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Old 26-07-2018, 12:43 PM
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mental4astro (Alexander)

mental4astro is offline
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,415
Hi Richard,

to IIS!

Eyepieces that are designed primarily for Newtonians (concave focal plane) are all more difficult to design and manufacture, and so cost more than for those scope with a convex focal plane. In the end it can come down to how deep your pockets are. But not always if you know what to look for.

However, do not confuse "eyepieces designed for a convex focal plane" as meaning that they are all crap in Newts. The performance of each individual eyepiece, NOT eyepiece design, varies tremendously when used in a Newtonian. Some of these eyepieces certainly will show a lot of aberrations related to being a significant optical mismatch. However, due to the complex nature of eyepiece designs, the same "design" in a different focal length can actually result in an eyepiece that is absolutely superb in Newtonians, and shows no aberrations what so ever, even if a different focal length of the same design show a lot of aberrations!

Also, the number and severity of any aberrations not only varies tremendously, but these also weigh differently on different PEOPLE. Some people expect some sort of mystical "text book perfect" image, and will then outright dismiss a perfectly good eyepiece only because the very edge show the slightest amount of say field curvature, but they somehow manage to forget that NO ONE does any serious observing at the very edge! So, for the sake of some one else's insistance that things NEED to be perfect, another person will forgo a perfectly good eyepiece just to satisfy the expectations and ego of someone else instead of being the pragmatist they would normally be.

There is nothing wrong with "text book perfect" if you can find it and that it MUST be that way for YOU! Just do not confuse a wee amount of aberration as an eyepiece that is trash. For even many "perfect" eyepieces will show their own set of aberrations and vary in performance depending on a whole bunch of factors, such as focal ratio - not so simple when the focal length of the scope dictates the radius of the focal plane, not the focal ratio which is a photographic ratio, and only serves as an indicator to the severity of the curve of the focal plane - thought it was that straight forward?

When you think about eyepieces for your scope, it isn't right for me just to rattle of a whole bunch of models that "you must get". What suits MY observing preference most likely WILL NOT match with yours. AFOV, price, eye relief, size, brand prejudice, observing habits, YOUR scope/scopes, all come into play.

I often mention the TMB Planetary Type II line because they are outrageously good for their price. Yes, they were primarily designed for refractors (convex focal plane), but they ALL do a good job at the very worst in Newtonians, and some individual focal lengths perform as good as more high end eyepieces. HOWEVER, this "good job" varies depending on the focal ratio of you Newtonian. The 6mm is the weakest of the 10 individual focal lengths, and while I wouldn't use one, a good mate of mine is most happy with his! Lucky man to have found his eyepiece nirvana so quickly and easily. Yet there are four individual focal lengths from these 10 separate pieces that are tack sharp all the way across the field of view in an 8" f/4 Newt. The 2.5mm, 4mm, 5mm, and 8mm. The others do show a tiny amount of field curvature along the very edge, and in an f/6 Newt, the amount of field curvature will be even smaller, even not there depending on the individual focal length. These all have the same generous 16mm eye relief (a hell of a lot more than your 9mm plossl), and the eye lens is the same large sucker in all pieces, and not getting smaller as the focal length reduces. For their price, these are all really good value. BUT you need to consider them individually with YOUR scope, and YOUR price bracket and YOUR preferences. I've had all 10 individual pieces, sold the ones I thought were too close to the same focal length, but I now regret selling as they all have their own niche place... Oh well...

And of course I have many other eyepieces besides the TMBs. Most of these are only focal-plane-specific, meaning I will only use them in one scope design and not in others. And this suit ME just fine But they are all selected for their individual performance in said scopes.

Below is a picture comparing the appearance of the 9mm TMB Planetary Type II with a stock standard GSO 9mm Plossl. BIG difference is appearance and performance. And beside it is a picture of the 14 separate eyepieces I tested, the entire 10 individual pieces plus four duplicates to test for QC.

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Now, how deep are your pockets? TV makes a wide range of eyepieces with Newtonians in mind. The different lines vary in AFOV, prices, and the individual eyepieces all do vary in performance, but as a whole are very blooming good in Newts. They are not all the same, and not everyone who is a TV fan and uses the Ethos line is as excited by the Delite line. From 52 AFOV (their plossl line) to 100. What do you want?

Explore Scientific also make very good eyepieces, with different lines designed for convex focal planes and others for concave. And these also vary in AFOV and performance.

The Pentax XW line is also very good.

Plossls, in terms of quality of transmission because of the few number of elements, are flaming da bomb! Designed for Newtonians, but their biggest problems are the tight eye relief that gets tighter as focal length shortens, and the eye lens also gets smaller, making them less and less comfortable to use. THEY ARE DESIGNED FOR NEWTS... But you are not happy with your 9mm plossl. TV plossls will perform better than really cheap plossls, and that's because TV will insist on better quality coatings, but the usage drawbacks are the same. While plossls are designed for a concave focal plane, they are a 150 year old design and optimized for very slow focal ratios. These begin to struggle with todays very fast Newtonians, and the faster the focal ratio the more significant the aberrations become.

Vixen also makes stonking good plossls, and other eyepieces that are also really, really good in Newts, but these are not well known here in Australia. Real shame.

Best thing to do if you are not sure what to get is to get yourself to an astro club/society meet, along with your scope, and look through other people's eyepieces and maybe try a few of them in your own scope, and see what most tickles your fancy and then start your own "wish list" that is based on YOUR EXPERIENCES, and not someone else's opinions, mine included


Last edited by mental4astro; 26-07-2018 at 01:43 PM.
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