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Lunar tracking rate vs Sidereal tracking rate?

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#1 DavidCarmichael

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Posted 04 April 2011 - 03:47 PM

Is there a big difference between the Lunar tracking rate and the Sidereal tracking rate?
The reason I ask is because I have an Alt/Az tracking mount that says it only has one rate - Sidereal - but it keeps the Moon centered in the eyepiece for at least 15 minutes.

#2 Mark9473

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Posted 04 April 2011 - 04:03 PM

The Moon shifts 360° over 28 days with regard to the stars i.e. about 1/2° per hour.

#3 Carol L

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Posted 05 April 2011 - 05:58 PM

My old Celestron Ultima 2000 had Sidereal, Lunar and King rates, but all i ever used was the Sidereal.. it seemed to do well enough for all of my visual observing needs.
Maybe the other types of rates are needed for astrophotography? :question:

#4 DavidCarmichael

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Posted 05 April 2011 - 09:15 PM

I'm just curious as to why the moon stays perfectly centered in the eyepiece when the mount is supposed to be moving at a sidereal rate?

#5 DaemonGPF

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Posted 05 April 2011 - 09:35 PM

Because over the course of minutes the difference is minimal and not something you'll notice visually. I actually do my lunar photography at sidereal rate with no issue, and I generally shoot tens of thousands of frames over the course of 30 minutes or so without any problems keeping the moon where I need it.

http://www.astrophot.../Wallpaper2.jpg

#6 Asbytec

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Posted 05 April 2011 - 10:30 PM

Yea, the moon is orbiting eastward across the sky very slowly, half degree per hour sounds right, and the stars are 'moving' westward due to the earth's rotation. This gives the moon a very slightly slower westward motion (lunar rate) as the earth rotates against the background stars (at the sidereal rate.) Reiterating what others have said, this difference in motion so slow as not to be a problem visually. So, no, there is not a big difference between lunar and sidereal rates.

But, what the heck is the king rate?

#7 pennyandchris

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Posted 06 April 2011 - 01:37 AM

But, what the heck is the king rate?


AFAIK it is named after Edward Skinner King and should be used at low declinations to take account of the apparent effect of air refraction. It is certainly available on Losmandy mounts. No doubt someone will be able to explain more fully with diagrams etc.

#8 Asbytec

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Posted 06 April 2011 - 02:47 AM

Ah, okay, thank you. That's close enough.

#9 Tony Flanders

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Posted 06 April 2011 - 05:20 AM

I'm just curious as to why the moon stays perfectly centered in the eyepiece when the mount is supposed to be moving at a sidereal rate?


It doesn't. Crank the magnification as high as your telescope can support, move the scope so that just a tiny edge of the Moon is visible, and you will see the Moon move in a matter of minutes.

When you talk about the Moon being "centered," it's clear that you're using ultralow magnification. Even at 150X -- a fairly modest magnification for most telescopes -- the Moon completly fills the field of view.

#10 michael hester

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Posted 06 April 2011 - 10:59 AM

The moon can be visibly seen to move across the sky in a telescope. Its enough that the moon ends up leaving your field of view in a matter of minutes with sidereal tracking rate.

My mounts have 3 rates: Sidereal, Lunar, and Solar
Sidereal: for stars and deep sky, and some planets
Lunar: for the moon
Solar (it wasn't called exactly this but it means for the sun): for the sun. Yes the sun does the same thing as the moon but slightly slower, slightly less than 1 degree per day, but if you're doing solar work it will be enough that the sun moves out of your field of view over the course of the day.

#11 DavidCarmichael

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Posted 06 April 2011 - 05:19 PM

Tony, the scope I am using is a Skywatcher 80mm refractor on the tracking multi-mount. It is the same scope as the 80mm GoScope you reviewed in The March edition of Sky & Telescope. I forget what magnification I was using but the Moon was entirely in view with just a small line of sky all around it. I left for 15 minutes and when I looked again the Moon hadn't moved at all. Still perfectly centered with the same small line of sky around it. I'm assuming that means the scope is tracking Lunar not Sidereal.

#12 Tony Flanders

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Posted 06 April 2011 - 05:37 PM

Tony, the scope I am using is a Skywatcher 80mm refractor on the tracking multi-mount. It is the same scope as the 80mm GoScope you reviewed in The March edition of Sky & Telescope. I forget what magnification I was using but the Moon was entirely in view with just a small line of sky all around it. I left for 15 minutes and when I looked again the Moon hadn't moved at all. Still perfectly centered with the same small line of sky around it. I'm assuming that means the scope is tracking Lunar not Sidereal.


Aha! Now I realize what's going on. Silly me, this should have been evident to me immediately.

The scope isn't tracking at any particular rate, as a traditional equatorial mount would. Instead, it's tracking the Moon in particular -- calculating its position periodically and doing a Go To to the new position.

Done properly, this method will keep anything centered, even an artificial satellite that takes just minutes to cross the whole sky.

#13 DavidCarmichael

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Posted 06 April 2011 - 06:47 PM

Tony, sorry if I got you upset. All I'm trying to do is figure out if my mount is tracking at a lunar rate or a sidereal rate.

#14 Tony Flanders

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Posted 07 April 2011 - 09:07 AM

All I'm trying to do is figure out if my mount is tracking at a lunar rate or a sidereal rate.


Lunar rate for the Moon, sidereal rate for the stars, solar rate for the Sun, etc. As I said, if you're using a well-functioning Go To, you can track anything.

#15 calibos

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Posted 07 April 2011 - 12:15 PM

Who's upset?? I don't see any upset Tony's??






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