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Eyepieces - the very least you need.


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Take your focal ratio, and multiply it by 3/4. So, if you have an f/8 scope, the result is 6. If you have an f/10 scope, the result is 7.5. This result is the length in millimetres of your high power eyepiece. It will give about 2/3 of the theoretical maximum power of your scope. This is the actual maximum if you do not always enjoy perfect seeing and transparency. If you have a 100mm scope, this eyepiece will give 133x.

Thanks very much - as a beginner and hopefully soon to buy, it's very informative and hopefully it will help me avoid spending money on things I don't need.

Once I'd worked out that the "100mm scope" you used as an example throughout was actually 1000mm focal length, I could do the maths.

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  • 2 weeks later...

Warthog,

I have an 8" Dob, F 5.9, 1200mm. I'm considering getting ONE 2" EP from Siebert Optical. Most 2" Siebert EPs have a FOV would be 70mm with 20mm eye relief. I'm primarily interested in DSOs. Any suggestions/guidance?

Many thanks.

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I have no experience with the Sieberts, but I visited their website and these are my thoughts: If you can live with 6mm exit pupil (above 35 years 5mm is advisable), then either the 34 or 36 mm will do. At 5mm exit pupil, you require 30mm focal length at most. I did not see such an EP on their website. You could check out the TMB Paragon 30mm, which has the same sort of features (69deg FOV, 20mm eye relief). I have the 40mm (4mm exit pupil on my F/10 scope) and am VERY pleased with it. No discernible pin-cushioning, no black-outs or kidney-beaning, VERY sharp edge to edge. The price is about the same as the Sieberts I have seen.

Just my tuppence

Cheers

Michael

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hello all first time on the site but wathched the nights sky every clear night i could for years love it any advise on telescopes for rookies thanks

Try posting this in a thread of it's own in this section - you will get plenty of suggestions then :blob10:

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  • 4 weeks later...
Thanks very much - as a beginner and hopefully soon to buy, it's very informative and hopefully it will help me avoid spending money on things I don't need.

Once I'd worked out that the "100mm scope" you used as an example throughout was actually 1000mm focal length, I could do the maths.

Not exactly. It is a 100mm aperture, and if you follow the formula I give, your ep will give 133x no matter what the focal length is. If you have an f/8 scope and are using a 6mm ep, then 800/6=133. For the f/10 scope, 1000/7.5=133, and for an f/6 scope, 600/4.5=133. This result surprised me a little when I was putting this article together.

Thanks to everyone for your kind comments. Family matters, health and work have kept me away from the ep and from the forums this year, but I am planning a resurgence.

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When you put a Barlow into the mix, you want to consider what focal lenghts it gives you. The 10 mm with your 3x Barlow will give you 3.3mm, which is close enough to the 3.75mm you get using my formula. Try it and see if it works; this article is meant to give you a starting point, not to be the final word. If the quality of the view suffers with the barlow, consider getting a 3 or 4mm ep.

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What a great thread. I've read it all but can't see whether this has been covered. Would a 3X barlow be "too much" for my 4", F/5 spotting scope? I currently use a 20mm & a 10mm lens & will hopefully soon have a 7mm.

Thanks

Many spotting scope objectives (unless you have very expensive ones) are not that well corrected for extreme magnifications, opting for portability (hence the fast F-ratio) and wide, flat field of view. If you have a barlow handy or can borrow one on a star party, give it a shot.

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My lowest power on my 13.2 inch is a 19mm panoptic which gives .73 degree actual field of view at X93. A lower power in my moderately light-polluted sky (south London) would make the eyepiece much too grey ( at X93 it is already quite unpleasantly grey). Other eyepieces; nagler X200 & 6mm X300. There is a large gap between X93 & X200 but the nag's wide field compensates somewhat I think.

X300 I need for v. close double stars and the planets. X200 doesnt quite seem enough for planetary details, in my experience, partly beacause I am near-middle aged too, I suppose. (BTW I mask aperture to 12inches for planets & double stars).

Am I missing anything by having such a large gap in the range of powers?

Alan

Edited by perrin6
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Am I missing anything by having such a large gap in the range of powers?

Alan

Only if you feel you are missing something. Are there times when 93x isn't quite enough, but 200x is too much? If there are, then you might want to stick something in the 12mm range in there.

I have a gap in my kit at 16mm, but I don't want it, and I get 12mm very occasionally by Barlowing my 24mm, but this is on very rare occasions. I sometimes use my Barlow with the 7.5mm to get 200x on the Moon, but usually I go straight from my 24mm at 31x to the 7.5mm (100x) or the 4.8 (156x).

I do mostly Lunar observing. If my skies were better, I might be interested in filling that 16mm gap.

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