Using a Rotator

Don't miss the Tips On Using a Rotator section! For a detailed engineering analysis of rotated guiding, see the paper Rotated Guiding of Astronomical Telescopes.

An image rotator can provide additional flexibility in guide star selection for internal/off-axis guiders and in composition of astrophotographs. When connected to a Rotator, ACP requires a third coordinate for all targets, the Position Angle. This is the rotation angle of the main imager against the sky. In other words, when ACP is using a rotator it becomes a three-axis system: RA, Dec, and PA. The only practical way to set up, or compose, images for rotated guiding is to use a field of view indicator (FOVI). This allows you to graphically determine all three coordinates.

ACP's observing plan format supports the #POSANG directive, which tells ACP to rotate the imager to the given position angle (PA) before acquiring the image. ACP's automatic meridian flip and guiding features manage the rotator as needed to produce images at the requested PA. If the imager is misaligned with respect to the rotator itself, ACP's imaging scripts will detect that difference after the first pointing update, and apply the necessary correction. Thus, the requested PA in the POSANG directive will be the true equatorial PA of each image regardless of GEM meridian flip state or rotator-imager offset.

Rotator control is provided via ACP's built-in rotator control logic. It provides the connection between ACP's observing logic and the rotator, via an ASCOM Rotator Driver. You must have the driver for your rotator installed. You can get drivers for the Optec Pyxis and the Astrodon TAKometer from the ASCOM web site, Rotator Downloads page. The RC Optical control system includes the RCOS rotator driver and must be updated to the latest version. Note that you do not need to run the RCOS TCC program, the ACP Rotator Controller serves the same purpose, using the RCOS rotator driver for control.

Setting Up The Rotator Controller

1. Make sure your camera and rotator are physically positioned so that 0 degrees PA is close to north-up. It does not have to be exact, ACP measures any error and applies corrections as needed.
2. In ACP's Rotator menu, click Setup... The ASCOM Rotator Chooser will appear. Select your rotator type.
3. With the Chooser still open, click Properties... The settings window for your rotator type will appear.
4. Make the settings needed by your rotator type such as the COM port.
5. Close the rotator settings window and the Chooser. You should see your rotator type displayed above the Setup button.
6. Power-up your rotator and wait until it has finished homing, if needed.
7. In ACP's Rotator menu, click Connect. The current position angle should show as Mechanical Angle: nnn. If you receive an error message, check the serial connections, cable, rotator settings, and make sure power is applied to the rotator. Don't worry about the Sky Position Angle value, it will become active after you start observing.
8. Enter a PA value next to the Set Angle button (in the Rotator section of ACP's main window) and click the button. Verify that your rotator turns to the newly selected mechanical angle.

Guiding Configuration Requirements

In ACP's Preferences, Guiding tab, there are three settings that are critical to rotated guiding success:

1. For Guide sensor configuration, select Internal (SBIG) or Off-Axis Pickoff, depending on your configuration. Obviously, External is not applicable to a rotated guiding setup!
2. You need to know the guide sensor angle (see Determining the Guide Sensor Angle below). Enter that value into the Guide sensor rotation angle field.
3. If your guider is an adaptive optics unit (SBIG, Starlight Xpress, or Orion) enable the Guider is an adaptive optics option (checkbox).

Routine Use

Once you have set up ACP's rotator control, ACP will manage your imager's position angle (PA) automatically in all phases of operation. If you have a German equatorial mount, ACP will automatically compensate for meridian flip. If there is a misalignment of the imager and/or rotator so that 0 degrees PA on the rotator is not really north up, ACP will correct for that as well (via plate solution and rotation angle extraction).

In your planetarium, rotate the FOVI and slide the sky around to compose your image and put a suitable guide star on the guide sensor. Read the J2000 RA, J2000 Dec, and the Position Angle (from the FOVI) and enter these three coordinates into the ACP web UI form. If you are using ACP Planner with TheSky X or Starry Night, you can automatically capture the three coordinates into a new target in Planner. In either case, that's all there is to it!

Assuming your FOVI is correctly scaled, the number one source of error is not using the J2000 coordinates of the FOVI center. ACP always uses J2000. If you are using TheSky X, you can set it up to display the J2000 coordinates of the screen center (the FOVI center as well) in the lower left in the status bar. In its Tools menu, Preferences..., select the Status Windows icon at the top, select Status Bar in the Status window list, then move the Chart Center RA and Dec out of the status window report, and replace with Chart Center RA (2000) and Dec (2000). The status bar should look like this:

This is also covered in our video Configuring [TheSky X] and Mouse Wheel Time Control.

If you're using the web interface and still have TheSky 6, note that TheSky 6 cannot display the J2000 coordinates of the FOVI center so you must use the little TheSky Quick Capture applet (part of ACP Planner) to get the J2000 RA/Dec and the PA and enter those numbers into the web form.

Tips on Using a Rotator

Thanks to Jim McMillan for this info.

• If using an SBIG camera and internal guide chip or external guide head on an off-axis guider: don't worry at all about the rotator orientation with respect to the OTA or the camera orientation with respect to the rotator. It does not matter what orientation the rotator/camera is in when you do your guider calibration. MaxIm is smart enough to guide properly as long as it knows what the actual camera orientation is, and ACP is smart enough to determine and provide that to MaxIm. This is really huge! It means you will never need to redo your guider calibration again when changing camera orientations for a mount flip or different target. We should thank both Doug (MaxIm) and Bob (ACP) for making their software really smart.

  Two side notes:

  1. If you do both manual and automated imaging (via ACP), it is easier to figure out the image orientation manually if you have North up for both the rotator and camera. But, it is not necessary to do it that way for automated operations with ACP.
  2. If you are using a rotator on the main imager and guide with a separate guide scope, MaxIm determines the camera angle for your guider when you calibrate it. In this case, the orientation of the separate guide camera does matter (as it always has.)
• Use ACP Planner in conjunction with Starry Night Pro or TheSky to develop your Plans. It is really easy to use and was developed specifically to facilitate this task. Or, at the very least, use it to determine the FOV/rotation angle for each target if you develop your Plans manually.
• Assuming you are using an internal or off-axis guider, when you first start out, adjust the FOV/rotation angle for each target such that the guide star falls on the center of the chip - at least until you get a feel for how accurate your mount/rotator perform. Throughout my testing, I found my setup to be accurate enough to put the guide star exactly where I wanted it, including after a mount flip - a tribute to both the hardware and software.
• Take the time to determine the faintest magnitude guide star that works for your filters (assuming you are using the internal guider). You need to know this when developing your Plans. If you don't know what the faintest magnitude is for each filter, you create the opportunity for unreliable unattended operations due to guiding failures. With my setup, I found I can successfully guide through my Blue filter (my "darkest" one) on a Mag 11.4 guide star.

Determining the Guide Sensor Angle Manually

 You should not do this manually under normal corcumstances. Use the CalibrateGuider script to take care of it the easy way.

ACP supports guider sensors with the guide sensor rotated with respect to the main imaging sensor. The "normal" configuration is with the guide sensor aligned with the main sensor. To confirm that this is the case, check that small motions of the scope result in the same left right motions on guide and main sensor. If this is the case, your guider angle is zero.

 The SBIG ST-X and ST-2000/4000 have their internal guide sensor rotated 90 degrees with respect to the main sensor. For these camera systems, set the Sensor Angle to 90.

1. If you have an AO unit, you must temporarily switch to conventional guiding for this engineering test.
2. Create an ACP plan for a target east of the meridian that will put a guide star on the guider at PA=0.
3. Run it via the ACP console (not the web) until ACP tries to start the guider.
4. Abort the plan. Since it was run from the ACP console, tracking will be left on and the guide star will remain on the guide sensor. You will see the TRACK light flashing, click it to keep the tracking on indefinitely.
5. Calibrate the guider (why are you doing this manually?)
6. Note the guiding angle. This is located in MaxIm 5 under the CCD Control, Guide Tab, Settings button, Settings tab, Manual Calibration area, Angle (deg).
7. If you have an AO, you can now switch back to AO guiding.

The guiding angle thus found is the number to enter in the Rotator Controller Sensor Angle field in ACP. Note that the guiding angle is automatically updated by the CalibrateGuider script.