How to Choose a Telescope for Your Family

By Irwin Horowitz, 11-01-07

The holiday season is fast approaching and some of my readers may be considering the purchase of a new telescope for their families to enjoy in the coming year.  Oftentimes, well meaning parents will spend hundreds or even thousands of dollars on a shiny new gift for their precocious child, only to see it sit in a closet unused or else to be overwhelmed by the complex instructions on how to set it up and use it.  As a result, interest in and excitement for astronomy fades away, and all that is left in its wake is a sense of frustration and resentment.  I would like to offer my services in order for you to avoid such an unfortunate situation.

The first question a prospective telescope buyer should ask is: How interested in this hobby is the recipient of my gift?  Are you trying to encourage him or her to take it up as an alternative to some other interest?  Have they expressed a direct interest in astronomy to you?  Have they asked for a new telescope?  Depending on the answers to these initial questions, you may choose to purchase anything from a subscription to one of the popular astronomy magazines (my recommendation for true beginners would be Astronomy magazine, while for more advanced individuals, a subscription to Sky & Telescope may be more appropriate) all the way up to the purchase of a serious piece of equipment that may cost several thousands of dollars.

Assuming you are going to purchase a new telescope, this leads to the next question: How much are you planning to spend on this gift?  The most basic of telescopes are still going to cost a few hundred dollars, and as mentioned above, the prices could get up towards the level of a good used vehicle!  This may be the single most important question to consider, as you certainly don’t want to spend more than you can afford on a lavish gift.  However, it is vitally important that you don’t try to get a cheap knockoff.  Quality does cost, but it is definitely worth it!

The third question to ask is what types of astronomical objects does the recipient of the gift wish to view with their telescope.  Are they interested in doing any solar observing?  (NOTE: NEVER POINT A TELESCOPE TOWARDS THE SUN WITHOUT PROPER FILTERS).  Viewing the Moon may be all that you plan on doing.  Perhaps the planets are what capture your interest.  Stars…nebulae…galaxies…different interests mean different telescope types.  This naturally leads to a discussion of these different types and their relative merits and drawbacks.

There are three basic types of telescopes (refractor, reflector and compound) and two main types of mounts (alt-azimuth and equatorial).  Go-to style mounts are becoming increasingly popular as they eliminate much of the frustration in locating objects in the sky, though you need to become adept at the procedure for setting up these types of mounts.

The most basic type of telescope is the refractor.  This type of telescope consists of a lens made of several types of glass and placed over the front end of a tube.  Glass lenses tend to suffer from the effects of chromatic aberration, where light at different wavelengths have different focal lengths.  This results in an inability to simultaneously focus all of the visible light from an object at one point.  To correct for this effect, manufacturers design lenses with multiple elements.

The two main types of refractors are known as achromatic and apochromatic.  An achromatic lens consists of two different pieces of glass glued together to correct for the effects of chromatic aberration at two different wavelengths (usually blue and red).  Apochromatics have multiple element lenses to correct at three wavelengths.  As you may surmise, the apochromatics give better performance but are also much more expensive.  In fact, for a given system aperture, refractors tend to be the most expensive type of commercially available telescope.

Refractors tend to be relatively easy to set up and use, as they should not require any optical collimation and often are mounted on simple tripods.  One thing to confirm when purchasing a refractor (or any telescope) is that it comes with its own mount.  Many of the systems available today consist solely of the optical tube and require a separate purchase for the mount.

A recent variation in refractor design is the short focal length (SFL) refractor.  These scopes are highly portable and are quite easy to set up and use.  They also provide better contrast when viewing bright objects like planets and stars.  However, they do tend to suffer more from the effects of chromatic aberration and their short focal lengths limit their ability to magnify objects like planets and double stars.  They are, however, well suited for viewing brighter deep sky objects like the Orion Nebula and the Andromeda Galaxy, as well as for lunar observing.

For those interested in getting the most “bang for their buck”, reflectors (particularly Dobsonian mounted) offer the ability to get large apertures for a fraction of the price of other designs.  As with some things in life, when it comes to telescopes bigger truly is better!  Many observers suffer from a condition known as “aperture fever” and at large gatherings of amateur astronomers it is often those folks with reflectors larger than 20” in diameter that tend to steal the show.  These scopes are ideal for viewing the faintest objects, as their larger mirrors will gather more light.

Reflectors do not use lenses to bring the image of the sky into focus.  Rather, they use curved mirrors to achieve the same result.  Mirrors have one big advantage over lenses…they don’t suffer from chromatic aberration.  They are, however, prone to other types of optical distortions like spherical aberration (what initially impacted the Hubble Space Telescope).

Dobsonian mounted reflectors are quite popular, as they provide the lowest prices for a given aperture.  For a small child (8-10 years of age), a 4”-6” Dobsonian reflector may be one of the better choices out there.  Once you gain experience with the collimation procedure, they are relatively simple to set up and operate, and they generally cost about $200-$500 for a good quality system.

There are some drawbacks with reflecting telescopes, most notably the inconvenient location of the eyepiece where you must view and the lack of tracking in most mounts for these designs.  For some of the largest reflecting telescopes out there, when viewing an object near the zenith you need to stand on a ladder that could be 8-10 feet above the ground just to reach the eyepiece!  That could prove to be a little dangerous, especially at night.  As for tracking, most reflectors use manual tracking, in which the observer literally uses his or her hands to move the telescope to follow the rotation of the Earth.  Reflecting telescopes also require proper collimation to align the primary and secondary mirrors, which add to the complexity of system set up.

Other issues related to these telescopes include the need to cool down the large mirrors to eliminate thermal distortions as the system adjusts to the cooler temperatures at night.  Also, the larger telescopes are less portable due to their huge mass.  This is especially relevant if the tube is solid.  Many manufacturers of the largest Dobsonian telescopes (over 16” diameter) are using a truss system to reduce the overall system mass and make them more portable.

The third basic type of telescope is known as the compound or catadioptric (“cat” for short) telescope.  This telescope uses a spherical mirror as its primary optical element which introduces spherical aberration into the design.  To correct for this, a specially designed glass plate is put in front of the mirror that introduces an equal amount of negative spherical aberration.  As a result, the combined effect cancels out.

The most popular cat design style is the Schmidt-Cassegrain (SCT) telescope though other versions have been gaining favor lately.  These designs tend to be easily portable, compact and very versatile.  They have longer focal lengths which permits greater magnification than the other designs.  The location of the eyepiece is also more convenient than on the large reflectors.  While not optimized for viewing any particular type of object, they are well suited for viewing nearly everything to be seen in the sky.  Some of the drawbacks include being more complicated to set up and operate and they are typically more expensive than comparable sized reflectors.

I purchased my SCT nearly 18 years ago (in fact I can register it to vote later this month).  It has served me well during the intervening time, but it is not a beginner scope.  I have had many years of enjoyment and use from this scope.  I particularly recall the Mars opposition in 2003, when I was at Washington State University.  For that entire week, we set up at the campus observatory for public viewing of the planet.  There is a 12” Alvin Clark refractor there and I had my 8” SCT set up outside for people to look through while waiting in line.  Several of those who viewed Mars through both scopes commented to me that the view with my telescope was as good as if not better than what they saw through the larger telescope inside the dome.

Yet another question to ask concerns the nature and quality of the mount for the telescope.  You want to get a mount that has sufficient mass to quickly dampen any vibrations caused by wind or by moving the telescope.  However, you don’t want a mount that is so massive that it makes it difficult and time consuming to set up.

There are two basic types of mounts.  The first is called alt-az, which stands for altitude-azimuth.  This is just a fancy way of saying that this mount moves the telescope up or down (altitude) and left or right (azimuth).  Dobsonian telescopes use this type of mount as do many refractors.  They are usually operated manually, but it is possible to connect an alt-az mount to drive motors and have it automatically track objects across the sky.

The second type of mount is known as the equatorial mount.  This design uses the rotation of the Earth for tracking objects across the sky.  Once the mount is properly polar aligned, it needs to move in only one axis to follow an object while the Earth rotates.  While these used to be the preferred mounts, in recent years with the advent of greater computer controls, the alt-az system has gained increasing favor.

One other consideration regarding the mounts is whether to purchase a system with a “go-to” computer controller.  Many of the newer telescopes include these as a means of aiding the observer with finding interesting objects quickly and efficiently.  They can prove quite useful on some of the larger scopes, but I believe they are overkill when used with small scopes that are too small to detect many of the objects located in their databases.

As you can see there are many issues to consider when choosing the particular optical design.  What aperture can you afford?  What is the focal ratio?  Does the purchase price include the mount and is it of good quality?  Is the finder scope easy to use?  How complicated is the set up?  What can I see and how well should I expect to see it?  Astronomy Magazine editor Michael Bakich has a good online review here that addresses many of these questions and others.

The November evening skies are relatively devoid of bright planets.  Jupiter sets shortly after the Sun, and both Uranus and Neptune are tiny and dim.  However, in just the past week, a comet has undergone a massive outburst, increasing in brightness by nearly a factor of 1 million!  Comet Holmes can be seen all night long in the constellation of Perseus, near the bright star Mirfak (alpha Persei).

Mars rises in the mid-evening hours as the month begins and early evening by the end of November.  It is high overhead for most of the later part of the night.  In the morning, Venus is the magnificently bright object we see rising in the east ahead of the Sun, while Saturn is located above it in Leo, just to the left of the bright star Regulus.  As the month progresses, these two planets will continue to separate, with Saturn rising earlier each night while Venus falls back towards the rising Sun.

For those with a clear eastern horizon, you may be able to spot elusive Mercury rising about an hour before the Sun for the next few weeks.  It will be at its brightest around the third week of the month before once again fading as it too falls back towards the rising Sun.

Don’t forget to turn your clocks back Saturday night before going to sleep as we end daylight savings time.  This means that sunrises will occur earlier in the morning while sunsets will be earlier in the evening for the next several months.

The Boise Astronomical Society will be holding their monthly membership meeting on Friday, 09 November at 7 p.m. MST in Classroom B of the Discovery Center of Idaho in Boise.  This month we will be hearing a presentation from two of our members who attended the launch of the space shuttle Discovery in August.  This was the mission where Educator-Astronaut, and former Idaho resident Barbara Morgan made her first voyage into space.