Can this be Improved?

[Kenn Hopkins]

Can this be improved?

I just obtain an OAG for my C-8 HD. I had been using Celestron’s 80mm 600mm FL scope as a guide scope, the results were good but I had heard an OAG would be better.

With the 80mm I was getting roundish stars, most of the time a little oblong, and a total RMS of 0.23 px (0.23 arc-sec). With the changing to an OAG I now get a total RMS of 1.26px (0.37 arc-sec) – I really do not know what the means.

My Polar alignment error in Guide assistance is 1.8 arc-min and the Backlash Results shows an excellent line, see attached pdf file.

I’ve included the log files from before the OAG installation and after the OAG installation. Is it to be expected for the numbers to be so different?

Attached pdf file contains screen shots of Guide Assist of Before OAG Installation and After OAG Installation.

Log files from before OAG Installation and After OAG Installation are at: https://openphdguiding.org/logs/dl/PHD2_logs_R3rg.zip

Kenn

[sarg314]

RMS of 0.37 is quite good.  I rarely get better than 0.5" (OAG  on Celetron 8" with 2023mm f.l. )

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Tom Sargent

[Bruce Waddington]

I don't know where you're getting these performance numbers, perhaps they are just impressions from watching the image display.  For your 10/19/2025 guiding sessions, done with the 600mm fl refractor and a Losmandy mount, your results look like this for total RMS:

0.68 a-s, 0.55 a-s, 0.62 a-s, 0.63 a-s, 0.63 a-s, 0.69 a-s, 0.68 a-s

These are perfectly good results, but nothing like the 0.23 a-s you're talking about.  You're simply not going to achieve that level of performance over extended time periods with that mount.  Of course, this was done while pointing at a Dec of 62 degrees, so your RA errors are suppressed by over 2x due to the spherical geometry of the sky.  

For the OAG test, you only ran a 15 min guiding session at Dec = 15 and you got too many lost-star events to draw any conclusions.  Your overall results were 1.01 arc-sec for that session, so I really don't know where you're getting the numbers you gave us.  If you want to evaluate performance, you should be using the PHD2 LogViewer tool.  The guider image scale of 0.3 arc-sec/px is too small, something the new-profile-wizard warned you about. With such a tiny pixel size (2.9 microns), the camera really needs to be binned to work well at the long focal length.  But it's unfortunately a color camera so the driver may not support binning in which case you may have painted yourself into a corner.  You may also need to spend more time trying to achieve critical focus on the guide camera because that's another source of lost-star problems.

The motivation for going to an OAG is not to improve the guiding statistics because it won't normally do that - it's the same mount with the same payload so you would expect very similar results.  The reason for going to an OAG is to eliminate differential flexure which was probably the cause of your elongated stars.  I think you should invest more time and effort with the OAG set-up to make it as effective as possible.  If you are going to do comparisons, they need to be done at the same sky pointing positions over longer time periods and without problems like lost-star events.

Good luck,

Bruce

[Kenn Hopkins]

Bruce, Thank You for the informative post to my query about my guiding using my new OAG. I thought I did bin 2 in the Camera setting but I see that was not the case; thanks for bringing that to my attention. And the reason I did choose to use an OAG was because of the elongated stars I was getting in my images; BlurXterminator helps but I was looking for better stars in the initial images.

The numbers I quoted were from the Guide Assistance of PHD2, the screen shots were in the attachment "Guide Assist". In the upper right section of the Guide Assist window there is the High-frequency Star Motion area and it shows RA RMS, DEC RMS and Total RMS; those are the numbers I cited.

I will admit that I may not be the 'sharpest knife in the rack' and I am always trying to learn better practices in using my equipment so again Thank You for taking the time to help shed some light into my dark corner.

Kenn 

[Bruce Waddington]

Ok, now I understand what you were looking at.  The Guiding Assistant doesn't actually measure guiding results because it disables guiding.  What it's trying to do is assess, to a limited extent, atmospheric conditions at the time and to a greater extent, how the mount performs on its own with no guiding.  So the fields in the "High-Frequency Star Motion" box are simply telling you how much the guide star is "bouncing around" from one frame to the next, mostly because of atmospheric seeing effects.  Those fields by themselves aren't particularly useful at this point but they are used as part of the calculation for recommended Min-Move values.  In any case, they only represent a snapshot view, typically only a few minutes worth.  To evaluate your guiding and tracking results during imaging, you should be using the PHD2 LogViewer tool and looking at longer time intervals.

https://openphdguiding.org/phd2-log-viewer/

Regards,

Bruce