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DRT

£1,000+ for an Eyepiece!

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This makes me think of a new question.

My other passion in life is Vintage Port and fine wine. Yes, I know how to needlessly spend money on things made of glass. Experience has shown me that fine wines benefit from being tasted from expensive glasses. The science or BS behind that is irrelevant to this discussion, but the principle is important.

Eyepieces and Telescopes are to astronomy what wine and wine glasses are to wine tasting. I find no pleasure in tasting a fine wine from a cheap and nasty wine glass. So is there any point in putting a £300 eyepiece in an entry level telescope?

Yes, alas there is. I have an amateur-made 20 inch Dob, F4.1, and I never make extravagent claims for it. It wasn't expensive and is, if you like, an entry level half metre scope. But - and here's the 'but' - when you put a Nagler or Ethos in it, it leaps up two grades. The top EPs clean up the edge of field no end. A plossl tends to look like a hangover seen from the inside but an Ethos looks terrific.

Olly

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That is interesting to know.

When you say it wasn't expensive, what ballpark are we playing in?

My original question came from my own experience of putting an Ethos 13mm in a £50 70mm travel refractor. I suspect your 20" mirror cost more than £50? ;-)

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I remember asking a salesman in a camera store if I could try my Vixen LV eyepieces in a cheapo F/4.3 Newtonian they had on offer, rather than the horrible plastic Huygens EPs it came with. He said OK, and when I showed him the result his jaw dropped. Objectives are generally simpler optical systems, compared to EPs, and hence are easier to get right cheaply. Many cheap scopes sport decent objectives but poor EPs.

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Great info, Michael.

So how does it go when you pair a very expensive scope with a cheap EP? Does one compensate for the other or is the EP the limiting factor?

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Just another thought.  There is one more reason why a cheap objective is generally OK, whereas cheap EPs aren't: Angle of incidence variations. A scope may have a true field of view of a mere 5-6 degrees (which is already stretching it for many, especially cheap ones with 1.25" EPs max). This means you only need to correct for aberrations over 2.5-3 degrees from centre axis (half the TFOV). Even a cheap Huygens EP has 25-30 deg FOV, and so it has to be corrcted for angles up to 12.5-15 degrees. Plossls need correction to 25 degrees, and Erfles  and SWAs even more. This is much harder to achieve, as aberrations like astigmatism rise very rapidly with angle of incidence. A cheap EP with more optical surfaces than the objective, and large maximum angle of incidence (or more properly angle of exit, but that does not matter), coupled to a good scope will probably fare worse than a cheap scope and a good EP. As a rule of thumb, the total width D of the central peak of the airy disk is roughly:

D = (D2objective + D2eyepiece)1/2

where Dobjective is the airy disk peak diameter of the objective (diffraction limit puts a lower bound on that, any deviation is caused by aberrations) and  Deyepiece is the spot size as seen in the EP gven a perfect point image. So to get the final diameter you sum the squares of these two contributions and take the square root of the result. It is not too hard to get the contribution of the objective close to the theoretical lower limit. It is much harder in the case of the EPs.

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Derek,

Just settle for the best combination, expensive objective and expensive eyepieces you know it makes sense.

BTW Michael, I have had to read your replies a few times to understand them but I know you know what your talking about. So in truth a Lzos objective is a very expensive luxury compared to say a quality Chinese scope with FLP 53 or whatever glass elements.

Alan

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Derek,

Just settle for the best combination, expensive objective and expensive eyepieces you know it makes sense.

BTW Michael, I have had to read your replies a few times to understand them but I know you know what your talking about. So in truth a Lzos objective is a very expensive luxury compared to say a quality Chinese scope with FLP 53 or whatever glass elements.

Alan

Not quite, although upgrading from my Chinese-made APM triplet 80/480 to the LZOS 80/480 is going to do FAR less than upgrading from a basic Plossl to a Nagler or Delos. Going from ST80 to Chinese triplet or ED scope is a much bigger jump, of course. The impact of a good objective is certainly there, it is just that cheap objectives are less horrible than cheap EPs, and there is a physical reason for that. Especially in fast scopes, cheap EPs fail VERY quickly towards the edges (centre performance can be quite acceptable). An LZOS triplet is of course closer to an EP is terms of the number of optical surfaces involved, which pushes up the price. The other thing in bigger chunks of optics to consider is thermal expansion, which is less of an issue in EPs, due to their small size. Several cheaper triplets and quadruplets had pinched optics problems due to thermal expansion effects.

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Thanks Michael, I am just thinking of getting a 130mm quality Chinese APO as well, the Lzos it rather a lot of cash but I know it will be nice.

Sorry for drifting off topic,

Alan

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Eyepieces confuse me only one will give the optimum exit pupil for a particular scope so wouldnt it be better to have one eyepiece and lots of scopes of differing focal lengths.

Alan

What, a zoom telescope? :tongue:

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I spent pretty much $1000 on my 21mm Ethos. That's aussie dollars, but I guess it's the equivalent to you guys spending 1000 pounds over there. Can't say I've used the Ethos anywhere near as much as I could or should have, especially considering the price, but I don't regret having bought it. The beauty of this astro gear we buy is, whether expensive or on the more affordable side, it doesn't spoil. Go off it for awhile and you don't have to worry about whether it is still going to work when you come back to it some time later.

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Joves,

Welcome back, not seen you for a while. It is an interesting point you make about the 21mm Ethos, I don't use mine much either. At first I thought it was the fact that the 20mm Nagler keep jumping up at me but I think it's the weight of it that makes me avoid it. One of those in you pocket and your trousers could fall down.

Alan

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I've been the same with my 21mm Ethos until recently when I've made a point of deliberately using it more often. It's finer points are starting to grow on me more and more as a get more familiar with it  :smiley:

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Seems strange 3 of us have an eyepiece to die for and all of us could do with using them more often. I have just put the M/N 190mm out and guess what the 20mm Nagler is out there, I need to be put down.

Alan.

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Better 1k on some quality glass than some desinger handbag :)

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Better 1k on some quality glass than some desinger handbag :)

Yeah, that last one really wasn't your colour.  :D

James

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Eyepieces confuse me only one will give the optimum exit pupil for a particular scope so wouldnt it be better to have one eyepiece and lots of scopes of differing focal lengths.

Alan

some might say im doing this oops

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Of course you need the right eyepiece in the right scope to do the job, this leads to a large ammount of combinations and piles a dust gathering gear as in theory ity never pans out like that.

I have one eyepiece which is a 40mm tv plossl, does the trick for me when i get the urge.

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The UK prices of Explore Scientific eyepieces are quite a bit higher than they can be bought for in other parts of the world. Nevertheless the 120 degree 9mm is still listed at $900 in the USA and there may be additional local taxes to add to that. 

The Nikon Nav HW eyepieces are a similar price:

http://www.apm-telescopes.de/en/eyepieces/more-74-ultra-wide-angle/other/nikon-nav-hw-17-mm-eyepiece-corrector-eic-14.html

I've never spent that much on a single eyepiece. My Ethos 21 is my most expensive but it was bought used from an SGL member for considerably less than these prices !

Just found this one on the same site as above/original post... http://www.apm-telescopes.de/en/used-Zeiss-products.html ...and advertised as 'used'.

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Something makes me think there will not be too many of the first one on the list about, some nice eyepieces on the site, the TBM supermonos are expensive and they only offer a very small FOV, I would not mind trying one but at that price I will not have a problem keeping the finger away from the button.

Alan

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The supermonos also require eye surgery to use at the shorter focal lengths.

Don't you just lick it and stick it to your eyeball? :D

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Don't you just lick it and stick it to your eyeball? :D

You first have to create a smooth slightly concave interface for best results, eliminating yet another air-glass surface :D

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I've owned a 5mm TMB Supermonocentric. The eye relief is actually a touch better than an ortho of that focal length. The eye lens is tiny and the AFoV was 30 degrees as I recall. It was one of the 2nd run TMB made and remains the best performing short focal length eyepiece that I have ever used, albeit by a small margin over the University HD 5mm I also had. Tracking at high power with undriven alt-az mounts and a 30 degree field is, er, challenging !

Back then they changed hands for around £100. I see from the APM site that some inflation has occurred :rolleyes2:

post-118-0-87362900-1410469491.jpg

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Don't you just lick it and stick it to your eyeball? :D

Yup, that's how we do it :D

Actually the 1.8 barlow that comes with them is superb so I tend to use the barlow most of the time which ups the eye relief. Not everyone's cup of tea though.

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