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Why are my diffraction spikes offset/misaligned?

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#1 Ty Williams

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Posted 25 August 2017 - 11:04 PM

A while back, we bought a new Astro-Tech AT-10D scope to replace our previous EQ-mounted Newt. It arrived just in time for a lot of things to suddenly change in our lives so we put it in the back of the closet without looking through it even once. A few years went by and the eclipse caused us to haul it back out and things are not good. Specifically, the view through it is much, much worse than our old 8" Newt. Yes, the new scope is properly collimated from first principles using all the Catseye toys. I am getting close to having examined and ruled out all the stuff I know of that can be wrong with a Newt but tonight I noticed something odd that maybe will give you guys a clue to help me find the solution.

 

The scope has a traditional 4-vane spider where the vanes travel straight across the tube through the center. This is supposed to result in 4 strong diffraction spikes every 90*. I'm getting eight spikes. They do occur every 90*, but instead of one bright spike, I have two slightly dimmer ones offset from each other a small amount. It's almost like looking through a rear-surface mirror where you get a reflection off the front of the glass and a reflection off the silver behind the glass and they're just slightly offset because of the thickness of the glass itself. The spikes are parallel but not coincident.

 

Any idea what this means?


Edited by Ty Williams, 25 August 2017 - 11:05 PM.


#2 Kunama

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Posted 25 August 2017 - 11:23 PM

You're not looking at double stars are you????cool.gif



#3 Ty Williams

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Posted 25 August 2017 - 11:30 PM

You're not looking at double stars are you????cool.gif

Well, after I googled for an answer to that, I'm less sure what the answer is! I was looking at Vega. Googling for information about Vega makes it sound like it's a double but that I shouldn't be able to split it or even get a hint that it's a double with this scope and this air blanket above me.



#4 Gipht

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Posted 25 August 2017 - 11:34 PM

To be confident that collimation is not the problem you should perform a star test.  I have seen double  diffraction spikes from each vane from bad collimation, but not in the manner you describe.  When this has occurred on my reflector, the two spikes are very close.  Have you tried two different eyepieces to be sure the problem is not in the eyepiece?



#5 Ty Williams

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Posted 25 August 2017 - 11:36 PM

I have only one eyepiece in that magnification range. My wide-field eyepiece doesn't show spikes brightly enough to notice whether or not they're doubled.



#6 GlennLeDrew

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Posted 26 August 2017 - 03:02 AM

My first guess; primary mirror clips. Any intrusion into the pupil can induce diffraction spikes. And don't overlook the focuser tube when fully or nearly fully racked in.



#7 Allan Wade

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Posted 26 August 2017 - 06:11 AM

I've seen this before. Your secondary mirror veins aren't perfectly straight across the entire width of the vein/tube. Get a piece of string/cotton and place it across the entire width of the vein and you will see which way it's not straight. You should be able to adjust the screws where the veins fasten to the tube to move the secondary hub and straighten the veins.


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#8 Cotts

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Posted 26 August 2017 - 06:31 PM

If the spider vanes are thick, say > 1/8", each edge of the vanes produces its own diffraction spike --> 8 spikes.  Looking at a picture of an AT 10 - it has noticeably thick vanes...  probably your culprit.

 

Also, If the vanes are not 100% perfectly in line with the incoming light that is the same as having thick vanes and you'll get 8 spikes...

 

Dave


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#9 Ty Williams

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Posted 26 August 2017 - 07:57 PM

Shoot, I meant to check that alignment today before I set the scope out to cool. I got caught up in trying to resurrect an old LXD75 mount and forgot to look at the newer scope.



#10 Oberon

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Posted 26 August 2017 - 10:48 PM

If the spider vanes are thick, say > 1/8", each edge of the vanes produces its own diffraction spike --> 8 spikes.  Looking at a picture of an AT 10 - it has noticeably thick vanes...  probably your culprit.

That can't be right. Thats not how diffraction works.



#11 Cotts

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Posted 27 August 2017 - 10:03 AM

 

If the spider vanes are thick, say > 1/8", each edge of the vanes produces its own diffraction spike --> 8 spikes.  Looking at a picture of an AT 10 - it has noticeably thick vanes...  probably your culprit.

That can't be right. Thats not how diffraction works.

 

A few years ago I had a Takahashi CN 212 which had spider vanes a full 1/8" thick (so as to support the interchangeable secondary mirrors).  The scope consistently produced four double spikes.  The 'double' spikes were very close together. But they were double for sure.

 

Even very thin vanes produce two spikes but, due to the thin-ness, the spikes will merge and appear to be single.

 

Sticking any sort of a straight line or 'corners' into the light path produces spikes and other diffraction artifacts.  See images produced by square tubes or mirror clips or what-have-you...

 

An experiment can be easily done.  Stretch variously thick objects across the objective of a telescope.  A wire, pencil, a ruler, etc.  Examine the diffraction produced.  I will try this next time I'm observing... 

 

And, to my point about the edge of the obstructing object being the producer of the spikes, a single edge always produces a diffraction pattern.  Every. Single. Time. How can I say this with perfect confidence?  Because the aperture of any telescope is a 'single edge' and produces a very familiar diffraction pattern - the airy disc and rings. 

 

Dave



#12 GlennLeDrew

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Posted 27 August 2017 - 10:03 AM

 

If the spider vanes are thick, say > 1/8", each edge of the vanes produces its own diffraction spike --> 8 spikes.  Looking at a picture of an AT 10 - it has noticeably thick vanes...  probably your culprit.

That can't be right. Thats not how diffraction works.

 

Dave's correct, but I don't see that in principle there is a thickness limit. Imagine a vane made from a bar which is thicker at one end, such that it has a visible taper when looked at along the optical axis. It will produce two spikes, each perpendicular to the edge forming it. The angular divergence of these two spikes will equal the angle of the taper on that bar.

 

The same will occur for a vane which has a twist, such that in profile as seen along the optical axis it presents in projection such a taper in thickness.

 

If any vane edge is not straight along its length, the curve will impart a fan-like aspect to a spike, of the same angular divergence as for the angle of arc presented by that curved edge across the pupil.



#13 Starman1

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Posted 27 August 2017 - 10:47 AM

https://www.cloudyni...4746_176849.jpg

Thicker makes spikes more intense, but not double:

http://www.beugungsb..._1min_field.jpg



#14 MitchAlsup

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Posted 27 August 2017 - 10:54 AM

Can you take an image looking into the scope from the EP position, and another one looking into the scope from where star light goes in?



#15 GlennLeDrew

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Posted 27 August 2017 - 12:57 PM

https://www.cloudyni...4746_176849.jpg

Thicker makes spikes more intense, but not double:

http://www.beugungsb..._1min_field.jpg

Two spikes are induced, but because the vane thickness is uniform they merely align. As I mentioned, install a vane that is tapered in thickness and you will get two angularly divergent spikes, one for each edge presented to the incoming wavefront.


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#16 Jon Isaacs

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Posted 27 August 2017 - 03:11 PM

 

https://www.cloudyni...4746_176849.jpg

Thicker makes spikes more intense, but not double:

http://www.beugungsb..._1min_field.jpg

Two spikes are induced, but because the vane thickness is uniform they merely align. As I mentioned, install a vane that is tapered in thickness and you will get two angularly divergent spikes, one for each edge presented to the incoming wavefront.

 

 

If I am not mistaken, a spider vane produces two spikes at 180 degrees. A three vane spiders produces 6 spikes. A 4 vane spiders produces 8 spikes but the can be aligned so you only see 4.. when they're aligned.

 

How important is it that they're aligned??  Doesn't seem that important though it maybe symptomatic of another misalignment.

 

Jon

 

Jon



#17 GlennLeDrew

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Posted 27 August 2017 - 03:29 PM

Diffraction spikes are principally an aesthetic concern, at least if the *total* diffraction is otherwise much the same. Well, if a spike aligns and interferes with some faint object one wishes to observe, then will the matter surpass the merely aesthetic. ;)

 

A 4-vane spider which has a pair of opposing vanes not parallel will, for those two vanes, produce two dverging diffraction spikes each of intensity 1/2 that for a vane pair which is nicely parallel. That is, total diffraction for the opposing pair is unchanged. When divergent in alignment the diffracted energy is divided between them; when parallel the two spikes overlap and hence have double the brightness of one vane's spike.


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#18 MitchAlsup

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Posted 27 August 2017 - 03:46 PM

 

How important is it that they're aligned??  Doesn't seem that important though it maybe symptomatic of another misalignment.

Say you had a single vane (stock); it will produce a diffraction spike on both sides of the focused image.

 

Diffraction from multiple vanes add with superposition.

 

If you have 4 vanes but they are not exactly at 90ยบ from each other, you will not get pairs of the vanes to line up, and you could see all 8 sets of diffraction spikes.


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#19 Oberon

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Posted 28 August 2017 - 07:19 AM

 

 

If the spider vanes are thick, say > 1/8", each edge of the vanes produces its own diffraction spike --> 8 spikes.  Looking at a picture of an AT 10 - it has noticeably thick vanes...  probably your culprit.

That can't be right. Thats not how diffraction works.

 

A few years ago I had a Takahashi CN 212 which had spider vanes a full 1/8" thick (so as to support the interchangeable secondary mirrors).  The scope consistently produced four double spikes.  The 'double' spikes were very close together. But they were double for sure.

 

Even very thin vanes produce two spikes but, due to the thin-ness, the spikes will merge and appear to be single.

 

Sticking any sort of a straight line or 'corners' into the light path produces spikes and other diffraction artifacts.  See images produced by square tubes or mirror clips or what-have-you...

 

An experiment can be easily done.  Stretch variously thick objects across the objective of a telescope.  A wire, pencil, a ruler, etc.  Examine the diffraction produced.  I will try this next time I'm observing... 

 

And, to my point about the edge of the obstructing object being the producer of the spikes, a single edge always produces a diffraction pattern.  Every. Single. Time. How can I say this with perfect confidence?  Because the aperture of any telescope is a 'single edge' and produces a very familiar diffraction pattern - the airy disc and rings. 

 

Dave

 

Spikes emanate from the star at an angle perpendicular to the diffractive edge. It doesn't matter how thick your spider or obstruction is, if the edges are parallel there will only be one spike in either direction, not a pair of spikes. The only circumstance where double parallel spikes appear is a double star, or perhaps an internal reflection (never seen one) or the like.


Edited by Oberon, 28 August 2017 - 07:22 AM.



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