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Eyepieces for XT8: a rule to select focal length?


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Hi everyone, new user here... I used to be into astronomy as a kid, and got to use some nice equipment when working as a student computing aide in an observatory.   Now much much older, very different profession but some free time on my hands and dark skies where I live, I would very much like to resurrect an old hobby.

I bought an XT8 recently that came with a 25mm Orion Plossl.  I am over 40, have to use glasses (or contact lenses).  I would like to start mostly with moon and planets, and move to deep sky objects as time goes.   So it looks like to get some eyepieces, and would like to build some collection.

First, is there an approximate rule which relates the focal length of the eyepieces someone might want to have in a set, for example, if I had a 6mm (200X) EP,  what could be the recommended next size up (lower magnification)?  Not sure I make a lot of sense here, just looking for a guidance that says that 7mm is still too close to 6, but 9mm would be useful to have.

Second, I narrowed my choice to Televue Delos line; the price still makes me cringe a bit but at least my wife won't grumble too much, these seem to have very good reviews here, and I could wear glasses and avoid a combo of contacts and dry eyes in the evening.  Any gotchas I should be aware of when using Delos in XT8?   Or maybe something else could be more appropriate?  For example, I read a lot of comparisons between televue Nagler (type 6) and Delos, people do not have any apparent preferece to either line.  Please chime in.

Delos are manufactured in focal length 3.5, 4.5 (probably too powerful for my scope), 6mm, 8,10,12,14, 17.  It would be nice to spread the pain and not to get something that I don't need, what magnification eyepieces would you get to just get started?

These questions have been covered in other topics on the forum; I did my reading and homework but still could not get definite answers to the above (maybe there is not one.

Thanks for reading.

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I cannot really suggest much at the price you are considering.

The Delos are certainly right for planetary observing, 6mm, 8mm, then likely 12mm and 17mm.

But at 72 degree they seem a bit "narrow" for the cost.

Alternative may be the ES 82's but they do not have the selection at the shorter end for planetary, but for future they do have some long focal lengths in 2" format to go with the 1.25" format.

If you really wanted to keep the cost doen I would have said get the Celestron X-Cel's.

There are Vixen SLV's but they have a 50 degree view but there is a good selection of focal lengths and they go to 25mm, just they are as narrow as plossl's.

The "rule" I follow is one at the f number so f/6 means a 6mm, one a bit below, one a bit longer, so 5mm and 8mm, then a medium and a long.

It is how I decide or at least what I look for, but you can make your own "rule" up. That is more fun.

If Delos is what you want then 6mm, 8mm ,12mm and the 17mm, you may not want or need the 12mm immediatly so that could come later, or not at all.

My concern of the Delos is the cost for a 72 degree eyepiece and they stop at 17mm. They need a 22mm or 25mm in the range.

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Getting odd, according to the prices on Optcorp TV Naglers ($310) are less then TV Delos ($350).

You could get the 7mm, 9mm, and 16mm Nagler, and later the 5mm for those good nights.

There is the brave/insane option - go for the new TV DeLites, they are 62 degree and presently in 7mm, 11mm and 18.2mm. :eek: :eek: :eek:

But those 3 should just about do you for now, $250 and nowhere seems to actually have them available. :rolleyes: :rolleyes: :rolleyes:

The catch is no idea what others TV will produce in the months to come.

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Thanks much for the replies.

Regarding TV type 6 Naglers vs. Delos, I don't understand the pricing either, since Televue website says Naglers a larger apparent field of view vs. Delos.

The Delos is a more recent design, derived from the Ethos and, despite it's smaller field of view, is a slightly better performer than the Nagler on both deep sky and solar system objects. The Delos also has 20mm of eye relief compared to the 12mm for the T6 Nagler so the glasses wearer will find them easier to use. Both excellent eyepieces of course but the Delos is just that touch better.

It's not all about the field of view :wink:

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Optimal focal length spread for 8" f/5.9 Dob will be like 4.5mm-6mm-8mm-12mm-18mm-24mm-35mm.  You can easily skip the 18mm. The rules are easy, just start form the *sweet spot* of 200x (6mm) or 100x (12mm) and maintain ~1.4x-1.6x jump or so between the eyepices.

Since you prefer wearing glasses when observing than the absolute min eye relief you need is around 16mm. I don't wear glasses when observing but I have to use them for reading/writing  and I tried my eyepieces with them on so I know well what will fit and what will not. Forget about the Naglers Type 6 or ES82, the eye relief is not enough for eyeglass wearers. Only Type 4 Naglers and longer FL Type 5 will meet your eye relief requirements.

If the Delos line is within your budget that's good. I'd suggest you buying first the 6mm, 8mm and 12mm Delos and one wide angle/finder eyepiece. As a wide angle eyepiece I'd recommend the 31mm Type 5 Nagler. It's really heavy (2 lb) and costs as two Delos EPs but has 19mm eye relief and will provide max TFOV possible so worth it, IMO. You can skip the 6mm Delos and buy the 2x ES Focal Extender  to barlow the 12mm, the Focal Extender is much cheaper than the Delos. Later you can get the 4.5mm Delos if you'll need it. If you want observing nebulae don't forget about the Lumicon UHC and OIII filters. For planetary observations I can recommend the Baader Moon & Sky Glow filter.

For some rules on eyepiece selection check out next links

http://scopecity.com/optics-course-0026.cfm?pn=HOW+TO+SELECT+AN+EYEPIECE

http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=96

http://www.cloudynights.com/page/articles/cat/articles/how-to/useful-magnification-ranges-for-visual-observimg-r535

http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/

http://www.astronomyforum.net/deep-sky-objects-forum/152002-treatise-optimizing-dso-observation.html#post1057637208

http://www.astromart.com/forums/viewpost.asp?forum_post_id=779780&poll_id=&news_id=&page=2

http://www.cloudynights.com/topic/479481-8-inch-f59ers-justify-your-eyepieces/

Hope, that helps.

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  • 1 year later...

Hello, this post has been helpful to me, because i was trying to decide between the 12 and 14 mm delos, especially since mr nagler on televue site recommended 14mm for 6-10 in dobs. I have an 8 inch dob. So, i think i should buy the 12mm  instead of the 14. I was aiming at 35mm tv panoptic. Might ck out the low power you suggest instead. I wear glasses. Good evening, pat

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Hi yanello -

I've noted you're in Pennsylvania, hence not far from a source of good eyepieces in the 2" format. And might like to save a little money? University Optics in Ann Arbor, MI has some true bargains, and have been doing a good business for over 50 years. And have a line of eyepieces in 26mm, 32mm, and 38mm. These have a 70° FOV:

http://www.universityoptics.com/2inch.html#WS70

I was out last night with these in my Maksutov, and I'm always impressed by how nice a view they provide - sharp stars edge to edge. And a comfortable amount of eye-relief for those of us who wear glasses. These same EP's have been sold by many outfits over the years. Orion-USA also offers these, with their brand-name on them:

http://www.telescope.com/Accessories/Telescope-Eyepieces/Orion-Q70-Super-Wide-Field-2-Telescope-Eyepieces/pc/-1/c/3/sc/47/e/66.uts

I thought you might like knowing about these.

Clear & dark skies,

Dave

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With respect to the eyepieces that Dave links to above, what works well in a slow maksutov may well not do so well, especially in the outer parts of the field of view, in an F/5.9 8" dobsonian.

You don't have to buy Tele Vue to get well corrected edge to edge views in medium to fast scopes (although it is their speciality). ES, William Optics UWAN's, Meade UWA's, Baader Morpheus etc will do a great job as well. Lower cost SWA / UWA eyepieces will struggle though, if field sharpness is important to you.

 

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Actually John, these EP's have been found to work very nicely in scopes down to F/4.5 Newtonians. I would imagine they're fine in F/5.9's as well. They are also fun in my F/5 ST80.

An inexpensive & wide-field & forgiving eyepiece is indeed a rarity!

Dave

 

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