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Thread: Help With Getting The Right Eyepieces

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    Default Help With Getting The Right Eyepieces



    I have a dobsonian reflector with a 254mm (10") aperture, a 1250mm focal length, and a focal ratio of f/4.92.

    While viewing Mars tonight with a 9mm 1.25" eyepiece, a 2x Barlow + a 1.5 Barlow, it seemed very blurry and star-like. This also occurred when I removed the Barlows and just left the eyepiece in the scope. If I switched to a 2" 30mm eyepiece with the 2x and 1.5x Barlow, the planet was very clear, but it was very small. What should I be doing to get a better and more clear view? Do I need a new lens?

    I have no problems with Saturn/Jupiter/Venus, just Mars.

    I frequently collimate the scope, so that is not the problem.

    If anyone who has a similar scope views Mars well, could you let me know what your setup is?

    (the links have to be copied because it won't let me put [img][/img] tags.)
    Yes; I understand I'm not going to be getting images like this:
    upload.wikimedia.org/wikipedia/commons/thumb/7/7d/Mars_atmosphere.jpg/250px-Mars_atmosphere.jpg

    But I'd at least maybe something like this:
    science.nasa.gov/media/medialibrary/2007/11/21/21nov_marsdoubles_resources/Friedrich-Deters1.jpg

    Thanks!

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    Default

    Hi Hawker14,

    Probably the main problem is the 9mm eyepiece, (cheapy supplied with the scope?). The 30mm with the mentioned stacked barlows, should yield a magnification of 125x.Good for Jupiter and Saturn but not enough magnification for the best viewing of Mars).

    A good reasonably priced eyepiece(I would recommend) to get that extra magnification is made by GSO. The GSO 1.25"15mm Superview 70deg FOV is of good quality, nice wide field of view, and its overall performance is the equal of some other oculars at 2x to 3x the price. With the 2x barlow you will get 167x, and with your stacked barlows, 250x(image of Mars will be of a reasonably size), you will have an exit pupil of 1mm. (This is ok for planetary viewing).

    Stephen.(44deg.S.)

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    Default

    Wow, thanks for all the info.

    What do you think about just going with a Vixen Optics NLV Lanthanum 2.5mm Eyepiece (1.25"), which would give me my max capabilities with my scope?

    I'm really looking to get the biggest picture with the clearest view, and I know that as you get to a scope's maximum magnification, things start to get very blurry. Should I go with the 2.5mm (which would give me x500), or should I go with the Vixen Optics NLV Lanthanum 5mm Eyepiece (1.25") (which would give me x250 + the barlow would take it to x500)?

    Also, with planetary viewing, would it be useful to have a 2", or should I just stick to the 1.25" eyepieces?

    Thanks!

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    It would be a bad idea to buy an eyepiece that maxes out your scope. That level of magnification has a couple of negative effects. One, atmospheric turbulence rarely allows for observation at over 300x -- at that point you're just magnifying the blurriness, not bringing out any more detail. Plus, a traditional 2.5mm EP (Plossl, orthosocopic, monocentric, etc.) is only going to give you 2-3mm of eye relief -- you'll literally have to squish your eyeball onto the top of the EP to see anything. The newer complex designs give you better eye relief, but do so by adding lots more lenses, which (can) reduce image contrast on bright objects like planets.

    Plus, with a 2.5mm EP and that 10" f/4.9 scope, you're looking at a .5mm exit pupil. At that size of exit pupil, the image will be dimmed, and "floaters" in your eyes will become distracting.

    My recommendation, if you want to have the best planetary viewing experience, is to purchase a quality eyepiece with a focal length that matches your f/ ratio. This is really the practical maximum for your scope, as it yields a 1mm exit pupil and 255x magnification, both of which are within the limits of quality visibility. On nights of superb seeing, you can add the 1.5x barlow to jump to 383x and a .66mm exit pupil. On nights of outstanding seeing (like once in a lifetime) you could add the 2x barlow and go to 510x and the .5mm exit pupil (but you still have to deal with the annoying floaters with that tiny of an exit pupil).
    Last edited by skaven; 02-08-2012 at 04:09 PM.

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    Quote Originally Posted by hawker14 View Post
    Wow, thanks for all the info.
    Also, with planetary viewing, would it be useful to have a 2", or should I just stick to the 1.25" eyepieces?
    A 1.25" EP is fine for planetary viewing. The area of light you're gathering is scarcely larger than a star. The 2" barrels are for capturing the maximum amount of light from wide-field views (such as extensive nebulas). As you ramp up the magnification, the effective light cone (the part of the light cone you are looking at) gets narrower, and that's why you see the higher magnification EPs sold in 1.25" formats. The 2" models are usually just 1.25" models with a screw-on adapter so they fit in a 2" focuser.

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    You might also find this link helpful: N.A.A. Telescope Math Calculator

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    If you get good seeing conditions where you live, it would be good to get an eyepiece that pushes your scope to the limit. For the 5-10% of the time the conditions are right to use it, you will be thrilled. For the 95% of the time though they just collect dust. So dont make it a priority unless you have extra funds laying around. Maybe not 500x for the 10inch but 400 or so.
    15 inch Obsession
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    High Power: Televue Delos 6mm ~285x @.25° : ES 4.7mm ~365x @.22°

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    Is the focal length of the lens what is usually listed (5mm, 9mm, etc), or does it have to be calculated?

    If it is in fact the focal length, from what you all have been telling me, I'll probably go with either a 4mm or a 5mm (1.25" or 2"). Which brands would you all recommend?

    Right now, I'm looking at either:

    The Tele Vue:

    Radian 3mm Wide Angle Eyepiece (1.25")
    Radian 4mm Wide Angle Eyepiece (1.25")
    Radian 5mm Wide Angle Eyepiece (1.25")

    Nagler Type 6 3.5mm Wide Angle Eyepiece (1.25")
    Nagler Type 6 5mm Wide Angle Eyepiece (1.25")

    What's really making it hard for me to decide is: Do I want higher eyepiece that I can use a barlow with and maybe get a slightly less clear view, or do I want a lower eyepiece that will probably be more clear and magnified, but I don't have the option of reducing magnification?
    Last edited by hawker14; 02-09-2012 at 02:30 AM.

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    For planetary viewing, the Radian is going to be better. But in general, people who spend a lot of time doing planetary observation stay away from the complex designs that you get from a Radian or Nagler. Planets are extremely bright compared to something like a nebula, and all those lens surfaces scatter a little bit of light. With a Radian or Nagler, you're looking at as many as 12-14 lens surfaces, each of which are scattering a little more light. The result is reduced contrast.

    Do any Google searches for "planetary eyepiece shootout" and every time you'll find that the simple Orthoscopic and Monocentric style eyepieces win (usually the ridiculously expensive ones from Zeiss and Pentax). But there are cheaper alternatives that are reportedly just as good (full disclosure: I've been doing my own hunting for the best planetary EP, but have not yet purchased one of these myself).

    The University Optics Orthoscopics always fare well in shootouts: Eyepieces - 1 1/4" Oculars from University Optics

    The Baader Genuine Orthoscopics review well as well: Baader Genuine Orthoscopic Eyepieces - OPT Telescopes

    I've also gotten a recommendation that the TMB Planetary series is nice, though it's a complex design that leaves me leery: TMB Planetary II series

    Note that all of these eyepieces are in the "planetary" category. That means they will tend to have narrow fields of view (40-50 degrees) and will exhibit aberrations when observing objects off-axis (especially in a fast scope). They're designed expressly for observing tiny, bright targets that you place dead center in the field of view of your scope. Thus, they aren't much use outside that specific use case.

    If you're looking for a high magnification eyepiece that you can use in other situations, such as observing the Trapezium in M42, you will be extremely happy with a Nagler or a Radian, both of which will be fantastic for all-around observing. It's surely going to be a marginal improvement from a Radian to an Orthoscopic (especially one that is 1/3 or 1/4 the price of a Radian) when doing planetary viewing.

    If you've got enough $$ that Radians and Naglers are within your budget, I recommend you get the best of both worlds -- buy an Explore Scientific 82 degree eyepiece at around 5-7mm for general-purpose high magnification observing, and you'll still have enough cash left over to get a nice 5mm Orthoscopic. The Explore Scientific 82 degree series eyepieces are high quality clones of the Televue Nagler line.

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    Quote Originally Posted by hawker14 View Post
    Is the focal length of the lens what is usually listed (5mm, 9mm, etc), or does it have to be calculated?
    Yes, the focal length of the eyepiece is the advertised "length" in mm.

    Quote Originally Posted by hawker14 View Post
    What's really making it hard for me to decide is: Do I want higher eyepiece that I can use a barlow with and maybe get a slightly less clear view, or do I want a lower eyepiece that will probably be more clear and magnified, but I don't have the option of reducing magnification?
    That's completely up to you. There are folks out there that like to have a full complement of eyepieces and avoid using barlows. Then there are those that swear by barlows and own several (1.5x, 2x, 3x, 5x) and only have a few long focal length eyepieces that they amplify with the right barlow for high magnification viewing.

    Personally I think that both extremes are ... well ... extreme. There are benefits to using an eyepiece that is the precise focal length you want (fewer lens surfaces, less image dimming, better contrast). But there are also benefits to using a barlow (better eye relief, multiplication of available eyepiece focal lengths). And good quality barlows really don't adversely affect the image all that much.

    So if you're looking for the absolute best image you're ever going to get of a planet, then I'd say definitely invest in a high quality orthoscopic eyepiece at ~5mm and not bother with the barlow (except under rare superb conditions). But this setup will *only* be useful on planets (and the moon, and splitting double stars).

    If you're planning on looking at *anything* else in the sky (which is very likely), then you should go for a more comfortable wide field complex eyepiece at a longer focal length that you can, on occasion, barlow up to the maximum magnification. With your ~f/5 scope, in my humble opinion, you should target the following EP focal lengths for the following targets:

    * a 30mm 82+ degree wide field EP (2" is essential). This is for wide-field hunting and for observing extended objects like M42, M45, M31 and M33 under dark skies. You will get a ~2 degree TFOV with this. Use a 2x barlow to bump this up to 15mm and ~1 degree TFOV for smaller clusters and nebulae. I recommend the Explore Scientific 82 or 100 degree lines for this.

    * a 9-14mm 60+ degree wide field EP (1.25" is fine). Use as-is for smaller objects like planetary nebulae, globular clusters, and some smaller open clusters. Use a 2x barlow with it when observing tiny planetary nebulae, splitting some double stars, and doing planetary observation of large planets like Jupiter. The Explore Scientific 82 degree line is good...I've also got the Meade Series 5000 HD-60 9mm and 6.5mm EPs and find them quite good at 1/2 the price (though only 60 degree AFOV).

    * a dedicated 4-5mm narrow field orthoscopic or monocentric planetary eyepiece specifically for high-magnification observation of Mars, Jupiter, Saturn, and splitting double stars. Use it with a 1.5x or 2x barlow under superb conditions to get the most detail you can out of bright targets, or to split stubborn double stars.

    So with a 2x barlow and just three eyepieces, you can pretty much cover all of your use cases. As you get more experience you'll inevitably desire some other focal lengths, but you can cross that bridge when you get there.

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