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Seeing and Transparency in Astronomy Defined for the Beginner

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What is Seeing In Astronomy Terminology?



There are some terms you need to be familiar with to understand comments you are likely to read about observing astronomical objects with your telescope.

One is seeing. Seeing is a reference to the ability of the telescope to show details through the atmosphere. It's often given on a 1 to 10 scale, 10 being perfect seeing, and 1 being very poor, with little detail visible.

Seeing is largely an atmospheric issue, but telescope optics plays a part. It is generally accepted that at the optics end, seeing is less disrupted by refractor optics than reflector optics. It takes refractors a shorter time to cool down, and one hypothesis is that as light passes through the different lens elements, each with different effects on seeing, the errors get averaged out a bit. The end result is that refractors tend to provide steadier images that do reflectors.

Atmospheric Scintillation

The atmospheric part of seeing is affected by movement of air cells in the atmosphere. The cells are often different temperature and possibly different in other atmospheric properties. When looking up through the atmosphere, one looks through many air cells.

These cells move around, causing the blurring and shifting in lunar and planetary images. The smaller the telescope you use, the larger are the scale of the disturbances. To the right you see an example of less than perfect seeing. This moon movie shows the shifting distortions that the atmosphere causes on moderate seeing night.



How Does Seeing Affect What I Can See With My Telescope?

With my 2 inch refractor and medium to poor seeing I can see waves of motion when I'm observing the moon. The more the turbulence, the greater the apparent motion. The motion disturbances are larger than the craters I'm trying to observe, so they don't limit the details I'm able to see. They are only a distraction, not a limit to the details I'm trying to perceive.

When I look at the moon with my 6 inch telescope, the waving phenomena are smaller in apparent size, being on the order of the smaller objects I want to see. In fact, the atmospheric motion forms a limit to the smallest details I'm able to resolve.

This explains why with medium to small telescopes, there are more evenings of acceptable seeing. The motion effects must be virtually gone for larger (8 inch or better) telescopes to be able to function up to their potential. An excellent source of additional information about astronomical instruments and observing issues is the book: The Backyard Astronomer's Guide.

You can minimize the effects of bad seeing by looking at your targets when they appear near their highest altitude. In this way you are looking through the least amount of atmosphere, which can reduce the turbulence. Also, trying moving your telescope, if it is portable, so that you aren't looking directly over a nearby roof. A low object viewed just over a rooftop will definitely be badly affected by turbulence.

Even if you are seeing a lot of turbulence, don't necessarily give up. The best lunar and planetary views often come to patient observers who keep looking. The atmosphere is dynamic, and sometimes it can steady while you are looking at your target, if even for just a few minutes.

Walther Crater Video

Check out the video above of the Lunar Walther Crater region. It was taken through a NexStar 5SE, using a Celestron NexImage Camera. You can see a lot of motion involved as the video plays. Pick out an area with tiny features and watch how they can even disappear momentarily. The NexImage camera allows me to record the movie, and I use a Linux utility to separate the movie into individual frames, then use a Yorick program to select, align, and average frames to get an optimized single image. With that technique, I can obtain some pretty good results. A section of the averaged version of the above video is shown below.

Walther Crater

Walther Crater w/NexStar 5SE


What is Transparency?

Another term you'll see when reading about observing is transparency. Transparency has to do with the ability to see dim objects through the atmosphere. There are many atmospheric phenomenon that can adversely affect transparency.

If you live in or near a city, pollution can reduce transparency. Thin, high altitude clouds can affect transparency. Forest fires can put up enough material to affect seeing over a very wide area. City or neighborhood lighting can brighten the background of the night sky, also limiting the faintness of objects you'll be able to see.

Transparency is also often indicated with a 1 to 10 rating, 10 being the best transparency.

If transparency is low, then your ability to detect dim objects is greatly reduced.

I've found, at least in my area, that transparency is better in the early morning than in the evening. I think it's related to traffic. In the evening there is still a lot of traffic, and that causes dust to be constantly churned up, some of it getting up into the air. But in the early morning, things have settled down, the air is usually less turbulent, and transparency can increase.

If you have the time and you have a portable telescope, you can resort to moving your apparatus to a better observing area, with less light interference and clearer skies. If you can see the Milky Way with naked eye, then you've found a good spot.

Incidentally if you have such a site not far away, you may find you can step down to a bit smaller, more portable telescope. You can easily gain a magnitude of star depth just by observing from a darker, more transparent site.



Recording the Observations

If you find you enjoy observing, you might consider keeping a journal of your observations. If you decide to do that, make the observations more useful by annotating (with a number from 1 to 10) the seeing and transparency conditions, as well as the telescope and other equipment you used. Consider posting your observations on your website, or join an e-group and share your experiences there. I can assure you that other amateur astronomers will enjoy reading about your observing adventures.

Why?

First, you'll enjoy reviewing your notes when you view the same or similar objects again later, perhaps through different conditions or a different telescope.

Plus, we all get clouded in now and again, and reading about someone else's observations helps make those days less terrible. Reading the observations of others, if they are well documented, is also helpful when you are shopping for another telescope. It helps you visualize what you might see with different sized and types of instruments.



Of course it's not only atmospheric conditions that affect what you will see in the night sky. The views of different object types is also a function of the type and size of telescope used. For observing those dim objects, you need good transparency and a large aperture, small f ratio instrument.

To view high resolution objects such as planets, transparency is less an issue, but you need good seeing and a telescope designed to deliver high resolution. That type of telescope is usually a long focus or high f ratio telescope.