Is PEC of value when using a guide cam?

[Demonperformer]

Given that, at any point in time, the total corrections that need to be applied to a specific mount to keep it on a specific target will be constant, is there any benefit to running a PEC recording and applying some of the corrections by means of that, in addition to running a guide camera?

My gut feelings are (1) the total corrections applied will be the same, so PEC is redundant, (2) having two programs trying to control the mount when one (the guide cam) can do on its own is asking for conflict [rather like a car with two steering wheels and two people trying to drive it down a windy country lane].

But what do I know?

So, does anyone use both? What are the benefits (if there are any) to doing so?

Thanks.

[i should say that I am looking at this from the pov of someone who does not have a permanent obsy, and so would need to run a new PEC calc every time I set up]

[graemlourens]

Hi.

I am currently experimenting with exactly that.

As i use EQMOD, using PEC there with Pulseguiding (not ST4) claims that is does not interfear, but actually work seamlessly together.

I did a few experiements the other night, and at a first look, it seems like the PEC does make a difference (taking my RA RMS from 0.6" to 0.5"), but to really say if that was it, i will have to rerun these tests serveral times, to straighten out seeing.

The main difference with PEC and guiding is that PEC will correct BEFORE the error has happened, and Guiding will only be able to to it AFTER, so the 'damange' is already done to your picture.

So i am leaning towards having PEC&Guiding at the same time, (with EQMOD & Pulseguiding)

My 2 cents.

Kind regards, Graem

[chrisshillito]

Yes PEC is of value in combination with autoguiding.

The problem with autoguiding is it can only correct errors that have already occurred and so to some extent your image may already have be slightly compromised before a correction is even made. PEC on the other hand will correct errors due to periodic error as they happen - but of course it can only do this for error signals that are truly periodic.

During the time it takes to capture the exposure the image and the guide image will be slightly smeared by the mounts periodic error. Depending on your guide resolution this smearing may not even be apparent in the guide image, if it isn't then that is bad because the autoguider isn't going to issue a correction. If the guide image is affected by periodic error then that means that the resulting centroid position used for calculating the guide correction will be based on the average position of the mount during the guide exposure and not on its position at the end of the exposure - so there is still a guide error.   Obviously the longer you're guide exposures the greater the error and in these situations PEC can act to keep the mount on track during the exposure time leading to a more accurate guide corrections.

Using PEC extends your options with regard to the length of guide exposures and/or guiding resolution. This in turn may increase you guide star options or allow you to achieve a better exposed guide image to the autoguider (the better star image the autoguide gets the better its centroid calculation is and the more accurate the corrections are - you really don't want a single, saturated pixel as a guide image!). Also because the PEC is taking out most of the slow moving error from the error signal the autoguider sees you may be able to adopt more aggressive autoguidng settings allowing a more rapid response to any non periodic bumps etc..

Of course to some extent any actual improvement in final image quality comes down to and accumulation of marginal gains and a lot will depend on your Individual setup (guide resolution, imaging resolution, mount PE etc.) With EQMOD's PEC implementation folks have reported that,if using guide exposures of three seconds or longer, the tightness of the stars in the raw unprocessed image is improved compared to not using PEC with the same guide exposure length (but I would caution that these things are often subjective!). Certainly for long exposure, higher resolution images PEC can be a useful tool to have in the box and if working correctly will do no harm.

Chris.

[Demonperformer]

Thanks for the responses.

The point about correcting errors before they occur is well-taken.

Also I had not really considered the question of how long a guide-exposure lasts (although it is a fairly obvious question now it has been pointed out to me!). I guess I have always thought of guiding as a fairly "instantaneous" affair, with corrections (if necessary) being shot off to the mount every second or so. Maybe not!

I am rather surprised that a single-pixel star is not what is wanted as a guide star. I would have thought that would be nigh-on perfect - any light spilling onto an adjacent pixel requires correction. So I repeat my comment from my original post: what do I know?

I guess that with practice, the processing of the raw PEC data into a useable file will get faster, but there does not seem to be a lot I could do to cut down the 48 minute record-time. Whether the benefits will outweigh losing an hour or so at the start of each session (not having a permanent setup), is something I will have to carefully consider. But at least I now know there is a benefit and should not discount the possibility completely.

Thanks.

[chrisshillito]

I am rather surprised that a single-pixel star is not what is wanted as a guide star. I would have thought that would be nigh-on perfect - any light spilling onto an adjacent pixel requires correction. So I repeat my comment from my original post: what do I know?

I guess that with practice, the processing of the raw PEC data into a useable file will get faster, but there does not seem to be a lot I could do to cut down the 48 minute record-time. Whether the benefits will outweigh losing an hour or so at the start of each session (not having a permanent setup), is something I will have to carefully consider. But at least I now know there is a benefit and should not discount the possibility completely.

Most autoguider software will use sub-pixel guiding and this allows the guide cam to work at up to an order of magnitude less than the imaging cam (it is sub-pixel guiding that makes finder guiders possible) Sub-Pixel guiding works by analyising the star image and calculating its "centre of mass" with respect to the intensity of the pixels it covers. So if you consider a star image two pixel wide, each pixel having the same intensity, the auto guide would calculate the star centre as being midway between the two pixels. If your star image is only one pixel then you loose this sub-pixel guiding and now you can only correct movements that result in a whole pixel movement in the guide image. Given that the guider image is usually considerably less resolution than the imaging resolution that means your image will have moved several pixels without a correction being issued.

I've always been of the opinion (others may disagree) that the bigger the guide star image, and the better exposed it is (no pixels saturated) the better and to this end I will sometimes defocus the guide cam slightly if the guide star is bright enough to take it.

You don't need a permanent site for EQMOD PEC . Moving the mount with the clucthes disengaged will not move the worm gear so as long as you park the mount at the end of one session PEC synchronisation should be restored when you unpark at the start of the next.

PEC is something you need to just find time to try and see how it goes - some folks just can't get it to work for them, some folks find it too much hassle with little observable improvement to their astronomy, others routinely use it all the time, and others just use it when they feel they are pushing the limits of what guiding alone can achieve. The nice thing about EQMOD's autopec is you can actually record PEC whilst imaging and guiding, and even whilst running PEC - so you don't have to loose observing/imaging time whilst recording it.

Chris.

[Demonperformer]

Thanks, Chris, for that explanation - I think I'm starting to get it. I will make sure I find some nice big star for my first attempts.

I've downloaded the latest version of EQASCOM (I was running 2.14) and will give PEC a go. I certainly like the way it automatically works out the right curve at the end rather than having to process it myself using PECprep.

If I record a second PC trace while my first is running, will the prog take all the data into account when computing the curve at the end of the capture, or will it just use the data from the second one and discard the old data when PEC.txt is replaced? My thinking is, if the former, there would be no reason not to capture more and more cycles every time I'm guiding - presumably that would just keep improving the error-prediction (subject to the law of diminishing returns)?
 

Thanks.

[r3i]

The nice thing about EQMOD's autopec is you can actually record PEC whilst imaging and guiding, and even whilst running PEC - so you don't have to loose observing/imaging time whilst recording it.

Chris.

I must get round to trying the AutoPEC - just one question if I may:  Will dithering between images affect the recording?

[Demonperformer]

Certainly will, if it's as much dithering as I have been doing before getting serious with the EQ mount

[sorry, couldn't resist - look forward to seeing a proper response]

[chrisshillito]

I must get round to trying the AutoPEC - just one question if I may:  Will dithering between images affect the recording?

Depends on how much you're dithering by and how long your exposures are. Autopec will record the mounts movement in its raw PE data, but before the PEC curve is constructed this raw data is linearly regressed and filters applied to remove seeing and wind effects - I can't see an occasional small dither movement having a significant effect on the final PEC curve.

[chrisshillito]

If I record a second PC trace while my first is running, will the prog take all the data into account when computing the curve at the end of the capture, or will it just use the data from the second one and discard the old data when PEC.txt is replaced? My thinking is, if the former, there would be no reason not to capture more and more cycles every time I'm guiding - presumably that would just keep improving the error-prediction (subject to the law of diminishing returns)?

AutoPEC works by monitoring the speed at which the mounts motors are running. Both PEC and autoguiding will be applying corrections to the motor speeds so as long as you are guiding successfully EQMOD will be recording exactly what it takes to achieve perfect tracking. Filters are then applied to this data to extract the elements that are periodic and remove any 'drifts' that might have been the result of polar alignment error correction. It is this filtered data that is used to build the new PEC curve. So if PEC is active when you record autopec then its output will certainly contribute towards the new PEC curve in addition to any further corrections the autoguide had to make.

You could certainly keep re-recording PEC but I suspect most folks don't as when they get something that works reliable they tend to want to stick with it and not risk change. Astronomy has so many variables in play, many that are out of your control, and I guess folks like to lock down as many as they can just to try an keep a handle on what is actually going on.

[MartinB]

The trouble with PEC is that it is another thing to faff around with and another potential weak link the the whole process. It depends how complicated you want to make your imaging! If you are getting sub arc second guide accuracy your guiding is likely to be comfortably within the best resolution of your imaging camera even if corrections are after the event. A poor PEC is worse than non at all and a good one may show no improvement if your guiding and tracking is already good.

[chrisshillito]

The trouble with PEC is that it is another thing to faff around with and another potential weak link the the whole process. It depends how complicated you want to make your imaging! If you are getting sub arc second guide accuracy your guiding is likely to be comfortably within the best resolution of your imaging camera even if corrections are after the event. A poor PEC is worse than non at all and a good one may show no improvement if your guiding and tracking is already good.

It used to be the case that amateur astronomers were just the type who relished faffing around to push the limits of what could be achieved with their equipment , sad if those days have gone.

It is true that out of sync PEC can be worse than no PEC but then badly calibrated guiding can also be worse than no guiding. The trick is to use good PEC and good guiding and  that only comes with experience, and experience is only gained by actually daring to use things. That good PEC may deliver no observable improvement isn't a reason not to use it - after all also implies that means it may show improvement!

I can't promise PEC will benefit any particular individual but if they express an interest I'd much rather encourage them to give it a go and see how it works out than simply dismiss it out of hand as an unnecessary and risky complication. After all if folks don't dare to experiment they won't progress.

Chris.

[laser_jock99]

With a fast/sensitive guide camera very short guide exposures are possible (0.05 - 0.2s) fast enough to cope with any drive errors, Although in this regime you are in danger of trying to correct for tube currents and bad seeing which isn't so good!

[Uranium235]

PEC what??

Never bovverd Guv'

[MartinB]

It used to be the case that amateur astronomers were just the type who relished faffing around to push the limits of what could be achieved with their equipment , sad if those days have gone.

It is true that out of sync PEC can be worse than no PEC but then badly calibrated guiding can also be worse than no guiding. The trick is to use good PEC and good guiding and  that only comes with experience, and experience is only gained by actually daring to use things. That good PEC may deliver no observable improvement isn't a reason not to use it - after all also implies that means it may show improvement!

I can't promise PEC will benefit any particular individual but if they express an interest I'd much rather encourage them to give it a go and see how it works out than simply dismiss it out of hand as an unnecessary and risky complication. After all if folks don't dare to experiment they won't progress.

Chris.

Crikey, I'm not suggesting people don't try things and experiment, the sub arc second guiding I mentioned doesn't come without considerable effort,  but I know that some people can get endlessly bogged down in adding complexity to their imaging process and literally loose sight of the hobby.  The KISS principle isn't about accepting mediocrity or being a luddite.  I think you've offered excellent advice on the use of PEC with guiding and I've no doubt it can be useful with some set ups.  There was not criticism of your comments at all.

[Demonperformer]

As someone who is just moving into this area, it seems to me there are a lot of issues that will only come with considerable effort.

My previous foray into PEC was when I had to record it in one program (PErecorder) and then move it into another program (PECprep) and then decide what needed to be kept/removed. I got completely bogged down. At the same time, I was finding PA to be totally impossible, and so finally gave up and went back to the SE mount for another two years.

I now feel that (if I am ever going to get beyond the 20-second-30%-useable-sub level) I need to get to grips with the EQ mount. I don't want it to be any more complicated than it needs to be, hence my original concern of whether going through the above process of getting a PEC curve would be worth it.

However, the goal-posts have shifted. The autopec feature on the latest EQASCOM program seems to mean that I can get a PEC curve at the press of a couple of buttons. This may, or may not, improve my subs, but it is hardly a considerable effort. And with the massive learning curve ahead of me, any encouragements will be welcome.

I would agree that, if I were already achieving sub-arcsecond guiding without it, I may be reluctant to introduce another variable into the system. But starting out, such a simple exercise as autopec, with the potential to give significant improvements, seems like a good option.

What's hard by the yard is a cinch by the inch.

[Uranium235]

I will re-iterate (except with more words) what I said earlier, messing about with PEC has little or no real benefit when guiding - especially if youre using an HEQ5 or 6.

Without any PEC at all, and good guiding (touch wood!) I get 100% useable subs, 100% of the time (900s +).

[Demonperformer]

Congratulations

[Uranium235]

I didnt mean that in an "im alright Jack" way, it just merely to state that you dont need PEC to do well - period. So your gut feeling (1) would be correct.

[ollypenrice]

It used to be the case that amateur astronomers were just the type who relished faffing around to push the limits of what could be achieved with their equipment , sad if those days have gone.

Chris.

Hmmm... I think a lot depends on the likely productivity of the faffing. Even with the number of clear nights I get here I would rather be imaging than faffing. I know you've advocated PEC plus guiding before and I respect that, but I've never tried it because I'm of the, 'If it ain't broke' school. I also try, wherever possible, to subtract rather than add complexities. 

Like anyone else's my guiding could be better but when it's good enough my philosophy is to get on with it. This is a farily typical graph from any of my mounts. (Lodestar, Bin 2, FL400mm, graph in pixels.)

With a lot of effort I could doubtless get it down to the values I obtain on the best of nights...

... but the effect on the resulting pictures would not, in my view, be discernible. (In fact the graph above made me jump out of my skin because I thought the darned thing had hung up!)

However, if the OP does have a problem and his system is 'broke' then trying feed-forward in conjunction with feedback makes sense. If ever I feel I need it I'll certainly try it myself.

Olly

[MartinB]

Just admit it Olly, you were guiding on a hot pixel!

[Vox45]

AutoPEC works by monitoring the speed at which the mounts motors are running. Both PEC and autoguiding will be applying corrections to the motor speeds so as long as you are guiding successfully EQMOD will be recording exactly what it takes to achieve perfect tracking. Filters are then applied to this data to extract the elements that are periodic and remove any 'drifts' that might have been the result of polar alignment error correction. It is this filtered data that is used to build the new PEC curve. So if PEC is active when you record autopec then its output will certainly contribute towards the new PEC curve in addition to any further corrections the autoguide had to make.

You could certainly keep re-recording PEC but I suspect most folks don't as when they get something that works reliable they tend to want to stick with it and not risk change. Astronomy has so many variables in play, many that are out of your control, and I guess folks like to lock down as many as they can just to try an keep a handle on what is actually going on.

Having gone through your (excellent) youtube channel I could only find tutorials on PEC and PECPrep ... I finally found one video called peccap3 that explains AutoPec here

Not sure why it does not show on your channel

Now, on using autopec: Is this a matter of choosing between 2 methods (PEC+PECPrep / AutoPEC) ? Is there an added value of using both methods + guiding with PHD2 ? I do believe that we should keep things simple but I would like to have your thoughts on this very interesting topic

Basically, is AutoPec meant to replace PEC+PECPrep in the future ?

[ollypenrice]

Just admit it Olly, you were guiding on a hot pixel!

Of course! The thing is, I can always find one on the Kodak 11 meg chip...

lly

[Davey-T]

As mentioned by others you can spend endless hours faffing about instead of imaging or at least capturing a few subs, guess it depends if you're a perfectionist or not, personally if the majority of my 10min subs are usable I'm happy, I can do 30 mins but the number of lost subs increases pro rata. As my images are mainly for my own amusement and not intended for publication they do for me.

If you've got designs on APOD then more effort may be justified as long as you enjoy it and it doesn't become a chore.

Dave

[Vox45]

In my case it is more about understanding and experimenting with the technology My faffing calender is open right now so that leaves me with a lot of free time for faffing related business.

I have to admit that PEC is my way of getting away from it all Nothing more relaxing than the sweet hum of my mount and the delicate graph of PHD at 2am .... good times ...