Gemini Model Builder v1.1.5 beta         

Gemini Model Builder is a software utility to aid in building a comprehensive Gemini pointing model. It works fairly automatically, requires few settings, and can produce excellent pointing results if used to map points throughout the sky region you will be using.

Disclaimer and Copyright  - PLEASE READ!

Copyright 2006 by Paul Kanevsky. All Rights Reserved.

GMB  is provided free of charge for all non-commercial use. Permission is given to distribute GMB in its original, un-modified form and only free of charge. The author accepts no responsibility for direct or consequential damage caused by the use of this software: use it at your own risk!

GMB is provided as-is, and although I will attempt to make changes and fixes as they become necessary, I provide no guarantees about its suitability for any purpose whatsoever.

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Building a model automatically

With a reasonably well polar-aligned mount, GMB can be used to build a complete model, from beginning to end. GMB performs the following sequence:

Building a model manually

There's also an option for helping to build a model manually: you move the mount throughout the sky to whatever position you want (doesn't have to have a bright star in the center), then press Add'l Align button to tell GMB to compute the correct position and to perform a Gemini Additional Align function. You can repeat this as many times as you'd like, and let GMB do the computations and perform the Additional Aligns for you.

You can also Sync manually on the current position. To do so, simply click One Sync button: GMB will take an image, do a plate solve, and then execute a Gemini Sync function to synchronize with the exact mount position. Again, any position in the sky will work: it doesn't have to have a bright star anywhere in the shot, and certainly not at the center.

Starting with version 1.1.5, GMB also gives you more manual control over what points in the sky are used for alignment. This makes it easy to avoid obstructions and to cover areas of the sky you'll be using more frequently. Add as many points as you want, then save the coordinates into a file that can be re-used many times in the future to build and to refine new Gemini models.

Testing Gemini model

Once a model is built (even if you didn't use GMB to do it) you may be interested in testing how good the model really is. To do this, simply click on the Test button. GMB will perform the same set of steps as it would when building a model, except that no Sync's or Additional Aligns will be executed. At each new location, GMB will take an image and compute the difference between where Gemini thinks it's pointing and where the telescope is really pointing to. A running total of this error as RMS (root-mean-square) and Peak values will be displayed with each step. At the end of the run, the error values will tell you how good the model did overall. On the average, models I've built with GMB have performed with accuracy of less than one arcminute RMS and peak error! This means that each Goto resulted in placing the desired object within 1 arcminute of the center of the image -- pretty good performance!

Again, you can let GMB pick the points to test automatically, or define a list of points that cover your visible sky: you're in control!

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Gemini Model Builder requires the following:

WARNING: Please check your safety limits settings, as GMB can push the mount to the limits, based on your settings. If the safety limits are not set properly, you can damage your equipment, so be careful! I also suggest that, at least in the beginning, you watch the mount while GMB is building the model. When it gets close to one of the safety limits, be ready to stop Gemini by pressing one of the direction keys to interrupt motion: better safe than sorry!

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Please download and install all the required components. As the last step, download and install Gemini Model Builder application. To run GMB, start it from the Windows Start menu.

Double check that CCDSoft is able to talk to TheSky by capturing a 10-20 second image of a region of the sky with at least 6 bright stars in it (more is better), and using CCDSoft's Research Incsert WCS/AutoAstrometry menu command. When prompted, enter image scale of your optical setup in arcseconds per pixel. This is the same image scale you will need to provide to GMB. If the plate solution is successful, you are ready to start using GMB. If not, please see the Troubleshooting section at the end of this document.

Check that the correct Gemini driver is selected under the GMB Telescope/Choose option. TheSky can be set to use either ASCOM Gemini driver or its native Gemini driver.

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Using GMB

Center a bright star on the CCD chip, and select Synchronize from the Gemini Align menu ( the way you'd normally do it when starting to build a model -- the star needs to be approximately in the center, this doesn't need to be precise). Use a star towards the East or North East horizon to start with.

Alternatively, predefine a set of map points you'll want to use in building the model by slewing the telescope to all the positions where you'd like to do an additional align, and pressing the Add Point button. GMB will record the coordinates for each of the points, and will use just these points to do the model building.

Make sure that no other software using Gemini serial connection is running, other than TheSky and GMB! GMB will manage the connection to Gemini, but this connection cannot be shared with any other software. Other software packages connected to Gemini driver through POTH should also be closed, or disconnected from POTH before running GMB.

Now start GMB. It will display some initial settings. Please verify that they are valid for your setup:

Map Points selects a predefined list of points to be used for model building. You must construct the list of points prior to doing the alignment. See below
Zig-Zag automatically selects the next mapping point based the parameters you specify, within the limits you select. See below

Initial Move in RA

first additional align should be done on widely separated position in RA. This is how far the first step in RA will be to accommodate this requirement.

Move Size in RA

step size that will be used to move the mount each time in RA after the first additional align

Move Size in DEC

how far the mount will be moved in DEC for each step

RA MAx Range

how far the mount will travel in the chosen direction from the starting point until it reverses direction. This sets the limit to how far in RA the mount will travel. Be careful to set a limit that will not take the mount below the horizon

DEC Max Range

how far the mount will travel in DEC before reversing direction. Initial motion direction is always North. Note that the motion will be 1/2 of DEC Max Range North from the starting position, and 1/2 to the South.

Start Motion in RA

set this to the initial direction of motion. If the mount is pointing East, set this to West, if pointint West, set it to East. The mount will move RA Max Range degrees in this direction, and then reverse to go back to the starting position


Number of points to map: the first point should always be a Sync, the rest will be Additional Aligns. For example, Points=15 will perform one Sync and 14 additional aligns before stopping

Choose Telescope

Allows you to pick a different ASCOM driver to talk to Gemini. Note that GMB uses a custom ASCOM interface created by Ajai Sehgal to execute native Gemini commands, and so using a driver not written by Ajai may not allow GMB to talk to Gemini correctly.      Currently, Ajai's is the only ASCOM Gemini driver out there, so this should not be a problem.

Camera & Plate Solution Setup

Camera control software selection, auto-dark or not, and plate solution settings.

    Pixel Scale

It is important to set this as close as possible to the correct, UNBINNED image scale of your setup. If you don't know what it is, you can either try to do a plate solve in TheSky to compute it, or get an approximate value from this equation:

imageScale = pixelSize * 206.2648 / focalLength

where focalLength is the effective focal length expressed in millimeters, pixelSize is the size of the CCD pixel in microns, and imageScale is the image scale in arcseconds/pixel.


The bin mode to use when taking images. I get best plate solution results with 2x2 binning, but this will depend on the size of your chip, and image scale

    Exposure Time

Ideally long enough to get a number of well exposed stars anywhere in the sky. I find that about 10 seconds works well for me. Longer exposures may be necessary with longer focal length or smaller chip, and shorter for really large chips with short focal length telescopes.


Number of seconds to wait after a slew before taking the next image. This gives the mount sufficient time to dampen vibrations, brake the motors, and finish backlash-related motion. If you find that the stars are trailed in the GMB exposures, try to increase this value.

To test that all the software connections all work in GMB, click on the One Sync button: GMB will take an image, do a plate solve, and try to execute a Gemini Sync command. If there's anything not set up properly, you will see an error message to that effect.

Now, that everything is working, select Zig-Zag option under Movement Settings, and click on START button, and off GMB will go to map the sky! Should you want to stop the run, simply click on the STOP button.

Watch the bottom right status window: it will be reporting the updated Azimuth and Elevation errors: these indicate the polar alignment errors as determined by the current model. These can start to converge within 3-5 points, but sometimes will not stabilize completely until approximately 20 or more points. It's up to you if you want to continue to keep building the model past 20 points: one possibility is to run the GMB Test and see how good the pointing really is. If it's close enough for what you are trying to do, then it's time to stop and do some observing!

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Movement Settings: Zig-Zag

Zig-Zag is the fully-automated model building option in GMB: GMB picks the points to do the alignment based on settings you specify. Once the mount reaches the limits specified in GMB configuration, the mount will reverse direction, and continue doing the alignment until all points are mapped. The mount is moving in RA, and DEC simultaneously at each step, by the amount you specify.

Movement Settings: Mapping Points

Mapping Points option provides for much more control over where in the sky the mount is moved to, and what portions of the sky are used to build the model. There's a little more work required on your part prior to building a model: you must select the points for alignment. Once selected, the list of points can be saved and reused as many times as you wish: the coordinates are saved relative to the Alt/Az coordinate system, and so will not change with time.

To add a new mapping point, slew the telescope to the desired next location, and click the Add Point button. You'll see the Alt/Az coordinates of the current location added to the list of points. Move the telescope a little more, and click on Add Point again. Continue to add points to cover as much of the sky as you'd like, include stars on both sides of the meridian, if desired. Make sure there are 5 or more points on one side of the meridian before picking a point on the other side.

When enough points are selected, click on Save... button and pick a file name where the points list will be saved. Next time you use GMB, simply click on Load... button, and pick the file you had saved earlier: you'll see all the mapping points in the list, and can start model building immediately.

Should you add a point that's too low to the horizon, or one that's obstructed or too close to the meridian, simply select the point in the list, and click the Delete button to remove it. Don't forget to save the list after making changes.

Once the list is created or loaded, simply click on START button to begin the alignment process. GMB will slew to points in the order listed. If you specify more mapping points than there are points in the list, GMB will complete the last point in the list, and will then start from the beginning of the list.

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Download GMB

Current version 1.1.5 beta of GMB is available to download as a WinZip file:

Click here to download GMB  v1.1.5 beta

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Release History

Release Date Version Number


06/17/2006 1.1.5b - Added a help file to the installation (contents of this page)
6/16/2006 1.1.5b - Added the ability to predefine mapping points
6/12/2006 1.1.3b - Added support for MaxIm DL
- Optional Auto-Dark setting


- Fixed ASCOM chooser dialog


- Now works with TheSky 6 Pro,
- checks and ignores poor plate solves,
- running log added
- fixed wrap-around error in TEST run error display


Initial beta release

Problems, suggestions, comments

If you're having problems using GMB or have comments or suggestions for improving it  drop me a line here.


If you like GMB, and would like to see similar software releases in the future, please feel free to encourage me with a PayPal donation. The amount is whatever you feel the software is worth to you:

Gemini Model Builder
Copyright 2006 by Paul Kanevsky. All rights reserved.
Revised: June 17, 2006 .