Guiding with the SBIG AO-7 or AO-L

There are three types of guiding available to CCDSoft users:

• Autoguiding with a separate camera (in a guidescope or using an off-axis splitter)

• Self-guiding with a guide chip built into the camera (for example, SBIG ST-7/8/9/10)

• Autoguiding with adaptive optics (that is, SBIG AO-7 or SBIG AO-L devices)

The first two methods work similarly: the guide corrections are calculated and CCDSoft makes corrections by moving the mount. This section assumes you are familiar with conventional (mount-based) guiding, and that you understand the concepts covered in chapter 4 of the CCDSoft documentation.

With the AO-7/L, an internal mirror tilts rapidly to make the guide corrections.

Quick Start

If you are already familiar with CCDSoft and the AO-7/L, follow these steps to start guiding with the AO-7/L. This section assumes that you are familiar with conventional guiding and with chapter 4 of the CCDSoft documentation. For more details, please read the following section Autoguiding with the AO-7/L. The AO-7/L must be connected to the camera and all cabling in place between the computer, camera, and AO-7 before you start.

Camera Control Panel, Setup tab

1. Click Camera | Setup and then choose SBIG with AO… as your imager camera. The correct autoguider selection will be made for you automatically when you choose your imager (Imager’s built-in Autoguider).

Camera Control Panel, Autoguide tab

1. Perform a drive calibration with the autoguider as you normally would when autoguiding without the AO. You do not have to uncheck AO Enabled. To access drive calibration, click the Calibrate button on the Camera Control panel, then click the Calibrate drive radio button.    Even if you calibrated your drive with your CCD camera or autoguider without the AO-7/L, you must recalibrate. The existing settings are no longer valid once you connect the AO-7/L.

2. Click the AO Settings tab. Click Use Defaults to set the default values for the parameters. For fine-tuning the device, see the section AO-7/L Settings below. Click OK.

3. Click Take Image to get an image using the guide chip. Adjust telescope pointing and exposure time until you have a suitable star for calibration. A good calibration star must be the brightest star in the field of view, and there must not be similar bright stars nearby that will move into the field of view during calibration. You can use TheSky to locate suitable calibration stars.

4. 5. Click Calibrate…. Click Calibrate AO. Verify that the AO deflection… setting is 100%. Click Calibrate. The AO-7/L will move its mirror, take exposures of the calibration star, and measure the movement in each direction.

5. Click Take Image to download an image using the guide chip. Adjust telescope pointing and exposure time until you have a suitable guide star. See chapter 4 of the CCDSoft User's Guide, for information about what makes a star suitable for guiding.

6. Click a star in the downloaded image to select it as the guide star, or click the Auto button to have CCDSoft select it for you.

7. Click Autoguide to start autoguiding. You can monitor progress using the information on the Autoguide tab, including X/Y tilt, Wander, etc. See following sections for details about these parameters.

Autoguiding with the AO-7/L

Once the AO-7/L is installed and connected according to the instructions from SBIG, you must do the following before you can start using the AO-7/L:

• Make the correct camera choice on the Setup tab of the Camera Control Panel

• Verify that the AO-7/L is enabled on the Autoguide tab

• Calibrate the AO-7/L and your mount’s drive.

Figure 1 shows the correct camera choice for using the AO-7/L: SBIG with AO (ST-7/7E/8/8E…). This setting will work with any of the listed SBIG cameras. If you choose any of the individual cameras instead, the AO-7/L will not be active or available.

Figure 1: Choosing the camera type for AO operation.

If you have been using another autoguider, take a moment to verify that the Autoguider selection is correct. Click the Autoguider radio button (at top right of the Setup tab). Make sure that Imager’s built-in autoguider is the selected camera (see figure 2). Click the Imager radio button, and then click Connect.

Figure 2: Selecting the Autoguider.

AO-7/L Settings

To access CCDSoft’s AO-7/L features, click the Autoguide tab to make it active (see figure 3).

Figure 3: The Autoguide tab.

The most frequently used AO settings are found at the lower left of the Autoguider tab. Check AO enabled to turn on the AO-7/L for guiding. Unlike most other features of CCD Soft, AO enabled (and Fan on) are real-time controls. This allows you to turn AO guiding and the camera fan on and off as appropriate. The following list explains all of the features found in this area.

Mount (Drive) Calibration

When guiding with the AO-7/L, there are actually two types of guiding going on. The AO-7/L tips and tilts its mirror to make rapid, precise guide corrections. Over time the guide star may drift and the AO will compensate for this drift as it guides. If the star’s drift is large enough, the mirror will have a larger tilt. When the tilt reaches a certain value, CCDSoft will move the mount using the camera relays. This puts the AO mirror closer to the center of its range of motion.

These two types of guiding (mirror tip/tilt and camera relays) are the reason that you must have both the mount’s drive and the AO-7/L calibrated before you can use the AO-7/L.

CCDSoft with AO-7/L support has a separate dialog box for Autoguide settings (see figure 4). The entries on the dialog box are similar to previous versions, but the Simultaneous X- and Y-axis relay activation setting is new. Some mounts will not guide properly with simultaneous XY corrections; contact your mount manufacturer for guidance if you have problems with simultaneous XY setting checked. The new layout puts tabs at the top of the dialog for quick navigation to both mount (drive) and AO-7/L settings and results.

Figure 4: The new Autoguide Setting dialog, showing the Drive Settings (mount) tab. (See also figure 5.)

For more details on mount calibration for autoguiding, please see section “Mount Calibration” in chapter 4.

Additional AO Settings

Figure 3 showed the basic AO settings available on the Autoguide tab. In most cases, you will not need to adjust any other settings to get good results with the AO-7/L. However, additional AO parameters are available on the Autoguide Settings dialog’s AO Settings tab (see figure 5). You can use these settings to refine the accuracy and performance of the AO-7/L. To access this dialog box, click Settings on the Camera Control Panel’s Autoguide tab (refer to figure 3).

Autoguide Settings dialog box

Figure 5: The AO Settings tab on the Autoguide Settings tab.

The Tracking Options section of the AO Settings tab contains the following settings:

The AO Exercise section allows you to test the movement of the AO-7/L mirror. You should remove the AO-7/L from the telescope when testing and observe whether the motion actually occurs. The AO-7/L will make a high-pitched sound while it is being exercised.

The AO Exercise section contains the following settings:

When you are done making changes to the AO Settings tab, click OK or Apply to save them, or Cancel.

AO-7/L Calibration

During drive calibration, CCDSoft takes an image, moves the mount in a given direction, and then takes another image. The difference in position of the calibration star between the two images tells CCDSoft how fast the mount moves in each of the four directions. Once calibrated, CCDSoft can issue guide corrections that are as accurate as the mount will allow. Various factors, such as backlash and rough gear surfaces, can reduce the accuracy of mount-based guiding.

With the AO-7/L, corrections are made by tipping and tilting a small, lightweight mirror. There are no gears to worry about, and the mass of the mirror is tiny compared to the mass of a mount. In addition to its adaptive optics capabilities, the AO-7/L provides very accurate guiding and can compensate, up to a point, for an inaccurate mount. The mount must be accurate enough to move via the camera relays. You can use a larger track box (see preceding section) to compensate for some of the mount’s inaccuracies - a larger track box makes it less likely that a mount error will push the star out of the track box.

As with any calibration, the key to success is finding a suitable star for calibration. A good calibration star must be the brightest star in the field of view, and there must not be similar bright stars nearby that will move into the field of view during calibration. You can use TheSky to locate suitable calibration stars. For more information about selecting stars for calibration, please see the section Choosing a Good Guide Star for Calibration in chapter 4 of the CCDSoft documentation.

Adjust the telescope position to put the calibration star roughly in the center of the image. This will prevent it from moving out of the field of view during calibration. Then click Calibration on the Autoguider tab of the Camera Control Panel. This displays the dialog box shown in figure 6.

Figure 6: Calibrating the AO-7/L.

Click the Calibrate AO radio button to active it. For most applications, use 100% as the AO deflection while calibrating.  You can reduce this number to see if it improves the perpendicularity of calibration. Use the AO Calibration Results tab of the Autoguide Settings dialog box to evaluate AO calibration (see figure 7).

Figure 7: AO Calibration results.

Note that the X/Y axes of the calibration results (red arrows) are not necessarily square to the reference X/Y axes. This is normal. Even if the camera and AO-7/L are installed exactly orthogonal to the mount’s axes, the vector showing AO movement will typically be non-orthogonal because of how the mirror moves. CCDSoft takes this into account and will guide accurately with the AO-7/L.

Autoguiding

Once you have calibrated your mount’s drive and the AO-7/L, you can use the AO-7/L for guiding. Start by clicking Camera | Take Image | Take Image.

As with calibration, the key to success is finding a suitable guide star. The guide star should be away from the edges of the chip, and if you are using color filters it should be bright enough to remain visible through all filters. The brighter the guide star, the faster the guide rate you can use. Since the AO-7/L works best with the brightest available guide star, it’s usually worthwhile to find the brightest one you can.

You can use TheSky to locate suitable guide stars. You can adjust the framing of your main image, or rotate the camera, to find the best available guide star. For more information about selecting stars for guiding, please see the section “Select a Guide Star” in chapter 4.

Adjust the telescope position to put the guide star at least 10 pixels away from the edges of the guide window (5 pixels in 2x2 mode). This will make it less likely that the guide star will move off of the guide chip during guiding. You can adjust the guide star’s position using the mount’s hand paddle; TheSky; or using the NSEW buttons on the Autoguide tab. The NSEW buttons will move the telescope at the current guide rate using the camera relays, and you can use them to make very small adjustments to position.

Before you start autoguiding, you can make final adjustments to guide parameters. Settings for Aggressiveness and Slew rate can be made right on the Autoguide tab. You may also need to adjust the Track box size to adjust for seeing conditions. The better your seeing conditions, the smaller the track box you can use. Figure 8 shows the appearance of the three track box sizes. Note that different zoom levels are used for the track boxes, so the track box stays the same size, but the relative size of the star changes. From left to right, they are Small, Medium, and Large. The Small track box requires very good seeing (steady skies), or the star will move out of the track box. The Medium setting is good for more typical seeing conditions, and the Large track box allows you to use the AO-7/L on nights when the seeing is below average.

Figure 8: Different sizes of track box. From left: Small, Medium, and Large.

Once the guide star is properly positioned, click Autoguide to begin guiding. CCDSoft takes multiple dark frames and averages them to reduce noise in the dark frame. This master dark is subtracted from all light frames to provide as clean an image as possible during guiding. If your exposure time is long (for example, 5 seconds), it will take longer to capture the multiple dark frames before the start of guiding. A 5-second exposure will not give you any benefits of the adaptive optics, but could be used instead of conventional guiding if your mount doesn’t handle guide corrections effectively.

During guiding, you can monitor the effectiveness of guiding by watching the values in the AO Enabled section of the Autoguide tab. X and Y tilt values show the current tilt of the AO-7/L mirror. These values should change over time as the mirror tilts to accommodate drift. When tilt reaches the maximum value allowed (default is 50%), CCDSoft will bump the mount and the tilt should go down. The amount it will go down depends on the aggressiveness setting of the mount. With low aggressiveness settings on the mount, smaller bumps will occur, and the bumps will therefore be more frequent.

Wander indicates how accurately the AO-7/L is guiding. If the AO-7/L aggressiveness is too low, wander will be high because the AO-7/L will under-correct. If the AO-7/L aggressiveness is too high, over-correction may occur and this will also increase the Wander value. Adjust Aggressiveness and Slew rate to obtain the optimal Wander value.

Note: Seeing conditions also affect Wander. Compare the Wander value with and without AO-7/L guiding so that you know the contribution to Wander from seeing.

You can also use First count and Count to monitor guiding. If the Count value drops, you may have clouds moving in or a focus shift. Focus can shift from changes in temperature, or from a physical slippage of the focuser.

The Guide rate may be lower than you expect, depending on the processing speed of your computer. For example, if you are using an exposure of 0.02 seconds, you might expect a guide rate of 50Hz (50 exposures per second). However, it takes time to download and display images, and it takes additional time to perform calculations on your computer. This time is added to each exposure, resulting in the actual guide rate. In this example, the guide rate might be anywhere from 6 to 40Hz depending on your equipment.

Autoguiding will automatically suspend during downloads from the main imaging chip. Autoguiding will resume when the download is complete. If you are using a Small or Medium track box, the guide star may have drifted out of the field of view during the download. Cameras with USB download are much faster, and it is more likely that you will retain your guide star using a USB-equipped camera. An extremely accurate polar alignment will also help you retain your guide star following a download.

If the guide star is lost, or if a different guide star is found, you can Abort the guiding and restart it with the desired star. Re-center the mirror if tilt is large. Click Take Image to get a new image using the guide star, click to select the guide star, and then click Autoguide to begin again. If you are taking a sequence of images, you may need to monitor the downloaded images and the guide star window to determine if the guide star is still available.

Note: If stars in the downloaded images have one or more small “ears” or are elongated, this could indicate over-correction. Reduce the aggressiveness setting and try again. A too-high mount aggressiveness setting can also cause this error.