|
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
refractor 120mm : f/5 or f/10 ?
"Wally" wrote in message ... well f7 or f8 would be ideal, if the achromat is well corrected. I made an f8 in 120mm dia once and I wish I had never given it away. I agree. and we can check this to manufacturers. The Meade achromats are f/8, f/9, AP apos are f/8 or higher, only TV sells a f/6.3 but it a apo as well.. I am even not sure that using ED lens on a 120 mm f/5 achromat we can prevent chromatiism. For a so short focal we need a minimum Fluorite lenses. So I think that I have to find anotehr 120 mm close to f/7-f/8 indeed. Thierry Thierry wrote: Hi, I would like to buy a small achromat (not sure to take an apo) OTA of 120 mm in dia. but I can't put my finger on the right focal length to select : 600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ? I know quite well optics and its problems, what change in the FOV, "speed", etc see at http://www.astrosurf.org/lombry/menu...el-acheter.htm (i translate ;-) how to select a scope) This new scope will be mainly used for visual ovservations (moon, bright planets, sun, starry fields...), and casually. I have the mount so I only need the OTA. I wonder if in purchasing a f/5 I will not regret the f/10 with its greater magnification on planets and if I buy the longer one, if it will not be too dark visually (I owned various scope but usually just in the middle, .f/7-8, no luck). But what is your feeling ? If you own a 100-120 mm refractor f:/5 or f/10, - if you had to make a new purchase, if you own a f/5 'd you buy the same one in f/10, and conversely - if you own a f/10, 'd you rather buy the f/5 ? and why 'd you change ? (magnification, brightness, problem of focusing with some accessories, other ?) Of course in changing from f/10 to f5, and essentially with a refractor, they are much chances that you improve a lot the chromatic aberration on a achromat. The prevent this I could also buy the OTA with ED lens (go with itn, but what f/ratio ? ). All your opinions will be appreciate to make my choice. (it will be easier and cheaper than buying both scopes and test them a year long...;-)) Thierry --- Auteur de "Un siècle de Physique, 1- La Physique Quantique", AEGEUS, 2005 http://www.astrosurf.org/lombry |
#12
|
|||
|
|||
refractor 120mm : f/5 or f/10 ?
On Thu, 14 Sep 2006 08:18:28 -0700, Alan Charlesworth
wrote: In article , William Hamblen wrote: On Wed, 13 Sep 2006 14:17:52 -0400, "Tom Royer" wrote: The one drawback to the shorter FL is that the scope may be more susceptible to false color. There's no "may" about it. A 120 mm aperture f/5 telescope achromat with common glass will have a lot of false color. You normally want the focal ratio on an achromat to be approximately the aperture in centimeters, or about f/12 for a 120 mm telescope. Bud Has anybody tried the new Orion 120 ED f/7.5 (price $2000)? Uncommon glass, or so they claim. -- The night is just the shadow of the Earth. |
#13
|
|||
|
|||
refractor 120mm : f/5 or f/10 ?
Thierry wrote:
"David Nakamoto" wrote in message news:Gc7Og.52096$Qb2.863@trnddc08... Hi Thierry, I actually got a 120mm f/5 refractor, and then got a 127mm f/12 Mak because I couldn't get enough magnification for planets and the moon, with eyepieces or a web camera. Even on deep sky objects, the slight contrast improvement going to the longer focal length is probably worth it. Yes. It is indeed what I am afraid of. the max useful magnoification is about 300x; thus using a.... 2 mm eyepiece ! Now, there is the psosibility to add it a quality barlow (I know that some also take into account a chromatic correction, they are not simple "apo" barlow) But you still have to deal with the chromatic aberration from the telescope itself and its short f/ratio. I think if the observer's main interest is Sun, Moon, and planets, then go with the longer focal length. --- Dave Thierry Just my humble opinion. Clear and Steady Nights ! --- Dave Thierry wrote: Hi, I would like to buy a small achromat (not sure to take an apo) OTA of 120 mm in dia. but I can't put my finger on the right focal length to select : 600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ? I know quite well optics and its problems, what change in the FOV, "speed", etc see at http://www.astrosurf.org/lombry/menu...el-acheter.htm (i translate ;-) how to select a scope) This new scope will be mainly used for visual ovservations (moon, bright planets, sun, starry fields...), and casually. I have the mount so I only need the OTA. I wonder if in purchasing a f/5 I will not regret the f/10 with its greater magnification on planets and if I buy the longer one, if it will not be too dark visually (I owned various scope but usually just in the middle, .f/7-8, no luck). But what is your feeling ? If you own a 100-120 mm refractor f:/5 or f/10, - if you had to make a new purchase, if you own a f/5 'd you buy the same one in f/10, and conversely - if you own a f/10, 'd you rather buy the f/5 ? and why 'd you change ? (magnification, brightness, problem of focusing with some accessories, other ?) Of course in changing from f/10 to f5, and essentially with a refractor, they are much chances that you improve a lot the chromatic aberration on a achromat. The prevent this I could also buy the OTA with ED lens (go with itn, but what f/ratio ? ). All your opinions will be appreciate to make my choice. (it will be easier and cheaper than buying both scopes and test them a year long...;-)) |
#14
|
|||
|
|||
refractor 120mm : f/5 or f/10 ?
Thierry wrote: Hi, I would like to buy a small achromat (not sure to take an apo) OTA of 120 mm in dia. but I can't put my finger on the right focal length to select : 600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ? I know quite well optics and its problems, what change in the FOV, "speed", etc see at http://www.astrosurf.org/lombry/menu...el-acheter.htm (i translate ;-) how to select a scope) This new scope will be mainly used for visual ovservations (moon, bright planets, sun, starry fields...), and casually. I have the mount so I only need the OTA. I wonder if in purchasing a f/5 I will not regret the f/10 with its greater magnification on planets and if I buy the longer one, if it will not be too dark visually (I owned various scope but usually just in the middle, .f/7-8, no luck). But what is your feeling ? If you own a 100-120 mm refractor f:/5 or f/10, - if you had to make a new purchase, if you own a f/5 'd you buy the same one in f/10, and conversely - if you own a f/10, 'd you rather buy the f/5 ? and why 'd you change ? (magnification, brightness, problem of focusing with some accessories, other ?) Of course in changing from f/10 to f5, and essentially with a refractor, they are much chances that you improve a lot the chromatic aberration on a achromat. The prevent this I could also buy the OTA with ED lens (go with itn, but what f/ratio ? ). All your opinions will be appreciate to make my choice. (it will be easier and cheaper than buying both scopes and test them a year long...;-)) If you are serious about either deep-sky and planetary observing, you need light-gathering power, so please consider a 200mm or larger Newtonian with quality optics. If you also need a very wide field of view, consider getting an -additional- telescope, with a short focal length, instead of trying to make your main scope serve all purposes. Achromatic refractors, except in short focal ratios and small sizes, are largely obsolete. Newtonians are cheaper, more powerful, and sometimes more portable. SCTs and Maks are less cumbersome and not much more expensive. Apochromats avoid many of the problems of the other designs (at a price) but are limited in aperture. |
#15
|
|||
|
|||
refractor 120mm : f/5 or f/10 ?
Thierry wrote:
I would like to buy a small achromat (not sure to take an apo) OTA of 120 mm in dia. but I can't put my finger on the right focal length to select : 600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ? This new scope will be mainly used for visual ovservations (moon, bright planets, sun, starry fields...), and casually. I have the mount so I only need the OTA. Well, I own a 100mm f/6 achromat, which presumably has only 2/3 as much chromatic aberration as a 120mm f/5 achromat. It's a superb deep-sky scope, but I would *not* recommend it to somebody who wants to spend much time viewing the planets. The problem isn't so much the colored fringes -- I can put up with those -- as the loss of contrast. If you put it side-by-side with a 100m f/6 apochromat, the difference is shocking. And when you're viewing Jupiter or Mars, you really need all the contrast you can get. It's not half bad on the Moon, though the violet fringes are *truly* garish. Still, the Moon has something to show at every magnification, and the 100mm f/6 beats my high-quality 70mm f/6.9 achromat hands down on such a high-contrast subject. On the other hand, a 120mm f/10 doesn't appeal to me at all -- just too darned big. I'd much prefer a 125mm Mak or a 150mm f/6 Newt. But obviously, other people feel differently. - Tony Flanders |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Apogee 4" f/10 refractor | elaich | Amateur Astronomy | 0 | September 21st 05 08:49 AM |
EP with widest actual FOV AND largest magnification for C5-S or C8-S? | Alen MacT | Amateur Astronomy | 75 | January 5th 04 08:40 PM |
Yet another mystery 'scope... | Alan W. Craft | Amateur Astronomy | 10 | November 30th 03 05:22 AM |