View Full Version : Discussion: What Telescope is Right for You?

2005-Feb-16, 06:14 PM
SUMMARY: Amateur astronomy may very well be the most popular and quickly growing scientific hobby in the world today. This popularity undoubtedly has a basis in the fact that astronomy covers a lot of "turf". Meanwhile the existence of the Internet has made it possible for observers to share their love and knowledge of the sky while working together and advancing the state of the art. Finally, the fact that telescopes and binoculars are so readily available in great diversity of cost, quality, and capacity doesn't hurt either.

View full article (http://www.universetoday.com/am/publish/what_telescope_is_right.html)

What do you think about this story? Post your comments below.

Anthony Jifkins
2005-Feb-17, 02:12 AM
I thought this was a very interesting article and it would have been helpful had I not recently bought my first telescope. I have been a keen sky watcher for as many years as I care to remember, but this is my first scope and I am having lots of fun seeing up close, some of the objects that I had heretofore only viewed with the naked eye.

I know from my astronomy software that there are loads of things to look at but I cant find them because I do not know how to set up a telescope with an equatorial mount.

Could you please do an article on how to use a telescope so that I and others like me can get more benefit from there scopes.

Thanking you in advance

Anthony Jifkins
Melbourne, Australia.

The Near-Sighted Astronomer
2005-Feb-17, 07:01 PM
Hi Anthony,

Sorry the article didn't come out before you bought a scope - but just maybe someone else will have read it first. I recently received an email from someone who received their first scope last Christmas 2004. Here was my reply:


First off right now the Moon is especially beautiful. So if you go out and the sky is clear, turn your telescope on the Moon using your lowest power (the largest eyepiece recieved with you telescope). To find the Moon in the sky is easy (if its clear out) but to find it in a telescope is harder. That's why your scope came with a smaller scope to help you line things up.) But if your smaller scope is not lined up with your bigger scope you will have some problems. So if that's the case then you will need to line the two scopes up - this means finding something on the Earth that is far away and adjusting the angle of the smaller scope so whatever you see in the bigger scope is right in the middle of it.

So OK, once you line things up, you will see the Moon in your scope with a lot of incredible detail. Things like "seas", and craters, and mountains, and ridges - all the kinds of stuff you would see on the Earth if there was no life on the Earth and no oceans - just rocks. But you may have a lot of trouble keeping the Moon in the field of view. To improve things you will need to line up your mount pointing the axis toward the north star (Polaris) to make tracking easier. (Check out your manual for this.)

Oh yah: When you put any eyepiece into the telescope you will need to adjust the nobs that change the focus the telescope - otherwise the views will be awful. Meanwhile you should be aware that sky conditions can make it very difficult to see things clearly...

OK, now once you can find the Moon the next thing is to see Saturn - BUT you will have to wait until around 9pm to give the planet time to rise high enough in the sky. How will you find Saturn? Well it will look like a bright star in line with two other bright stars (Castor and Pollux in the constellation Gemini). You will see these three if you go out at 9:00PM and look up and toward the East. Saturn will be the star more toward the south from Castor and Pollux - but go ahead look at all the "bright stars" you can find with your telescope to the East - that will be fun anyway!

Once you find Saturn you will see it is not a starlike point of light. It will look more like a disk with ears. And that's when you will need to put the smallest eyepiece (highest power) you have in the telescope - in order to see the rings.

There is a lot more to see and once you find the Moon and Saturn you may want to go to a local star party to learn things from other amateurs as well.

Good Journeys!


All of the above is for an absolute beginner of course. Something I write up for UT would probably be for more advanced telescopists and would be more of an overview since I am limited on how long such an article would be.

A great start on this would be to have folks who are amateurs leave some comments on the kinds of things they would have liked to have known before they got started...

This forum could help!

Robert Kedoin
2005-Feb-19, 01:38 PM
Hello. I'm only a star-gazer wannabe. My daughter's received a telescope in 2003 and we've tried it out a few times.

I was doing a good job following your article until the fifth and sixth paragraph from the end when you started to talk about fast and slow telescopes. I got completely lost in "F7-" and "F12+" and had no idea what I was reading at that point. I imagine one is a larger telescope than the other, but as I said, I got lost.

Our biggest problems with our current telescope are getting what we want to see in the telescope. (Usually, I get stuck with this job and then run into the house to get them to see). The next problem is keeping the item there until they get back to the scope. I think it was Saturn that we were trying to view one night and everytime I ran in to get them, by the time I came back Saturn had moved out of the field of view causing disappointment all around.

Thank you for the article. It sheds some light and helps me realize there's still many more questions I need to ask about telescopes before I understand what's going on. :D

The Near-Sighted Astronomer
2005-Feb-19, 09:31 PM
Hi Robert,

Thank you your comments on the article. It is difficult sometimes trying to find just the right level of explanation to appeal to all readers.

The concept of focal ratio has to do with how far the lens or mirror sends the rays of light from the star or planet before they all converge. Think of an "ice cream cone" (without the ice cream). The tip of the cone is where the rays come to focus and the mouth is where the rays are collected. Focal ratio is the length of the cone divided by the width of the mouth. So say the cone is four inches long and has a mouth 2 inches in diameter, the ratio of length to mouth is 4 inches to 2 inches of 2:1. If the mouth was a mirror (or lens) and the tip was where the beams of light come to focus this would be a very fast "F2" telescope.

From this you get the idea that an F12 scope would be very long compared to its aperture and an F7 much shorter. In general, optical quality is better with longer focal ratios but usability (and wide fields) are better with shorter ones. As telescopes have improved there has been a tendency to continuously shorten focal ratios since portability and wide field views are very desirable. Right now you can buy newtonian telescopes in the F4 to F8 range. Achromatic refractors in the F5 to F15, apochromatic (high quality images - low "false" color) refractors in the F6 to F9 range. SCTS and MCTs are an interesting type they are very short (like F2-F3 telescopes) but due to their "folded" design are actually quite slow (F10 - F15). Another type of scope (the MN or "Mak Newt") is typically F6. Each type of scope has its advantages and disadvantages. Perhaps I shall write something up on this - if folks are interested...

And oh yes - be sure to use your lowest power eyepiece to find stuff (30-50x) . That way Saturn will hang around in the field of view for a couple minutes while you track down the fam. Then once they are together switch to a higher power (150x) to show them the details.

Carpe Noctem,


Dave Mitsky
2005-Feb-20, 01:06 PM
In general, I agree with what Jeff stated in his article. However, there are some points with which I beg to differ.

"Meanwhile the author has also watched fellow observers take nearly an hour setting up a large truss-framed Newtonian telescope on a relatively simple (dobsonian) mount. (All the while the sky darkened and stars drifted a full fifteen degrees across the heavens.)"

This gives an erroneous impression that all truss-tube Dobsonian Newtonians take a long time to prepare. Perhaps the very largest apertures commercially available today may take that long (with ancilliary gear such as equatorial platforms or stepper motor drives) but the most common ones (11 to 18 inches) are far easier and quicker to set up than a large refractor, Newtonian, or Schmidt-Cassegrain on an equatorial mount. Accurate polar alignment takes time. My 12.5" truss-tube Starsplitter Dob is ready to go in 15 minutes or less. I've helped two friends of mine get their truss-tube Dobs, a 14.5 inch Starmaster and 15 inch Obsession, ready many times in a span of 20 to 25 minutes. It seems a moot point to me anyway, since the majority of observers that I know set up their Dobs during daylight.

"In contrast to quasars, the brightest celestial study is the Sun. Due to its brilliance, it takes but a few inches of aperture to get decent views of spots, faculae, granularities, and other fine features. (The Sun is so intense that direct inspection without a solar filter will permanently damage the retina!!!) Solar observing is best pursued with small scopes due to the reality of daylight sky conditions. As the Sun heats the atmosphere, super-fine detail is lost. Because of this, three inch instruments deliver all detail possible (except when observing at high elevations)."

It's not the Sun's brilliance, which is the very thing that solar filters diminish, but its large apparent size of half a degree that allows a small telescope to provide fairly good views. (The same is true of the Moon which subtends a similar angle.) The seeing (i.e., atmospheric steadiness) tends to degrade as the temperature rises during the day as you mentioned but I have seen full aperture, white light views of the Sun in apertures as big as 11 inches that have been absolutely stunning and not from mountain tops either. There are other locations, southern Florida being one obvious example, that have uniformly excellent seeing. As far as H-alpha solar observing is concerned, the very best views that I have ever had have been through a 7.1 inch Astro-Physics Starfire apochromat that belongs to Jim Sweeney, a fellow DVAA member, and the 9 inch Clark refractor at Bucknell University.

"(Although fast newtonian models are available, such scopes often show pronounced coma at wide angles. In general, scopes that include light-handling refractory collector elements (refractors, Maksutovs and Schmidt's) give superior off-axis image quality to all but the slowest pure reflector models."

Coma is not a major problem with Newtonians that have f/ratios of f/6 or above. The coma in fast Newtonians is easily ameloriated with the use of a Tele Vue Paracorr coma corrector. Many people are not bothered much by it anyway and don't shell out for Paracorrs. Coma is more pronounced when low power, wide-field and ultra wide-field oculars are used. The astigmatism present in most of these eyepieces is a far greater optical aberration than coma anyway.

You seem to sell the Newtonian design short. Well-made Newtonians with small central obstructions and curved spider vanes can provide stunning images. The problem is that the major manufacturers such as Meade and Celestron aren't interested in selling high-quality Newtonians due to the large profit margins possible with easily manufactured designs like the Schmidt-Cassegrain. Off-axis Newtonians, which have no central obstruction, produced "refractor-like" images without the chromatic aberration that achromatic refractors exhibit. I've done some observing with an 11 inch achromat and the false color was so bad on bright objects that the owner had to stop it down to 8 inches.

All telescope designs have optical aberrations of one sort or another.

"Meanwhile some very fast scopes (F7-) can lack the kind of optical quality needed to specialize in lunar-planetary-double star observing."

Fast telescopes are generally considered to have focal ratios of less than f/6. Scopes with f/ratios between f/6 and f/10 are called "normal" . Slow telescopes have f/ratios greater than f/10.

I've looked through quite a few "large" Maksutov-Cassegrains including the 7 inch Questar and apochromatic refractors including one of the very few 8 inch Astro-Physics Starfires ever built but the very best planetary views that I've ever had have been through large premium truss-tube Dobs and classical Cassegrains. A 24 inch f/4.3 Starmaster with one of Carl Zambuto's legendary mirrors gave me an absolutely exquisite view of Saturn and a 25 inch f/5 Obsession provided an equally remarkable image of Mars.

Dave Mitsky

The Near-Sighted Astronomer
2005-Feb-21, 05:28 AM
Hi Dave,

Wonderful that you have added your comments to the forum - and I take no exceptions to them in the particular.

Let's keep in mind however that it makes more sense to give folks an overview with some simple rules they can use generally to guide their choices. There are exceptions to everything and you've done a fine job of listing them.

There are some wonderful optics at F6 and below that give stunning views.
There are some observing conditions and times of day when larger apertures will give fine views of the Sun. There are some superb fast newtonians that give very sharp views of the Moon and planets at large apertures. (I saw a view through a 14.5 inch Starmaster of Jupiter that completely blew away my own MK-67 - and the seeing was marginal at the time.)

But in general if you want flat fields don't get a fast newt. If you want image quality in an achromat get something F10 plus. If you want portability plus aperture get an SCT.

Meanwhile for those of you who can follow Dave's input here - super! Otherwise ask some tough questions of yourself about what you want to see and about setup, and tough questions of vendors about what they are selling.

Make an art of buying that last telescope first - its the one that will test how much sustainable interest yo0u have in our High Art and Science.

Carpe Noctem,