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Diffraction spike issue


Carl M
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Hi all,

Last night seemed like a good night to try and get collimation of my Quattro 8s sorted once and for all (or so it seemed), but rather than sorting it I've opened up another can of worms! It was the first night where I'd completely ditch the laser for collimating the primary mirror and collimate on a star instead. So I pulled the star (Capella) out of focus and into the middle of the frame to get the concentric rings. The primary was not collimated because the secondary shadow was not centred. So I centred it and took a 30 sec exposure. What I got was a star with the 4 normal diffraction spikes, but on the same axis mirrored quadruple ones!

collimation.thumb.png.9a0e34072a6c8b6ea53ae721d0a57cd6.png

I'm fairly sure nothing was intruding the light path as Capella was pretty much directly overhead. I've had a search around online and seen that extra diffraction spikes can come from twisted vanes but the effect never gave this many diffraction spikes.

Other interesting thing to note was that when I moved the star out of focus again the secondary shadow had moved and was no longer central. But when I moved the focuser the other way to defocus the star on the other side of focus it was centred! Would that indicate a focuser that is not sitting square?

Any help would be much appreciated!

Thanks,

Carl

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I would say that your spider vanes are not inline with each other. Each vane creates a spike at 90 deg but on both sides so that when the vanes are inline they overlap and you only see 4 well defined ones. The vertical spikes are showing this very slightly but the horozontal ones more so.

Make sure that the secondary holder in centred in the tube by measuring the length of each vane and getting them all the same.

 

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Just give your vanes a bit of a twist to get them straight. When getting your secondary central just put your scope end up on a piece of paper and draw around it. Cut this circle out then fold in half and half again and snip of the tip. Open up and put this over you scope. You can look through the hole and make sure its right. I am sure you should use a vernier but I found this worked okay for me.

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Maybe spider vanes but more likely your focusing tube is intruding into the light path. Its a common issue with DSLR's as they have lots of back focus. If tweaking the spider vanes does not help then try moving the primary up the tube on its adjustment screws.

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Thanks for the suggestions, rather worryingly I've found that these additional spikes are coming from the actual secondary mirror itself. I put my laser in and can see at the bottom of the telescope (on the primary) these same spikes appear. I thought it may be from the mirror being too tight, but I've loosened the collimation screws and the centre bolt but these same spikes are still apparent.

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1 hour ago, Carl M said:

Thanks for the suggestions, rather worryingly I've found that these additional spikes are coming from the actual secondary mirror itself. I put my laser in and can see at the bottom of the telescope (on the primary) these same spikes appear. I thought it may be from the mirror being too tight, but I've loosened the collimation screws and the centre bolt but these same spikes are still apparent.

Highly unlikely. What you are most likely seeing is the results of a rubbish laser diode. Very cheap laser diodes have terrible beam divergence in one direction due to their construction. It gets worse the more power you put through the diode, try dialing it down to the minimum brightness setting and you should get a nicer spot....

However if its this piece of rubbish or another derivative of it then you may still get that effect even on minimum power:

http://www.rothervalleyoptics.co.uk/skywatcher-laser-collimator-1252.html

Dont believe me then place it on a flat surface turn it on and fire it at a wall about 4 meters away, you will see it produces a cross pattern as its a rubbish diode. Then consider that the effect of bouncing it off the primarys curved surface is to enhance that effect and you will understand why you are seeing what you are seeing.

 

 

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20 hours ago, Adam J said:

Highly unlikely. What you are most likely seeing is the results of a rubbish laser diode. Very cheap laser diodes have terrible beam divergence in one direction due to their construction. It gets worse the more power you put through the diode, try dialing it down to the minimum brightness setting and you should get a nicer spot....

However if its this piece of rubbish or another derivative of it then you may still get that effect even on minimum power:

http://www.rothervalleyoptics.co.uk/skywatcher-laser-collimator-1252.html

Dont believe me then place it on a flat surface turn it on and fire it at a wall about 4 meters away, you will see it produces a cross pattern as its a rubbish diode. Then consider that the effect of bouncing it off the primarys curved surface is to enhance that effect and you will understand why you are seeing what you are seeing.

 

 

Hmm you are correct, not the secondary mirror itself. I pointed it down just the open end of the tube to the primary and still saw the pattern. If anything it seemed like the primary was too far up the tube as the mirror wasn't touching all of the locking screws, I'm not sure if having the primary too far up the tube would cause issues? I'm fairly positive that the focuser hasn't obstructed the light path before but will check again on next clear night.

I've also had two additional star spikes like this before, I wonder if these issues are connected to the same problem? Close up it looks like there may be more than two additional spikes, they just happen to be closer together unlike the first image. Quite hard to tell. 

58dc19db044e8_mirrorspike.thumb.png.4e8e6a553e16b8053d8e6dab2a1dd5d3.png

Spider vanes are measured and all the same length, last thing to check is for the bent spider vanes. Can't do much to check any changes until the some clear skies come along!

Thanks for help once again.

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On 29/03/2017 at 21:53, Carl M said:

Hmm you are correct, not the secondary mirror itself. I pointed it down just the open end of the tube to the primary and still saw the pattern. If anything it seemed like the primary was too far up the tube as the mirror wasn't touching all of the locking screws, I'm not sure if having the primary too far up the tube would cause issues? I'm fairly positive that the focuser hasn't obstructed the light path before but will check again on next clear night.

I've also had two additional star spikes like this before, I wonder if these issues are connected to the same problem? Close up it looks like there may be more than two additional spikes, they just happen to be closer together unlike the first image. Quite hard to tell. 

58dc19db044e8_mirrorspike.thumb.png.4e8e6a553e16b8053d8e6dab2a1dd5d3.png

Spider vanes are measured and all the same length, last thing to check is for the bent spider vanes. Can't do much to check any changes until the some clear skies come along!

Thanks for help once again.

That is too far out to be a twisted spider vane mate, its definitely something else in the light path. This is what the focus tube in the light path looked like on my setup before I moved the primary up. Excuse the quality of the image it was my second ever.

Problem is that yours is 45degrees out....so the edge causing it would have to be 45 degrees out from the spider vanes......

processed-horses-head-v4.jpg

Edited by Adam J
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That's really helpful thanks!

I wish I'd also collimated the primary with my laser when I was out there now to see if I got the same result as these two horsehead images show. All my other images have these same two additional spikes on bright stars when I was using the laser for collimation. Then it would have pretty much confirmed it to be the focuser intruding into the light path. It may also explain why the shadow of the secondary shifted when I collimated the primary using a defocused star while outward focusing, then when I defocused the star again using inward focusing it was out of collimation again.

I don't think the angle means much. If you rotate the camera in the focuser the spider vane spikes will move position, but the additional spikes stay static. I diagnosed this while I was out there. Also, when I took the image with the spider spikes intersecting the additional spikes the additional ones were coming from the same axis as where the focuser was. I remember meddling with the vanes while I was out there because I thought it might have been them! Seems as though all aspects point to the focuser :D

rotation.thumb.png.50d8f673739129c00ea522b6c982a159.png

Edited by Carl M
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Ah well if the spider vanes are rotating and the additional spikes are static then that means they can only have been generated by something that is rotating along with the camera. 

I see that you have the Baader MPCC coma corrector.....this would be static in respect to the camera when you rotate. I am going to guess its an original coma corrector MKI and not the current MKIII....if this is the case then that is your entire problem the older original design was notorious for causing reflections on bright stars. Not much you can do about it without putting your hand in your pocket and buying a new coma corrector. 

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