Upgraded from an ASI120mm to ASI220mm – Maybe a Problem & Questions

[Jim Waters]

As the title says, I just upgraded.  I have been using the ASI120mm Mini for ~8 years without issues.  I set the Gain to 95% and adjust the Gamma slider when necessary.  My backyard is Bortle 8.5ish and I have problems finding guide stars at times. I switched to the ASI220mm Mini because I wanted a larger sensor / FOV with better sensitivity. My guide scope is 200mm. I am using PHD2 2.6.14.

Here’s the problem / question.  At a Gain of 95 the exposure is 100% saturated at anything over 0.5 seconds.  There’s a tremendous amount of background noise.  Yes, I took Darks.  I need to dynamically adjust the Gain between ~50 to 65 depending on the sub length (0.5 to 2.0 seconds).  There’s also a large amount of grainy noise in all the displayed subs.  I have to adjust the Gamma to reduce the displayed noise.  The ASI220mm Mini is set to 16bit.  Is this typical of the ASI220mm Mini?  What’s the ideal Gain setting? I have never seen these problems with the ASI120mm.

There's no log to upload.

Thanks - Jim

[steve]

Hi
Unity gain -and where HCG mode kicks in- on a 220, is 106 so best to begin troubleshooting in phd2 at around camera gain 35%. Take new dark frames at this gain and make certain you have the focus absolutely perfect by observing the HFR value as you tweak the guide focus. Allow settle time before taking the reading.

But with only a 200mm guide telescope, you'd almost certainly get better guiding with the smaller pixels of your 120mm using gain 48.

Otherwise, post your logs. There are always logs.
HTH and CS
Steve

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[Bryan]

Jim

When you say "Saturated at anything over 0.5 s" are you referring to the screen display or the Star Profile? I suspect you mean the former.  However, if the latter, lower the Gain.  0.5s is very short.  I suggest you leave the exposure at 2 s.  

Adjusting the gamma slider (which is labelled SCREEN brightness) does not change the data that PHD2 uses; the slider only changes the displayed image.  You will not improve your star selection or guiding using the gamma.

Finally, as Steve notes, there are always logs. 

To get help with PHD2, please upload your log files - here's how: How to ask for help with PHD2.

Bryan

[Jim Waters]

Bryan, Steve

Last night I was just focusing the new setup and not guiding.  When I say "Saturated at anything over 0.5 s" I am talking about the screen.  When it's not saturated the screen background is very noisy.  I understand that Gama is basically the screen brightness.  I don't recall what the Gain setting was when I took Darks.  I will redo the Darks at Gain 35.  What Gain should I use for guiding?  I am not overly concerned about the difference between 3.75 vs. 4.0 pixel size.

 If it's clear tonight I will set up an imaging run and do guiding.  I will get back to you later.  Thx

Jim

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[Bruce Waddington]

Let's not make this harder than it needs to be.  Gain isn't something that normally requires a lot of fiddling around for PHD2 use.  Keep in mind that the gain setting in PHD2 is a percentage of the min to max range of the camera.  There are a few zillion models of guide cameras and it isn't practical for us to keep track of all the vendor-defined gain settings.  You should basically be able to set it once and forget it.  What you see in the image display window doesn't generally make much difference because you should be using the 'Auto-select' button for choosing guide stars.  In order to do multi-star guiding, PHD2 uses a statistically-based algorithm for choosing the optimal set of guide stars, taking various things into account that probably wouldn't occur to you.  Those may not look "the best" to you and often might appear very dim.  None of that matters, let the algorithm do its work. Just make sure the guide camera is well-focused and use guide camera exposure times that are appropriate.  Use a dark library that matches whatever gain setting you end up with and be sure the Min-HFD reflects the reality of the smallest and faintest stars you can expect with your setup.

Good luck,

Bruce

[Dale Ghent]

A few points:

Gain values turns out to be rather important these days due to the now-overwhelming prevalence of dual gain domain on modern chips. Most (but on all) guide camera vendors who offer camera that use such chips (which is pretty much all of them now) have the camera set up to have a single contiguous gain range from 0 to some value. The lower half of that range will operate the sensor in is Low Conversion Gain domain. As gain value increases, the well depth shortens, read noise and dynamic range goes down. Eventually, at some gain level, the camera will turn off the LCG amplifiers and turn on the High Conversion Gain amps. At this point, the DR and read noise "resets".

You can see this effect on an example, ZWO's ASI462 guide camera:
https://i.zwoastro.com/wp-content/uploads/2023/05/412f7215cd23349759f5f9b29c6eff51-768x1024.png

The gain levels involved can vary from camera to camera even from the same vendor as these are tied to gain operation of the specific sensor. It's not like CCDs where things tended to be more homogenous. This creates a situation where one would preferably operate at a specific gain level to get the best low-noise performance from the sensor; something which doing gain by percentage makes a bit bothersome to do. This is why I added current, min, and max gain and offset display to PHD2's QHY camera driver a few years ago.

As for knowing a camera's min and max gain (and other such stats), the vendor-provided SDKs provide that information. There's no need to keep track of such stuff in the way you describe. Getting these values is just an SDK call.

-Dale

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[Jim Waters]

Thanks for the education and reply's everybody.  

Jim

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