Night Vision - Part 2

The more I read about telescopes this past fall and winter, the more confused I got about which type of scope I wanted to get for our family. It took me a long time and a lot of research to find one that met my requirements. There are 3 main types of scopes.

Refractors are what one typically thinks of when imagining a telescope. They have a big lens in the front (the part facing up) and an eyepiece in the back. The lens magnifies the image and focuses it down to the eyepiece where you see it. A small refractor is what Galileo used to see the moons of Jupiter for the first time. They typically offer smallish diameter lenses anywhere from 1" to 6" and can be anywhere from 2' to 4' long (although I have seen refractors more than 10' long, but they are rare). They require a very sturdy tripod to keep the scope stable.

Reflectors were invented by Newton. They are tubes with a bowl shaped mirror in the rear end which reflects and focuses the light back up to an eyepiece in the front. Amateur models are typically 6" to 12" in diameter and 4' to 5' long and very heavy. They have considerably greater light gathering power than refractors, but are bulkier. Also, Dobsonian models do not require a sophisticated tripod, but have a rather simplistic (and less expensive) base.

Cassegrain models have both a lens in the front end AND a mirror in back. This allows the light to pass back and forth along the tube offering a longer focal length in a smaller tube. They are lighter and more portable than the other models, but require regular adjustments to keep the lenses and mirrors all lined up. The process of lining up all the lenses and/or mirrors is called "collimation".

Focal Length simply means the distance between the lens/mirror and the eyepiece. Focal ratio is the ratio between the diameter of the lens/mirror and the focal length. For refractors and reflectors this just means the diameter of the lens/mirror divided by the length of the tube. Cassegrain models have a longer focal length because light is bounced 3 times back and forth from one end of the tube to the other. This means that Cassegrain models have smaller physical tubes, but have much longer focal lengths than reflectors or refractors. Refractors will typically have a focal ratio usually ranging from f/7 to f/9 with f/8 being about average. Reflectors usually have a shorter ratio at around f/5 and Cassegrain models have a longer ratio at about f/10 to f/12. In all cases there are models with longer or shorter focal ratios than average, but these are about average as far as I can tell.

Focal length is important because the smaller the ratio the wider the field of view you achieve but with less magnifying power. The larger the ratio the more amplifying power you achieve. Longer focal length scopes are capable of greater magnification, but narrow fields of view. Also, as you increase magnifying power you decrease the brightness of an object.

Magnifying power is calculated by dividing the focal length of the telescope by the focal length of the eyepiece you're using. So, with the same eyepiece an f/10 scope will deliver more magnifying power than an f/5 scope. But the f/5 scope will deliver a wider field of view with a brighter overall (although smaller) image with that same eyepiece.

What this means is that, if you are looking at a small, bright object (like a planet) you want to have a long focal length and a narrow field of view. That way you can achieve a high magnifying power and make the planet look really big and get a lot of detail in your image. The high magnifying power will make the planet look less bright, but planets are very bright anyway so being less bright isn't a big deal.

However, if you want to look at large, dim objects like nebulae, star clusters or galaxies, a high magnifying power will reduce their brightness so much that you may not be able to see them anymore. In these cases you want to have a short focal length and (because some of these objects are quite large, you want to have a wide field of view. Thus you want to have a low magnifying power to increase the brightness (rather than the size) of the object.

What all of this means is that telescopes with a long focal length are very good at giving you a large, detailed image of planets and other telescopes are better at giving you a wider field view of dim, further away objects like galaxies and nebulae and star clusters.

However, all other things being equal, the larger the diameter of the telescope, the more light gathering power it has and the more it will show you of just about everything. So, as a general rule of thumb, refractors and Cassegrain telescopes have longer focal lengths and give better images of planets than they do of other Deep Space Objects (DSO's). Big, heavy Newtonian scopes are much better at bringing out the details of DSO's but have a pretty short focal length and don't necessarily have the magnification power of longer focal length scopes. But, because they have so much more diameter, Newtonians are better all around at seeing everything you might want to see in the sky. Also, Dobsonian Newtonian scopes are the least expensive scope to buy because their mounting bases are so easy to manufacture and because mirrors are much cheaper ot make than lenses are. But, these types of scopes are also really big and heavy. They don't call them "light buckets" for nothing.

It took me several weeks to fully understand everything I just wrote up above. Once I started to grasp these basics it helped me to decide what I wanted out of a scope for our family. After a while I came up with these criteria for what I wanted:

1. Portability (I want to be able to take it camping with us)
2. Ability to see DSO's along with planets
3. Low cost (or as low as possible / reasonable)
4. Low maintenance and will last a lifetime
5. Quick set up and tear down time (Michigan winter nights are COLD)

After I came up with this list it took me a while to find the scope I wanted. Basically, I wanted a cheap, easy to use scope that I can carry around without killing my back, that I can set up quickly and that will let me see everything in the sky from planets to other galaxies. Regarding this last point I wasn't looking for perfection in viewing ability, merely competence. That is, I'd be happy with "good" rather than "perfect". I'm not willing to pay for "perfect" views of interstellar objects. Telescopes that offer "perfect" images are WAY too expensive for me.

Elements that did not make it to my list of criteria included imaging (astro-photography) and computerized mounts. Some day it might be nice to take pictures of objects in space, but that adds a HUGE amount to the cost of the scope and also requires a lot of experience. I originally toyed with getting a computerized scope for ease of use. These are scopes on a tripod with a motor and a hand held remote control. All you have to do is select name of the object from the computers list of objects and the telescope will automatically pivot and swing to point exactly at it.

I decided against computerized scopes for two reasons. First, it seemed like it would take all the fun out of research and exploration and second, it would add considerably to the price. I don't mind spending a bit of money on a scope, but I want that money to be spent on maximizing opitical quality and not on computerized bells and whistles.

Next time I'll describe what scope I chose and how it fits into my criteria. I ordered it a few days ago and am still waiting for it to arrive. Hopefully by next weekend!