Good guiding but out of round stars

[Bruce Braunstein]

Hi,

I've been struggling with my star shapes lately and want to eliminate my guiding if possible. This is an MX+ with a QHY OAG and an ASI 290. How do my guide & debug logs look?

I also may have discovered a bug in v2.6.11. I tried to change my guide rate to 0.80 in the advanced settings tab but it kept reverting back to 0.50. This was after having changed it in the mount firmware.

Thanks,

Bruce

https://openphdguiding.org/logs/dl/PHD2_logs_CWYr.zip

[Bruce Waddington]

I don’t see anything in the log files that would  imply frequent or consistent star elongation.  But I think you’re going at this backwards.  It’s better to find a few specific frames where you see star elongation, note their start and end times, then look at the guide logs to see if anything happened with the mount or with guiding.  In the first log, you were getting a lot of lost-star conditions but I assume you know that and aren’t looking at images affected by those events. 

 

You haven’t discovered a bug in 2.6.11.  PHD2 interrogates the mount driver to see what the mount guide speed settings are, that happens every time guiding is started.  Throughout both nights of 3/6 and 3/7, those guide speeds were reported as 0.5x sidereal.  So you may think you’ve changed them but they’re still the same as before.  Further, all of the calibrations you did are consistent with mount guide speeds of 0.5x sidereal.  PHD2 never sets the guide speeds in the mount, it only reads them.

 

Regards,

Bruce 

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[Brian Valente]

Just to emphasize what Bruce said, which is a common misconception regarding PHD

>>>PHD2 never sets the guide speeds in the mount, it only reads them.

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/003101d83311%2434be0a80%249e3a1f80%24%40earthlink.net.

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Brian 

Brian Valente

portfolio brianvalentephotography.com 

[mj.w...@gmail.com]

One of the old Meade LX200GPS Drivers would reset the mount Guide Speed setting to the default 0.66 as soon as PHD2 connected to the mount.

Okay if the guide Rate was altered AFTER connecting PHD2.

Might be a similar situation with the MX Driver ?

Michael 

Wiltshire UK

[Bruce Braunstein]

There were unexpected clouds night before last and images during that time were not saved. If the logs are otherwise good then I'll continue to pursue optical issues. This with an FSQ106EDXIV using the stock focuser and rotator which has been linked to sag by some users. I'll look for signs of tilt and sag and see what I come up with.

Before I entered the new guide rate in PHD2, I used the Software Bisque TCS to change the guide rate in the mount. I may have neglected to disconnect PHD from the mount at that point and tried to enter the new guide rate in PHD2 which didn't stick. I was using NINA and the guide rate was still being reported as half sidereal so I jumped back to PHD2 and found that although I had updated the guide rate in advanced settings, it had returned to 0.5. Maybe if I'd reconnected PHD2 to the mount , it would've updated to 0.8. I changed it back to 0.5 in TCS because it seemed like PHD2 was not retaining the setting and I wanted them to match. I suppose since there's a field to enter the guide rate, that would be useful if a mount driver doesn't report it to PHD2? Maybe it could be grayed out when used with a mount that does report the guide rate?

Is there any advantage to increasing the guide rate from 0.5 to 0.8 for a mount of this type?

I've seen it mentioned in some of the presentations on optimizing PHD2 that having a minor PA offset can result in better guiding by preventing DEC reversals. My mount reports very little DEC backlash so am I correct that introducing PA error isn't helpful in my case?

I notice at times that my DEC error may be 0.2 or so, while the RA error may be 0.4 or more. Would that account for oblong stars because the difference in error is too great?

Thanks So Much For Your Help,

Bruce

[David Majors]

Have you looked at your physical equipment?

If your guiding does not show excursions then those elongated stars could well be due to differential flexure or mirror shift- more likely both. My guiding camera Orion SSAG +240 mm guidescope is particularly susceptible to the former-especially from the contact points of the aligning screws.  My 8" Celestron Edge HD is vulnerable to the latter. One way to see this is to turn your sub-frames into a movie and watch what happens. I use Pixinsight for my processing which has a function called Blink that does this for you. I found that my object was systematically drifting off to the side of the frame with occasional sudden shifts even when the guiding was dead on.