MCCD Fast guider use

M. F. Waterson 1997 Aug 06

The MCCD fast guider was built so that it could operate on either a bright field/dark spot or a dark field/bright spot guiding object and was extensively lab tested by Buzz Graves and Herb Ryerson in both modes. However it is important to be mindful of the fact that they were using an artificial image that was very well defined in their tests; real life is seldom so nice.

Inner and outer loops

The fast guider sensor itself does not distinguish the image on the sensor, rather, when the loop is closed simply tries to drive the voltage difference between the opposite edges of each axis to zero by moving the fast mirror proportionate to the difference voltage multiplied by the gain pot setting. (Thus, if the gain is set to zero, the mirror will not be moved). At short intervals, the software executes a monitoring loop that samples the mirror drive voltage in each axis, and if it is close to full scale, drives the Coude mirror in a direction that is supposed to reduce the required drive, centering the image. The light level is also checked, and if the large error or low light persists for a (settable) time, the loop is opened automatically. One might ask how large the error had to be to start the Coude mirror drive; basically, if the X or Y meter is pegged and the gain is turned up even to 0.1 there will be a large enough drive signal to cause the coude system to try correct it.

Gradient compensation

Because almost every potential target will have an intensity gradient across the guide sensor, a feature called "Limb Darkening Correction" is built into the system. Despite the fancy title, this is nothing more than a command that measures the instantaneous error voltage at the preamp outputs, and injects an opposite DC level to (supposedly) cancel it out. If this is done on a quiet sun region near a spot on which you will attempt to guide, the (quiet sun) error will basically be dominated by the limb gradient and thus will allow you to effectively flatten the field at the spot. This useful feature can be used to help many other problems as well, but one needs to remember that all it is doing is measuring the error voltage (with no offset active), then setting a DC DAC voltage into the summing junction of the loop. It does not check to see if the correction was effective.

Bright or dark guide spot

The ability to guide on a "dark" or "bright" spot is provided by enabling the system to change the sign of the loop feedback by a software flag; this effectively reverses the sense of the mirror correction movement (rather then swapping the sensor "+" and "-", for example). This means that the sense of the error signal as seen on the meters will be "backwards", but that is seldom particularly confusing in operation. What can be confusing is the definition of "Bright" vs "dark"- it may seem as though the "invert loop gain" flag (image stabilizer screen #2 on the MCCD) is backwards to normal logic - just use the opposite if the obvious choice doesn't work!

Tricks, treats, traps

Finally, closing the loop depends on having a reasonable error voltage when the command is given; this system has many degrees of freedom and it is easy to find a non-linear combination (!). There are a few tricks I've found that make this more or less easier to achieve.

First, the system will guide on all the photons in the guide sensor, despite how ugly the image is. if you have vignetting, ghosting, etc., you need to think about how that will sum into the (square) guide sensor area; sometimes you can use the asymmetries to advantage. Also, if you have a marked intensity difference in one axis relative to the other (such as a ghost at the top of the field) you may have different optical gain in that axis; the gain pots should always be adjusted independently (see below)
Second, when to try to close the loop, you should have only a small error voltage for best results. Since the scale of the meters is quite expanded around the zero point, this is tricky, especially if you do have a substantial image gradient, however by careful movement of the beam-splitter ("Buzz mirror") you can roughly locate the electrical center of the sensor by observing the meter crossovers. While it is helpful to have the meters actually reading on-scale rather than pinned, this is not always possible to achieve in a reasonable time if you are trying to guide on a difficult object. In this case, true limb darkening is not likely to be relevant anyway, so I use the limb darkening function to help me out: I put the image as near the crossover as I can find and activate the limb darkening choice. (NOTE - it is a Good Idea to manually remove the limb darkening correction by setting the line to N if you need to make large adjustments in the image setup or position as the old offset can cause the guider tuning to be more difficult). The limb darkening function will attempt to remove the large static offset you have (from not being in the right place) with a much better precision than you can achieve with the Buzz mirror. If you have both meters on-scale (hopefully near zero!) when the function completes, you are all set and can go ahead and close the loops; it should be possible to adjust the gains as described below. If either meter is still off scale, remove the darkening by setting the line to N and try again. . . but maybe you will have to change some optical parameters (Zoom, e.g.) to get things in a more reasonable configuration.
Third, when you do close the loop, do so with the gain pots set to ZERO.ZERO so that the loop is effectively still open. This will let you slowly increase the loop gain while watching the spot and/or the meters; as you turn up the gain the spot will either stabilize or jump off to the side. If the meters peg and it jumps to a limit position in both axes, OPEN THE LOOP IMMEDIATELY! If you leave it closed, the Coude mirror will begin driving to try to center the image, but it will be heading in the wrong direction, and you'll have to manually drive it back before proceeding (which is a Really Good Reason to write down the position of your desired observation position referred to the coude encoders before you start this whole process!) Be careful you don't accidentally correct a Coude movement with the Buzz mirror--that only moves the image on the guider! If you have been reasonably careful about the center position and limb darkening, you probably have the polarity wrong, and should try reversing that choice first, going through the whole business again from the start.
If, when you turn the pots up a little (no higher than 0.3), the meter moves towards the center (or at least does not peg!) you are on the right track, and should try to adjust the loop gain further. Slowly, turn up the gain of each channel a little (no more than 1 turn at a time) iteratively, watching the behavior of the meter. You should see the meter stabilize near the center zero point, but you will soon reach a point beyond which there is no visible improvement; reduce the gain somewhat (to prevent oscillations) and go around to the optics bench to listen for a high-pitched, raspy whine. That noise is the mirror mechanically oscillating, and is a Bad Thing--reduce the gain, in both channels, until it goes away! It will blur the image badly, but you can't see it as it is faster than the video frame rate. I have found that things will work better than you visually realize using the minimum gain possible while still keeping the image quiet on the monitor. Gain is like that--while some is good, more is not necessarily better.
If it seems as if the spot is not guiding in the center of the detector, but you can't remove the offset, you can try to apply the limb correction with the loop closed. This is useful of you have a very diffuse spot, or are so near the limb that you can't find a good quiet-sun region to use. The loop may lose lock when the initial offset is removed for the measurement if it was a large offset, but I have generally had good luck when I tried it; because the loop is closed, the error voltage measured will be more stable that if it was open, so the compensation will come closer to nulling.
Finally, the Coude Mirror drive feature is there to allow the system to take out large drifts in the image position, and usually works well. However, if you are guiding at extreme zoom levels you may have to adjust the length of time that the mirror is driven to make its movement match the image scale of the guider (too long will drive it out of the guider field, too short and the loop will open because the error persists too long. The value is set on screen #2, at the bottom, in hundreds of a second; think about how fast the image moves on the guide video when you use the paddle to estimate a setting (< .1 sec is probably too short, 5 sec is probably too long). You'd like the mirror to move the image back towards the "center" by no more than 1/2 the sensor width at a time.