Don Pensack

Eyepieces show a flat field to the eye when their field curvature matches the scope exactly. Since manufacturers cannot know the exact degree of field curvature in the instruments in which the eyepieces will be used, flat fields are typically designed (witness how many eyepieces now have the name "flat field" or "ultra flat field"). However, many if not most scopes have negative field curvature, so naturally the 3.5mm through 10mm XWs would test better since their curvatures match the scopes better. Eyepieces in the line from 14-40mm have positive field curvature, so are more likely to see comments about field curvature. The 30mm and 40mm are actually quite flat, but because their field stop diameters are larger, they extend farther out into the curved fields of the scopes and people comment about the field curvature. If you look at the curves, though, it's easy to see why the 14mm and 20mm get the most comments about field curvature. Personally, I use a dob with a long focal length that is flattened by a Paracorr (as well as corrected for coma), and I only notice one aberration in any of these--astigmatism in the outer edges of the field. If you look at the charts, the farther the meridional and sagittal curves separate, the more astigmatism there is (worst is the 3.5mm).

It was the pre-production prototype brought to the star party by Al Nagler himself.

As of September 12, they are in production and the initial run of ~30opcs will be delivered to TeleVue some time in the Fall. Alas, at this writing, we still have not retail price.

They are a modern updated equivalent of an Abbe orthoscopic, that is they are a narrow field (42°) with high resolution, intended for high power views of planets, moon, double stars. There was a brief production quality-control problem with the 3.4mm, though that is now fixed. The shorter focal lengths did not experience that. They are good alternatives to using other eyepieces with, say, a Barlow lens, to achieve ultra high powers in small scopes of short focal length.

There is an empirical way to find the right setting for an eyepiece. You only need to do this for 1 eyepiece. For every other eyepiece you own or will own, you simply insert the other eyepiece and focus using the Tunable Top. Whatever setting results is the correct setting for that eyepiece and you can pre-dial it before you put the eyepiece in in the future. So, Insert the eyepiece with the tunable top set to its maximum out setting (H). Focus the scope and look at stars at the edge of the field (or move a moderately bright star to the edge of the field). Still see coma in the star image? Then dial the top in one setting to G , focus the scope and repeat your evaluation of a star at the edge (it's easier to see coma on a brighter star, like Polaris, than a dimmer star). Still see coma? Keep moving in one setting until, when you focus, the star at the edge of the field displays no coma. That is the setting you'll use for that eyepiece in the future. For every other eyepiece, leave the focuser where it is and focus using the tunable top. Write down the settings so you know what they are. You can do this with any eyepiece from any company. If the eyepiece needs more outward movement of the tunable top, you can add a parfocalizing ring to the eyepiece to raise it a bit. If the eyepiece needs more inward travel of the focuser, you will have to settle for the inward most setting (A): example Nikon HW 17mm. What you do by doing this is to set the Paracorr in the light cone from the primary. There is one more way to do this: insert the Paracorr in the focuser in setting E. Place translucent scotch tape across the opening of the Paracorr in an X shape where the eyepiece would normally go. Point the scope at the moon and focus the telescope with the moon's image on the translucent tape. When the Moon is in focus, you have found the perfect setting of the Paracorr in the light cone from the primary mirror. Lock the focuser in this position, and insert each one of your eyepieces in sequence, focusing on the Moon with the tunable top. The settings each eyepiece uses is the correct setting for that eyepiece. I have done both and am happy to report that, at most, I was about a half setting off on an eyepiece or two that way, and a half setting is a very small error (about a mm). The problem with both methods is if the observer is nearsighted. In that case, perform the test(s) with glasses on that correct your vision. Otherwise the settings for each eyepiece will be off. Later, when you view without glasses, the distance between the eyepiece and Paracorr will have already been accurately determined and even if you have to move the focuser in to focus, the correction will be approximately correct.

Since the Ethos comes in 21mm, 17mm, 13mm, 10mm, 8mm, 6mm, 4.7mm, and 3.7mm while the APM/Lunt HDC XWAs come in 20mm, 13mm, 9mm, 5mm, and 3.5mm. it would appear there is only one focal length in common. Some enterprising soul needs to acquire a complete set of each and do in-depth comparisons for us all (and don't suggest I do it--I don't get enough observing time in as it is!).

You must have a long focal length scope. I typically use a 13mm or 10mm eyepiece on that group (my scope has an 1825mm focal length). 20-21mm would seem to be a bit low in power.

The same eyepiece is also sold under the Astromania, Knight Owl, Olivon, Omegon, and TMB labels as well as BST and Sky Watcher. They are quite competent eyepieces and work well in scopes of f/6+ (shorter focal ratios will induce some edge of field astigmatism) Among the various brands, they are available in 2.5mm, 3.2mm, 4mm, 4.5mm, 5mm, 6mm, 7mm, 8mm, 9mm, 15mm, 20mm, and 25mm If I were starting out again, instead of the junk I used then, these would be on my radar screen as a "best buy" eyepiece, along with the Celestron X-Cel LX, Meade HD60, and the various brands of BST eyepiece sold as Astrotech Paradigms, BST Starguide, etc. (about 6 different labels for that one). This inexpensive 58-60° type of eyepiece is more comfortable to use, has a wider field, and general better mechanical structures than the ubiquitous inexpensive Plossls sold under 50 brand names. I like the eyecups on these (once the oil underneath is wiped off).

Just a note: In the pressure chambers where the eyepieces are assembled, it doesn't matter what gas is used. However, under nitrogen the seals in the machine fail more often when nitrogen is used than when argon is used. Since refitting the machine with new seals means down-time and lost production, argon, though more expensive than nitrogen, results in increased production and lower per-unit cost than using the less expensive nitrogen. Any claim this makes any difference to the end user is advertising hype. ES has entirely changed over to Argon, though they still have stock of 20x100 in N2 version. From the standpoint of amateur astronomers, it makes no difference which gas is used. My own eyepieces are pressurized with a nitrogen-oxygen-argon mix to 14.7psi. Always have been.