Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

PHD Calibration Data Question


Kaliska

Recommended Posts

The attached image was taken on Friday night after i calibrated in PHD. My guiding was actually pretty good after, with no real issues throughout the night, but I'm interested in what is actually going on here (AZEQ6 GT, SW80ED, ST80 Guide Scope, GPCAM2 on a side-by-side setup) and if i may have an issue with the mount. I made sure there was no wire snagging the mount etc. I tried recalibrating, but got pretty much the same results.

 

CalData.jpg

Link to comment
Share on other sites

  • Replies 27
  • Created
  • Last Reply

Calibration calculator? Erm...no, I'm pretty sure my setting up was using the guiding assistant for a couple on minutes, then adjusting the values suggested, then running the calibration program straight after. Yes, the focal length of the ST80 is 400mm according to google.

Link to comment
Share on other sites

I have the same mount/guidescope set up (although not the same guide camera) and agree that guide speeds particularly the DEC might benefit from being a bit higher.  (My last calibration below).

Also, when the orthogonality error is largish, the older version of PHD2 used to give a warning about polar alignment being suspect.  Not sure if this is the case here, but might be worth checking next time out?

new-1.jpg.f319418ffa6e4a22e1badec5845f4acc.jpg

Link to comment
Share on other sites

It looks like you calibrated your DEC at the moment where periodic error in RA made it move.

Mount has some periodic error in RA, and while it tracks without guiding or PEC (periodic error correction), star "oscillates" in RA direction around its true position. This is fairly slow oscillation, depending on type of periodic error it can have components of oscillation that are minutes long, and sometimes it has short period components - such mount is hard to guide (one with fast changing periodic error).

Now imagine that calibration star is while it is moving in DEC due to calibration - moving in RA also due to PE - you will get calibration data like that.

You will be able to spot it on RA part of the graph but it looks different. If you have minimal PE or slow changing PE it will be much more difficult to spot. It shows as uneven spacing of dots - dots should be evenly spaced if only thing that moves the dot are calibration pulses. But since there is inherent motion in RA - dots will be more separated if RA periodic error is working with pulses (pushes the star in same direction) and less separated if it is on part of oscillation that goes the other way around. So if you see uneven spacing in RA calibration - it also means PE.

When you repeat calibration you will end up with different graph - because periodic error will very likely be out of sync with time it takes to do calibration.

Ideally you will want to do PEC if you have graph like that - it signals that you have somewhat faster oscillation in RA and hopefully it will be harmonic to worm so you will be able to correct it with PEC.

HTH

Link to comment
Share on other sites

2 hours ago, vlaiv said:

It looks like you calibrated your DEC at the moment where periodic error in RA made it move.

Mount has some periodic error in RA, and while it tracks without guiding or PEC (periodic error correction), star "oscillates" in RA direction around its true position. This is fairly slow oscillation, depending on type of periodic error it can have components of oscillation that are minutes long, and sometimes it has short period components - such mount is hard to guide (one with fast changing periodic error).

Now imagine that calibration star is while it is moving in DEC due to calibration - moving in RA also due to PE - you will get calibration data like that.

You will be able to spot it on RA part of the graph but it looks different. If you have minimal PE or slow changing PE it will be much more difficult to spot. It shows as uneven spacing of dots - dots should be evenly spaced if only thing that moves the dot are calibration pulses. But since there is inherent motion in RA - dots will be more separated if RA periodic error is working with pulses (pushes the star in same direction) and less separated if it is on part of oscillation that goes the other way around. So if you see uneven spacing in RA calibration - it also means PE.

When you repeat calibration you will end up with different graph - because periodic error will very likely be out of sync with time it takes to do calibration.

Ideally you will want to do PEC if you have graph like that - it signals that you have somewhat faster oscillation in RA and hopefully it will be harmonic to worm so you will be able to correct it with PEC.

HTH

I've been toying with the idea of getting my mount serviced and even tuned up via Darkframe Optics. Would this sort out the problem?

Link to comment
Share on other sites

Have you done PEC? It is simple procedure ...

I would say that tuning the mount tends to help with PE, at least it did in my case, but my mount is HEQ5 and belt mod and tuning did help.

As far as I can see your mount is AZEQ6 so it should already have belt drive, so not sure how much tuning might do for it.

Link to comment
Share on other sites

Huh :D

It involves about hour or so of data gathering and then fiddling a bit with software. You are using EQMod, right?

You need to capture guide log without guiding - just fire up phd2 and let it track the star and create log, but be sure to disable guide command output. Then load this log into PECPrep software (part of EQMod project) and analyze data and create PEC curve. Then you just load that PEC curve into EQmod and you are ready to go ...

There are tutorials for this online, let me see if I can quickly find that for you.

 

Link to comment
Share on other sites

2 hours ago, vlaiv said:

Have you done PEC? It is simple procedure ...

I would say that tuning the mount tends to help with PE, at least it did in my case, but my mount is HEQ5 and belt mod and tuning did help.

As far as I can see your mount is AZEQ6 so it should already have belt drive, so not sure how much tuning might do for it.

A question Vlaiv, since our recent exchange about phd guiding and Az-EQ6, I've been going to sort the PEC on my own mount, but was going to use the mount handset to do it as the mount has PPEC, and from reading forums, this appears to be stored in the mount, not the handset, so would PPEC here work better than PEC through EQmod?

Huw

Link to comment
Share on other sites

1 minute ago, Horwig said:

A question Vlaiv, since our recent exchange about phd guiding and Az-EQ6, I've been going to sort the PEC on my own mount, but was going to use the mount handset to do it as the mount has PPEC, and from reading forums, this appears to be stored in the mount, not the handset, so would PPEC here work better than PEC through EQmod?

Huw

Huh, not sure if I can correctly answer that question. I will however give you info that I possess, hopefully it will help you in further research and decision.

PEC through EQMod has a benefit of working together with pulse guiding rather than "against". Since EQMod controls both PEC and guide correction, eqmod can examine both and decide to combine them and issue single "correction". If for example PEC is saying that mount should be moving a bit faster then sidereal (because worm is such that at that point it is trailing behind), but guide command is to push it a bit back, eqmod can see that these two are conflicting and decide not to do anything at all in that moment (or calculate proper motion from two, thus applying single value).

I've read somewhere that in "regular" scenario, you might end up with guide commands fighting PEC, or rather in terms from above, because two different systems control what should be applied, both "commands" get applied and that might cause more trouble than solve things. But this is something that I've only read, I don't have any experience in this regard (I only guided via EQMod, with and without PEC, PEC indeed helps a lot).

On the other hand having PPEC is something that I really wish I had on my HEQ5. EQMod PEC on HEQ5 is not permanent and this means that you need to park mount between each session, and can only use mount with EQMod.

Sometimes I wish to bring out mount to just observe, but using mount without EQMod (just hand controller) means that sync would be lost and I would have to do PEC again when I plan to image. Somehow I don't feel like loosing an hour or so of imaging time doing PEC every so often. There is additional problem of loosing sync in case of something going wrong, like power outage, or laptop crashing or something. It happened to me once, I'm on mains power for my rig and power went out - great for visual (neighborhood in dark :D ) but PITA if you loose exposure, set point cooling and PEC sync in the middle of the night.

Final note, I have no idea if EQMod will recognize PPEC capable mount like AzEQ6 equipped with encoders and will one need to park after each session or will it use encoder information for PEC.

HTH,

Vladimir

Link to comment
Share on other sites

31 minutes ago, Horwig said:

A question Vlaiv, since our recent exchange about phd guiding and Az-EQ6, I've been going to sort the PEC on my own mount, but was going to use the mount handset to do it as the mount has PPEC, and from reading forums, this appears to be stored in the mount, not the handset, so would PPEC here work better than PEC through EQmod?

Huw

Here is EQMod pdf that explains a bit how is PEC implemented in EQMod (it is sort of user manual but has some explanations at the beginning). It goes a bit more in depth about how PEC + guide is implemented in EQMod and why is it different from standard PPEC + ST4 guiding.

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

Link to comment
Share on other sites

24 minutes ago, vlaiv said:

Final note, I have no idea if EQMod will recognize PPEC capable mount like AzEQ6 equipped with encoders and will one need to park after each session or will it use encoder information for PEC.

That is a good question, will do some more reading.

Another question then, what are your opinions of the PEC guiding algorithm in PHD2, one piece of software guiding and sorting out PEC does seem good

Link to comment
Share on other sites

1 minute ago, Horwig said:

That is a good question, will do some more reading.

Another question then, what are your opinions of the PEC guiding algorithm in PHD2, one piece of software guiding and sorting out PEC does seem good

Not sure what is PEC guiding algorithm in PHD2?

If you mean EQMod PEC and pulse guide (one app manages both PEC and guide commands, but PHD2 issues guide commands), then I can say that I'm using it and it did make quite improvement over pure guiding (without PEC) on my HEQ5. I'm now able to go 3-4s guide exposures without loosing anything over 1-2s exposures in terms of guide precision (it helps with seeing to go longer). Stars are pretty tight and round. I can guide down to 0.5" RMS, seeing and wind dependent. I still have not fully tested my latest setup yet. Since the beginning of using this HEQ5, I've made considerable progress in guide precision, but at expense of numerous mods and tweaks. I've changed all bearings on mount, stripped / re greased it, added belt mod, changed tripod to Berlebach planet and replaced saddle plate for larger Vixen+Losmandy with better grip / surface clamping.

Main difference related to PEC and guiding was, of course, belt mod. It made PEC curve so much smoother and max PE much smaller.

Link to comment
Share on other sites

19 minutes ago, Horwig said:

Very interesting, I've not tried it yet, but I suspect it might have a positive impact on guide performance. I will be sure to try it out. I was not even aware it is implemented.

Some heavy math behind it, but idea is really simple. I did encounter discussion about adding single frequency compensation / prediction in PHD to address high frequency oscillations present in some mounts. This sounds way better. My only concern is duration of learning phase. I would expect at least couple of worm periods to get good model, and that is half an hour or so of guiding. Papers state that it needs 1-2 worm cycles before benefits start to show.

I'll experiment with it and see what it does for my guiding. I'll probably throw in 10-15min "warmup" time before each session to let it build at least some of it's model.

I also suspect that it will complement regular PEC and will not clash with it since it will gather model of current mount PE (that being corrected for main worm harmonics). It will probably be able to pick up other frequencies that are not harmonics of worm period. These show up in PE analysis but need to be ignored since PEC only works with worm harmonics.

Link to comment
Share on other sites

2 minutes ago, vlaiv said:

I'll experiment with it and see what it does for my guiding. I'll probably throw in 10-15min "warmup" time before each session to let it build at least some of it's model.

I've only just found out about it, but constant cloud has stopped me trying it out. Starting an imaging run by capturing RGB data would seem to be the way to go, leaving the capture of Luminance till later in the session when the model is fully built

Link to comment
Share on other sites

16 hours ago, ChrisEll said:

I ran PHD2's Predictive PEC for half an hour but it made no difference. My mount's PEC playback was turned off during this run.

Quote from PHD2 Guide Algorithms:

"If you’re following best practices, you will have programmed periodic error correction in your mount".

PPEC looks for and tries to minimise residual PE from a mount's PEC.

So no surprise you didn't see any difference.

PPEC made a big difference to my LX200GPS with it's PEC on.

Michael

Link to comment
Share on other sites

  • 2 weeks later...

I thought I would follow up on this post as this is the first night I've been able to get out and do some imaging in ages (in Ha tonight). I ran the PHD Calibration first, which looked a lot better than last time (although im still no expert). Ran the guiding assistant and adjusted to recommended settings. Ran Auto PEC in EQMOD. Things are looking a little better than last time.

5a725bc2ef99f_PHD31_01.2018(2).thumb.jpg.8e6868aecdcba1bfc73d586510d9ffc3.jpg

CalData(2.1.18).jpg

Link to comment
Share on other sites

  • 2 weeks later...

Just a quick update.

I now run the EQMOD ASCOM PEC routine before i do any imaging and have turned up both pulse guise rates to 0.5 and increased the exposure time to 2.5-4 seconds (in PHD2). Everything looks so much better now. Next time i will read the instructions, I promise!

I will eventually run the PECPrep routine to see if that improves the guiding even more.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.