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Double Diffraction Spikes Normal Out of Focus?

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#1 Kent10

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Posted 01 October 2015 - 10:39 AM

A quick question.  Are double diffraction spikes normal on a bright star when slightly out of focus or does this mean my spider vanes are not perfectly aligned?  The spikes are just slightly double out of focus and when in focus they line up.

 

Thanks, Kent



#2 xiando

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Posted 01 October 2015 - 11:21 AM

Yes to the first. It just means you need to tweak the focus a bit in one direction or the other until the doubled spikes merge. If you're seeing double spikes only in some of the pattern, (for instance, when focused, one of the spikes still shows a bit of doubling) it means you have a collimation issue as well.

 

edit: content clarification


Edited by xiando, 01 October 2015 - 11:22 AM.


#3 Nils Olof Carlin

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Posted 01 October 2015 - 11:56 AM

This could happen when the out-of-focus star is a "donut", due to large obstruction and perhaps some spher abb added. Thus the image is brighter along the "tangents" than along the diameter, causing parallel spikes. But with a bright enough star to cause visible spikes, the star image itself is bloated.



#4 Kent10

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Posted 01 October 2015 - 03:53 PM

Thanks xiando and Nils.  I was using Vega so it was a bright star.  My Starmaster 16.5" scope has a 3.5" secondary.



#5 Lulu66

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Posted 05 October 2015 - 09:49 AM

I seen those double drffraction spikes, but on my 12" sw thet are not exactly parallel. They seem to  merge at the star and again at the end of the spike. More like 2 opposing arches per spike. I havnt got a chance to troubleshoot it, but im guessing its twisted spider vanes.



#6 Nils Olof Carlin

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Posted 06 October 2015 - 01:43 AM

The likely cause is non-parallel opposite vanes. Spikes are perpendicular to the vane halves, and if these are not parallel, the spikes won't be either, but they share the origin at the (focused) star. Should be easy to check, but you need to figure how to remedy it, if worth bothering at all.

Nils Olof



#7 Allan Wade

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Posted 06 October 2015 - 08:06 AM

Get a piece of cotton or similar and hold it at both ends of the spider vain where they attach into the tube. Now check to see if the vain is dead straight across the full width of the tube using the cotton as your guide. Out of the factory mine were not straight and that gave me double diffraction spikes on either side of focus. After I straightened everything up the problem disappeared.



#8 Kent10

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Posted 06 October 2015 - 09:51 PM

Get a piece of cotton or similar and hold it at both ends of the spider vain where they attach into the tube. Now check to see if the vain is dead straight across the full width of the tube using the cotton as your guide. Out of the factory mine were not straight and that gave me double diffraction spikes on either side of focus. After I straightened everything up the problem disappeared.

I checked mine using a string and also a long ruler.  I could see one vane was just slightly bowed.  Also, one pair where they meet in the middle was just slightly out of alignment.  I am not sure it is easily remedied.  I assume this won't affect views.  I am just trying to understand my scope and how it all works. Thanks.




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