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Old 15-07-2019, 04:20 PM
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pmrid (Peter)
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mirror lenses for astro

I recently purchased (from #Bay of course) a mirror lens rated at a fixed f6.3 and a FL of 500 mm (mirror is 80 mm diameter). With it came a x2 doubler giving an all up FL of 1000 mm at f12.6.
I know it won't be much use at deep space objects at f12.6 but it may do better at the shorter FL and f6.3.

In any case, I bought it to try some tripod-based images of the Moon and some night-scape objects as well.

Has anyone had a play with this sort of gear?

I'm using it with a Canon 40D but have an adapter that would allow me to couple it to a conventional astro-camera such as my SX and SBIG gear with filters etc. Could be a lot of fun since I now live in suburbia and will have to go the NB route from now on.

Peter
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Old 15-07-2019, 04:31 PM
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Atmos (Colin)
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Haven't ever tried one myself but I have heard reports of people using them as they've been reasonably soft. Fairly cheap and lightweight lens' so they're not bad for the price.

For astro I'd expect some not so flattering distortion and large stars.
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Old 15-07-2019, 04:53 PM
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I have one of those..
http://www.iceinspace.com.au/forum/s...ghlight=opteka

My specimen has a problem with coma.. I think it should be fixable, but I haven't found time for experimenting with moving elements to and fro..
https://www.cloudynights.com/topic/4...ens/?p=7942081

I was talking with couple of other owners (from Astrobin) and it seems my specimen is unique in that respect (that is what gives me hope I will be able to fix it someday).
https://www.astrobin.com/search/?q=500mm+f%2F6.3


Some reviews on web mention focus problem... it is not easy to focus this lens but there is a way around, see last post in the first link.
To make focusing even more precise, I added eccentric cylinder as focus stop. This arrangement with springs not only removes backlash, but with eccentric cylinder (acting as mechanical reductor) it compensates for way too steep focus thread of this lens.


All in all, when (if...) I remove the coma problem, it will be useful as it is very light. and if not, I will use it for guiding.
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Last edited by bojan; 16-07-2019 at 12:16 PM.
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Old 15-07-2019, 08:03 PM
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Ukastronomer (Jeremy)
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I was trying to work out what a "mirror lens" was, it is a CATadioptric . old, once the thing for sports about 35 years ago, always wanted to try one but the fixed aperture was a total put off

https://www.whatdigitalcamera.com/te...-it-work-64667

http://www.astropix.com/html/i_astrop/lenses.html
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Old 16-07-2019, 08:16 AM
Wavytone (Nick)
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Peter Iíve had three over the years
- a cheap compact and lightweight 500mm f/8 picked up for a song at auction,
- a Sigma 600mm f/8 and
- a Russian MTO 1000mm f/10 likewise found at auction.

None were sharp enough to be considered as a telescope - at high power they all showed a quite messy star image nowhere near as good as an equivalent scope.

I did use the little 500 f/8 as a finderscope though for many years with a 25mm erfle. For finderscope duty most cheap telephoto lenses usually work quite well too.
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Old 16-07-2019, 01:33 PM
Mickoid (Michael)
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Peter, I've owned one of these for a few years now, got it second hand for a reasonable price. It's a Tamron 500mm f8 and as well as using it for birdlife in our garden, I've chucked it on my Nano Tracker and pointed it at a few astro objects. It does an OK job, a bit tedious to focus but mine holds well once that's achieved. It occasionally gets some strange flaring off bright stars but you can fix that in PS or just crop them out. Below are a couple of examples taken with the mirror lens.
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Old 16-07-2019, 01:53 PM
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Tamron 500mm f8 is different kind of fish... much better optics than Samyang/Opteka/Bower/Rokinon/etc. mirror lens...

Also, it seems Opteka is Samyang (Korea), but manufactured in China (mine is Opteka.. not sure if coma in my specimen is quality issue?)

Last edited by bojan; 16-07-2019 at 03:00 PM.
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Old 17-07-2019, 12:02 AM
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pmrid (Peter)
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Thank you gents. I don't have any great expectations of this lens but hope that it will provide a decent basis for some lunar images. And of course, we have a full moon tomorrow(Wednesday) night so I will begin the experiment at the surf club at Woorim on Bribie ISland where, I think, the moon will rise behind the northern point of Moreton Island. I'll post the results after some playing with Registax and PS. Maybe!!

Peter
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Old 17-07-2019, 01:49 AM
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Ukastronomer (Jeremy)
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These days for things like astronomy they should be fine, just not day to day use
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