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Stars with spikes?

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#1 James Cunningham

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Posted 17 August 2014 - 02:30 PM

What would cause small spikes all over stars even though they are in focus?  The stars also shimmer.  Thanks.

jim



#2 Starman1

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Posted 17 August 2014 - 04:48 PM

Astigmatism in your eye or the scope--your eye or the optics producing spikes--most likely your eye if its a low power.

Seeing issues in your observing sky, including an overhead jet stream

Thermal issues in the optics--glass not sufficiently cooled

So:

--try the power one up from your lowest power.

--wait until after midnight, when seeing is best at most sites, and/or do not observe below 30 degrees off the horizon.

And don't view directly over a roof, where heat waves from the roof interfere.

--leave the scope outside for a couple hours before starting to use.  If your scope is 6" or larger, use fans on the mirror.

 

Given your scopes, I'd say heat is a big culprit.

Look into this:

http://www.lymax.com/sct/


Edited by Starman1, 17 August 2014 - 04:49 PM.


#3 James Cunningham

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Posted 17 August 2014 - 05:07 PM

I have used a Lymax cooler and the eyepiece is a 6mm eyepiece.  I think it's something in the optics.  I can get a decent collimation.  I only see this in my 9.25 SCT.  I do not see spikes with the 11 inch SCT.



#4 Starman1

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Posted 17 August 2014 - 05:27 PM

Ah,

Well, then it may also be reflections from something in the train.

Like the inside of the central baffle, or some shiny screws.

If none of these, then perhaps the diagonal?  Or are you using the same diagonal in both scopes?

(If not, switch them to see).

Spikes indicate scratches or reflections or a hideously rough mirror surface.

You can go outside of focus and look at the rings to gauge correction and see it spikes are everywhere from the outer ring on one

side of focus and not on the other.

There are many issues that could be at fault.

But, if the same care taken with both sees no issues in the 11" and does in the 9.25", then.........



#5 James Cunningham

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Posted 17 August 2014 - 06:07 PM

I do not even use a diagonal.  I almost exclusively image.  However, I see the spikes with the camera and with eyepieces.  I have looked for scratches and spider webs but can see none.  I even used a vacumn cleaner to suck out anything inside of the tube.   



#6 Starman1

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Posted 17 August 2014 - 06:53 PM

The scratches I'm referring to would be sleeks in the glass, maybe before coating, and might not be visible except close up with a high power loupe

or in a Foucault test.

Have you adjusted back-focus distance with your camera to optimize the distance setting between primary and secondary mirror?

I believe the 9.25" has a somewhat shorter optimum back focus distance.

This would result primarily in spherical aberration, though, not spikes so much.

I presume the same camera on both scopes, which would rule out the camera.

 

You know, a slightly tilted corrector could do it, too.

Make sure the spacers perfectly center the corrector plate in its cell and that the corrector is perfectly seated before you snug up the pressure ring that holds it in place.

Remember the plate can be warped by too much pressure on the ring's screws, too.  They should be snug--snug enough to hold the plate in place--but not too tight.

Err on the side of a little loose if you're not sure.

 

But, if the corrector is centered and seated and collimation is good, then the difference has to be optics.



#7 dave brock

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Posted 17 August 2014 - 11:30 PM

What would cause small spikes all over stars even though they are in focus?  The stars also shimmer.  Thanks.

jim

 

The spikes on these stars were caused by rough optics with a turned down edge

Attached File  253 f9.5.JPG   36.14KB   0 downloads.

 

Dave



#8 James Cunningham

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Posted 18 August 2014 - 03:07 AM

That looks a little like mine but the spikes are much, much smaller.  They just peek out from the body of the star.  When you say rough optics and a turned down edge, just what so you mean.

Jim



#9 dave brock

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Posted 18 August 2014 - 07:17 AM

I mean the mirror's surface was not polished smooth and the edge zone focused too long.

 

 If your spikes are much smaller then it may be from something else.

 

Dave








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