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DSalters
sage
Reged: 03/03/07
Posts: 438
Loc: High Ridge, MO (St. Louis)
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I am not terribly new to astronomy, but there is a concept that is boggling my mind: The idea that long FL Newtonians that are good for planets and why they are called "Planet Killers".
Now, I know that a long f/ratio scope is not best for wide-field views and a fast scope is. However, isn't a 6" f/5 going to show the same image as a 6" f/10 at the same magnification?
The only difference to me is the eyepieces needed to achieve the same magnification:
To see 50x:
6" f/10 ~31mm EP
6" f/8 ~24mm EP
6" f/5 ~15mm EP
Thanks for helping clear this up with me, because I'm baffled.
Daniel
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6" f/8 Bushnell Voyager Dob. Rebuild
10" f/5 Highe Dob. variant
(UNDER CONSTRUCTION: 5" f/5, 8" f/6, 13" f/4.6)
Baader Hyperions--Primary Eyepieces
"The sun has one kind of splendor, the moon another and the stars another; and star differs from star in splendor." -1 Corinthians 15:41
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jg3
sage
Reged: 05/27/07
Posts: 209
Loc: near Auburn, CA
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Good question. The differences in views from such two scopes are subtle, assuming both have good optics. These differences are from:
1) Coma. This is a natural aberration of even perfect Newtonians. Newtonians focus perfectly only one point in the sky, which, if the scope is well collimated, is focused in the center of the eyepiece field. Every other point in the sky is focused imperfectly, with a smear out from where it should be. The size of the smear is approximately proportional to the angular distance off center, and inversely to the square of the scope's focal ratio.
2) Secondary size. The secondary obstruction broadens the diffraction pattern, softening contrast of features that already have low contrast. Secondaries under 20% (minor axis) of the aperture are almost indistinguishable from the same size unobstructed, but anything bigger loses contrast. The smallest secondary to illuminate a small planetary field is smaller with the f/10 than the f/5. Furthermore, with small Newtonians, you can't have both wide fields and small secondaries, so it just makes sense to outfit an f/5 for wide fields, and a f/8 to 10 for planets.
3) It's easier to make longer focal ratio mirrors than faster for a given optical standard. Or conversely, for a given effort or cost, you can expect closer to perfection from a longer focal ratio. Planet viewing demands the best optics.
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InkDark
scholastic sledgehammer
Reged: 10/29/07
Posts: 1461
Loc: Montreal, Canada
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What jg3 said plus Newts don't have any color aberrations like a achro. refractor might. Newts only have two optical surfaces which may help in reducing light loss (no diagonal and most of them don't have a correction plate).
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Jimmy
"Rarely Have So Many Understood So Little About So Much" - Palle Yourgrau
"...since that time, I have not complained about the weather one single time. I’m glad there is weather." – Alan Bean, Apollo 12
What do you mean by “Saving the Earth”? The Earth is not in danger! Don’t worry about the planet it will be here long after we are extinct...
Edited by InkDark (09/24/08 12:21 PM)
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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 2062
Loc: Cambridge, MA, USA
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Quote:
I am not terribly new to astronomy, but there is a concept that is boggling my mind: The idea that long FL Newtonians that are good for planets and why they are called "Planet Killers".
Now, I know that a long f/ratio scope is not best for wide-field views and a fast scope is. However, isn't a 6" f/5 going to show the same image as a 6" f/10 at the same magnification?
No, very similar but not quite the same -- all other things being equal. There are several differences.
First, if you want to avoid vignetting the edge of the field of view, you need a significantly larger secondary mirror for a 6" f/5 than a 6" f/10. That makes the view visibly less contrasty.
Second, most simple eyepiece designs exhibit significant aberrations at f/5, but almost all designs (including the much-disdained Huyghenian) work nearly flawlessly at f/10. Moreover, to achieve decent planetary magnifications (around 200X) with a 6" f/5, you need an eyepiece with a focal length less than 4 mm. That gives exceedingly short eye relief with simple eyepiece designs. And complicated eyepiece designs introduce more glass and more surfaces, which can't help and might hurt the image.
Third, it's really easy to make a top-quality 6" f/10 mirror, and much harder to make a really good 6" f/5.
Finally, focusing tolerances are twice as tight with the f/5, so you need a higher-quality focuser to achieve the same results.
Having said all that, the difference in planetary views between a good 6" f/5 and a good 6" f/10 is really not that big. And an 8" f/4 (of suitable quality, with good focuser and eyepieces) handily beats either one.
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Tony Flanders
eyeglasses
6x15 and 8x32 monoculars
8x25, 7x35, 10x30 IS, 10x50, and 15x70 binoculars
70mm and 100mm achromatic refractors
4.5", 7", and 12.5" Dobs
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DSalters
sage
Reged: 03/03/07
Posts: 438
Loc: High Ridge, MO (St. Louis)
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You guys have all sure cleared this up for me. There are clearly several differences that all add up.
I figured that coma would be a certain factor along with eyepiece choice. Like you said, Tony, with a fast 6", you'd need a very short focal length eyepeice and that would cause even more problems to enter into the situation.
Subtle contrast differences due to secondary size is something I had not thought about in this situation.
Thank you everyone, these factors all add up to make it pretty clear now.
Daniel
--------------------
6" f/8 Bushnell Voyager Dob. Rebuild
10" f/5 Highe Dob. variant
(UNDER CONSTRUCTION: 5" f/5, 8" f/6, 13" f/4.6)
Baader Hyperions--Primary Eyepieces
"The sun has one kind of splendor, the moon another and the stars another; and star differs from star in splendor." -1 Corinthians 15:41
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ckwastro
professor emeritus
Reged: 11/23/05
Posts: 724
Loc: AZ
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Don't forget too that collimation tolerances are much more forgiving in an f/10 than an f/5.
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Kerry
- 92mm AP Stowaway (f/7 version, uh...actually f/6.57
 )
- Takahashi Mewlon 210
- Giro 3 Twin View
- 10x42 Audubon Equinox
- Bogen 3221 tripod / 3047 3-Way Head
- Denk Std BV
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