Night Vision - Part 3

So, after all the research I've described in previous posts below, one overarching fact became obvious to me. If I wanted the best possible views of planets, nebulae, star clusters and galaxies in a relatively inexpensive package, then I should buy the biggest possible Dobsonian Reflector.

Here's an example of a 10" Dob from Orion Telescopes. It only costs about $500.00 before any accessories are added. And the only extras it might need are additional eyepieces, a better finder scope and maybe an improved focuser if you're picky about such things. The only trouble with this is that it is 10" in diameter and 4' long and weighs 55 pounds.

If portability wasn't an issue for me, then this is probably the telescope I would have purchased.

But, because I really wanted something portable and have been having fantasies about taking a telescope camping with us, I had to come up with another plan.

I started looking at either refractors or cassegrain scopes, but those tend to have either small diameter apertures or long focal lengths which mean they are better at viewing planets and the moon than they are at DSOs like nebulae or star clusters. Since I wanted to look at everything I could in the sky I knew that a standard refractor or cassegrain probably wasn't going to work for me.

Eventually I decided to purchase the 120ST refractor from Orion telescopes. The 120ST can either be purchased as a package, with a mount, tripod and eyepieces or it can be purchased as a simple optical tube assembly (OTA). The name of this telescope, the "120ST" means that the objective lens is 120mm in diameter (about 4.7") and that it is a Short Tube scope. The focal length is only 600mm long which gives it a focal ratio of f/5.

The fact that this is a 120mm f/5 scope means that it has some interesting characteristics. Some of these are benefits and others are drawbacks. The 120mm diameter lens is a definite plus because that is large enough to start getting some somewhat good views of deep space objects. At the same time, because the tube is so short, at only a bit longer than 2' and because the OTA only weighs about 8.6lbs, it is pretty darn portable. It's what amateur astronomers like to call: "Grab-n-Go". Also, because it is a short f/5 it should provide very wide fields of view, allowing the user to see large swaths of the sky at low to medium levels of magnification.

However, there is a big draw back. A scope like this will not be nearly as good as an f/8 or f/9 scope at viewing the planets and the moon. There are two reasons for this. First, because magnification power results from dividing the focal length of the OTA by the focal length of the eyepiece, a short tube scope has a hard time generating big magnifications. It isn't impossible, it just means that you either need a very small eyepiece or a special "barlow" lens which provides additional magnification.

The bigger drawback is this: Color Aberration (CA). This happens when you point a short tube refractor at a bright object (like a planet). What happens is that the light coming into the lens is bent at much sharper angles in an f/5 refractor than in an f/9 refractor. When the light is bent like this some wavelengths at the purple end of the spectrum are sent off at a slightly different angle than the rest of the light. The result is that you will see a purple halo around bright objects.

So, I had to decide if this was a big deal for me. In truth there are only a limited number of really bright objects in the sky to look at. Planets are interesting, but Mercury is hard to see anyway, Venus is kind of boring to look at and Uranus, Neptune and Pluto are both hard to find and not very bright. So, the only problems the 120ST would really have would be for The Moon, Mars, Jupiter and Saturn. That's only 4 objects out of all the hundreds of other less bright objects in the sky.

(Addendum: This isn't precisely true. Stars can also be interesting to look at, especially if they are in clusters or if you are trying to "split" close binary stars. That does add a lot more bright and possibly problematic objects I might want to look at through the scope.)

But, even if that is a problem, it turns out that there are two solutions to fix (mostly if not completely) the color aberration problem. First, you can get a special filter which fits on your eyepiece which filters out all the purple so you can't see it anymore (or at least you see a lot less of it.) These filters cost between $50 and $150 dollars depending on how good an effect you want. I've been eying a more expensive filter here.

Second you can also partially solve this problem by changing the focal ratio of your telescope. If you can somehow turn your f/5 scope into an f/7 or f/8 scope, then a big chunk of the color aberration problem goes away. This is how it works. The lens of the 120ST is 120mm in diameter. The focal length is 600mm. 600/120=5 which gives you an f/5 scope. But, if you place a cap over the main lens with an 80mm hole in the center of it, then now you have an 80mm lens. 600/80 = 7.5. So now, just by covering up part of the lens you have magically converted an f/5 scope to become an f/7.5 scope which should have much fewer problems with color aberration. It is true that, because of the smaller aperture, the image will be a bit dimmer, but planets and the moon are pretty bright anyway so that shouldn't be a big deal.

So, the long and short of it is that I decided on the 120ST because it was light, portable and, in theory, should provide nice images of deep space objects. Its ability to view the moon and the planets is problematic because of the fore mentioned problems, but these difficulties are not insurmountable and there are cost effective workarounds to solve them.

When I came to these conclusions I was finally settled that this was the scope I wanted. So I made a post on the Cloudy Nights forums describing my decision making process and asking for advice on how to optimize this portable scope to be good (but not necessarily great) multipurpose scope for viewing the moon, planets, nebulae, star clusters and galaxies.

I got some great advice from that thread, which helped me finalize all my decisions about exactly what I wanted, including eyepieces and other accessories. That thread can be found here. I won't bother to summarize it all again since this post is already long enough.

Suffice to say that after months of research I finally decided on exactly what I wanted out of a telescope and last week I ordered it. As I write this my new scope and almost all the accessories are on a FedEx truck to be delivered later today. The only thing I won't have yet is the eyepiece I bought, which was on back order and will be shipped later this week.