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What is a planetary eyepiece?


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This was discussed on CN recently, under the heading "There is no such thing as a planetary eyepiece". That was just an attention-getting ploy which sparked some debate at least.

Actually, I think it's pretty simple.

The bottom line is that pretty much any eyepiece that's got reasonable transmission and good sharpness is good enough for looking at DSO. However, the performance requirements on eyepieces used for viewing planets are much, much tougher:

A "planetary" eyepiece should have:

  • No ghost images
  • Control of stray light so there's no reflections off lens edges etc.
  • No eyeball reflections
  • Razor-sharp focus
  • Low narrow-angle light-scatter
  • Low non-narrow-angle light-scatter
  • Low chromatic aberration for Dob owners
  • Very high contrast
  • Minimal glass impurities

Some say planetaries only need to be sharp on-axis. Dobsonian owners - naturally - disagree! :o Similar contention applies to Apparent Field of View. Dob owners say it should be wide for manual guiding, purists say you shouldn't be guiding manually when observing planets...

Edited by great_bear
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So what are generally considered to be the best Planetary EP's then?

I thought Zeiss Ortho's, Pentax XO's and TMB Monocentrics were considered the "bees knees" of planetary eyepieces ?.

Personally I love the Nagler T6's although I'm sure they would not satisfy a nunber of the "criteria" - they work well for me though :o

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This was discussed on CN recently, under the heading "There is no such thing as a planetary eyepiece". That was just an attention-getting ploy which sparked some debate at least.

Now I am surprised!

Similar contention applies to Apparent Field of View. Dob owners say it should be wide for manual guiding, purists say you shouldn't be guiding manually when observing planets...

And what do "purists" know??:o

I suppose "purists" wouldn't speak to us Dob drivers at a star party because we don't do Goto!

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So what are generally considered to be the best Planetary EP's then?

Oooh, there's 18 pages of CN flamewar right there :o

PERSONALLY my favourite is the Pentax XW with the Ethos a close second, but I don't like orthoscopics and the like with minuscule eye relief. The purist would probably pick the Zeiss Abbe Orthoscopics, Astro-Physics SPLs or somesuch

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So what are generally considered to be the best Planetary EP's then?

THAT opens a whole can of worms!! As usual, in depends. In slow scopes the requirements are less strict, and the focal lengths needed longer (7-10mm on my F/10 scope), on fast scope you will need say 3.5-5mm (for an F/5 scope). If you do not need long eye relief, the contenders include

- Zeiss Abbe Orthos (and several QUALITY copies)

- Monocentrics (smaller AFOV, bummer for Dobs)

- Brandon EPs

- just a single triplet APO lens for certain purists (or masochists, the AFOV and eye relief are really awful)

If you do need long eye relief the main contenders would be:

- TMB Planetary EPs (also sold as TS and various other brands)

- TeleVue Radians (I have one and LOVE it)

- Pentax XW.

The latter three all have larger FOV as well. The older Vixen LVs and their newer counterparts the NLVs are also very useful for planets, but the Radian beat my old LVs clearly.

The TMB Planetaries are quite a bit cheaper than the Radians (which are cheaper than the Pentax XW), but I do not know how they compare.

Michael

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The TMB Planetaries are quite a bit cheaper than the Radians (which are cheaper than the Pentax XW), but I do not know how they compare.

I've owned all three and would place the XW comfortably ahead of the Radian, which is itself comfortably ahead of the TMB. The reason I don't currently have the XW is partly cost - at a touch under £100 used the Radian hits the right value for money spot for me - and also size, the XW is a much bigger eyepiece. But with my AP130 under good seeing, the planetary performance of the 5mm XW was stunning.

The TMBs offer good performance and brilliant value for money, but Radian's they aren't.

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Recently I have been using my 6mm Ethos and 3-6mm Nagler Zoom for Jupiter and this has got me wondering if there is anything SIGNIFICANTLY better or would we be talking about such small increases that I would need to be mad to spend more ££££

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In my opinion

is there is anything SIGNIFICANTLY better

No.

or would we be talking about such small increases that I would need to be mad to spend more ££££

Yes.

But that's my opinion, your mileage may vary. Would suggest trying out some very good ones at a star party one day, that cured me of any desire to use anything that's more contact lens than eyepiece. :o

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if there is anything SIGNIFICANTLY better or would we be talking about such small increases that I would need to be mad to spend more ££££

A worthwhile read is Bill Paolini's 6mm Lunar/Planetary Eyepiece Comparison <click>, and I think the first bullet point of his conclusion answer your question:

  1. Very good atmospheric seeing and transparency are required to bring out many of the rankings and differences noted. On multiple evenings which did not prove good enough to conduct the evaluations, all eyepieces generally showed little difference between them.
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I think I must be a Quasi-Purist then! I love using Ortho's for planetary viewing, but sometimes the extra comfort and field of a Pentax XW or my 13mm Nagler T6 kicks in, and I know I can resort to using them with little or no loss in detail. It can depend on the scope being used too.

Ant

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It all depends how much you want to extract any last possible bit of detail particularly when seeing is very good.

There is a reason why, say, a zeiss monocentric costs $800.

one, its very rare, and two, it has the best possible sharpness and contrast, coupled with very high transmission and exceptional lens polish.

As stated, the differences can be subtle in normal seeing, but for example, a zeiss jena ortho will clearly best a circle T, in superior scatter control and contrast.

Without in any way meaning any offence, alot of Nagler, XW, and similar high end widefields have never had the experience of using Zeiss jena, Zeiss Abbe,Supermono, Pentax orthos, or AP SPL's.

These are eyepieces optimised for bringing out very subtle planetary detail, and as such are valued, which drives their used prices very high.

As i say, Nagler, XW, XL are very fine, but for the specialised role of planetary use, there are better.

Mike

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I bought a SkyWatcher 9mm long eye relief EP with 66deg FOV, but I sent it back after looking at Saturn. The internal reflections were horrible. Now I have a TS Planetary HR 9mm and I have hardly any reflections and a nice sharp image. But I don't just use it for planets, it is my only 9mm EP.

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Without in any way meaning any offence, alot of Nagler, XW, and similar high end widefields have never had the experience of using Zeiss jena, Zeiss Abbe,Supermono, Pentax orthos, or AP SPL's.

I've not used Zeiss Abbe's, but I've used the Pentax XO and AP SPL with a TEC 160FL in Pickering 9+ seeing in Arizona. And personally I'd use the XW any day. But that's because i'd prefer an extended spell at the eyepiece to catch those moments when the seeing turns (even more) perfect, I really don't like the (very) limited ER of high powered Orthoscopics or the SPLs and don't find them comfortable to use for long. So overall I do better from a complex design with comfortable ER. But YMMV.

Alongside the absolutes of transmission etc., there's a strong - and quite possibly stronger - element of personal preference. Threads like this are a fun debate for eyepiece fans, but all you can really do is try out as much as you can under both 'typical' and 'best' conditions, work out where your budget goes, and make up your own mind.

Edited by Ben Ritchie
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Now I am surprised!

And what do "purists" know??:o

I suppose "purists" wouldn't speak to us Dob drivers at a star party because we don't do Goto!

Then that would be their Loss mate, and not a worry to you should it be.

Ron.:)

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Ben, I envy you your spell with the TEC160 and the Pentax in Arizona!

What is a lttle crazy here is that the simple, dedicated planetary designs - planetary in having few elemants, limited ER and narrow field - ought to be cheaper than the complex widefield designs. But, being rare, they seem not to be!

Olly

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Ben, I envy you your spell with the TEC160 and the Pentax in Arizona!

The 20" (or was it 24"?!) Obsession trumped it :o

Part of my immediate family's from the desert south of Tucson, AZ and the desert there gives a perfect combination of mag 6.5+ skies, transparency and near-perfect seeing. Problem is that it almost kills UK astronomy for me, as you come back to the haze and light pollution with a sense of "why do I even bother".

What is a lttle crazy here is that the simple, dedicated planetary designs - planetary in having few elemants, limited ER and narrow field - ought to be cheaper than the complex widefield designs. But, being rare, they seem not to be!

Worth reading Roland's comments on the SPLs and why they aren't likely to make any more - sounds like the effort in getting that last fraction in performance isn't easy

Edited by Ben Ritchie
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I tend to prefer the view through the Ethos for everything with my dobs, including planets. it's just so easy to use, the contrast is superb and the sharpness great. for the money it should be. I think when people say this is better or that is better when looking at top end eyepieces of any kind, they should always point out that the differences are so small as to be down to personal preference rather than optical quality in most cases. I conceed that I have never looked through a good Ortho though, although I fear the ER would be unacceptable to me. That said, I love my Nagler 6-3 zoom, especially on the moon. maybe I should try an ortho some time.

the wide field of an Ethos is like a dream on a manual dob.

my next eyepiece will be a 10mm Radian and then it's a shoot out between that and my 8mm Radian : winner gets sold for the equivalent Ethos. :o

Disappointed with Radians - no way Jose. (obviously note the Mexican accent). :)

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Hi Ben,

Your point regarding comfort is perfectly valid.

Whilst i have used all of the specialised planetary eyepieces i mentioned, i accept that not everyone would like them.

The Pentax 5XO is a good example. incredibly sharp, but most cannot live with 3mm of eye relief. I can personally, but i see the point of view that alot would not tolerate that for extended viewing.

Mike

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