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Old Dec 4, 2006, 8:20 PM   #1
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Unfortunately my budget does not run to new lenses so I buy from on-line auctions. My usual technique is to decide what I think a lens may go for then put in an auto-bid for some lesser some. Sometimes I get the item, sometimes I miss out. Sometimes I get the item and wish I had missed out!

The last box of goodies to arrive from the postman included a Sigma 600mm f8 mirror lens. It was very cheap but as often happens with cheap lenses this one had it's crop of mushrooms.

I did a gentle clean with alcohol and amonia based window cleaner and it looks quite good although the images it produces are indeed rather 'soft'. I have some discolouration of the coating on the front glass and a few specks on the small mirror. The big mirror is clean as a whistle after the treatment. Now I just need a nice bright day to mount it on my DS and go try it on some wildlife around here! (mainly sparrows and seagulls :sad




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Old Dec 4, 2006, 10:10 PM   #2
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john,
are you disassembling these lenses? if so, how about a tutorial....

roy
i've gotten a few lenses with the same technique.. happy sometimes, disappointed others..
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Old Dec 5, 2006, 12:16 AM   #3
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OK, how to get into you Sigma 600mm mirror lens.

Carefully roll back and remove the rubber focus grip sleeve. Remove the piece of metallic adhesive tape you see wrapped around the lens. Now unscrew as you would altering the focus, continue to unscrew and it comes apart in your hands!

There, you have the lens apart with no tools and you can do gentle cleaning from right there.

The front glass has a coated rear and should be treated accordingly. The big mirror is thick glass with the mirror coat on the rear so no special cleaning worries there, as far as I can see.

The little convex front mirror is another issue,I cant confirm by looking at it that it is back or front surfaced, I treated mine very carefully with a soft bristled brush and alcohol. I carefully dried it with a microfibre cloth.

There is at least one small lens in the central tube which I cleaned with my brush and the rolled up cloth.

Reassembly is easy, just screw the front part back on then mount it on a camera body. Focus at a known, or estimated, distance and align index ring accordingly and reapply the tape. I was able to reuse my tape. Slip the rubber sleeve back on and the job is done!



Most of the fungus on mine was on the front glass and there are some discoloured spots that I think are permanent and fungus on the big mirror which was about 30% or more covered in growth. I used the ammonia window cleaner on that.



No warranties expressed or implied!:-)




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Old Dec 5, 2006, 1:09 AM   #4
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This fellow was screaming abuse at me from a building across the street!



Sigma 600mm f8 mirror lens.

100% crop 1/350 second, ISO 200. Taken at about 06:50 on a dull gray morning!

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Old Dec 5, 2006, 10:10 AM   #5
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thanks for the tut, john. one thing i do know is that the internal mirrors in all lenses and telescopes are front mirrored. otherwise you get distortions from the light going thru the glass. enjoy your lens.

roy
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Old Dec 5, 2006, 1:40 PM   #6
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Quote:
one thing i do know is that the internal mirrors in all lenses and telescopes are front one thing i do know is that the internal mirrors in all lenses and telescopes are front mirrored.


Errr, dont think so, as far as I can tell the concave mirror in this lens in not front mirrored. When I put my cleaning wand on the surfacethere is still a sizeable gap between it and the reflection. I was able to scrub quite assertively to get the fungus off the mirror without damage to themirror.
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Old Dec 5, 2006, 6:44 PM   #7
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that's the first timei've heard of it.. the reason for using a mirror is light transference. putting more glass in the way is defeating it.. also you have to shape the glass on 2 surfaces instead of one which is another reason for using a mirror-- cost.

roy
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Old Dec 5, 2006, 8:41 PM   #8
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Dangnation Royyou are forcing me to take another look inside and maybe photograph that thick mirror! :-)
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