Orion 70MM AZ 700mm25mm =, What to Expect, Objects to Observe, Magnification Limits

To calculate the magnification, or power, of a telescope-eye- piece combination, simply divide the focal length of the tele- scope by the focal length of the eyepiece:

Telescope Focal Length (mm) = Magnification

Eyepiece Focal Length (mm)

For example, the Observer 70, which has a focal length of 700mm, used in combination with the 25mm eyepiece, yields a magnification of:

700mm25mm = 28x

Whatever you choose to view, always start by inserting your lowest-power (longest focal length) eyepiece to locate and center the object. Low magnification yields a wide field of view, which shows a larger area of sky in the eyepiece. This makes finding and centering an object much easier. Trying to find and center objects with a high power (narrow field of view) eyepiece is like trying to find a needle in a haystack!

Once you’ve centered the object in the eyepiece, you can switch to a higher magnification (shorter focal length) eye- piece, if you wish. This is recommended for small and bright objects, like planets and double stars. The Moon also takes higher magnifications well.

The best rule of thumb with eyepiece selection is to start with a low power, wide-field eyepiece, and then work your way up in magnification. If the object looks better, try an even higher magnification eyepiece. If the object looks worse, then back off the magnification a little by using a lower power eyepiece.

What to Expect

So what will you see with your telescope? You should be able to see bands on Jupiter, the rings of Saturn, craters on the Moon, the waxing and waning of Venus, and many bright deep-sky objects. Do not expect to see color as you do in NASA photos, since those are taken with long exposure cam- eras and have “false color” added. Our eyes are not sensitive enough to see color in deep sky objects except in a few of the brightest ones.

Remember that you are seeing these objects using your own telescope with your own eyes! The object you see in your eye- piece is in real-time, and not some conveniently provided image from an expensive space probe. Each session with your telescope will be a learning experience. Each time you work with your telescope it will get easier to use, and stellar objects will become easier to find. Take it from us, there is big difference between looking at a well made full-color NASA image of a deep sky object in a lit room during the daytime, and seeing that same object in your telescope at night. One can merely be a pretty image someone gave to you. The other is an experience you will never forget!

Objects to Observe

Now that you are all set up and ready to go, one critical deci- sion must be made: what to look at?

Magnification Limits

Every telescope has a useful magnification limit of about 2X per millimeter of aperture. This comes to 140X for the Observer 70. Some telescope manufacturers will use misleading claims of excess magnification, such as “See distant galaxies at 640X!”. While such magnifications are technically possible, the actual image at that magnifica- tion would be an indistinct blur.

Moderate magnifications are what give the best views. It is better to view a small, but bright and detailed image than a dim, unclear, oversized image.

A. The Moon

With its rocky surface, the Moon is one of the easiest and most interesting targets to view with your telescope. Lunar craters, marias, and even mountain ranges can all be clearly seen from a distance of 238,000 miles away! With its ever- changing phases, you’ll get a new view of the Moon every night. The best time to observe our one and only natural satel- lite is during a partial phase, that is, when the Moon is NOT full. During partial phases, shadows are cast on the surface, which reveal more detail, especially right along the border between the dark and light portions of the disk (called the “terminator”). A full Moon is too bright and devoid of surface shadows to yield a pleasing view. Make sure to observe the Moon when it is well above the horizon to get the sharpest images.

Use an optional Moon filter to dim the Moon when it is very bright. It simply threads onto the bottom of the eyepieces (you must first remove the eyepiece from the focuser to attach a filter). You’ll find that the Moon filter improves viewing comfort, and also helps to bring out subtle features on the lunar sur- face.

B. The Sun

You can change your nighttime telescope into a daytime Sun viewer by installing an optional full-aperture solar filter over the front opening of the Observer 70. The primary attraction is sunspots, which change shape, appearance, and location daily. Sunspots are directly related to magnetic activity in the Sun. Many observers like to make drawings of sunspots to monitor how the Sun is changing from day to day.

Important Note: Do not look at the Sun with any optical instrument without a professionally made solar filter, or permanent eye damage could result.

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