The Minor Planet Observer
and
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

MPO Connections – Guided Images

Guiding support in Connections extends to include any supported camera, including one working independently of the imaging camera. For example, one might use an FLI camera for imaging while using an SBIG camera for guiding through an off-axis guider or separate guide scope.

The current drivers for SBIG and FLI cameras allow running more than one camera at a time by storing a handle to each camera and setting the handle before commanding the camera to do something. While this adds flexibility to Connections in that it can support two cameras at once, it also increases the complexity of programming. More important, it does not circumvent the issue that, for parallel cameras at least, one cannot download an image from both cameras at the same time. Data corruption is almost guaranteed under such circumstances.

This problem is handled in Connections by having the imaging camera tell the guiding camera to suspend guiding when the imaging camera wants to download an image. Once the download is completed and appropriate, the guiding camera is allowed to continue. While this issue is not so important using USB, to keep operations as simple as possible, this process of suspending the guiding camera is used regardless of the type of connection the camera is using.

Guiding via Camera Relays or Telescope Motion Commands

MPO Connections can control guiding one of two ways. First by using the guider relays of the SBIG cameras or by simulating pushing buttons on a guide box for the telescope. This means you can use any supported camera for guiding as long as Connections can control the scope for guiding.

For the LX-200 GPS/RCX scopes, a new Meade protocol that allows "pulse guiding" is used. This allows control down to 1 millisecond precision. More so, it means that if you have an LX-200 GPS/RCX telescope that you can use any camera supported by Connections for guiding and that it can be external to the imaging camera itself. For example, in tests at the Palmer Divide Observatory, the internal guiding chip of an ST-9 was used to take guiding images but the camera relays were not used. Instead the pulse guiding commands were sent to the telescope. Image of up to 30-minutes showed excellent guiding.

How Guided Images Works

The general idea of a guided image is simple: while the imaging camera is taking a single image, the guiding camera is taking a series of images, measuring the position of the guide star on each image, computing the guiding correction required, and adjusting the scope position by minute amounts to keep the guide star in its original position on the frame.

Guiding Before, During, and After an Image

If taking a single image, Connections gets the guiding started by taking image with the guide camera and finding a suitable guide star. Once that’s done, a "subframe", a portion of a full-frame centered on the guide star, is found and shot. This subframe's much smaller size allows for fast downloads, something required for good guiding.

From there, Connections runs the guiding in a separate subprocess of the one that controls the entire job of taking an image (for programmers, guiding is done in a separate "thread"). After this thread has started, the imagining camera is told to open its shutter and begin taking an image.

Once the imaging camera exposure is complete, the guiding thread is "suspended" momentarily so that the imaging camera image can be downloaded with a minimum of delay and to avoid data corruption.

Guiding Between Images

If the exposure parameters are set so that a single command takes a series of guided images and if guiding between images is set - or the guider is operating independently, Connections continues guiding between images. This feature is available only if using an SBIG camera with internal/external guider or a guiding camera that is not behind the imaging camera shutter.

In the case with the SBIG guider, while the camera shutter is opened and closed for each exposure, the shutter is fairly tolerant of such frequent cycling. The same is not really the case for FLI shutters. Naturally, if the guiding camera is independent of the imaging camera or its getting its light via an off-axis relay that is before the imaging camera, there is no need to cycle the imaging camera shutter to keep guiding going.

Internal versus External Guiders (Independent Guiding)

As noted above, MPO Connections allows you to use the internal chip of an SBIG camera or any supported camera for guiding. When using an external camera, including the external guider of the SBIG LF cameras, guiding can be started independently of the imaging camera using Camera | Guiding Camera | Independent guiding from the main menu or using the Start/Stop Guiding commands in a script. As before, Connections will try to find a guide star and, if found, start taking a rapid series of subframe images and adjust telescope tracking. This ability to have an independent guider means, for example, that you could use a CCD camera to guide a telescope with a digital or film camera attached. See "Taking Guided Images" below for more details.

Internal Guiders and Independent Guiding

As the old expression goes, "never the twain shall meet." One of the common mistakes is to try to Independent guiding or the Start/StopGuiding commands to control an internal guider. Often, you'll end up with a blank image when the guider form is displayed. That's because the internal guider and full imaging chip of the SBIG cameras are both behind the same shutter and so the shutter must be worked according to the needs of the imaging camera (even though physically it and the guiding camera are the same).

Taking Guided Images

The way to take guided images, manually or within a script, is to first define an exposure parameters set using the Exposure Parameters form. With that, you tell the program that you want to take one or more guided images as part of a single "exposure" and set the guiding parameters such as the guiding exposure and whether or not to track between images if taking more than one image. As you can see by looking at the Exposure Parameters form, you can do much more than just tell Connections you want to guide. This is where the power of MPO Connections comes to the forefront.

Once you've defined an exposure parameters set, you can start taking the one or more guided images by clicking the start exposure speed button on the Connections camera control toolbar. In a script,  you would simply call the TakeImageEx command using the name of the exposure parameters set. It's that simple.

When to Use Independent Guiding

Just because you're using an independent (external) guider that doesn't mean you always need to use independent guiding. This feature was designed primarily for those cases where the imaging camera could not be controlled by MPO Connections – CCD, film, or digital. However, there are some other times when you might be able to use the feature even when the imaging camera is under control of MPO Connections.

Say you're shooting an asteroid. These objects move and so you might include steps in your script to periodically update the position of the telescope to keep the asteroid centered. This means the guide star used when you started the nightly run might drift out of the field of view of the guider at some point. Also, the attempt by MPO Connections to keep the guide star centered would be in conflict with repositioning the telescope.

In general, you would approach this problem by using a StopGuiding and StartGuiding in a script just before and after the repositioning GotoAsteroid command in a script. A quick example is shown below.

1. This step stops independent guiding. It doesn't hurt to call the command if guiding hasn't been started. However, it's very important that this command be here. The reason is that calling this command forces Connections to re-find the guide star when StartGuiding is called in a few moments.

2. GotoAsteroid sends the telescope to the asteroid. This command is going to be called periodically to keep the asteroid near the center of the imaging camera frame.

3. SyncLastPos assures the telescope is aimed where it should be. A quick image is taken with the imaging camera and AutoMatched to the chart centered on the last GotoAsteroid position. If necessary, the scope's aim is adjusted by up to 15 arcminutes to make sure the target is centered as best as possible.

4. StartGuiding starts the independent guiding, which includes finding the guide star. Since the telescope may have moved some distance to keep the asteroid centered, there would be no guarantee that the guide star was still in the field of view. The earlier call to StopGuiding and then this to StartGuiding helps assure that Connections will have a guide star even after the telescope is re-aimed.

5. SetFrameMode forces the imaging camera back to the mode of taking a single dark and then merging it automatically to every light image until the command is encountered again.

6. SetResolution forces the imaging camera to 2x2 binning. This and the SetFrameMode command should always be called immediately after a SyncLastPos since that command forces the camera to DKLTKeep and 2x2 binning. Even though that happens to match the revised settings, the exposure time of the sync image is almost certainly different, and so you want to force Connections to shoot another dark of the proper exposure. Of course, you could just take light frames and merge a master dark afterwards but before you measure the images. You would still want to include these two Set commands. If you get in the habit of doing it, you'll never be wrong.

7. ResetLoop tells Connections to repeat steps 8-11.

8. JumpAfterEvent is the means to break out of the loop when a certain condition is met, usually when the target is 30° high in the west or morning twilight has started. When the condition is met, the script will jump to Step 12. The reason this command is inside the loop is to make sure that Connections doesn't keep shooting well after the intended time. If you checked only once, just before the loop, then the script could go well past the time it should stop.

9. TakeImage takes a single image of 45s, gives it a base name of A696 (the filter and auto incrementing number are appended), and the file is stored in C:\MPO\IMAGES\A696\20050918.

10. The script pauses for 60 seconds

11. The script checks how many times the loop of steps 8 through 10 have been executed. If the count is 10, the script returns to Step 1, which stops the independent guiding. Steps 1 through 11 are repeated until the JumpAfterEvent condition in Step 8 is met. Then the script jumps to Step 12.

12. ShutdownCamera turns off temperature regulation, waiting 3 minutes, before going to the next step.

13. SendToHome sends the telescope to its park position. For many scopes, this means that tracking stops and so the scope stays in that position. Classic LX-200 scopes do not have a Park feature per se. However, you can issue a command as part of the Connections SendToHome command that puts the classic in Land mode, meaning the motors do stop. You'd have to be sure to reset the scope to Polar mode when you power up the scope again.

Download the MPO Connections manual (PDF)