ZWO camera offset setting & cooling

[bomberbaz]

Hello all.

I am currently in the process of gathering data on my camera's (ZWO 183MC Pro) optimum settings for data gathering.  Following on from watching the excellent video of Dr Robin Glover (Link Dr Robin Glover ), I decided to make the very modest investment into Sharpcap Pro again, (previous subscription expired) this so I can use the tools available in the programme, these being sensor analysis and smart histogram. 

Following this I discovered to my surprise I had been vastly over doing it in terms of exposure length, especially from my home.  I knew they were a little overdone following watching the video link above, but not to the extents of what it was.

The actual result wasn't realistic in terms of data gathering (2-4 seconds), however previous times of 2 minutes at home can comfortably be reduced to 12-15 seconds, any less and the number of subs for a five hours session will just become very silly. I also found my optimum unity gain should be slightly lower (-4) but I will leave it unchanged to the ZWO default. I have yet to settle on the final sub setting from home but I can't imagine it will change much from said 12-15.

So whilst on with this research of optimum settings, I started to dig into things further and keep coming across the use of offset during the taking of subs to avoid clipping of data but cannot seem to find settings for the alteration of offset either in sharpcap or ASIAir+, am I missing something very simple here?  I am actually wondering if this has been automated in some sort of algorythm, can anyone help?

On cooling there appears to be nothing in sharpcap or the wider forums that advises the best setting of this, (maybe missing this too but it is getting quite overwhelming hence this post) but I am thinking that for 15 second subs, cooling will not be such an issue anyway!

Cheers for any advice on this matter.

Steve

[drjolo]

The amount of the dark (thermal) signal  depends on the total integration time. It does not matter if you collect 1000x15s or 15x1000s of frames - you will collect the same amount of thermal noise. The only way to limit it is to cool the chip. So you may use the same temperature preset as before. Each 6-7C of temperature difference doubles the thermal signal. Thermal noise in modern cameras is not a big issue, unless you reach some larger positive values (like +10C or more), or you capture data with narrowband filters. 

As for 2-4 seconds subframes it was probably calculated for light polluted sky, so for your specific case the read noise amount becomes not significant when you use 2-4s subframes, or longer. You can make subframes longer, and then you need to consider the pixel capacity (so they will be saturated) and your tracking/guiding capabilities.  Collecting few hours of frames in 2-4s subframes probably is not reasonable, unless you would like to do a kind of lucky imaging and then select best frames to stack this selection into better resolved final image. 

PS - offset is as far as I remember called "black level" in SharpCap currently

Edited January 24, 2023 by drjolo

[bomberbaz]

50 minutes ago, drjolo said:

The amount of the dark (thermal) signal  depends on the total integration time. It does not matter if you collect 1000x15s or 15x1000s of frames - you will collect the same amount of thermal noise. The only way to limit it is to cool the chip. So you may use the same temperature preset as before. Each 6-7C of temperature difference doubles the thermal signal. Thermal noise in modern cameras is not a big issue, unless you reach some larger positive values (like +10C or more), or you capture data with narrowband filters. 

As for 2-4 seconds subframes it was probably calculated for light polluted sky, so for your specific case the read noise amount becomes not significant when you use 2-4s subframes, or longer. You can make subframes longer, and then you need to consider the pixel capacity (so they will be saturated) and your tracking/guiding capabilities.  Collecting few hours of frames in 2-4s subframes probably is not reasonable, unless you would like to do a kind of lucky imaging and then select best frames to stack this selection into better resolved final image. 

PS - offset is as far as I remember called "black level" in SharpCap currently

Actually that makes perfect sense. End picture is a result of lots of stacked data, so thermal will be the same. I shall maintain things as they have always been.

Thanks for the tip ref offset but there is no control for black level either. I should point out that A. I am on V4 sharpcap, recentl;y released and B. there is no mention of an offset value on the sensor analysis report and that there is no black level in ASIAIR either.

[bomberbaz]

With ZWO camera's, OFFSET is simply brightness. Took some finding.

[andrew s]

1 hour ago, bomberbaz said:

With ZWO camera's, OFFSET is simply brightness. Took some finding.

Not sure what you mean but see this:

"

Offset is exactly as its name implies: it is an offset that is applied to the histogram curve which shifts it – offsets it – farther to the right on the histogram by some amount. This ensures that the minimum value that any pixel might record is not underexposed and is therefore clipped in the blacks. This is especially important in astrophotography because of the nature of it – some pixels on the sensor might be positioned to look at purely dark space and therefore will collect far, far fewer photons than pixels that has the light from a star, nebula, or other light-producing object pouring into it.

Avoiding clipped-black pixels means that you avoid having pixels with insufficient signal or, worse, a value of 0. This is the opposite predicament of an image that has pixels that are clipped-white. Both types of clipped pixels effectively have no data that can be meaningfully acted upon during processing."

 

From here https://daleghent.com/2020/08/understanding-camera-offset

Regards Andrew 

[bomberbaz]

30 minutes ago, andrew s said:

Not sure what you mean but see this:

"

Offset is exactly as its name implies: it is an offset that is applied to the histogram curve which shifts it – offsets it – farther to the right on the histogram by some amount. This ensures that the minimum value that any pixel might record is not underexposed and is therefore clipped in the blacks. This is especially important in astrophotography because of the nature of it – some pixels on the sensor might be positioned to look at purely dark space and therefore will collect far, far fewer photons than pixels that has the light from a star, nebula, or other light-producing object pouring into it.

Avoiding clipped-black pixels means that you avoid having pixels with insufficient signal or, worse, a value of 0. This is the opposite predicament of an image that has pixels that are clipped-white. Both types of clipped pixels effectively have no data that can be meaningfully acted upon during processing."

 

From here https://daleghent.com/2020/08/understanding-camera-offset

Regards Andrew 

I mean there is nothing called offset in the zwo settings andrew, see this How do I change offset - SharpCap Forums

It was hard finding this information, I had to look all over until I found this.

However I still don't see a brightness setting in the ASI app, I will have to carry on looking. 

cheers for the explanation though.

steve

[andrew s]

Well there is in the software I use including ASCOM,  The Sky X and Voyager. It must be a hangover in SharpCap.

However,  I now see what you meant. 

Regards Andrew 

[rickwayne]

IIRC others have commented that ASIAir does not provide a user-settable offset.

[AstroMuni]

23 hours ago, bomberbaz said:

I also found my optimum unity gain should be slightly lower (-4) but I will leave it unchanged to the ZWO default. I have yet to settle on the final sub setting from home but I can't imagine it will change much from said 12-15.

So whilst on with this research of optimum settings, I started to dig into things further and keep coming across the use of offset during the taking of subs to avoid clipping of data but cannot seem to find settings for the alteration of offset either in sharpcap or ASIAir+, am I missing something very simple here? 

The way I went about this is look at the data provided by ZWO on my camera. In your case https://www.firstlightoptics.com/zwo-cameras/zwo-asi-183mc-pro-usb-3-cooled-colour-camera.html the graph says unity gain is at 120 and read noise drops to 2 at 150 gain and dynamic range is 10ish so not too bad - so thats the gain I would aim for.

In terms of cooling the dark current drops very well beyond -10 so you could set to that level of cooling.

I set my Offset to 70 for my ASI533mc pro in Ekos but cant comment about ASIAir or Sharpcap. Here is a good discussion on this topic of offsets https://www.cloudynights.com/topic/542048-gain-and-offset-settings-for-zwo-cameras/

You could also work it out yourself - once you have your gain and exposure chosen, capture some dark frames with the selected gain/exposure. Adjust the offset to make sure that the peak in the image histogram is clearly separated from the left hand side of the graph.
 

HTH

[bomberbaz]

15 hours ago, rickwayne said:

IIRC others have commented that ASIAir does not provide a user-settable offset.

Thanks for that, it was as I suspected. It must be algorhythm controlled somehow!

1 hour ago, AstroMuni said:

The way I went about this is look at the data provided by ZWO on my camera. In your case https://www.firstlightoptics.com/zwo-cameras/zwo-asi-183mc-pro-usb-3-cooled-colour-camera.html the graph says unity gain is at 120 and read noise drops to 2 at 150 gain and dynamic range is 10ish so not too bad - so thats the gain I would aim for.

In terms of cooling the dark current drops very well beyond -10 so you could set to that level of cooling.

I set my Offset to 70 for my ASI533mc pro in Ekos but cant comment about ASIAir or Sharpcap. Here is a good discussion on this topic of offsets https://www.cloudynights.com/topic/542048-gain-and-offset-settings-for-zwo-cameras/

You could also work it out yourself - once you have your gain and exposure chosen, capture some dark frames with the selected gain/exposure. Adjust the offset to make sure that the peak in the image histogram is clearly separated from the left hand side of the graph.
 

HTH

I have since posting found out that the gain for mine is 115 according to sharpcap tools although I want to recheck that under darker skies. Also I have been given info from ZWO which suggests -15 is the cooling to aim for so only 5 degrees from the -10.

Thanks for taking time to reply though and tbf, I wouldn't be far wrong had I used those figures of yours anyway.

[Elp]

I have used all sorts of gain, and find rarely does it make an iota of difference to any of the images other than with a 294 camera sensor, there's always noise which needs to be post processed out with some sort of software filter operation. Adding more hours on the target makes more difference.

[bomberbaz]

5 hours ago, Elp said:

I have used all sorts of gain, and find rarely does it make an iota of difference to any of the images other than with a 294 camera sensor, there's always noise which needs to be post processed out with some sort of software filter operation. Adding more hours on the target makes more difference.

Well what I am doing at the moment is following the science. Up until now I have worked on the basis of longer exposures/dark site & shorter exposures where there is LP and no idea as to best cooling if I am honest.

So I am gathering data on objects I already data of from previous projects but this time I will get my subs using a more science based approach and see how things compare, I am currently gathering on the iris nebula and will run a side by side, see where that takes me.

Steve

[rickwayne]

Ok, here's some science! I don't think this video has yet been name-checked in this thread,  but try 

He shows the results for exposure, cooling, etc. Again and again the message is "good enough is good enough". 0 deg. C is a huge win over +20, -10 a bit better, -20 hardly distinguishable from -10. Diminishing returns step quickly in for almost everything.

[AstroMuni]

On 30/01/2023 at 15:42, rickwayne said:

I don't think this video has yet been name-checked in this thread,

The OP has beat you to it in the first post 🙂

[rickwayne]

Oh man, that is pretty much Peak Rick, there. Sorry all.