Help with PHD Graph

[ianaiken]

Hi all,

I managed to get some data the other night of IC434 (I'm at 54 55N) and decided I would try to do something about my eggy stars (all uniform to heading west) and spikes on PHD. I am wondering if anyone can offer any pointers or has seen something similar on their or someone else's setup?

You can see the spikes on this PHD graph, exposure 2.5s.

You can see that the guide graph is reasonably good throughout, but there is a repetitive spike every 100 seconds on my NEQ6 with my side by side setup (finder guider in the middle). I have performed a drift align and I will recheck this.

Does anyone have an idea what is causing these spikes? Is it PE?

I ran the subs through DSS to see if I was getting Field Rotation but the angle between subs was not changing. The dX and dY did show difference which means the image is drifting, maybe due to PA.

Finally here is the PHD log from PECPrep.

I'm really at a loss so if anyone can provide any pointers it would be greatly appreciated.

I made a change last night to PHD parameters and set RA Agg to 120, Hys to 20 and pointed at M51 (instead of IC434 which is what the above data is for) and the spikes in the graph existed although they were not as strong. The scopes were pointing more toward the Zenith opposed to 0 Dec in the South. Unfortunately IC434 was clouded over so I couldn't test the PHD settings there to see if that made a difference.

I also tried guiding in AA5 which resulted in eggy stars.

One train of through is that my PHD settings need to be set correctly for my finder guider to guide out that PE and my PA checking for the drift. I could also have differential flexure but I have appear to have equal drift from both cameras either side of the finder (but still could be DF because guider is separate from the two imaging rigs).

[Tim]

I'd guess at differential flexure there, that will give you eggy stars in one direction usually.

What focal length is the imaging scope, and what for the finder guider?

Guiding works best when around the same FL as the imaging rig, and the ultimate way is to use an off axis guider to guide through the same scope as you image. You can try firming everything up till kingdom come, but bad DF will really drive you mad and isn't easy to sort out.

It goes without saying that you should get your polar alignment as accurate as possible, which will always help in any case.

My advice would be to forget about PE for now, good guiding will get rid of that without you worrying about it anyway, and concentrate on getting your PHD settings right for the finder guider. Then you will see for certain how bad the DF is, and what would be the best route to take.

Off axis guiding saves you so many headaches, although it can seem hard at first. But I can guide my 2800mm focal length scope for hours on end with no PE correction in PHD with the right settings and an off axis guider, so dont depair, it CAN be done

HTH

Tim

[ianaiken]

And here is a PEMPro output from earlier in the night prior to imaging (or not since I cannot add the jpg keeps saying no file was selected for upload).

Tim - thanks for the input and direction. I think I am probably looking at too many things and not sorting anything out. PA is pretty accurate so I guess this is flexure here.

The imaging rigs are 100ED+QHY8L @ 2.06 arcsec/pixel and an 80ED+ATIK314L+ @ 2.55 arcsec/pixel. The guider I think samples at 5.36 arcsec/pixel. Its a 50mm finder with QHY5.

If I get a clear night I will point it back at IC434 and see how it performs with the new PHD settings. I will also check my PA again in case all of my fiddling has knocked it out.

An OAG would be very nice on the Atik, which also has an EFW2 connected.

[Tim]

One thing, the RA graph shows that PHD is over-compensating for the error. You can see this by the blue line going past the central guide line when it makes an adjustment. Play with the aggressiveness until it just brings the mount back to the line, not past it.

the graph isnt too horrible though. Did you have DEC guiding turned off?

[ianaiken]

Dec guiding was on. I did reduced aggressiveness but this made matters worse for the majority of the guiding. In fact, last night I had the aggressiveness set to 120 and the graph was even better.

I forgot to mention both scopes are focally reduced with the SW 0.85x reducers.

[ianaiken]

Tim - Actually something just clicked in my head with what you have said on the RA aggr. I will try changing these values next time on the aggressiveness rather than in EQmod with the RA Width Gain. Is it possible if you go too low you won't get enough movement and then insufficient correction?

[chrisshillito]

Tim - Actually something just clicked in my head with what you have said on the RA aggr. I will try changing these values next time on the aggressiveness rather than in EQmod with the RA Width Gain. Is it possible if you go too low you won't get enough movement and then insufficient correction?

Take a look at your PHD log - the duration of the guide pulses being issued are in there and those will be proportional to the aggressiveness you are using.

If a guide pulse is less than half the minimum duration value set in EQMOD then the pulse itself will not be performed (as EQMOD couldn't time it accurately) - however this will just lead to a greater error being measure on the subsequent sample and a longer correction pulse being issued. If the pulse direction is less than the EQMOD minimum but greater than half the minimum then EQMOD will apply a pulse of duration equal to the minimum with will in theory apply an over correction which again should be compensated for on the next sample. In any case these corrections and relate to such minor tracking errors that they shouldn't result in the kind of movement you are seeing. What guide rate are you using by the way?

From the PECPrep screenshot there doesn't appear to be any significant frequencies present that are associated with the usual mechanics of an EQ6 mount.

Chris.

[ianaiken]

Chris, thanks for your input. Guiderate was 0.3x but progressively I have been changing this up to 0.6x to see what difference it makes. Min pulse width is 30. I noticed that I could have an influence on the overall graph but I couldn't get rid of the spikes seen. I checked the PHD guide log and overall the guide pulse duration was greater than the minimum set. But yes I agree. I can get a nice flat graph albeit these peaks which suggests something mechanical in my very limited experience.

From the PECPrep screenshot there doesn't appear to be any significant frequencies present that are associated with the usual mechanics of an EQ6 mount.

What do you mean? The NEQ6 mount is second hand with a few mods like saddle plate and extension bar, but as far as I know untouched inside. Do you mean it looks OK or it looks odd in some way?

[Tim]

Do you calibrate in PHD before altering anything in EQmod?

Thats the way I guide, using the RA width settings in EQmod to filter the PHD response. At least I think thats how it works

I really think your problem is flexure though, and the best guiding in the world wont fix that

[ianaiken]

Hi Tim, I calibrate in PHD before altering anything in EQmod and I always ensure that the pulse width is set to 100% for both RA and DEC and PEC is turned off (not that I had PEC enabled in this instance). Flexure wouldn't explain the spikes which appear consistently every 50 seconds or so, or would it?

[chrisshillito]

What do you mean? The NEQ6 mount is second hand with a few mods like saddle plate and extension bar, but as far as I know untouched inside. Do you mean it looks OK or it looks odd in some way?

Hi Ian,

Sorry I wasn't clear - it looks fine .

The periodic error of the mount is a result of the summation of errors induced by all to the mechanical components in the RA drive. For the NEQ6 these have periods of: 10.2 seconds (tooth contact), 122 seconds (stepper gear), 366 seconds (transfer gear) and 479 seconds (worm). Non of the peaks shown in you frequency spectrum tie up with these values and so we can conclude that all the periodic error from these "usual suspects" has been successfully guided out! The peaks that you do see are most likely PECPrep trying to make sense out of what is actually a non periodic signal - so you see peaks across the frequency range - if you collected more data you would probably see this merge into one broad band of frequencies.

Chris.

[ianaiken]

Maybe jumping to something here but if teeth have a period of 10.2 seconds and I have peaks at approx 50 seconds then I could have some bad teeth? I don't even know how a NEQ6 should perform and what its typical error is like, I'll do some searching. I think maybe seeing that may help me to understand and maybe not try fixing something that may be quite normal.

[Tim]

Ian, have you the bits to guide through one scope and image through the other just to test it? might be useful to see.

If you get good polar alignment, and turn off guide output, how does the graph look then? If you target a star on the eastern or western horizon and just allow the mount to track round at sidereal rate, how long does the star stay central on a camera or eyepiece? You should be able to get it to stay central for at least ten minutes and actually a lot longer.

Does it do exactly the same thing every time you guide, on any star?

[ianaiken]

Hi Tim both imaging cameras could be used for guiding while imaging on the other. They both also have ST4 although I'm not using that.

While not tracking the guide star, at least when pointing South near 0 dec, remains reasonably steady. On average it's pretty flat. However, when I do a sub of 120 seconds I do get trailing on the Atik which does suggest flexure or PA is off. I will do some tests connecting PHD to that imaging camera and see what happens. I will also do a test on the other side.

The peaks occur on any star. The intensity appears to change depending on where in the sky everything is pointing. Bigger intensity closer to the horizon and smaller when near the Zenith (this is based upon limited testing).

Next time it's clear I will point it at East and West like suggested and test and I will record more accurately what is occurring from the perspective of both imagers. I could do this in PEMPro or PHD simultaneously.

Lastly I got to thank you both for all the help and advice provided so far! Many thanks!

[chrisshillito]

Maybe jumping to something here but if teeth have a period of 10.2 seconds and I have peaks at approx 50 seconds then I could have some bad teeth?

I can't see that would work. The 10.2 seconds the the contact period of the teeth for all three gears in the drive but 'bad teeth' would show up as signals with periods that are integer factors of the gear periods - 50 is not such a factor (neither is 32 sec which is the most significant signal that PECPrep has detected in your data)

If you happen to be an EQMOD group member the latest test release of EQMOD includes the ability to record your mount's PE to a pecprep compatible file whilst actively imaging under PHD guiding (you don't have to apply the result as PEC if you don't want).

There are a number PECPrep screenshots showing typical PE traces in the EQMOD groups photo section. The screenshots in the PECPrep manual itself are also made using real data.

Chris.

[ianaiken]

Thanks Chris I will head over and get the latest test result and use that. Giving up precious image time is hard especially since I can still obtain fairly reasonable subs. I will post the results at the earliest opportunity once I have verified my PA.

Hopefully that may lead somewhere.

[ianaiken]

Hi Chris I assume that I just need to click the record button on 1.26g and record data for at least 50 mins for my NEQ6?

[chrisshillito]

Hi Ian.

Yes to initate data capture you just hit the record button whilst you guiding. There are options on the autopec config screen to set how many cycles of data to capture and whether you want to automatically generate and apply PEC (which you probably don't want to).

The data capture process won't affect your normal guiding/imaging.

Chris.

[chrisshillito]

...... I forgot to mention that if you take a look in the EQMOD groups files/documentation section you'll find a draft copy of the vs-pec workflow document that covers the recent AutoPEC features.

Chris

[ianaiken]

Hi Chris,

Thanks. I will check out the documentation too.

I intend to check PA through thebmain imagers and then check an unguided sub then I will record PE while guiding.

Now I have a plan I just need a clear sky.

[ianaiken]

Well here are two files from EQMOD capturing PEC data while guiding. Not sure what to make of the data yet.

pecapture_01172013005620_EQMOD.txt

pecapture_01172013023842_EQMOD.txt

[chrisshillito]

Hi Ian,

There are two files because you must have taken two logs - each filename includes a timestamp.

What the data shows is that your PE signal consists mostly of contributions from the worm (around 16 arcsec peak to peak), its harmonics and the transfer gear (around 6 arcsec peak to peak). There are no significant signals present of periods around 50s region so if these are present in the PHD's guiding graph I can only assume the exist only at the PHD level and not at the mount ???

Chris.

[ianaiken]

Hi Chris,

Thanks for looking its very appreciated

Actually during these two sessions, one pointing south the other near zenith, the peaks were not present as before and the graph , although not as flat, was good enough for now with no significant peaks. I think the rebalance and polar align has helped. Dec was quite unbalanced prior.

Does the graph look typical for an neq6?