Monday, July 29, 2013

The Smaller Telescope, Pt. II - Capabilities

Can this little instrument see galaxies and nebula? Read on.
Let's look at what a small telescope is capable of.
Most of the work done in the early days of telescopic astronomy was done with instruments of small aperture. Even when those instruments had fairly large apertures, the optics of the time usually left a lot to be desired. The modest, low end instruments we find today are at the least their peers, and more likely their superiors.
A telescope really does two things; it magnifies, and it catches light. Many novices get drawn in by magnification without consideration to the telescopes aperture, its diameter. There is a good rule of thumb when dealing with telescopes and their recommended top end magnification. Take the telescope's aperture in milllimeters and multiply it by two. Simply put, a 60mm telescope should be able to handle up to 120x maginification easily. Yes, you could push beyond that, but the images will become very murky and dark. 
The other things telescopes do is capture light and concentrate it back to the viewer. Many astronomers, amateur and otherwise, refer to telescopes as "light buckets". A better description might be "light funnel". A telescope takes the light and concentrates it. 
Not a light bucket, but a light funnel, that concentrates light back to the eye.

The larger the funnel, the more light it can concentrate. This means you are able to see fainter objects.
Of course, the more they will cost as well.
Let's look at what some smaller telescopes are capable of, using the online "Telescope Limiting Magnitude Calculator". "Limiting magnitude" refers to how faint an object the telescope can see with the conditions and parameters set out in the calculations.
The parameters I'm going to lay out here are for typical deep suburban conditions, using my middle aged (currently 50 year old) eyes. I'm also going to include telescope apertures that aren't that common, the two smallest ones. Let's assume that we are on the edge of civilization, the outskirts of a suburban area, with naked eye magnitude near 4.5, which while not terribly bright is not terribly dark either. On all of these apertures, we are assuming a modest 25x magnification. To give you an idea of what the telescope should be able to see, we are using the Messier object list at Wikipedia, compiled in the 18th and 19th centuries by Charles Messier and his assistant Pierre M├ęchain. This is considered one of the most important lists in astronomy, and is something of a stepping stone for amateur astronomers. Using the aforementioned, we arrive at the following results -  

  • A 30mm telescope can see down to magnitude 9.8. This is theoretically capable of viewing 74 of the 103 objects on Messier's list.
  • A 40mm telescope can obtain 10.1 magnitude. This means we can view 80 Messier objects.
  • A 50mm, a common size, should be capable of viewing down to 10.3 magnitude. We are up to 81 Messier objects.
  • The 60mm telescope, one of the most common sizes, can view down to 10.5 magnitude. We are now up to 89 Messier objects.
  • We end with the 70mm telescope, the largest size we will deal with here, and still fairly common. We are at 10.6 magnitude, and we are should be able to view 89 to 91 Messier objects.

There is one telescope I want to touch on by itself, a nice little beginner's telescope from Celestron, the 76mm FirstScope. This is a baby Dobsonian. It is capable of reaching down to 10.7 magnitude at 25x. Considering its price, it is one of the best little telescopes one can buy.

For its price and aperture, the Celestron 76mm FirstScope is one of the more clever telescopes available.

Now that we have established what the smaller telescope can see, we'll next look at how to find these objects. We're going to use tools designed for binocular astronomy.

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