Star Saturation Detection Settings

[André Vilhena]

Hello,

This setting (Advanced Settings > Camera > Star Saturation Detection) is usually set at 255, which corresponds to saturation ADU for an 8-bit camera. Should we adjust this setting accordingly to each camera depth? 

Reading the manual I am under the impression that yes, but when I tried to set at 4095 (for a 12-bit camera), this value does not stay and is reset to 255.

Is this the expected behaviour?

Thanks,

André

[Brian Valente]

Hi Andre

your camera is going to operate in either 8 bit or 16 bit, depending on your camera/driver settings

most 12-bit cameras can operate in either 8 bit or 16 bit mode (depending on the driver setting), so it would be 16 bit value. 

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Brian Valente

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[André Vilhena]

Hello Brian,

Thanks for the feedback. The camera in question is the ASI 120MM and as far as I see, the output bit depth is 12-bit. I haven't checked the drivers but I'll do it soon.

However, if I can adjust to either 8 or 16-bit, would any them be better than the other?

André

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[Richard Beck]

André,

My experience/understanding is that ASI's driver will drop the 120MM from 16-bit to 8-bit without notice if the 16-bit option is selected.  Selecting the 8-bit option has resulted in rock-solid performance.  I use the ZWO OAG on an f6 scope and have several stars with decent SNRs which allow multistar guiding to work well.

Richard

[George Shoup]

Richard:

Remember me?  It's George.

Here is an observation about my ASI224 guide camera.

Sometimes I guide with my C11 when I'm imaging with a smaller piggyback scope.

On a long focal length guide scope (C11), the 16 bit setting (vs 8 bit) results in much smaller and crisper star images and reduced FWHM values, and better guiding.

When guiding through an 80mm f6 short focal length scope the bit setting does not make much of a difference in star images and guide performance.

But this is observation and I have not tried other guide cameras.

You are the bit master, so please correct me if this does not make sense.

George

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[Richard Beck]

George,

Of course I remember you :-D.  The issue I mention in my post only affects the 120s.  It does not affect the other ASI cameras.  For my RC 8, I purchased an ASI 174MM Mini to go in a Celestron OAG.  I have run that camera in 16-bit mode the one time I have used it and plan to continue to do so.

Generally, most 12-bit camera drivers deliver 16 bits by shifting bits left by four and padding with zeros.  This is equivalent to multiplying the 12-bit decimal value by 16.  For a 12-bit native camera, the maximum value is 65520 (2^16 = 4096, therefore ranging from 0 through 4095, 16*4095=65520).

It's not clear how the 12-bit camera drivers deliver 8-bits, but I suspect the least significant 4 bits are dropped. If the most significant 4 bits are dropped, saturation would occur more quickly.

If the 16-bit driver is stable, I would urge using 16-bits since there is no information lost.

[andfar...@gmail.com]

George and Richard,

 

Thank you so much for your informative discussion.

 

Indeed, as far as I see ZWO mention the output of this specific camera is 12-bit but either PHD2 and SharpCap only allow to choose 8 and 16 bit. I have changed for 16-bit output and now will wait for some clear skies to try and see of there’s any difference.

 

One of the thing I find odd is that I usually only see very few stars in PHD2 and the selected one is always bordering saturation in the Star Profile tool (at 255 ADU’s). Hence I was wondering if having the camera at 8-bit was somehow lowering the saturation threshold.

 

Thanks,

André

 

Enviado do Correio para Windows 10

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[Richard Beck]

André,

With an 8-bit driver and a camera which is at least 8-bits, saturation will always be at an ADU of 255 (2^8-1).  With most 16-bit drivers I am familiar with, saturation is the native bit level times the difference in bit level.  For a 12-bit camera and a 16-bit driver, the saturation is 65520 [2^4*(2^12-1) = 65520].  If the camera were 14-bits (e.g., ASI 294MM in 2x2 bin mode), saturation would be [2^2*(2^14-1) = 65532.  For a 16-bit driver and camera (none of which I am aware), saturation would be 2^16-1 = 65535.

I want to reiterate that you may find trying to use a 16-bit driver either not possible or leading to unannounced drops to 8-bits with the ASI 120 (and only an issue for the 120).  If the 16-bit driver is available in you set  up, it may drop to 8-bit causing PHD2 being unable to find stars because the maximum value in 8-bits is so small.  This is exactly what  happened for me using the INDI driver.  When I raised the issue on the forum, the lead INDI maintainer told me that dropping to 8-bits was a "feature" of the ASI driver (from ASI) and only pertained to the 120s.

Best wishes as you get out under the stars.

Richard

[Brian Valente]

>>>dropping to 8-bits was a "feature" of the ASI driver (from ASI) and only pertained to the 120s.

Not sure I will ever understand that "feature"

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