Advice for an iOptron CEM40EC

[Bill Stent]

Hi Brains trust, Bill here

I've borrowed an iOptron CEM40EC mount from work for a bit of testing and learning. My normal mount is a belt modded NEQ6, so I have had a bit of fun getting to grips with the encoders.

The first night I didn't bother changing the PHD settings, so I'd used PPEC, which meant that guiding was crazy. However, last night I tried a few different things. After a bit of reading, I plumped for LowPass2 on RA and ResistSwitch on Dec, and after a while I tried Hysteresis on RA.

Please disregard my first two logs, I had the focal length set for my guide scope and I hadn't changed it back for my OAG!

My log shows a reasonable calibration, but log 5 (the long one) shows some pretty weird stuff in RA.The total error was largely made up of RA. That puzzles me, as the encoder should have sorted that out, shouldn't it?

My Polar alignment was OK, I'd done it in iPolar, and then SharpCap, but the guiding assistant in PHD told me it was about 1.1 arc-min out, so reasonable, but not awesome.

I'd balanced the mount in both RA and Dec, but possibly east heavy might have improved things. The iOptron is very easy to move, so balance might have see-sawed. I also had dangling cables, as it's a borrowed mount and my refractor has a loom which hung off the west side of the rig. My test subject was M8, using 300s exposures in Ha. Some of the small stars in the centre of the image were a bit egg-shaped. I think (I haven't checked) the elongation was in RA.

Can anyone give me some suggestions (for PHD, not for cables!)?

[Michael Borland]

I struggled to guide a CEM40EC for quite a while before learning that iOptron EC mounts suffer from "sub-divisional error" on the encoders. In my CEM40EC, this resulted in the mount oscillating in RA with a 6-second period and a peak-to-peak amplitude of 0.6 arc seconds. There was no way to guide this out. The best I could do was use a long interval (3 or 6s) to try to average out the wobble so it didn't inject noise into the guiding. Eventually, I bought a CEM60 (no EC) and sold the CEM40EC to someone who was imaging at a larger scale.

--Michael

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