Starwatcher Azi-GTI mount (equatorial mode) - polar alignment versus "star alignment"

[Dave Wilden]

Hi,

Last night, with my Az-GTI in equatorial mode,  despite achieving "excellent" polar alignment ( less tha 10 arc sec overall error, according to Sharpcap's Polar Alignment Tool)  the longest sub exposure I could get away with before I saw elongated stars  (with 420mm scope) was just 8 secs. This is frustrating, as I really want to be using 30 to 60s subs, othewise the deadtime transferring data from the camera to Sharpcap becomes excessive.

So I'm trying to understand what affects the tracking accuracy of  my AZ GTI in equatorial mode:  is it just the accuracy of polar alignment, or does the "2 star" alignment accuracy also impact on tracking?

My process has been:

1.  Level tripod using spirit level built into the Star Adventurer wedge.

2.  Roughly align mount  with north pole (with mount turned off at this point)

3. Perform Polar alignment using Sharpcap polar alignment tool (manually rotating the scope through 90 degrees when requested)

4.  Put mount  back into approximate equatorial home position (pointing to N pole with counterweight down)

5. Power on the mount, connect to Synscan and perform 2 star alignment.

Last  night I got the polar alignment to 10 arc sec accuracy (as measured by the Sharpcap tool) at step 3 yet  when I later selected to slew to named objects,  the scope was often 5 degrees of more out (as determined by plate solving).   Am I right in thinking that the "slew to object error" is due to the fact that the  mount was only roughly  in the correct  "home position" at step  4?  If so why didn't the 2 star alignment process correct for this? 

This is not a major problem as I can use Sharpcap 's "Plate Solve and realign" to correct the error. A much bigger problem for me is that despite getting "excellent" polar alignment (accoing to Sharpcap)  the longest sub exposure I could get away with before I saw elongated stars  (with 420mm scope) was just 8 secs. In  equatorial mode, I thought that all the mount does is rotate the RA  axis at the siderial rate, so as the RA axis was accurately aligned to the North Celestrial pole, why am I still  getting elongated stars for such short exposures?  Or is there more to it than that -  does the "2 star" alignment error some how impact on tracking accuracy as well? Or is this just an inherent error in the mechanical gearing that does the sidereal rate rotation?

Any suggestion gratefully received

Dave
 

 

Edited August 29 by Dave Wilden

[StevieDvd]

PA and alignment are not the same. 

PA should level the mount but your alignment can be affected by the Date/Time/Location or BST/GMT settings.  Also, see if the software has the same settings as your phone/tablet/pc - one may override the other.

Check those and if all okay try a 3 star alignment and that nothing is left loose(clutches etc).

 

[Tomatobro]

What is the tracking speed set to? Is tracking turned on? Is the scope balanced and the clutch locked?

[Elp]

I used an azgti in EQ mode for nearly four years. If it's PAd, usually that's it, you don't then do a star alignment on top of that, though I'd see the use of it if you weren't using a computer controller.

I was however using it with an Asiair and autoguiding.

Something definitely going wrong, I could image in alt az mode with 9s exposures at a similar FL to yours. Are you sure it's using eq mode in the Synscan pro app, it usually asks what mode you want to use when you start the app.

[Elp]

Usually a mount also needs to have a known home position (how it was before you performed the PA), if you declutched and moved it manually, then possibly retightened and slewed you will have changed this home position and the mount won't know where it is in real terms. There's an auto home button in the app, I have a feeling this sends it to the home position (I've never used it), it also has a PA button on the same screen among others.

Edited August 29 by Elp

[happy-kat]

What's the weight of your telescope and accessories?