Motor position for PEC

[Guest]

I have an HEQ5 which does not have absolute encoders. The motor position is reset to zero each time you switch the mount off and on again. So does that mean that in order for PEC to work you have to make sure that the mount is in exactly the same position every time you switch off the mount? So I guess the correct procedure is that at the end of each session you have to return the mount to the home position before switching off. And if you ever switch the mount off when it's not in that position then you'd have to redo your PEC curve?

Cheers

Steve

 

[Dr_Ju_ju]

Nothing is stored 'inside' the mount, and I take it that you are using the handset to control/drive the mount ??  therefore the start-up/initialization procedure will be to polar align and PEC alignment...

[Guest]

I use EQMOD to control the mount and I'm just starting to use the PEC utility which is built in to EQMOD. As you say, at the start of a session I polar align. PEC within EQMOD uses a PEC file which effectively plots correction against motor position. I'm trying to understand how this PEC facility knows what the absolute motor position is.

Steve

 

[newbie alert]

Periodic error comes from the worm drive.. not too sure on the ratio of the HEQ5 but it has a worm period of 638 seconds.. so I'd suggest it won't matter where the motor is in regard to the worm position.. home position is like a start position for star alignment

I've not tried pec training any mount that I've owned.. anyone chip in with if it makes any difference?

Phd2 also has predictive periodic error correction... not tried that yet either

[Dr_Ju_ju]

Personally, I don't use PEC on my AZEQ6/EQMod setup, as the Plate solving / PHD2 routines and the supposed PEC alignments point will, over time, diverge, and so it ends up in a fight between the two, and you start losing valuable imaging time....

[Guest]

6 hours ago, newbie alert said:

Periodic error comes from the worm drive.. not too sure on the ratio of the HEQ5 but it has a worm period of 638 seconds.. so I'd suggest it won't matter where the motor is in regard to the worm position.. home position is like a start position for star alignment

I've not tried pec training any mount that I've owned.. anyone chip in with if it makes any difference?

Phd2 also has predictive periodic error correction... not tried that yet either

You are right. The HEQ5 has a worm period of 638 seconds. I've also had a discussion about this on the EQMOD forum. You can take a PHD2 guide log file and analyse it in PECPREP to produce the PEC file. The PEC file contains a table of worm position against correction value. EQMOD/PEC looks up the worm position to find out what correction value to use. But the worm position is given in terms of motor pulses. In the case of HEQ5 there are 66844 motor pulses per rev of the worm. So 1 turn of worm =638 seconds =66844 motor pulses. The table wraps around so that the last point equals the first point. But it needs to know the exact position of the worm and since the pulse counts are incremental and not absolute it assumes a start position. This means that the start position (zero count) when you record the PEC file must be the same start position when you look up the PEC file.

Anyway, the upshot is that the best way to do this is to always return your mount to the home position at the end of a session before switching off.

It is possible to screw up the system like this - Move the RA using the motor. Then switch off. Then open the clutch and manually rotate the DEC so that it's at the home position. The next time you switch on the mount will be in the home position but the worm will be in a different position to where it was when you recorded the PEC file and PEC won't work right and you'll have to record a new one. I've done this occasionally when I've been doing some work on the mount in the daytime and generally messing about.

Guiding will take care of PE but if you also use PEC then it has less work to do. 

[newbie alert]

Thanks for that info, but isn't it always best to start off from home position  for star alignment anyway...I mean for mounts without absolute encoders ..

[Guest]

Yes. You are right. Always start from the home position.

[wimvb]

Software PEC is tricky, because it works by dead reckoning. As long as the mounts worm/motor is where eqmod thinks it is, it works fine. In theory. But in any mechanical system, over time, there is drift, and the two systems will differ. At that point, phd will need to undo eqmods pec pulses. Imo, just use phd predictive pec and forget about software pec. As long as the periodic error is smooth(-ish), phd can handle it.

Edited September 27, 2021 by wimvb