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Extreme asymmetry in diffraction spikes

Sky-Watcher Explorer 150PDS Troubleshooting Artifacts
John Walshandrea tasselli
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John Walsh avatar
John Walsh avatar
Hi all,
I just installed a primary mirror mask to hide the mirror clips as well as a CNC spider from backyard universe.
Had about 10 mins of clear sky to test the result and am puzzled by the extreme asymmetry in the diffraction spikes as can be seen in the images below.
I emailed the manufacturer and he advised to apply velour to the spider struts to eliminate this effect (eliminate reflections from the cnc material)
I will do this as soon as I can but am just wondering how reflections from the spider would cause 2 of the spikes to be longer than the other, especially considering that the images were taken in completely different parts of the sky.
I should note that there is no coma corrector in place and there was no time for the scope to cool down.
Collimation was done before taking it out and then verified with a star test.
Any advice would be appreciated
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Jordan Morley avatar
Jordan Morley avatar
Get yourself a corrector. A ts optics gpu is a good option for the price. I do my colimation with a camera. I personally use an ocal colimator and Android phone.

I dont use any kind of mirror mask. Just sricky felt velor and blackboard paint. i dont think its your spider either colimation, some pinched primary or something is tilted like focuser or secondary
andrea tasselli avatar
andrea tasselli avatar
I guess that:

1. The spider isn't centered
2. The mask does a poor job of suppressing clips diffraction (see flaring on bright star)
John Walsh avatar
John Walsh avatar
andrea tasselli:
I guess that:

1. The spider isn't centered
2. The mask does a poor job of suppressing clips diffraction (see flaring on bright star)

Thanks Andrea. I assumed the spider would be centred with a CNC spider but perhaps it is not.
I can’t see the clips at all with the mask in place but the focuser tube intrudes 25mm into the OTA which I suppose causes its own issues
John Walsh avatar
John Walsh avatar
Jordan Morley:
Get yourself a corrector. A ts optics gpu is a good option for the price. I do my colimation with a camera. I personally use an ocal colimator and Android phone.

I dont use any kind of mirror mask. Just sricky felt velor and blackboard paint. i dont think its your spider either colimation, some pinched primary or something is tilted like focuser or secondary

Thanks Jordan. Yes I am looking at that exact CC, if for nothing else than it extends the focus out by 20mm and might mitigate the problem of my draw tube intruding into the OTA so much
andrea tasselli avatar
andrea tasselli avatar
John Walsh:
Thanks Andrea. I assumed the spider would be centred with a CNC spider but perhaps it is not.
I can’t see the clips at all with the mask in place but the focuser tube intrudes 25mm into the OTA which I suppose causes its own issues


Well, something is there with a 3-fold unsymmetrical pattern beside the spider diffraction and that doesn't look like an effect from the focuser tube protruding into the light path (which is showing in the first picture). 

Anyway, the best course of action is actually measure the spider thing.
andrea tasselli avatar
andrea tasselli avatar
John Walsh:
Thanks Jordan. Yes I am looking at that exact CC, if for nothing else than it extends the focus out by 20mm and might mitigate the problem of my draw tube intruding into the OTA so much


*I very much doubt it. The 20mm increase in the new position of the focus point is correct but it comes at the expense of 100mm of CC length which means that is very likely that it would protrude even more into the light path. A lot depends on the actual design of the draw tube and its actual 2" length (most have a raised feature to prevent things from falling inside the tube).
John Walsh avatar
John Walsh avatar
andrea tasselli:
John Walsh:
Thanks Andrea. I assumed the spider would be centred with a CNC spider but perhaps it is not.
I can’t see the clips at all with the mask in place but the focuser tube intrudes 25mm into the OTA which I suppose causes its own issues


Well, something is there with a 3-fold unsymmetrical pattern beside the spider diffraction and that doesn't look like an effect from the focuser tube protruding into the light path (which is showing in the first picture). 

Anyway, the best course of action is actually measure the spider thing.

Thanks Andrea. I will measure it today as soon as I get home.
Sorry for the stupid question but what do you mean by “3-fold” asymmetry?
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andrea tasselli avatar
andrea tasselli avatar
John Walsh:
Sorry for the stupid question but what do you mean by “3-fold” asymmetry?


A 3-pointed pattern.
Tony Gondola avatar
Tony Gondola avatar
It's the spider, clips and focuser intrusion w and the other stuff won't give you that fine extended spike IMO. If you have a refractor take a moment and play with string and tape and observe the patterns you get. I suspect that it's an error in the machining of the vanes of the new spider. The last thing I would want to do is get a fancy machined spider and then thicken the vanes with valore. You may as well trade your primary in for one with a lower strehl ratio because that's what you'd be doing.
John Walsh avatar
John Walsh avatar
Tony Gondola:
It's the spider, clips and focuser intrusion w and the other stuff won't give you that fine extended spike IMO. If you have a refractor take a moment and play with string and tape and observe the patterns you get. I suspect that it's an error in the machining of the vanes of the new spider. The last thing I would want to do is get a fancy machined spider and then thicken the vanes with valore. You may as well trade your primary in for one with a lower strehl ratio because that's what you'd be doing.

Thanks Tony.
Well that’s disappointing, considering how expensive the spider was. 
I think I’ll put back in the old stock spider vanes that came with the scope so
Christoph Paulus avatar
Christoph Paulus avatar
I also have a spider from Backyard Universe for my Skywatcher 200 PDS. Definitely worth the money. I also have a custom (self-made) mask which not only covers the clips but some millimeters more (just in case). I also cut the focuser tube to an absolute minimum. Causes me some headache sometimes as I wish I kept it 5-10 mm longer but I don't have any intrusion. I don't have any velour but I covered some bright/blank pieces with TS-Optics' black paint.

Here's a photo of the Pleiades where you can see symmetrical spikes: https://www.astrobin.com/2v0ert/

Maybe something's wrong with your spider? Have you checked if all dimensions are correct and symmetrical?
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John Walsh avatar
John Walsh avatar
Christoph Paulus:
I also have a spider from Backyard Universe for my Skywatcher 200 PDS. Definitely worth the money. I also have a custom (self-made) mask which not only covers the clips but some millimeters more (just in case). I also cut the focuser tube to an absolute minimum. Causes me some headache sometimes as I wish I kept it 5-10 mm longer but I don't have any intrusion. I don't have any velour but I covered some bright/blank pieces with TS-Optics' black paint.

Here's a photo of the Pleiades where you can see symmetrical spikes: https://www.astrobin.com/2v0ert/

Maybe something's wrong with your spider? Have you checked if all dimensions are correct and symmetrical?

Lovely stuff! Certainly was the type of effect I was expecting upon purchasing it. 
I will remove it today and measure the dimensions. I find it hard to imagine that a machined unit of that price could possibly be off in dimensions
Peter Graf avatar
Peter Graf avatar
Almost same setup here. 150PDS, Backyard Universe CNC spider and the mask. I can confirm Christoph, it works really well. As I started with a DSLR, I am still using the Baader MPCC Mark III.
The stock spider that came with my 150PDS was little bit twisted, I could not get it straight, the spikes looked really weird on bright stars.
Here also my take on the Pleiades. Great target for spikes... :-) https://app.astrobin.com/i/5eo3z4

To be honest, I also tried putting velours on the spider vanes, but then the spikes were almost gone. So I decided to remove it again. But that is personal taste...

So if there is any trouble with the spider, I would check it with Michael from Backyard Universe and return it - maybe for a new one.
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John Walsh avatar
John Walsh avatar
Thanks Peter. Lovely image!
I was in touch with Michael alright and he suggested that valour would solve the asymmetry. 
If it is in fact solved by some issue with the dimensions of my particular spider, I’m trying to think what would actually cause it? 
would it be if the struts were of differing thicknesses?
andrea tasselli avatar
andrea tasselli avatar
Thicker spider vanes make for fatter but shorter diffraction spikes, which is what covering them with velour would do, to a degree.
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Tony Gondola avatar
Tony Gondola avatar
andrea tasselli:
Thicker spider vanes make for fatter but shorter diffraction spikes, which is what covering them with velour would do, to a degree.

Exactly, I remember that the big dob visual guys would make spiders out of a network of very thin wire under tension. That would make the spikes just about disappear, at least visually. The spikes are extra bright with a 4 vane setup because one vane causes two spikes at 180 degrees to each other. That reinforces the intensity of the spikes. Going with a three vane setup would give you 6 spikes but they would be dimmer. Another alternative would be to go with curved vanes. That would eliminate the spikes but it would cause a very dim diffuse glow around bright stars so I'm not sure that would be a good photographic solution although I'll  admit, I've not seen it tried. All if these options could be tried my masking a refractor to see what you like best.
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andrea tasselli avatar
andrea tasselli avatar
Tony Gondola:
I've not seen it tried


Tried (done) many times. Just for the record.
John Walsh avatar
John Walsh avatar
andrea tasselli:
Thicker spider vanes make for fatter but shorter diffraction spikes, which is what covering them with velour would do, to a degree.

Makes sense. Does this therefore imply that 2 of my vanes are already thicker than the other 2? Is that what could lead to 2 orthogonal spikes being so much brighter and longer?
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CraigT82 avatar
CraigT82 avatar
For what it’s worth I don’t see how this can be a diffraction effect.

If you had one vane machined slightly different to the rest it would create two different spikes 180 degrees opposed, not orthogonally.

If you had two vanes different and they were 90 degrees to each other, than all 4 spikes would look the same.

 I think it’s likely a reflection issue as the BYU chap suggests.
andrea tasselli avatar
andrea tasselli avatar
Of course it's a diffraction effect.
andrea tasselli avatar
andrea tasselli avatar
John Walsh:
Makes sense. Does this therefore imply that 2 of my vanes are already thicker than the other 2? Is that what could lead to 2 orthogonal spikes being so much brighter and longer?


*I don't think so unless they are prismatic.
CraigT82 avatar
CraigT82 avatar
andrea tasselli:
Of course it's a diffraction effect.



Please explain what is wrong with the vanes to cause two orthogonal long spikes. 

You cant.

It’s reflection from the vanes, it is covered on the BYU website
Christoph Paulus avatar
Christoph Paulus avatar
Is the paint ok on the vanes? I haven't done anything on my spider, I just put it in and collimated the scope.
Maybe it's really the focuser tube?
John Walsh avatar
John Walsh avatar
Christoph Paulus:
Is the paint ok on the vanes? I haven't done anything on my spider, I just put it in and collimated the scope.
Maybe it's really the focuser tube?

Seems ok Christoph. Maybe the focuser tube so. I don't know. It's very frustrating. I will apply the velour anyway as the manufacturer suggested and see if it helps