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Old 21-03-2014, 02:52 PM
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billdan (Bill)
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Sidereal Rate confusion

Hi

I'm a little bit confused about sidereal rate, a thread was just started on Cloudy Nights about refraction and sidereal rates. Its off the air now so I can't read the conclusions.

It appears that the 15.041 arcseconds per sec rate is only applicable at when pointing at zenith. Any altitude below Zenith, refraction of the air slows this rate down and is slowest at the horizon.

Apparantly you have to calculate a 'King' rate to compensate for this.

So does my Synscan controller do this in firmware based on its internal pointing model?

My drift alignment is now spot on and I can turn DEC guiding off, but it appears I will still need RA guiding to compensate for the change in Sidereal rate. ( discounting PEC errors)

Confused

Bill D
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Old 21-03-2014, 03:16 PM
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alistairsam
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hi

Have a read through this thread and Gary's Posts. might help

http://www.iceinspace.com.au/forum/s...=sidereal+rate

Cheers
Alistair
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Old 21-03-2014, 04:55 PM
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billdan (Bill)
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Thanks for the link Alistair and it does clarify a few things for me.

Gary suggests that we should align our telescopes to the refracted pole and not the true pole and the fixed sidereal rate of the RA motor will be close enough.

I wonder if by performing an accurate drift alignment we are in fact aligning with the refracted pole as opposed to polar aligning with the 2 star setup recently introduced in Synscan V3.35. Otherwise how do you find the refracted pole?

Anyway it appears that the error is minimal and only on a sub-pixel level on 15 - 30 minutes subs.

Regards
Bill
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