Basics of Go-To Mounts

What’s in a Name?

There are a variety of names used for mounts that can automatically position themselves to point to objects in the sky. The one in most common use is “go-to mount” (because the mount will go to any object you tell it to).

The spelling “GOTO” is also popular – mainly, I think, because the word “GOTO” has an important historical meaning in computer science and many amateur astronomers have that background. However, I will be avoiding spelling it that way, because GOTO is also the brand name of a Japanese optical company that makes planetarium systems and once made telescopes. So “GOTO mount” has another meaning in the astronomy field, and could lead to confusion. (Should you come across that excellent gear, Goto is a proper noun and is pronounced with two “long o” vowels – i.e. the pronunciation is “go tow” not “go too”.)

Introduction

This article is a basic discussion of go-to mounts for your telescope (also known as “computerized mounts” and other names). Much of it will also apply to Digital Setting Circles (DSCs, also known as “object locators”, “push-to mounts”, and other names).

If you already have a go-to mount, this article will not replace your user manual. There are brand- and model- specific details with every go-to mount, and you will need the manual. If you didn’t get a manual with a used mount, you can download one – I am not aware of any brand of go-to mount for which a manual isn’t available online. I intend this article for people who haven’t bought a go-to mount yet and want to understand the concepts, or people who have a go-to mount and are looking for another perspective to supplement their user manual.

If you are a beginner who found this page via search engine, you might prefer to start at the beginner scopes page or the setting up page.

Go-To v.s. DSC

I mentioned above that this article is about go-to mounts but that much of it would also apply to digital setting circles (DSCs). What is the difference between those?

First, the similarities. On both go-to mounts and DSCs, the mount is equipped with encoders: precise measuring devices that report movement of the mount to a central computer. So, after you help the computer get its bearings, by keeping track of the signals from the encoders, it always knows where, in the sky, the telescope is pointed.

The difference is simply this:

• DSCs are just encoders and a computer. The motion of the mount comes from you: you will push the telescope, or use its slow-motion controls or tracking motors, to move it to the desired location in the sky, watching the computer display to know which way to push, and how far.
• A go-to mount also includes motors connected to the encoders (usually as integrated units, called servo motors), so the computer will move the mount to the desired location. You don’t push it manually – in fact, you must not push it manually or you will confuse the computer.

Some advanced mounts (e.g. the Gemini system, as an extra-cost option) have both servo motors and separate encoders. With these, you can push the mount manually without messing up the computers. This feature is rare – as far as I know it is not an option on the majority of entry-level mounts from Meade, Celestron, Skywatcher, Vixen, etc. Assume you cannot move your mount manually, once the go-to system is in use, unless the manual very clearly says you can. (And by “cannot move your mount manually”, I mean loosening the clutches and pushing it. It’s OK with all mounts to use the mount’s slow-motion controls to move it.)

The setup and operation of DSCs and go-to mounts is almost the same – the only difference is whether you stand back and watch the mount move itself, or move it yourself while watching the computer display.

Basics of Set-Up

Why?

We’ve already covered the most important point about go-to mounts that will help you understand why you need to do some set-up.

Your mount contains sensitive electro-mechanical measuring devices called encoders that send information to the mount’s computer. However, encoders report motion, not position.

So, your mount’s computer gets very accurate reports of the motion of the mount. (“I have just moved 2.073 degrees toward the North.”) But that information does not tell the computer where the telescope is pointing now unless it knows where it started.

The set-up procedure for your go-to mount or DSCs is used to give the computer enough information to understand where it is presently pointed in the sky, and the precise orientation of the sky, so the motion information from the encoders can be used to track where the telescope is pointing.

We call the data the computer collects and stores, that allow it to understand where it is pointing in the sky, its sky model.

How?

All go-to and DSC systems have the same general approach to establishing a sky model. You will:

• Tell the computer the date, time, and your location on the planet – or this information will be supplied by a GPS (Global Positioning System) on mounts equipped with that feature;
• Manually put the mount in some defined start-up reference position, and tell the computer when it is in that position, so it can note it as a reference point for the motion information from the encoders;
• Finally, point the telescope at one or more known objects in the sky (with most systems, 2 or 3 stars), carefully centring them yourself, and then telling the computer “there ya go, star such-and-such is now centred”.

With this information, the computer can build a sky model that allows it to know where the telescope is pointing and the precise layout of the sky. With some systems you can continue to add reference points as you do your observing, allowing the computer to refine its sky model as the evening progresses.

What the Mount Needs to Know

Your mount needs two pieces of information in order to accurately point the telescope: the position of the mount, and the position of the sky.

Position of the Mount

To know where the telescope is pointing in the sky, the mount needs a reference position from which to measure movement. This is simple – your mount will require that you place it in some standard and easy to achieve configuration either before you turn it on or in response to a prompt. Typical examples of this reference position include

For alt-az mounts, having the optical tube perfectly level and pointing due north is a common reference position.
Some modern alt-az fork mounts are able to align without starting with a reference position because they have internal sensors for North and “levelness” – check your manual.

For equatorial mounts, polar aligning the mount, then having the counterweight shaft vertical and the optical tube pointing precisely along the polar axis is a common reference position.

Check your manual – the startup reference position for your mount might be something different; these are only examples of typical mounts.

Note that on some alt-az fork mounts, finding the reference position is automated. Built-in sensors will automatically level the optical tube and point it due North. On others, you will need to position the tube to its startup reference position yourself.

Position of the Sky

Then, to know where things are in the sky at a given moment, the mount computer needs to know the position of the sky. Since the sky rotates with the rotation of the earth, and object positions are also dependent on the date (because the sky rotates with the season and planets move on their own schedules), the mount needs to know the precise date and time, and your location on the planet.

Your mount will prompt you, on the hand control, to enter the date and time, then to select a nearby city from a list, or to enter your latitude and longitude. If your mount is GPS-equipped (optional on most mounts, standard on some), this information will be obtained directly from the GPS and you won’t have to enter it.

If you have a mount where the date and time are entered manually, take extra time with that section of your manual. Getting this wrong is probably the most common cause of problems for beginners with Go-To mounts. Make sure you know

• whether the time is entered in UTC or local time;
  • If you specify UTC (GMT), you also need to think about the fact that the date may be different. If you are working in the evening in North America, there is a good chance that it is already the next day in UTC time, so you may have to enter “tomorrow”‘s date.
• whether you have to specify what time zone you are in, and how to do this;
• the format for dates. Especially if it is MM-DD-YY or DD-MM-YY etc.

Take the time to thoroughly understand this section of your mount’s manual before you go on. It will save you from a lot of frustration.

In theory, if everything were perfect, this would be enough information. If you know the date, time, and your precise location, you know the sky. (After all, this is all sky charts ask of you.)

But pointing a telescope is a very high-precision task, and the above information is rarely precise enough. Date, time, and location will be approximations, altitude will be unknown, your reference position for your telescope will be off a bit, the mount will not be perfectly square, etc.

So the mount will now ask you to help refine its approximate sky model by pointing the telescope to one or more known objects. Typically you or the mount will pick a well-known bright star, move the telescope there, and you will use the finder, then an eyepiece in the scope, and the hand control’s motion buttons, to precisely centre the star in the eyepiece. On simple mounts, you do this with one, two, or three stars (depending on your mount model) and your alignment is done. On more complex mounts, you use a minimum of 2 reference stars, but can use as many more as you like and the more you use, the better the sky model.

If you have a GPS scope, you still have to do the 2- or 3-star alignment. The GPS only allows you to skip entering the date, time, and your location – it does not replace this final alignment step.

On some very recent mounts, the process of star alignment is also automated, through a camera and star position database built into the mount.

Typical Set-Up Sequences

The above description has been kept at a general level. In order to be a little more specific, the following somewhat more detailed setup sequences are for different types of mounts.

What about a Dobsonian example? Sorry, I have never owned a Dob, so I can’t share that experience. However, you would normally use a Dob with Digital Setting Circles, so the procedure would be almost exactly as described in the alt-az mount example above, except with a vertical tube instead of a horizontal tube for the initial position.

Using the System After Set-Up

After you have completed the alignment procedures, you are ready to start using your scope. Typically, you will spend most of your time doing one or more of the following things:

• Browse through the mount’s object catalogue and, when you find something you would like to observe, select it and tell the mount to go there (or tell the DSCs to guide you while you move the scope there).
  • Note that the mount’s object catalogue will normally only offer you objects that are visible at that time. But beware: “visible” only means they are above the horizon. The mount doesn’t know if a part of your sky is obscured by a building or trees; and it doesn’t know how big your telescope is. It’s quite possible to have the go-to mount point the telescope at the side of your house, or at something that your specific telescope is incapable of seeing.
  • In fact, I think it’s safe to say that most of the objects in your object catalog will be beyond the cabilities of your telescope. So you will have to gain some familiarity with the objects in the catalogue; most catalogues give you a brightness rating on each object so you can estimate whether your scope will be capable of seeing it.
• Let the mount recommend objects. Most beginner-oriented go-to mounts have a “tour” feature that will offer you a series of interesting, easy-to-see objects visible at your time and location. These will be brighter objects, so they almost certainly are all visible in your scope.
• Move around the sky yourself, using the mount’s electronic hand control. If you find something you can’t identify, you can ask the mount what it thinks you are now pointing at.
• Refine the sky model. With most mounts, after you go-to an object and adjust the pointing (using the electronic hand control) so the object is perfectly centred, you can instruct the mount to add the now-centred object to the sky model, improving the accuracy of subsequent go-tos. This is easy and worthwhile – usually it is as simple as:
  • Use the go-to to go to the object of interest;
  • Use the hand control to make small adjustments to perfectly centre the object;
  • Enter a command on the hand control that tells the computer to add this object to the sky model.

Once Go-To, Always Go-To

Once aligned, it is important you understand one warning:

With a go-to mount, once aligned, you must not loosen the clutches and move the mount yourself by hand. If you do this, the encoders will not be sending motion information to the computer, so the mount will lose track of where it is pointed, and you will have to do the entire alignment procedure again from the beginning.

(The only exception to this that I am aware of is the Losmandy/Gemini system where, if you add the extra-option external encoders, you can manually move the mount without losing the go-to alignment.)

With DSCs, you can do whatever mount motion still drives the encoders. With all the encoder setups I have seen, it is safe to loosen the clutches and move the mount manually.

General Advice

The following are some miscellaneous points I offer as additional bits of advice to the new go-to user.

1. I strongly recommend that you practice using your telescope semi-manually before you tackle the go-to. By semi-manually, I mean do whatever you need to do to have the electronic hand control’s movement keys work – polar align the mount, turn it on, etc. Possibly even do the 2-star alignment. But then use the finder and motion controls to point the telescope at something easy and interesting, and get used to using the hand control, finder, and eyepieces.
   • See your telescope work and get comfortable with it as an optical instrument before adding the challenge of the sophisticated computer features.You want to avoid the common experience of the total beginner who sets up the scope, does a go-to, sees nothing in the eyepiece, and then has no idea where to start looking for the problem.
2. Again, carefully read your manual to understand whether, when entering the date and time during set-up, you need to use local time, or Universal Time Coordinates (UTC, aka GMT), how you specify time zones, whether you account for Daylight Time, etc. And if you are required to enter the time in Universal Time, learn how to convert your local time to UTC, and don’t forget that, in the evening, converting the time to UTC often moves the date to the next day too.
   • If your first go-tos with your mount are way off in the sky, there is a good chance that you have misunderstood how the date and time are to be set.
3. Your telescope can be set to move at different speeds when you use the motion arrow keys on the hand control. Learn how to do this. Some mounts automatically select a fairly high speed when you are looking through the finder, and a much slower speed when you are looking through the telescope. On others, you will want to make this adjustment yourself. Aligning the telescope when the motion is too slow or too fast is very frustrating.
4. Most mounts have a certain amount of backlash – small slop in the motion of the internal gears, that shows up as a small delay between when you push one of the direction buttons and when the scope starts to move.
   • Backlash can seriously degrade the accuracy of your initial 2-star alignment. For the most accurate alignment, you should form the habit of always approaching the alignment star with the same two direction buttons. For example, you might decide to always approach the alignment star with the “Up” and “Right” buttons – never the “Down” or “Left” buttons. If you need to come at the star with the Down or Left buttons, overshoot it and then back up with Up and Right.
   • This habit means that your alignments to the mount computer always have the same backlash, so the computer can eliminate that error. If, on the other hand, you approach your first alignment star using Up and Right, and the second alignment star using Down and Left, you are sending information containing significant pointing error to the computer, and your go-tos will be off.
5. On some go-to mounts, you have the option of having the mount select the initial 2 or 3 alignment stars, or of selecting them yourself. You’ll probably use the automatic selection for your first couple of evenings, until you learn the names of some of the bright stars.
   • However, it is worthwhile to learn the names of the 5 or 6 bright stars that make good alignment stars in any given season, and to select them yourself. This allows you to immediately select stars you recognize, and that are not obscured by buildings or trees. (And you avoid the situation where your mount automatically selects a star, then does an initial go-to that leaves you between two bright stars, and you’re not sure which one is the alignment target.)
   • Better still, learning the 5 or 6 stars that are good for the current season is not too hard. By the end of the season, with 3 or 4 months of familiarity, you’ll know them thoroughly, then you can learn 4 or 5 more for the next season. By the end of your first year, you’ll know 20 or more alignment stars by name and be able to find them easily.