DIY Neutral Density & Lunar Eyepiece Filters

This isn't the first time I've used this little trick, but since I needed to make a few more filters for some upcoming solar observations, I thought I'd share.

You know those disposable sunglasses you get after an eye exam? They are an excellent source for neutral density filters, if they are simple dark tinted, or lunar filters if they have a brown tint. Simply cut out 1 1/4" (a little less than 32mm) disks from the material. They have the potential to supply up to six disks, which is more than adequate for most amateur astronomers. They can always be layered to add more filtration as needed. The way you use them is to put them ahead of the eyepiece if you have a diagonal. If you are using a telescope that doesn't use diagonals, say a reflector, you may need to do a little engineering, perhaps making a hood that attaches over the back of the eyepiece, putting the filter between the eye and the optics.

They help to tone down the Moon for lunar observations, and can be used in conjunction with solar filters to provide additional contrast (NEVER use these filters alone for observing the Sun, even layered. Always use proper solar filtration).

So, next eye exam, don't toss these little disposable sunglasses, add them to your astronomical tool kit.

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 Méchain. 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.

The Smaller Telescope, Part I - In Defense of Smaller Telescopes

As an advocate for smaller telescopes, I frequently find myself trying to find better methods and procedures for the amateur and small telescope enthusiast. As I have grumbled many times, many amateur astronomers tend to put anything with an aperture less than 152mm (6 inches) into the category of toy, save for a few, high end exceptions. But as their light gathering capabilities are usually similar to these higher end smaller instruments, I find this a bit puzzling.

Instead of trying to find resources for smaller telescopes, perhaps a different approach is needed. That approach is to use resources and tools developed for binocular astronomy.

In short, binoculars are nothing more than small, twin telescopes, held together. They are all almost always low power, with 7x to 10x being perhaps the most common, regardless of aperture. Most books for binocular astronomy therefore tend to concentrate on objects that can be easily observed at lower powers, such as open clusters.

A small telescope, 70mm aperture or less, has two viewing advantages immediately. The first is, of course, magnification. With a modest assortment of eyepieces, one can choose their magnification, but normally they can have higher magnification than comparable binoculars. The other advantage is a steady mount. You can buy tripods and tripod adapters for binoculars, or other devices to steady the view. However, telescopes, even inexpensive ones, almost always include some sort of mount. 

The combination of magnification and steady mount makes a small telescope complimentary or preferable in many ways. When  combined with observer guides for binoculars, the small telescope enthusiast is now armed with the tools necessary to do a fair amount of observing. 

Perhaps most binocular astronomy guides should carry "and small telescope" in their titling.

Next, we will look at some of the best resources for the smaller telescope.