Guide Setting


Ned Smith
 

I have used SiTech for years and have had very little trouble.  I have always ignored the "Guide" speed.  Don't really know what it is for.
Now I am finding that in long exposures the image slowly drifts off the screen. Can the Guide speed be used to overcome this drift?  How?  

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Ned Smith


Don W
 

Hi Ned,
The guide speed is the speed that guiding uses to correct the position - only with a guide camera.  Usually the guide speed is equal to the sidereal rate = 15 arc-seconds per second.  Autoguider software usually have a factor called aggressiveness which can reduce the guide speed.  Also some users set the guide speed to about half sidereal to minimize excursions of their mount.  That is equivalent to setting the aggressiveness to 5 (on a scale of 0 to 10).

From your description, you must be doing unguided imaging.  Unfortunately, the motion of stars varies across the sky, mostly due to refraction.  Pure sidereal rate is only near the zenith.  The movement due to refraction is mostly i the east and west ( I think it is slow in the east and fast in the west).  This effect is strongest at low altitudes.  This is why it is generally recommended that imaging be at altitudes greater than 30°.

Don W