TRG – Telescope Review Guide

There’s a new comet passing nearby and it’s name is Lulin. This is when and how to find it.

On the night of February 23rd, in a moonless sky and near its peak brightness, Comet Lulin passes 2° south-southwest of Saturn.

Lulin’s closest approach to Earth, 61 million km, occurs on February 24th, when the comet may be at a peak of magnitude 5.0. By now it’s high up by late evening.

On the night of February 25th the comet goes through opposition, nearly 180° from the Sun in our sky. This means there mght be an “opposition effect”

brightening of its dusty coma and dust tail!

Path of Comet Lulin during February

After that Lulin moves away from both Earth and the Sun, fading quickly. The evening of February 27 will see it at about 6th magnitude within 1° of Regulus. Moonlight starts interfering again around the 28th.

Tails and Antitails

Because the comet stays nearly on the ecliptic, its tail (which points away from the Sun) aligns with the ecliptic and with the comet’s own direction of motion across the sky.

Moreover, because Earth remains in the comet’s own orbital plane, we see the comet with a very thin tail and an antitail, a spike pointing in almost the opposite direction from the main tail. Why? In three dimensions a comet’s dust tail is often wide but it’s always thin, confined to the comet’s orbital plane. When we are in or near this plane, we can sometimes see parts of the wide, thin dust tail on opposite sides of the comet’s head. We pass through most comets’ orbital planes briefly. This time, the situation will last and last.

A comet’s blue-green gas tail, on the other hand, always points nearly in a straight line away from the Sun in space. Cometary gas is blown directly away from the Sun at high speed by the solar wind.

MARCH: Following Lulin Out

Comet Lulin crosses from Leo into Cancer at the beginning of March and, having passed opposition, is now better seen in the evening than the morning hours however observers will have to contend with moonlight from about March 1st through 11th.

As Comet Lulin recedes, its passage across our sky will slow.  By May’s end it will be lost in the afterglow of sunset.

Comet Lulin won’t return again to the inner solar system for more than a thousand years.

Last of a series

Your telescope

No matter what telescope you select, choose one that will meet your precise needs and interests. The planets, the Moon, and close stars require high power, good contrast, and sharp resolution, and if these are the objects of your attention, a refractor or reflector would be a good choice. Because very faint objects like galaxies and nebulae need a large aperture, you should invest in a big reflector telescope to view these. An all-purpose midrange telescope should serve best, for example a 6″ to 8″ reflector or an 8″ Schmidt-Cassegrain.

Keep checking back. Future articles will delve into filters used to cope with light polluted skies, astrophotography, astronomy book and software reviews, astronomical binoculars, and reviews of specific telescope models.

Next to last in my 6 part series on choosing your first telescope.

Telescope mountings

In the first four parts, I’ve discuse aperture, power and the different types of telescopes. Now I’d like to talk about an often overlooked but very important aspect of using a telescope – the mounting. A shaky view is all it takes to a fanatastic viewing session! A good mount can enhance your views. There are two basic telescope mountings:
ﾷ The equatorial and
ﾷ The altazimuth.
An equatorial mount is designed so you can easily track the motion of the sky as the Earth turns and its motions indicate celestial directions in the eyepiece. This helps when you’re trying to find your way among the stars with a map. The altazimuth mounts are simpler and just swing up, down, left and right. You have to move the scope along every so often to follow the stars, moons and planets. An altazimuth mount is both cheaper and lighter for the same degree of stability.

Come back for my concluding article when I’ll wrap everything up.

Catadioptric telescope

Catadioptric telescopes use a combination of mirrors and lenses to fold the optics and form an image.  There are two popular designs, the Schmidt-Cassegrain and the Maksutov-Cassegrain.

In the Schmidt-Cassegrain, light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube to be intercepted by a small secondary mirror. The mirror then reflects the light out the back of the instrument where the image is formed at the eyepiece.

Advantages

ﾷ Most versatile type of telescope
ﾷ Best near focus capability of any type telescope
ﾷ First-rate for deep sky observing or astrophotography with fast film cameras or CCD’s
ﾷ Excellent for lunar, planetary and binary star observing plus terrestrial viewing and photography
ﾷ Closed tube design reduces image degrading air currents
ﾷ Compact and durable

Disadvantages

ﾷ More expensive than reflectors of equal aperture
ﾷ Slight light loss due to secondary mirror obstruction compared to refractors
The Maksutov-Cassegrain telescope design has basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt’s giving it slightly better resolution for planetary observing.

However, the Maksutov is heavier than the Schmidt and because of the thick correcting lens, it takes a long time to reach thermal stability at night in larger apertures. The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt Cassegrain.

Welcome back to part 3 of my series on how to buy a telescope.

The reflecting telescope

Reflecting telescopes use a parabolic mirror at the back of the telescope to gather and focus the light to a flat secondary mirror that the image out of an opening at the side of the main tube. You observe through an eyepiece on the side of the tube up near the top.

ﾷ Easy to use
ﾷ Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters because their larger apertures collects more light
ﾷ Low in optical irregularities and deliver very bright images
ﾷ Reasonably compact and portable
ﾷ A reflector costs the least per inch of aperture compared to refractors and catadioptrics

Disadvantages

ﾷ Not suited for terrestrial applications
ﾷ Slight light loss due to secondary obstruction when compared with refractors
ﾷ The tube is open to the air, which means dust on the optics even if the tube is kept under wraps

Check back often for the continuation of my series on how to buy a telescope. Part 4 will discuss catadioptric telescopes.