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Thread: Choice of Eyepiece

  1. #21
    not_Fritz_Argelander's Avatar
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    Default Re: Choice of Eyepiece



    Quote Originally Posted by rwt View Post
    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.
    It’s not a problem though. Beginners don’t know and in particular don’t know how to distinguish good from bad intel. The seductive thing is power. Eventually it all works out.
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  2. #22
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    Default Re: Choice of Eyepiece

    Thank you not_Fritz,

    I have already forgotten about the post #19 in that Barlow thread.
    Should you allow a friendly joke with my very best intention and with the highest respect towards you,
    you have memory like an elephant in India.

    I forget everything, and always must re-invent nearly all with math, that's what the math is good for.

    Best,

    JG
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  3. #23
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    Default Re: Choice of Eyepiece

    Quote Originally Posted by j.gardavsky View Post
    Thank you not_Fritz,

    I have already forgotten about the post #19 in that Barlow thread.
    Should you allow a friendly joke with my very best intention and with the highest respect towards you,
    you have memory like an elephant in India.

    I forget everything, and always must re-invent nearly all with math, that's what the math is good for.

    Best,

    JG
    Thanks. JG. It’s not as good as it used to be though. In my youth after I entered the workforce I was nicknamed “Dr Memory”. I’d recount minutiae of meetings verbatim and cite documents including page numbers. Can’t do that anymore!

    Since I feel closer to 19 than 70 it can’t be due to age. Rather I think I just filled up the memory banks and didn’t clear enough useless data out.
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  4. The Following User Says Thank You to not_Fritz_Argelander For This Useful Post:

    j.gardavsky (08-20-2018)

  5. #24
    Don Alvarez's Avatar
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    Default Re: Choice of Eyepiece

    Quote Originally Posted by rwt View Post
    Wow! I'm amazed at someone joining a forum, asking advice, and then disregarding the advice given.
    I'm not sure that's the case. I think that post can be easily misread to seem that way, but I took as more "I believed x because I had previously read y, but now I have new information to consider."

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  6. #25
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    Default Re: Choice of Eyepiece

    Quote Originally Posted by Don Alvarez View Post
    I'm not sure that's the case. I think that post can be easily misread to seem that way, but I took as more "I believed x because I had previously read y, but now I have new information to consider."
    I agree. In fact I couldn't work out why anyone thought he was disregarding advice: I thought he was expressing gratitude... He is a (fellow) Aussie after all, and what he said made perfect sense to me!

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  7. #26
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    Default Re: Choice of Eyepiece

    Quote Originally Posted by not_Fritz_Argelander View Post
    The maximum resolution obtainable with any scope is given by a 2mm exit pupil. That is a fact of human physiology.
    I'll take your word on this nFA, but I'm sure I read somewhere that the maximum resolution of a scope was given by a 1mm exit pupil, which roughly corresponds with the aperture size in millimetres. I also believe (probably mistakenly) that this is related to the Dawes' limit in some way. Bear in mind that I'm an arts graduate though and physics and maths aren't really my forte.

    We may have had this conversation before ... my memory isn't what it was either lol.
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    Default Re: Choice of Eyepiece

    Listen to these folks. This is great advice. I'm only a year into this hobby, but they have never steered me wrong. I rarely have a need to go beyond 100X. With Mars in the sky, I've been up to 200X once or twice, but 100X-130X is a sweet spot for my taste and planets.
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    Default Re: Choice of Eyepiece

    Let me add another note. I have never gotten rotten advice off this site, however, Being in this for just a year, what I wish I understood a year ago is this:
    #1) I use around 60x power, more than any other, and an inexpensive, ($30-$35) 20mm GSO Plossl, gives me that (best view) in my (f8) scope.
    #2) Eye Relief is important to me. 13mm - 20mm is perfect for my eyeball comfort.
    #3) Don't under estimate the value of a good wide-angle low power eyepiece.

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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?

    All the best,

    Dean
    Hi All,

    Just a quick follow-up from the above. I have just been looking at the big five above me tonight: Venus, Jupiter, Saturn, the Moon and Mars (the latter 3 at around 60-75 altitude at the moment!). The atmosphere is quite steady so I dropped the TV 3-6mm into my Tak TSA102. This gives me magnifications from 136x to 272x, with exit pupil from 0.75mm to 0.38mm... I also tried my Tak LE 7.5 eyepiece: mag. 109x, exit pupil 0.94mm.

    There is no doubt the best contrast was at the lower magnifications. However, I found that I could make out the most detail on Jupiter (altitude was around 40, so getting a bit fuzzier) with the zoom at 5mm (163x, exit pupil 0.63mm). Saturn showed best detail for me at 5mm (163x) and 4mm (204x, 0.5mm exit pupil) in the zoom, and surprisingly Mars was easily best at 4mm (204x). In fact I could make out a dark line around the edge of the polar cap on Mars at 204x much easier than at 163x (the 4mm). The other surface features on Mars are still muted, no doubt due to the now-fading dust storm, but again I could see them best with the 4mm and 5mm. At the lowest power (the Tak LE7.5 at 109x) I could only just make out the polar cap as a bright spot, but I couldn't see any extra detail like the dark edge of the polar cap because the view was simply too small. I didn't bother with a 16mm eyepiece (2mm exit pupil) as I simply can't see that level of detail at 50x on the planets, even if the contrast might have been better. (I tried the 3mm, at 272x, but the atmospheric conditions weren't up to that sort of power: also I find that power/exit pupil is when I really start to notice the "floaters" in my eye.)

    So, the bottom line is that for me a 0.5mm exit pupil (204x) can work very well on the planets in my Tak refractor. A 1mm exit pupil (102x) is very sharp, but the planetary details are getting a bit small: eg. I can make out some detail in the GRS at 204x, but it is a lot harder at 102x, even though the GRS itself stands out a bit more due to higher contrast.

    Looking at some of the theory on resolution and best exit pupil/eye dilation size (thanks JG and not_Fritz for some of the links), some sources talk of a best pupil dilation from 2-4mm, with the best lateral resolution obtained at around 2.5mm (SD 0.66mm) but the spread of results is quite broad: with a large standard deviation of 0.66mm (see, eg: http://www.breault.com/sites/default...imal_pupil.pdf) Best resolution at this eye dilation size makes a lot of sense, as this is roughly what our eyes dilate to during the day, so we want to see best then.

    However, for me personally I think if I want to see good detail on the planets I really need to be able to look at from around 150-200x, and there is no way I can do that with a small scope (like my Tak) without using a much smaller exit pupil than 2-2.5mm! So I compromise by making sure I am using a good scope and good eyepieces- and I can get really nice detail with a 0.5mm exit pupil and around 200x (on a night of good seeing of course). Ideally I would want a 1mm or even 2mm exit pupil: but that requires a much slower (ie longer) scope at that aperture, which is unwieldy, or a much bigger aperture, which means it is not portable.

    Just a few thoughts on a very tricky and often subjective subject...

    All the best,

    Dean
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  11. #30
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    Default Re: Choice of Eyepiece

    The subjective bit has to do with factors such as ease of use and how well developed the observing skills are. The 2mm exit pupil resolution limit is based on physics and physiology. If one needs a smaller exit pupil to see a detail that is an indication that one might need more practice developing visual observing skills to get the most out of the eyes you have.

    It's always easier to use a higher magnification to get a detail. The benefit of developing the skill to see it at lower magnification comes on nights when the seeing is poorer and high magnifications are not possible.
    Last edited by not_Fritz_Argelander; 08-21-2018 at 01:22 PM.
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