EQ6-R What RA accuracy should I expect?

[Mikael Lindkvist]

Hi,

Im a newbie astrophotographer, last winter was my first season and I was quite happy just getting any images with my new equipment: EQ6-R. Celestron 9.25 XLT, ASI533MC Pro, Celestron OAG, ASI290MM (for guiding). 

This season I have started to investigate my guiding since my total RMS is almost always > 1" but at the same time I see other people with the same mount reporting much less error. I have also noticed that my RA error is always much worse than my DEC error.

I have been going over the PHD docs and tried some tweaking without any significant improvements so Im starting to get a bit worried that there is a mechanical problem with my mount? 

Before starting to tweak the actual mount, grease it, loosen the worm gear etc, I would be really grateful if someone with more experience could have a look at the base log I have created and uploaded: https://openphdguiding.org/logs/dl/PHD2_logs_4X8v.zip

I have followed the instructions in How to create a baseline for guiding results using OpenPHD2, i.e.:

1. Fresh install of latest dev version of PHD, new profile using wizard.

2. Balanced mount (slightly east heavy) and good polar alignment

3. Calibration

4. GA a few minutes and apply suggestions

5. Guiding 30-40 mins

6. Optional run with GA to determine "raw" mount performance.

The result from the point 6 above looks pretty bad on RA from what I can see but on the other hand Im not sure what to expect?

Regards 

Mikael
 

[Bruce Waddington]

Hi Mikhael.  Thanks for following the procedures and best practices, it make it much easier for us to see what’s going on.  It looks to me like you’ve already avoided the most common mistakes that people make.  Unfortunately, your current limitation is the RA tracking accuracy of the mount and that is unfortunately a mechanical limitation.  The overall periodic error of the mount is actually pretty reasonable but you are suffering from a common, high-frequency tracking error that some of these mounts have:

 

 

You can see this 10-sec recurring tracking error creates guide star excursions of nearly 1 arc-sec, so it’s going to be hard to ever get below this value during guide.  The frequency of the error is generally too high to handle well with guiding – it represents more of an “impulse” error rather than a smooth error that ramps up and back down over time.  I think if you check on one of the EQ or CloudyNights forums, you can probably learn exactly which component of the RA drive system is causing this and probably some tips on how to improve it.  Not all of these mounts have this problem and can therefore produce better guiding results out of the box.  There is another periodic error that occurs on 120-sec intervals (to the right) which will be more amenable to correction, either by mount-based periodic error correction or by the PHD2 PPEC guide algorithm.  If you want to try the latter, just specify a period-length of 120 sec and un-check the “auto-adjust” checkbox.  You can also try to reduce the guide camera exposure time so long as you can retain the use of multiple stars and have sufficient SNR that you aren’t losing stars frequently.  You can try stepping down the exposure time until you get down to 1 second – we don’t advise going lower than that.

 

If you can get the RA tracking improved, the mount will probably benefit from using PHD2 Dec backlash compensation which should improve the Dec guiding results.  But doing that now won’t help and will probably just introduce elongated stars in your main camera images.

 

Good luck,

Bruce

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