The Halos that Aren't

[Stefan B]

Thanks for your detailed responses, Ivo and Mike.

With the image at hand, I cannot readily observe detailed diffraction patterns (which is a useful cue in itself!). At full resolution, there appears some blotchiness to the outlines of stars, and it is clear the stacker was unable to properly align the two channels (causing differently colored fringing).

I should have added that the image doesn't only have duo NB data but I merged RGB stars into it. And if you have a closer look you will notice that my masking has been far from perfect

Probably I have a wrong definition of diffraction pattern in my head. Apart from the spikes which are diffraction patterns (right?) I see a lot of light from the star that's reaching almost to the end of the spikes.

star2.jpg (92.91 KiB) Viewed 4867 times

See the blue star on the left. I guess this comes from some kind of diffration. Accordingly I thought this might qualify as a pattern. Probably not? Does pattern only describe these rainbows in the spikes from Newtonians?

What I am wondering is, where does this thing come from? Is it light diffracted by the secondary mirror? Is this an unusually big surface covered since I don't see this in other images? If I stretch manually with curves in PS the stars do not show this "pattern" so heavily than with AutoDev. But since I guess it's true signal that's rather a feature of ST than a bug, right? Can I modify my imaging train so that the area gets smaller?

You don't need star cores to be overexposed to appear colorless! You just need - after color balancing of course - star cores to be of equal brightness across all channels (as an 8-bit example, R:G:B 64:64:64 will appear just as colorless as R:G:B: 255:255:255). No matter how little or much you stretch something that is colorless, it will remain colorless.

I get that. But have a look at this little guy:

star1.jpg (20.74 KiB) Viewed 4867 times

The center looks pretty white to me and the outer areas rather blue. The center has RGB ratios of 150:211:250, a bit farther away you get 47:145:220. So red isn't overexposed in the center, blue is close. Outside of the center blue stays high, red gets down rather fast so it appears more blue than the center. But I would have thought the center would be less bright, but it's obviously already near the 1:1:1 ratio. Or ST fades to white since G and B are already near the top. That's an interesting bit of information on ST!

That said, I think it's a couple things here. One is sort of the diffraction "halo" (just for Ivo!) around the star - the smaller spikes radiating out in all directions. Perhaps depending on seeing and atmosphere, that region dances around a bit, thus filling it in as sort of a ball around the star.

The balls around the stars is exactly what I am after, Mike And you gave a quite nice example with your image. In the upper left the brighter stars have balls which are as big as the spikes like in some of my images. Why is that. And why is this e.g. different with the brightest star in the image? Does this tell me something about your imaging train? I don't know.

By the way, do you have an aperture ring around your primary mirror in order to cover the mirror holders? If not, this might be something to consider. It massively improved the star shapes in my images.

Regards
Stefan

[Mike in Rancho]

Hey Stefan,

I'll have to think about that more. I am leaning towards it still being diffraction though. Perhaps from whatever is setting the outer circumference of the aperture. Any hard edge, no matter how perfectly smooth and circular, is going to diffract the point source, no? Thus, the star balls. I am out of my league and just guessing here.

Now, the less perfect the aperture edge, the more one probably gets the more well-defined shorter spikes that one can see within the "ball" perimeter, mostly. Since those can be resolved even after stacking and processing, I'm going to say they are very small perturbations of the circumference. Or perhaps even things like screws and the focuser tube? Will have to get back into the Synth module and see what can be created with the options. Although I actually don't think my tube sticks into the light path when I am at the DSLR focus plane.

And finally, the spider vanes create the massive diffraction spikes -- the pretty ones!

I have not altered the scope at all, but thanks for reminding me of the clips. Do you have any before-after, and how did you mask it off? I'd hate to lose aperture though!

Along with my light leak fixes, today I was thinking of removing the secondary in order to then remove the spider. I need to get the vanes straight and in line with each other to stop these split spikes, and I can see the bends and misalignments of the vanes. Then I'll reinstall, center it as well as I can, and recollimate. It would almost seem a good time to clean the mirrors too, but I don't think I have enough distilled water on hand. Alas, today is turning into a big cleanup day after some crazy winds all night long.

[UlfG]

Hi, I have a question relating to the below:

With the image at hand, I cannot readily observe detailed diffraction patterns (which is a useful cue in itself!). At full resolution, there appears some blotchiness to the outlines of stars, and it is clear the stacker was unable to properly align the two channels (causing differently colored fringing).

Since to my knowledge we are dealing solely with OSC images here, how could channels ( as in RGB channels?) become misaligned?

Regards
Ulf

[happy-kat]

Hi, the post at the top of this page said the image was NB and merged with RGB stars too so not a OSC source.

[UlfG]

Hi, the post at the top of this page said the image was NB and merged with RGB stars too so not a OSC source.

Hi,
I think Stefan should be the one to answer this, but it just so happens that I have had some discussions on this topic before with Stefan, we discussed this subject because his imaging setup is identical to mine, which means: 6" f5 Newtonian with Canon EOS 1100D modded camera and Optolong L-enhance filter. So "narrowband data" here means OSC camera with Optolong narrowband filter. And "RGB stars" means OSC image without narrowband filter. I take the liberty of posting an AB link to Stefans Cygnus wall image, which specifies his setup:
https://astrob.in/ctbere/0/

For the clarity of my involvement in the subject, I also post an AB link to my own Cygnus wall image, which has very similar star fringes.
https://www.astrobin.com/pq39qg/E/

Regards

Ulf

[Stefan B]

Thanks, Ulf, that's exactly what I would have replied

I am leaning towards it still being diffraction though. Perhaps from whatever is setting the outer circumference of the aperture. Any hard edge, no matter how perfectly smooth and circular, is going to diffract the point source, no? Thus, the star balls.

That's probably at least part of the answer. But why are they so big? Have a look at the spikes in these images of @Startrek :

viewtopic.php?f=11&t=2285

The optics are similar although it's an 8 inch instead of 6 inch. But also GSO and f5. Why don't have the stars those balls? Upon close inspection there might be halos (maybe from the l-extreme which is said to cause those) but not these "balls". Is there something different in the train apart from the bigger aperture or is the processing just better?

And finally, the spider vanes create the massive diffraction spikes -- the pretty ones!

Just to be sure: I knew that

I have not altered the scope at all, but thanks for reminding me of the clips. Do you have any before-after, and how did you mask it off? I'd hate to lose aperture though!

Here are two images without ring:

And here are two images with bright stars after installing an aperture ring:

It made a major difference in terms of star shape and I would always trade a bit of speed for this improvement of stars. Of course a matter of personal taste.

Regards
Stefan

[Mike in Rancho]

Hi Stefan,

Yes a very improved star shape after your mod. Interesting, it almost seems like there are four "wedges" in the halos/diffraction/spaceballs in the pre-tweak version. Do you have three clips or more? I had aperture issues with my refractor similar to that before I took it apart and fixed it. Again, good shape though. What material was used to create and affix the aperture or clip mask? I haven't looked up procedures for that.

Good examples all around on the spaceballs though. I believe I can in fact see them in Startrek's images, albeit quite faint. We might have to inquire if he specifically processed them out, perhaps a masked fringe killer?

I'm still kind of putting my money on some sort of light-is-a-wave interference, though waiting on Ivo's expertise.

I think it can also be seen in things like the Pillars of Creation, old or new version, though with the amazing resolution and narrow field of view one can probably also make out the actual Airy disk outer rings. At least I think that's what those are.

I'm still chasing down my light leaks, which have proven more stubborn than I expected. Even as much as I try to block off with canopies and tarps, I think I just have a lot of stray light around here. But I'm getting it bit by bit. Also took my spider apart last night to straighten and square up the vanes, and then perfectly center the whole thing in the OTA before recollimating. A quick test showed round stars across the field, without the usual MPCC anomalies. I will test again, as it could be other things including very short integration, no guiding, or the horrible seeing we've been having with the high winds.

[Stefan B]

Yes a very improved star shape after your mod. Interesting, it almost seems like there are four "wedges" in the halos/diffraction/spaceballs in the pre-tweak version. Do you have three clips or more? I had aperture issues with my refractor similar to that before I took it apart and fixed it. Again, good shape though. What material was used to create and affix the aperture or clip mask? I haven't looked up procedures for that.

It is a 3D printed one. It is the cheap one which isn't additionally painted black:

https://www.liquidart-shop.at/de/Astro- ... eskop.html
(Sorry, german website)

The mirror had three clips, but the focuser tube may have protruded into the light path a bit. After the mod it looks like this:

Regards
Stefan

[Mike in Rancho]

Thank you, Stefan!

Interesting. I don't notice any wedge shadows from my clips that I have now seen this can improve, but it also does seem to clean up the overall diffraction region. I did some more poking around after looking at your links, and saw that people have done this in any number of ways. I may look into it. My clips seem to be just barely poking into the mirror surface, so if sized right I won't lose much area. I also should examine the primary closer with it out of the OTA, to see what the edge/bevel looks like.

So far my star shape seems more affected by my beginner collimation efforts, focus errors, and maybe some wonky guiding too. And right now I am trying to chase down some awful gradients (referenced in another thread by Ionia who seems to be experiencing the same), possibly from stray light bouncing all over inside this silly telescope.

Back to the original matter at hand, I have done some searching around on star profiles. Be sure to add the word Airy or something to google searches otherwise you just get Hollywood stuff. There are some good discussions and debates by experts on CN, though it fairly quickly starts getting into things beyond Airy, like FWHM, Moffat, beta, and more. Oh and PSF, of course. Can't forget that!

Suffice to say, the point source is spread out, but should I believe be a center tending towards white (not colored), then spreading into colors, and then there's diffraction spiking as well, both large (vanes) and small (edges).

What I still can't find though is some sort of good annotation of an actual star from an image, explaining all the different parts we are seeing or could be seeing, including the colored ball. The sample images in things like Airy disk articles just don't match up well enough to our actual images for me to figure it out or know if there's more to it. All my searches to find a fully annotated star image like that have been in vane. ( he he).

[Stefan B]

Hi Mike,

I did almost experience no light leaks in my Newtonian (although I maybe haven't looked closely enough) with the notable exception of the rear end of the tube. It's known that Newtonians tend to let light through from behind the mirror. I improved the situation with a flexible dust cap:

As stated at TS (https://www.teleskop-express.de/shop/pr ... fnung.html), you can also use it as a light shield at the end of the tube. It's not 100% lightproof but it's better. And I can easily take it off and on during collimation.

As you already noted I, too, didn't find any spot on explanation of the "star balls" and how to reduce their size. Probably I will have to live with them until I use a completely different setup and they maybe miraculously disappear

All my searches to find a fully annotated star image like that have been in vane. ( he he).

Regards
Stefan