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How accurate are GoTos


W. Christopher Moses
 

Hi,

Somebody on CN is asking about how many star aligns you need to do and how good the gotos are.  


I use TheSkyX and have it automatically center the object via a plate-solve, so I have no idea.


How many stars do you people use and how accurate are the gotos?


hubbell_jerry@...
 

Hi Chris,

I am curious to see how accurate most users are getting with the raw pointing of the G11 with an accurate polar alignment via the Polemaster or equivalent, but I can give you some insight into the raw point testing we did at the factory. 

At the factory, we mounted a telescope on the G11 and the EXOS 2 with a cross-hair eyepiece and we configured the mount so that we could rotate the DEC axis (which is pretty much identical to the RA axis) 360 degrees. We positioned the telescope via the clutches to center a target on the far wall of our warehouse and then zeroed the motor counts. The test was to slew the mount 360 degrees in DEC to recenter the target and record the motor counts. 

We did this a total of 20 times slewing 10 times clockwise and 10 time counter-clockwise, centering the target each time around and recording the total counts. The G11 total motor counts for each axis is 4,608,000 for 360 degrees of motion. After taking the 20 measurements, I did a mean value and a standard deviation calculation and the end result was a raw pointing accuracy of < +/-4 arc-sec with a 2-sigma precision of < +/-1 arc-minute. The EXOS 2 mount is <+/-10 arc-sec with a 2-sigma precision of < +/-1 arc-minute. 

So the raw pointing of both system is about +/-1 arc-minute. Add onto that the error in time for RA pointing (1 arc-minute error for 15 second time error) and any flexure that may occur between the mount and the camera, plus any additional lash that may exist in the mount, and the overall result should be < +/-5 arc-minute pointing with a perfect polar alignment. I have seen this demonstrated at our observatory.

Overall, I think you should be able to expect the mount to place your object near the center of your camera's field of view when pointing anywhere in the sky. I have yet to hear anything bad about the raw pointing from customers.



Jerry Hubbell
Director Electrical Engineering
Explore Scientific, LLC.


ji.theriot@...
 

Using CDC, after 2 or 3 really good syncs, the go-to is pretty good on mine. Good enough to get my target on the tiny chip in f my ASI290 mini guide cam at f4.6. Obviously more is better in that regards and I try to sync 5-7 targets. That first one is usually around 30’ or so, assuming my PA is spot on; close enough to make it obvious in my finder scope but usually not close enough to be in my guide scope fov.


r_hoskin@...
 

I just posted on the thread.  Said much the same, plus a small overview of how the pieces can go together. 

Post-sync goto is the least of my worries.  I really must get around to getting CdC, BYN, and Astrotortilla integrated.  Stepping out to Astrometry.net for my plate-solving syncs is pretty time-consuming.  Astrometry's an enormous help and I'm grateful it's available, but...  :-)


W. Christopher Moses
 

Hi,

Thanks for the info.  I will relay that to CN.

Thanks,
Chris


W. Christopher Moses
 

That is pretty much how I did things for a long time. I agree with you about Astrometry.net.

Eventually, I just bit the bullet and bought TheSkyX. It isn't cheap, but it is easy and integrated