Trying to improve guiding numbers with PHD2

[John K]

Hi everyone,

I have a new rig that I am trying to optimise guiding with. 1700mm FL and a S/H AP mount.

Attached are some guiding graphs snapshots from PHD2 from last night.

As far as I could tell, there was no high level cloud when these were taken, but hard to tell with Melbourne's light polluted skies.

Overall tending to get round stars, but the guiding graphs can vary in quality and wanting to smooth these out if I can, but questioning whether it is the seeing/sky conditions or the parameters I am using.

Tend to run the PHD2 guiding assistant at the start of imaging runs to decide on parameters.

The mount is well polar aligned and normally I don't get swings in Dec.

Advice welcomed.

Clear skies.

John K.

[glend]

Your RMS Error numbers look high to me? What exactly did the guide assistant suggest that you should change?
Also, at that focal length, balance of rhe load is important and setup issues like cable drag can impact guide results. Were you using a guide scope or OAG?

[John K]

No other changes recommended by the guiding assistant - I normally run this and press apply for both axis.

Interesting that even as the RMS sometimes approaches 1" I can still get good round stars with some subs.

Setup uses an OAG.

[Shiraz]

looks pretty good for average seeing John - the seeing appears to have varied a bit over a fairly short term, but that happens.

The DEC axis looks like it has a bit of backlash (it has more variance than the RA and needs a few hits to correct some of the larger scale wander). That could probably be tuned out and there are guides on the web on how to do it. I think that you can check backlash using the check cal tool? For interest, does it have spring loaded worms?

If the guide variability is just down to seeing, the stars will stay round as the guiding goes to pot - they just get bigger

[Slawomir]

Hi John. When seeing is poor, longer exposures for autoguider might give better overall results. Sometimes a change from 3 to 4 sec makes a big difference.

[Marke]

There was a recent long discussion on the AP Yahoo group about using PHD I have used it for awhile and had issues like you . I have just gone back to Maxim for guiding and getting way better results RMS <0.1 for a lot less effort so I tend to blame the algorithms in PHD for the moment.

[John K]

Gents,

thank you for all the advice and comments.

All makes sense.

I think as well at 1.7 FL the seeing definitely is king, combined with my ASI600 which has small pixels, means sky conditions have to be be at or above average.

Much better looking graph results tonight.

Clear skies.

John K.

[codemonkey]

Mostly looks like seeing to me based off the fact that Ra and Dec are pretty close on most of the graphs.

Could be worth disabling Dec guide output to see how it fluctuates. Given that Dec isn't tracking like Ra is, barring any cable snags and assuming a good calibration, all you should see is drift due to polar misalignment and seeing. If it's bouncing around a lot, with guide output disabled you can be pretty confident it's seeing. I think the guide assistant will give you a good idea of this too.

As a side note, have you definitely got the right focal length / pixel size set up in PHD? I note the imaging scale seems oddly close to 1"/px. If you're using the 174 (?) with 1.7m FL that should be 0.71"/px

[John K]

The mount has been polar aligned with Pempro so there is little/no movement in Dec even though it's a portable set up.

The guide camera is a Lodestar so with it's 8.2 mu pixels it gives me 0.99" per pixel in terms of resolution.

One thing I have thought about doing is to use my ASI290mm camera as the guide camera so with it's 2.9mu pixels which would lower the resolution to 0.35" per pixel but this may be overkill as I will be limited by seeing anyhow.

With my ASI1600 my theoretical imaging resolution is 0.46" per pixel which is right on the Dawes limit and below most of the seeing conditions I will generally get.

Further thoughts welcomed.

[gregbradley]

I agree with Mark.

PHD2 on the surface seems good but I also had variable results with it.
I get more reliable results with CCDsoft or The Sky X.

Pempro will get you close but Sky X accurate polar alignment with Tpoint gets you bang on. There is a slight difference and gain to be had from a perfect polar alignment.

Greg.

[Merlin66]

My 2c

I use a C11 @ f10 (2500mm fl) with a reflective slit plate for spectroscopy.
I regularly use PHD2 and AstroArt for guiding with a Lodestar on an NEQ6 mount.
I haven’t experienced any difficulties using PHD2 giving 10 min subs for hours of exposure.

[Camelopardalis]

I use my ASI290MM for guiding and it works well, although my focal length is a lot shorter ;)

Even so, the difference in seeing is quite noticeable, both in the size of the stars and in how PHD2 responds. My mount is a mere modded EQ6 but it's a consistent performer, unlike the seeing.

And the 290 often shows the target object in the guide image. I was most surprised when I had one of the Leo Triplet front and centre in 2 second subs.