Setting Up a Celestron AS-GT Go-To Mount

Introduction

This article is intended to be read after the general introduction to go-to mounts. It uses a specific mount, the Celestron CG-5 AS-GT, as an example to give a more detailed account of the procedure of setting up and using a go-to equatorial mount.

I’ll be using a 9.25″ SCT on this mount, with a Telrad unit-power finder added.

(I must confess that I no longer own this mount — I sold it about a year ago to fund a Losmandy GM8 mount. But I wanted to include this very popular beginner mount, so this article is written from memory, old photos, and the manual.)

Mount Summary

Various features of go-to mounts are mentioned in the general introduction. On this mount, the specifics are:

Mount geometry
Equatorial
Reference Position
Marked on mount body: counterweight down, OTA along polar axis.
Alignment
3-star. Mount selects or user selects.
Features
Object catalogue, guided tour, identify object.
Refinement
Early versions can only replace one of the 3 alignment stars. Newer versions can add stars to the model.
Backlash
Significant — use only up and right buttons when aligning.

Setting Up

Let’s go outside. It’s late evening in July, and we have couple of hours to observe. We are planning to look at some globular clusters, and maybe explore the region around Sagittarius, where we have heard the sky is densely filled with interesting objects.

We have assembled, balanced, and polar aligned the mount, and aligned the Telrad unit-power finder, so mechanically, everything is ready to go.

Initializing the Go-To System

Before powering-on the computer, we place the mount in its reference position (the AS-GT manual calls it the Index Position). This is the normal counterweight-vertical, OTA-along-polar axis position.

There are index mounts on the body of the mount which are lined up when the mount is in this position.
Strangely, the Right Ascension index marks do not precisely correspond with having the OTA pointing straight along the polar axis. There is much discussion of this on the Internet user groups, and the consensus is that this is inaccurate manufacturing, not a deliberate design. Many people have moved the index marks and say that was a good move. Others swear that they need to be left in their slightly offset positions. In my experience, it doesn’t matter — the position of the index marks only affects the accuracy of the first alignment go-to, which you will quickly correct.

Aligning the Go-To System

Having set up the mount, it’s time to do the 3-star alignment. The mount can pick 3 alignment stars automatically, but I don’t like that option — it will sometimes pick a star I’m not familiar with, or one that is inconveniently placed, like behind my house. I prefer the option where I choose the alignment stars. To pick good alignment stars, let’s have a look at the sky.

At our fairly dark location, looking up, we can see the spectacular spread of stars in the night sky.

It’s a little confusing at first.

But after picking out a few bright stars, and then some familiar constellations, we feel oriented.

There’s the Big and Little dippers, Cassiopeia, Cygnus, and others. Down in the South is the familiar Teapot of Sagittarius.

We already found Polaris when we set up and polar-aligned the mount.

That’s good, because we need it now to identify an important imaginary line in the sky.

Imagine a straight line from Polaris, passing directly overhead, and ending straight South.

This is the Meridian, conveniently dividing the sky in half.

The ASGT requires that our first two alignment stars be on the same side of the Meridian, and the third star be on the other side of the Meridian. (All go-to mounts need stars on both sides of the meridian, because when you cross the meridian, internal components in the telescope will fall “the other way”, and the mount needs to see this movement in order to compensate for it.)

We pick 3 stars that are easy to see and find, that are not too close together, and that we’re certain we can identify.

Let’s use brilliant Arcturus, and Dubhe (easy to find as the last star in the Big Dipper’s cup) on one side of the Meridian, and bright Deneb in Cygnus on the other side of the Meridian.

Loosen the clutches and move the mount so it is sitting in its reference position. The counterweight shaft is straight up and down, and the OTA (Optical Tube Assembly) is pointed straight along the polar axis. The Index marks on the two axes are lined up. Then tighten the clutches securely.
We will line up the Index marks precisely even if the tube is not pointed precisely North. We’re pretty sure their misalignment is just an assembly error, but it doesn’t matter since it will only throw the initial alignment go-to off, and that doesn’t matter because we chose alignment stars we can recognize and find.

We now turn on the power to the mount, watch the messages on the hand control, and start responding to its questions.

  • It says LINE UP INDEX MARKS THEN PRESS ENTER. Having already lined them up, we press Enter.
  • Next, the mount asks us to confirm or modify the date and time, displaying those on the hand control. We use the hand control to modify the date and time so they are correct.
  • Next the mount asks us to confirm our location as Ottawa (it remembers this from last time). Since we haven’t moved, we press Enter to confirm his location.Finally, the scope suggests doing an AUTO ALIGN. That’s the option where it chooses the alignment stars, and is not what we want to do. We use the Scroll Down button (the “6” and “9” keys on the keypad are labelled “up” and “down”) to change the option to AUTO THREE-STAR ALIGN, which is the option that allows us to pick the alignment stars. Press Enter.

First Alignment Star

The mount now displays the name of a possible alignment star — the first in its alphabetical list. Its suggestion is ARCHERNAR tonight, but we’re going to use Arcturus for our first alignment star, so we use the Scroll Down button on the hand control until the menu reads ARCTURUS, then press Enter.

The motors fire up, and the scope slews over to point to the general position of Arcturus. The hand control says to CENTER ARCTURUS IN THE FINDER.

Looking through the Telrad, we see we’re in the right general area, but certainly not centred on Arcturus. This is because of the various errors in the reference position, and the approximation of using Ottawa as our location. Good thing we know which star is Arcturus.

We use the hand control to move the scope until the Telrad is centred on Arcturus. (The computer has automatically selected a fairly high moving speed for the telescope while we are using the finder.) With Arcturus centred in the Telrad, we press Enter.

The computer now asks us to CENTER ARCTURUS IN THE EYEPIECE.

Now we switch to an eyepiece. I use a special 12mm eyepiece that has an illuminated cross-hair reticle in it for accurate centring.

If you don’t have a reticle eyepiece, just use a relatively high power eyepiece and judge the centre of the field by eye.

Arcturus is in the field of view but not centred.

We use the hand control to carefully centre Arcturus. (The mount has automatically switched to a slower movement speed to make the centring easy.)

Ensure that the final movements to centre the star are made with the Up and Right buttons, not Down or Left. Overshoot and back up if necessary.

After Arcturus is centred, we press Enter to move on to the second alignment star.

Second Alignment Star

We use the Scroll-Up and Scroll-Down buttons on the hand control to select our chosen second alignment star, Dubhe in the Big Dipper.

Pressing Enter, the scope slews to that part of the sky and, again, says CENTER DUBHE IN THE FINDER.

Looking through the Telrad, we see the initial slew has taken us quite close to Dubhe. Not perfect, but considerably better than the first slew to Arcturus.

The first alignment star improved the sky model, so the go-to is already more accurate.

We use the hand control to centre the finder exactly on Dubhe.

Press Enter, and the scope responds with CENTER DUBHE IN THE EYEPIECE.

Now we look in the 12mm cross-hair eyepiece.

Again, we see Dubhe is in the field of view, but not perfectly centred.

We did not expect the centring to be any better this time, since it is dependent on our centring of the Telrad, not on the mount’s sky model.

Using the hand control, we centre Dubhe in the eyepiece.

Ensure that the final movements to centre the star are made with the Up and Right buttons, not Down or Left. Overshoot and back up if necessary.

When Dubhe is centred, press Enter. The scope now asks for the third alignment star.

Third Alignment Star

The scope is now ready for a third alignment star, and presents us with the alignment star menu again. This third star should be on the other side of the Meridian and, as we decided, we select Deneb, and press Enter.

The scope slews to that part of the sky and says CENTER DENEB IN THE FINDER.

Looking in the Telrad, we see we are almost exactly on Deneb on this first slew.

The minor error is because, on the other side of the Meridian, the system’s weight has shifted to the other side, so gears and mirrors are now leaning on the other side of their supports. Giving the mount a chance to see this error and account for it is the reason we use alignment stars on both sides of the Meridian.

We centre the Telrad on Deneb, then press Enter.

The scope now instructs us to CENTER DENEB IN THE EYEPIECE.

As before, we switch to the cross-hair eyepiece, centre Deneb, and press Enter.

Having responded to the third alignment star, the mount now has all the information it needs. It displays CALIBRATING on the display for several seconds, then ALIGNMENT SUCCESSFUL. We are now ready to use the mount to go to objects of our choice.

Using the Go-To System

I’m anxious to show you the spectacular globular cluster M13. To get there quickly and easily, we call up the Messier catalogue from the mount, by pressing the “M” key for “Messier”. (It’s the 1 on the keypad.)

Then we type “13” on the keypad and, with the display now reading M13 we press Enter.

The scope slews to where it believes M13 will be.

Now we look in our 6×30 magnifying finder and see that M13 is close to being centred. We use the arrow keys on the keypad to bring it to the centre.
Finally, we switch to a medium-power eyepiece (a 17mm Nagler, my favourite with this ‘scope) and, sure enough, there is M13 in the field of view.

We’ll use this same procedure to explore a number of other objects tonight. But this will do to demonstrate the use of this go-to mount. Thanks for watching.

4 Comments

  1. This is **THE** clearest guide on how to do alignment with the AS-GT I have found anywhere!

    A point others might like to know is that the alignment stars are by name, not Bayer designation.
    For example, it’s Aldebaran, not Alpha Tauri. There are a limited number of available named stars
    you can use. A BIG help is to have that list- perhaps Richard will post it or you can google it.
    Then you use your fav apps, maps,etc. to figure out what’s up that night. That way you can
    plan in advance for a clear night!

  2. I send you as many thank-you’d as there are stars! I have not long bought this setup and have been scratching my head t all the different potions.this appears to bea very clear ‘Dummies guide’ on how to get things ready, can’t wait for a clear sky to try again. Thank you from a very excited Englishman

  3. Great review, very clear, and done like a story teller. So good it has converted me…now I want a go-to mount.
    It sounds like fun. Seems kind of lazy, but I suppose it doesn’t mean we star hoppers will forget what we have learned about the sky.

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