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Thread: What's a great eyepiece upgrade for a 10" dobsonian?

  1. #1
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    What's a great eyepiece upgrade for a 10" dobsonian?

    Hi,
    I think I'd like to get something like the Orion XT10 classsic Dobsonian as an entry into DSO observing. I've thought seriously about other designs but the 10" Dob seems perfect for what I want to do and my circumstances.

    We live out in the woods and just want to star gaze a bit.

    Maybe later I'll gets rious about a refractor and an excellet equatorial mount... but not now. :-)

    One thing I'm very interested in is the idea of collecting a few top of the line 2" eyepieces to go with the Dob. It seems like I can always use them with other scopes later.

    I'm looking at 2" Televue eyepieces at the Orion website and I'm open to investing in a few... one a time probably :-)

    I understand that these eyepieces might cost more than the reflector assembly but that doesn't seem too crazy to me.

    I don't mind spending extra to get wide field viewing and lots of comfort with regards to exit pupil and eye relief.

    Does anyone have any thoughts to share about that approach to getting started?

    Maybe there's a favorite eyepiece to consider?

    Perhaps there's a combo that seems useful but isn't in practice?

    thanks very much,
    mike

  2. #2
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    I have the XT10 (great scope by the way), and the eyepeices I use most are these two:

    1) Orion 'Stratus' 30mm for low power, wide field scanning. I'd link to it but either the Orion site is having problems, or they've drastically reduced their eypeice selection. I like this eyepiece for it's price/performance ratio. It's a 2" eyepiece and gives me 40 power and 1.7 true field of view. I've tried the high end Televue ep's, and they are nice, but IMO I get 80% of the Televue performance for roughly half the price.

    2) Vixen Lanthanum Wide 17mm for DSO's, esp. galaxies. This eyepiece yields about 70 power and has a wide enough field that DSO's are well resolved and nicely framed. This is my favorite eyepeice in my collection. Link: http://www.optcorp.com/product.aspx?...-6811&kw=&st=0 Link to the Vixen Lanthanum line: http://www.optcorp.com/productList.a...=30-718-76-813

  3. #3
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    Thanks for the detailed response and the links.

    Are there any sizes to avoid? Can you go too wide?

  4. #4
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    Quote Originally Posted by mike_mccue View Post
    Thanks for the detailed response and the links.

    Are there any sizes to avoid? Can you go too wide?
    I wouldn't go any higher than a focal length of 30mm in a 10" scope, unless you're very young, say early 20's. The reason I say that is because you don't want too high of an exit pupil. Exit pupil is the size of the light cone that passes out of the eyepiece into your pupil. Your pupil can dialate out to a maximum of 7mm or so when you're young, but goes down to 5 - 6mm as you age. You don't want the exit pupil to exceed the maximum diameter of your pupil, that will just be 'wasted' light. In my experience, having too wide an exit pupil can result in loss of contrast.

    To find the exit pupil of a given eyepiece, divide the aperature of your scope by the magnification of the eyepiece in question. For example, my 30mm eyepiece yields 40 power, since my scope's focal length is 1200mm. Divide the aperature of the XT 10 (250mm) by 40, you get an exit pupil of 6.25mm.

    My eyepieces are the following focal lengths: 30, 25, 17, 9.5, and 7.5mm. I also have a pretty good barlow.

  5. #5
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    You need to read up on eyepieces. There are several primers on the net. As power goes down the image contrast does as well. Most people see highest contrast at about a 1mm exit pupil. Lowest power is where the exit pupil matches your eye's pupil. For a young eye that's at about 7mm, middle age about 5mm. Lower power may give a wider FOV but it won't be any brighter and the secondary shadow can be an issue. So with reflectors it is best to avoid going any larger. Highest useful power depends on the atmosphere but even when perfect 0.5mm is likely the limit.

    Notice I said nothing about focal length. Exit pupil is universal to any amateur sized scope. Power is not, its specific to certain focal lengths. Exit pupil is the focal length of the eyepiece divided by the f ratio of your scope. Yours is f/4.7 but to keep the math clean here I'll use f/5. The difference is immaterial. So the eyepiece for a 5mm exit pupil would be 25mm (5mm x f/ratio), 7mm would be 35mm (7x5), 1mm would be a 5 mm eyepiece.

    Notice I haven't mentioned how wide the field is. That has nothing to do with the contrast or brightness of the image. Width of field is determined by the design of the eyepiece and the size of the lenses. You can only see the image that fits down a 1.25" or 2" tube. Since the inside diameter is smaller make that about 1" or 1.75". This limits a 1.25" eyepiece to about a 1.2 degree true FOV (25mm opening x 57.3 / 1200; 25 being the max opening of a 1.25" eyepiece, 57.3 degrees in 1 radian and 1200 the focal length of your scope). For your focal length that happens about at about a 30mm eyepiece with a 50 degree FOV (40/50=1.25; 40 being the power of a 30mm eyepiece in your scope and 50 degrees being the eyepieces apparent FOV, this is approximate as eyepiece distortions may make minor changes here but its close). So for a wider field at 30mm focal length you will have to go to 2" eyepieces and more costly designs than the plossl.

    Since contrast goes up with higher powers, many prefer to achieve the max FOV by using more expensive eyepiece designs than the plossl to get a 65, 70, 80 or even 100 degree FOV. Televue has a good discussion of this associated with their Ethos line.
    http://www.televue.com/engine/page.asp?ID=312#majesty
    While I'd strongly argue against an Ethos at this stage in your career and this is an ad for the Ethos, it does do a good job of covering how contrast and power should enter into your eyepiece selection, not just the field of view.
    Also see:
    http://www.televue.com/engine/page.asp?ID=277
    http://www.televue.com/engine/page.asp?ID=131
    http://www.televue.com/engine/page.asp?ID=221
    Again they are pushing Tele Vue but it applies to any brand of eyepiece.


    But for a beginner simple plossls are often a good inexpensive choice. Later, with experience as to what you like and dislike in viewing requirements you can move up to more expensive designs. I still do 90% of my viewing the my 10" f/5 using 40 year old orthoscopic eyepieces with a 45 degree apparent FOV. My exception is a 20mm erfle, a design not really seen today though many of the 65 degree eyepieces of today are based on improvements in its design. We've had good luck at Hyde Memorial Observatory, where I was supervisor for 27 years, with Orion's Expanse type 65 degree eyepieces. Yes, there's some distortion at the edge of the FOV not seen in far more expensive designs. But most find it not much of a problem. A coma corrector like a Paracor will go a long way to fixing that. Also it will vanish when using a barlow for high power work. There a wide field is especially useful on an undriven Dob.

    Eye relief is an issue at a public observatory and a wider FOV useful but our budget limited. If you wear glasses (no contacts) eye relief is important as if it isn't sufficient the full FOV of the eyepiece can't be seen. Shorter focal length eyepieces often (not always) have shorter eye relief. Some of today's designs keep eye relief constant at 20mm or keep it at least within what a typical eye glass wearer can handle. Those without glasses sometimes feel thats too far and can't old their head in the right spot. Such eyepieces often come with roll down or adjustable eye cups that hold the non eyeglass wearer's eye in the right place.

    For higher powers I prefer to use a barlow rather than high power eyepieces. A good 3 element barlow can double your eyepiece collection. I use the Klee and also have a PowerMate, each has its place. Pick your eyepiece and barlow such that you don't duplicate powers!

    Rick

  6. #6
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    Thanks very much for the detailed reply. I'll reread it a few times... especially about the field of view calculations.

    I have been reading eyepiece primers and I have some experience with Optics.

    I suppose I'm being to casual about the contrast issue... I would have said that I prefer a bit wider exit pupil for comfort... I don't think of it as wasted light, but now I see that it's at the expense of contrast. Thanks!

    I have seen some casual references about reflectors scopes and the secondary mirror vane. I get the impression some combinations of eyepiece and scope specs result in either visually noticeable distortion or apparent obstruction.

    That was why I thought perhaps you could go too wide? Maybe it's the opposite? Maybe these problems show up if you go to tight?

    Should I take concerns like that seriously?

    best regards,
    mike

  7. #7
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    You can have eyepieces that are too low powered to be useful with a given scope, and eyepieces that are too high powered to be useful with a given scope. But neither of those really has anything to do with the secondary and spider.

    Edited to put in my nomination for the Panoptics. I have the 27mm in the 2 inch size and like it a lot with my 10 inch Dob. I also have a 35mm Panoptic, but my scope is f5.6, and if you are going f4.5 you may not want to go any lower than the 27mm.

  8. #8
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    Thank you for sharing examples that you enjoy.

    The links to the Televue eyepiece articles have been really helpful... I just read the Ethos and Eyepiece primer and now I see they have one dedicated to small Dobs and I'm about to dive in and read it.

    edit to add: when you say "lower than 27mm" do you mean lower magnification? In other words something like a 40mm or 55mm would be wasted because of the oversized exit pupil?

    best regards,
    mike
    Last edited by mike_mccue; 2009-Feb-02 at 03:25 PM. Reason: add a question

  9. #9
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    Yes, refer back to Rick's discussion of exit pupil.

    And when I said lower, I meant lower power.

  10. #10
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    Ok then :-)

    I think I just had an aha moment. You can easily take the f stop figure and multiply that by any suggested exit pupil measurement and determine the focal length of the eyepiece.

    That makes it seem easy :-)

    4mm x f4.7 = 18.8mm
    5mm x f4.7 = 23.5mm
    6mm x f4.7 = 28.2mm
    7mm x f4.7 = 32.9mm

    I noticed in one of the Televue articles that Mr Nagler referred to an ideal collection of eyepieces as "having one of each exit pupil".

    Some how the phrase and my aha moment seem related :-)

    I'm still having some trouble understanding the relationship between the Nagler features and the Ethos features.

    How does it work in real life? It seems like for practical purposes when you finally buy the fancier glass that for any comparable true field of view the nicer glass will replace the lesser. It seems like each level in design family/price point provides more magnification, a wider field of view and more comfort.

    Is that a reasonable assumption?

    Can one predict where they overlap in suitablity? In other words What Nagle replace what Panoptic? What Ethos replaces a certain Nagler? For example; does a 17mm Ethos replace (I'm just using a famliar name here) a 26mm Nagler because they have a similar true field of view but the Ethos is more comfortable to view?

    I've been looking at the Panoptics and Plossils as well... but the Naglers and Ethos fascinate me. The idea of reasonably high magnification AND field of view seems like a great feature. Am I missing something?

    best regards,
    mike

  11. #11
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    Quote Originally Posted by mike_mccue
    I've been looking at the Panoptics and Plossils as well... but the Naglers and Ethos fascinate me. The idea of reasonably high magnification AND field of view seems like a great feature. Am I missing something?
    Mike, I just came into a Nagler 9mm Type 1 (the new ones are Type 6's), and the main thing that's different on them from most other wide-field eyepieces is the sheer size of the field stop. If my eye is positioned juuuust right, I can see almost the entire field (I can glimpse the field stop through the now-fuzzy edge of the eye-lens). More realistically, you'll have to move your eye around to see the entire field. With the Ethos, you'll definitely have to move your head around to see everything.

    Since the Ethos and Nagler you're comparing give you roughly the same true field, the main difference is obviously the magnification offered. The Ethos' image will appear about 1.5x as large as the Nagler's.

    I'd follow other posters' advice here and try before you buy. You may find that you don't like the Ethos as much as the Nagler or Panoptic, for example. Or you might want the Ethos to the exclusion of everything else. And especially find out what you really like to observe first, before making any purchases. If you like planets, the Radian line or similar are better choices than Tele Vue's other lines. For DSOs, things like the Nagler and Panoptic, depending on the object, are better.

    Quote Originally Posted by redshifter
    I'd link to it but either the Orion site is having problems, or they've drastically reduced their eypeice selection.
    The Stratus line now only goes up to 24mm.

  12. #12
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    Thanks for making it seem easier to understand the choices.

    I'm sure I appear precocious with my interest in the fancy glass...

    FWIW, I work in TV and Film production so the budget I'm considering on enjoying this seasons good skies seems familiar and comfortable compared to the money I have tied up in my Cameras, lenses, and all the support equipment.

    It also happens that I make a hobby of high accuracy target shooting... that's a sport where people are desperate for an adequate exit pupil... the primary objective of a rifle scope can only get so big for practical purposes so at higher magnification they get difficult to sight thru. Many people can't even find the light in the scope I use for my practice.

    My point is that from these perspectives I can see a lot of value in having a telescope eyepiece that had the features of Naglers and Ethos models for reasons of comfort and enhanced casual enjoyment. I don't think I'd mind having to move my eye around a bit.

    I'm really interested in looking at DSO for sheer entertainment and the joy of learning about the sky map. I live in a National Forest with dark skies... I can view the southern sky from backyard. It's just seems like this could be a really rewarding "keep it simple" hobby for overly techie guy like myself.

    I don't really need another technical hobby that requires a lot time considering gear upgrades as I learn why the next best thing is better than what I already have... my job is just like that :-).

    It seems I really should try before I buy... thanks for the good advice... I hope I can be disciplined about it :-)

    Thanks very much to all who responded.

    best regards,
    mike

  13. #13
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    I'm realizing that I don't really understand the relationship between apparent field of view and true filed of view.

    I'm learning that something like the TV Panoptic 41mm has a wider true field regardless of the lower apparent fields in the other designs.

    more to learn :-)

  14. #14
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    Apparent FOV is the field you'd see at 1x. It's just the angular size of the "window" you are looking through. Some designs can support a bigger apparent field of view than others. Design gets more complicated the wider the apparent field of view requiring lots more elements and larger lenses so cost. size and weight go up rapidly. Many scopes need counterweights to handle the weight of these eyepieces.

    True field of view is just the area of the sky you see. It is about equal to the apparent field of view divided by the magnification the scope is running at. So at 50x you see about a 1 degree true field of view in a plossl with a 50 degree apparent field of view. With the Ethos you'd see a 2 degree true field of view and thus a 4 times greater field by area.

    With an undriven scope a wide field that is also very sharp across it is an advantage as you don't need to move the scope as often but you pay dearly for this! I find a good dob so smooth and solid this isn't a problem at all, others disagree of course.

    At a club star parties you can see a wide variety of eyepieces. It will save you a bundle when buying eyepieces if you find you are happy with a basic plossl. The one professional astronomer in our club still uses Kelner's from the 50's and sees no reason to spend more money. Others have over $5000 in their eyepiece collection. Both see the same thing however, one just looks through a bigger picture window.

    Rick

  15. #15
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    Thanks again for all the great advice.

    I just impulsively purchased a Zhumell brand 12" Dobsonian and it will arrive next week. I'm comfortable with handling the 12' size and I'm thinking it will be a fun way to start looking at DSO.

    I made this chart up using the Televue eye piece calculator:



    and it really seems helpful to me. I thought perhaps the idea might be useful to others.

    best regards,
    mike

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