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stringer
journeyman
Reged: 08/16/08
Posts: 6
Loc: Southcentral PA
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As a novice with my first scope, a 10" Orion Dob, I have the two EPs which shipped with it, the 10mm and 25mm plossls. In an effort to "boost" the quality of my EPs, I have also purchased a 7mm BO/TMB planetary EP for that purpose. The Teleview and Pentax line are out of the question due to wallet size.
However, I want to puchase a higher quality EP for DSOs. I've been looking at the Baader Hyperion line and like what I've read about their high quality and wide FOV. My qaundry is what focal length EP to purchase. Baader's line has 68 degree AFOV. I've been reading many forums where the consensus is that eveyone should have a low power EP, i.e, 21-30mm or higher. But my question is this: If the AFOV is the same, why purchase a low power EP when a moderate power EP would provide higher magification? It is thus why I am trying to choose between the 24mm and the 17mm, both with the same AFOV.
I would appreciate some experienced opinions on this subject. thx, walt
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Kolenka
professor emeritus
Reged: 06/01/08
Posts: 589
Loc: Seattle Area, WA, USA
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The apparent field of view is not the same as the true field of view.
a 17mm 68-degree eyepiece has a smaller true field of view than a 24mm 68-degree eyepiece.
Also, the lower magnification eyepiece will have slightly brighter images, so for larger DSOs, I would definitely get the 24mm, IMO. I just received mine, and it just lands in that sweet spot for me with my 10".
Your needs will be slightly different.
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Meade 10" LX200R
Orion 80ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XSi, TIS DMK 31AF03
Northwest Astro Photoblog
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Mr. Mike
scholastic sledgehammer
Reged: 11/08/05
Posts: 855
Loc: Churchville, NY
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Quote:
The apparent field of view is not the same as the true field of view.
a 17mm 68-degree eyepiece has a smaller true field of view than a 24mm 68-degree eyepiece.
Also, the lower magnification eyepiece will have slightly brighter images, so for larger DSOs, I would definitely get the 24mm, IMO. I just received mine, and it just lands in that sweet spot for me with my 10".
Your needs will be slightly different.
What he said... True FOV changes from EP to EP. For instance, to get the same true field of view from a higher-power eyepiece compared to a lower-power one, the higher powered eyepiece would have to have a larger Apparent field of view.
So, a 13mm 82 degree eyepiece has roughly the same TRUE field of view as a 17mm 65 degree eyepiece.
I think thats what I meant say.... 
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Stellarvue NG 80mm ED
Meade 7x50 Binos
Pentax XW 5mm
Meade 5K UWA 8.8mm
Vixen LVW 13mm
Vixen LVW 22mm
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Lard Greystoke
super member
Reged: 07/27/08
Posts: 184
Loc: Ohio
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Quote:
But my question is this: If the AFOV is the same, why purchase a low power EP when a moderate power EP would provide higher magification? It is thus why I am trying to choose between the 24mm and the 17mm, both with the same AFOV.
I would appreciate some experienced opinions on this subject. thx, walt
Either the 24 or 17 will replace the 25mm plossl. The 24 will give you about the same magnification and contrast with a wider field of view; the 17 will gave you (I'm guessing) about the same field of view, with higher magnification and better contrast.
So, take some good looks through the 25. Is your view too narrow? Go with the 24. Too washed-out, too hard to recognize objects? Go with the 17.
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Lard Greystoke
10" Odyssey Compact
"With Tantor, the elephant, he made friends. How? Ask me not."
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RussL
Music Maker
Reged: 03/18/08
Posts: 1572
Loc: Cayce, SC
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I have a 40mm Sirius Plossl that I use in my 8" SCT, and I tell ya, even with the narrow 43-degree AFOV, I love that view. It gives me 51x, and I wish I could go even lower. I recently bought an ST80 which yields 16x with that eyepiece. I have fallen in love with low power. I'm not used to powers between 10x and 51x. It is another world with a whole new perspective, like starting out new all over again in astronomy after 49 years of observing. Now I want a 6.3 focal reducer for the 8" SCT. 
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--Russell
"Akita mani yo." Observe everything as you walk. (--Lakota)
Celestron Celestar 8 Standard SCT, f10
Celestron 80mm Wide View ref., f5
Criterion RV-6 Dynascope, Newt., f8, (c. 1962)
Sears Discoverer 60mm ref., f7, (c. 1973)
Celestron Ultima DX 10x50, 6.5 TFOV
Tasco 7x35 wide
Several mediocre eyepieces
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10408
Loc: Amargosa Valley, NV, USA
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Tough choice. One thing you should keep in mind is that these will probably not be your final eyepiece purchases -- eventually you'll want a whole range of good glass. My gut feeling is to recommend the 24mm Hyperion. It can be the anchor for future acquisitions down the line. However, for the uses you would put it to, the 17mm wouldn't be bad either. And even the 13mm looks great on DSOs.
Like I said, tough choice.
There are other good eyepiece choices besides the Hyperion, but there's no doubt that they are a great value.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4619
Loc: Illinois, US
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Quote:
Now I want a 6.3 focal reducer for the 8" SCT.
Until I read that line, it's exactly what I was going to suggest to you...  Well, I was going to suggest an f/5.
Chris
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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RussL
Music Maker
Reged: 03/18/08
Posts: 1572
Loc: Cayce, SC
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Quote:
Until I read that line, it's exactly what I was going to suggest to you... Well, I was going to suggest an f/5.
Yeah, Chris, my ST80 is f5, but I understand that another inch of aperture will make a big difference with a 100 or 102 f5. I've been looking at them. Man, but the SCT 8" at f6.3 would be killer, I'll bet. And no CA.
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--Russell
"Akita mani yo." Observe everything as you walk. (--Lakota)
Celestron Celestar 8 Standard SCT, f10
Celestron 80mm Wide View ref., f5
Criterion RV-6 Dynascope, Newt., f8, (c. 1962)
Sears Discoverer 60mm ref., f7, (c. 1973)
Celestron Ultima DX 10x50, 6.5 TFOV
Tasco 7x35 wide
Several mediocre eyepieces
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4619
Loc: Illinois, US
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Quote:
Yeah, Chris, my ST80 is f5, but I understand that another inch of aperture will make a big difference with a 100 or 102 f5. I've been looking at them. Man, but the SCT 8" at f6.3 would be killer, I'll bet. And no CA.
Well, remember that the focal ratio is (of course) relative. The larger scope will provide a higher magnificaiton at f/5 than the smaller one... Even so, focal reducers work quite well in catadioptric scopes, as far as I've seen, and it will allow much lower power at f/5 than at f/10.
Chris
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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RussL
Music Maker
Reged: 03/18/08
Posts: 1572
Loc: Cayce, SC
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Quote:
Well, remember that the focal ratio is (of course) relative. The larger scope will provide a higher magnificaiton at f/5 than the smaller one...
Yep. It won't be much, though, since I'll only be going from 400mm FL to 500mm FL. The cool thing is that I have that 40mm plossl. Right now it makes my ST80 yield 10x. My 25mm give me 16x. In a 102mm at 500mm FL, that would be 12.5x and 20x respectively. Of course the difference is more noticeable at higher powers where a 10mm would bring in 40x and 50x. But all of this with an extra inch of aperture in the 102 at f5. Not great for the moon and planets. So maybe a 102 Mak would come later. (I'm into the grab-n-go thing lately).
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--Russell
"Akita mani yo." Observe everything as you walk. (--Lakota)
Celestron Celestar 8 Standard SCT, f10
Celestron 80mm Wide View ref., f5
Criterion RV-6 Dynascope, Newt., f8, (c. 1962)
Sears Discoverer 60mm ref., f7, (c. 1973)
Celestron Ultima DX 10x50, 6.5 TFOV
Tasco 7x35 wide
Several mediocre eyepieces
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4619
Loc: Illinois, US
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Quote:
Not great for the moon and planets. So maybe a 102 Mak would come later. (I'm into the grab-n-go thing lately).
You and me both. Who has time to pull out a large scope, especially if you have to leave it sealed for half an hour for it to warm up past the point where it will gather dew?
Chris
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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stringer
journeyman
Reged: 08/16/08
Posts: 6
Loc: Southcentral PA
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I can see that my query elicited very helpful responses, but aroused a discussion elsewhere, which is also good. Thanks to all who volunteered their input.
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JT5
sage
Reged: 11/08/07
Posts: 261
Loc: Ozarks of Missouri
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In my opinion the best eyepieces for a DOB are wide fields of view. I have a 33 mm eyepiece for my 12 inch Lightbridge. The narrower your field of view the more you will need to realign. Also finding a particular object increases with difficulty for narrow fields of view. High power observing is best with a computer driven SC or MAK scope in my opinion.
John
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A wife that shares my love of the night skies and tolerates all of my other hobbies.
Televue Pronto
Meade 2045D
Meade ETX-125PE
Meade 12" Lightbridge
Meade, Swan and Televue Lenses (33, 26, 20, 15 & 9mm), Televue Barlow
Catsperch Observing Chair
Member: Astronomical Society of Eastern Missouri
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Bob Griffiths
Post Laureate
Reged: 10/10/05
Posts: 4116
Loc: Frederick Maryland
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Yep low powered eyepieces are my favorite EP for viewing DSO for sure... in my SCT I do use a Focal reducer and use 2" eyepieces of 26 mm 30 mm 35 mm and 42 mm all but the 42 ate 68-70 degree Eyepieces but the 42 will show some vignetting with the focal reducer which is honestly not a big deal for me..
90 percent of my viewing is really done from .40x to 200x
so if it were me I'd cover that range as completely as possible...buy one eyepiece at a time ..then use it and over time think about what one you want to buy next...never a rush .because there is never an end to the "I gotta haves"
Bob G
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CPC1100
Nexstar 8i + GPS & Rays Brackets
Denk S1 power switch
Orion 100 mm Refractor
Meade LXD 55 ...AR-5 127 mm Refractor
Exploradome Observatory S.I.E. (Smiling Irish Eyes)
39*21'03" N
77*28'12" W
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wirenut
sage
Reged: 09/21/06
Posts: 462
Loc: m'dale Pa
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the TFOV with the 21mm & 17mm hyperion/stratus is the same due to the 21mm doesnt have a true 68 like advertized it's closer to 60 AFOV.the 24 is differant. it's been reported the 24mm is as well corrected as the rest so i dont know if it's good for under a F5 scope. the 21mm is said to be the one that works best without nose piece making for a 32mm wide field EP I've tried this & it works very well for me but opinions very
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8"GSO dob
8,17 mm hyperions & FT rings
21mm stratus
25,15,9 mm plossls
ultima barlow
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Doug L
super member
Reged: 08/17/08
Posts: 122
Loc: Ottawa, Canada
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Quote:
As a novice with my first scope, a 10" Orion Dob ... I want to puchase a higher quality EP for DSOs.
I love my 10 inch Dob. Over the years I've purchased a lot of various eyepieces for it over the years, but eventually settled on only a few. I tried to match my eyepieces to the resulting exit pupil (another use of the initials 'EP') and wanted a significant change in magnification (about double) with each. I wish I had done this initially - would have saved me big bucks! My 32mm eyepiece (2in. diameter University Optics Konig) gives me about x40 power magnification, 1.25 degree true field of view, and an EP of 6mm. Put the 32mm. into my x2 power barlow lens (2in. diameter Orion) and I now have x80 mag., .6 deg. fov (perfect for just framing the entire moon or sun), and 3mm exit pupil. My 9mm (Televue Nagler) gives me x140 mag, .4 deg. fov, and about 2mm EP (my favourite exit pupil for looking at many deep sky objects - great contrast). With the x2 power barlow, I get x280 mag and a 1mm EP (my favourite eyepiece for planets when the atmosphere is steady). So basically, 2 eyepieces and a barlow gives me all I need or want. I encourage you to use the eyepieces you have for a while until you discover what objects you really like looking at most, and get to a star party or join a club so you can try other brands of eyepieces in your scope before you purchase them. It may save a lot of $$ in the long run (unless you try some of the premium ultrawides and get hooked). 
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Bino reality... 1 + 1 > 2
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boyd
sage
Reged: 11/30/06
Posts: 307
Loc: Florida
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might read this:
http://www.cloudynights.com/item.php?arch=1&cy=2005&cm=12&cmn=December&item_id=1293
and this:
http://www.cloudynights.com/item.php?item_id=1451
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XT8
ETX70AT
C80EQ
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