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Don't want to chase the seeing, but....

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#1 birddog99

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Posted 26 January 2020 - 11:36 PM

I am using my new ASI290mm guide camera and a 50mm Orion mini Guide scope. Seeing here in Houston is iffy most of the time and combine that with the heavy light pollution, guiding below 1.0 total RMS is always a challenge with my Atlas EQ-G. It was recommended that I reduce the gain from 100 to 50 on the guide camera. Did that. SNR on the guide star was always above 100 using 100 gain. Tonite at 50 gain the star SNR in PHD2 was consistently above 100. (115-134). So I reduced the gain to zero and the SNR would fluctuate between 85-100. As I understand it any SNR above 100 probably would result in PHD2 not being able to accurately determine the center of the star. Seeing wasn't that good tonite, so per all the recommendations I should have increased the exposure to 3, maybe 4 seconds. I get that, but when I increased the exposure to 3 seconds, the SNR went up into the 150-175 range, which, of course caused issues trying to guide. The clear skies forecast called for average seeing, but I did notice slight twinkling of the bright stars. So I suspect the forecast was off somewhat. It was a clear night with no clouds. What does one do in this situation. I don't think seeing was really that bad, but maybe it was.Does anyone out there have to reduce gain to zero to keep the SNR at a reasonable level? Thanks for the input in advance.



#2 Cfreerksen

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Posted 26 January 2020 - 11:50 PM

It's not the SNR that will cause the poor guiding on the star AFAIK. It's when you get the SAT error in red at the bottom of the screen. That means you have hit maximum output of the camera. It is then that PHD2 cannot accurately determine the center of the star.

 

I guess you could barlow the guidescope. That would reduce the amount of light hitting any given pixel. Or just select a dimmer star.

 

Chris


Edited by Cfreerksen, 26 January 2020 - 11:52 PM.

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#3 bobzeq25

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Posted 27 January 2020 - 12:58 AM

I am using my new ASI290mm guide camera and a 50mm Orion mini Guide scope. Seeing here in Houston is iffy most of the time and combine that with the heavy light pollution, guiding below 1.0 total RMS is always a challenge with my Atlas EQ-G. It was recommended that I reduce the gain from 100 to 50 on the guide camera. Did that. SNR on the guide star was always above 100 using 100 gain. Tonite at 50 gain the star SNR in PHD2 was consistently above 100. (115-134). So I reduced the gain to zero and the SNR would fluctuate between 85-100. As I understand it any SNR above 100 probably would result in PHD2 not being able to accurately determine the center of the star. Seeing wasn't that good tonite, so per all the recommendations I should have increased the exposure to 3, maybe 4 seconds. I get that, but when I increased the exposure to 3 seconds, the SNR went up into the 150-175 range, which, of course caused issues trying to guide. The clear skies forecast called for average seeing, but I did notice slight twinkling of the bright stars. So I suspect the forecast was off somewhat. It was a clear night with no clouds. What does one do in this situation. I don't think seeing was really that bad, but maybe it was.Does anyone out there have to reduce gain to zero to keep the SNR at a reasonable level? Thanks for the input in advance.

As said above, what you need to watch out for is saturation.  That's a flat top to the star profile, and that "SAT" warning.

 

The SNR for saturated will vary a lot depending, no doubt, on a number of factors.

 

But it's easy to get a non saturated star, at any exposure, any gain.  Just have PhD2 pick it.  I think the setting is under "Tools", don't have it in front of me.  Sometimes transparency increases and it becomes saturated.  Having PhD2 pick another, or reducing the exposure by 0.5 second works.  But I've never noticed a small amount of saturation hurt guiding.


Edited by bobzeq25, 27 January 2020 - 01:01 AM.

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#4 birddog99

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Posted 27 January 2020 - 06:37 AM

Got it. Thanks




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