How to fix AZ GTi systematic drift (AZ Mode)

[Artik]

I have AZ GTi in AZ mode with a small and lightweight 60/400 scope that weights ~1kg and asi224mc.

The setup is very lightweight well withing limits of AZ GTi with 12V power source from the wall rated for 3A.

I do 3 point alignment - usually Mars, Rigel and Cih that are well separated.

I noticed that there is a significant systematic drift of ~2pixel between 5s frame when I capture M1 (see image with uncorrected green hot pixel drifting over stack of 12 frames) it basically limits the exposure to very low values (of ~2.5s) if I want to prevent star trails.

 

 

Here another measurement I did over 40 frames capturing flame nebula, the drift over AZ axis is 0.66 arcsec/s and in Alt much better of only 0.08 arcsec/s.

Considering that camera pixel size is 3.75um over 400mm FL gives ~1.9 arcsec resolution and basically the drift is 1 pixel in 2.9s which is very high and limits exposure.

Now I know that AZ GTi and Alt-Az mounts are capable of much better results. And another point is that the drift is very linear and consistent as you can see from the trend line I added to the graph that shows it is very consistent.

Now before you say "move to EQ" setup - I don't want to because I want to keep my EAA rig as simple as possible and I know from many reports of other users that much longer exposures aren't that problematic (of course till it gets to field rotation)

How can I fix/improve the accuracy?

 

 

 

 

Edited January 30, 2023 by Artik

[Elp]

The furthest I managed to push this is 10s exp at 360mm FL. The longer the FL the more error you'll see. 

I believe the more you conduct the alignment, the better the tracking becomes. Larger targets like the moon/sun tend to track very well.

I wanted to do deep sky though so EQ mode allows up to 2-3 minutes in good seeing, but I typically do 60s.

Edited January 30, 2023 by Elp

[Artik]

1 hour ago, Elp said:

The furthest I managed to push this is 10s exp at 360mm FL.

With what Pixel size?

Anyway  I'll take 10s at this point since I do EAA. Currently it looks like 5s is too much.

 

The accurate linear behavior tells me it is more model related than actual gear accuracy. 

[Elp]

6 minutes ago, Artik said:

Pixel size

4.3

[wuthton]

9 hours ago, Artik said:

How can I fix/improve the accuracy?

Buy a wedge.....

I'm sorry but the fact remains, cameras belong on EQ mounts. Your mount is tracking on both axes so double the error before you even get to field rotation.

In EQ mode have it slightly unbalanced and polar alignment slightly off and it should be tip-top.

[Artik]

1 hour ago, wuthton said:

Buy a wedge

Since I know it is coming here what I have written 

10 hours ago, Artik said:

before you say "move to EQ" setup - I don't want to because I want to keep my EAA rig as simple as possible

The goal is to get ~10s exposure. Usually it good enough not to get field rotation in a single frame.

 

Once again. I'm not looking for AP but EAA

[happy-kat]

With my virtuoso v1 I don't get good tracking if the OTA is balanced, I've found I need to ensure it is tail weighted bias when East facing and nose weighted for West.

Do you end your alignment movements with up and right.

 

[Artik]

Yes the scope isn't very balanced but I assume it is good to prevent backlash 

 

7 hours ago, happy-kat said:

Do you end your alignment movements with up and right.

Yes it is required by synscan app

[Artik]

Ok,

yesterday I did EAA session and I prepared the mount as following:

1. Make sure it is level (using bubble level on the top of the AZ GTi) - I adjusted tripod legs to level it properly
2. I did 3 point alignment when for each point I made sure that the star is in the middle of the frame
3. I did large (at least 1/8 of frame) movement right/up in synscan as final movement for backlash handling

Result:

1. Each and every object I looked for was in the center of the frame or at least very close to it
2. Tracking was almost perfect - no more than few pixels error from the start to the end of the session of even 8 minutes - frame to frame movements where almost non-existent!

I'm really pleased!

 

[happy-kat]

Sounds good.

Has this improved your exposure length for your EAA?

[Artik]

Yes I could easily go much longer, also I have found that I don't really need to go past 2s under Bortle 8 sky and my camera, see lecture below:

https://www.youtube.com/watch?v=3RH93UvP358

It is bit long but very informative, also for Bortle 5 I'll need 5s exposures so having AZ GTi support them will be very helpful.

[Artik]

Below the graph I created of tracking error

With pixel size of 1.9 arcsec, I can easily go up to 10s exposure time