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# Adjusting for Declination when measuring PE

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### #1 Tim C

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Posted 10 August 2009 - 08:16 AM

Hi,

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.

Tim

### #2 jrcrilly

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Posted 10 August 2009 - 09:13 AM

Periodic error is a function of inaccuracies in the RA worm. It's not affected by anything else so it doesn't matter what the declination is of the star used to measure it (except that it's hard to measure when too close to +-90 degrees DEC); the RA rate is always the same.

-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

### #3 freestar8n

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Posted 10 August 2009 - 09:33 AM

If you are just passively logging the position of a star and you measure the amplitude of its wiggles on the ccd over time, in arc-seconds, then you need to divide that amplitude by cos(dec) to correct for declination. Dividing by cos(dec) will make the amplitude get bigger - to reflect what it would have been for a star at the equator.

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.

Frank

### #4 Tim C

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Posted 10 August 2009 - 12:14 PM

hmm, i thought ive read where a PE calculation using software like PecPrep would understate the peak to peak PE measurement if you were ising say a star at 30 deg declination. What is the "adjust for declination" checkbox for if it doesnt matter? Pempro also points you to the equator as well just west of the meridian as the ideal part of the sky. What I'm doing is using unguided phd logs to create a PE curve in excel.

### #5 freestar8n

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Posted 10 August 2009 - 01:56 PM

Right - what you describe is the case where you need to correct for Dec.

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).

Frank

### #6 Tim C

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Posted 10 August 2009 - 03:30 PM

Thanks very much Frank

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