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Old 16-12-2021, 10:17 PM
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xa-coupe (Jeff)
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Exposures differing on PHD2

This is more of a general question than a problem, but as I watch my guiding I note that subsequent exposures can vary markedly in how over/under/correctly exposed they are. Nothing really blows out and guiding (when it's not testing me) goes about it's thing.


Anyone know why?
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Old 17-12-2021, 09:48 AM
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Drac0 (Mark)
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You sure there isn't some light cloud or mist moving through? This makes a huge difference to the looping images.
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Old 17-12-2021, 11:06 AM
Startrek (Martin)
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Don’t worry too much about the live image view when guiding, high cloud and atmospheric conditions can alter the view
If guiding is affected by heavy cloud , passing air craft or other obstructions like trees , PHD2 should alert you with an annoying beep until the obstruction is cleared
The Star profile tool ( found in the View tab ) is your best tool to observe your guide Star which then reflects your guiding performance
A Star profile with a sharp peak in the graph is what your after for good guiding
You can watch your guide Star being moved back to the lock position or central target
Also your SNR ( located on the task bar ) of your guide Star generally should be between say 20 to 50

See attached photo of my Star Profile on a night of good guiding

Cheers
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Old 18-12-2021, 08:42 PM
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xa-coupe (Jeff)
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Thanks, it happens all the time so it isn't clouds and the guiding (although sometimes quite problematical) isn't affected with the different exposures. Up until the last couple of nights I have recently been getting exceptional guiding.... it was during my last couple of stress tests that I thought about it and couldn't find an answer to this question.
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Old 19-12-2021, 02:09 PM
Startrek (Martin)
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Just a thought , could it be your camera buffering due to signal or bandwidth loss in your USB cable
Is your guide camera USB2 or USB3 ?
How long is your guide camera cable ?
Is it running through a hub or ASI Air ??

The older ZWOASI120MM USB2 cameras had driver issues and connection/ buffering issues, the newer and latest ZWOASI120MM-S USB3 camera which I’ve used for 3 years , no issues at all , mine runs direct to Laptop (5m) via a Startech Active USB3 cable (cable cost me $180 , money well spent ) I tried a few $20 to $60 standard USB3 cables and they all had connection issues.

Just a thought ?
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Old 19-12-2021, 08:21 PM
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xa-coupe (Jeff)
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It could well be that. I am using the 290mm but a 2m cable and through a powered hub.



I do have an ASI Air on order so it'll be interesting to see if that still has it.
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Old 21-12-2021, 10:56 AM
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The_bluester (Paul)
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PHD2 appears to auto stretch the guide images as they come in, minor differences in seeing seem to produce large differences in the stretch applied. I am reasonably sure that the image to image change in brightness on screen are changes in the auto stretch applied.

I sometimes wish that the stretch level could be limited as with the 290MM mini in marginal conditions it sometimes tries to guide on noise, but then if seeing is poor enough that it is doing that then your target images will be rubbish anyway.
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