Improving Guiding

[Steve789]

With a weekend of clear nights and a full moon I decided to set myself the task of improving my guiding.

My subs have always been limited to 8 minutes before star trailing starts with 10 minute subs almost always showing elongated stars. Here's a 20min sub I did as a test last night.

My set-up includes a HEQ5 carrying a SW 150pds with a 350mm focal length guide scope piggy backed using some home made guide rings. All my connections seem rock solid and I've modified the focuser of the guide scope to minimise flexure. My guide camera is a ZWO ASI120mm and I use EQASCOM pulse guiding with PHD2. It's not a permanent set-up so I place the tripod legs on marked positions and use EQASCOM polar alignment tool each session to get a reasonable polar alignment.

So for the last 2 nights I've been experimenting with PHD to see if any tweaks can improve my sub length. I've tried guidecam exposures up to 5s, changed pulse guide rate from 0.2x to 0.5x, changed aggression from 70% to 100% tried different calibration steps and recalibrated numerous times. I've also taken special care with balancing to the point of changing the balance slightly during a session with no noticeable difference in results.

If I look at my subs over the course of an evening there seems to be a consistent drift in RA with perhaps a slight drift in DEC.

I realise its hard to identify problems with someone's set-up over the internet but all suggestions as to what I should try next are welcome. I've attached the PHD2 log file for those people that understand such things.

PHD2_GuideLog_2015-09-25_215105.txt

[Leveye]

Do a drift alignment in PHD to finetune your mount's alignment. If that does not help then cut back on exposure time till you get better results. 

[Freddie]

Looking at your log, there are multiple calibrations. Some take ~26 steps to calibrate (which is quite a lot but will be fine) and one I saw only took 3 steps (which is too few and may cause issues) so you may want to adjust your calibration step size to try and get ~8-10 steps each time.

You say there is a drift in RA through the evening but do you know if the star elongations on each sub are in RA, DEC or a bit of both?

[Steve789]

Thanks Leveye, I've tried drift aligning tonight,  something I've been putting off for a long time. I can't say it was hugely successful. I found the trend lines were bouncing above and below the centreline and no matter what I did I couldn't get them level. The altitude adjusters on my mount aren't exactly easy to use. When I finally decided I must be close a 1 minute unguided sub showed awful star trails so I gave up and went back to using EQASCOM polar tool. I had been a bit lazy with not checking the polar home position in EQASCOM and I realise now that it was slightly out and this probably caused my polar alignment to be a bit off.

I've also tried unbalancing my rig a bit more to see if that helps.

The first few 10 min subs are looking good so thanks for pointing me in the right direction. I'm now trying a 15 min sub to see how far I can push it...

[Steve789]

That last post must have taken over 30 minutes to write while playing with my mount etc I didn't see you're response Freddie.

Yes every time I changed a setting that I thought might affect calibration I recalibrated hence the different number of calibration steps. I've settled on 12 calibration steps now.

The star elongations were mostly in RA with a bit of DEC.

All signs are that a bit more care with polar alignment has made a marked improvement.

[Steve789]

One more question I have is how important is it to have the guide scope and imaging scope exactly aligned? At the moment they are almost definitely not exactly aligned but probably within a degree or two of each other.

[Thalestris24]

Hi

It looks like something's not right... Try running the PHD2 Guiding Assistant to see where the drift is. The heq5 isn't easy to PA adjust even with the upgrade bolts... Tiny movements! Unless you're imaging in narrowband, there's little or no advantage in taking long subs. Better to stack lots of shorter ones. It's probably a good idea to have dithering off to save the complications until you sort it out.

Louise

EDit: PA errors get worse, the longer the sub and the further your guide star is from your image frame

[Steve789]

Thanks Thalestris24, I've just started venturing into narrowband hence my search for longer subs

"PA errors get worse, the longer the sub and the further your guide star is from your image frame" - That's interesting to know, I think I'll use the moon tonight to get the two scopes aligned.

[Thalestris24]

Hi again

I'm not sure you have a PA error problem since your stars seem to be all elongated in one direction. As I say, the Guiding Assistant should give you some useful info.

If you're interested, there's some useful PA theory and calculations here

Louise