PHD2 insist on picking bad stars as guide star

[Miguel Tremblay]

I am using a OAG, guiding at 2.5s exposure. Ran guide assistant for 15 minutes. Getting good guiding once I get it to pick good stars. My CEM70 mount was guiding at a steady 0.2/0.3 arcsecond on the ASIAir, never picking bad stars, could go to sleep every night knowing it would make good decisions. I switched to NINA on a PrimaluceLab EAGLE5 Pro and now have to play with PHD2 directly.

I see plenty of stars in my guide camera with a lot of signal. However PHD2 insist on auto-picking the only visible feature that isn't even a star in the image. On this particular target I have to spam the "Auto-select star" button for 1-2 minutes before it decide to select a real star, and still, it will often pick the lowest signal star that will result in star being lost every 10 seconds or something the shape of a triangle 2px from the border. As far as I know I'm forced to rely on the Auto-select feature to be able to use multi-star guiding and there doesn't seem to be a way to tell it to stop picking a particular star. I tried setting the "Minimum star SNR for AutoFind" to anything between 12 and 30, it will still insist on picking the worst star in the field, or the one closest to the border even if I set the ROI to 60% in NINA (???).

What was the ASIAir doing with PHD that I am not doing, or how can I solve my problem?

[wave...@talktalk.net]

Hello Miguel,
I'm not one of the experts, but I'll give you my comments on what you're reporting and what I think I see in your Debug log.
I think the first thing to do is install the latest PHD2 development snapshot v2.6.11dev6), which you'll find  here: https://openphdguiding.org/development-snapshots/

The ASI174 guide camera you have should be running in 16bit mode (Camera Tools button in the Connect Equipment interface). In the Brain/Camera tab, set the Star Saturation Detection ADU to 65500 and not 255 as indicated in your log. Almost all the stars PHD2 had to work with were likely saturated at 2.5s exposure and ADU255 Max. The guide star flux density display should show a pointed profile and not the flat-topped ones you have been getting.  Advice on the camera settings is given in the User Guide and Best Practices literature.

You comment that PHD2 always selects 'bad' stars when you can see 'good' stars on the display. This is a subjective response to the bad settings being used, PHD2 always selects the most appropriate stars from what is gets from the exposure (try 1-2sec), even though you would select something different. Let PHD2 make the decision to get the best outcome. If you can't see the selected stars in the camera display, move the gain slider to the left. Ensure it isn't selecting hot pixels by creating a Bad Pixel Map before starting the Calibration, etc.

So, run the latest PHD2 dev 6 version, do a Calibration using the new Calibration Wizard and then a 10-15min Guiding Assistant run, upload both Debug and Guide logs with your questions. Use the automatic log uploader provided in the PHD2 Help menu. The experts will then be able to tell you a lot more about your setup and how to get better results.
Cheers,
- Jack T

[Miguel Tremblay]

Hi, I didn't even doubt that it was possible for it to default to 8bits and I always assumed 16bits. This is a huge mistake and the correct settings should help a lot. Before going deeper with debug logs I will try it under the next clear sky as the main issue was mainly the choice of guide star. Tracking was already good. Thanks!

[Brian Valente]

PHD chooses stars based on your criteria, so if your max ADU does not match your actual max ADU, that can cause guidestars to be selected that are saturated. 

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[Bruce Waddington]

The 8 to 16-bit conversion will help but it probably isn't your only problem.  For each of the auto-find ops you did in the log you posted, over 40 field stars were ignored because they were saturated.  Those were all the stars you decided were "good" based on squinting at the display.   Even if you change the dynamic range of the camera, you may still be getting saturated stars because of overly long exposure times or maybe a gain setting.  Next, you obviously have an internal reflection that is causing the light streak you were unhappy about - probably in the OAG.  You need to fix that as well.  If you upgrade to the 2.6.11dev6 release, you can try a Max-HFD feature that has just been introduced.  If you look at the star statistics using the Star Profile tool, you will see that the HFD of that streak is very high, usually over 6 pixels.  But all your real stars were smaller than that with HFD values below 4 and mostly below 3.  So try setting the Max-HFD value to something that will exclude the reflection but still keep the real stars. When you make the change to 16-bit mode, you should do that in a new configuration profile using the new-profile-wizard.  When the config work is done, create a new dark library for 16-bit operation.

Bruce

[Miguel Tremblay]

Thanks for the input Bruce,

- I will go with 2s exposure but lower the gain as I'm just trying to replicate the excellent results I was having in the ASIAir for now. I was running at 2s with the ASIAir but I'm not sure how the absolute gain value of my camera translate to the % we see in PHD2. Ideally I'd set the equivalent. I will indeed rebuild my dark library after this.
- The light streak we're seeing is either a very bright filament from Cygnus Loop or parasite light from a off-field star. I don't remember which panel of my 2x2 mosaic I was capturing in this screenshot but one of them has parasite light on the RGB filters coming from either 52 Cygni or Epsilon Cygni. I normally carefully frame my targets with that in mind but I can't account for that on a 2x2 mosaic. Is it still considered internal reflection? The scope is a William Optics FLT120 and I think they applied a lot of countermeasure for that in their scope.

- I will download the new version ASAP and will only attempt to play with the Max-HFD setting if I see the problem persist.

[Bruce Waddington]

Yes, that is an internal reflection.  It isn't likely to be coming from the scope itself, it's more likely to be something behind the scope such as the OAG.  All it takes is some part of a fitting that has a somewhat reflective surface  - a fitting that isn't blackened, something like that.  There's nothing wrong with setting a Max-HFD value that will eliminate it, that won't have any effect unless the problem shows up again.  Obviously, this sort of thing is very sensitive to where the scope is pointing and what the surrounding star fields look like.  If this is a one-time problem for you, it may not be worth trying to eliminate if the Max-HFD setting solves it.  The gain setting in PHD2 is a percentage of the min-gain to max-gain range of the camera:

https://openphdguiding.org/man/Advanced_settings.htm#Camera_Tab

These settings are all described in the user guide.

Bruce

[Miguel Tremblay]

Bruce,

I understood that it is a percentage of the min-max gain of my camera. What I cannot find is the range of my camera. The manual doesn't specify it and I cannot find the information on google: https://i.zwoastro.com/zwo-website/manuals/ASI174_Manual_EN_V1.5.pdf

But I think I found a workaround. I opened the Planetary Imaging app in ASIStudio and their gain slider let me go up to 400 max so I will assume this is my range. I had 240 in ASIAir so that should give me 60% in PHD2.

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[Bruce Waddington]

Look at the graph on page 8 of the ZWO manual.