Pros and Cons of Computerized Telescopes

Do You Need a Computerized Telescope?

It's a good question, especially if you are just getting into astronomy. It's even a good question for those who've been in the hobby for awhile but have yet to experience a computerized telescope.

What is a Non-Computerized Telescope?

Before we delve into computerized telescope, perhaps we should first talk about the old standby telescopes of past and present that are not computerized. They exist is all types, in many different types of mounts. The simplest is the telescope on an altazimuth mount. below is an example of a simple, home-made in fact, Pipe Fitting Altazimuth Mount.

Simple Altazimuth Telescope Mount

This type of mount lets the telescope move around a vertical axis (azimuth) and up and down (elevation). It is the simplest to implement, cheapest to purchase, and generally one of the most steady. The Orion 10105 VersaGo II Altazimuth Telescope Mount is an example of a commercially manufactured altazimuth mount. If you view it, notice that it has setting circles on both axes. The pipe fitting mount I made also has setting circles, shown below:

Altazimuth Mount Setting Circles

You can find most any type of telescope available on an altazimuth mount, including refractors, reflectors (like Dobsonian telescopes), and even Cassegrains.

Orion Starblast 90mm AltAz Refractor
Orion StarMax 90 AltAz Maksutov
Orion SkyQuest XT6 Plus Dobsonian

These telescopes are certainly not the only altazimuth mounted telescopes on the market, but represent some examples, from refractor to Maksutov, to Dobsonian. Each has the basic altazimuth function of being moveable in azimuth (left/right) and elevation (up/down). Note in each case the mounts look very simple, and they are. Most any altazimuth mounted telescope is a good bargin if optics is your concern. The reason is that they the mounts in each case are inexpensive to manufacture, yet are generally quite stable. So the bulk of the purchase is in the optics.

So what's the pros and cons of having a telescope on an altazimuth mount?

Altazimuth mount pros:

Least expensive mount

Except for mounts on cheap 60mm scopes, generally very sturdy

Very simple to use

Generally quite portable

Altazimuth mount cons:

Cheap ones can be too flimsy

Do not compensate for earth rotation

Not useful for astro-photography

Equatorial Mounts

In the past, one common solution for the inabilty of simple altazimuth mounts to track celestial objects was the equatorial mount. My f/5 Newtonian telescope, shown below, is on such a mount:

Newtonian Telescope on Equatorial Mount

You see immediately that this mount looks more complicated. It is basically an altazimuth mount with the vertical axis tilted towards the earth's spin axis. In the northern hemisphere, this basically means that what would be an altazimuth mount's vertical axis is tilted toward Polaris.

In this orientation, the axis aligned with the earth's spin axis is called the Polar axis or Right Ascension (RA) axis. The other axis is called the Declination axis. These axes are analogous to the azimuth and elevation axes of the altazimuth mount, but by being tilted so, the RA axis can compensate for earth's rotation. A clock driven (motorized) equatorial mount just has a motor that rotates the RA axis in the opposite direction of the earth's rotation at earth rate, and thus keeps the telescope pointed at a celestial object.

The implications of this is two-fold.

First, when observing with this type of mount, the user sees an object stay pretty much in the same place in the field of view, and he/she doesn't have keep making adjustments to re-center the object of interest.

Second, the idea of taking pictures of astronomical objects becomes possible. The length of exposures a person can make through a clock driven telescope depends upon the quality of the mount, and the alignment accuracy of course.

The equatorial mount is still sold, most often on 4.5 to 8 inch Newtonians like mine shown above. Below are some examples of what's available with this type mount:

Orion StarBlast II 4.5 Equatorial Reflector  Orion 9024 AstroView 90mm Equatorial Refractor

The above examples include common options in the reflector and refractor models. The particular ones displayed give less tedious observing because only the RA axis needs repeated adjustment, and clock drives are available for many models. So what are the pros and cons?

Pros of classical equatorial mounts:

Visual tracking less tedious since only one axis needs repeated adjustment

Clock driven versions keep target in field of view with little operator effort

Clock driven versions allow astro-photography

Cons of classical equatorial mounts

More expensive than altazimuth mounts

More complicated setup and operation than altazimuth

Heavier than altazimuth mounts

Less portable than altazimuth mounts

Fork Mount Telescopes

The advent of short focus Cassegrain telescopes, similar to my ETX 90 Maksutov  (shown below) ushered in the fork mount. The image below of my ETX 90 shows the typical fork mount:

Fork Mount with ETX 90

The fork mount is in actuality a variation of the equatorial mount. The base is tilted to point the rotational axis at, you guessed it, the earth's spin axis. So the tilted base creates a right ascension and declination axis. Like with the classical equatorial, in this orientation only the polar axis needs to be adjust to remain on a target. The short barrel Cassegrains made possible the very compact fork mount, many with a clock drive mounted in the base. Even this modest Meade ETX 90 telescope has a clock drive and drive gear mounted in the base.

For a time the fork mount in the equatorial orientation was quite popular. It was much easier to implement a clock drive into the base of a compact Cassegrain than to implement a clock drive on a heavier classical equatorial mount hosting a large refractor or Newtonian telescope.

Here are the pros and cons of the fork mount:

Pros of the fork mount

Makes for a very compact and stable telescope package

Makes a very portable mount

Easy to implement a clock drive in the mount base

Less expensive than a comperably sized telescop on a classical equatorial

Cons of the fork mount

Fork mounts are awkward to use when pointing near the earth's pole

The larger telescopes needed a rather large wedge to hold the telescope in the tipped orientation, making the setup less portable.

Awkward observation toward polar objects

Going Modern with Computerized Telescope

The big equatorial mounts and compact fork mounts dominated the amateur telescope world for a number of decades. In the last couple of decades what has come to prominence is the computerized mount. These mounts use a small onboard computer to hold catalogs of thousands of celestial objects, plus do the computations to help align the telescope and track objects. They look like a basic altazimuth mounts in that the telescopes move in azimuth and elevation. An example is my Celestron NexStar 5SE:

Notice the simple straight up and down style, no tilted axis. The magic that makes this design possible is that the mount has two motors instead of the older classical equatorial mounts and fork mounts with their one motor. The reason for two motors is that the earth isn't rotating around the observers local vertical (unless he/she's at one of the earth's poles). So the rotation rates of azimuth and elevation must be constantly adjusted to make up for that.

That's another area where the onboard computer comes in. Once the computer knows the telescope's geographical location and that the mount is aligned, the necessary drive rates are computed and applied by the computer for any pointing angle.

So in practice, one uses a hand controller attached to the mount to align the telescope for the evening, then simply selects objects with the same controller and the computer positions the telescope to the selected targets.

Pros of the Computerized Mount

Targets are simply selected from the controller

The computer controls the altazimuth mount to track properly

With no tilted axis, the footprint of the telescope is smaller

The simple vertical axis design is more stable and lighter

The telescope and mount are more portable

Cons of the COmputerized Mount

The mount needs power to operate

The mount can only be pointed via the controller

Any given sized computerized telescope is more expensive than an altazimuth one

So When Do You Need a Computerized Telescope

The cost tradeoff is clear, a more sophisticated mount costs money. So a computerized telescope costs more per aperature than a simpler mounted telescope. For example, at the time of the writing of this article, you could purchase an 8 inch Dobsonian telescope on it's simple wooden mount for about $600, or you could purchase a 5 inch computerized telescope for about the same money.

An 8 inch telescope has a resolution of about 0.56 arc-seconds versus a resolution of about 0.9 arc-seconds for a 5 inch. So you can detect (seeing permitting) lunar and planetary detail of nearly half the size with the 8 inch.

An 8 inch telescope has about 2.5 times the light gathering power of a 5 inch, which reveals another magnitude of stars. So you can see that for the money, optical power is definitely more available with a simple, altazimuth mount.

But on the flip side, a smaller computerized mount may actually let you see more objects, because it will locate objects for you. If you've ever struggled to star-hop a non-computerized telescope to a target that isn't naked eye visible, then you know what I mean.

A smaller computerized telescope will certainly be more compact, lighter, and more portable. Plus, the computerized mount will give you some short-time (30 seconds or shorter) photographic capability.

And most of the modern computerized telescopes can be operated in an equatorial mode (tipping the vertical axis toward the earth's pole), allowing photography with longer exposure times. My Celestron NexStar 5SE has a base that has a rod that can be used to hold the base at the proper equatorial angle. Some other models require purchase of a wedge that sits on top of the tripod, and has the telescope base mounted to it. In that way, the telescope is in equatorial mode.