Some guiding oddities with PME

[Paul Haese]

I have now mounted my RC12 and TSA with cameras on my PME (see image; also cabling is now managed correctly)

I have found that guiding in X (using maxim) looks a bit like a slow sine wave peaking from +1 pixels to -1 pixels over a period of 1-1.5 minutes. The dec is rock solid and stays pretty much on the center line.

This guiding is all done in the TSA using the OAG on the QSI.

When the mount just had the TSA on board this sine wave did not seem to exist.

I did a PEC recently and applied that, but wonder if I got it the right way around. PEC seems to be around 0.9 arc seconds at present. Using precision PEC and applied it shows this figure in the sky

I have the mount balanced perfectly, with no imbalance at any point of the sky.

I think this "sine wave guiding error" is the cause of the slightly elongated stars in my RC images, but I am not discounting flexure and have ordered a new STi camera for the moag (guiding through the internal guide chip is problematic and produces all sorts of corrections).

Would the spring plungers need adjustment? I think there is currently around 37kg on the mount. There appears to be some slight movement in RA (in rotation of the axis) when I put torque on the counter weight bar. I mean very slight movement, nothing like what I had before.

Guide settings are aggression 5 in X 4 in Y (I have tried a few other settings but nothing really makes a lot fo difference). Other settings are default.

The bottom line is that the TSA images have round stars but the RC12 images don't. Although the TSA star images are slightly bigger than before. The sign wave guiding is my main concern here though.

Any suggestions will be considered.

[rally]

Paul,

You seem to be guiding within a total range of 1.5 arc secs ?

Turn down the aggression if you think this is a problem and see what happens, but I would not have thought this was a problem at all.

I am guessing you are probably operating within the limits of your seeing.
Any PE cycle is going to much longer than 1.5 minutes on the P-ME unless you created your own harmonic PEC curve and actually added it back in !

Rally

[Paul Haese]

thanks Rally for the reply.

I have tried a lower aggression setting but this also failed to make an impact.

I would have thought that seeing would produce less regular results. This is like s smooth wide sine wave, which is quite weird. I take your point about PE, so that seems to rule that out.

Seeing can often be around 0.7 arc seconds at my place and also sometimes around 1.5" (this is fairly infrequent though) and my guide interations are 3". I might try 1.5 seconds, 2 seconds and 4 seconds to see what that brings.

I just wonder if I have the PEC around the wrong way? Being 0.9 would suggest though I have it the right way around.

Any suggestions as to settings for OA guiding with the RC12?

[gregbradley]

If applying the PEC curve causes errors to be worse then you have it back to front - ie scope pointing west checked when it was actually pointing east.

Try reversing the PEC and see if that changes it.

Greg.

[Paul Haese]

Will do that Greg.

[frolinmod]

Not only that, but you really do have to orient the camera very near perfectly while collecting PE data for best results. I didn't used to think it was so effing sensitive, but found out it really is after Daniel slapped me upside the head for thinking otherwise. I checked it and yes indeed, much better results having the camera oriented within one degree of perfect as opposed to within five or ten degrees.

[Paul Haese]

Thanks Ernie, I am about a degree from the north point, but I will check this just in case. I had not thought this would be an issue. I will try to dig up the imager log.

[gregbradley]

That's a good tip.

Greg.

[rally]

Paul,

My comment was that you are attempting to try to see and correct something that is potentially within the noise level of your system and the seeing.

Can you post a small section of your resultant image at full resolution and the guide log so we can see what you are talking about.

What level of guiding is it that you are actually seeking ?

Rally

[Peter Ward]

These are extraordinary figures Paul....virtually on a par with the seeing at Mauna Kea! See
http://www2.keck.hawaii.edu/optics/ScienceCase/TechSciInstrmnts/Products_SeeingVarMaunaKea.pdf

I rarely get better than 2-3 arc sec, being pretty typical of most sea level sites. How are you determining these numbers?

[Paul Haese]

We've done DIMM measurements to get the figures. We have tested the seeing in SA for our new 36" scope and are looking for relevant sites. My site was tested some time ago. So far most sites were like you suggest, mine does have extra-ordinary seeing. My Saturn and Mars shots this year came from that site.


Rally I just want to get good round stars and as tight as possible. I will try to hunt down a guide log and an image. My guess is that I have flexure in the system but I am trying to hunt down the other possible variables. I will need to do some testing for flexure too. :)

[Peter Ward]

Problem I have with differential seeing tests is: it is too easy to confuse an momentary figure with say an RMS or average.

Planetary seeing (and imaging) looks at fleeting moments in the former, (in which case I'd claim similar numbers in Sydney) but the rub is: deep sky looks at the latter.

To be blunt, I'd suggest 0.7 arc sec seeing for deep sky simply isn't credible. (if it was, you have to question the professional ethic of putting domes on laminar flow covered high mountains)

But, if this is proven, I'd expect to see professional domes popping up like mushroom's at Clayton! :)

[Paul Haese]

Yes that may well be and seeing here is not just momentary Pete. There are long minutes of high seeing values. There is little if any wobble during that time. Your statement actually shows how much DSO imagers think they know about seeing. At long focal lengths (ie 12m) the seeing must be sustained for long periods to get great images. We cannot make great images from fleeting moments. No offence intended Pete.

We get laminair flow here because the high pressures push straight up from the southern ocean. There are no terrain features to cause eddies, only the jet stream can cause an issue. It gets that still here that you can here the ocean 6kms away crashing into the beach on many nights. It is unique and plenty of people who have been here can attest to the quality of the seeing here. Also your comment clearly suggests that great seeing cannot be obtained at sea level. Exmouth and high up the West Coast of Australia produces this same sort of seeing and is probably at higher values. I have seen the videos and it looks just like looking out of a space ship at video of Jupiter. Flinders rangers has been professionally tested too and is not high peaks and produces incredible seeing. Ask Wysiwig why he put his dome there. The only reason why all the pro observatories on the east coast were not put in the Flinders was because of its remote location at the time.

Guide stars don't bounce here, don't discount this being real. Suggesting it is not credible is suggesting I am lying.;)

[Peter Ward]

Not at all Paul.

But I think some clarification is needed.

Small aperture, iosplanic cells can and do drift over several arc sec.

Clever software like .Avistack and Registax follow that movement, giving the illusion of sub-arc second seeing.

Take a simultaneous deep sky image covering say, 1/4 a degree of sky you'll see the drift..or seeing induced... datum distributes around a bell curve, you will see also over some minutes, a totally different figure.

A simple measurement of (deep sky) FWHM's verify this...and on casual inspection, your (posted) deep sky images are, sorry to say, on a par with everyone else's...ie around 2 arc sec.

I'm not having a go at you...just trying to point out that "seeing" is not that simple.

P.S.
Still not convinced? A simple, non-aligned sum, of say 2-300 planetary images should also prove the point ;)

[Paul Haese]

Back to the central issue, I adjusted the spring plunger screws in the RA tonight and the guiding has improved out of sight. Now instead of a slow sine wave I have the following. Excuse the rudimentary image.

That is the sort of guiding I was after. Now I can test for flexure when imaging with the RC12 and guiding through the TSA.

[Peter Ward]

Glad to see the PME is working well once more!

Hint: To capture your track-log, press the "print screen" key, run Photoshop, drop down: file "new" . accept defaults. click OK. Control V (paste)

Now you have an image of your screen ;)

[Paul Haese]

Yeah normally I would do that and paste into irfan view but I don't have any program like that on the capture computer. It just has stuff for imaging. Only connects to the net for time updates. Otherwise I would have given you a better image to view. That was taken on the iphone. :rofl:

[Peter Ward]

i-phone 5?? :lol:

[Paul Haese]

Nah only the 4, I will wait for another generation yet.

[rally]

Paul,

The guide log lists Peak X at 2.17 and Y at 1.8 ?

That may just be a case of the seeing conditions being different at different times.

But if you are happy thats good !

Have you got a full res crop or subframe of one of the images to look at the stars.
Not compressed into a jpg !

Rally

[Paul Haese]

Not at present Rally but the stars are still a little eggy tonight. Some flexure present but I think I can live with it at present. So long as guiding remains the same now I think it will be ok. Just doing some work on NGC253 at present. Next some on NGC1365. I will not be able to add much until after the eclipse. I will get an image crop for you to take a look at though.