Often you will see telescopes advertised with a focal ratio such as f/6 or f/10, but what does this mean for viewing and astro imaging?

The lower the number of the focal ratio the lower the power of magnification and the wider the field of view you can see through an eyepiece, or image with a camera. An additional bonus of a low (also known as fast) focal ratio is that a given amount of light can be gathered with shorter exposures than that of a telescope with a larger (slow) focal ratio. However, telescopes with larger focal ratios will be able to produce higher magnifications than a lower focal ratio telescope if both are using the same eyepiece. A larger focal ratio telescope will also produce a smaller field of view.

So, how do you determine a telescope’s focal length (the distance light travels from the main mirror or lens to the eyepiece)?

Simply multiply the telescope’s focal ratio by its aperture (diameter). For example, my 8”LX200R has a focal ratio of f/10 and the aperture is 8”. You do however need to change the aperture from imperial to metric (2.54cm to 1”), so for my scope;-

8 x 2.54cm = 20.32cm

So, my LX200R has an aperture of 20.32cm but I’ll round this down to 20cm for ease. Now we have a figure of 20cm for the aperture we can multiply it by the focal ratio;-

f/10 x 20cm = 200cm or 2000mm (normally expressed in mm)

Now you have the telescope’s focal length you can determine the magnification any given eyepiece will produce. You will see eyepieces are rated in millimetres (mm), 9mm, 40mm etc etc. Divide the focal length of the telescope in mm by the eyepiece in mm to work out the magnification, for example;-

2000mm focal length divided by 30mm eyepiece = 66.6 times magnification

2000/30 = 66.6
2000/20 = 100
2000/12 = 166.6
2000/6 = 333.33

So in a telescope with a focal length of 2000mm a 30mm eyepiece will give you a magnification of 66.6 times whereas a 6mm eyepiece in the same scope will give you a magnification of 333.33 times.

Also, scopes with different focal lengths will give different magnifications with the same eyepiece. Below is a list of Meade LX200 scopes from 8" to 14" with their respective focal lengths and the magnification each one will produce when using a 26mm eyepiece.

8" LX200R focal length 2000mm/26mm eyepiece = 77x magnification
10"LX200R focal length 2500mm/26mm eyepiece = 96x magnification
12"LX200R focal length 3048mm/26mm eyepiece = 117x magnification
14"LX200R focal length 3556mm/26mm eyepiece = 136x magnification

So, what focal ratio is right for you?

If you want to view and image the planets then a telescope with a larger focal ratio is preferable (f/10 is better for planets than f/6). If you’d prefer to view and image deep sky objects then a telescope with a lower focal ratio is preferable (f/6 would generally be better than f/10). Many deep sky objects are in fact very large and would not fit in the eyepiece of an f/10 scope. If you wanted to buy only one scope then you could either get maybe a larger focal ratio scope to use on planetary viewing, and use it in conjunction with a focal reducer that fits between the tube assembly and eyepiece. For example, I use a Meade f/6.3 focal reducer (about £100) with my 8”LX200R that lowers the focal ration from f/10 to f6.3. This lowers the magnification and gives me a wider field of view. Alternatively you could buy a lower focal ratio scope and use it in conjunction with a 2 x Barlow lens (fits just before the eyepiece), so an f/6 scope would become f/12.

As with anything there is always a trade-off. The higher the magnification the harder it is to resolve (focus) an image. Too high a magnification will be impossible to resolve. Basically, I own a 6mm eyepiece that I rarely use as my scope has difficulty resolving the image. The highest I normally use is a 9mm eyepiece.

A telescope with a larger aperture will find it easier to resolve smaller objects than an identical scope with a smaller aperture. It’s not just the aperture that makes it difficult to resolve an image; the atmosphere also plays a huge part. If you point your scope directly upwards it has to look through fewer layers of atmosphere than it would if you pointed your scope just above the horizon; the less atmosphere you look through the better, which is why it’s always best to view objects when they’re as high in the sky as possible. Sometimes the night sky may appear clear with the naked eye but can be poor when viewed through a scope, but an hour later you might be lucky and the seeing (astronomy term for the blurring effects of air turbulence in the atmosphere) could be really good.

So in some respects it’s best to buy the largest aperture scope you can for your money if you want to resolve smaller objects. A larger aperture scope will also be able to see fainter objects, the bigger the aperture the dimmer items it will be able to show you.

An increase in aperture also means an increase of weight of the scope. My 8” LX200R is quite heavy and the next one up in the range, the 10” is much heavier. Personally I wish I’d bought a larger aperture scope, especially as I now have it permanently mounted. But, if you’ll be taking your scope in and out of the house then don’t get a scope that’s too heavy for you to comfortably handle or you’ll never use it. Go to a retail store and try handling it first, walking with it and getting it on and off the tripod. Make sure you get a scope you’ll be able to move easily unless you’re planning on mounting it permanently or you can get someone to help you move it each time.

Before I mounted mine in an observatory I sometimes used to pack it all up in my car and take it to a darker site. I think a 10” LX200R would have been about as large as I could have handled on my own (and I’ve been a regular weight trainer for many years), as it's tricky getting the scope on and off the tripod. There are however lighter scopes made, a 10” Meade LX90 is far lighter than an equivalent LX200R.



Remember, with an LX200 (pictured above) or LX90 you're not just lifting the tube assembly alone, the tube assembly is permanently attached to the fork mount and base (basically everything that sits on top of the tripod). Celestron's (and others) equivalent systems might be easier to handle as the tube assembly separates from the mount. If I could start again I'd probably opt for a Celestron C11 SGT on an equatorial mount (pictured below) or larger.