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Question on Sidereal Tracking Rate

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#1 orlyandico

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Posted 24 March 2013 - 12:04 PM

Hi all,

Does anyone here know how the various mounts implement sidereal tracking?

I assume the non-GoTo mounts all track at the average King rate, which is 86188 seconds per sidereal day.

However, the GoTo mounts know where they are pointing in the sky, so in theory they would be able to vary their tracking rate to account for atmospheric refraction. I'm fairly sure that the Paramount does this, since it is totally reliant on its PC and pointing model, might as well do the right thing.

But what about all the other GoTo mounts - Nexstar, Synscan, Gemini 1 and 2...?

#2 Hikari

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Posted 24 March 2013 - 01:06 PM

Atmospheric refraction is not an issue--it is simply how fast the Earth spins.

#3 orlyandico

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Posted 24 March 2013 - 06:37 PM

Errr, no. Look up the King rate.

I figure that a mount tracking at sidereal rate (1 rotation / 86164 seconds - earth's rotation) would accumulate 1 arc-second of error in 240 seconds of unguided exposure, vs. if it tracked at the average King rate (1 rotation / 86188 seconds).

The average King rate is only valid close to the zenith, and varies as the pointing altitude decreases. But this variation is rather small, it would take about 3 hours to accumulate 1" of error if tracking at the average King rate.

Of course with a very long FL scope, this difference would become obvious much sooner.

And of course this only matters for long exposure unguided. Even the error between sidereal and average King rate is easily swamped by periodic error, unless you have an AP or Bisque mount.

#4 orlyandico

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Posted 24 March 2013 - 07:50 PM

It also looks like drift in the crystal oscillator clock of the telescope drive would produce about ~ 1" of tracking error per hour, unless a temperature-compensated crystal (TCXO) is used, which would reduce the error to about 1" / 4 hours (or about the same magnitude as the error caused by using the average King rate instead of the actual King rate).

#5 korborh

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Posted 24 March 2013 - 10:32 PM

Flexure, non-perfect PA, refraction conditions (pressure, etc.) will prevent knowing exact apparent sidereal speed. For Paramount you will need a very good model plus no mirror based flexure or other random effects. Bottomline - high accuracy long-exposure needs autoguiding unless one is willing to live with the limitations of modeling.

#6 orlyandico

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Posted 25 March 2013 - 03:56 AM

Just found this in the Mach1 manual, which answers my question:

the sidereal tracking rate is exact in the mount (it is crystal controlled and checked here for accuracy). however, the stars
do not move at exactly the sidereal rate everywhere in the sky. the only place they move at that rate is straight overhead.
As soon as you depart from that point in the sky, the stars will be moving more slowly, especially as you approach the
horizons. thus, it looks like the mount is moving slightly faster than the sidereal rate. Just because you have done a classic
drift alignment, does not mean that the stars will now be moving at the sidereal rate everywhere in the sky.
in order to increase the area of sky from the zenith that will give you fairly good tracking, you will need to offset the polar
axis by a small amount. the amount will depend on what your latitude is. the other approach is to vary the tracking rate for
different parts of the sky. ray Gralak’s Pulse Guide will allow you to dial in an exact tracking rate for any part of the sky.


Although the "Smart Guide" functionality allows prolonged unguided tracking in a certain patch of the sky. Very similar I believe to the unguided tracking of the ASA mounts, except this one only uses the handpad and only uses two points (beginning and ending).

#7 rmollise

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Posted 25 March 2013 - 08:37 AM

Hi all,

Does anyone here know how the various mounts implement sidereal tracking?

I assume the non-GoTo mounts all track at the average King rate, which is 86188 seconds per sidereal day.

However, the GoTo mounts know where they are pointing in the sky, so in theory they would be able to vary their tracking rate to account for atmospheric refraction. I'm fairly sure that the Paramount does this, since it is totally reliant on its PC and pointing model, might as well do the right thing.

But what about all the other GoTo mounts - Nexstar, Synscan, Gemini 1 and 2...?


Some mounts may implement King rate, but I don't know of a single one that defaults to it.

As for the RA drives of go-to mounts? No, they don't vary their speed in any way while tracking.






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