Weird guiding issue

[Saturn488]

Last night was imaing the Tarantula Nebula (tends to stay more or less in the same spot in the sky) and my guiding was good and bad throughout the night.

My guiding was bang on for a bit, then it went off in the DEC or RA then recovered. Every now and then it said it couldn't make sufficient adjustments so I am thinking maybe the object wasn't high enough?

I do have the log which I can provide but it is quite big.

Any ideas why guiding can be up and down?

[AdamJL]

Backlash? Have you checked your mount to see if there's any play in both axis?

How long were your guiding exposures as well?

[Startrek (Martin)]

Imaging below 30degrees in Alt has a higher risk of unstable guiding due to atmospheric instability but doesn’t generally cause random big excursions in Dec or Ra

Random big excursions in Ra and Dec can be caused by -
Dec and Ra Backlash
Period error in your worm drive
Wind gusts
Cable snags
Passing Cloud
Incorrect Guide Settings ( calibration, guide exposure, min mov etc.. )

If your previous guiding has been ok for a while then on the next clear night try some test guiding on a star in the East up at 50 to 60 degrees and see how the mount performs
Also , if using PHD2 , try the PPEC algorithm and guide for 2 or 3 worm cycles ( 20 mins or so ) to see how the mount performs
Are you using PHD2 ?
If so, are you using Version dev 4 or later with Multistar guiding feature ?

Cheers
Martin

[xelasnave]

Check for cable hang ups...
Alex

[Startrek (Martin)]

Originally Posted by Startrek

Imaging below 30degrees in Alt has a higher risk of unstable guiding due to atmospheric instability but doesn’t generally cause random big excursions in Dec or Ra

Random big excursions in Ra and Dec can be caused by -
Dec and Ra Backlash
Period error in your worm drive
Wind gusts
Cable snags
Passing Cloud
Incorrect Guide Settings ( calibration, guide exposure, min mov etc.. )

If your previous guiding has been ok for a while then on the next clear night try some test guiding on a star in the East up at 50 to 60 degrees and see how the mount performs
Also , if using PHD2 , try the PPEC algorithm and guide for 2 or 3 worm cycles ( 20 mins or so ) to see how the mount performs
Are you using PHD2 ?
If so, are you using Version dev 4 or later with Multistar guiding feature ?

Cheers
Martin

Attached are some docs I put together a while back on PHD2 guiding
They may be of some assistance moving forward if you use PHD2
Cheers
Martin

[multiweb (Marc)]

Originally Posted by Saturn488

Last night was imaing the Tarantula Nebula (tends to stay more or less in the same spot in the sky) and my guiding was good and bad throughout the night.

My guiding was bang on for a bit, then it went off in the DEC or RA then recovered. Every now and then it said it couldn't make sufficient adjustments so I am thinking maybe the object wasn't high enough?

I do have the log which I can provide but it is quite big.

Any ideas why guiding can be up and down?

When you're imaging the tarantula there's always a period of time where your counterweight bar is vertical so when your RA worm lost its preload your mount will oscillate within the backlash until it's on an angle again and the worm rests on the other side of the spur gear.

[Saturn488]

- No cable snags, all good
- Using PHD2 with 1 second exposures
- Weather seemed alright here last night in Melbourne

I was imaging the Helix a while back and the guiding was perfect so I thinking that the Taran. is too low.

Thanks for those docs Ill have a look through them.

Tonight looks to be another clear one so I will locate an object much higher up and see how guiding performs. All the good stuff is too low to the horizon!

[AdamJL]

1 second is very short
Try longer subs as well

[Stonius (Markus)]

Originally Posted by AdamJL

1 second is very short
Try longer subs as well

Same - I thought 2sec was recommnded minimum otherwise you're chasing seeing

[Startrek (Martin)]

Totally agree , guide cam exposure of 1 sec is short and can lead to “chasing the seeing”
2 seconds is a more practical exposure time

[Drac0 (Mark)]

Must have been a night for guiding problems. Mine would work well for 10-15mins then DEC would go haywire. Thanks for the pdf files Martin, will look into them.

[Startrek (Martin)]

Originally Posted by Drac0

Must have been a night for guiding problems. Mine would work well for 10-15mins then DEC would go haywire. Thanks for the pdf files Martin, will look into them.

A couple of thoughts on the issue which are in my docs -

Are you using ST4 guiding ( guide camera to mount ) ? If so , ditch it and use Ascom pulse guiding ( guide camera direct to laptop) enables you to guide on both sides of the meridian without issue, allows you to save a good Calibration for reuse over months and months , easier to analyse and diagnose PHD2 logs

Check Backlash and Stiction in both Dec and Ra , worms may need adjusting ?

Ensure you achieve good calibration at Dec 0 ( Calibrate near celestial equator and close to north meridian)

Check balance in both Ra and Dec

How’s your Polar Alignment ? Under an arc minute ?

Run the Guide Assistant in PHD2 , it may pick up the problem

Cheers
Martin

[Drac0 (Mark)]

Are you using ST4 guiding ( guide camera to mount ) ? If so , ditch it and use Ascom pulse guiding ( guide camera direct to laptop) enables you to guide on both sides of the meridian without issue, allows you to save a good Calibration for reuse over months and months , easier to analyse and diagnose PHD2 logs

Pulse guiding through ASCOM.

Check Backlash and Stiction in both Dec and Ra , worms may need adjusting ?

Doing that tonight - RA guides fine, just DEC - I'm thinking this is the main issue.

Ensure you achieve good calibration at Dec 0 ( Calibrate near celestial equator and close to north meridian)

Will redo the calibration & see once the skies clear again.

Check balance in both Ra and Dec

How’s your Polar Alignment ? Under an arc minute ?

Both good.

Run the Guide Assistant in PHD2 , it may pick up the problem

Will do. Thanks.

[Drac0 (Mark)]

Checking too if my cheap Officeworks USB3 extension cable is part of the problem too - I know PHD2 sometimes loses the camera & wondering if some of the guiding pulses go missing too...new one from Lindy arrived today.

[Drac0 (Mark)]

Seemed to have pretty much fixed my problems. Tweeked the worm gears and recalibrated and all seems pretty good. Still a little backlash visible when the mount changes between north/south guiding but well within acceptable limits - not sure I'll tighten it more as I want to avoid binding. I don't know if the new USB3 cable helped with guiding, but the camera never dropped out once all night.


Cheers,
Mark

[Startrek (Martin)]

Originally Posted by multiweb

When you're imaging the tarantula there's always a period of time where your counterweight bar is vertical so when your RA worm lost its preload your mount will oscillate within the backlash until it's on an angle again and the worm rests on the other side of the spur gear.

Marc,
Just following in from your advice, I was imaging the Tarantula last night in Sydney
Started imaging the nebula at 33 degrees Alt , guiding was good at 0.80 arc sec total , improved to 0.70 at 40 degrees and ended up at 0.63 at 47 degrees , stayed excellent until high cloud rolled in around 1.30am and called it a night
My Skywatcher mounts, years ago with the HEQ5 (both main and worm gear driven ) and in the last 3 years with my two EQ6-R mounts ( belt and worm gear driven ) at both Sydney and South Coast have always guided well near the south meridian / SCP.
Just thought I’d mention my experiences
Cheers
Martin

[Nikolas (Nik)]

Weird I imaged Tarantula over 2 nights with the asiair pro and guiding was spot on perfect, less than 0.6 RMS error. Conditions were very good. Was your rig perfectly balanced?

[Nikolas (Nik)]

Originally Posted by Saturn488

- No cable snags, all good
- Using PHD2 with 1 second exposures
- Weather seemed alright here last night in Melbourne

I was imaging the Helix a while back and the guiding was perfect so I thinking that the Taran. is too low.

Thanks for those docs Ill have a look through them.

Tonight looks to be another clear one so I will locate an object much higher up and see how guiding performs. All the good stuff is too low to the horizon!

Did you recalibrate when you went to tarantula? You should as it is lower on the horizon that helix which is more or less overhead. Also are you using multi star guiding then 1 second is fine otherwise 2 secs or more for single star guiding.

[multiweb (Marc)]

Originally Posted by Startrek

Marc,
Just following in from your advice, I was imaging the Tarantula last night in Sydney
Started imaging the nebula at 33 degrees Alt , guiding was good at 0.80 arc sec total , improved to 0.70 at 40 degrees and ended up at 0.63 at 47 degrees , stayed excellent until high cloud rolled in around 1.30am and called it a night
My Skywatcher mounts, years ago with the HEQ5 (both main and worm gear driven ) and in the last 3 years with my two EQ6-R mounts ( belt and worm gear driven ) at both Sydney and South Coast have always guided well near the south meridian / SCP.
Just thought I’d mention my experiences
Cheers
Martin

For sure if you have very little or no backlash in RA with your mount it is a non issue. Guiding close to the SCP is also always easier. The only thing that can become a problem is field rotation but generally guiding is better.