SiTech Mesu and image centering


When centering an image in SGP a test image is taken and plate-solved.
This is then compared to a reference image. SGP then slews the mount to
where it thinks the image is centered and another test image is taken
and plate-solved. Everytime this happens, the SiTech control system pops up the init

My question is, should I select on Offset Init or just cancel the window?


Steve, not fully my area this is Julain’s area more than mine but should hit OK to that prompt I think.

There are some other settings that are required to get centering working well with sitech, linked which set of encoders are in use, you may know about it.

I have pointed Julian this so hopefully will answer when he gets back from IAS


By default, Mesu is set to polite mode (on the scope encoder tab of sitech.exe config window).
I found that SGPro centering cannot get closer than 50 ~ 100 arcsec.
This is because the scope encoders on Mesu200 has resolutions of 2.16 arcmin.

When I set Mesu to ignore mode, SGPro centering get me within 1 ~ 2 arcsec within 2 tries.
In the ignore mode, Mesu only uses motor encoders which resolution is much higher than the scope encoder.


I use the same equipment. If you think about the way the SItech II controller works with the popup window, by default it counts down in seconds (unles you’ve changed it), does an offset init by default then closes the window. So if you intervene manually in this process, you need to click offset init and OK (need both) otherwise you are not refining your pointing accuracy each time a plate solve succeeds.

I follow the process I described above (using Platesolve 2) and it works fine. I image at F8 with a celestron C11 and Sbig STT8300, which gives a detailed image scale for galaxy imaging. I ask SGP to iterate until it gets to within 90 pixels and it does this fine in two iterations.

I just think 90 pixels is fine when you think about how many pixels there are on your computer monitor, and this avoids unecessary iterations too, which all take time (and risk failure to solve).



Stephen - as has been said when the init window pops up you should allow it to do an offset init. I run a 64 point model and like donghun I use the encoders set to ignore mode - this uses the more accurate motor encoders - the only drawback could be if the scope slips (possibly during a long slew such as a meridian flip) - this has never occurred in the 6 weeks since I have set it this way - in order to stop this happening I have carefully balanced my system.

I do agree with Paulkirk that the level of accuracy that this gives is not really necessary but I have got used to this level of precision from sequence generator - the scope will now centre to an accuracy of below 3 pixels in normally 1 attempt occasionally requiring 2 attempts. I run a dual scope system which comes in at about 27 kgs.



This is not related to SGPro but do you see any advantage building a point model using Mesu 200?
I’m currently only using offset inits and rely on SGPro centering and I’m wondering what modeling can give me other than more accurate initial slew.
It may compensate polar alignment error and some RA tracking for DEC coordinates but we more likely need to guide Mesu without absolute encoders so I’m not sure those corrections are really worth given the guiding performance of Mesu.



there is not much real use to use a model with sgpro - it does mean that I can park the mount accurately (needed in my obs as there is not much room from the pier to the roof) it also means the initial platesolve is quicker and more accurate as the scope knows where it is.

As I had done a model I decided I may as well use it - the model did not take long to do - I did it at twilight so I was not wasting imaging time. I agree the mesu guides really well with or without the model.



Julian, yes I agree it’s worth building the model if you have a permanent setup like you.



I have ordered the Mesu 200 and eagerly waiting for it be shipped. While I chew through my nails, perhaps you can help with a naive question.

I am reading here from JulianR and donghun that having a model helps. How does SGPro behave or use or refine the model ? Or is the solve-slew-solve approach essentially ignoring (wasting away) the model?



Hi Vipin - as long as you do an offset init after making your model, then sitech will continue to do offset inits - this means that any syncs you do will not be added to your model. After make your model you do not want to do any more cal stars as this will clear the model you have already made - of course you could reload the model which you have previously saved.

The model helps me as when it does a meridian flip the ra dec is more or less correct and platesolve works quickly and accurately.


Thank you Julian.

Can’t wait to try all this out…



I wanted to bring this topic back to the top.

I have the same combination of a MM200 with SGP.

My autocenter fails.

During autocentering what I have noticed is that SGP platesolves and then issues a sync based on the platesolve co-ords. This pops the sync window in Sitech and as a result the subsequent correcting slew from SGP is failing. Is anyone else noticing this?


I used to have problems with my Mesu 200.
I originally had the Scope Encoders set to polite mode, but because my ticks per rev was inaccurate the mount would oscillate. This meant that when it issued a snyc the mount had move away from the image location.
I was sometimes struggling get within 1000 pixles after 15 attempts.
I finally set the Scope Encoders to ignore.
I now centre to less that 10 pixels (often sub pixel) with less than 3 attempts.


Hi Nigel, I do not seem to have the oscillation problem. I have Gurley 500k encoders and with the mount model through maxpoint my gotos are already with a few arc seconds. Problem is with the auto centering where sgp fails saying that the slew after the plate solve failed and I am guessing this is because there sync window is during there with a 14 second timeout.


You can reduce the length of the timeout.
Open Change Config
Init Window Timeout
I have mine set to 5 sec.


Perfect, this is exactly what I needed. Everything works fine now. Thanks Nigel!


Glad to have been able to help.