7 replies to this topic

[bob11746]

PHD2, ZWO ASI120MM, Guide Camera Autoexposure

 

Background: I have my guide camera set to 0.5, 1, or 1.5 seconds exposure, but it seems obvious as you pan across the sky the camera is in autoexposure mode.  As a bright star passes through the field, the background gets darker, and when passed, the background gets lighter and the noise level goes up (exactly like you'd expect autoexp to work).  I go to the camera tab and there are auto exposure settings, but nothing that says manual exposure.  I've never gotten auto to work, maybe it's my settings.

 

Question:  Is there a definitive setting for manual exposure?  Why are the camera settings greyed out?  Are there settings that I am missing or glossing over?

 

Thanks!

Bob

[t-ara-fan]

The exposure is what you set.
You are seeing auto stretch making the background brighter or darker.

0.5 sec is very short and you will be chasing seeing.

Edited by t-ara-fan, 04 June 2023 - 10:44 AM.

[bob11746]

Thank you.  That makes sense.  Is there a way to control or disable the autostretch?  Or, is that just a bad idea?

[AstroVagabond]

 

Question:  Is there a definitive setting for manual exposure?  Why are the camera settings greyed out?  Are there settings that I am missing or glossing over?

 

In my view no definitive setting. Setting will vary based upon many factors.

 

The PHD2 manual is at this link if you have not seen it yet:

 

https://openphdguiding.org/man/ 

 

Bill

Edited by AstroVagabond, 05 June 2023 - 01:47 PM.

[t-ara-fan]

Thank you. That makes sense. Is there a way to control or disable the autostretch? Or, is that just a bad idea?

The stretch doesn't matter. The shape of the star profile with FWHM / HFR shown is what is important. You want a pointy star profile that isn't saturated.

[Drakkith]

Thank you.  That makes sense.  Is there a way to control or disable the autostretch?  Or, is that just a bad idea?

I've never needed to alter it at all. PHD2 is highly automated and I don't think I've ever needed to try to alter the stretch. Just to reiterate, stretching is solely for 'looks' for us human beings. It does absolutely nothing for the computer, which works directly with the given data, not by how bright or dark a pixel looks on the screen.

[happylimpet]

Thank you.  That makes sense.  Is there a way to control or disable the autostretch?  Or, is that just a bad idea?

You can use the gamma slider but you cant fully control the screen stretch. As said, it makes zero difference to the internals.

[bob11746]

Yes, PHD2 is one of the more hands free programs in a typical astrophotography suite, it usually only screws up when I interfere!  It's more of visual aspect where it goes from light grey to black, especially when slewing. The gamma will only adjust the brightness, it will not adjust the background response from a dim to bright star.  But I get it, it is what it is, I just wanted to make sure there was no adjustment I was missing.

 

Thanks to all who responded.

Bob