the square of the aperture, so even if you have 99.9% instead of 89% now (not even considering current enhanced coatings), you'll just get your 20" up competing with a 21.2" having 89% coatings. Perhaps non-metallic coatings will get low-stress enough to be used on curved substrates, making for a more durable and reflective coating (currently non-metallic dielectric coatings are pretty much only useful on flat substrates, and even then they warp the figure near the edge).

I can imagine some interesting advances in electronic eyepieces blurring the boundaries between visual and photographic astronomy.
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Mike

Requirements:
1. focal length 5mm to 6mm
2. 1.25" barrel
3. >= 50 degree apparent field.
4. eye relief 10mm or more w/o a built-in Barlow.

If there's a product like that, I don't know of it. Why the prohibition of a built-in Barlow?

The 3-6mm Nagler Zoom comes close with 10mm of eye relief and a 50 degree apparent field. It probably does have a negative field group, but it has just 5-elements total so maybe you will cut it some slack on that score. It does, I note, fit the 5-6mm focal length range literally. :-) The 5mm Tak LE is also a good choice. I think it has 9mm of eye relief and 5-elements (not sure), but it does have that negative field group, too.
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Mike

Robert, it depends on so many things. What objects were you hoping to use the eyepiece on? If I had to pick just one Radian for an 8" f/10 SCT, I think it would be the 10mm for 200x, which is very useful for planetary observation and globular clusters. With a 10mm in hand the 12mm is kinda close, so the next longer would be 14mm, but maybe it would be best for you to keep the 15mm Plossl and think about the 18mm Radian. I guess I'm saying that either of those two Radians you missed would have been fine, but for my tastes I'd prefer the 10mm for high power and 18mm for medium power, especially with the 15mm Plossl available in between.

I suggest you first formulate a plan and a wish list and THEN shop astromart. Generally you can take your time and ask a lot of questions on big ticket items before committing, but with eyepieces, which sell quickly if the price is right, you have to be decisive and quick. (But I'm not advising you to take extra risk. It can be frustrating, but I usually don't even bother with poorly written ads that don't answer my questions about condition, shipping, methods of payment accepted, etc.)
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Mike

The 4mm Tak-HI ortho and 7mm Tak ortho would just meet your eye relief criteria and cost around $150 each ($141*2 + 1.25" adapter). I'm not particularly recommending (or not recommending) that, but it's a possibility.

The Tak LE 7.5 is a good choice, but definitely go with the Tele Vue Barlow, not the Ultima. The Tele Vue will be nearly parfocal with the Tak, making it easier to switch. Plus I think the Tele Vue 2x is arguably the best 1.25" 2x Barlow in current production.

For $80 more you should consider the 3-6mm Nagler Zoom instead of two eyepieces. That covers an ideal 0.5mm to 1mm exit pupil range on your scope, and most reviewers have had glowing words for it and placed in a category with some of the best planetary eyepieces around. I had one and thought it very very nice, but my eye relief minimum requirement is 12mm. I sold it, but I think I may someday buy another.

Think hard about that Nagler Zoom...
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Mike

I've probably spent more than a hundred hours comparing views with and without Barlows (at the same magnification) and really don't feel the same as you about it. In the 1.25" barrel size my vote goes to the 2x Tele Vue among all those 2x Barlows I know of that are in current production. The 2.5x Powermate has its advantages, and I've heard the Tele Vue 3x is good, too. I don't use 2" Barlows because they protrude into the light paths in my 7" and 10" Newts, each with low-profile focusers. Nevertheless, the Tele Vue 2x Big Barlow is a good one I've tried, and the 1.8x (I think) AP Barcon is also highly regarded. I haven't tried either of the 2" Powermates.
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Mike

Not dumb at all.

Spherical aberration implies an inability to focus light properly. Most commonly, rays handled in outer zones of the optics focus at a different place than rays handled in the inner zones. It means you can't find a pinpoint focus.

Coma refers to a similar inability to focus light properly, but it is progressive, from no aberration on-axis, i.e. in the middle of the view increasing as you look farther and farther off-axis. The name is reminiscent of the shape of the blur one gets from a point source imaged off-axis. It looks like a little comet with the head pointed towards the center of the view.

Astigmatism can occur anywhere in the view and is due to asymmetry in the way light is refracted or reflected. For example, a spoon-shaped cornea causes astigmatism in your eye and is corrected with a lens with an asymmetry to cancel. Astigmatism often occurs with constrictive primary lens cells in Newtonians during cooldown (e.g. when the mirror is hard glued in as is too often done in inexpensive imports) and with thin mirrors that are not properly supported. In eyepieces it may occur anywhere in the view (the result is a lack of sharpness), but most often it is not present on-axis but increases off-axis, somewhat like coma but growing faster generally. People often mistake off-axis astigmatism in eyepieces for coma. There's a nice interactive tutorial on this at

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

Some of the other demos/tutorials on this site are quite good. Look for others on

http://www.microscopyu.com/tutorials/java/index.html .
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Mike