SFn8CieR Can you explain this in laymans terms if you can please.
Unfortunately, no, if you do not know what a derivative is, then it is impossible to explain.
The crudest way to explain autoguiding is this: lets say you update the loop once every three seconds, i.e., take a 3 second exposure and then depending upon where the star has moved, send a correction to the mount to move the same amount in the opposite direction.
The autoguider tells the mount the amount to move by initially setting up the guide rate. The rate the mount moves is usually given as rate relative to the rate the stars move ("sidereal rate"). Stars move 360 degrees in 23 hours 56 seconds. Or about 15 degrees per hour of time, or about 0.25 degrees (15 arc minutes) per minute of time, or 15 arc seconds per second of time.
For example, a guide rate of 0.5x sidereal would correspond to moving the mount by 15*0.5 = 7.5 arc seconds per second of time.
To move the mount by a certain amount, the computer basically tells the mount to start moving (at this rate), wait N seconds, and then then tell the mount to stop moving. This is known as a pulse that is N seconds long. If you want to move the mount by 1 arc second, and if your guide rate is 1x sidereal rate, then, you would send a pulse to the mount that is 1/15 second, or 66 milliseconds long. if your guide rate is 0.5x sidereal rate (the most common guide rate for reason you need to buy a good book to find out;I really don't have time to explain everything) then you would need to send a pulse that is 132 milliseconds long to the mount.
Thus, for the most common rate of 0.5x sidereal rate, if your mount has drifted by no more than 1 arc second between the sampling rate (3 seconds in our example), then the pulse would never need to be more than 132 milliseconds long. This is the max rate parameter. The previous couple of paragraphs explains why max guide duration is in units of time, not angle.
How much your mount moves every 3 seconds is the max derivative of the periodic error of your mount multiplied by 3. This is where the derivative comes in. The periodic error itself is only useful for visual astronomy. For autoguiding, it is the derivative of the periodic error is the important parameter. I can't explain derivatives to you if you do not know calculus. You need to read a book on it.
The periodic error curve of a good mount looks very much like a sine wave. Therefore the derivative is a cosine waveform.
To determine the max pulse duration (132 milliseconds in our example above), you simply need to know the largest derivative of the periodic error curve. And with a good legacy mount the largest derivative of the periodic error curve is likely going to be less than 1 arc second (angle) per second (time); and that implies that the max pulse duration need not be more than 132 milliseconds when using 3 second exposure time.
ZWO should do this for you via the ASIAIR as they know the FL, camera, pixel size, tracking rate etc, but for whatever reason
ZWO just wants to make money, not help you understand how things really work. I recommend some other manufacturer if you want decent documentation. A large part is that ZWO employees do not understand anything themselves, and just use open sourced software that is written by others.
Chen
