Hi Pierre,

I wrote a small (and primitive) script to derive the RA and DEC drift

rates for PG.

Basically, I take two images separated by an interval of a few

minutes, plate solve each and compute the difference in plate center,

then divide by the elapsed time. Then feed to PG.

For the RA rate, is this what PG expects or should I take into

account the DEC cosine ?

PulseGuide expects the RA/Dec rate to be in units of arc-sec/sec. e.g. A

value 1.00 would move Dec axis 1 arc-sec per second, and the RA axis 1/15

arc-sec seconds per second (since RA is measured in 24H format instead of

degrees).

So, if your plate solves returns RA/Dec then you can just subtract the RA

values divide by time and multiply by 15. So, you should not need to adjust

for declination (Cosine of Dec) in this case.

And second question: I understand that one possible usage of PG is

to compensate for (some amount of) polar misalignment

- such as the difference between true and refracted pole.

Am I pushing it too far if I also apply it for a misalignement of

several minutes (say 10). Of course, I understand that

there can be many factors (exposure time, declination, latitude...).

So there might be no simple answer.

The possible rate changes are rather complex to calculate and dependent on

which direction the polar misalignment is directed. As you expected there is

no easy formula to go by. You can measure the rate and null polar

misalignment for a good interval of time, maybe as much as 30-40 minutes

before re-measuring. The problem is possible field rotation, which is not

corrected for even by a rate change.

-Ray