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Equipment Discussions >> Cats & Casses

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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Why would I have a diffraction spike on a C11? new [Re: Phil Hosey]
      #5747014 - 03/21/13 07:39 AM

That sounds like the problem because the diffraction pattern of a line will be perpendicular to it. This whole thing was puzzling to me if nothing was evident on the corrector - but if there is something on the primary that would explain it better. It may help just to mask it off with a thin strip of black paper just to try. You may need to put another on the mirror 90 degrees away to help balance it. Masking will introduce its own diffraction spike - but it may not be as bad, particularly if balanced with another one at 90 to it.

You might even be able to draw over the scratch with a black marker - but that would be harder to undo than a simple paper cover.

Frank


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Eddgie
Postmaster
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Reged: 02/01/06

Re: Why would I have a diffraction spike on a C11? new [Re: freestar8n]
      #5747179 - 03/21/13 09:27 AM

I have my doubt that a small scratch would cause a diffraction spike. Maybe.

To the OP..

A star test will amost always show sources of diffraction.

Have you use aimed it at a bright star and racked it way in and way out of focus?

If the scratch were bad enough to cause some kind of diffraction artifact, I would think it would be glaring in the star test.

You want to be out 20 or 30 waves in each direction.


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Phil Hosey
Carpal Tunnel
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Reged: 10/29/08

Loc: LaGrange, GA
Re: Why would I have a diffraction spike on a C11? new [Re: Eddgie]
      #5747389 - 03/21/13 11:24 AM

Quote:

I have my doubt that a small scratch would cause a diffraction spike. Maybe.

To the OP..

A star test will amost always show sources of diffraction.

Have you use aimed it at a bright star and racked it way in and way out of focus?

If the scratch were bad enough to cause some kind of diffraction artifact, I would think it would be glaring in the star test.

You want to be out 20 or 30 waves in each direction.




Eddgie, I have had it way out of focus on both sides but never noticed anything, but then again I wasn't looking for anything either. I'll try this again and this time I'll be specifically looking for the issue.


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Why would I have a diffraction spike on a C11? new [Re: Phil Hosey]
      #5747427 - 03/21/13 11:47 AM

What your images indicate is a very faint streak that only appears next to bright stars. There is no reason this would show strongly in a visual star test near focus since it appears to be faint - and its faintness, plus its perpedicularity to the scratch, for me points to the scratch as the cause.

Unless there is something else going on inside - it is the only lead to this unusual effect I have heard that is at least plausible.

Frank


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: Why would I have a diffraction spike on a C11? new [Re: Eddgie]
      #5748168 - 03/21/13 05:56 PM

Quote:

I have my doubt that a small scratch would cause a diffraction spike. Maybe.






My MN-66 has a 1" scratch in the corrector it does cause a faint diffraction spike on the brighter planets. Can't say I have noticed any spiking on the stars.

I can actually see two faint spikes and was initially puzzled since I had bought the scope knowing only about the scratch. The illuminated pupil test showed two apparent scratches but the other scratch was nowhere to be seen on the corrector, primary or the secondary. Turned out it was a stria in the corrector and very hard to see directly but easy to spot in the illuminated pupil. So I believe that if there's anything on the corrector or primary, the OP should've been able to spot it easily.

This test, BTW, will also easily spot any oil leaking onto the surface of an oiled objective.

Tanveer.


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