What the hell is going on here? Guiding with AZ Gti

[edjrgibbs]

I'm struggling! When guiding with the AZ GTI the movement is all over the place, more so than with no guiding! Setup is AZGTI, guiding with Samyang 135mm, ASI533  and guiding with ASi120  mini. All run through an ASI Air pro. 

This is a guided image. Guiding is all over the place. Unguided at 60s has no movement at all!

 

 

 

[ONIKKINEN]

The DEC axis is having trouble with backlash, it looks like. That unresponsive series of corrections and then a sudden jerk that overcorrects is what happens when your worm gear has cleared backlash. This jerky movement could also affect RA if its too aggressive. The RA error could indicate bad guide settings, mechanical issues, balance issues, calibration issues or just be a side effect of a weak DEC axis.

 

I can think of multiple issues and fixes that i have also come across but really there are too many unknowns to give good advice.

 

Did calibration succeed, what exposure time are you using, how is your mount balanced, is the guider focal length set correctly etc?

[edjrgibbs]

5 hours ago, ONIKKINEN said:

The DEC axis is having trouble with backlash, it looks like. That unresponsive series of corrections and then a sudden jerk that overcorrects is what happens when your worm gear has cleared backlash. This jerky movement could also affect RA if its too aggressive. The RA error could indicate bad guide settings, mechanical issues, balance issues, calibration issues or just be a side effect of a weak DEC axis.

 

I can think of multiple issues and fixes that i have also come across but really there are too many unknowns to give good advice.

 

Did calibration succeed, what exposure time are you using, how is your mount balanced, is the guider focal length set correctly etc?

Calibration succeeded, I used 60s exposure, mount is well balanced and the fl is set correctly as far as I can tell. Odd that it tracks well without guiding but when guiding goes haywire!

[StevieDvd]

It may help others to give a better answer if you tell us the firmware version of the AZ-Gti and if you are using it in AZ or EQ mode.

Here's the readme from firmware version 3.36 showing the firmware changes. Release Note_RightArm.txt

[ONIKKINEN]

4 hours ago, edjrgibbs said:

Calibration succeeded, I used 60s exposure, mount is well balanced and the fl is set correctly as far as I can tell. Odd that it tracks well without guiding but when guiding goes haywire!

I meant the guide camera exposure, too short and youre chasing seeing = good guiding impossible  too long and there is not enough time for corrections. I would recommend  2s exposure at minimum to even out seeing, but not much longer.

 

You see these errors while guiding because unguided your DEC axis does nothing at all and with a decent polar alignment it does not move. RA is only tracking and experiencing periodic error, which can have a small or an unnoticeable effect on 60s 135mm fl subs. Guiding attempts to correct for periodic error and DEC drift because of polar misalignment etc.

 

I think i remember seeing that the AZ-GTI needs a firmware update to work in EQ mode properly. You are running in EQ mode with a wedge and counterweights? Because that is a must.

[michael8554]

Looking at the screengrab,

RA = 9.80 arcsecs,  Dec = 29 arcsecs.

Such hugely different figures indicate very poor guiding, resulting in Dec elongation of stars

Dec is wobbling at least 8 arcsecs,  it looks like it's going to end up way beyond the limit of the vertical scale.

And the guiding software is struggling to bring Dec back, taking 8 x exposure secs to correct.

Your Dec has a lot of Stiction, so when that pent-up energy is finally released, the mount is catapulted to the other end of the backlash.

And all this probably repeats.

RA is 300% better, but still poor.

So many things could be wrong, with little info on settings to work with.

Guide stars look out of focus, HFD = 6.06 is too high.

Michael

 

Edited October 24, 2021 by michael8554

[GoldTop57]

What firmware version do you have installed on the GTI?

[edjrgibbs]

Thanks for all the advice and sorry for the lengthy delay in responding. Firmware version is 3.26 and i;m running in EQ mode. Next clear night i'm going to try a better polar alignment and play with the guiding settings. Also sounds like I need to focus my guide cam better.

[AstroNebulee]

5 hours ago, edjrgibbs said:

Thanks for all the advice and sorry for the lengthy delay in responding. Firmware version is 3.26 and i;m running in EQ mode. Next clear night i'm going to try a better polar alignment and play with the guiding settings. Also sounds like I need to focus my guide cam better.

I'd definitely upgrade the firmware to 3.36 as this ironed out a whole heap of issues with the AzGti. What is your dec and RA aggression settings when guiding?

I have a skywatcher 72ed ds pro on AzGti mount in EQ mode, with an asiair pro and my Canon 600D, guidescope is the zwo 30f4 with my zwo asi120mc-s for guiding. I have my dec aggression to 60% and RA to 40% but this can depend on each night but its a rough figure I work from and I set the dec duration to 500ms and RA to 500ms (this really helps the settle time after a dither for me) and combined with the 3.36 firmware update, I've so far gone to 3 mins guided perfect round stars and probably easily go longer, just haven't tried it yet. Guiding is a dream with it now. But definitely update the firmware. I did also take my mount apart and regreased it all and checked the worms and gears. If your confident enough. I focused my guidescope during the day to save time, it doesn't need to be perfect though, as good as you can get it is enough. I guide at 50 gain and 2s exposures mainly. When PA with Asiair pro I generally go to the 3rd time the circle increases (zooms in) if you know what I mean and get to total rms of around 0.38"with PA (I think) or lower if possible. This mount and scope have been brilliant for me lightweight and easy to carry from my first floor flat. The below rms figures are the best I've had but normally around 1.5"

Edited October 27, 2021 by AstroNebulee

[david_taurus83]

The motors on these mounts have little gearboxes on the output shaft. Inside there are 3 small nylon gears with loads of play between the splines. There is no way to reduce backlash there unless you fabricated something custom. The worm gear is spring loaded onto the ring gear so minimal backlash issues there. I wish I had taken a picture of mine when I opened it up as nearly all the complaints and issues people have is with the Dec axis.

The best way to guide these mounts is to polar align with some error and guide Dec in one direction only so it doesn't need to go the other way. Forget dithering in Dec also.

Despite its short comings there, its a cracking little goto portable mount considering it's price and it was only ever intended as an Alt/Az platform.

[AstroNebulee]

19 minutes ago, david_taurus83 said:

The motors on these mounts have little gearboxes on the output shaft. Inside there are 3 small nylon gears with loads of play between the splines. There is no way to reduce backlash there unless you fabricated something custom. The worm gear is spring loaded onto the ring gear so minimal backlash issues there. I wish I had taken a picture of mine when I opened it up as nearly all the complaints and issues people have is with the Dec axis.

The best way to guide these mounts is to polar align with some error and guide Dec in one direction only so it doesn't need to go the other way. Forget dithering in Dec also.

Despite its short comings there, its a cracking little goto portable mount considering it's price and it was only ever intended as an Alt/Az platform.

I have images of my mount when I took the covers off if it's any use to you David. 

[david_taurus83]

@AstroNebulee 

 

Inside that little gearbox are the 3 nylon gears with AU's of backlash!

[edjrgibbs]

Ok, upgraded firmware. I think the aggressiveness was at 90% or so. Need some clear skies to play.

[AstroNebulee]

2 hours ago, edjrgibbs said:

Ok, upgraded firmware. I think the aggressiveness was at 90% or so. Need some clear skies to play.

Hopefully the upgraded firmware and lower aggression will sort the majority of your issue, let us know how you get on when you have clear skies 👍

[edjrgibbs]

On 28/10/2021 at 14:42, AstroNebulee said:

Hopefully the upgraded firmware and lower aggression will sort the majority of your issue, let us know how you get on when you have clear skies 👍

It worked. The lower aggression and firmware sorted the guiding issues. I also refocused my guide cam. Thanks everyone for the help!

[AstroNebulee]

2 hours ago, edjrgibbs said:

It worked. The lower aggression and firmware sorted the guiding issues. I also refocused my guide cam. Thanks everyone for the help!

Brilliant! So pleased it worked for you. 👍