Gemini Level 4

These directions apply specifically to the Gemini Level 4 controller. With PEMPro you can:

1. Turn PEC On/Off when connected.
2. Read and display the current and saved PEC tables.
3. Upload a PEC curve to the current or saved PEC tables.
4. Save a curve to a file.
5. Read a curve from a previously saved file.
6. Clear the mount's PEC.
7. Adjust the final curve before sending it to the mount.


1. When you connect to the mount the worm period and phase are automatically determined from the mount. PEMPro is automatically synchronized to the mount.

2. Whenever you start Acquiring PE data PEMPro automatically synchronizes to the mount. This allows you the freedom to move the mount around without worry of forgetting to re-synchronize.

3. Hovering over the PEC status text box will display a tool tip of the entire PEC status byte. Clicking the text box will update the PEC status.

4. On the Gemini tab the firmware version and guide rate are displayed.

Note: Some pictures on this page show "PEMPro v1.50" in the title, however these pictures still apply to v1.70 and later.

Basic Steps to use PEMPro with your Gemini Level 4 Mount

You must follow these basic steps to use PEMPro:

1. Use the mount wizard to create an instance of a Gemini Level 4 mount in PEMPro. You only need to do this step once.

2. Each time before acquiring periodic error data you should run the Calibration Wizard.

3. Acquire 5-6 worm cycles of periodic error data with PEC turned off.

4. Create a PE curve.

5. Adjust curve if needed and upload the curve directly to your Gemini

6. Check that PEC is reduced by turning PEC on and acquiring a couple worm cycles of data.

Each of these steps is explained in more detail below:

Setup Gemini mount with the Mount Wizard

The first thing you must do is to create an instance of a Gemini Mount in PEMPro. The easiest way to do that is with the Mount Wizard. Press F8 or go to the Wizards Menu and select Mount Wizard...

When the Mount Wizard comes up enter a unique name for your mount. When you exit the Mount Setup wizard the name you type will appear in drop down list box to the right of the Connect Scope button on PEMPro's setup tab.

Press Next> to continue. Select Gemini Level 4 in the drop down list in Step 2. Click Next> to continue.

The last step will automatically select the Gemini ASCOM driver so you just need to click Finish.

The next thing you should do is verify your ASCOM driver is setup is OK by clicking Edit Driver Settings. At the time of this writing the latest ASCOM Gemini driver is 4.2.21 and its setup page looks like this:

Run the Calibration Wizard before acquiring data

Once every session, before acquiring periodic data, you should run the Calibration Wizard. The Calibration Wizard sets all of the important parameters used by PEMPro to accurately calculate periodic error. Note that you don't need to run this wizard if you are analyzing data previously collected.

Some of the settings established include:

Before starting the Calibration Wizard you should center a 6th-8th magnitude star in the center of your CCD camera. The star should be just west of due South and at 0 degrees declination or higher if you are in the Northern hemisphere.

If you are in the Southern Hemisphere, the star should be just West of due North and 0 degrees declination or lower. In either case it is best to pick a star within 45 degrees of the celestial equator (in Northern hemisphere: 0 < declination < +45, in Southern Hemisphere: 0 > declination > -45).

Make sure that Adjust Data for Declination is checked in the Settings Menu. While you are at it also make sure that De-Rotate Data is also checked. If either is not checked then enable them.

Connect to your Gemini equipped telescope by pressing the Connect Scope button on PEMPro's setup page:

Select your camera control program and connect to your CCD camera by pressing the Connect Camera button. Also, make sure that Use Autoguider Port to Move Scope is unchecked.

To active the Calibration Wizard select its menu option:

If you are not familiar with the steps of the Calibration Wizard then you can find how to use it here.

Acquiring Data

Make sure that you are connected to the mount and the CCD camera as described above. You will know you are connected when the scope and camera drop down list boxes are grayed out and the buttons say Disconnect Scope and Disconnect Camera as shown here:

Also make sure that Gemini.Telescope appears in the text box next to Configure Scope/Mounts.

Important: Do not connect to the Gemini through an ASCOM hub such as POTH. Make sure that the text field to the left of the Configure Scopes/Mounts button says Gemini.Telescope. If not, press Configure Scopes/Mounts and change the ASCOM driver to be the native Gemini driver.

One other thing you will want to do is to make sure PEC is off. PEMPro has convenient buttons to turn the PEC on/off. If PEC is on then simply press the PEC OFF button.

Then click the Acquire Data tab and press the Start button. If you have Ask for Filename and Comments when Acquiring Data checked in the Settings menu, you will have an opportunity to change the default filename and add optional comments to the file.

Before PEMPro takes any images it automatically queries the mount for the current worm position and synchronizes the data that it is about to collect to the mount's worm phase. This means that you can freely move the mount before acquiring data without the risk of forgetting to synchronize PEMPro to the mount.

Now, just sit back and let PEMPro collect 5-6 cycles of periodic data. The horizontal axis of the graph is worm phase and the vertical axis is the measured PEC. After each worm cycle the color of the data will change.

Click Stop when you are done collecting data. If you have Ask for Filename and Comments when Acquiring Data checked in the Settings menu, you will have a final opportunity to add comments to the file or delete the log file and start over. If you don't delete the file you will be prompted to go directly into the create curve dialog:

Click Yes to continue.

Create a PE Curve

You can create a PE correction curve immediately after acquiring data or use the log file created in a previous session. PEMPro places all log files in the directory specified in the Logs and Images Dir field on the Setup tab of PEMPro's main window.

When the create PE Curve dialog opens it may take a few seconds to calculate an optimum curve. In most cases you won't need to do any modifications to the curve. Advanced users may wish to add or remove fundamental frequencies to see how that effects the curve's fit, the measure of which is the RMS Error in the upper right of the dialog.

Before leaving this dialog take a moment to check that the Worm Period, Image Roll, Declination, and Image Scale look correct. If anything looks wrong click Cancel and go back to the Setup and Calibration Wizard to setup the parameters again. Otherwise press Create PE Profile and Close if you are satisfied with the curve.

Adjust Curve and upload to the mount

After the Create PE Curve dialog closes in the previous step, PEMPro will automatically switch to the Gemini L4 tab and ask if you want to upload to the mount. If you click Yes the curve is immediately uploaded to the mount and you are done!

However, if you are an advanced user you may to click No so that you can first adjust the settings in the Curve Tweaking group. Controls on the Gemini L4 tab are explained below.

At the top of the graph information about the curve is displayed and dynamically updated if you use any of the controls in the Curve Tweaking section.

Peak to Peak PE: This is the total periodic error of the curve.

Curve Fit - RMS Error: This is a measure of how well the curve to be uploaded to the mount matches the optimized curve that PEMPro created. The lower this number the better.

Max Error: This is the maximum deviation at any point in the curve to be uploaded from the optimized curve that PEMPro created.

Check Corrected PEC

After you upload the new PE correction curve you will want to turn PEC On and acquire at least 2 worm cycles of data. If you see that the periodic error is worse than before you can go back to the Gemini L4 tab and check the Invert Curve checkbox and then click To Mount to send the curve to the mount again. Then start the acquire data again. The mount's periodic error should be significantly reduced.

If, after checking the corrected periodic error of the mount you find that you want to try for further improvement you can use the Refine feature.

Gemini L4 Tab features

VERY Important: The Gemini Level 4 PEC data is directly tied to the guide rate. If you program PEC to the mount then you use a different guide rate than what was in effect when PEC was programmed the PEC data will no longer be valid and will need to be reprogrammed (or guide rate returned to the original rate).

Below is the Gemini L4 tab before connecting to the mount. All the buttons and controls are disabled except the From File button and the Show Saved PEC checkbox. If you click From File you can load in a previously saved PEC curve. The table will then fill with entries from the file and a graph of the curve will display.

Once PEMPro is connected to the mount on the Setup tab, the From Mount and To Mount buttons are enabled. Also note that the firmware and guide rate are also displayed.

Once a curve has been created in the Create PE Curve dialog, the From Created PE Curve button and all of the controls in the Curve Tweaking section are enabled.

Here is a summary of all of the controls:

From Created PE Curve: loads a PE curve that has just been created or loaded.

A previously created curve (*.ppc) can be loaded through the Refine page. Just load a curve with the Curve 1 button then go to the Program Mount section and press Use Curve. You can then import the curve to the Gemini L4 tab by pressing the From Created PE Curve button:

From Mount: loads the current and saved PEC curves from the mount. You can toggle between the current and saved PEC curves with the Show Saved PEC checkbox.

To Mount: This will save the currently displayed PEC curve to the mount's Current PEC or Saved PEC table. Select the table and press OK to program the table or Cancel to quit without saving.

If you have a fast serial port you may want to set the Delay between writes to mount to 0 milliseconds. The default is 20 milliseconds but the range is 0-100 milliseconds. In general, if any error message pops up when writing to the mount then try increasing this delay value.

From File: will load a previously saved PEC file. Gemini PEC files have the extension ".gpe" and are simple ASCII text files.

To File: will save the currently displayed PEC data to a file. Gemini PEC files have the extension ".gpe" and are simple ASCII text files.

Show Saved PEC: if the From Mount button was pressed this will toggle the displayed curve between the Current Curve and the Saved Curve.

Clear PEC: this will clear the current PEC curve.

Sampling: this controls the sampling size that PEMPro uses to create the curve. The lower the value the finer the resolution but the more table entries that are created (longer upload/download times for PEC curves). The default is 10 steps but lowering to 1 might produce a slightly more accurate curve.

VERY Important (repeated from above): The Gemini Level 4 PEC data is directly tied to the guide rate. If you program PEC to the mount then you use a different guide rate than what was in effect when PEC was programmed the PEC data will no longer be valid and will need to be reprogrammed (or guide rate returned to the original rate).

GuideRate: this controls the guide rate that the PEC is calculated for. When the curve is uploaded to the mount the guide rate (and the PEC guide rate) are set to this rate automatically. In general the lower the guide rate the more accurate the PEC curve at the expense of a limited correction speed.

Phase: this allows you to slightly alter the phase of the curve (up to 5 seconds forward or backward). Usually this is set to 0 but slightly adjusting this value (usually to the negative) will compensate for any timing errors in the acquired data caused by camera download times.

Re-center: this will reset the Phase to 0.0. After adjusting Phase it is sometimes hard to reset it to 0. By the way when one of the track bar controls is selected the left and right arrow keys can be used to adjust the control one step at a time.

Invert Curve: Inverts the displayed curve. Use this to flip the curve if the periodic error of the mount is doubled (after correction) instead of reduced.