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HOW TO USE YOUR TELESCOPE

  Scope City's Optics Crash Course
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PART VII: HOW TO USE YOUR TELESCOPE

MAXIMIZING WHAT YOU SEE

Magnification is the basic advantage a telescope has over the eye. The telescope forms an image on its final focal plane and the eyepiece acts as a simple magnifier to increase the size of that image. Since the telescope gathers more light than the eye, the image thus formed is also brighter than we see with the naked eye. Brighter and larger equals more visible.

Objects in the sky come in all sizes and shapes-from large to tiny and from very bright to exceedingly faint. This is the primary reason we want to have more than one magnification for our telescope (accomplished with multiple eyepieces or Barlow lenses).

The sky's natural brightness seems to diminish as we magnify-its light gets spread out over a larger area, reducing the brightness per unit area. As a result, fainter objects are often more visible at higher powers, but there is always a compromise between "large enough to see" and "too spread out to be visible". It's desirable to experiment with magnifications to get the best view possible.

Low powers in a telescope are considered to be from 3X per inch of aperture to 10X per inch (e.g. on a 3" telescope, 9X to 30X). These powers are used for large objects and the widest true fields possible.

Medium powers are from 10X per inch to 29X per inch. These powers are used for inspection of larger objects, or for viewing most objects. These are typically the most comfortable powers to use, and combine the right combination of magnification with object visibility. (e.g. 30X to 87X on a 3" telescope)

High powers are from 30X to 50X per inch of aperture. These powers are used to split double stars, examine the planets and Moon closely, or to see details in a small, bright, object. To use these powers, the Seeing has to be good (we'll go into that later). This is 90X to 150X on that 3" scope.

Extra high power (60X per inch and higher) are rarely used except on double stars or the Moon, and only under conditions of exceptional stillness in the air (Superb Seeing). The number of nights where these powers can be used, over a decade, would number on the fingers of one hand, typically.

Averted Vision is a technique that amateur astronomers use to see very faint details in an object, or even to see a very faint object. The eye's rods (the part of the retina we use in night vision) don't occupy the center of our retinas, yet they are incredibly sensitive to light. Accordingly, our peripheral visions are more sensitive to light than our central vision. Rods are most heavily concentrated in our retinas just slightly outside the central area (known as the fovea centralis), so a slightly "averted" vision will often allow us to see a LOT more than we can see by looking straight at the object. Fainter stars become visible, and the full size of a faint object may become apparent when we look slightly to the side of the object. The typically most sensitive area of the retina can be employed, for the person who is using the right eye, by looking slightly to the right of the object, or, for a left-eyed observer, slightly to the left. Normally, this puts the object's image directly on the part of the retina that is most sensitive to light.

Phase of the Moon: This influences what we can see. The Moon acts just like a big city nearby, and its light makes the sky quite a bit brighter. When the Moon is out, we simply cannot see objects or stars as faint as we can see when the Moon is down. So always try to observe when the Moon has set or has not yet risen--unless you're observing the Moon, of course. The best time of the month to observe all night is when the Moon is new.

Dark adaptation: When we first go from a lighted room or car into the darkness, our eyes have not adjusted to the dark. The eyes' sensitivity to light will gradually increase to their maxima for about 45 minutes after stepping into the darkness. What's happening is that the body is naturally producing a chemical in the retina that is incredibly sensitive to light. This chemical is destroyed by light, so we have to be in the darkness before it begins to build up. It goes without saying that if we want to see objects as well as possible through our telescopes, we should wait this period until our eyes have fully dark adapted. One easy "rule of thumb": if the sky appears black, you are not dark adapted. When fully dark adapted, the night sky is seen as silvery-gray. The eye is said to go from photopic vision, in which the cones in the eye are turned on and we see color, to scotopic vision, in which the cones are turned off so we lose color vision but the huge increase in sensitivity of the rods (over 90,000 times!) gives us the ability to see fairly well in the darkness.

To stay dark-adapted, never use a white light to read by, or to see with. Only use a deep red light (our eyes are not very sensitive to deep red), like a red LED flashlight, or a regular flashlight with several layers of red cellophane taped in place over the lens. Red flashlights can be obtained at telescope stores or many sporting goods merchants.

Local Light Pollution: This is the bane of the urban astronomer. Certainly, when you view the sky, you'll extinguish all exterior lights on your house and pull the shades so interior lights don't bother you, but there's little we can do about street lights or neighbor's yard lights. To maximize our observing experience, try to always set up your telescope where you are not bothered by such lights. And when you are at a dark site, remember to turn off the interior lights in your car. A 1-second blast from a white light may require 15 minutes for your dark adaptation to recover.

Where to Observe in the Sky: When we look straight up, we are looking through about 10 miles of air, with ten miles of water vapor, dust, and light scatter. When we look at an object on the horizon, that column of air is extended to almost 100 miles, with a huge increase in atmospheric aerosols. Everything at the horizon is a lot fainter and harder to see than when the objects are higher in the sky. The highest above the horizon that any object achieves is when it crosses the meridian (astronomers say the object Culminates, or reaches its highest elevation), the imaginary line running from due north to due south, passing over our heads. To see the most of each object in the sky, try to observe each object when it passes the meridian, and try to avoid looking at objects close to the horizon-especially the 30 degrees or so from the horizon up, where the air is thickest. The best place to observe any object, of course, is the zenith. We get the best view possible of objects that cross at or near there.

When to Observe during the night: The atmosphere quickly loses heat into space when it first gets dark. This causes turbulence in the atmosphere and blurs the images in our telescopes. But later, the atmosphere settles down and heat loss becomes quite uniform. It is this time the sky is steadiest, and images are the best. Additionally, the moisture in the atmosphere will settle out as dew, causing the air above us to dry out and become clearer. Plus, many outdoor lights go out after midnight, making the sky darker. At most locations, these improved viewing conditions occur after midnight and until dawn. These are the best hours to observe at most sites. It may not always be possible to view during these hours, but some steadfast observers will take a nap earlier and get up to observe during the morning hours.

Seeing and Transparency: or HOW ASTRONOMERS RATE THE QUALITY OF THE ATMOSPHERE (understanding how the air influences the view through a telescope)

The atmosphere interferes with the telescope's ability to see. Every beginner learns that axiom very quickly. First, we blame the scope: "I just didn't spend enough money to get a good one." But soon we talk with other amateurs or telescope shop employees and we quickly learn we're all in the same boat. Young or old, big telescope or small, we are all limited by this thick soup of an atmosphere we look through.

Though there are many factors at play in our abilities to penetrate that thick soup, there are two definitions that stand out in nearly every beginner's text on the use of a telescope:

Seeing and Transparency.

Seeing refers to the steadiness of the atmosphere. It's the "Great Equalizer" that makes the big, expensive scopes perform like smaller scopes, and limits even the biggest observatory scopes' resolution to quantities similar to an amateur's small scope. It's as if we were viewing the pebbles on the bottom of the shallows in a small lake-the quieter and steadier the water, the sharper and easier the view of those pebbles. But let a small amount of turbulence enter into the water above the pebbles and it becomes hard to tell what's even there. So it is with the atmosphere-just a little turbulence in the air between your telescope and space and the star images dance around and send out spikes in every direction until they become large fuzzy balls, denying us the clear view of the skies we paid our money for when we purchased our telescopes in the first place.

Seeing limits the magnification we can use; sometimes to quite low powers of 50X to 60X. Don't worry-it's probably not the telescope. Just be persistent in your observing and those moments when you can use higher powers will come.

The Seeing differences have been scaled by various observers in the past. A good example of a Seeing Scale (of horrors) can be seen in the Pickering scale of Seeing, a ten point scale that allows you to estimate how good your Seeing is. It can be found on the web at: http://uk.geocities.com/dpeach_78/pickering.htm Unfortunately, Pickering 1 is all-too-common, and Pickering 10 is quite rare.

Probably, more than anything else, this is what limits our abilities to see detail on the planets and the Moon. The good news is that out of every minute of horrible Seeing, there will be a few seconds of relative calm and the images will sharpen up, revealing to us that we did indeed buy decent scopes and that it's just the crumby atmosphere preventing those scopes from seeing more.

Patience! Keep at it, and sooner or later you will have one of those mystical moments when the Seeing steadies and allows us to see the Universe through a veritable Hole in the Atmosphere. Those of us out West and those of us in Florida see more of those nights than the people who live east of the Mississippi, but all of us are eventually treated to one of those nights of especially good Seeing.

One clue: the Earth's atmosphere steadies, in most places, more completely between the hours of midnight and dawn. If your interests in planets, Moon, or double stars makes you seek the higher powers, these are your best hours to observe.

Transparency is the other atmospheric issue for us amateur astronomers, according to the books. It is usually defined as the darkness of the night sky. The farther you are from civilization, the more Transparent is the sky, or so it is said.

Let's deviate from the book definitions, here, and refer to Transparency as being actually a combination of two atmospheric characteristics, which can be defined as Darkness and Clarity. We could use the same term, transparency, for the second characteristic, but this might be confusing.

Darkness is what is often referred to as transparency. But it doesn't take very many trips to a dark site to notice that the stars don't always appear equally bright-especially toward the horizon. The stars overhead may appear just as impressive, but the fine details in the Milky Way seem to be less obvious and the sky slightly less impressive on some nights compared to others.

Many have used a sky brightness meter to actually measure the brightness of the night sky, and these nights of seemingly lower clarity can measure just as dark as the nights when the Milky Way looks like big, cumulus, clouds in the sky. An obvious case in point is that if you are at a site well away from civilization and it's overcast, the sky is quite dark, yet the atmospheric Clarity is terrible.

So, while Darkness is an important thing to seek out-it determines how "deep" our telescopes reach, both in magnitude and into the Universe-it becomes obvious to the sky observer that it has to accompany atmospheric Clarity to really be called Transparent.

Clarity is probably best explained as the absence of aerosols and dust in the atmosphere. When water vapor or wind-blown dust is present between your telescope and space, you will just not see the faint details in deep-sky objects that a clear atmosphere allows. There will be more Extinction (the reduction in your ability to see faint stars with lower altitudes) in the air, and more light scatter. At its worst, the sky seems to take on a silver sheen and the Milky Way's details are lost.

These two characteristics can be exclusive. Here in the LA area, we often get strong winds that blow all water vapor and dust and smog out to sea, and the visibility of distant mountains and buildings is superb-as if they were just next door instead of many miles away. We have wonderful Clarity. Yet, we still have the worst light pollution in the nation, and the skies are not dark at all. It is true during these times that somewhat fainter stars can be seen, but the dimmest are several magnitudes brighter than what can be seen in a dark site, and the Milky Way is never visible.

So the stargazer has to accept that conditions will not always be perfect when he/she heads for the dark place to observe the sky. But is there a way we can help ourselves experience better Seeing, Darkness, or Clarity?

Yes there is.

Seeing is often better in those places where the wind, if it blows at all, blows in a smooth manner without turbulence. You can look for sites near a body of water, or in a saddle-shaped mountain valley. There's a site in LA famous for its wonderful Seeing-Mount Wilson. You may recall they built a big observatory there over a hundred years ago.

Darkness is usually better the farther from cities you are. State or National Parks and Monuments excel in this regard. Look at the information for your local areas on the website, www.cleardarksky.com to find the nearest dark site. Be prepared to drive a bit if you live in a big city.

Clarity is best at high altitudes (one of the advantages Westerners have), so if you live at low altitudes, the days just after a front passes may be the best days to observe the fainter objects. Seeing can be turbulent at these times, but there is usually a day or two after the front comes through when the air has not yet "steamed up" again, yet the air is also not strongly turbulent. That's the day to go out.

This simple explanation will hopefully help you understand what you are seeing when you take your telescope out under the stars. This is a hobby that rewards perseverance-observe often and the magical combination of good Seeing, Darkness, AND Clarity will be encountered a lot more often.

Cooling of the Optics: The surface of good optics are accurately figured to just a few nanometers-if a telescope mirror were stretched out to the size of the United States, there would be no bump higher than an inch-we're talking SMOOTH. So, it probably comes as no surprise that when warm optics meet cold air and start cooling, that surface accuracy is distorted by the process of cooling. The optics flex and warp enough to reduce the quality of images from our telescopes until the optics come to thermal equilibrium with the surrounding air. This can take quite a while, hours, in fact, for larger optics, and even an hour for smaller scopes. So, if the quality of the image is important (and, on planets or the Moon, it is especially), it is important to remember to let the telescope cool down before viewing. This is ultra-important for high-power viewing in particular.

Fortunately, for those of us at the usual latitudes of the US, there is about 90 minutes between sunset and complete darkness (astronomical twilight falls when the sun is 18 degrees below the horizon) that we can use to let the telescope cool down. Set the telescope outside during this period, and you can begin your observing with a telescope that is already cooled.

Collimation of Optics: This will be explored in detail later on, but suffice it to say that no telescope will deliver its best possible images without having its optics aligned. This process, called collimation, is simple and requires a fairly simple tool or tools.

Cleanliness of Optics: Though a small amount of dust does not hurt the image quality from our scopes, grease, oils, and fingerprints do, as well as possibly damaging the fine coatings on lenses and mirrors. We'll go into this more, later. Clean optics, though, are essential for quality images through the scope. At the very least, eyepieces should be inspected before every outing, and eyelash oils or mascara carefully cleaned from the lenses. The rule, here, is to NEVER CLEAN OPTICS THAT AREN'T DIRTY.

Observing Site Optimization: There are a couple of don'ts that should be remembered:

  • Don't look at objects directly above a roof. Heat rising from the roof will distort the images.
  • Don't set the telescope up on concrete, if possible. Concrete holds its heat and releases it all night, causing heat waves visible in the scope. Grass or dirt is the ideal material to have underfoot.
  • Don't store the telescope where it can get very hot. This might damage the optics. At the least, it will slow the cooling of the optics after dark.
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Vixen R130SF - 5.1 inch f/5 Newtonian Reflector OTA
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