Orion 9549, 9541, 9909 Using Your Telescope, Choosing an Observing Site, “Seeing” and Transparency

Figure 10: Mounting the TableTop base on a photo tripod

sharp focus until the image starts to blur again, then reverse the rotation of the knob and stop when sharp focus has been achieved again.

Using Your Telescope

Choosing an Observing Site

When selecting a location for observing, get as far away as possible from direct artificial light such as streetlights, porch lights, and automobile headlights. The glare from these lights will greatly impair your dark-adapted night vision. Avoid view- ing over rooftops and chimneys, as they often have warm air currents rising from them. Similarly, avoid observing indoors through a window, either open or closed, because the tem- perature difference between the indoor and outdoor air will cause image blurring and distortion. Window glass may also introduce glare, internal reflections, or double-images into your view.

If at all possible, escape the light-polluted city sky and head for darker country skies. You will be amazed at how many more objects are visible in a dark sky!

“Seeing” and Transparency

Atmospheric conditions vary significantly from night to night. “Seeing” refers to the steadiness of the Earth’s atmosphere at a given time. In conditions of poor seeing, atmospheric turbu- lence causes objects viewed through the telescope to “boil”. If, when you look up at the sky with your naked eyes, the stars are twinkling noticeably, the seeing is bad and you will be lim- ited to viewing with low powers (bad seeing affects images at high powers more severely). Planetary observing may also be poor.

In conditions of good seeing, star twinkling is minimal and images appear steady in the eyepiece. Seeing is best over- head, worst at the horizon. Also, seeing generally gets better after midnight, when much of the heat absorbed by the Earth during the day has radiated off into space.

Figure 11: Making optional adjustments to the azimuth tension

Especially important for observing faint objects is good “transparency” – air free of moisture, smoke, and dust. All tend to scatter light, which reduces an object’s brightness. Transparency is judged by the magnitude of the faintest stars you can see with the unaided eye (6th magnitude or fainter is desirable).

If you cannot see stars of magnitude 3.5 or dimmer then con- ditions are poor. Magnitude is a measure of how bright a star is – the brighter a star is, the lower its magnitude will be. A good star to remember for this is Megrez (mag. 3.4), which is the star in the “Big Dipper” connecting the handle to the “dipper”. If you cannot see Megrez, then you have fog, haze, clouds, smog, or other conditions that are hindering your viewing.

Tracking Celestial Objects

The Earth is constantly rotating about its polar axis, complet- ing one full rotation every 24 hours; this is what defines a “day”. We do not feel the Earth rotating, but we see it at night from the apparent movement of stars from east to west.

When you observe any astronomical object, you are watch- ing a moving target. This means the telescope’s position must be continuously adjusted over time to keep an object in the field of view. This is easy to do with the TableTop because of its smooth motions on both axes. As the object moves off towards the edge of the field of view, just lightly nudge the telescope to re-center it.

Objects appear to move across the field of view faster at high- er magnifications. This is because the field of view becomes narrower. Objects seen through the eyepiece will be upside down and reversed left-to right on reflectors. On telescopes that accept a diagonal the image will be right side up. Star diagonals will still be reversed left-right, correct image diag- onals will provide images that are right side up and non- reversed, as seen with the naked eye.

9