Mon, 09/17/2012 - 03:13
I have found a particular brand and style of eyepiece that is basically perfect for me. I have only the 25mm eyepiece which currently I use 100% for my visual LPV observing. Now I have seen discussions where observers talk of having a range of powers available to them useful, I assume when one is working very close to the scopes limits.
So anyway I am in a position to purchase one more eyepiece, so I am wonder what the seasoned observers would suggest. I have the choice of 18mm 15mm 12mm 8mm or 5mm. Personally I am thinking the 15mm or 12mm - but what do you say?
Thanks Glen
Hi Glen,
The choice of ocular depends on the focus of your telescom too. In my home villagage I have only an 15mm Erfle for my 270/1470 Dobsonian ("The Boiler"), but an 12mm and a 9mm ocular would extend the limit of my telescope.
At Polaris Observatory I mostly use a 15mm and an 9mm ocular for the 250/1250 GSO Dobs. Sometimes I use 25mm and 6mm oculars too. These are Skywatcher LEW Goldline oculars -- not too expensive ones (~$50 or 39 EUR), but perfect for variable stars:
http://www.teleskop-austria.at/shop/index.php?lng=hun&m=2&kod=oku1-lew-…
or in German: http://www.teleskop-austria.at/shop/index.php?lng=de&m=2&kod=oku1-lew-0…
If you use an SCT telescope with 2000-3000mm focus, a 15 or 12mm ocular would suite for you. If you have shorter focus (near 1200-1500mm) I'd recommend a 15mm ocular and a 9mm too.
Yes, something I should have mentioned. I have an SCT 2000mm. And thank you for the reply
To properly select ep's you need to know the f/ratio of the telescope as well as afov of the ep. Another factor to consider is the optical quality of the telescope and your seeing conditions.
For telescopes of average optical quality, a good rule of thumb is minimum ep fl in mm = 1 to 1.5 x telescope f/ratio. So for a typical f/10 SCT a 10-15mm ep would be the limit. Superb optics can go down to 0.5 x f/ratio, but SCT's rarely fall into this category.
For most typical seeing conditions 300x or so is usually about the max magnification. Excellent seeing can allow 500x or even above 1000x on rare occaisons. An average quality 8" SCT is not going to be seeing limited, typically.
And the ep field of view makes a difference too. A 25mm plossl can be replaced by a 12.5mm Ethos or other 100deg afov for twice the power and the same true field of view. This will markedly improve visual performance, limited by optics and seeing of course.
Mike LMK
Here's a google search page with links to some calculators that may be useful ,
https://www.google.com/search?q=eyepiece+calculator
I use an 8 inch sct sometimes and the 3 eyepieces I use the most are a 45mm plossel for low power, wide fields when the star is bright , a 18mm radian for general use (and used the most) and a 9mm nagler for the faintest stars. I have a 56 2" plossel for the widest field but rarely use and 7mm that the seeing limits its use most of the time.
Richard Campbell
A lifetime of VSO has shown me that the observer can probably get by quite well with just three high quality eyepieces. The specific choice will, of course, depend on factors like the quality of the person's telescope, the size of its fully illuminated field, and prevailing local seeing conditions.
In my own case, over the past 40 years I have used 12.5" and 16" Newtonians of moderate optical quality that I designed with purposely large secondary mirrors. At my location the quality of the seeing has almost always been something of a limiting factor. I've found that my best basic low-power eyepiece is one with a very wide field and about 60x to 75x (with due consideration toward the size of the fully illuminated field). Lacking GOTO ability, this allows for quick, easy and accurate identification of the larger star patterns in the variable's field and the surrounding comp stars, as well as useful when making estimates when the star is fairly bright.
My workhorse higher-power eyepiece produces 175x-200x magnification. It darkens the sky background considerably and typically gains me an additional 1.5- 2.0 magnitudes over the low-power view. Here again a wider than average field of view is desirable so as to include the sometimes widely spaced comp stars found on certain charts.
On unusually clear and dark nights with very good seeing (the latter a rarity at my location) one additional special eyepiece should be available to press into service. This one should produce something like 300x. Mine will gain me yet another 0.5-0.75 magnitudes over my moderate power eyepiece. This modern eyepiece has an 80-degree field design and can pull out very faint stars, such as some of my faint CV's and a few of the old novae I follow.
J.Bortle (BRJ
I have just come
Hi John,
I have just come to the conclusion that the traditional "wide field is best" may not be all it's cracked up to be - not for location purposes but for optical ones. I have a super-wide field eyepiece (used with an f/5 36cm Newt) but the star images aren't really good enough to capture enough of those precious photons. I'm not saying the images are bad, just that I'm not getting enough sharpness which I am assuming is a combination of fast optics and the Wide Field Eyepiece. Just wondering if anyone else has had troubles in this direction? It wasn't a bad night last night but I found myself struggling to see below 14th mag with the WFE in. Using my previous 'standard field' ortho the usual ZLM would have been about 16, though of course with a rather smaller field. Telescope is purely Dob and manual (GoTo... Pah!!)
Having tried a wide variety of ep's over the years, I found the Televe Ethos line stands above the rest. They are "state of the art" optical design, and with 100+ degrees of apparent field, its almost as if your field of view is unencumbered by an artificial limit! The large AFOV means you can get about twice the magnification as a typical narrow ep, yet the same true field. This darkens the background substantially, increasing contrast and allows fainter stars to be seen. They are also very well corrected for aberrations, especially useful for fast optical systems, without introducing undercorrection, as many classical older designs such as Plossl's and orthos do at f/4-5.
While Ethos are fairly pricey, my view about this issue is they are a lifetime investment, if cared for properly. Why not spend half a months rent/mortgage for a "keeper" eyepiece?!
Several competitors are now producing 100 deg ep's too, such as Explore Scientific, though I have not tested their quality for comparison. They are quite a bit more affordable, and may be the best "bang for the buck"!
Mike LMK
[quote=lmk]
Several competitors are now producing 100 deg ep's too, such as Explore Scientific, though I have not tested their quality for comparison. They are quite a bit more affordable, and may be the best "bang for the buck"!
Mike LMK
[/quote]
I have an Explore Scientific 82 degree eyepiece, and am very happy with it.
/Gustav, HGUA
I wholeheartedly
Hi All
I wholeheartedly agee with Mike regarding The Televue Ethos eyepieces. I recently purchased a 3.7mm SX Ethos and in my 16" reflector that works out to be a magnification of 494x which I was quite nervous about in terms of getting reasonable images. It's winter in Australia at the moment and therefore the seeing is usually better in Perth this time of year compared to summer and last night I had my first session with the eyepiece and I was amazed. The large field of view in combination with the high magnification worked superbly even in my 23 year old telescope with less than perfect optics. In bright moonlight I was seeing just under 15th magnitude which I've never done before. I think I may be retiring my 20 year old 4.8mm Nagler. Thanks for your long years of service!
Therefore provided your seeing conditions allow, I can recommend this eyepiece for visual observers trying to chase the fainter limits in their scopes.
Regards
Andrew Pearce (PEX)
Hi Glen,
If I may chip in here: I have a 203mm SCT, like yours. I employ just two attachments; a 16mm TeleVue Nagler eyepiece and a F/6.3 focal reducer.
This eyepiece has an 82° apparent field; and at a magnification of x127 delivers a real field of 38'. This normally gives a limiting magnitude of around 12.5. If I need a larger field, to bring In useful comparison stars, I put in the focal reducer. This gives a one degree field; a magnitude of x80 and still has a limiting magnitude of about 12.0.
For fainter objects you need a larger scope. For large fields, and brighter targets, use your finder. It's as simple as that. Don't get bogged down by tortuous discussion.
Incidentally there's a very good article on eyepieces in the September S&T.
Best wishes, Bill Wilson.