Please Explain Motors to Me

[nheather]

Hi,

New to astronomy. I have just bought a 150PL second hand at a good price to get me started. The idea is to buy something reasonable (but cheap) and then if I get hooked I can buy something decent later on. I figure that I haven't spent much and if I do decide it is not for me or I want to upgrade I can sell it for more or less what I paid for it.

But for now I have a question about motors. My mount isn't motorised but there are kits to do so.

I understand the basic principle that the sky moves in relation to the earth and that an object (especially at high mag) will move out of the field of view. The motors move the telescope in sympathy so that a properly aligned scope will keep the object in the field of view.

But I notice that you can buy single and dual axis motors for my mount.

What is the difference? If you need dual-axis what is the point of selling single axis.

Can anyone explain it simply for me.

Cheers,

Nigel

[JamesF]

If you have the telescope mount RA axis aligned with the axis of rotation of the Earth then the target will appear to rotate about the RA axis, so you only need one motor to drive the telescope in the opposite direction. In that sense only one motor is absolutely required. Two is often handy however, especially if you don't have the RA axis perfectly aligned.

James

[AstroAdam]

As I understand it, single axis simply allows the scope to track on the RA axis once you've polar aligned it and manually moved it onto an object. This would allow for good visual use, and basic imaging. A dual-axis motor is usually present in GOTO systems, as these require motors on both axes to move the scope to the right part of the sky. A dual-axis stepper-motor system is also what's needed if you want to do guiding via a guidescope and guide cam for imaging purposes. This allows the guiding system to correct the drift in both axes to keep the image stable if you have a guide-compatible mount.

[michael.h.f.wilkinson]

Hi Nigel,

A 150PL is a very nice scope. A 6" F/8 is very versatile, and will show excellent detail on planets, along with a very large range of deep sky objects. I have an equatorial mount with just a single motor on the RA axis, and it is very pleasant during observation. It is also fine for planetary photography (and not just basic!). The shot below was made with just the RA motor

[ronin]

As AA said you need an RA motor to track the objevt that you have centered the scope on, assumes that the mount is polar aligned. After which the object should remain in the field of view of the scope so that you can observe it for longer without having to move the scope.

However that presumes that the object is in view.

For the "next" object you will have to move the scope in both RA and Dec to locate and centre it.

If you have one RA motor then you have to move the scope manually in Dec at least - release clutches or use the twiddly bits - then move the scope in RA - again release clutches, twiddly bits or use the motor.

It is simply easier to buy both RA and Dec motor together, fit both then use the handset and motors to move the scope between objects. Cost is not greatly different either.

I suspect that most often a person may choose a goto but really the convenience of the tracking is by far the greater practical advantage. Just makes "observing" easier and so a bit more pleasurable.

[michael.h.f.wilkinson]

Tastes differ. I always do basic motions with clutches released (which I find MUCH faster than the slew rate of a motorized mount), and then refine Dec with a knob, and RA with motor control. I would use a dual motor system in much the same way: large motions manually with clutches released, refine with RA and DEC control on box (I did this with the TEC 140 of Olly Penrice, for me it is faster than goto systems). This has bagged me some 630 deep sky objects to date, so it cannot be that bad.

This is not to advise against a goto system or any other system with two motors: I would prefer to have two (and would not mind goto) but getting a dual motor version (with goto) was horrendously costly when I bought the scope 17+ years ago. With an EQ mount, you can ignore goto when you do not feel like going through the trouble of star-aligning it. You can then just polar align (roughly is sufficient) and use it in the way I describe. The advantage of the dual axis motor systems (with goto) is that they allow guiding for astrophotography as well.

[redneon]

As a complement to Nigel's image of Jupiter, this shot was taken last night with a 150PL and a single axis motor drive:

ronin: You don't have to release the clutches if you have a slow motion cable for DEC, do you? I know I don't. If the thing I'm looking at drifts too much I just control it in RA using the drive and in DEC using the slow motion cable.

[michael.h.f.wilkinson]

Nice image Darrell

[redneon]

Nice image Darrell

Cheers I'm happy with it given the conditions last night. I just think it looks a bit bright and washed out.

[nheather]

Many thanks for the help and the photos. I understand it now - it was puzzling me before.

It was really out of interest at the moment as I don't want to get caught up with upgraditis before I've even used it.

But assuming I do I will probably get a dual axis for the sake of an extra £30.

Not interested in GOTO as it would be an expensive upgrade and I would prefer to learn the sky myself.

Cheers,

Nigel