Equatorial mount? – What’s one of those? This article isn’t about polar aligning a telescope mount. It’s all about understanding the mechanics of an equatorial telescope mount and how it works.
When somebody tells you their equatorial mount is tracking the sky, you may say – “ok, that’s cool” and think nothing more of it…….If you then decide to go out and buy yourself a mount capable of this very thing, you may ask yourself the same question again, but this time what you really should be asking is how?
I’ve been thinking about this recently and have realised that in order to get your head around this, it’s best to actually understand the mechanics of how the scope moves around on the mount, how the scope is positioned and why. I don’t know about you, but whenever I’m learning something new, I always want to know why it is, rather than being told it just is and accepting it! This always gives me a better and more complete understanding, whilst also helping me to remember it. The very fact that some beginners to Astronomy don’t get this, can put them into a frame of mind where they think it’s too complicated to understand and end up struggling to move out from this foggy patch in their learning curve, perhaps even putting them off moving forward all together!
Once you can visualise the mechanics of what’s happening here, everything will just click into place, I promise you!
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Equatorial mount part 1
The first thing to understand if you don’t already, is that the sky gives the impression that it moves, only because the Earth is moving in relation to the sky. As the Earth rotates you can’t feel it moving, so it’s not obvious the sky is the one moving and not you! This may be basic knowledge to most, but please don’t be embarrassed if this is news to you. For more info about this, please read my new to Astronomy blog first. If you already get all that and you also know that the sky rotates around a central point, then stick with me and keep reading 🙂
Equatorial mount part 2
The second thing you need to know is that if you want to point a telescope at the sky and enable it to track the sky no matter where in the sky it’s pointing, the telescope must be fixed to something with a rotation axis pointing exactly to that central point in the sky. This ‘something’ would be the axis that would be driven by a motor and gearbox set at the correct speed to match the speed the sky is moving. Remember, this is actually the speed the Earth is rotating (mixed in with a bit of an allowance for how the Earth moves around the Sun too)! Don’t worry about this too much, all you need to know is that this speed is one complete revolution in 23 hours, 56 minutes and 4 seconds.
Just imagine for now, that there’s a shaft/pole/bar, whatever you want to call it, is suspended in mid-air pointing directly at this central point in the sky. From now on I will use the correct terminology of the Celestial pole or CP (also known as either the NCP or SCP depending if you are in the northern or southern hemisphere). Then imagine that this shaft is rotating slowly at the correct speed. Now, if you fixed a lazerpen to this shaft so it shines a beam onto the sky quite close to the CP (path 2), the spot in the sky it’s pointing to would slowly start drawing a small circle while moving with that part of the sky. Now if you imagine re positioning the lazerpen so it’s pointing to a part of sky further away from the CP (path 1), it would be drawing a larger circle than before. The fact that this second circle is larger means that the lazerpen beam would be drawing a line in the sky which is moving faster, because if you think about it……both the small and the large circle are only turning around once in just under 24 hours. Therefore the path of the larger circle would need to be drawn/plotted faster to keep up due to the circle being bigger. So from this, you can appreciate that the speed the sky moves varies from very slow (near the point of rotation) to fastest (at a right angle to the shaft). I suppose you could visualise a potters wheel spinning. If you put your finger near the middle it will travel round fairly slowly. If you try to put your finger at the outer edge of the wheel, you will find it hard to keep up with it as it’s moving much faster due to the larger size of the outside edge of the wheel (even though the speed of the motor remains the same!). Once you start pointing to a part of the sky past this right angle (in either direction), the sky will start to slow down again as the circle drawn in the sky starts to reduce in size. The sketches to the left show what I’m on about.
Hopefully you are keeping up here and understand why different parts of the sky move faster than other parts and the fact that no matter where you point the imaginary lazerpen, the shaft rotation speed does not change.
Basically all we have done here is used the lazerpen in place of the telescope! The rotating shaft in mid-air would in reality be held in place by the mount housing and to get that shaft to point at the central point in the sky there is an up and down adjustment (Altitude or elevation) and a left and right adjustment (Azimuth) on the mount too. Once this shaft (it’s about time I start using the correct terminology here too) the RA axis or right ascension axis is pointing exactly to the right place in the sky (the CP), the mount is aligned and will stay in the same position whenever the telescope is moved around to a different part of the sky.
Equatorial mount the final part
The final part to this jigsaw is to understand how the scope is moved to point to different locations in the sky. Obviously you don’t want to be moving the mount around on the ground in any way or make any further adjustments to the elevation or the azimuth, otherwise all the lining up of the RA axis (also known as polar alignment) will be undone! The telescope is moved in two planes or axis. The first one is simply rotating the RA axis itself, and the other is by rotating another axis called the DEC or declination axis. The DEC axis is fixed at right angles to the RA axis. It enables the telescope to be moved in a different way. Using both the movement of the RA and the DEC axis, the telescope can be pointed to any position in the sky. Once the telescope is moved to the desired position, both these axis are locked and the following will happen.
The DEC axis will lock to a fixed position and the RA axis will be locked to the clutch connected to the motor and gearbox driving that axis at the correct rate. The result will be that the telescope will always track any part of the sky it’s pointing at for as long as the motor is running. The sketch to the left shows all the moving parts on an equatorial mount.
Well…….it’s easy as that! I know it took a lot of words to explain, but hopefully it was clear and really has unlocked the mystery of how a telescope can track the sky!
Now you are ready to go onto the next stage of polar aligning your mount with an extra level of knowledge and confidence. If you have a Telrad finder scope
you can read my Polar alignment using a Telrad Finder Scope blog. Please send me any comments or questions, I would be happy to answer them 🙂
I must finish off by pointing out here that I have only used the lazerpen as a way to explain things. There are certain spec lazerpens that should never be used to point into the sky at any time, which could cause a hazard to planes overhead etc. There are strict laws about these and some types are classed as weapons, and the improper use of these can actually lead to imprisonment!