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.