Anti-spike mask?

[Larry 1969]

Hello everyone!
I haven't posted anything in a while. The weather has been terrible, but I'm still thinking about my astro addiction.

I came across a Facebook post that mentioned eliminating diffraction spikes from a reflector with these little things attached to the spider vanes. I bought a 3D printer and made a sample using white plastic. I've since made 4 of them out of black matte plastic. Just waiting to try them out.

I'm not opposed to diffraction spikes but sometimes it really takes over the image. Like Alnitak...

I'll post back after I get to use them.

Larry

[Gordon]

It will be interesting to see the results!

[KathyNS]

I actually like my spikes. But I am following out of curiosity.

[Larry 1969]

I actually like my spikes. But I am following out of curiosity.

I do too Kathy. There are just a few instances where I find it a bit much. I'm going to try this mostly out of curiosity.

Larry

[JayTee]

Color me curious.

[Larry 1969]

Color me curious.

Me too! If the skies ever clear...

Larry

[seigell]

Those look substantial enough to cause problems with Flats.

[Larry 1969]

Those look substantial enough to cause problems with Flats.

How so? I wouldn't think it would be a problem if I left them installed for the flats as well as the lights.

Larry

[seigell]

Those look substantial enough to cause problems with Flats.

How so? I wouldn't think it would be a problem if I left them installed for the flats as well as the lights.

While the Spikes are generated by Interference Patterns, they don't cast Direct Shadows.
On the other hand, these Add-Ons to the Spider Vanes have substantial cross-sectional area that could result in mild shadows in the Flat.

[Makuser]

Hi Larry. Here is a link to the explanation of what causes the spider vane diffraction spikes and why the curved spider vanes are superior:
https://www.telescope-optics.net/spider.htm
There is a popular manufacturer of curved spider vanes here at:
http://www.destinycomp.com/Online%20store%20menu.htm
The problem is the outlay of funds, getting the right size and fit, getting the secondary mirror remounted, and then the collimation when you are done. What you have here looks like an inexpensive and easy install device. But as others have said, this could present a new problem in astrophotography. I hope this helps Larry and looking forward to your report on how this mask works.

[Larry 1969]

While the Spikes are generated by Interference Patterns, they don't cast Direct Shadows.
On the other hand, these Add-Ons to the Spider Vanes have substantial cross-sectional area that could result in mild shadows in the Flat.

Thanks! I think I understand. Maybe if I shot my light frames with them and removed them for the flats? Really just experimenting for now.

Larry

[Larry 1969]

Makuser,

Thanks for the links. I did read a little about those. The problem is that I actually like the diffraction spikes mostly. It's just a few targets that it seems the spikes take over the image. This is easily reversible which is what I like.

Now, just to see if they actually work ...

Larry

[Larry 1969]

Update:
Here is a screenshot of a single, 60 sec image of M109 with Phecda deliberately put in the FOV. Phecda is a magnitude 2.4 star, so it's very bright and there is no real sign of diffraction spikes. I kept the exposures down to 60 sec due to the close proximity of the moon. Not expecting a lot from this other than to test the anti-spike mask.

Larry

[Larry 1969]

For the record, I did take flats at 30,000 ADU with these in place and my target is pretty close to tonight's 58% waxing gibbous.

Larry

[OzEclipse]

@sdbodin and I recently discovered that we both did some work in our youth at about the same time (early 1980's) during our respective Physics degrees in USA and Australia on something called apodization masks. They are masks that modify the incoming wavefront to change diffraction patterns.

There are various "solutions" to diffraction issues. All of them modify the intensity profile of the incoming wavefront so that the modulation transfer (the optical transform) disperses the diffraction into a different form. The diffraction pattern observed in any optic is the incoming intensity profile with a mathematical integral transform called a Fourier Transform applied to it. You can't eliminate a diffraction spike but you can redistribute the light in the diffraction spike into more scattered light with these types of masks.

In this diagram, I have attempted to illustrate what I remember of work I did 40 years ago. It was based on an unobstructed refractor aperture which is the most simple to analyse mathematically. The profile across a simple refractor lens is a square pulse aka a "Top Hat" function. It transforms into a Bessel function of the first kind. The Besselian profile is recognisable as the intensity profile of the Airey Disk and diffraction rings that observers are so familiar with.

The aim of my work back then was to create a Gaussian intensity entrance profile approximated using a radial density mask which in theory transforms to a diffractionless image.

Once you throw spiders, vanes. and vase shaped modifiers into the mix, the math becomes much more complicated.

I'll be interested to see your results but my guess will be that the sharp spikes will be smeared so that they don't contrast and show up so strongly against the other diffracted scattered light around the stars. It may not give you the results you desire, but I hope it does.

In the end, there are no free lunches or magic wands. The Gaussian profile has the hidden catch that it removes most of the light from the outer part of the optic thus resulting in a lot of light loss and some possible reduction in resolution. However, there is another optical principle, which I knew then and the name of which I can't remember now, that states that the diffraction pattern of an aperture is the same as it's inverse. So the mask could be made with a dark centre graduated to a transparent exterior or a transparent centre graduated to an opaque exterior.

cheers

Joe

[bobharmony]

Update:
Here is a screenshot of a single, 60 sec image of M109 with Phecda deliberately put in the FOV. Phecda is a magnitude 2.4 star, so it's very bright and there is no real sign of diffraction spikes. I kept the exposures down to 60 sec due to the close proximity of the moon. Not expecting a lot from this other than to test the anti-spike mask.

Larry

To my eye it looks like a lot of short spikes instead of 4 long ones. For a moment I was thinking you used a camera lens - the pattern looks similar to me as the one that comes from the iris of a stopped down lens. I look forward to seeing what more time and a stack of images generates. Interesting experiment for sure!

Bob

[Larry 1969]

Thanks Bob.
I did notice the many small spikes and you're right. It does look very similar to a camera lens. Clouds rolled in and it was pretty close to the moon so I'm not expecting much. Just curious to see how that star turns out...

Larry

[Larry 1969]

I finally got around to processing this and I think it turned out OK considering I was shooting very close to a 60% moon.
I originally processed it in StarTools but the moon caused significant gradients that I couldn't manage so I ran it through APP and, having never used APP for post-processing, I was quite pleased with it. The light pollution removal tool worked great and I saturated it a bit as well. I applied some final sharpening and noise reduction in Topaz DeNoise AI and exported it as jpg and png. The jpg was 10.1 Mb and too large to attach, so I used IrfanView to reduce it and it lost some quality...

I did shoot 30,000 ADU flats with the masks installed and it seemed to correct the image just fine.

I'm going to go ahead and post this in the DSO images part of this forum as well.

Thanks for the input!

Larry