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Thread: Choice of Eyepiece

  1. #11
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    Default Re: Choice of Eyepiece



    I have the 3mm Explore Scientific. I too have an 8". I use it (the 3mm lens) mainly on the moon. Planetary use is pretty well restricted to those few and very rare excellent seeing nights. You've got to have excellent collimation as well. I *really* wanted to see Mars up close with the 3mm but alas, the weather local and on Mars prevented me. I used the 2" 9MM ES with great results. I also tried a 4.7mm (Don't recall the manufacturer of it) but it blurred so badly I quickly went back to the 9mm for all four of my "Mars Parties" I hosted for various outreach programs. <sigh> *IF* you have money to spare then, sure, buy the 3mm - but if not shy away from it. I use my 9mm for planetary almost all the time. The image may not be big but it is crisp and that counts more then size IMHO. Good luck in your choice!
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  2. #12
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    Default Re: Choice of Eyepiece

    Quote Originally Posted by Barney85 View Post
    My apologises, i was after maybe a 3mm eyepiece to get the most out of my telescope as that is the max it is capable of. Any lower would be pointless.
    No you have it backwards. A 3mm eyepiece is pretty pointless since the atmosphere will only be steady enough to use it well 2 or 3 nights a year. A 12mm is more generally useful. A 6mm eyepiece will be useful less often than the 12mm but much more often than the 3mm.

    It's not just the scope, the atmosphere is part of your optical system too! You can only very rarely get the maximum your scope can deliver.
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  3. #13
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    Default Re: Choice of Eyepiece

    Thank you for your thoughts. It’s exciting getting into this. Having forums like this makes it so much easier to get feedback from thing and to save me some cash. I discovered last night that my collimating was out which could account for my blurry images, except for the moon. Off to the shop today to get a hex set to adjust it. Hoping it solves my issue of being dissatisfied with images. I’ve read numerous other forums where people talk about getting 2.5mm-4mm to get up close to a planet. I didn’t think it would distort the image that much though like you guys have said.

  4. #14
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    Default Re: Choice of Eyepiece

    No, 32 years of visually observing planets means nothing for giving eyepiece advice....
    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
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    Ya gotta keep this Apo/
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  5. #15
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    Default Re: Choice of Eyepiece

    Quote Originally Posted by Gabby76 View Post
    No, 32 years of visually observing planets means nothing for giving eyepiece advice....
    I agree, youngster. I have over 60 years visually observing and clearly I have no basis for giving advice either.
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    Default Re: Choice of Eyepiece

    Quote Originally Posted by Barney85 View Post
    Thank you for your thoughts. It’s exciting getting into this. Having forums like this makes it so much easier to get feedback from thing and to save me some cash. I discovered last night that my collimating was out which could account for my blurry images, except for the moon. Off to the shop today to get a hex set to adjust it. Hoping it solves my issue of being dissatisfied with images. I’ve read numerous other forums where people talk about getting 2.5mm-4mm to get up close to a planet. I didn’t think it would distort the image that much though like you guys have said.
    The mistake you are making, and it is definitely a mistake, is thinking only about the focal length of the eyepiece as though the f-ratio of the scope doesn't matter. One needs to think about both to design a suitable set of eyepieces. Your scope has an f ratio of about f6. The maximum resolution obtainable with any scope is given by a 2mm exit pupil. That is a fact of human physiology. It is not a rumor on some board somewhere.

    To get the focal length of eyepiece that corresponds to a 2mm exit pupil you MULTIPLY by the f ratio, in this case 6. So 2mm x 6 = 12mm. You will not see any more detail in a shorter focal length. You will just make the same details look bigger. ALSO a fact of human physiology is that at an 0.5 exit pupil or less you start to see biological defects in your eye, like floaters, etc. Thats at 0.5mm X 6 = 3mm. So you run the risk with a 3mm of seeing more of the defects in your own eye modifying the image.

    So believe the rumors if you like. Some folks need to waste money to learn the lesson.

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    Default Re: Choice of Eyepiece

    Quote Originally Posted by not_Fritz_Argelander View Post
    ...

    To get the focal length of eyepiece that corresponds to a 2mm exit pupil you MULTIPLY by the f ratio, in this case 6. So 2mm x 6 = 12mm. You will not see any more detail in a shorter focal length. You will just make the same details look bigger.
    However when the seeing is good it is sometimes helpful to see those details a bit bigger, especially with planetary imaging. There is of course a loss of contrast the higher you go in terms of magnification, and while the resolution may max out at a 2mm exit pupil, I have often found that the magnification provided by a 1mm exit pupil is very useful to tease out detail, especially in my Apo refractor. This is a major reason that I have found my 3-6mm zoom so useful.

    BTW, not_Fritz: I would be interested in any research that you have found regarding exit pupil and maximum resolution. Is there any correlation with individual eye "defects" like astigmatism, where a smaller exit pupil (within reason) will give a sharper image if you aren't wearing glasses? Can you send any links please?

    All the best,

    Dean

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    Default Re: Choice of Eyepiece

    Quote Originally Posted by DeanD View Post
    However when the seeing is good it is sometimes helpful to see those details a bit bigger, especially with planetary imaging. There is of course a loss of contrast the higher you go in terms of magnification, and while the resolution may max out at a 2mm exit pupil, I have often found that the magnification provided by a 1mm exit pupil is very useful to tease out detail, especially in my Apo refractor. This is a major reason that I have found my 3-6mm zoom so useful.

    BTW, not_Fritz: I would be interested in any research that you have found regarding exit pupil and maximum resolution. Is there any correlation with individual eye "defects" like astigmatism, where a smaller exit pupil (within reason) will give a sharper image if you aren't wearing glasses? Can you send any links please?
    Better than links is logic based on data. The most recent best current expression of that comes in this thread

    Looking for a barlow

    Post #19 in the thread by j.gardavsky has all most folks need to know.

    Also on the biology of the human eye and how it affects telescope use see Articles: Clarkvision.com

    Also
    Clark, R.N., Visual Astronomy of the Deep Sky, Cambridge University Press and Sky Publishing, (book of 355 pages), 1990.
    is a wonderful resource.

    I'm beginning to feel like I'm repeating myself.
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    Default Re: Choice of Eyepiece

    If it were my scope I would look in the IceInSpace classifieds where MortonH is selling a mint condition Pentax XF8.5mm eyepiece for $150.....
    It is an excellent eyepiece, one of my favourites, in fact I use a pair in my binoscope.

    In an 8" SW Dob it gives 141x with exit pupil of 1.44mm and a true field of 0.44, just under the full moon diameter. It is a very very sharp eyepiece.

    Whereabout in Oz are you Barney?
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    Takahashi TSA102S F8.0

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    Default Re: Choice of Eyepiece

    Wow! I'm amazed at someone joining a forum, asking advice, and then disregarding the advice given. I've only been at this hobby for 6-7 years, but that's enough experience to know that I've never used my 7mm eyepiece with my 2X barlow (3.5mm performance) because the seeing conditions during the times I happen to be viewing have never been good enough to permit it. IMO, the OP would be wasting money chasing a 3mm.
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