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stars_in_my_eyes
scholastic sledgehammer
Reged: 03/24/08
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Dob Focal Length vs. Cost of Eyepieces
#2429082 - 05/30/08 12:34 PM
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While weighing the choice of which Dob to buy, I am wondering what focal length is the dividing line between standard low/medium cost eyepieces and the more expensive eyepieces that are needed for shorter focal length scopes.
Any thoughts on this subject?
Thanks
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InkDark
Carpal Tunnel
Reged: 10/29/07
Loc: Lanaudiere, Quebec, Canada
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: stars_in_my_eyes]
#2429143 - 05/30/08 12:58 PM
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I don't have a lot of experience with different scopes but what I've read is that F/6 and higher is forgiving on medium quality EPs and F/5 and lower needs better corrected EPs.
BTW, make sure you distinguish focal length and focal ratio.
Focal ratio = Focal length of the telescope (in mm) / diameter of the primary mirror (in mm)
Although it is possible to find 10” Dobs with F/ratios of 6 (read F/6), you will find more 8” F/6 Dobs than the previous.
Clear skies!
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stars_in_my_eyes
scholastic sledgehammer
Reged: 03/24/08
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: InkDark]
#2429212 - 05/30/08 01:22 PM
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Thanks for the response.
Right you are about the focal length and ratio.
I meant focal ratio.
Thanks
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Jarad
Postmaster
Reged: 04/28/03
Loc: Atlanta, GA
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: stars_in_my_eyes]
#2429274 - 05/30/08 01:40 PM
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It also depends somewhat on what type of viewing you want to do. If you don't care about wide AFOV, then moderately priced plossls perform quite well even in fast scopes. In general, where you see problems in fast scopes is with cheap super-wide designs (Efle, Konig, or similar). Narrow-field designs (orthos, plossls, etc.) generally do fine even in fast scopes.
In general, I would aim to keep the eyepiece low enough to avoid a ladder with small-medium scopes, or to minimize the ladder height in larger scopes. In small-medium scopes, that means you can aim for f6 or slower with no trouble. When you get much above 10", then it is worth it to me to go faster to avoid ladders. Above 18" or so, there is really no way to completely avoid a ladder, but I lean towards the faster is better camp (I would rather spend a bit more on eyepieces than lug a tall ladder around, and teeter on top of it trying to observe). And those big scopes are expensive enough that a few hundred on eyepieces really doesn't make it that much more over all (if the few hundred breaks your price limit, drop down in aperture by 1-2" rather than skimp on eyepieces).
Jarad
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ajg
member
Reged: 03/05/08
Loc: South Yorkshire, UK
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: Jarad]
#2429299 - 05/30/08 01:50 PM
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I picked up the Naglers on fleabay for not a great deal of money. I felt that for the higher magnification it was worth it - the wide AFOV means that you don't have to keep nudging the scope along quite as much.
The Meade QX is OK, although it ain't all that compared to the Naglers.
The GSO suffers a bit towards the edges but I live with that because it's only 44x for me.
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Jan cz
member
Reged: 09/02/07
Loc: Czech Republic, Jistebnik
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: stars_in_my_eyes]
#2429827 - 05/30/08 05:25 PM
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In my case cost of eyepieces does not really depend on focal length. It is usually a multiple of the price of my scope, no matter what scope I own.
Like some other amateurs who are more into tweaking their equipment instead of using it to look at the sky, I have also developed kind of "equipment fever". It breaks down into aperture fever (scopes) and eyepiece fever and can be cured by buying new, better stuff. Since buying a scope in 12" neighbourhood can be a substantial investment possible only about once per decade, people end up buying eyepieces. After some time total cost of your eyepieces gets higher than the cost of your scope no matter what eypieces you buy.
Over the years I progressed from noname plossls to Naglers and binoviewer.
In my F5 12" scope all my low cost eyepieces show astigmatism (V-shaped distortion of stars) in outer part of field. The worst is Meade QX 36, but as a finder eyepiece it os ok.
My Naglers and Pans are clearly better. Their greater field of view also helps with finding objects, tracking and higher contrast due to darker background.
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camvan
Carpal Tunnel
Reged: 05/02/05
Loc: British Columbia
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: Jan cz]
#2429877 - 05/30/08 05:49 PM
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I've read time & time again that the Hyperion's are very good for the price. also, it's been said that the Hyperion Zoom EP is one of the best out there...some have commented that it's even better then TV's zoom. for $200, I'd say get one...then you have a single EP that takes through most of the realm one views and perhaps invest in some low mag EP's like a 32mm or so.
I know my next EP is going to be a Hyperion Zoom! 
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sixela
Postmaster
Reged: 12/23/04
Loc: Boechout, Belgium
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: stars_in_my_eyes]
#2430254 - 05/30/08 09:01 PM
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Quote:
While weighing the choice of which Dob to buy, I am wondering what focal length
Focal ratio
Quote:
is the dividing line between standard low/medium cost eyepieces and the more expensive eyepieces that are needed for shorter focal length scopes.
It depends - both on your tolerance to aberrations and on how cheap you take "standard cost" eyepieces to be. A Hyperion does OK at f/6 to my eyes (but just), but the cheap 80° Chinese widefields take f/8 at least for me to like them (and so does the WO SWAN).
f/6 is usually where the battle is being fought. And f/5 is undoubtedly "fast" (even though some people like cheap eyepieces in them, I certainly don't), and an f/8 is undoubtedly slow (even though I can still see aberrations is a lot of cheap widefields).
If you're talking about Plössls, anything over f/5 is slow, and even at f/4.5, it's hard to see the effects of the fast f/ratio on stars, as opposed to seeing it in the out of focus star test.
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Rick Woods
Postmaster
Reged: 01/27/05
Loc: Inner Solar System
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Re: Dob Focal Length vs. Cost of Eyepieces
[Re: sixela]
#2430279 - 05/30/08 09:15 PM
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If you want wide field of view, don't skimp on the eyepieces no matter what focal ratio telescope you go with. Invest once, wisely, and they will serve you well for the rest of your life.
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