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Stars become comets in Chinese BV

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#1 Usquebae

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Posted 02 January 2014 - 09:12 PM

I have found my Arcturus BV (same as WO, etc.) unusable in my Newt. I was trying a couple nights ago for Jupiter with 3x Barlow and 20mm Expanse EPs in the F/5.9 dob, 179x. Jupiter looked like Venus at high mag near the horizon, only worse, and its moons were streaking arrowheads, maybe commonly called comets or teardrops? Same effect on stars. A hideous sight.

Collimation and cooling of the Newt were proven near optimal when I got a decent mono view of Jupiter at 521x - far, far higher than I have ever before achieved. My two previous attempts at using the BV in the Newt were similarly plagued, though not quite so bad as this last time. I had assumed it was from poor mirror collimation, or pinched mirror, but that possibility is now eliminated. Oddly, I've had pleasing views with the BV in my C90 Mak using the same 3x Barlow. It does seem the stock 30mm plossls work better than the 20mm Expanses, but I don't think I can blame the EPs for the problem.

Any ideas about what's going on? Being new to binoviewing (and astronomy altogether) I would appreciate some troubleshooting guidance. Everything I can find online seems to be about merging issues, but I seem to be blessed with a good tolerance for binocular misalignment and don't think that's the issue.

:help:

#2 GlennLeDrew

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Posted 03 January 2014 - 01:46 AM

I wonder if there is very significant sagging at the focuser, which throws alignment out so much that you're examining a part of the field which is well off axis? Your aberration reads like severe coma...

#3 beatlejuice

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Posted 03 January 2014 - 02:30 AM

Does it look any better on either side with one eye closed?

Eric

#4 Usquebae

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Posted 03 January 2014 - 07:49 AM

The effect was present in each barrel independently, though I don't know if the effect was equal.

Sagging seems possible. The Arcturus Barlow nosepieces are oddly tapered, and the positioning in the Newt would be different from the C90. I've also got compression rings on one scope and set screws on the other. Will test tonight.

Thanks.

#5 Eddgie

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Posted 03 January 2014 - 09:46 AM

Try rotating the BV on the eyepeice axis and see if the aberrations rotate with them.

If they do, it is the BV. If not, it is either sag or some other factor.

And you can just press in on the back of the diagonal to hold it flush against the visual back to check that it is not sagging there.

But I have had this problem with reverse tapered diagonals. I won't own another one.

And in my Mark Vs, the heavy ES eyepecies with the reverse taper sag too.

One of those great ideas on paper that does not always work as well as what went before it.

#6 Usquebae

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Posted 04 January 2014 - 02:37 AM

[mis]Alignment was indeed the issue. Aberration was the same in each eyepiece and did not move with the viewer when I rotated the entire unit. I screwed a GSO 2x Barlow cell into the 1x Arcturus nosepiece, which is not tapered, and the problem vanished. Got my first good, confirmed view of the Great Red Spot - thanks to you CN gurus!

:thanx:






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