PEC recording

[Rob Astro]

Can someone give a quick rundown on how to record a PEC curve? I'm unsure if you just run the worm drive and record or do you need to image an object? If so what program do you use image and record the PEC error. Its all new to me and just noticed the PEC recording in EQMOD.

[vlaiv]

You can record it via PHD2.

Start by calibrating guide scope like you normally would - close to meridian and on equator (DEC 0).

Then turn off guide corrections and start guide session. Make sure you have logging enabled in PHD2. In EQmod at some point press timestamp button (this is used for synchronization).

You need to record about 1-1.5h of data (several worm cycles). You don't need to image during this time and best time to do it is when the moon is full and it's clear outside. You probably won't image at this time so you can put aside couple of hours to do this.

After you finished your run of data gathering - stop guiding and close PHD2. Now run PecPrep software (part of EQMod - http://eq-mod.sourceforge.net/pecprep/). Load phd2 guide log into it and start your analysis and preparation of PEC file.

Alternative to all of this is to simply use EQMod and record PEC file while you image something. You again need to be guiding - but this time don't disable guide corrections - just have regular imaging session with guiding for example. At some point hit "record" PE button in EQMod and after again - several worm cycles (about hour to hour and a half) - just start using that PEC. EQMod will automatically do same thing you would manually do in PecPrep.

If you want to go PecPrep route - check youtube for tutorial on how to do it (there are several videos covering basic procedure).

Check out this document as well:

http://eq-mod.sourceforge.net/docs/eqmod_vs-pec.pdf

It should give you brief overview of EQmod pec and how you can record it while imaging.

[Rob Astro]

Thanks very much, its was just something new to me and wasn't sure  how to go about it. Thanks.

[mgutierrez]

On 11/10/2021 at 22:00, vlaiv said:

You can record it via PHD2.

Start by calibrating guide scope like you normally would - close to meridian and on equator (DEC 0).

Then turn off guide corrections and start guide session. Make sure you have logging enabled in PHD2. In EQmod at some point press timestamp button (this is used for synchronization).

You need to record about 1-1.5h of data (several worm cycles). You don't need to image during this time and best time to do it is when the moon is full and it's clear outside. You probably won't image at this time so you can put aside couple of hours to do this.

After you finished your run of data gathering - stop guiding and close PHD2. Now run PecPrep software (part of EQMod - http://eq-mod.sourceforge.net/pecprep/). Load phd2 guide log into it and start your analysis and preparation of PEC file.

Alternative to all of this is to simply use EQMod and record PEC file while you image something. You again need to be guiding - but this time don't disable guide corrections - just have regular imaging session with guiding for example. At some point hit "record" PE button in EQMod and after again - several worm cycles (about hour to hour and a half) - just start using that PEC. EQMod will automatically do same thing you would manually do in PecPrep.

If you want to go PecPrep route - check youtube for tutorial on how to do it (there are several videos covering basic procedure).

Check out this document as well:

http://eq-mod.sourceforge.net/docs/eqmod_vs-pec.pdf

It should give you brief overview of EQmod pec and how you can record it while imaging.

looking for info about pecprep I found this thread. There is only a thing I don't fully get. Why do we need to disable guide output from phd2? If we enable the output, corrections are also written to the log file and hence the error could be computed afterwards, no? why pecprep is not able to do it, but autopec via eqmod can?

Edited October 7, 2023 by mgutierrez

[vlaiv]

7 minutes ago, mgutierrez said:

looking for info about pecprep I found this thread. There is only a thing I don't fully get. Why do we need to disable guide output from phd2? If we enable the output, corrections are also written to the log file and hence the error could be computed afterwards, no? why pecprep is not able to do it, but autopec via eqmod can?

Pecprep wants to know how much mount trails or leads sidereal rate (what the actual error is) and it computes that based on guide star position versus start position.

It can do this from variety of logs and all need to do the same - have guide star be displaced from original position.

If you enable guide output - you always return guide star to (near) its original position. Calculations then need to account for this and you need to sum up thousands of small corrections.

In ideal world - these two approaches would yield the same result - but there is significant difference between the two approaches and that difference is how error behaves. In each step there is some error. Each time guide star is measured there is some error in its position. Mount itself won't react 100% perfectly to correction so guide correction and guide pulse won't completely match.

When you add 1000 corrections with this sort of error - error accumulates, but when you simply track the star - each measurement will have some small error - but these don't accumulate over time. For this reason it is much much better to simply record how mount performs rather than to try to correct it and assume your correction is perfect and then calculate result (and do so a thousand times per recording).

Autopec needs to do this because it does not have idea where the star is - it relies on corrections (how ever imperfect those are) to calculate where it thinks guide star is. Autopec is less reliable than measuring PE on your own and then calculating PEC curve, but it is only way it can be automated while the mount tracks and guides.

[mgutierrez]

loud and clear @vlaiv. Fully understood. Thanks for the detailed explanation.

m

[mgutierrez]

@vlaiv can I please make you another question? In order to test and understand how eqmod autopec works under the hoods, I performed a test recording the pec curve with eqmod only tracking; no guiding (no capturing nor running phd2 or similar; only tracking with eqmod). Obviously while generating the curve, it was completely flat as expected. However the corresponding .txt data contains values !=0 on the PE column. I expected to find zero. Do you know how autopec calculates these values and why they are !=0, even being the curve flat?

pec.txt:

# EQMOD HEQ5/6 V2.00w
!WormPeriod=479
!StepsPerWorm=51200
# time - motor - smoothed PE
0 43740 -0.0275
1 43848 -0.0287

...

peccapture_EQMOD.txt:

# EQMOD HEQ5/6 V2.00w
# AUTO-PEC
# RA  = 23.4058906013238
# DEC = 90
# PulseGuide, Rate=0.1
!WormPeriod=479
!StepsPerWorm=51200
#Time MotorPosition PE
1.000 8389308 -0.0058
2.000 8389415 -0.0116

...

[vlaiv]

53 minutes ago, mgutierrez said:

Do you know how autopec calculates these values and why they are !=0, even being the curve flat?

No idea of what is going on there.

Did you have previous autopec / pec curve loaded? Maybe it just repeated earlier corrections?

[mgutierrez]

7 minutes ago, vlaiv said:

No idea of what is going on there.

Did you have previous autopec / pec curve loaded? Maybe it just repeated earlier corrections?

no. I'm an indi user. Almost first time I run eqmod and first time for autopec