StarTools

Mike in Rancho wrote: ↑Sat Sep 10, 2022 7:51 pm Thanks Ivo. Yes I've read that in the docs and even quoted it (loosely) in my post. I'm just trying to dig deeper into the apod mask colors - and perhaps am looking for meaning that isn't there. Wouldn't be the first time.

I also forgot to mention that the red/yellow/green/yellow/red coloring is a sliding scale, so you may see shades of pure green, banana green, banana yellow, etc.

Panel 1 is your basic red ball. I assume these are not to be used for SVD (I've never tried, actually). Problem is, sometimes my entire apod mask is pretty much only stars just like this.

If all you have is these, try picking the brightest ones.

Also. consider whether a pure synthetic PSF might just be better, if good quality samples are not getting you where you want to be. This may well get you better results than chasing samples in crowded wide fields. At the very least, the Moffat models should quite effectively undo the effects atmospheric turbulence. Any other imperfections/distortions in the PSF (whether local or across the entire image) may not even be visible at some scales.
Usually, atmospheric influences are the biggest contributors to out-of-the-ordinary PSFs, if the rest of your gear is dialed in reasonably well.

Panel 2 has some yellow in the core, no green. Ostensibly then okay for a secondary choice if I have nothing else in the area?

Correct!

Panel 3 has some green. Not a ton, but pretty close to the best that I typically see. Assume this should be selected.

Correct. This appears to be a good sample. You may even want to increase the apodization mask so that more of the stellar profile is included.

Panel 4, (if one ignores the adjoining star), is green but red pixeled in the center. Not to be used even if it was by itself.

Ideally not, but if you are quite sure that the star isn't over-exposing and your sensor's (and stack's!) response stays linear with stars this bright, then maybe.

Panel 5, (again ignore the double star for now), is starting to show a green center but there's a yellow pixel (though not gone red) in the very center. Still okay? I think the Features and Docs examples of green stars actually may have a central yellow dot themselves, if I am peeking at it correctly?

Still OK. It looks like the center is just creeping into the (possibly) non-linear danger zone.

More about the "danger zone"; this danger zone is, as said, the brightness where linearity might be getting iffy. This can be due to CCD well saturation (e.g. photons don't get converted into electrons as much due to the wells becoming "full"). This can also be due to stacking data of variable quality. For example, a star under poor seeing will not saturate as quick, because its light is "smeared out" over multiple pixels. Under good seeing conditions, however, stars will over-expose quicker as the light gets concentrated into fewer photo sites. If you stack a mixture of these, you will get stars that have cores that have unreliable data (due to averaging over-exposing cores with non-over-exposing cores).

Does the above help?

Actually it does, thanks Ivo. I'm getting there. But of course have follow up questions.

I do indeed revert to synthetic modeling when I have to (it can still be pretty good), though usually only when SVD causes a menagerie of polygon stars. That's also a work in progress, as you of course know.

Thanks for the tip on the red stars. I believe I have oft been leaving regions unsampled due to lack of any green or yellow. So my new epiphany is, that SVD can still read and utilize the stellar profile of a red sample star. But it would be the last choice and I should pick well-shaped brighter ones.

As such, no need then to fiddle with the linearity slider to give those red stars yellow or green cores? I have not yet experimented with that, but may not bother if it is superfluous.

Lastly, in regards to increasing the apod mask of a sample star as you mentioned. Until now I have only modified the mask for deringing purposes. But this raises the question of what exactly the blue box versus the white outline actually mean, as I am unsure and could be confusing them. Also, my sometimes wonky star shapes plus diffraction spikes can make mask touch-up difficult, and dependent upon just what the auto-apod came up with on its own. Would I run the danger of creating a phony stellar profile by using the "pixels lighter than" selection or even "drag circle," or does SVD still read the "real" stellar profile within the confines of the mask's white outline?

I must say this has been a great week of SVD education.

Mike in Rancho wrote: ↑Mon Sep 12, 2022 6:14 am As such, no need then to fiddle with the linearity slider to give those red stars yellow or green cores?

No need.

But this raises the question of what exactly the blue box versus the white outline actually mean, as I am unsure and could be confusing them.
Also, my sometimes wonky star shapes plus diffraction spikes can make mask touch-up difficult, and dependent upon just what the auto-apod came up with on its own. Would I run the danger of creating a phony stellar profile by using the "pixels lighter than" selection or even "drag circle," or does SVD still read the "real" stellar profile within the confines of the mask's white outline?

The apodization mask indicates which pixels may be included in a sample and which may not.
The blue box indicates the location and area that will be sampled. Any pixels inside the blue box AND inside the apodization mask get included in the final sample. Anything outside the box is excluded from the sample. Anything inside the box, but not set in the apodization mask is also excluded from the sample.

You can certainly touch up the mask to include any part of the stellar profile you wish (or do not wish) to include.

Does that help?

admin wrote: ↑Mon Sep 12, 2022 6:35 am
No need.

The apodization mask indicates which pixels may be included in a sample and which may not.
The blue box indicates the location and area that will be sampled. Any pixels inside the blue box AND inside the apodization mask get included in the final sample. Anything outside the box is excluded from the sample. Anything inside the box, but not set in the apodization mask is also excluded from the sample.

You can certainly touch up the mask to include any part of the stellar profile you wish (or do not wish) to include.

Does that help?

Yes!

That helps clarify why we don't want the outline of a nearby star within the sample's blue box, which we already knew, but also that I would need to be rather smart and deliberate regarding apod mask touch-up on a star I want to sample. Probably zoomed way in.