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Old 26-07-2018, 12:52 PM
Biggus (Richard)
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Planetary eyepiece for a 203mm F/6 Dobsonian

Hi all, been lurking around reading threads for a short while and it's time for me to put my hand up and ask a question. I've just started getting into this whole stargazing lark and to this end recently purchased a Bintel (GSO) 8"/203mm Dob which came with 9mm and 15mm Plossls and a 2" 30mm Superview. I also subsequently got the 2" 2x ED barlow and a 35mm 2" extension tube since the Superview wouldn't focus unless you had the eyepiece hanging about 20mm out from a proper seating position.

I wear glasses but tend to observe without, and thus far I'm comfortable with the 30mm and 15mm eyepieces, the 9mm not so much since the eye relief is a little tight for my liking. In another thread here somewhere, I think it had to do with TMB planetary eyepieces, a clearly knowledgeable member stated something to the effect that these eyepieces were designed for telescopes that produce a convex focal plane such as cassegrains and refractors whereas newts produce a concave focal plane (I hope I remembered that correctly).

What wasn't touched upon from memory was precisely which eyepieces did suit Newtonians for planetary viewing, so.......looking for some eyepieces, at least a couple I guess since I don't much like the 9mm Plossl, so maybe a 9 and a 6 with nice eye relief to suit an 8" F/6 reflector. Another possibility I guess is to get a Baader Hyperion 8-24 zoom if that would be suitable.

Thoughts please.
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Old 26-07-2018, 01:43 PM
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mental4astro (Alexander)
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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

Alex.

Last edited by mental4astro; 26-07-2018 at 02:43 PM.
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Old 26-07-2018, 03:08 PM
Biggus (Richard)
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Thank you Alexander, I appreciate the effort you've made to give me a comprehensive answer. The more information the better.

I probably won't get much of a chance to get to star parties to look at other people's gear since as far as I can tell it's a couple of hundred kilometers to the nearest astronomy club, so lurking on astronomy fora will have to suffice. I don't think my eyepiece fever is at Televue levels quite yet but the fever is rising and it may well be inevitable that it will in due course.

Since my scope isn't a goto I'd prefer a wider FOV than with a plossl, so maybe one in the 68 degree range, and being realistic there would have to be compelling reasons to pay more than $200 a pop per eyepiece at this relatively early stage.

Where does one get these TMB eyepieces? I can't seem to find a vendor in Oz?

Thanks in advance.
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Old 26-07-2018, 03:41 PM
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mental4astro (Alexander)
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There is no one here in Australia that sells these eyepieces any more. The best place to buy them is through ebay:

https://www.ebay.com.au/sch/i.html?_...0&LH_PrefLoc=2

There's a whole bunch of other eyepieces in the above search, but the TMB's all have the same appearance as those in the pictures I attached in my earlier post. The AFOV of all of these is 58, more than the plossls. These will all serve you very well.

There is one particular eyepiece that is 68 that is really good in Newtonians, and second hand is between $100 and $150, the Baader Hyperion 5mm. The Hyperion line is designed for a convex focal plane, and of the entire line, ONLY the 5mm is really good in Newtonians. If you have a refractor, SCT or Mak the entire Hyperion line is really good, but only the 5mm is great in Newtonians.

The same eyepiece as the Hyperions is the Saxon Superwide, the Celestron Duo, and the Orion Stratus. Same eyepieces, different cloak.

Other people may have other suggestions for your 8" f/6 Newt.

Alex
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Old 26-07-2018, 05:26 PM
astro744
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Instead of the 15mm Plossl I recommend the 15mm Superview. See https://www.bintel.com.au/product/bi...s-1-25-2-inch/

It represents excellent value and works well at 15mm and also when combined with your 2x Barlow will give you 7.5mm and 160x which is better for planets than the 133x from the 9mm alone. The apparent field of the 15 Superview is a nice 68 degrees. If you get one try it on Omega Centauri in your 'scope without the Barlow first then with.

The 15mm Plossl and 2x Parlow should work well to with 50 deg field.
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Old 26-07-2018, 05:36 PM
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mental4astro (Alexander)
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Quote:
Originally Posted by astro744 View Post
Instead of the 15mm Plossl I recommend the 15mm Superview. See https://www.bintel.com.au/product/bi...s-1-25-2-inch/
Yep, brilliant inexpensive eyepiece! I've got one too, plus its 30mm big brother! Superb outreach eyepieces too as they are so easy to use. Not "perfect" in a Newt, but great to start with
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Old 26-07-2018, 05:58 PM
Biggus (Richard)
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Thanks once again. There's a shop up in Perth that has Hyperions, Saxon SWA and Celestron Duos as well, priced between $150-$230. Are these the same internally quality wise?

Also I presume the unbranded SWA 58 degree eyepieces in that link you gave me are TMB clones. If so are they any good or a rubbish copy? They have those also up in Perth on sale for $99 atm, supposedly on special from $199.

Once again, thanks in advance.
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Old 26-07-2018, 06:13 PM
Biggus (Richard)
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Quote:
Originally Posted by astro744 View Post
Instead of the 15mm Plossl I recommend the 15mm Superview. See https://www.bintel.com.au/product/bi...s-1-25-2-inch/

It represents excellent value and works well at 15mm and also when combined with your 2x Barlow will give you 7.5mm and 160x which is better for planets than the 133x from the 9mm alone. The apparent field of the 15 Superview is a nice 68 degrees. If you get one try it on Omega Centauri in your 'scope without the Barlow first then with.

The 15mm Plossl and 2x Parlow should work well to with 50 deg field.
Thank you Astro, I've read good things about the 15mm Superview. I'm kicking myself that I didn't get Bintel to send one over when I got the Barlow and extender. On the basis that it would be wasteful to pay postage on just the Superview I now feel I must get something else.

Hey, they sell Televues, don't they?

I think I'm getting the hang of this astronomy thingamajig.
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