Star clipping vs shorter exposures

[Datalord]

So, I have a problem. Not entirely sure I want to fix it, but I would like some input.

So, I have had a problem with clipped stars. But it is isolated to the big ones. I snipped an area chart from Maxim:

This is a 600s bin2 red on the 16200 from the RC12.

Now, The obvious choice would be to reduce my exposure time, but I find my SNR (obviously) suffers more than I'm willing to accept. Or I was willing to accept. I'm also quite worried about losing detail on fainter targets, especially the galaxies in my cluster images and nebulae.

What is the consensus amongst the congregated stargazers here?

[fireballxl5]

well I'm dealing with the same problem on my current project (Cat's Eye nebula). I've resolved to two or three exposure durations, 600s for the outer shell and as short as 20s for the inner core. I'll merge them in a similar way to when processing M42.

[waylander]

7 hours ago, fireballxl5 said:

well I'm dealing with the same problem on my current project (Cat's Eye nebula). I've resolved to two or three exposure durations, 600s for the outer shell and as short as 20s for the inner core. I'll merge them in a similar way to when processing M42.

So that sounds like what most people do for M42 which is a large scale structure. So you can just cut out the blown out core and replace with the other shorter exposure segment- but the OP is talking about "stars" in general, how easy is it to combine the properly exposed stars from one image with the galaxies from another. Can Pixinsight do this do we know? I'm facing similar issues, short exposure not enough DSO detail, long exposures blown out stars with no color.

[fireballxl5]

13 minutes ago, waylander said:

So that sounds like what most people do for M42 which is a large scale structure. So you can just cut out the blown out core and replace with the other shorter exposure segment- but the OP is talking about "stars" in general, how easy is it to combine the properly exposed stars from one image with the galaxies from another. Can Pixinsight do this do we know? I'm facing similar issues, short exposure not enough DSO detail, long exposures blown out stars with no color.

I see what you mean. My current problem is the blown core of the PN, not the stars so much, so the M42 approach may not work in @Datalord's case. I'm using PI's HDRcomposition process, which has worked very well for my M42 and the initial checks that I've performed on the luminance datasets I've captured for the Cat's Eye now. Blown stars are a common problem so it'll be interesting to see what else is advised.

[Datalord]

9 hours ago, fireballxl5 said:

may not work in @Datalord's case

I really, really prefer not to have to do different exposure sets. That would mean a massive increase in imaging time for what is usually not the primary target in the images.

[ollypenrice]

On 06/05/2020 at 19:39, Datalord said:

I really, really prefer not to have to do different exposure sets. That would mean a massive increase in imaging time for what is usually not the primary target in the images.

Not necessarily. Because the stars are bright you don't need many subs to get a good SNR. For stellar cores what would you need? 3x15 seconds per colour? Pretty quick.

Another thing that works well is to use your RGB-only for over exposed parts of the image rather than the luminance. There's less signal in the RGB, normally. You can convert it to greyscale and combine with the luminance or work in RGB in Layers.

In fact I don't worry about saturated stellar cores because I find I can pull the colour into the core from the outside using Noel's Actions Increase Star Colour or by doing it longhand. 

Olly

[wimvb]

On 06/05/2020 at 10:13, waylander said:

Can Pixinsight do this do we know?

Yes. Have a look here. It's an extreme example, but it shows the technique quite nicely. 

https://pixinsight.com/tutorials/NGC7023-HDR/index.html#High_Contrast_Small_Scale_Structures

[Datalord]

On 08/05/2020 at 18:27, ollypenrice said:

For stellar cores what would you need? 3x15 seconds per colour? Pretty quick.

That sounds incredibly short, but I get your point. I need to find the right exposure to get the stars below clipping point, then mask the core in to the final image.

On 08/05/2020 at 18:27, ollypenrice said:

In fact I don't worry about saturated stellar cores because I find I can pull the colour into the core from the outside using Noel's Actions Increase Star Colour or by doing it longhand. 

Yeah, I don't actually have a problem getting colour into the center of the stars. I only have a problem with them getting large while processing.

14 hours ago, wimvb said:

Yes. Have a look here. It's an extreme example, but it shows the technique quite nicely. 

https://pixinsight.com/tutorials/NGC7023-HDR/index.html#High_Contrast_Small_Scale_Structures

I've used this before. I'm not sure it gets me what I want. Somehow the stars become fuzzy blobs. Colourful fuzzy blobs, but blobs nonetheless.

[ollypenrice]

13 minutes ago, Datalord said:

 

Yeah, I don't actually have a problem getting colour into the center of the stars. I only have a problem with them getting large while processing.

 

Ah yes, that's a universal problem. It's not just the saturated core you're talking about, then. One thing that looks promising is using Starnet or Straton to de-star a copy of the stretched image then put the linear original on top as a layer in Blend Mode Lighten and stretch in situ till you have the stars as large/bright as you want them. In Blend Mode Lighten only the stars will appear during the stretch.

Olly

[Datalord]

51 minutes ago, ollypenrice said:

Ah yes, that's a universal problem. It's not just the saturated core you're talking about, then. One thing that looks promising is using Starnet or Straton to de-star a copy of the stretched image then put the linear original on top as a layer in Blend Mode Lighten and stretch in situ till you have the stars as large/bright as you want them. In Blend Mode Lighten only the stars will appear during the stretch.

Yes, but that messes up any hope of diffraction spikes... 

I should probably be content. My images are getting pretty good, but Hubble is always grinding my eyeballs! 😂

[ollypenrice]

5 minutes ago, Datalord said:

Yes, but that messes up any hope of diffraction spikes... 

I should probably be content. My images are getting pretty good, but Hubble is always grinding my eyeballs! 😂

I hadn't thought of diff spikes! Curses!!

Olly

[wimvb]

2 hours ago, Datalord said:

Yeah, I don't actually have a problem getting colour into the center of the stars. I only have a problem with them getting large while processing.

So, it's not so much about data capture as it is about processing that data. In that case, I'd not change the exposure time. Although, if each of the colour channels shows enough details, you should probably shorten the exposure time for luminance (if it's the same as your colour exposure time). L captures all the colours at once, and there's more risk of bloated stars. You should be able to at least halve your L exposure time, and double the number of exposures.

[Datalord]

33 minutes ago, wimvb said:

So, it's not so much about data capture as it is about processing that data. In that case, I'd not change the exposure time. Although, if each of the colour channels shows enough details, you should probably shorten the exposure time for luminance (if it's the same as your colour exposure time). L captures all the colours at once, and there's more risk of bloated stars. You should be able to at least halve your L exposure time, and double the number of exposures.

Hmm, I usually do twice the exposure time, but bin1 instead of bin2, so that should account for half?

But, definitely something I can reduce drastically, at least as a test.

[wimvb]

3 minutes ago, Datalord said:

Hmm, I usually do twice the exposure time, but bin1 instead of bin2, so that should account for half?

Do you mean you expose L twice as long as RGB?

From some of your images, I gather that for RGB you use 600 s @ bin 2x2, and for L, 1200 s @ bin 1x1. That seems a massive exposuretime, but is of course dependent on your gear and local conditions. To compare, I have a friend who uses 500 s with a Moravian G2 8300, a f/5 scope (8") and a sky darkness of 20.5, for his L and for RGB, all at bin 1x1.

In this image, you used shorter times. How did that compare to your usual settings?

 

[Datalord]

Well, as I mentioned, I'm much more comfortable with the longer exposures because it makes a difference in the fainter areas of the images. That said, my M82 I posted last night uses 600s bin2 L and 180s bin2 RGB. But, 1200s bin1 Ha. 

Comparing the cameras, I have a little bigger pixel size, but also 41,000 full well compared to 25,000 on the 8300. I should be able to cope with a bit longer exposure time. But, differences in aperture etc etc. 

Honestly, I think I'll just go back 180s bin2 RGB and 300s bin1 L. It seems like the most sane approach, especially for galaxies and star fields.