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#21
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Mirror lenses
Check out the sci.astro.amateur newsgroup and the various Meade yahoo
forums as a resource on using a SCT (Schmidt-Cassegrain) design scope for daytime photography. These scopes have highly curved fields which will make it virtually impossible to get focus at the center and edges of the frame, although this will be less evident with a sub-35mm size digital imaging chip. Also, as you move away from the optical axis, aberrations such as coma and astigmatism get worse. The f/6.3 designs are worse than the f/10 designs for off-axis performance. Focus is critical, as is using a very high shutter speed. The baffling is pretty poor on most of these, allowing stray light in to reduce contrast. You can improve this area of performance, however. You will get better results by using an MCT (Maksutov-Cassegrain) design scope, but they are more expensive. They have flatter fields, fewer off-axis optical aberrations, and higher contrast. The Questar is the best known of this design, but the best values in 6-10 inch apertures come from Russian optical companies. Evan Miller Brian Stirling wrote: On Thu, 15 Jan 2004 21:40:14 -0700, "Roger N. Clark (change username to rnclark)" wrote: Brian Stirling wrote: Roger, I have been toying with the idea of using a Meade 10" F6.3 1600mm telescope ( 10" LX200GPS-SMT) for a very long telephoto lense (as well as for astrophotography) and have had mostly unfavorable responses from virtually everyone I've asked about it. I can see some potential pitfalls of course, manual focus, fixed aperture being the most obvious, but I also wonder about the minimum focus distance with this setup. I have two cameras I plan to use with it: Nikon D100 (6MP DSLR) and Nikon F100 (35mm film) and I'd need to be sure I can adapt them to the telescope at the prime focus. I would also wish to be able to mount my cameras, using my 50mm F1.8 lense to the top of the tube and take advantage of the tracking system to do longer exposures without blurring due to the earths rotation. I am a little uncertain about the long exposure ability of this mount however as it does not look like an equatorial mount and without that I believe you need a 3-axis tracking system to include rotation of the tube along the long axis in addition to the other two axis. So, I don't know if you know much about this Meade telescope but if you do or know where I can find someone who does, I'd be very thankful. I'm confused. The LX200GPS-SMT is a 16-inch f/10 Schmidt-Cassegrain: http://www.meade.com/catalog/lx/16_lx200gps.html (Nature photographers: you thought you had it hard carrying around the 600 mm f/4, see the above!) The Meade LX200GPS-SMT is a LINE of telescopes and they make two that are 10 inches. One of the is a 2500mm F/10 and the other is a special order 1600mm F/6.3. Do you mean a Meade Schmidt Newtonian? E.g.: http://www.meade.com/catalog/lxd55/lxd55_series.html The lxd55 SN-10 is a 10 inch aperture 1016mm focal length f/4 system. The tube assembly is around $1100. I've been thinking of this one for astro. You can always move the mirror forward to get the focal plane into position. One of the advantages of the SCT design is that because focusing is done by moving the main mirror you can get a very large range of focus. I do not, however, know for sure just what the minimum focus distance is. The problem I see for general photographic work is focusing. Modern autofocus cameras to not have a good microprism viewfinder (like older 35mm cameras), so accurate focusing is very difficult. If your Nikons have interchangeable screens, then you might do OK. But following action will be extremely difficult. And if you can't focus accurately, you would probably be better off with a real autofocus lens, even if it is shorter focal length, like 300mm f/4. But for static subjects at long distance, a 1016mm f/4 would be amazing. It would be great for hawks and eagles in distant trees, especially with a 1.4 or 2x TC. I've been waiting to see reviews and photos from users of the Meade Schmidt Newtonians before I buy. I saw one comment that they were soft. But was it focus error, or what? Roger If a 1016mm f/4 would be sweet with a 1.4 or 2x TC how much sweeter would a 1600mm f/6.3 be without a TC? As I said, I don't think I would shell out $3K US for a limited use telephoto lense (telescope) alone but getting a bright (f/6.3) telescope of 1600mm FL and computer tracking and alignment as well might just be the icing on the cake. If only I could be sure of the minimum focus distance... Later, Brian |
#22
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Mirror lenses
On Sat, 17 Jan 2004 01:37:17 -0500, Brian Stirling
wrote: On Thu, 15 Jan 2004 21:40:14 -0700, "Roger N. Clark (change username to rnclark)" wrote: Brian Stirling wrote: Roger, I have been toying with the idea of using a Meade 10" F6.3 1600mm telescope ( 10" LX200GPS-SMT) for a very long telephoto lense (as well as for astrophotography) and have had mostly unfavorable responses from virtually everyone I've asked about it. I can see some potential pitfalls of course, manual focus, fixed aperture being the most obvious, but I also wonder about the minimum focus distance with this setup. I have two cameras I plan to use with it: Nikon D100 (6MP DSLR) and Nikon F100 (35mm film) and I'd need to be sure I can adapt them to the telescope at the prime focus. I would also wish to be able to mount my cameras, using my 50mm F1.8 lense to the top of the tube and take advantage of the tracking system to do longer exposures without blurring due to the earths rotation. I am a little uncertain about the long exposure ability of this mount however as it does not look like an equatorial mount and without that I believe you need a 3-axis tracking system to include rotation of the tube along the long axis in addition to the other two axis. So, I don't know if you know much about this Meade telescope but if you do or know where I can find someone who does, I'd be very thankful. I've got an older 8-inch Celestron "Big Eye" that appears to have been designed as a huge terrestrial scope, then promptly discontinued. It can be mounted to a tripod like any big tele. While it can take some impressive photos when conditions are right, that doesn't happen very often. It is probably the reason why no one has jumped in and made anything similar since then. With bags of lead shot piled on it and the mirror locked up, sharp photos are difficult at best, since at the ranges where it's effective there is usually significant amounts of haze and atmospheric effects. Years ago I had a Nikkor 500mm f/5, but it seemed to have low contrast and poor sharpness. The smaller Nikkor 500mm f/8 has usually shown better results, image-wise, and is quite tiny. The Nikkor 1000mm f/11 is also a good performer, and it's probably my fault that it doesn't get taken out more often. The f/11 aperture is limiting except on sunny days, and we get precious few of the here... |
#23
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Mirror lenses
KBob wrote in
: I've got an older 8-inch Celestron "Big Eye" that appears to have been designed as a huge terrestrial scope, then promptly discontinued Most of the Celestron, Meade, Questar etc. telescopes tend to be offered in a "spotting scope" format, without the astronomical mounting and accessories, for those who either want them for such purposes, or for those who have their own ideas about how to mount and just want the OTA with as little extra cost as possible. -- Colin J Denman N 51º 54' 38" W 00º 29' 45" Elev: 125m email: -- use my first name home: http://www.cjdenman.freeserve.co.uk |
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