Saturday, November 19, 2011

Observing with a 60mm Telescope: Technical Details

When determining what you can do with any telescope, there are some equations which give you an idea of what you can expect from your telescope. These equations are estimates, as they don't take into account the construction details of your particular scope. The better your scope is optically, the closer it will come to these estimates.

But in any case, real, practical observing and the skill of the observer will be more important than numbers turned out of an equation. So all these numbers should be taken with a shaker-full of salt. Like the gas efficiency numbers for a car, they're a basis for comparison but not reality.

Lowest Useful Magnification

The equation for this is the diameter of the telescope divided by the observer's pupil size when open to its widest. Basically it measures how large the image can get before light is lost outside the observer's pupil:

Lowest Useful Magnification = D / p

Where D is the telescope's objective diameter, and p is the pupil diameter. The rule of thumb version is:

D(in inches) x 4 for a 6mm pupil (older adult average pupil)
D(in inches) x 3.6 for a 7mm pupil (younger adult average pupil)
D(in mm) / 6 for a 6mm pupil,
D(in mm) / 7 for a 7mm pupil.

With a 60mm telescope, that gives us a minimum useful magnification figure of 9 or 10 powers.

This is tempered by the practicality of getting an eyepiece that's one ninth or one tenth of the focal length of your telescope's objective. If you have a 60mm f/10 telescope that would be about a 60mm focal length eyepiece! Typically the longest eyepiece focal lengths you'll find in a size for your 60mm telescope are about 45mm. This would give the 60mm f/10 a magnification of 13.

A 60mm f/6 telescope has a focal length six times 60mm, or 360mm. A 36mm eyepiece would give 10 power in this telescope. A 42mm eyepiece would give a magnification just under 9 powers.

What does this mean?
What this means is that it's just about impossible to go to powers that are too low for your 60mm telescope to be useful. As a matter of practice, it's not really a problem to go below the "lowest useful magnification" by a bit, as in the example of the 60mm f/6 with a 42mm eyepiece. You'll lose a little light outside the area of your pupil, but the eyepiece will also be a bit more forgiving about how you position your eye next to it. It also adds to the "spacewalk" experience when your eye can get image even when you move around a bit at the telescope.

Low powers are also where you'll typically get the widest field of view from your telescope. They are well suited for looking at dim deep sky objects, concentrating their light into a smaller area in your field of view, making them easier to see in spite of your limited light gathering power.

Minimum Magnification to See Finest Detail
This is a calculation to estimate what magnification you should use to make out the finest detail in well-illuminated objects like planets and the Moon. If you are below this magnification, you're missing some detail, theoretically. If you're above this, the light on dim, marginal objects will be spread out, possibly causing some detail to be lost.

In practice, this is a decent estimate of the highest magnification for the scope for general use, and the highest usable magnification for the scope will be about twice as high with brighter objects.

The calculations assume that the observer can see detail down to about 100 arcseconds without a telescope.

A simple way to calculate it is 100/R, where R is the resolving power of the telescope in arcseconds. For a 60mm scope, R is about 3.0 in practice, as low as about 1.9 in theory. This gives us values from 33 powers to 52 powers.

This result again emphasizes that the 60mm scope is at its best at low powers of magnification. It gives us a practical top magnification for image details at around 70 to 100 powers depending on how good conditions are. Better conditions allow higher powers. The lower powers are the magnification that will give the finest detail on planets, the Moon, in nebulas, and so on. The higher powers are how high you can normally expect to go without losing lots of internal detail. In objects where internal detail is not as important, such as a galaxy where you're trying to find the outer extremities, you can go to lower power to raise surface brightness. Along the Moon's terminator, where contrast is very strong, you can probably still see plenty of detail at higher powers.

Maximum Useful Magnification
This is another useful rule of thumb. It's also based on the aperture of the scope. This is one of the things that causes "aperture fever", where amateur astronomers keep going for larger and larger telescopes. In fact, the contrast of the image will determine what can be seen with the scope and how well it will work with higher levels of magnification as much or more than the aperture. But contrast is tricky to measure, while all it takes to measure aperture is a ruler and the open end of a telescope.

Standard rules of thumb for this figure are as follows:
M = 20 x D(inches) or
M = 0.8 x D (mm)

For a 60mm telescope, this gives us a maximum magnification of about 48 powers. The rule of thumb also states that typical magnification should be about half this value to get maximum brightness and contrast.

Does that mean your scope is useless at powers higher than about 50? No way! This is a rule of thumb more applicable to larger scopes (6 to 30 inches) than it is to smaller scopes. Also, this number will vary dramatically depending on the optical quality of your scope. The highest quality refractors in 60mm aperture can give great detail at over 300 powers under the best conditions. But this isn't too bad a measure for a scope of moderate performance. Basically, while the number it produces is low, it again emphasizes that the best performance for a small scope is at relatively low powers. You'll see more if you use your scope in the way that it works the best.

Theoretical Limiting Magnitude
This is the theoretical dimmest star you can see with your telescope. For your 60mm this is a magnitude of 10.6. The highest quality scopes will outperform this, and poorer scopes won't do this well. My experience is that the contrast of the image the scope gives, and the quality of the sky--how dark the background of the sky is--have the strongest effect on this.

For "extended objects", that is, anything that doesn't have all its light in a single point like a star, the apparent magnitude is a measure of all its light put together. So if you find a galaxy in a catalog that has a magnitude of 10.4, it may well not be visible in your 60mm telescope, even though your scope can theoretically see even dimmer objects. The problem is that the light is spread out over a larger area, so that the light at any point is too dim for the telescope to show it.

Putting It All Together
Upshot for a 60mm telescope, keep your powers low to see the most. Pick bright objects to observe. Get to dark skies if you can. Get the best optics possible, and remember that optical quality doesn't matter at all if you're not working on a good, solid mount that holds the scope on target.

Saturday, November 12, 2011

Observing Galaxies with a 60mm Telescope

This article is part of a series I'm doing on observing with 60mm telescopes. 60mm telescopes are among the most common of first telescopes. My own first telescope was a 60mm which I used for over 10 years, before "moving up" to a 75mm, then over 20 years later to a 200mm telescope. The points in this article are applicable to larger scopes than 60mm, though it is written about 60mm scopes.

In the first article in this series I listed a number of galaxies that can be seen with a 60mm scope. Here I'll cover some of the best, along with tips on how best to observe them with a small telescope.

Conditions for Observing
With a small scope, small things can make a difference between success and failure when observing faint objects. The sky must be right. Low atmospheric moisture levels mean better contrast and more light from the galaxies you want to view. The telescope must be right. It should be in good condition, with clean optics, and most importantly of all, a stable mount that holds the telescope securely on its target, and allows the scope to be moved and pointed easily. The observer should be in a relaxed, unhurried mood.

Bright Galaxies
There are a few galaxies that are bright enough to be seen easily, even with a 60mm telescope. The Andromeda Galaxy, M31, is one of these. In the southern hemisphere the Large and Small Magellanic Clouds are visible to the eye, so they're easy to find and point the scope at. They're also bright enough to show a lot of detail. M31 is too large to fit into the field of view of a normal telescope, but the brightest parts of the halo, the disk part of a spiral galaxy, and the nucleus, the ball of stars at the center of a spiral galaxy, are visible. It is a good observing exercise to see how far from the nucleus of M31 you can go while still being able to tell the halo of M31 from the background sky.

Low powers will give the best contrast in a small telescope like a 60mm. It is more important to be able to make out the galaxy against the background to see it than it is to magnify the detail in them. The higher the level of magnification, the more the light from the galaxy gets spread out, lowering the contrast. The brightest galaxies, like M31, LMC and SMC, M65 and M66 will allow some moderate magnification. Still, they are best viewed at the lowest power possible with the telescope.

If you don't have an eyepiece with a longer focal length than 25mm, you may consider getting one just for observing deep sky objects at low powers with your telescope. either 32mm or 40mm are fairly common. My 40mm eyepiece is my favorite for viewing galaxies, and once it goes into the focuser for the evening, it almost never comes back out.

Dimmer Galaxies
There are many dimmer galaxies visible with a 60mm telescope. Don't expect them to look like the photographs. They'll look like fuzzy gray spots. But they'll have different shapes, brightnesses, and distributions of light across their visible form. Some will have bright centers and dimmer areas around, others will be equally bright all across, others yet will be splotchy or broken into multiple sections.

Near M31 there are two other galaxies, M32 and M110. They are much, much smaller companions of M31, like the LMC and SMC are companions of our galaxy. They are easy to see because they have a high surface brightness. This is the most important determinant for whether a galaxy will be visible in a small scope.

The apparent magnitude is the brightness listed in sky catalogs for galaxies. It's a misleading number, because it is the measure of how much light it would put out if all its light were gathered into one point of light, like a star. But the light is spread out over the whole face of the galaxy.

How much light is put out by any specific part of the galaxy you see is the surface brightness. A galaxy may be bright, but have a low surface brightness. The Triangulum Galaxy, M33, is like this. It is visible in a 60mm scope, but only with difficulty, because any part of it is very dim. Its total light is spread out over a large area of sky. M110 is very bright at any point. It is compact and well defined, making it easy to see compared to M33. So when reading descriptions of galaxies, look for those with a high surface brightness to view with the 60mm scope.

M108 and M109 in the Big Bear both have a high surface brightness. They are spiral galaxies that we see edge-on, so they appear as a short line of light or small lens shape near different parts of the Big Dipper.

M65 and M66 are bright galaxies in the leg of Leo, The Lion, that can be seen together in the same low power field of view. So you can see, and show others, two galaxies at once! There is a third galaxy nearby that's a bit dimmer, but if you can see it, you'll have three galaxies at once.

Sunday, November 6, 2011

Observing the Deep Sky with a 60mm Telescope

"Deep Sky" observing is not normally associated with a telescope as small as a 60mm (2.4 inch). There are many nice objects that can be viewed well with a small telescope, however. This is part of a series of articles on this blog about observing with the 60mm telescope. The information also applies to larger scopes, which can get even more out of these objects with their greater observing power.

There are a good number of objects that most any 60mm scope can show well, so long as they have a decent mount. The Lagoon and Trifid Nebulas, the Hercules Cluster, the Great Orion Nebula, the Crab Nebula, and many star clusters are all wonderful objects to show off the abilities of your small scope.

60mm scopes with the highest quality optics and very good mounts can show objects that are usually thought of as objects requiring larger telescopes. Better mounts give a steadier view, and allow you to relax more while looking. Better optics improve the contrast of the image, which makes it easier to find and see the object you're looking for, and to see more of the detail in it.

Star Clusters
Star clusters are the deep sky objects your small scope is most suited for. They are relatively bright, and very numerous, meaning there'll almost always be plenty of them in the sky to choose from. There are two basic types of star cluster, the "open" cluster, which is a general group of stars that happen to be near each other, and the "globular" cluster, which is a group of stars whose mutual gravity has pulled them together into a globe shape.

Both types are visible in your 60mm telesccope. In my first article in this series I listed a good selection of star clusters that can be viewed with a 60mm scope. Here's I'll describe some of my favorites, and observation tips that apply to all clusters.

Globular Clusters
M13, the Great Hercules Cluster, is a showpiece object for northern hemisphere astronomers. It's a big glowing ball of stars in the Keystone of Hercules. Different globular clusters look different. Some have a very even brightness across the face of them, others are brighter at the center then the brightness tapers off as you go outward. M13 is one of these, much brighter at the center. Its brightness drops off regularly from center to edge. Individual stars can be picked out at the edges, with tendril-like streams of stars flowing outward around M13s boundaries.

Compare this with M22, near the top of the Teapot in The Archer, visible to observers north and south. It is about the same brightness as M13 overall, but the bright wash of its center extends well across its diameter. Only the outer parts dim. Southern observers can also enjoy the enormous globular Omega Centauri. M13 and M22 are both visible to the naked eye, but not like Omega Centauri! In the telescope, Omega Centauri shows enough detail to spend a lot of time enjoying it.

Open Clusters
Star Clusters with no particular form are also beautiful in a 60mm. A good night with high contrast skies will show these the best, making more stars visible in the cluster as well as making them stand out clearly from the background sky.

The Beehive Cluster and Alpha Persei Moving Group are beautiful groups of bright yellow stars. As is the case for star clusters in general, they should be viewed at the lowest power you've got for your scope. This would be the eyepiece with the longest focal length. This is the number shown on the eyepiece, usually given in millimeters (mm). Sometimes a viewing angle or other number is listed as well, but the focal length is usually listed most prominently in this case. The one with the largest number gives the lowest power views. For example, a 25mm eyepiece will give lower power magnification than a 15mm eyepiece.

The Seven Sister, or Pleiades (M45) are spread out too far to be seen all at once in nearly all telescopes. But the area can be scanned at low power. About 40-some stars are visible in all under the best conditions, and faint nebulosity (cloudiness) is visible in this area as well.

M39 is a nice, tight cluster at the edge of the area of sky that holds The Swan. It is easily contained in the telescope's field of view, and stands out nicely from its background.

Nebulas are clouds of gas and dust. There are several different sorts. Planetary nebulas are ones that tend toward being round, looking a little like "planets", which is why they have that name. They have nothing to do with planets other than looking a little like the disk that a planet shows. Supernova remnants are clouds of material that have been blown into space by an exploding star. Most commonly seen nebulas are general clouds of gas and dust in the spiral arms of our galaxy. Many of these are places where new stars are being formed, so they are often associated with nearby star clusters.

There are a few bright star clusters that look really good in a 60mm telescope. Most, however, are faint, show little detail, and are very hard to see in a small scope. The Great Orion Nebula near the belt of Orion, and the Lagoon and Trifid Nebulas in The Archer are among the best. On a clear night with a dark sky they show up clearly, with traceries of their gas and dust forming streamers and shapes inside and around them. At their best, they can show a faint greenish color, though they'll usually just show shades of gray in a 60mm.

The Crab Nebula, M1, in Taurus, is a fine supernova remnant for small telescopes. It is relatively easy to find, near the tip of one of The Bull's horns.

Fainter nebulas can be seen as small cloudy shapes. They are a good way to develop your skill as an observer, both in finding them and in observing their form. NGC 6334 lies near the star cluster M6, and is a good starting point for seeking more challenging nebulas.

Many planetaries are very nice to observe in a 60mm scope. Their compact form and well defined edges make them easier to see than a lot of the more "gaseous" looking nebulas.

The Saturn Nebula, NGC 7009, in Aquarius the Water Bearer, is very bright and has a nice green color visible even in small scopes when the sky is good. The Eskimo Nebula, NGC 2392, in Gemini the Twins, is fainter, but its form can be made out easily once found. The Ring Nebula, M57, in Lyra is a favorite of small scope owners. It looks like a smoke ring. It is bright enough to take your scope to its highest magnification, as is the Saturn Nebula.

In the next article I'll cover the final sort of deep sky object you can see with a 60mm: galaxies.
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