Plate Solving
(Getting dead-on GoTo's)

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For many years I've been using EQMOD to control my mount (currently an Orion HDX110 EQ G) .  Usually, at the start of  session I would slew to a reasonably bright star in the region  that I'm interested in  hoping that  it will be in the Field of View (FOV) . For the most part it is. If not, Manual intervention is required.

 I would have to:

  1. Look thru the finder scope and slew the mount till the star is seen in the view finder.
  2. Next I would use the computer to center the star in the picture
  3. Finally I would sync the scope to the  current location

Now the mount knows exactly where it's pointing. GoTo's after that  are usually very close. I I were to move to the other side of the meridian, it's possible that I would have to go thru the above steps again.

I had read about Plate Solving some time ago and tried it but didn't have any luck getting it to work so I gave up.

I finally bit the bullet and made an attempt to try it again. This  time I had success with the help of a few acquaintances I met on one of the forums.

Ok, so you may ask , what made me try it again?  EQMOD is a program that's been around for many years and works just fine but there is very little incentive for the person supporting the application to update it and incorporate some of the newer features of the EQ8 and HDX110 mounts. So an ambitious and dedicated programmer and astro enthusiast, Rob  Morgan, decided to write a new application that  would mimic the main features of EQMOD and incorporate some new stuff. In as little as a year and a half he has come up with a super package that he calls Green Swamp Server (GSS) . You can get a copy by joining the Yahoo group 
( ) .

GSS handles the mapping of the sky differently than EQMOD and so doing the initial goto's may be off and require the manual intervention steps above. Rob is used to plate solving  which overcomes this issue. I was interested in using Rob's program so I was determined to get plate solving to work for me. With  help  from Rob, Colm Brazel and Lenny Shaffer we got it going.

So what is plate solving? It's a process controlled by a program that compares and image taken by your equipment with known images of the general area in an attempt to identify the actual coordinate at which your telescope is pointing. This allows you to then synchronize you equipment  with the actual location in the sky. Once this is done your goto's  to an object should put the object near the center of the FOV.

How to get started:

I'm using a WINDOWS 10 PC.

  1. Download a copy of  AstroTortilla version 0.8 Alpha-1. It's available from in either 32 bit or 64 bit version is available
  2.  When loading AstroTortilla load only the program DO NOT  load the CYGWIN program nor  the INDEX files.
    Default all the other options  during the install.
  3. Next install the Program that  does the actual work CYGWIN_ANSVR and the INDEX files used to do the solving from . Installation LINK and instructions can be found here.
  4. After the program installs it will attempt to install the INDEX files. You system may complain and block the PERL.EXE from downloading them. Tell the system to allow this program thru the firewall.
    The space needed for the index files is considerable so it will take a bit to download them
  5. Having installed the AstroTortilla and CYGWIN_ANSVR and all the index files, you now need to change a setting in the AstroTortilla program to point to the proper location for  CYGWIN_ANSVR.


Enter the following in the AT setup window line 8  C:\Users\{user id}\AppData\Local\cygwin_ansvr\bin\bash.exe --login -c "%s" exactly placing your system login ID in the proper place {user id}.

The process now will change to:

  •  Initially aim the scope in the direction of your intended target. With guiding OFF
  • Turn on AstroTortilla and do a plate solve and sync of that part of the sky
  • Slew to your intended target which should be near the centre of the FOV. If need be center the object and sync the Scope to the planetarium program.
  • Turn on auto guiding if you are using it and commence with the imaging session.

If your scope is remote ( in the back yard  and you are controlling it from inside your house) all this can be done without going out to the telescope.

 An Excellent tutorial on how to use AstroTortilla can be found here .  You will note that  the instructions reference the CYGWIN program. this has been replaced by the CYGWIN_ANSVR

If you have question or looking for some answers regarding AT you can go to SourceForge at
You can peruse the site. If you want to ask a question you have to join SourceForge. No cost involved.

Take note that  images taken with narrow band filters may be difficult to solve as may images take at very high elevations.

(Last changed 10/04/2019)

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