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star drop
Guilty as Charged
Reged: 02/02/08
Posts: 2890
Loc: Cattaraugus Co., NY
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What would happen if one took an 8" SCT F/10 and wanted to get a rich field effect by using a long focal length eyepiece yielding a magnification between 25x and 35x? Would the views be identical to a newtonian or would there be problems?
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Ted
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hudson_yak
sage
Reged: 11/15/07
Posts: 445
Loc: Hyde Park, NY, USA
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The upper end of that range is certainly reasonable, giving you an exit pupil around 5.5 mm. Pushing lower (which would require using a focal reducer) will start to get you into more severe vignetting and an annoyingly large central obstruction in the exit pupil.
Mike
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Meade 8" LX10
Orion ED100 + SVP
Meade 12" LightBridge
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 1960
Loc: Massachusetts
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The max TFOV is determined by the focal length of the scope and size of the hole at the back of the scope. After that, it's limited by the diameter of the field stop in the EP. Something like a 35mm Panoptic will provide the widest TFOV possible. It will have minor vigneting toward the edges but it's not noticeable. You can put a 6.3 focal reducer on the scope and get wider TFOVs but the vignetting becomes much more of an issue with EPs with large fieldstops, like a 35mm Pan. It's still usable though.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
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letimotif
no complaints
Reged: 05/20/07
Posts: 1072
Loc: Far Flung Isles of Langerhans
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I happened to be out at an observing site with some other club members last night. At one point we briefly compared one member's TMB 152 with a Nagler 31mm to my humble CPC 1100 with a Hyperion 36mm.
I was using a focal reducer and we took a peek at the Double Cluster. The 36mm showed the black hole central obstruction, but the views were remarkably good just the same.
Honestly, I'm intimidated a bit by big, expensive eyepieces, and since I didn't know the other member that well, I didn't borrow his Nagler to do a more direct comparison. Have to try that sometime.
So sad, though, the seeing was remarkably good, while the Moon was soooooooooooooooooooooooooo remarkably bright. 
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Celestron CPC 1100
Celestron C6R-GT
Orion 120ST
B&L Banner Astro 80mm
Astro Telescopes 'tis a mystery 80mm ED
Sundry Oddments
Ah, but a man's reach should exceed his grasp, or what's a Heaven for?
Ad astra per aspera
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Joe Ogiba
Post Laureate
Reged: 02/14/02
Posts: 3340
Loc: NJ USA
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Just use your 25mm Ethos. 
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Pentax PF-80ED
Meade 102ED APO
Orion EON 72
120ST
Apex 127
C6 XLT
CR150
C9.25
XT10
Zeiss 7x42 FL
Canon 10x42L IS WP
15x50 IS
12x36 IS II
Garrett Optical 28x110 HD-WP Signature Series
Oberwerk BT-80 45
Apogee RA-88-SA
Denk II Power x Switch binoviewer w/13mm Ethos, 20mm Pentax XW's, 20mm Widescan III's.
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star drop
Guilty as Charged
Reged: 02/02/08
Posts: 2890
Loc: Cattaraugus Co., NY
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Quote:
Just use your 25mm Ethos.
I'm expecting it in the mail any day now.  
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Ted
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star drop
Guilty as Charged
Reged: 02/02/08
Posts: 2890
Loc: Cattaraugus Co., NY
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Thanks for the responses all. Disreguarding the loss of edge of field brightness and the "Black Hole" central obstruction do the stars visible at such low powers exhibit the sparkly bright pinpoint images in a SCT as in a reflector or refractor? After looking through such a scope at around 100x I have not really noticed the field curvature problem that others have mentioned. Perhaps my mediocre seeing conditions mask this effect. Does the field curvature become more prominent when going to lower magnifications or to the high quality 82 degree apparent field of view eyepieces? I'm trying to figure out if is this is an attainable goal with this scope design before investing in 2" diameter opening backs and 2" diagonals or focal reducers.
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Ted
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CHASLX200
professor emeritus
Reged: 09/29/07
Posts: 690
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Most of the SCT's i have owned show very sharp stars with my 1.25" 32mm Televue, and my 10" and bigger cats with a 2" 40mm WF Televue. A cooled scope, good seeing and being collimated does help alot. I cant see going lower than a 40mm most times, other than using C14 i would use a 2" 55mm Televue.
Chas
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Kolenka
professor emeritus
Reged: 06/01/08
Posts: 593
Loc: Seattle Area, WA, USA
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A 24mm Hyperion (1.25" eyepiece) did a pretty good job of providing pinpoint stars in my 10" SCT at 2500mm. A 31mm Hyperion was even better. This was at 80-105x or so.
The lowest I've run this scope was at ~1250mm using a 24mm Hyperion, or around 50x. It is possible to get it down to 40x, but the central obstruction was just too noticeable for me.
An 8" SCT should be able to get a little lower than I can. But when you are in this magnification range, vignetting will be affecting your field of view. At 50-40x with these eyepieces, I was seeing the field stop of the baffle tube, and the field stop of the eyepiece.
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Meade 10" LX200R
Orion 80ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XSi, TIS DMK 31AF03
Northwest Astro Photoblog
Edited by Kolenka (10/12/08 04:32 PM)
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Midnight Dan
professor emeritus
Reged: 01/23/08
Posts: 558
Loc: Brockport, NY
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I was just reading an article about minimum usable magnification (MUM). It said MUM is calculated by dividing the telescope aperture by your eye's dark-adapted pupil size. For an 8" SCT, if you have a 5mm pupil size, that would be 203mm/5mm = 40.6x. That assumes you're an old dude like me. Younger folks may have a 6 or 7 mm pupil.
The author of the article was using a piece of software called Virtual Observer to plot various comparisons of features in the optical system. One such plot was brightness vs. magnification. Everyone knows that when you increase magnification you decrease brightness. But what the plot showed was that below the MUM, brightness dropped off VERY rapidly. So, that doesn't mean you can't use lower mags, but you will loose a lot of light because it is being spread over a larger area than your eye can collect in its given pupil size.
Just another factor to consider in addition to the vignetting and CO visibility.
-Dan
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Scopes: Celestron NexStar 8, Orion EON 72mm ED/APO on Astroview mount (EQ3)
Eyepieces: Celestron 40mm, 25mm, Baader Hyperion 13mm, 8mm, 5mm
Other: Telrad, 2x Barlow, 0.63x Focal Reducer, Dew-not strips, DewBuster controller
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letimotif
no complaints
Reged: 05/20/07
Posts: 1072
Loc: Far Flung Isles of Langerhans
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Quote:
Thanks for the responses all. Disreguarding the loss of edge of field brightness and the "Black Hole" central obstruction do the stars visible at such low powers exhibit the sparkly bright pinpoint images in a SCT as in a reflector or refractor?
Just relating my experience last night, I was paying particular attention to the DC's stars around the edges as well as in the center. Have to say that they looked pinpoint to me and the other folks who took a gander. That would have been about 50X. I say "about" because I've got my FRFF mounted to the OTA first, and use and adaptor to hold my refractor diagonal behind hit. No way I'm at f/6.3 precisely.
I know other folks don't like to use a FRFF for a variety of good and valid reasons. I just find with the Hyperions they make a pleasing field view and quite sharp to the edge. Of course, as I mentioned earlier, the seeing was exceptional last night. I inadvertantly picked up from my ep case my 5mm for a while and at 350 +/- things were crystal (doggone Moon).
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Celestron CPC 1100
Celestron C6R-GT
Orion 120ST
B&L Banner Astro 80mm
Astro Telescopes 'tis a mystery 80mm ED
Sundry Oddments
Ah, but a man's reach should exceed his grasp, or what's a Heaven for?
Ad astra per aspera
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star drop
Guilty as Charged
Reged: 02/02/08
Posts: 2890
Loc: Cattaraugus Co., NY
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Again thank you to all who contributed. It seems that at the lower magnifications that I inquired about it boils down to that it IS possible. But personal preferences as to how much vignetting one can tolerate and to how objectionable the central obstructions presence has become will play a big part in determining how low one goes. It also appears that an excessively large exit pupil would not be a concern with an eyepiece focal length of 40 mm or less.
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Ted
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 1960
Loc: Massachusetts
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In my experience, stars are just as pin-point and sparkly in a decent SCT as a refractor at equal powers. Small exit pupil and low power is what makes the view in small refractor so nice. A 4" refractor will have an Airy disc twice the size of that in a 8" SCT. The SCT will put more light into the first ring but still is sharper than the 4" refractor.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
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