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Karl_Bonner_1982
sage
Reged: 05/13/09
Posts: 442
Loc: Springfield, Oregon (4.5ish ma...
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(feel free to move to Refractors forum if deemed appropriate)
We all know that short achros like Orion's 120ST are not recommended for planets, but some people use aperture stops to raise the effective f-ratio and cut down on CA. But this reduction in aperture also severely curtails the Dawes' limit. A 120ST f/5 with a 60mm aperture stop won't be any different from a 60mm f/10 refractor of the same lens quality, at least I don't think so (correct me if I'm wrong!)
If limiting CA is the only reason for stopping a fast refractor, then why not go even further and use only a 40mm mask, or for that matter a 10mm one? Clearly, CA isn't the only factor at play. After a point, any further reductions in the mask's aperture would make the image worse rather than better.
For something like a 120ST, what do you think is the ideal mask aperture for using on planets? And how good are the planets at this mask size?
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Current equipment:
Orion 8x40 fully coated binocs
Orion 90mm f/10 achro refractor
Celestron 102mm f/5 rich field refractor
Eyepieces: 25, 20, 12.5, 10, 7.5 (all plossls), borrowing 40mm Plossl and 2x shorty barlow
"We're here, we love refractors, get used to it!"
Edited by Karl_Bonner_1982 (11/19/09 04:24 AM)
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brianb11213
Post Laureate
Reged: 02/25/09
Posts: 3288
Loc: 55.215N 6.554W
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Why not try a minus violet filter instead?
The basic rule for "achromatism" of a doublet using normal glasses is that the f-ratio should be at least 5 times the diameter in inches. Your 120mm f/5 scope has a focal length of 600mm (say 24"). Trying various apertures:
D=2" f-ratio = 24/2 = 12 but need 5x2 = 10 longer therefore OK
D=2.4" f-ratio = 24/2.4 = 10 but need 5x2.4 = 12 therefore too short
The optimal size for resolution & achromatism is somewhere between, around 2.25".
An aperture that small will not show much detail on planets even with perfect optics.
Edited by brianb11213 (11/19/09 09:04 AM)
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7331Peg
scholastic sledgehammer
Reged: 09/01/08
Posts: 968
Loc: North coast of Oregon
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The two main reasons for stopping down a scope's aperture are reducing CA on a bright object - primarily a bright planet like Jupiter - or improving seeing on a turbulent night.
For Jupiter, stopping down to 100mm from 120 ought to be about enough to eliminate most of the CA and still not lose much resolution.
From the standpoint of seeing, 90 to 100mm should be about right - if that doesn't work, you're better off to call it a night, or give up until conditions improve. Yes, you'll lose some resolution, but it's the lesser of two evils in this case.
Sometimes the western Oregon skies just ain't all that friendly.
John
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Karl_Bonner_1982
sage
Reged: 05/13/09
Posts: 442
Loc: Springfield, Oregon (4.5ish ma...
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Not planning to use a 120ST for a Mak replacement, of course. This was just one of those geeky idea questions for those who happen to end up with a short refractor as a sole scope, and what suggestions to make in order to help them with the planets.
Perhaps some day when I have quite a bit more $ and perhaps a stronger mount, a fast achro will become part of my by-then-burgeoning scope collection. That, of course, would be after I have a big-aperture scope. But getting a stronger mount would also give me incentive to upgrade the 80ED to a 4-5" ED or a large achro for planets - but then having the 120ST might become redundant - but then....well you get the point. 
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Current equipment:
Orion 8x40 fully coated binocs
Orion 90mm f/10 achro refractor
Celestron 102mm f/5 rich field refractor
Eyepieces: 25, 20, 12.5, 10, 7.5 (all plossls), borrowing 40mm Plossl and 2x shorty barlow
"We're here, we love refractors, get used to it!"
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jg3
sage
Reged: 05/27/07
Posts: 332
Loc: near Auburn, CA
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It seems the more CA you got, the more you can benefit from good color filters. In effect, "stop down" the spectrum instead of the aperture, and really get the most possible resolution. Or to keep some amount of natural color, try minus-violet or "fringe killer" filters, which try to narrow the spectrum around the optimal range.
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7331Peg
scholastic sledgehammer
Reged: 09/01/08
Posts: 968
Loc: North coast of Oregon
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On my six inch achro, I've tried both a four inch mask and a minus-violet filter on Jupiter. Most nights I prefer the four inch mask, but if seeing is reasonably good, the minus-violet does pretty well at full aperture. Color filters haven't been a great success at full aperture.
Give both a try, Karl. You never really know what will work best and under what conditions until you spend some time with both methods.
John
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SteelStar
member
Reged: 11/08/09
Posts: 13
Loc: Oklahoma
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I stop my ST120 down to 90mm for the planets. Yes, there is still some CA but it is not to bad for me.
I have never liked MV or colored filters much. I would rather have a natural looking planet with a light purple halo instead of a greenish or other color disk and no halo.
I stop down my 6" refractor to 112mm ( thats the size of my center cap on mine).
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XT12 Classic Dob
C6R refractor on Atlas mount
120ST refractor
PST solar scope
Orion 9x63 binos
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mikey cee
Post Laureate
Reged: 01/18/07
Posts: 4103
Loc: bellevue ne.
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Try a Baader Semi-Apo. The only filter that works on Mars and Jupiter. Kills the CA but leaves the planet's natural colors intact. Some dimming is to be expected but overall it's a home run for me.  Mike
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7x35 and 10x50 sears tower binocs, 3" f/10 edmunds reflector, 2.4" f/11.7 manon refractor, 6" f/8 jaegers refractor, "The 8 Ball" 8" f/13.3 brandt refractor, 3" f/15.8 sans&streiffe refractor, 3.1" f/15 selsi refractor(towa 339), 2.4" f/15 sears refractor, selsi 30x30mm spyglass, criterion 5-draw 25x45x75x spyglass(1957), 4.25" f/14.8 tasco 20te.
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