Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3421
Loc: 51°N 4°E
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Hi all,
I have 32 years in this hobby, but so far my largest scope has been a 90 mm refractor. I'm now ready to move up to something showing me more detail on the planets, especially Jupiter.
I live in Belgium, where the weather is very unsettled most of the year, so I have to account for viewing sessions being short and few. When it's clear, the seeing is normally "not so good" at best; really steady seeing only happens less than 5 times a year or so.
So I'm thinking I need something with good cool down characteristics, and shouldn't count on using more than 150x - 200x regularly. I also want something that's not too heavy and relatively easy/quick to set up.
Here's what I'm looking at as far as my next telescope:
- 6" f/11 or 8" f/8 newtonian on a smooth dob mount (Orion UK)
- 105 - 120 mm doublet ED refractor, also alt-az
Any comments about those two options?
Many thanks.
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD
APM 107mm f/6.5 on GR3-DX
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Rick Woods
Postmaster
Reged: 01/27/05
Posts: 6199
Loc: Inner Solar System
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I recommend the 8" Newtonian, based on my experiences with a very good 8" f/6.
Also a tracking mount - a simple GEM with a clock drive would do it.
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- Rick
14" LX200GPS
Dyslexics Untie!
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bobhen
super member
Reged: 06/25/05
Posts: 190
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Of the choices you listed…
The 8” F8 Newtonian would be an excellent choice. But you must select a high quality, very smooth mirror and first-rate focuser with provisions for aggressive cool down and thermal management. Put your money into the OTA if it’s going on a Dob mount.
The 120 mm ED refractor is also an excellent choice of those that you listed. It has all the advantages of quick cool down, thermal management and good mechanics and a 2-speed focuser already built in. Its only disadvantage is slightly less aperture.
So the choice depends on your observing conditions and style. If you need to bring the scope outside from a warm house, like to grab views whenever you can under less than ideal conditions when seeing is mediocre or the planets are not optimally positioned and some of your sessions are quick 45-minute sessions and you want as little fuss as possible, I would select the refractor.
The reflector will be more finicky and you will need high quality components but if you want a scope that “might” deliver more detail if only rarely and you want to deal with more fussing and waiting than the reflector is the scope.
I would also recommend adding tracking at some point.
Bob
Oh – if it were me, with your conditions, I would select the refractor.
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3523
Loc: Doylestown, PA
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The best planet images I've ever seen through a scope was with an 8" f/8 Newtonian.
This would be a good choice. Just don't skimp on the quality of the primary or secondary.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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Brian Schmidt
sage
Reged: 01/25/09
Posts: 241
Loc: Roswell, GA
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Quote:
The best planet images I've ever seen through a scope was with an 8" f/8 Newtonian.
This would be a good choice. Just don't skimp on the quality of the primary or secondary.
I second that. I have a 8 f/6 dob and use that as my planetary. I'd also recommend a cross hair ep to get the mirrors lined up. Collimation is critical, but you can obtain very "refractor like" views through such a scope, especially if you have high quality Pyrex mirrors that are cooled down. I use a house fan for that purpose....counted 6 cloud belts in my 13 and 7 T6 Naglers and that scope the other night. 
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Brian
Edited by Brian Schmidt (11/24/09 09:08 PM)
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starrancher
Pooh-Bah
Reged: 06/09/09
Posts: 1304
Loc: Northern Arizona
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Quote:
Quote:
The best planet images I've ever seen through a scope was with an 8" f/8 Newtonian.
This would be a good choice. Just don't skimp on the quality of the primary or secondary.
I second that. I have a 8 f/6 dob and use that as my planetary. I'd also recommend a cross hair ep to get the mirrors lined up. Collimation is critical, but you can obtain very "refractor like" views through such a scope, especially if you have high quality Pyrex mirrors that are cooled down. I use a house fan for that purpose....counted 6 cloud belts in my 13 and 7 T6 Naglers and that scope the other night.
What really amazed me was that I bought my 8 inch Schmidt Newtonain for DSOs' & the best Saturn I've ever seen was at 381x through this f4 scope . It puts up an awesome Moon & a better Mars than my 5 inch Refractor . Hard to believe for a fast Newt with a fairly large CO .
The DSO views that this scope has produced is another thread altogether .
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LXD75 AR5
LXD75 SN8
Series 4000 Plossls
Misc. other stuff
Fort Rock , Az .
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Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3421
Loc: 51°N 4°E
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Thanks all for your comments. Some more pondering to do at this end...
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD
APM 107mm f/6.5 on GR3-DX
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Scott Beith
SRF
Reged: 11/26/03
Posts: 37858
Loc: Frederick, MD
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For planetary you will want a tracking mount.
Either an 8" f/6 newt or the 120ED would be great scopes.
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   SLAP Observer (TMB130SS, SV102V{LOMO Lens}, SV80ED)
"The only thing necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke.
"People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." -- George Orwell
"The measure of a man’s greatness is not determined by what he accomplishes for himself, but by what he accomplishes for others.” -- Some Bald Guy
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brianb11213
Post Laureate
Reged: 02/25/09
Posts: 3158
Loc: 55.215N 6.554W
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Quote:
Either an 8" f/6 newt or the 120ED would be great scopes.
But the Newt would be greater (more aperture) and an 8" f/8 with 1/10 wave PV optics would be better still.
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Scott Beith
SRF
Reged: 11/26/03
Posts: 37858
Loc: Frederick, MD
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I agree.
Even the 8" f/6 should have an Atlas/CGEM level mount.
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SLAP Observer (TMB130SS, SV102V{LOMO Lens}, SV80ED)
"The only thing necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke.
"People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." -- George Orwell
"The measure of a man’s greatness is not determined by what he accomplishes for himself, but by what he accomplishes for others.” -- Some Bald Guy
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Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3421
Loc: 51°N 4°E
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Thanks guys. I'm not so hot on the tracking equatorial mount. I've found that when it takes 3-4 trips to get my gear out, it doesn't get used as often.
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD
APM 107mm f/6.5 on GR3-DX
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Rick Woods
Postmaster
Reged: 01/27/05
Posts: 6199
Loc: Inner Solar System
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Quote:
Thanks guys. I'm not so hot on the tracking equatorial mount. I've found that when it takes 3-4 trips to get my gear out, it doesn't get used as often.
But you'll quickly find that unless you have a tracking mount, you don't see as much of the planet at high power because you constantly have to nudge the scope. The tracking is well worth the extra trip. Planetary observing has different imperatives than DSO observing.
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- Rick
14" LX200GPS
Dyslexics Untie!
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Ziggy943
Carpal Tunnel
Reged: 08/11/06
Posts: 1814
Loc: Utah
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Quote:
Quote:
Either an 8" f/6 newt or the 120ED would be great scopes.
But the Newt would be greater (more aperture) and an 8" f/8 with 1/10 wave PV optics would be better still.
The math of the optics doesn't always equate to quality at the eyepiece. An 8" F/8 can be an incredible telescope. "Can be.."
You may also want to consider a 6" F/15 or greater F ratio refractor. At F/17.5 you get virtually perfect color correction. The advantage of the extremely long refractor is the image stability. It just doesn't bounce. It sits there and you can study it. If I were looking for a 'consistent' planetary performer in less that south Florida conditions then I would go with a long focus refractor.
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May your skies always be clear,
Ziggy
www.slas.us
4" Mak
#1, 160mm F8 TEC (born 1-18-2007, 27 lbs, 45.5" long), on AP900
6" F17.5 (D&G lens) (first light 6-7-2008)
9" F/14.9 Alvan Clark (1915), on Byers 812
14.25 F/5.5 Newt in a roll off observatory
Others, that have come and gone
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3523
Loc: Doylestown, PA
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Quote:
Quote:
Thanks guys. I'm not so hot on the tracking equatorial mount. I've found that when it takes 3-4 trips to get my gear out, it doesn't get used as often.
But you'll quickly find that unless you have a tracking mount, you don't see as much of the planet at high power because you constantly have to nudge the scope. The tracking is well worth the extra trip. Planetary observing has different imperatives than DSO observing.
Absolutely, tracking is well worth the extra effort if viewing at high magnifications. I have the ServoCat on my 18" and will go to over 700x on the planets and catch those fleeting moments of steady seeing. without tracking I would never see as much detail.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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Jim Rosenstock
Post Laureate
Reged: 07/14/05
Posts: 4122
Loc: MD, south of the DC Nebula
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Quote:
Thanks guys. I'm not so hot on the tracking equatorial mount. I've found that when it takes 3-4 trips to get my gear out, it doesn't get used as often.
I find a smooth, well-balanced Dob no problem to manually track up to about 300X....beyond that, it gets to be work. I do have an equatorial tracking platform for my Dob, but I use it only a fraction of the time, when I know I'm going to do extended viewing at 300-400X.
Like, all next month with Mars! 
I also heartily recommend a long-focus Newt....
Cheers,
Jim
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Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3421
Loc: 51°N 4°E
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Thanks all! My next step will be to try and look at some option in the flesh. There's no way to judge handling and such from an online catalog.
If I understand correctly, as a rule of thumb, a well made newtonian (and well collimated) is approximately as good as a refractor with aperture equal to the reflector's aperture minus the diameter of its secondary mirror. Any comments on this?
This would almost rule out a 6" reflector now that 4.5-5" ED refractors have become so widely available at reasonable cost.
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD
APM 107mm f/6.5 on GR3-DX
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brianb11213
Post Laureate
Reged: 02/25/09
Posts: 3158
Loc: 55.215N 6.554W
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Quote:
If I understand correctly, as a rule of thumb, a well made newtonian (and well collimated) is approximately as good as a refractor with aperture equal to the reflector's aperture minus the diameter of its secondary mirror. Any comments on this?
Yes, it's rubbish. The central obstruction reduces light grasp by a totally insignificant amount (square law for areas, remember) and actually tightens the size of the central diffraction disk. True a bit more light goes into the diffraction rings but the effect of this is to reduce contrast, not resolution.
With modern super high reflectivity coatings (another thing which used to impact the contrast of reflectors was coatings which scattered light) there is next to no difference between the views you will get between a reflector with a 25% central obstruction and an unobstructed apochromatic refractor with the same aperture and the same quality optics. There is, however, a big difference in the numbers on the price ticket.
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Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3421
Loc: 51°N 4°E
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Thanks, I'll try to forget that rule of thumb. With my memory, that should be no problem. 
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD
APM 107mm f/6.5 on GR3-DX
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lionel
member
Reged: 08/12/07
Posts: 83
Loc: Delaware
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Quote:
If I understand correctly, as a rule of thumb, a well made newtonian (and well collimated) is approximately as good as a refractor with aperture equal to the reflector's aperture minus the diameter of its secondary mirror. Any comments on this?
Yes. It's a rule of thumb for contrast only, not light grasp or resolution. A well-made 6" newtonian will still outresolve and show brighter images than a smaller refractor.
Lionel
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Tak Mewlon 250
Powerstar C8
1960s Bushnell 60mm f12 refractor
Sphinx SXD
CG5-ASGT
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Mark9473
Post Laureate
Reged: 07/21/05
Posts: 3421
Loc: 51°N 4°E
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Thanks for clarifying that, Lionel.
I'll be honest, I'm 99% decided to get a 120 mm ED refractor. Thanks for all the advice though.
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Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63; Docter 15x60
WO Megrez II 80 FD
APM 107mm f/6.5 on GR3-DX
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