Developing NGC7000 using ST: can I do better?

[polslinux]

Some days ago, while on a trip on a Bortle 4 location (SQM 21.63), I decided I wanted to image NGC 7000.
I didn't plan the session before, so I just picked the car up and drove to the top of a nearby hill. The night sky view was amazing
TL;DR:
- StarTools 1.6.400M1 on openSUSE Tumbleweed.
- iOptron SkyGuider Pro
- Nikon D7500
- Tamron 150-600 G2 @ 150mm f/5
- ISO 1600
- 77 lights @ 55 seconds
- no darks, flats, biases as it was a cold and unplanned night (and the next morning I had to woke up very early for a long hike). Next time I will take them, I swear

All lights were stacked and kept by ASTAP.

This is the stacked fits: https://drive.google.com/file/d/1qOB4sX ... sp=sharing
And this is a single NEF file: https://drive.google.com/file/d/13TOA0O ... sp=sharing

While this is the config I used:

This is what I did with ST:

Code: Select all

autodev -> keep
bin -> 60% -> keep
crop -> keep
wipe -> 78% -> do -> keep
autodev -> redo global stretch -> keep
contrast -> dark anomaly filters 3 -> do -> keep
Sharp -> next -> keep
Color -> fill mask -> dark saturation 3.10, saturation 230% -> keep
Life -> less=More -> keep
Entropy -> Dark/Light 70%/30% -> do -> keep
Tracking off -> grain removal -> next

My result:

Given that I'm a super newbie in both astrophotography and StarTools, do you think I could achieve a better result here?
Or should I now move to Starnet++ and GIMP for some follow up stuff? (I honestly wouldn't even know what to do more than this...)

[admin]

It sometimes blows me away what rank beginners do with ST.
It is a really nice image, achieved without calibration frames no less!
The coloring is spot-on - looking at the dataset it appears you even managed the slight chromatic aberration of your lens very well too (there are techniques to deal with that if it becomes a problem).

I can see some walking noise - dither between frames if practical to solve that. StarTools 1.7 now has walking noise reduction if needed.

For star-crowded widefields like these, signal fidelity is not so much an issue, however, it may be worth finding out if ASTAP can turn off white-balancing.
The white balancing causes luminance noise to increase before StarTools can track and mitigate it.

I'm not a massive fan of StarNet++ for a few reasons. Intuitively star removal sounds like a great idea; remove the stars, work on what's "underneath", put them back again. However in practice the results are quite the opposite; I can almost always pick images that were processed (e.g. stars removed and then layered back in) using StarNet++ as they usually contain many artefacts and detail that doesn't actually exist in reality. I actually really hope it is a fad that will go away soon. It is somewhat useful for star mask creation for applications that don't have decent tools (e.g. PI), but ST does and all its modules are star-aware. Truth be told, I cringe every time I see an image where it was employed. That's just me though and it's a free world! There is a guide for using StarNet in conjunction with StarTools here.

It is better to keep the stars in situ and modify them in-place if you are so inclined. There are two main ways you can do that in ST. You can use the Life module's Isolate preset (selected by default) with full mask set ("green"). This will push back a busy star field and re-focus the attention on the bigger structures. The other method is to put the stars in a mask and use the Shrink module (revamped in 1.7). Or... combine them.

Great work!

[polslinux]

Hello Ivo,
thanks for your answer and, most importantly, for your software

The coloring is spot-on - looking at the dataset it appears you even managed the slight chromatic aberration of your lens very well too (there are techniques to deal with that if it becomes a problem).

I'm happy to know that I manged to correct the chromatic aberration, but honestly I don't know how I did it I just played around with ST settings. Do you have any idea on what I could have done to fix it?

I can see some walking noise - dither between frames if practical to solve that. StarTools 1.7 now has walking noise reduction if needed.

Yea, I saw that too unfortunately, I have no idea how to dither with my skyguider pro. When I will buy the ASI Air pro for guiding, I will be able to dither in RA. Maybe this will help?

For star-crowded widefields like these, signal fidelity is not so much an issue, however, it may be worth finding out if ASTAP can turn off white-balancing.
The white balancing causes luminance noise to increase before StarTools can track and mitigate it.

Interesting, I'll have a look at it. Worst case I'll spawn up a Windows VM and install DSS. Unfortunately, DSS doesn't work well on WINE

I'm not a massive fan of StarNet++ for a few reasons. Intuitively star removal sounds like a great idea; remove the stars, work on what's "underneath", put them back again. However in practice the results are quite the opposite; I can almost always pick images that were processed (e.g. stars removed and then layered back in) using StarNet++ as they usually contain many artefacts and detail that doesn't actually exist in reality. I actually really hope it is a fad that will go away soon. It is somewhat useful for star mask creation for applications that don't have decent tools (e.g. PI), but ST does and all its modules are star-aware. Truth be told, I cringe every time I see an image where it was employed. That's just me though and it's a free world! There is a guide for using StarNet in conjunction with StarTools here.

It is better to keep the stars in situ and modify them in-place if you are so inclined. There are two main ways you can do that in ST. You can use the Life module's Isolate preset (selected by default) with full mask set ("green"). This will push back a busy star field and re-focus the attention on the bigger structures. The other method is to put the stars in a mask and use the Shrink module (revamped in 1.7). Or... combine them.

I've played a bit with it in the past and, honestly, I'm not a big fan of it too. I'm more than happy with what I'm achieving with ST, so I see no need to complicate my life I should spend my spare time to better learn ST instead

Great work!

Thanks a lot
And going a bit off-topic, just so you know, the top 2 reasons why I bought ST are:

1. you: always active and always ready to give us suggestions! Thanks a lot and keep up the amazing job you're doing
2. ST's user-friendliness and its results: I was blown away with how simple it is to post process an image with ST compared to other tools

Clear skies

[admin]

Your kind comments mean a lot!

The most effective way of removing chromatic aberration is this;

After running the Color module, run the Filter module.
Create a "Fat Stars" mask (or just a general star mask) and make sure all the stars that show CA are selected in that (e.g. Grow once or twice).
Then back in the Filter module, set the Filter Mode to "Fringe Killer". Now zoom into one of the stars and keep clicking on the purple (or any other offending color) halo pixels around the stars. They should start to disappear.

You can see how out of focus the red channel is versus the green;

CAsbs.jpg (136.13 KiB) Viewed 6065 times

This is what causes the fringing and greenish star cores (if not remedied).

Re:ASTAP, I have heard the author is very open to new feature requests (ASTAP may well be able to turn off white-balancing, I just don't know the software very well).

Cheers!

[F-Astro]

You can see how out of focus the red channel is versus the green;
CAsbs.jpg
This is what causes the fringing and greenish star cores (if not remedied).

Hi Ivo,

Probably a dumb question, but with a DSLR like the OP has (or a OSC camera ), how did you obtain the red and green channel images you showed?

I assume there is nothing one could do after the fact to mitigate the focus differences? I know why it happens, but seeing it is believing!

Fran

[polslinux]

After running the Color module, run the Filter module.
Create a "Fat Stars" mask (or just a general star mask) and make sure all the stars that show CA are selected in that (e.g. Grow once or twice).
Then back in the Filter module, set the Filter Mode to "Fringe Killer". Now zoom into one of the stars and keep clicking on the purple (or any other offending color) halo pixels around the stars. They should start to disappear.

Good to know, thanks

You can see how out of focus the red channel is versus the green;
This is what causes the fringing and greenish star cores (if not remedied).

Ah, interesting! That's why my image seems a little bit blurry/out of focus. This is due to the red channel being out of focus then, right?
What did I do wrong here to get this blurry red channel? Didn't I focus correctly using live view? Or Lens is not good enough for astro? Or too aggressive PP?

Re:ASTAP, I have heard the author is very open to new feature requests (ASTAP may well be able to turn off white-balancing, I just don't know the software very well).

I just wrote him, and he almost immediately replied this is how to disable white balancing on ASTAP:
"For colour there is an option "auto levels" in tab "stack method". If checked it will make the stars average white and the background average gray. Uncheck this one."

Thanks!

[admin]

What did I do wrong here to get this blurry red channel? Lens is not good enough for astro?

You did not do anything wrong at all! It's indeed a characteristic of your lens. It's a very common thing with lenses meant for terrestrial use. It's not the end of the world, just something to be mindful of. The blue channel suffers from precisely the same thing. It's textbook axial/longitudal chromatic aberration.

In fact, you did everything right; the green channel (which is the most prominent, visible and most reliable) is in good focus. The green part of the spectrum is the most important for visual spectrum datasets (particularly when shot with a DSLR or OSC). It's no coincidence DSLRs/OSCs sensors allocate 2 green samples vs just 1 red and 1 blue sample per 2x2 patch.
Being able to turn off white balancing will actually allow ST to re-weigh the green channel higher for luminance/detail purposes (e.g. import your stack with the second option), which will make the chromatic aberration a little less visible.

I just wrote him, and he almost immediately replied this is how to disable white balancing on ASTAP:
"For colour there is an option "auto levels" in tab "stack method". If checked it will make the stars average white and the background average gray. Uncheck this one."

That's fantastic - the AP community is full of passionate people! If not too much trouble, would you be able to re-post the stack by any chance with that setting disabled? I'm keen to see the difference. If it indeed fixes that issue, that would move ASTAP a lot higher up the list of preferred/recommended stacking solutions. That would allow me to update the docs accordingly.

Thank you!

[admin]

Probably a dumb question

No such thing!

but with a DSLR like the OP has (or a OSC camera ), how did you obtain the red and green channel images you showed?

The Compose module will automatically extract the right channel from a color dataset.
E.g. if you click "Red/S-II" and then load a color dataset, it will extract the red channel. The "Luminance, Color" and Channel Interpolation settings do the rest (e.g. make it mono and/or make the other channels inherit the same data for the purpose of "color" rendering).

I assume there is nothing one could do after the fact to mitigate the focus differences? I know why it happens, but seeing it is believing!

Deconvolution restoration would theoretically be the "proper" go-to tool for this, and you can indeed choose to separate each channel in its own dataset, apply deconvolution, restore the dataset to its linear state, and save the new dataset. Once you're done with all channels, you can re-composite the 3 new linear datasets into one linear color dataset and... hope for the best.

In practice, your mileage may vary considerably, as deconvolution restoration will never be perfect. Given that CA is particularly visible around highlights and deconvolution cannot work around overexposing star cores (there is no data in them; they are singularities), it's still not a perfect solution.

As said, making sure the dataset is not white-balanced allows ST to re-weigh the green channel properly, which will reduce the problem. The rest of the issue can be fixed in the color domain to a reasonable degree through the method mentioned above (e.g. Filter module).

Hope this helps!

[polslinux]

You did not do anything wrong at all! It's indeed a characteristic of your lens. It's a very common thing with lenses meant for terrestrial use. It's not the end of the world, just something to be mindful of. The blue channel suffers from precisely the same thing. It's textbook axial/longitudal chromatic aberration.

Interesting, thank you very much! I'm happy to know that I did everything right, but way less happy to know that, for astro, my lens is not as good as I thought
I guess it's time to start looking around for a lightweight APO refractor then

That's fantastic - the AP community is full of passionate people! If not too much trouble, would you be able to re-post the stack by any chance with that setting disabled? I'm keen to see the difference. If it indeed fixes that issue, that would move ASTAP a lot higher up the list of preferred/recommended stacking solutions. That would allow me to update the docs accordingly.

Thank you!

Sure, no problem

Stacked with DSS: https://drive.google.com/file/d/1B__N92 ... sp=sharing
Stacked with ASTAP: https://drive.google.com/file/d/1Ru3xV5 ... sp=sharing

This is the full ASTAP process:

1. Select "Stack"

1.jpg (52.06 KiB) Viewed 6033 times

2. Select "Images" and then "Browse" . Add all your lights.

2.jpg (58.07 KiB) Viewed 6033 times

4. Repeat procedure for Darks, Flats and Flat darks (I didn't have any )

5. Go to "Stack Method" and then:
5a: select "sigma clip average"
5b: disable auto levels
5c. select "bilinear interpolation"

3.jpg (164.96 KiB) Viewed 6033 times

6. Click on "Stack (Sigma clip average)

Thanks!

[F-Astro]

Probably a dumb question

No such thing!

but with a DSLR like the OP has (or a OSC camera ), how did you obtain the red and green channel images you showed?

The Compose module will automatically extract the right channel from a color dataset.
E.g. if you click "Red/S-II" and then load a color dataset, it will extract the red channel. The "Luminance, Color" and Channel Interpolation settings do the rest (e.g. make it mono and/or make the other channels inherit the same data for the purpose of "color" rendering).

I assume there is nothing one could do after the fact to mitigate the focus differences? I know why it happens, but seeing it is believing!

Deconvolution restoration would theoretically be the "proper" go-to tool for this, and you can indeed choose to separate each channel in its own dataset, apply deconvolution, restore the dataset to its linear state, and save the new dataset. Once you're done with all channels, you can re-composite the 3 new linear datasets into one linear color dataset and... hope for the best.

In practice, your mileage may vary considerably, as deconvolution restoration will never be perfect. Given that CA is particularly visible around highlights and deconvolution cannot work around overexposing star cores (there is no data in them; they are singularities), it's still not a perfect solution.

As said, making sure the dataset is not white-balanced allows ST to re-weigh the green channel properly, which will reduce the problem. The rest of the issue can be fixed in the color domain to a reasonable degree through the method mentioned above (e.g. Filter module).

Hope this helps!

Ivo,

Thanks for your detailed reply. It was all very clear. I’m still new with this, so I’m happy to sometimes stumble upon some information that I didn’t know anything about. I plan on keeping my OSC camera for a bit longer, so my next question is this. If I wanted to create a purely mono image with my OSC, which RGB channel would you suggest processing? I assume that by processing a single channel it would eliminate the focus/wavelength issues discussed above, creating a sharper image than a full color image converted to black and white?

Thanks,
Fran