Astronomical Observing 101 (or before) Part 3

OK, so I lied! I had ended the last blog with "Next time I'll look at the 3 types of reflectors and give some examples of the prices of some popular models (two of which I have plus one that is on loan to my brother)."

Instead, I find I need to cover just a little bit more on terminology and on the types of telescopes.

Refractors have an Objective lens (the big one) and an eyepiece (or ocular). Reflectors have a primary mirror (the big one) and either a secondary mirror or a diagonal, and the eyepiece. The size of the objective lens or the primary mirror is known as the aperture. The focus point of lens/mirror is based on the amount of curvature of the lens/mirror(s). The distance from the lens/mirror/eyepiece that the image comes to a focal point is known as the focal length. This is important when calculating the magnification achieved with certain telescope/eyepiece combinations. The focal length divided by the aperture is known as the focal ratio and for a 150mm telescope with a focal length of 1200 would be shown as having a focal ratio of f/8. If you are into photography, this will seem familiar as camera lenses have a focal ratio assigned to them in the same way and it defines how "fast" the lens is. The lower the number, the faster the lens and thus requires a faster shutter speed to properly expose the picture. It has similar ramifications in doing astrophotography!

When you look at the design of reflector telescopes, you will see that the effective aperture usually is not the same as the diameter of the primary mirror. Following is a chart from "Norton's Star Atlas and Reference Handbook" that I referenced in my first Astronomy blog.

This diagram shows the focal point and not the eyepiece. It must be understood that eyepiece is located just beyond the focal point and in case you can't read the fine print, here is the explanation:

Different types of telescopes. For telescopes of similar apertures, the path of light ray from a celestial object to the focus F is shown through (a) a refractor, (b) a Newtonian, (c) a Cassegrain, (d) a Cassegrain-coude', (e) a Schmidt-Cassegrain, and (f) a Maksutov-Cassegrain.

Note that I did not mention the Cassegrain and Cassagrain-coude' earlier because they are rarely found. There is actually a rare 7th type which tilts the primary mirror of the Newtonian, reflects the image off a mirror which is outside the the path of the incoming light, and passes it through lenses that correct the astigmatism caused by the tilt of the primary. Although more difficult make, it offers brighter images than possible through the other reflectors for the same size primary mirror. This is known as a tilted-component telescope. With the exception of the this last type, you will notice that the incoming light has part of its path blocked by a diagonal or mirror, and in the case of several of them, a hole in the primary mirror. This is what reduces the physical aperture to an effective aperture that may be as little as 60-70% of the actual one.

One last term that you may come across is the catadioptric telescope which is a generic term for the telescopes like the Schmidt-Cassegrain and Maksutov-Cassegrain.

The last item to cover in this blog is magnification. Eyepieces have their own focal length and when used in conjunction with a telescope with its own focal length, defines the magnification achieved. Magnification is determined by dividing the focal length of the telescope by the focal length of the eyepiece. Here is a table of the magnifications from my eyepieces and telescopes. There is an additional lens that can be used called a Barlow which doubles the effective magnification, hence the nnnn.x2 columns.

Next time I'll REALLY look at the 3 types of reflectors and give some examples of the prices of some popular models (two of which I have plus one that is on loan to my brother).