Autoguiding Issues

[VPAstro]

Hi All, Hoping someone may be able to help.
I have been autoguiding for years using a Meade dsi ii and an Orion st80 using a GPUSB and PHD2. My imaging scope is and ED80 on a HEQ5 and I use a polemaster for polar alignment.
I have been able to do 10min subs without any trailing using this setup and my PHD graph is usually pretty stable.
I switched my guide cam to an Orion SS autoguider last night, connecting it directly to the st4 port and had major issues. The phd graph was all over the place alternating in both RA and Dec making corrections of sometimes greater than 3". I thought maybe it was something to do with the ST4 port on the guider, so I reconnected the GPUSB, and got the same result. I was getting trailed stars on one minute subs. After struggling with this for a few hours, I decided to go back to the Meade DSI II and PHD went back to normal operation with. Just wondering if anyone has experienced this before, or know of settings in PHD that be causing this behavior.
Thanks
Andrew...

[peter_4059]

Maybe lower signal to noise ratio with the orios ss causing error in guide star centroid calculation? What length guide camera exposure are you using?

[VPAstro]

Thanks Peter.
I am using 1 second, (the same as the Meade) but did change up to 3 seconds with no change. Still wild corrections.

[peter_4059]

Do you know the pixel size for the orion - maybe 5 microns? I think the dsi is arount 8.5 microns. If the orion pixels are small this will magnify the errors in a similar way to using a longer focal length guide scope.

[VPAstro]

Hi Peter,
Yes, the dsi is 8.3 x 8.6 and the Orion 5.5 I think.
If the smaller pixels are causing the errors to be amplified, maybe I am better sticking with the dsi.
It does seem strange that is was getting such wild adjustments even though I had pretty good polar alignment. Thanks
Andrew

[traveller]

Andrew, did you use the set up wizard in PHD2 for the SSAG?
Assuming you set up the guider correctly (FL of the guidescope, pixel size of the new cam, calculated the steps required etc), you should get similar readings.
The other point to remember is to use "On camera" under mount setup.
http://openphdguiding.org/man/Basic_use.htm
What was the RMS error on the SSAG compared with the DSI II? That should tell you a better story.
Hope that helps.
Bo

[VPAstro]

Hi Bo,
Thanks for that.
No, I did not use the setup wizard for the ssag, I just swapped it out.
I set the mount as on camera, but also disconnected the st4 port on the ssag and put it back to the GPUSB with the same results. From memory the rms error on the ssag was 3" compared to .2" for he dsi

[traveller]

Run the set up wizard and see how you go. 3" is not really usable.
I connect the ST4 port from camera to the mount and usually set the time to 2-2.5 seconds. My mount is still not running right, gets sharp spikes in RA now and then. I suspect I need to shim the RA axis soon.
Good luck.
Cheers
Bo

[Slawomir]

Perhaps I'm saying something obvious, but did you run a fresh calibration routine after swapping the cameras? If orientation of the chip is different and pixels are smaller then corrections would be all over the place.

[VPAstro]

Hi Suavi and Bo,
Yeh, I did do a new Calibration routine when changing the cameras.
I have just set up a new equipment profile for the SSAG using the wizard. Unfortunately, I can't test it until the next clear night. (hate wasting imaging time with these kinds of issues.
I realised that I had logging on, so I have attached a log of both the meade and the ssag at the same scale.

Andrew

[traveller]

Interesting. Can you post a screen shot of the orthogonality graph with the SSAG, under review calibration? How many calibration steps in the RA axis?
I had similar issues with my guiding (large errors in RA, RMS over 1.5" at times), so I am tinkering with the mechanical side of the mount to see if I can fix it (worm meshing and adjusting for back lash etc).
Bo

[VPAstro]

Hi Bo,
I think this is the graph you were after..
Thanks
Andrew...

[traveller]

The graph looks spot on, so no mechanical issues (unlike mine...)
So it looks like a software issue.
Try a reinstall with the wizard and see how you go.
Bo

[VPAstro]

Thanks Bo,
Hopefully tonight.
I will post back

[Slawomir]

Hi Andrew,

I have noticed two things that I will try to comment on.

1. You have pixel size set to 1micron for the Meade camera. From memory it should be about 8microns, thus guiding errors represented on this graph are really eight times larger. That will make both graphs comparable, but does not explain elongated stars with SSAG.

2. SNR for the star you have been guiding on with SSAG is way too small IMO. I'm getting over 100 to close to 200 with my autoguider. You either need to make your guiding exposures longer or find a brighter star or improve focus. From my experience with SSAG, 1 second might not be enough and seeing will cause your graph to go all over the place. Gradually try longer exposures until the blue graph stops alternating from exposure to exposure and you start getting consecutive corrections in the same direction. Then you can tweak aggressiveness, min movement etc to fine tune it. And since seeing varies even during one night, sometimes 1s guiding exposures will be fine, while other times you will need longer (2-4 second) guiding exposures. I would suggest trying 2s exposures and go from there.

Unfortunately HEQ5 may give less round stars with longer autoguiding exposures, but apart from fine tuning it mechanically there is not much more that can be done about it.

One more thing, those occasional spikes in Dec (red graph) will definitely spoil data - I think they highlight a mechanical / flex problem.

Please keep us informed.

Suavi

[VPAstro]

Suavi,
Yes you are right. It is set to only 1 micron. On looking back at previous logs, looks like it has been that way for a very long time. (and I thought I was getting great guiding because of the flat graph....)
Strange that I was very happy with the results.
I will try your suggestion later tonight, but due to heat seeing will not be good.
I will let you know.
thanks
Andrew...
PS, I think the DEC spikes were dithering at the end of each 5 min exposure

[Slawomir]

No worries Andrew :)

Yes, the spikes look to happen regularly, so you are probably right and that is good news. In phd2 you can dither every second sub - it sometimes helps with guiding. It definitely looks like the guide graph from Meade, in spite of being scaled down, also alternates directions for the better part in RA, probably because of seeing moving star's centroid around.

Another possible reason for star elongation is differential flexure between guide scope and the main scope. Orion ST80 is a substantial guidescope and there may be flex between the two telescopes. Is your guide camera supported by a ring or is it simply attached by an extension tube to ST80's focuser? A smaller (and lighter) 50mm guide scope would be perfectly sufficient for guiding ED80. In the years long gone by, before I started using an OAG, my 50mm guide scope was be supported by 3 Losmandy rings and one of them directly supported SSAG so it would not flex.

[VPAstro]

HI Suavi, Flexure is a possibility.
No ring, just ext tube and last night, I failed to lock down the focus tube after focusing, so there could have been movement there.
I have a 50mm finderguider, so I could try that tonight.
I am interested to see if I still have issues tonight.
I have attached a 100% crop of a 5 min sub from last night to show the shape of the stars.

[Slawomir]

Andrew, forgetting to lock down the focuser tube would very likely cause flexure with ST80. The focuser in ST80 is not that awesome, so it needs whatever help you can provide :)

If all stars in your subs are elongated in the same direction, then to my knowledge either flex or poor guiding in one axis only (RA or DEC) are your prime suspects. If you can securely attach a 50mm guide scope then it will almost certainly give you tighter stars to guide on (better SNR) and it will be less prone to flexing.

[VPAstro]

Great, I will see how I go tonight.

[VPAstro]

Here come the clouds.
An hour ago it said clear until midnight.... :(

[VPAstro]

Clear night last night, although poor seeing, and 32km/h wind. Thought I would give it a go again with the new settings, and achieved much better results. Even with the wind, I got round stars this time.
Thanks for all your help.
Andrew

[traveller]

Very respectable results Andrew.
What did you do? Re-installed using wizard?
Any changes to your settings?
Bo

[VPAstro]

Hi Bo,
Thanks for that.
I did create a new profile and use the wizard. I also used 2 second exposures, but I think the biggest improvement was to lock down the focus.
Hopefully on a less windy night, I will get even better results.
Thanks
Andrew...

[traveller]

Forget tonight and Friday night.
Sat and Sunday nights are looking good though.
I am imaging NGC 300 and the Running Chicken at the moment, can only grab 30-40 mins on each due to obstructions and moonrise etc.
Good luck in yours.
Bo

[Slawomir]

Great news Andrew, really happy your mount guides better now :thumbsup:

SNR is MUCH higher in the recent graph and overall the graph looks better too. Sometimes it is tiny things that can make a significant difference :-)