Help with spikes and general suggestions

[Phillip Locke]

Hello,

I have a new iOptron CEM 70 mount and I'm a little disappointediOpTron CEM70_pjl guide log with guiding.  I seem to have some spikes also and want to see if someone can help/offer suggestions.  I have it setup so there are no wires hanging so snagging is not a issue:).

I appreciate any help on this.

Thanks,

-Phil

[wave...@talktalk.net]

Hello Phil, I'm still learning too, so I'll take a stab at what I see in your GuideLog. Hopefully the PHD2 experts will be around later and we'll both learn a bit more.

Anyway, I reckon things are looking pretty well OK with your new mount and with a little experience, I'd say you're good to go. Your first guiding session achieved RMS 1.10" over the 2h20m run which isn't so bad taking into account the loss of the guide star. Your Calibration at Dec 52.5 degrees for this and the subsequent session hasn't made things too easy for guiding.You'll see in the User Manual and the Best Practices Guide that you really need to Calibrate on or close to the Celestial Equator (Dec 0 degrees) and near your meridian.

However, the three hour second session was rather better giving and overall RMS of 0.74". This was despite losing the guide star every few minutes (yellow line).

Don't know whether this was due to cloud or an inappropriate star mass setting - the gurus will know.

It might be worth reading up on Periodic Error reduction. This is the RA movement over the three hours.

And here's the frequency analysis of the above.

You have around 5.6 arcsec Peak-Peak of native worm gear (?) error, which isn't too bad and PHD2 does cope, but any reduction at source might show a benefit.

I'd strongly suggest doing a Baseline Calibration session over say six worm cycles and posting the logs. 

I wish my LX200 gave results this good! (Still working on it).

Cheers.

- Jack

[Bruce Waddington]

Hi Phil.  I doubt the big guide star excursions are coming from the mount drive system because many of them are Dec excursions and the Dec motor isn't running.  I think the source is probably what's riding on top of the mount - something that's moving or shifting on its own.  I assume it isn't wind because you would presumably be aware of that.  I don't know what your guide scope arrangement is but that's perhaps the first place to start - thumbscrews, Delrin-tipped fasteners, all of that.  I suppose another possibility could be something with the imaging gear that's causing vibration - I can see one short sequence where small excursions happened at 6-minute intervals. You have to keep in mind the scale of things here - the worst disruptions around midnight were equivalent to mechanical displacements of 7 microns while the spikes in the last 3 hour session were equivalent to 4 micron movements.  A human hair is about 50 microns thick.  You may find that such small errors have little or no effect on your final images so you should look into that before you get involved in a giant detective project.

Your overall guiding results look pretty good.  You can eliminate the star-lost/HFD events by updating to the latest 2.6.11dev1 release which will allow you to set the MinHFD parameter below 1.5px. 

Hope this helps,

Bruce