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07-24-2008, 04:45 AM   #16
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I may be wrong here, but aren't the PF and CA only present with refractive systems and (at least mostly) nonexistant with reflective optical systems ? Glass refracts different wavelenghts differently but no such problems with reflections IIRC.

07-24-2008, 04:49 AM   #17
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QuoteOriginally posted by procyon Quote
I may be wrong here, but aren't the PF and CA only present with refractive systems and (at least mostly) nonexistant with reflective optical systems ? Glass refracts different wavelenghts differently but no such problems with reflections IIRC.
There are few lenses in a mirror scope...
07-24-2008, 04:54 AM   #18
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Almost all "mirror" lenses for cameras and many telescopes have a combination of mirrors and lenses. This is the reason, why "catadioptric" lenses would be the correct term for these compound systems.

The lenses are usually thin and don't introduce much colour, but in some cases it can be noticeable, though this is a rare occurance with quality optics.
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07-24-2008, 05:02 AM   #19
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QuoteOriginally posted by Ben_Edict Quote
I don't know these small Meade scopes, we have a new 16-inch in our observatory and I know the 12- and 8-inch modells. But in scopes you usually use screw-in filters at the back. Not directly on the scope, but somewhere in the camera tube. There are so many different camera tubes, that there is no general rule, but they should use a 2-inch filter (48mm screw thread), for which you find loads of NDs and colour filters.

As I don't know this scope, you should have a look at the front lens. If it is very thick, it should be a Maksutov lens, if it is very thin, it will be a Schmidt lens. The latter might be a little bit better (I assume, it will be a Schmidt, as most Maksutovs are slower than f/10), but need regular collimation, after transport. If you have never done that, you should ask a local astronomy club to explain that process.

A proper lens hood will be as long as the whole tube assembly, but will improve contrast significantly. Some people use flexible sheets of black plastic or even cardboard, which are simply wrapped around the front and fixed with velcro.

How is the focusing mechanism. Does this scope/lens sport the focusing knob on the lower right backside?

regards
Ben
Ben more great info. First I don't see a drop in spot for filters but maybe there were different tubes available for it at one time. Possiblly if I took off the T mount tube they can be screwed in that way but it would be a pain in the field to change ND's or other filters. I'd love to have a Polarizer for it though. The case hasn't got any slots in the foam for extras (like rear tubes or a range finder) and it's a Meade case.
The front element is a Schmidt. It's not very thick and the name is on the front ring. I'll have to look into the lens adjustment but it seems sharp and I live in the middle of nowhere when it comes to Astro clubs and stores that might help in that regard.

The focus ring is around the back tube right at the back of the body of the lens. It's not a knob style.

I was thinking the same thing. Taking a peice of black bristol board and fitting it to make a sliding hood. Hoods make a big difference in regular lenses and I assumed that it would really help this lens as well.

The weather is getting better here today and maybe tonight I can get some real use shots with this lens.

Thanks for the info. Now time for a Google search on DIY collimation.

07-24-2008, 05:11 AM   #20
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QuoteOriginally posted by Ben_Edict Quote
Almost all "mirror" lenses for cameras and many telescopes have a combination of mirrors and lenses. This is the reason, why "catadioptric" lenses would be the correct term for these compound systems.

The lenses are usually thin and don't introduce much colour, but in some cases it can be noticeable, though this is a rare occurance with quality optics.
Ben

I can't tell how many lenses are inside. But clearly there is a group in the center between the front mirror and the rear tube. But I did a number of CA test with bright sky and tree branches etc and this lens holds up really well. CA and PF isn't really noticable. The only weakness is the contrast level is lower than a 'normal' photo lens but to be expected. The hood should help that and PP can help as well.

I hope all this assists others in the hunt for a long lens. If you find something like this at a reasonable cost, I'd say give it a try. I'll post more images as I use the lens to give a more 'real world' experience with it. Now I might have to find a gimbal for the tripod....
07-24-2008, 05:57 AM   #21
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QuoteOriginally posted by Peter Zack Quote
B First I don't see a drop in spot for filters but maybe there were different tubes available for it at one time. Possiblly if I took off the T mount tube they can be screwed in that way but it would be a pain in the field to change ND's or other filters. I'd love to have a Polarizer for it though.
There are no drop-in filters on scopes, never. The next best thing you can use is a filter slider, but it will leave the unused filters exposed. The slider goes between the camera and the adapter tube: Multiple Filter Selector 2in - Excellent Quality Astro Accessories

Ben
07-24-2008, 11:58 AM   #22
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I took the lens out today again for another test shot or 2. It's a lot sharper than I realized the first couple of times. This was hand held at 1/160th with SR to the max 800. With enough contrast (ie sun) the AF confirmation will work. I just wanted to test PF and there is basically none (look at the chrome). The sun was directly behind me and this bike was about 125 feet from the shooting position. No tripod and a little sharpening, ISO 100.
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This is a 100% crop. I'm thinking of revising my web site and becoming a private dick. "taking pictures of your cheatin' husband/wife" NOT!
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07-24-2008, 11:59 AM   #23
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Btw thanks Ben for the Web site info. I will be doing some research there for sure.

Just for comparison here's the same scene taken with 2 other good lenses

Sigma 400mm f5.6 APO at 1/500th and f11
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100% crop
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Pentax FA*300mm f4.5 at 1/500 and f11
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Clearly in most respects the Pentax is the best performer but all are quite good. The telescope has the best PF control of the 3 which compared to the FA* is really quite amazing. But this is about as tough on PF as you could ask. Dark stuff with chrome in bright sun.


Last edited by Peter Zack; 07-24-2008 at 12:50 PM.
07-24-2008, 12:01 PM   #24
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Simply Amazing. !!
07-24-2008, 02:01 PM   #25
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Peter, these images are stunningly sharp. It must be one of the best mirror (catadiotric) lenses, of which i have seen images and that handheld (which is proof of your arm's steadiness).

Ben
07-24-2008, 02:26 PM   #26
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Peter

Rig is very similar to my C90 1000mnm F11

I hope you have better luck than I have had with my celestron C90 and vibration.

I have finally gotten all the vibration out of the mount and will be back out soon with it.

By the way, although hand held is almost impossible, I'm willing to bet a monopod would be a great idea. just leave SR on.

After reading the comments about CA etc. the corrective lenses in this 'scope are not the normal thin ones, they correct and add to the magnification. They also usually double for the support for the secondary mirror.

One thing to look out for, is whether the mirrors are coated or not. If not they are prone over time to degradation due to oxidation. Most scopes use either silver or aluminium coating on the lenses as the reflective surface. Some astronomical ones use gold, but they would add a color tint.
07-24-2008, 02:40 PM   #27
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Thanks for the info Lowell. I'll have to study some more on how to care for it and make sure it stays dry inside, temperature controlled etc. But as noted above, it's easy to clean (but I want to avoid opening it too much). The padded case is excellent for protection. I had to tighten the mount as well for vibration.

I'll use the tripod or monopod most of the time I'm sure. It's not easy to focus or hold steady hand held. Not heavy but the FOV is so darn narrow. Feel like a sailor on a boat.
07-24-2008, 05:09 PM   #28
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can i ask what may be a stupid question - are these mirror lenses fixed aperture? ie- is it always f/10 or can it be stopped down to f/32 etc? just curious, as i have never handled, or even seen, one.

i reckon it would be a great lens for moon shots.
07-24-2008, 05:17 PM   #29
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Damn it, Peter, I'm almost regreting not bidding on that auction! I would'a had a fast 28 and a slow 1000

You were complaining about no focus confirmation due to lack of Pentax contrast, but I think that on a cloudy day, and at f/10, it's more a matter of lack of light as you did get confirmation when it was bright and sunny.

Hey, now you should be able to get some icebergs on the horizon next time you're in Newfoundland.
07-24-2008, 05:17 PM   #30
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It's a fixed aperture. There is no aperture blade assembly inside the lens. It works with mirrors. So the light comes in the front element and goes to a mirror in the back that is curved. The mirror then sends the focused light back to the front where there is another small mirror in the middle. That one sends the light down the center and through a group of Glass lenses to allow focus etc. Then on to the sensor for pickup.
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