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Recent Questar Advertising.
On Thu, 10 Jul 2003 09:54:33 +0100, "Roger Hamlett" ...reflected:
"Alan W. Craft" wrote in message .. . On Mon, 7 Jul 2003 23:42:37 +0000 (UTC), William Mc Hale ...reflected: Shawn Grant wrote: Anyone who buys a Questar is stupid plain and simple. Meade ETX is just fine. In fact its optics are better. The optics in the ETX are good, not sure I would say better than a Questar, the Questars I have looked through have been more or less perfect. In any case, optics are only part of the reason to buy a Questar, and in my opinion, in these days of beautiful Apos in the 3.5-4", the optics are the least compelling reason. A Questar represents a beautiful precision instrument whose mechanical components are as well thought out as the optical components and will, with some care last a lifetime or more of use, not sure anyone would say that about the ETX. Have fast apochromatic refractors rendered the Maksutov's redundant, especially considering the latter's seconday obstruction? Wasn't their primary purpose to achieve slow-refractor-esque performance in a tube half or even less in length? After all, a secondary obstruction is ALWAYS just that, and never a boon to performance; a necessary evil, if you will. In one part, you are answering yourself. I fancy myself a master at rhetorical questioning; just ask the denizens throughout Usenet, particularly within the religious groups... ....and the not-so-religious... Generally, an APO, will be quite 'fast' (perhaps f/10 or lower, especially on designs with larger objectives). A Mak with a small CO, will be designed to give a long focal length, and be relatively 'slow' (f/15 or higher). There are some people who have built/used such APO's with focal ratios like this, since long focal length can make them excellent for some types of planetary observation, but once you have tried handling a f/20 6" APO (120" long...), you soon realise just where the Maksutov wins!... Unfortunately, many of the commonest Maksutov designs sold, are at the 'low end', of the sort of focal ratios which were originally built (Maksutov did designs up to about f/35), and hence lose out by needing slightly larger CO's. It is also worth realising, that the last line, that a 'secondary obstruction is ALWAYS just that, and never a boon to performance; a necessary evil', can at very small obstruction levels be argued about!. At lower obstruction levels than are common for most Cassegrain designs (perhaps 18 - 20% or less), the degradation is tiny, and can result in a sharper fall off at the edge of the Airy disk. Oh, but there's no denying an utterly obstruction-free, even if it's a pinhead in size, REFRACTOR... Newbie: "Uh...what's ~THAT~ in the center?" Dealer: "Oh, that's just the secondary...O-B-S-T-R-U-C-T-I-O-N!!! O-B-S-T-R-U-C-T-I-O-N!!! O-B-S-T-R-U-C-T-I-O-N!!! (Is there an echo in here?) Newbie: "Ooooooo...what's that over there, glistening? Dealer: "Uh...a refractor." Newbie: "Sold." Lastly, just who exactly would turn his back on a nudie show yet watch it nonetheless with his wife's borrowed compact in hand? And no, I did not type from personal experience. When I want to see stars, I look DIRECTLY at them. When I want to look at myself, which is often admittedly, I employ a mirror. With any obstruction, energy is shifted from the centre of the Airy disk outwards, but at low obstruction levels, little energy actually shifts outside the Airy disk, instead moving just towards the edge of the disk, and resulting in a slightly sharper edge. Historically, some observers have even tried adding small obstructions to APO's, to produce this effect!. With the larger CO's, that are common, the effects are negative, but if you observe through a design like a f/20 8" Mak, with a small CO, you will find just what Maksutov was trying to achieve!. Now the Questar, has a low enough focal ratio, that the CO, will be having a noticeable effect. However against this, you have the very small amount of chromatic aberration from a basically reflecting scope, against the best that an APO can achieve, which is by no means 'perfection'. In a sense, both designs are compromises. If however you look at the price of their 7" Mak, and then try to find a 7" Flourite APO, their products may well start to look quite a 'bargain'. The 3.5" scope, is in a sense, a 'niche market' product for somebody who wants the best small scope, in a very portable package. It is worth remembering also, that the current 'crop' of 4" APO's, didn't really exist, only a very few years ago, with the units that were on the market being relatively a lot more expensive. Best Wishes Not if you're advising a Mak over a Tak, you're not, you're not, you're not. Toodles! Alan |
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Recent Questar Advertising.
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