Uneven diffraction spikes

[BrendanC]

Hi all,

Now that I've fixed the 999,999,999,999 other problems I've had over the past year, I'm working on the next one. I keep noticing that the diffraction spikes on my stars tend to be longer along one axis than another - example attached. To my eye, the vertical spikes are longer than the horizontal ones. I've collimated the heck out of my 130PDS - even bought a Concenter to help with the secondary mirror and the out-of-focus star test yields concentric circles - so does anyone know what could be causing this? It's not a huge issue but it does sort of bug me.

Thanks, Brendan

 

Edited July 19, 2021 by BrendanC

[happy-kat]

Do you notice it on other images taken by other 130PDS users?

[BrendanC]

Good point! I haven't really looked. I know of the 130PDS thread on this forum so I'll take a look. Totally prepared to believe it's just something that happens with this scope. I know it's a very minor issue but after the long, hard struggle to get this far, I'm now considering these ongoing cosmetic irritations.

[Fraunhoffer]

Have you checked the images earlier in the post processing chain?

 

[BrendanC]

Nope, but that's another good idea. I could check the raw subs, and stretch them in Photoshop to see what's going on. My feeling is that they'll still be there, but I could check. All I've done is process in StarTools and Topaz DeNoise, so I don't see how they would have done this to the spikes.

Btw the 130PDS thread shows some shots with uneven spikes and some with, so I guess it's down to individual setups. I have a strong feeling it's because the secondary might be very slightly canted in one direction, so I guess I'll have to get the Concenter out and give that pesky secondary another go. I hate collimating.

[wimvb]

If you destroy the diffraction of two opposite vanes, you can figure out which need adjusting. Just wrapping something around two vanes should do the trick.

[BrendanC]

Thanks!

 

[happy-kat]

Maybe the answer to this turned up yesterday on a 130p-ds imaging post, and that was effectively a thin aperture mask that was used in the primary mirror to effectively cover the mirror clips. It reduces the defraction spikes to just 4 pretty much.

Took ages to find this thread to share but can't now find the other thread with the details that were posted.

Edited September 6, 2021 by happy-kat

[vlaiv]

This can happen for two reasons:

1. one spider vane being longer than the other (here different vanes means different direction, so as "vertical" one would be considered both sides of secondary for example).

2. one spider vane being thicker than other - it does not need to be mechanically thicker - it can be optically thicker - like slanted with respect to incoming light

 

[BrendanC]

@happy-kat I know what you mean about that 130PDS thread, I was looking for it too yesterday. Anyway, I have actually fitted a primary mirror baffle and it has improved my stars.

@vlaiv I'm fairly sure the vanes are the same length and thickness. I think that, at the end of the day, the 130PDS isn't the last word in quality so I've decided it's really not a problem and that I can live with it.

 

[vlaiv]

1 minute ago, BrendanC said:

I'm fairly sure the vanes are the same length and thickness.

I'm also sure that is the case, however - they can present uneven "profile thickness" to incoming light if they are rotated a bit:

[BrendanC]

Interesting - I've had twisted vanes before but was able to get ointment for it I thought I'd fixed it, so I'll check again.

Still, I've decided it's not a real problem. Thanks anyway!

[Adam J]

On 19/07/2021 at 13:45, BrendanC said:

Hi all,

Now that I've fixed the 999,999,999,999 other problems I've had over the past year, I'm working on the next one. I keep noticing that the diffraction spikes on my stars tend to be longer along one axis than another - example attached. To my eye, the vertical spikes are longer than the horizontal ones. I've collimated the heck out of my 130PDS - even bought a Concenter to help with the secondary mirror and the out-of-focus star test yields concentric circles - so does anyone know what could be causing this? It's not a huge issue but it does sort of bug me.

Thanks, Brendan

 

You cant do much about it its because one of the two spider vane pairs is partially blocked by the focus tube and hence less diffraction will take place. Also the secondary is offset so wider in one plane than the other again blocking a portion of the spider.  Have you cut your focus tube down??

Edited September 8, 2021 by Adam J

[BrendanC]

Interesting, never really thought it could be because of that, but it does make sense. I haven't chopped the focus tube, but I do know that's one of the fixes. I've got half a mind to invest in a TS optics non-reducing coma corrector which might help. Thanks for the suggestion!