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surplusshed 127/1200 lens

Started by Phil Barker, 07/19/2012 07:47AM
Posted 07/19/2012 07:47AM Opening Post
My brother secured me a 127/1200 mm surplus shed lens which I was able to first light this week.

The lens came in a metal adjustable push pull cell a nice feature.

It looks like it has been removed from a tube so maybe was a second for some reason. I mounted it in a homemade PVC tube with a nice gso focuser baffles etc etc. I'm using it on a SkyWatcher HEQ5 mount.

Looking at the lens it looked like something was up with the coatings. A non centred circular pattern was visible. I wasn't too worried about this and put it in the tube and tried looking at a 1st magnitude star. I was horrified with what I saw and my heart sank as quite clearly a large non centred zone clearly related to the issue above was visible on each side of focus and seemed to be causing ghosting around objects in focus.

I removed the lens seperated the elements cleaned reshimed the gap to see the possible coating flaw mentioned earlier was completly gone.

Looks like R2 and R3 were tounching and the shims initially in there weren't doing there job.

I retested the lens and it turns out to be rather good with no trace of the circular zones mentioned earlier in the star test.

Antares split at 130 X and Saturn was good at 190 X. I noticed at 190 X with barlow a bit of violet around Saturn barely noticeable and similar to a 6 inch f-12 D & G a friend owns. Barlows do introduce a bit of false colour with an achromat I may need to get a triplet barlow.

In focus at 190 X on a bright star overhead and that lovely lean airy disc typical of a good refractor was clearly visible.

This lens turns out to be a bargin and I'm already planning to get a decent 4-5mm eyepiece to exploit the lenses potential. I won't be buying a CA filter as frankly it isn't too bad its there and I can cope with it.

Made me think perhaps it was a second and rejected because of the initial flaw I found. The flint lens has some revealing numbers around the outside in pencil showing it was tested for wedge.

Posted 07/19/2012 12:42PM #1
Posted 07/21/2012 02:22AM #2
I look forward to looking through this scope.

The 130X to split Antares sounds promising. Another good test is Gamma Centauri in good seeing.t should be a peanut in this lens if the spherical figure is good enough.

I am glad you were able to sort out the Newton's rings scenario. I read somewhere (possibly Cloudy nights) that earlier versions of the 127/1200 has a rubber ring between the crown and flint elements.
In theory a 5 inch f 9.4 is similar to a 6 inch f-12 if you crunch the numbers. The 6 inch f-12 you mention was the one you wrote about on CN which I owned. It was too big for my Zeiss 1b mount but I remember the open cluster images were outstanding.

I reckon 5 inches of aperture is optimum in New Zealand conditions.