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07-25-2017, 09:59 AM   #16
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QuoteOriginally posted by wanderer2 Quote
..................On Amazon and B&H I also found many non-mirror f8 - 32 manual lenses in 500 mm and several of these include a doubler for 1000mm reach for a bit less than $100. A description is below (I assume these are all made by the same manufacturer). These would seem ideal for the eclipse but I wonder about the IQ. When a 500 mm standard type lens from Sigma, etc. costs $5-6000 it's difficult to imagine a $90 lens would provide much quality. I'm not knowledgeable about optics and have no idea what "catadioptic mirror lens" or " achromatic refractor design" mean. The description:......................
Those are the ones.

The design uses a thin lens up front with a small circular spot in its middle that's aluminized to reflect light from the main mirror at the back. The rear mirror is spherical which is low cost to make and the lens corrects for most of the spherical aberrations of the mirror. These are also referred to as a "folded" optical system and are shorter than just having the main mirror in a tube like a Newtonian (standard type) reflecting telescope. It allows a focal length of 500mm in a lens much shorter than 500mm.

If the manufacture does their job well, this can be a pretty good lens (though less contrast than a refractive lens but that can usually be corrected in post-processing). The "CATS" that B&H carry are of Asian origin and may or may not be constructed so well. I really can't advise since I haven't used any and there may be variations between brands or even individual lenses, but there are some promising reports elsewhere (some in the Pentax forum) you might check if interested. Try Googling also.

The image quality would probably be on a par with most low cost telescopes and maybe even better since the "CAT" has better aberration correction (theoretically). They are certainly easier to handle because of their compact design. As you surmised, they probably won't stack up to a Sigma prime 500mm and they are slow compared to f5.6 or f4 (and have fixed aperture and no autofocus capabilities).

I would get or make a solar filter that fits over the front of the CAT. Alternately, you can buy a skylight or UV filter, remove the retaining ring with a spanner wrench and put a disc of solar filtering material in place of the glass, making sure there aren't any gaps at its edges when the retaining ring is screwed back in. I just mention the latter because of the short time, you may want to order a filter mount at the time you order the CAT if you choose to do so. The slip-on type filter changes faster than a screw-in filter though.


Last edited by Bob 256; 07-25-2017 at 10:04 AM.
07-26-2017, 01:26 PM   #17
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QuoteOriginally posted by Bob 256 Quote
Those are the ones.

The design uses a thin lens up front with a small circular spot in its middle that's aluminized to reflect light from the main mirror at the back. The rear mirror is spherical which is low cost to make and the lens corrects for most of the spherical aberrations of the mirror. These are also referred to as a "folded" optical system and are shorter than just having the main mirror in a tube like a Newtonian (standard type) reflecting telescope. It allows a focal length of 500mm in a lens much shorter than 500mm.

If the manufacture does their job well, this can be a pretty good lens (though less contrast than a refractive lens but that can usually be corrected in post-processing). The "CATS" that B&H carry are of Asian origin and may or may not be constructed so well. I really can't advise since I haven't used any and there may be variations between brands or even individual lenses, but there are some promising reports elsewhere (some in the Pentax forum) you might check if interested. Try Googling also.

The image quality would probably be on a par with most low cost telescopes and maybe even better since the "CAT" has better aberration correction (theoretically). They are certainly easier to handle because of their compact design. As you surmised, they probably won't stack up to a Sigma prime 500mm and they are slow compared to f5.6 or f4 (and have fixed aperture and no autofocus capabilities).

I would get or make a solar filter that fits over the front of the CAT. Alternately, you can buy a skylight or UV filter, remove the retaining ring with a spanner wrench and put a disc of solar filtering material in place of the glass, making sure there aren't any gaps at its edges when the retaining ring is screwed back in. I just mention the latter because of the short time, you may want to order a filter mount at the time you order the CAT if you choose to do so. The slip-on type filter changes faster than a screw-in filter though.
Thanks once again for all the useful info Bob. I noticed that Samyang/Rokinon sells a 500 mm CAT. Given the good quality of the prime lenses made by Samyang perhaps the quality of the CAT would be superior to most others. Or perhaps not? I wonder if Samyang actually manufactures theirs - all of them sound very much alike in the descriptions.

Do you think there is any reason to not use pixel shift, other than the large file sizes which won't be a problem for me as I will have some large capacity cards? Thanks again.

Btw, do you live in the zone of totality? I noticed you live in Idaho as I do and I am in the zone, although not in the center.

Mike
07-26-2017, 01:58 PM   #18
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That's true about Samyang Mike. I think I heard somewhere that they make a couple of the CAT lenses but I'm not sure which ones. Probably a better bet than one of the unknown brands.

I don't know how much you would gain with PS. Unless you have a really sharp lens, it probably wouldn't be of benefit. There isn't that much detail in the partial phase and the total phase will show the corona and maybe some prominences which are kind of "whispy" anyway though better sharpness might be better to see fine details in the prominences. A complication is that the successive shutter actuations of PS could introduce some vibrations countering any improvements. I would test it ahead of time on a target like the moon and see if there is any advantage to PS shots (take a look at the small craters). The moon will also move like the sun so you can see if that introduces PS problems. Also, you can get some thermally induced distortions (heat waves) with the sun that could change each of the PS frames slightly causing ghosting artifacts. I don't plan to do any PS shots but will do substantial bracketing (another complication with PS is if you want to bracket).

I live in the totality zone near the Oregon border but plan to move north so I can get the full 2 minutes of totality. Where I am will only have 1 minute. Half of Boise might be moving with me so it could be a crazy day, especially after the eclipse. Will take camping gear & supplies just in case.
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