Diffraction Eliminator Mask?

[kbrown]

Ran across this idea of a mask that you put on top of the spider vanes on a newt on facebook earlier today to "eliminate" the diffraction spikes on bright stars. The guy had 3D printed one and claimed it works great. Has anyone here tried one?

Here's a French article about them: http://serge.bertorello.free.fr/antiaigr/antiaigr.html

Google translate version: https://serge-bertorello-free-fr.translate.goog/antiaigr/antiaigr.html?_x_tr_sch=http&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

[John]

Very interesting idea and one that looks relatively easy to try out 🙂

My 12 inch dob used curved vanes which worked quite well but I was only visual. 

 

[LuckieEddie]

I tried these a while ago. I found that on my 8" Newtonian they did almost eliminate the diffraction spikes but in doing so spread that extra light evenly around. This meant that the stars went from tight points with sharp spikes to softer and larger without spikes. For me I considered that a failure.

[kbrown]

The guy who posted it on FB said it worked great when he was imaging the cat's eye nebula as the centre of it is very bright and the diffraction spikes would have ruined it.

Suppose if you could easily put the mask on and take it off it might be worth it depending on the target.

[kbrown]

2 hours ago, LuckieEddie said:

I tried these a while ago. I found that on my 8" Newtonian they did almost eliminate the diffraction spikes but in doing so spread that extra light evenly around. This meant that the stars went from tight points with sharp spikes to softer and larger without spikes. For me I considered that a failure.

Did you try something like deconvolution in PixInsight on them?

[vlaiv]

Yes, like in commend above - I would not call it diffraction eliminator mask - I'd call it diffraction reshaper mask.

Diffraction happens on edges. It is always perpendicular to the edge. It happens on clear aperture of telescope as well because edges of mirrors or lens also produce the same effect.

Long straight edges gather all the light into perpendicular direction and that creates strong spike. Curved edges have many adjacent perpendicular directions - so no direction reinforces the other directions.

This is how this mask behaves - it contains full circle but spread into segments. That way there is many "small diffraction spikes" - each of them at a different angle and they all combine to form a "glow" rather than noticeable spikes.

Total diffraction is greater than with regular configuration with spider - simply because diffraction edge is longer (total edge that diffracts light when combined). This means that more of light from a star is spread into halo then if we had star and spikes only (spikes contain less total photons than halo, but since halo is spread over larger surface - it is harder to notice).

In any case - it works to remove spikes but it does reduce contrast compared to normal configuration.

[vlaiv]

3 minutes ago, kbrown said:

Did you try something like deconvolution in PixInsight on them?

Too low SNR for effective deconvolution. Also - since it is very specific kernel, regular gaussian PSF won't be of a much help for deconvolution.

[Captain Scarlet]

I believe @Stu got one, like the one in fig7 on that linked site, and recently inadvertently blind-tested it by being terribly disappointed by poor views he was getting of Jupiter in his 8” newt. He then remembered he’d left this mask on from a previous session, removed it, and Jupiter was suddenly much better. I’m sure he’ll tell the story more faithfully when he reads this,  but that was the gist that recall.

Magnus

[Stu]

11 minutes ago, Captain Scarlet said:

I believe @Stu got one, like the one in fig7 on that linked site, and recently inadvertently blind-tested it by being terribly disappointed by poor views he was getting of Jupiter in his 8” newt. He then remembered he’d left this mask on from a previous session, removed it, and Jupiter was suddenly much better. I’m sure he’ll tell the story more faithfully when he reads this,  but that was the gist that recall.

Magnus

Exactly right Magnus! I forgot I had left them in place and thought my scope was broken! Removing them improved things dramatically so I won’t be trying them again.

I have always wanted to try either the circular supports that John had on his 12”, or the Protostar type single curved vane which supposedly minimises diffraction as it is shorter overall.

I had an 8” Portaball with three curved vanes and was never totally convinced, I suspect the circular supports may well work better at totally smearing out the spikes.

[vlaiv]

For anyone wanting to try some of different designs - I remember there was an app where you put in image of your spider and it will produce diffraction pattern for you.

I can't seem to find it now, but there is alternative.

In software that is capable of doing FFT and squaring the image pixel values  - you can produce diffraction pattern yourself for any sort of obstructed aperture. Just draw obstructed aperture, make FFT of that image and square resulting pixel values (math operation of square).

[vlaiv]

Ok, managed to find reference to that software, it could be tricky to find a download for it:

https://www.cloudynights.com/topic/547413-where-to-find-maskulator/

[kbrown]

2 hours ago, vlaiv said:

Too low SNR for effective deconvolution. Also - since it is very specific kernel, regular gaussian PSF won't be of a much help for deconvolution.

I was mainly thinking the mode in PI where you sample the PSF from your image rather than a synthetic Gaussian PSF. Nonetheless, it sounds like I'm going to pass testing this idea.

I don't mind small diffraction spikes anyway. I always keep my CCD rotated about 45 degree angle (apparent) off the spider vanes. Reduces the effect a bit as the pixels don't align with the spikes without sacrificing anything.

[Captain Scarlet]

My own effort to reduce spikes/help local contrast has seen me buy some 0.2mm thick stainless steel sheet and I’ll get my local metalwork company to guillotine it into 15 or 20mm wide strips. I’ll also reduce the number of vanes on my 8” from 4 to 3. These will replace my current 4-vane spider whose vanes are 1.2mm thick. The idea is that fewer vanes will reduce total length of diffraction edges, and the thinness will make the spikes longer and fainter, throwing most of the diffraction far beyond a planet.

[kbrown]

3 minutes ago, Captain Scarlet said:

My own effort to reduce spikes/help local contrast has seen me buy some 0.2mm thick stainless steel sheet and I’ll get my local metalwork company to guillotine it into 15 or 20mm wide strips. I’ll also reduce the number of vanes on my 8” from 4 to 3. These will replace my current 4-vane spider whose vanes are 1.2mm thick. The idea is that fewer vanes will reduce total length of diffraction edges, and the thinness will make the spikes longer and fainter, throwing most of the diffraction far beyond a planet.

I might be wrong but I thought that 3 (straight) vanes would produce 6 spikes since they're all pointing at different directions?

[Stu]

5 minutes ago, Captain Scarlet said:

My own effort to reduce spikes/help local contrast has seen me buy some 0.2mm thick stainless steel sheet and I’ll get my local metalwork company to guillotine it into 15 or 20mm wide strips. I’ll also reduce the number of vanes on my 8” from 4 to 3. These will replace my current 4-vane spider whose vanes are 1.2mm thick. The idea is that fewer vanes will reduce total length of diffraction edges, and the thinness will make the spikes longer and fainter, throwing most of the diffraction far beyond a planet.

I think I actually prefer six dimmer diffraction spikes to four brighter ones. Will be interest to hear how it turns out.

[John]

3 minutes ago, kbrown said:

I might be wrong but I thought that 3 (straight) vanes would produce 6 spikes since they're all pointing at different directions?

That is my understanding as well.

My 12 inch dob vanes were like this:

 

Edited May 3, 2023 by John

[Captain Scarlet]

6 minutes ago, kbrown said:

I might be wrong but I thought that 3 (straight) vanes would produce 6 spikes since they're all pointing at different directions?

Yes that’s true, but all told less diffraction though. A 4-vane spider in fact produces 8 spikes, but in 4 pairs which overlap/“double-up”. With a planet, for any given point on it the total amount of vane-induced-diffraction-softening from that point’s neighbouring “pixels” is reduced both by the one fewer vane and the fact of thinner vanes.

I’ll probably start off by adding one vane with the original spider still in place, to compare spikes side by side 

Edited May 3, 2023 by Captain Scarlet