Doug's Telescope Page

Doug's 10"F4.5 Starfinder (reflector)

I built my first telescope, a 6", f8 Newtonian Reflector. I liked the large aperture and bright images, so I got a 10" f4.5 Meade Starfinder, pictured above.

For those of you who aren't aware, 10" is the diameter of the primary mirror, and f4.5 is the focal length. This gives a focal length of 4.5*10", or roughly 45" from the primary mirror. Why this is important is that it provides all of the combinations of 'power or 'zoom' you will get through various eyepieces. By taking the focal length of the telescope (generally in mm), you divide by the eyepiece's focal length (always in mm) and that gives you the 'power' of the scope for that particular eyepiece.

"Resolving power" = FL_telescope / FL_eyepiece

Below, is a drawing I made of the light meter I recently built. There are several types of circuits that will accomplish the same functionality of the light meter, but I used one from a Sky and Telescope article awhile back (I can't find the article, now). The plans I used make the light meter too sensitive (it doesn't use the full range of current values), but it works for now. If I find the plans I will post them here.

[Click for larger image] Light Meter Drawing

In case you don't know what a light meter is, here goes. When you go outside at night, you see a dark sky with a certain number of stars (assuming it's not cloudy). If you've ever gone to the desert, forest or other remote site, you probably noticed there are "a lot of stars". The stars you've seen before are now VERY bright, and there are tons of stars you don't see when you're at home.

The fact that most of the stars in the sky are masked from an observer in the city, or even near street lights or normal house lights, is due to "Light Pollution". In order to figure out a measure of "how dark the sky is", you need something like a light meter. A light meter uses an ambient background 'screen' and allows the observer to compare the brightness (intensity) of the background sky against a green LED. In order to match the green LED, a filter is placed over the aperture of the light meter tube, as shown in the drawing. You turn a dial controlling the current to the LED, and adjust until the light of the LED matches the light of the background sky. Once it matches, you read the current meter (micro-amps) and record the value. Now you have 'quantified' the amount of pollution the surrounding lights have created. This allows you to compare observing sights and helps to identify reasons why you might not be able to observe a dim object with or without a telescope.

More later...

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