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PHD2 Saturated Stars

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#1 Travmay

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Posted 26 March 2020 - 01:05 AM

Hello all,

I’ve been trying to improve my guiding, and I am having issues with saturated stars. I am using an SVBONY guide scope with the QHY5III-224 guide camera. No matter what settings I use (gain, exposure, etc), my stars have the SAT icon at the bottom of the screen. This goes for the auto-selected star, and also any star that I manually select.

I have been guiding on saturated stars since I couldn’t figure it out (getting around 1.5 arc sec). Now I am trying to run the guiding assistant and it will not let me run it on a saturated star...

Thoughts?

#2 james7ca

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Posted 26 March 2020 - 03:55 AM

What does the star profile look like? Is it shaped like a smooth bell curve or does the top look square? If square it is clipped/saturated, otherwise it should be okay.


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#3 Madratter

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Posted 26 March 2020 - 06:54 AM

Hello all,

I’ve been trying to improve my guiding, and I am having issues with saturated stars. I am using an SVBONY guide scope with the QHY5III-224 guide camera. No matter what settings I use (gain, exposure, etc), my stars have the SAT icon at the bottom of the screen. This goes for the auto-selected star, and also any star that I manually select.

I have been guiding on saturated stars since I couldn’t figure it out (getting around 1.5 arc sec). Now I am trying to run the guiding assistant and it will not let me run it on a saturated star...

Thoughts?

By any chance are you using the camera in 8 bit mode?


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#4 Travmay

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Posted 26 March 2020 - 10:25 AM

What does the star profile look like? Is it shaped like a smooth bell curve or does the top look square? If square it is clipped/saturated, otherwise it should be okay.

I’ve been trying my best to select a star without a clipped/squared profile, but either way I still get the SAT notification at the bottom of the screen telling me my star is saturated. Thank you! 

 

By any chance are you using the camera in 8 bit mode?

I may be! Is this changeable in PHD2? Or would I change it in the ASCOM driver? I see the option to change it in SharpCap when I am doing my polar alignment. I will look into that today/tonight. Thank you! 


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#5 Madratter

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Posted 26 March 2020 - 10:47 AM

I’ve been trying my best to select a star without a clipped/squared profile, but either way I still get the SAT notification at the bottom of the screen telling me my star is saturated. Thank you! 

 

I may be! Is this changeable in PHD2? Or would I change it in the ASCOM driver? I see the option to change it in SharpCap when I am doing my polar alignment. I will look into that today/tonight. Thank you! 

You would change how many bits are used in the ASCOM driver.



#6 Der_Pit

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Posted 26 March 2020 - 12:05 PM

Hmm, whether it saturates or not should not depend on the bit depth.  That is rather the full well of the camera.  I use my 290MMmini in 8bit, without issues.

 

So what gain/exposure time do you use?  Is this the 60mm guide scope?  In doubt consider a grey filter for the camera....

 

Other explanation could of course be a bad camera.  IIRC PHD2 has an option to save guide frames.  Maybe you can take and share one?


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#7 Travmay

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Posted 26 March 2020 - 12:50 PM

You would change how many bits are used in the ASCOM driver.

I will try this change and see what happens!

 

Hmm, whether it saturates or not should not depend on the bit depth.  That is rather the full well of the camera.  I use my 290MMmini in 8bit, without issues.

 

So what gain/exposure time do you use?  Is this the 60mm guide scope?  In doubt consider a grey filter for the camera....

 

Other explanation could of course be a bad camera.  IIRC PHD2 has an option to save guide frames.  Maybe you can take and share one?

I have tried 1,2,3,4 sec exposures and anywhere from 0 to 50 gain. It is the 50mm guidescope. I currently have a UV/IR cut filter and a light pollution filter stacked. I was having this issue before installing those, so I do not think they would be the issue, but I will try again without them/separately. I will look into a gray filter if nothing else works!

 

I have used this camera on the moon with pretty decent results, how would I tell if the camera is bad? I will save some frames from tonight's session and save them.
 



#8 rgsalinger

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Posted 26 March 2020 - 01:01 PM

Well, if you are getting saturated stars at 2 seconds of exposure then you need to put a delay in between exposures. This is from the PHD manual:

 

"Time lapse' - imposes a fixed delay between guide exposures.  This can be useful if the guide exposures are very short and you don't want to overload either the mount or the camera link with very high traffic rates."

 

Just use a 1/2 second exposure with a 2.5 second delay and voila you're done. 

 

When you only have 8 bits to play with you have only 8 stops between too dim and too bright even though the FWD is 14k, you've just got 8 stops. That's the problem. 

 

Rgrds-Ross


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#9 KLWalsh

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Posted 26 March 2020 - 01:11 PM

“I will look into a gray filter if nothing else works!“

If you have a gray filter that can be easily installed, try it.
If you need to buy one, a quick and inexpensive alternative would be to use an aperture mask over the front lens with 1/2 the diameter of the objective lens. That would reduce the number of photons by 75%.
If the aperture mask is made to slip onto the front, it could be easily removed when you’re trying to find guide stars in a star-poor field.
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#10 Travmay

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Posted 26 March 2020 - 01:11 PM

Well, if you are getting saturated stars at 2 seconds of exposure then you need to put a delay in between exposures. This is from the PHD manual:

 

"Time lapse' - imposes a fixed delay between guide exposures.  This can be useful if the guide exposures are very short and you don't want to overload either the mount or the camera link with very high traffic rates."

 

Just use a 1/2 second exposure with a 2.5 second delay and voila you're done. 

 

When you only have 8 bits to play with you have only 8 stops between too dim and too bright even though the FWD is 14k, you've just got 8 stops. That's the problem. 

 

Rgrds-Ross

Nice! I'll try this and report back after tonight. Thanks!



#11 bobzeq25

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Posted 26 March 2020 - 01:36 PM

There's a _really_ easy solution.  Of the "why didn't I think of that" school.   <smile>  I do this often.

 

Select an exposure 1 second longer than you think you'll want.  If you think you'll want 3 seconds, do 4.  Auto-select a guidestar.

 

Dial the exposure back to 3.


Edited by bobzeq25, 26 March 2020 - 01:37 PM.


#12 Der_Pit

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Posted 26 March 2020 - 02:14 PM

I have tried 1,2,3,4 sec exposures and anywhere from 0 to 50 gain. It is the 50mm guidescope. I currently have a UV/IR cut filter and a light pollution filter stacked. I was having this issue before installing those, so I do not think they would be the issue, but I will try again without them/separately. I will look into a gray filter if nothing else works!

 

I have used this camera on the moon with pretty decent results, how would I tell if the camera is bad? I will save some frames from tonight's session and save them.
 

If you used it on the moon it's OK.  Then the issue is something different.  I just had a look at PHD2, the only thing I could imagine is that you have (in the brain->Camera tab) set the Star Saturation Detection to a too low value.  Or maybe it's still set from an 8bit camera to 255 and you use 16bit now.

So have a look at that, in doubt use 'Saturation via Star Profile'


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#13 Travmay

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Posted 26 March 2020 - 03:15 PM

There's a _really_ easy solution.  Of the "why didn't I think of that" school.   <smile>  I do this often.

 

Select an exposure 1 second longer than you think you'll want.  If you think you'll want 3 seconds, do 4.  Auto-select a guidestar.

 

Dial the exposure back to 3.

Added to my list of things to try!

 

If you used it on the moon it's OK.  Then the issue is something different.  I just had a look at PHD2, the only thing I could imagine is that you have (in the brain->Camera tab) set the Star Saturation Detection to a too low value.  Or maybe it's still set from an 8bit camera to 255 and you use 16bit now.

So have a look at that, in doubt use 'Saturation via Star Profile'

You beat me to it! I just found that setting too. I am going to try the Saturation via Star Profile and see how that goes... it was set to 255 but who knows.
 



#14 Travmay

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Posted 28 March 2020 - 01:11 PM

Update: I was able to get around 1" - 1.2" error last night, which is much better than my previous 1.5-1.8". Still going to aim for under 1" but this is at least reasonable. I changed the saturation method (previous post) in order to get away from the saturated stars warning.

 

I also trained my PPEC and that is what helped the most I think. I did an initial training followed up with two updates, and saw immediate improvement with each training.

 

Thanks for the help everyone! Currently stacking my M51 data from the last two nights...


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