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Astronomy Hints



General Telescope Tutorial

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Astronomical Telescope Tutorial

The image below is of the Lunar Apennine mountains, and was taken with my ETX90 Maksutov telescope and a Celestron NexImage webcam. It is a stack of about 50 frames, selected for their clarity. More images taken with that setup are at ETX90 Astrophotos.

Such images are very reachable with any quality small telescope using either a purchased web cam astrocamera such as the Celestron NexImage or a homemade web cam conversion like that on the Webcam Astrocamera web page.

Lunar Apennines With Meade ETX90 and Celestron NexImage

Yet another tutorial on telescopes

Why read another tutorial? Because:

  • There's always a new tidbit in a page you've not encountered before.
  • Telescope tutorials are biased by user experiences (including this one), so they are not all alike.
  • This tutorial is based upon my first-hand experience as well as other documents on telescopes.
  • This tutorial has some handy tables and calculators for helping you plan equipment purchases.
  • The holy grail, the perfect telescope

    Telescope Use Survey Results

    Most Used Telescope Survey Results

    As the two charts above taken from the Amateur Astronomer Survey suggest, what constitutes the perfect telescope certainly varies from user to user. The top-most chart shows what telescope survey takers listed as telescope types they owned. The lower chart shows what survey takers listed as the telescope types most often used. It's an instructive pair of charts, with some expected results and some confusing ones.

    Both charts agree that refractors seem a popular favorite, but there's a lot of big Dobsonian users out there also. More curious is the indication that while slightly more survey takers own Newtonian telescopes than Dobsonian telescopes, in actual use they seem to prefer the Dobsonian telescopes nearly 2 to 1 over the Newtonian telescopes. I would speculate that the ease of use of the Dobsonian over the Newtonian, especially in sizes over 6 inches, accounts for this result. Having owned a homemade Dobsonian for many years, I can attest to how handy they make the use of large aperture telescopes. I enjoyed my old clunky design, which wasn't nearly as slick as the modern commercial DOBs, like the Orion 8945 SkyQuest XT8 Classic Dobsonian Telescope.

    I'll preface my discussion of what constitutes perfection with a few opinions that I've settled on after some 50 years of amateur astronomy. First, I'm a believer in the oft made statement that every telescope has its sky, assuming that its optics are reasonably good. I also haven't been able to find that "perfect" telescope. It seems, after all this time of telescope use, that all telescopes are either somewhat specialized some way, or are a compromise.

    There are many trade offs, including viewing interests, tolerance for maintenance, portability, and price. It is the interplay of all of the factors that makes different telescope users have different requirements. For that matter, ones telescope needs, as I can attest to after all these years, change over time. So I caution you, don't let anyone tell you that you need a certain telescope because it's the best. You're the only one who will know how all of the factors combine in guiding your telescope preference.

    If You Know What You Want To View ...

    Astronomy Survey of Favorite Targets

    There are many different types of targets that draw people into the hobby and profession of astronomy. My survey participants so far show most interest in observing the moon, planets, and Deep Space Objects (DSO's). Somewhat less popular are double stars and comets. Some people enjoy observing the sun, but solar observing requires use of proper safety equipment. The deep space objects consist of many interesting objects, both external to our galaxy (other galaxies) and internal to our galaxy, such as diffuse nebulae, planetary nebulae, open star clusters, and globular star clusters. As beautiful as comets are to view, I'm sure that the reason for their lower popularity is that really good ones become visible infrequently, typically every couple of years.

    These many and varied targets make different demands upon the user and his or her equipment. That's certainly part of the variance in the types of telescopes preferred -- the targets of most interest. The book The 50 Best Sights in Astronomy and How to See Them: Observing Eclipses, Bright Comets, Meteor Showers, and Other Celestial Wonders gives any new observer a great starting place, describing some of the best of the multitude of targets that entice amateur astronomers. This humble web page gives you at least some ideas about the hobby.

    Shown below is a chart to help you decide what type of telescope you might be shopping for if you already know what kind of observing you find interesting. From left to right the chart starts with wide-field telescopes good for sweeping views of rich star fields, to high resolution telescopes most capable for observing the moon and planets.

    From bottom to top the chart goes from compact and portable telescopes to large and non-portable telescopes.

    You'll notice that going from bottom to top yields larger apertures, and from left to right yields higher f ratio (longer focal length) telescopes in general. Going from the lower left in any direction tends to also mean higher prices.

    So if you're interested in a portable telescope for star gazing, start in the lower left corner. If you're a planet hunter and portability isn't an issue, start in the upper right, etc.

    The chart doesn't state hard and fast rules for telescope use, but presents the general features amateurs often consider.

    Telescope/Observing Preference Table

    (Small Instruments At Table Bottom)

    Wide FieldGeneral PurposeNarrow Field
    15" f/4.5 DOB10" f/10 DOB12" SCT
    12" f/4.5 DOB10" f/6 DOB6" Refractor
    6" f/5 Newt8" f/10 SCT6" Maksutov
    6" f/5 DOB6" f/8 Newt6" f/10 Newt
    3.5" f/8 Refractor4" f/11 Refractor4" f/15 Refractor
    4.5" f/4.5 DOB5" f/10 SCT5" f/15 Maksutov
    2.4" Refractor4.5" f/10 Newt3.5" f/15 Refractor
    Binoculars3" f/10 Refractor3.5" Maksutov

    What's Cheezy, Scope-wise?

    If you're not yet sure enough to use the shopping chart, here's some more information to help you understand the type of telescope that might be right for you.

    The mistakes new buyers make are usually consequences of not knowing the factors I've cited. You may buy a telescope that is far to cheesy for you to see what you desire. You might just as well error on the other extreme and buy a behemoth that is so bulky and difficult to handle that it ends up in a garage sale in just a few months. But if you choose well, you'll find the hobby of astronomy enormously enjoyable, if not a little humbling.

    There are many good brands out there, including Meade, Celestron, Vixen, Tele Vue, Orion, Zhumell, Questar, Parks, and others. Stick with these brands or similarly priced brands and you'll likely come out well. For a time, when Chinese made telescopes were making their way into our market, the Chinese labels were a good warning sign. But more recently the Chinese made instruments have improved significantly. In fact, many of the telescopes sold by some of the major distributors are imported from China.

    A oft stated general rule, and one worth considering, is don't by a telescope from a department store. That rule isn't quite as cut and dried today, since some major manufactures such as Meade and Celestron market through places like Sams or Walmart. But while the optics in department store telescopes has improved, it's still a good idea to avoid such telescopes.

    Particularly, stay away from telescopes that are on flimsy mounts. And unfortunately, that's exactly what department store telescopes tend to have. The cheap mount may work at low magnification, but at higher power, or with the slightest breeze, you'll see an image maddeningly jumping around as you try to see details. Sometimes this can be remedied with some bracing added to the tripod legs, but it's best to avoid telescopes with undersized mounts. Either that, or consider making your own Pipe Fitting Tripod.

    Steer away from scopes that are advertised by their magnifying power, like 500 x 60mm, suggesting a 60mm diameter telescope that magnifies 500 times. These telescopes usually do include a Barlow lens and a very short focal length eyepiece (4mm or so) that actually produce a magnification of 500x. The problem is, the listed magnification is way beyond what is useful for that size of telescope even if the optics were superb, and these telescopes usually have sub par eyepieces and Barlow lenses.

    The general rule of thumb is that the maximum useful magnification for any instrument is about 50 times the objective diameter expressed in inches. The objective is the main image forming lens or mirror. So a 60mm telescope is about 2.4 inches in diameter, yielding a maximum useful magnification of 120 times.

    This common stated rule is a good rule when applied to mid-size and larger telescopes. Good quality, smaller refractors can often tolerate up to 75x or even 100x per inch of objective diameter and still give good images if the target object is bright enough, and seeing conditions are exceptional.

    That magnification might not seem like much, but it is enough to see countless moon craters, cloud bands on Jupiter, Saturn's rings, and some detail on Mars during favorable (close) oppositions. The following calculator presents the maximum useful magnification for some popular sized telescopes.

    Select Telescope Diameter
    Telescope Size in mm Maximum Useful Magnification

    There is a caveat here. The maximum useful magnification is a number based upon the telescope alone. In actual use, the atmosphere places further restrictions on the maximum useful magnification. This is particularly true for bigger instruments, say 8 inch and larger. It's a rare occasion for an 8 inch telescope to be actually be useful at 400x. A factor of 30x to 20x per inch of objective diameter better applies to telescopes 8 inches and over. In fact, regardless of telescope size, magnifications of over 200 to 300 are rarely used.

    For you older folks (like me), you may find that the higher magnifications present another problem. With higher magnifications, the light from the target object gets spread over a greater and greater area, making the target appear dimmer. When this happens, you may find that the floaters in your eye start to be visible and quite distracting when straining for those fine details. A little less magnification will brighten the image and cause considerably less distraction from floaters. Of course, a bigger telescope using the same magnification will brighten up an image, possibly making floaters less of a problem.

    I find that though images through my 60mm refractors and ETX90 are very good up to even 200 power, the small apertures of these instruments makes images at such magnification dim enough to really accentuate my eye floaters. But at the same 200 magnification, my 6 inch telescopes make images bright enough to make the floaters much less of a problem. That's probably one reason for the popularity of the compact Cassegrain telescopes, like the Celestron NexStar 6 SE Telescope. These telescopes give big enough apertures to provide that high magnification with bright enough images to reduce the nuisance of floaters. They are also compact enough to be very portable.

    Another option for dealing with floaters, should you have the funds, is to purchase a binoviewer accessory, like the Celestron Stereo Binocular Viewer for Telescopes. The binoviewer goes into the focuser and directs the incoming light to two matched eyepieces, making the view available to both eyes. Since the floaters aren't the same in both eyes, a binoviewer helps eliminate them from your view. A binoviewer will also greatly reduce fatigue during long viewing sessions. Just be sure to purchase one of good quality. Note also that to use a binoviewer, you need matched sets of eyepieces, since there must be an eyepiece for each eye. This also ups the cost of using the binoviewer.

    A excellent site with more telescope buying considerations is Which Scope To Buy.

    How many telescopes is enough?

    I caution you that if you choose well and really get into the hobby of astronomy, you'll likely either migrate through a number of telescopes as your interests change, or end up owning more than one at a time. To the consternation of my spouse, I own six.

    Why six, you ask? In my defense I want to point out that many amateur astronomers own much more than that. And the reasons vary. For some, it's just that there are so many types, and they want to experience them all. Many telescopes have specific uses, such as a planetary telescope, a light bucket (star telescope), a travel telescope, etc.

    My telescopes and reasons for them are as follows:

  • A 60mm mid-length refractor, received as a gift, great for quick setups and and short viewing sessions.
  • A 60mm long-focus refractor, constructed cheaply and used to experience the classic 60 mm size.
  • A 90mm ETX Maksutov, very portable, great for quick setup, planetary observing, and short exposure photography.
  • A Celestron NexStar 5 SE Telescope , portable, great for quick setup, good for general observing and short exposure photography.
  • A 6" f/5 Newtonian reflector like the Celestron Omni XLT 150mm Telescope Newtonian Reflector 31057 , great for wide-field star work, and still quite portable.
  • A 6" f/10 Dobsonian reflector, superb for planetary observing.
  • Do you get the idea? Different telescopes have different advantages, and we'll go over many of them here.

    Some important parameters

    Regardless of telescope type, there are a few parameters you should be aware of as they pertain to what you'll see through the telescope you choose.

    Resolution is a term used to describe a telescope's ability to deliver detailed views. The larger the diameter of an instrument, the better the telescope's theoretical resolution. Strictly speaking, resolution is a function of the telescope diameter and the wavelength(s) being observed. Telescope resolution in arc-seconds is approximated by the following formula:

    Resolution = 4.5/diameter(in)

    This equation inherently assumes that the wavelength is that of the middle of the visible spectrum. If wavelength was included in the equation, it would be in the denominator. Thus, longer wavelength would give less resolution for any given instrument. Or conversely, for any given resolution, receiver size must increase with wavelength. That explains the enormous size radio telescopes must be to give good resolution at millimeter wavelengths.

    The main image forming element in a telescope is called the objective. As I'll discuss, it is a lens in some instruments, and a mirror in others. A lens of twice the diameter has twice the resolution capability -- if you ignore the atmospheric limitations. As it happens, for the typical backyard observer, the atmosphere creates a resolution limit of about one arc-second (1/3600 of a degree). That relates to about a one mile diameter crater on the Moon as seen from Earth. Seeing occasionally gets better, and often gets worse. Fleeting glimpses of better clarity often occur during an observing session.

    For a given diameter, a refractor is generally considered to give higher contrast views, especially on low contrast objects such as Mars, Jupiter, or low contrast nebula and galaxies. On high contrast objects, like the moon with its stark shadows or Saturn with its rings, refractors have no particular advantage over a reflector of similar size. The reason for the refractor's better contrast is that it has no obstructions in the light path between the objective and the eyepiece.

    There are reflector telescope designs that also avoid obstructions in the field of view, but these aren't commonly available to amateur astronomers. Most reflector telescope designs must have a mirror in the light path to deliver the image to the eyepiece. This secondary mirror is responsible for the lower contrast views produced by reflectors compared to their similar diameter refractor brethren.

    For those low contrast targets, a pretty good rule of thumb is that subtracting the diameter of a reflector's secondary from the diameter of its primary gives the size of refractor that would deliver as contrasty a view. So when looking at Jupiter with an 8 inch Schmidt Cassegrain that has a 2.6 inch secondary, a 5.4 inch refractor (8 minus 2.6) would give a similar contrast result. The image in the 8 inch would be brighter, but the contrast would be similar to that in the 5.4 inch refractor.

    On high contrast targets, the 8 inch would deliver better resolved images, and certainly would deliver better on dim objects. This topic is discussed in detail at The Effects of a Central Obstruction.

    That brings up Light gathering power, another telescope attribute. Think of a telescope as a funnel. If you put a test tube out in the rain, it will collect some water, but will take awhile to fill up. Now put a funnel in the tube, and the wider opening of the funnel will collect water over a much greater area and fill the test tube much quicker.

    That's essentially what a telescope does for the eye. It collects light over a much bigger area than the eye pupil and funnels it into the eye. Just like a funnel, the light gathering power of a telescope is a function of area, which is proportional to the diameter squared. Thus a telescope of twice the diameter will collect four times the light (make things four times brighter).

    The following calculator illustrates the relationship between telescope diameter, resolution, and light gathering power.

    Select Telescope Diameter
    Resolution in arc-seconds Observable Star Magnitude
    Smallest Resolvable Moon Crater (mi) Magnitude Ratio to Naked Eye

    In the preceding calculator, note that the resolution is a function of diameter. Twice the diameter will resolve something 1/2 as big.

    Note that star magnitude is a logarithmic scale, with each magnitude being about 2.5 times brighter than the next larger magnitude. That is, a magnitude 5 star is 2.5 times brighter than a magnitude 6. Incidentally, under dark skies the naked eye is limited to about magnitude 6.

    The following table shows the parameters and best uses of some common telescopes.

    Telescope Characteristics Chart

    TypeSize RangeF ratiosUseCost
    Dobsonian4.5" to 30"f/4 to f/10General, Visual, Light Bucket$240 - $20,000
    Newtonian EQ4.5" to 10"f/4 to f/8General, Visual, Photography$200 - $2000
    Maksutov3.5" to 7"f/13 to f/15Lunar, Planetary, Visual, Photography$300 - $2000
    Schmidt Cass.5" to 16"f/10 to f/11General, Visual, Photography$1500 - $16000
    Refractor2" to 6"f/6 to f/15General, Visual, Photography$150 - $5000

    This astro-customized search engine can help you shop at some excellent telescope outlets:

    Custom Search

    Another useful chart standardizes on the 6" size to better give the price differential of the different telescope types.

    Type vs Cost for 6" Telescope

    TypeMotor DriveCost
    Newtonian EQNo$420
    Newtonian EQYes$500
    Schmidt Cass.Yes$1000