M33 last night (updated to add a 2nd night of data)

[Stuart1971]

3 hours ago, The Lazy Astronomer said:

That formula can work, it depends how the stars only image was created.

The post I linked to explains how to unscreen stars using starnet (I think the latest version of starX has an option to create an unscreened star image automatically). The pixelmath

~((~starless)*(~stars))

is used later to add the stars back in

What is the screening option for, I have never used it…?

[StuartT]

On 04/10/2022 at 15:08, The Lazy Astronomer said:

The joys of image calibration!

The first thing I would think it would be is improperly subtracted darks. Did you take darks (for the lights) and flat darks (for the flats)?

By the way, I figured out the problem. I used ABE rather than DBE (note to self... don't be lazy)

[StuartT]

edited

Edited October 6, 2022 by StuartT
dupe

[StuartT]

5 hours ago, The Lazy Astronomer said:

That formula can work, it depends how the stars only image was created.

The post I linked to explains how to unscreen stars using starnet (I think the latest version of starX has an option to create an unscreened star image automatically). The pixelmath

~((~starless)*(~stars))

is used later to add the stars back in

Ok, I tried your pixelmath expression (on the left) and my simple additive one (one the right). They are indeed subtly different. I think yours has lightened the sky a little more. Not sure which I prefer to be honest

 

[Stuart1971]

2 hours ago, StuartT said:

Ok, I tried your pixelmath expression (on the left) and my simple additive one (one the right). They are indeed subtly different. I think yours has lightened the sky a little more. Not sure which I prefer to be honest

 

The one on the right is better, the galaxy has lost a small amount of detail in the left image too…👍🏻

[The Lazy Astronomer]

4 hours ago, StuartT said:

Starnet2 and StarX both have the same options. You can either simply remove the stars, or you can remove them and generate a star only image too.

 

4 hours ago, Stuart1971 said:

What is the screening option for, I have never used it…?

Yeah, starnet just does a simple subtraction of "original - starless" to get the stars only image (as does StarX, although I had heard it had an unscreen option; I don't use StarX though, so don't actually know - it may well just create an unscreened star image by default now).

Subtraction works fine when the stars are just sitting in front of background, but the problem with subtraction is that stars that were over brighter areas end up being dimmer and possibly have their colour altered (depending on the colour of the star, and the colour of whatever was behind them). Unscreening gets around this, and extracts the stars at pretty much their full brightness and with minimal impact on their colour (if any), regardless of what was behind them.

The 2 stars only images below, taken from the pixinisight forum post I linked to, show the difference between simple subtraction and unscreening.

Subtraction:

Unscreened:

 

To get the bottom image, you create a duplicate of whatever you're working on, run starnet/StarX, then do

~((~original)/(~starless))

Continue and do whatever you want to your stars only and starless images, then recombine with the pixelmath I posted previously. 

Edit: I should also mention rescreening the stars back in to the image puts them back at basically their orginal brightness (i.e. before they were extracted). Addition will obviously just add the star brightness to the starless image brightness, which can then cause clipping of the stars if you increased the level of the stretch on the starless image. A multiplier applied to the stars only image during addition recombination can somewhat mitigate this, but it's an imperfect solution, as you've likely brightened or darkened different parts of the image in different ways.

By the way, I take absolutely no credit for this, I merely stumbled across the post on the PI forums.

3 hours ago, StuartT said:

By the way, I figured out the problem. I used ABE rather than DBE (note to self... don't be lazy)

P.s. nothing wrong with being lazy! 😉

Edited October 6, 2022 by The Lazy Astronomer

[The Lazy Astronomer]

3 hours ago, StuartT said:

Ok, I tried your pixelmath expression (on the left) and my simple additive one (one the right). They are indeed subtly different. I think yours has lightened the sky a little more. Not sure which I prefer to be honest

 

I'm going to hazard a guess that StarX created the stars only image by subtraction in this case, so the stars (and other bright areas it misidentified as stars) that were extracted from within m33 are dimmer than they should be, and where you've rescreened the stars back in, they've been put back in at this dimmer brightness value from the stars only image.

[StuartT]

14 hours ago, The Lazy Astronomer said:

I'm going to hazard a guess that StarX created the stars only image by subtraction in this case, so the stars (and other bright areas it misidentified as stars) that were extracted from within m33 are dimmer than they should be, and where you've rescreened the stars back in, they've been put back in at this dimmer brightness value from the stars only image.

I'm not sure I'm following you here. I don't know how StarX removes stars. I assumed it only had one mode of operation. I don't know what 'unscreening' means. I assumed it just took the stars out and then I add them back in, but I've not really looked into what goes on under the hood

[Stuart1971]

1 hour ago, StuartT said:

I'm not sure I'm following you here. I don't know how StarX removes stars. I assumed it only had one mode of operation. I don't know what 'unscreening' means. I assumed it just took the stars out and then I add them back in, but I've not really looked into what goes on under the hood

Read the reply to my message above, where I ask “What is the screening function” from @The Lazy Astronomer it explains what the screening is and does….looks very good 👍🏻