Siril histogram

[Pitch Black Skies]

I've noticed that my histogram is way over to the left. Does that mean I'm clipping my frames? The image is 24 hrs of 300s @ unity gain under Bortle 4. I'm using 130P-DS and 533MC-PRO.

There is a ton of detail in the auto- stretched image.

 

Edited April 7, 2022 by Pitch Black Skies
typo

[ONIKKINEN]

This is normal, actually expected from skies as dark as yours and that camera.

Your camera has a huge dynamic range which makes saturating signal very difficult. The result is that all of the faint signal is usually in the black parts of the shot. I have had images that contain on average just 25 photons per pixel per sub and there is no issue.

But like you said stretching is no issue so there is no problem here with the histogram.

By the way the siril autostretch function is quite aggressive, especially for low noise images like yours so when actually applying the stretch to the histogram i recommend walking the midpoint slider a bit back from what the autostretch function wants.

[alacant]

43 minutes ago, Pitch Black Skies said:

my histogram is way over to the left

Correct. Because you haven't stretched it. It is your job to bring it to life by moving it away from where it is.

Remember that selecting AutoStretch from the dropdown does nothing to the data. There's also a handy Auto button on the Histogram window if you're feeling lazy. Notice that in both examples, the Linear mode is chosen.

But be careful. Do as much as you can first WITHOUT stretching. 

Here is an example using non colour balanced dslr data from a dark sky:

 

Edited April 7, 2022 by alacant

[Pitch Black Skies]

19 minutes ago, ONIKKINEN said:

This is normal, actually expected from skies as dark as yours and that camera.

Your camera has a huge dynamic range which makes saturating signal very difficult. The result is that all of the faint signal is usually in the black parts of the shot. I have had images that contain on average just 25 photons per pixel per sub and there is no issue.

But like you said stretching is no issue so there is no problem here with the histogram.

By the way the siril autostretch function is quite aggressive, especially for low noise images like yours so when actually applying the stretch to the histogram i recommend walking the midpoint slider a bit back from what the autostretch function wants.

It's just that when moving the midpoint slider, the histogram is moving very erratically. Do you think it would be better in future if I increased the gain to say 200 instead of unity 100?

 

[ONIKKINEN]

The histogram does this weird dance that looks like a seismograph when you move the sliders, its normal. If you see posterization after moving the slider that still remains, you have stretched the image too far, or are processing a 16-bit file (save as 32bit always). I wouldn't bother with upping the gain, as you see there are diminishing returns from doing so. With your 5min subs i dont see how there could be any noticeable positives from doing so, but star cores would start to saturate earlier and so you would have fatter stars (the brightest ones).

This button stretches the image with the autostretch function, the same as the lower tool bar thing that only visualizes the data for you. But like the autostretch preview, it is very aggressive so i recommend you move the midpoint slider a bit back towards the middle after pressing the button. You will probably need to zoom in on the histogram to see where the slider even is after the autostretch by the way.

[Felias]

Following step by step the Siril tutorial will teach you how to stretch your image: https://siril.org/tutorials/tuto-scripts/#tuto-3

Personally, I have found recently that successive asinh stretches, done carefully, make it redundant to tamper with the histogram any further. But my data are always very noisy, so there's that.

[Padraic M]

+1 for multiple asinh stretches. I'm just starting out on my Siril journey but also a bit scared by the crazy histogram stretch control.

I use that tutorial that @Felias linked as my go-to checklist for processing.

[Pitch Black Skies]

39 minutes ago, ONIKKINEN said:

The histogram does this weird dance that looks like a seismograph when you move the sliders, its normal. If you see posterization after moving the slider that still remains, you have stretched the image too far, or are processing a 16-bit file (save as 32bit always). I wouldn't bother with upping the gain, as you see there are diminishing returns from doing so. With your 5min subs i dont see how there could be any noticeable positives from doing so, but star cores would start to saturate earlier and so you would have fatter stars (the brightest ones).

This button stretches the image with the autostretch function, the same as the lower tool bar thing that only visualizes the data for you. But like the autostretch preview, it is very aggressive so i recommend you move the midpoint slider a bit back towards the middle after pressing the button. You will probably need to zoom in on the histogram to see where the slider even is after the autostretch by the way.

Ah I see, I was trying to move the midpoint before I had even pressed the auto-stretch button 

I see why the 32bit is so important now.

My thinking about the gain is if I increased to 200 I would have lower read noise and shorter exposure times leading to tighter stars possibly?

[ONIKKINEN]

5 minutes ago, Pitch Black Skies said:

Ah I see, I was trying to move the midpoint before I had even pressed the auto-stretch button 

I see why the 32bit is so important now.

My thinking about the gain is if I increased to 200 I would have lower read noise and shorter exposure times leading to tighter stars possibly?

You could actually keep the gain as is and still lower your exposure to 1 or 2 minutes and still swamp read noise by maybe 5x. If your mount has issues then definitely worth doing just that.

If youre chasing tighter stars then stacking with siril and rejecting the softest subs would be effective too. DSS produces softer stars than siril, i believe because it uses bilinear interpolation for alignment of frames (adds blur). You have 24 hours, of which you could reject half of and still have a deep image left. The list of things to do in search of sharpness is endless, so just throwing ideas now.

 

[Padraic M]

I wouldn't be inclined to up the gain. Stars are so bright compared to everything else in a DSO image that they're likely to saturate early even at unity gain. I normally use 30 or 60-sec subs @unity for RGB stars. There's a logic for using lower gain for RGB and higher gain for narrowband as the NB filters block a lot of the star light.

Good discussion on CloudyNights if you search for "Have I understood CMOS camera gain correctly?"

[900SL]

Lukomatico does a great intro to SiriL on YouTube, including how to do successive manual stretches. The autostretch is useful for getting a quick idea of what you have, but manual has more finesse

 

[900SL]

On 07/04/2022 at 13:26, Pitch Black Skies said:

I've noticed that my histogram is way over to the left. Does that mean I'm clipping my frames? The image is 24 hrs of 300s @ unity gain under Bortle 4. I'm using 130P-DS and 533MC-PRO.

There is a ton of detail in the auto- stretched image.

 

Are you using any filters?

 

[Pitch Black Skies]

15 minutes ago, 900SL said:

Are you using any filters?

UV/IR filter

[900SL]

Ok, I'd say at unity gain in Bortle 4 at f5 you may be overexposing at 300s exposures, if only a UV IR filter

 

I normally cool to 0C. I set gain to 105 to ensure I'm the right side of the boost.

Take a bias frame to check average ADU (usually around the 2000 and something mark), then do a trial exposure to increase the ADU by 1500 - 2000. This is more than enough to swamp the noise 

Last time I was in bortle 4 my exposures were 120s to 150s and the histogram well off the LHS no clipping

 

 

 

 

 

[Pitch Black Skies]

21 hours ago, 900SL said:

unity gain in Bortle 4 at f5 you may be overexposing at 300s exposures, if only a UV IR filter

But wouldn't the histogram be over on the RHS then?

 

I will try the bias method, sounds very straight forward.

[900SL]

No, because the bulk of the signal is still background sky, but you could be clipping a lot of the highlights (stars) 

Theres a good YouTube by Cuiv the lazy geek on the ADU exposure method, and a more detailed video by Dr Robin Glover

[AstroMuni]

On 07/04/2022 at 11:26, Pitch Black Skies said:

I've noticed that my histogram is way over to the left. Does that mean I'm clipping my frames? The image is 24 hrs of 300s @ unity gain under Bortle 4. I'm using 130P-DS and 533MC-PRO.

Check the default offset that is applied by the driver for your camera at that gain. When you open histogram transformation tool if you notice there is a lot of data at the lower range, then you can increase the offset in the camera so there is less risk of clipping. You can increase the zoom level by increasing the level in the box on top left.

As an example I apply an offset of around 100 for 180 gain but 120 for 240 gain on my asi224mc.

Edited April 11, 2022 by AstroMuni