Adjusting for Declination when measuring PE
Posted 10 August 2009 - 08:16 AM
Does anyone know how much to adjust a periodic error calculation for the declination of the star you are using if it's not on the equator (i.e., dec zero)? I know PE measurement programs can do this for you but I was creating some graphs in excel and need to know how to do this calc myself. Pretty nerdy question I know but I'm sure someone out there has the answer.
Posted 10 August 2009 - 09:13 AM
-edit- On review, it's obvious that I answered the wrong question rather than the question which was asked. Sorry. Frank has provided the appropriate response :o
Posted 10 August 2009 - 09:33 AM
If you are actively guiding and you are tracking the corrections made to the RA axis to keep the star centered while autoguiding, then you don't need to correct that log for dec. at all because it is measuring the motion of the axis itself.
Posted 10 August 2009 - 12:14 PM
Posted 10 August 2009 - 01:56 PM
If you have a log of a star position and you are not guiding - then the wiggles you measure will be smaller than the true PE. You need to divide the wiggle size by cos(dec) to get the true PE - which will be bigger.
This assumes the logging software you used didn't already do this scaling - which I think is the case.
To convince yourself you can always do a log of a star at 80 degrees or so. The wiggles will be smaller with increasing dec., and need to be corrected by 1/cos(dec).