A good telescope is a wonderful gift but where do you start?
To buy a suitable starter telescope, it helps to have a basic understanding of how a telescope works, so here is a quick crash course.
A telescope is a relatively simple instrument which works in a two-step process.
Step 1: Capture Image: Firstly the light from a distant object enters the telescope through the objective lens. The light is refracted or bent as it passes through the lens and is focused to form the captured image of the object within the telescope.
The distance between the Objective lens and the point where the captured image forms is known as the focal length of the lens and a telescope has to be long enough so that the captured image is within the telescope tube which is why many telescopes are long and thin.
Step 2: Magnify Image: Secondly, the captured image is then viewed through the eye piece which is basically a magnifying glass allowing us to enlarge or magnify the captured image. By changing the eyepiece we can magnify the image by a greater or lesser amounts.
So, even though we associate the telescope as primarily a device that magnifies, it’s most important function is to capture an image that is bright and clear, which, when magnified is still bright a clear enough to be seen.
Brightness: The diameter of the objective lens determines how much light enters the telescope and therefore how bright the captured image will be. When an image is magnified 10 times it is 100 times dimmer and so if the captured image is not very bright, then we can only magnify it a small amount before it becomes too dim to see.
Resolution: The quality of the objective lens determines the quality and resolution of the captured image. If the lens is of a low quality, the resulting captured image will be unfocused and fuzzy and magnifying it will only enlarge the poor detail, not improve it.
You can have an eyepiece that might allow you to magnify the capture object 100’s of times, but if it is too dim to see and of poor quality or resolution, it is pointless.
Even if the objective lens is of the highest quality, there is a limit to how much you can magnify the captured image. The rule of thumb is, 50 times /inch of objective lens but usually this figure is closer to 30 times.
Finally, even if you create a clear + viewable magnified image, if the telescope is not on a sturdy tripod, then the image will bounce around with the slightest breeze or touch and be impossible to view. Remember, objects are constantly moving across the sky and when magnified the movement appears greatly increased so you have to constantly change the direction the telescope is pointing to keep the object in view and an unstable telescope will wobble and ruin your image as you guide the telescope.
So the features of a good telescope are:
The objective lens size
The lens quality
A solid mount
Remember the absolute maximum magnification you could hope to get is 50 times per inch of diameter of the objective lens, although in reality this will probably be nearer to 23-30 times per inch.
The price is also an indicator. Now this is not as absolute as the other features. There are good bargins for scopes out there but in general, if you are spending much less than €100, I would be cautious.
As with any other quality product, there are reliable brands out there. The biggest and most popular ones are Celestron, Sky Watcher and Orion. There are others and it is worth checking out on-line reviews of the different brands. This can be an important purchase and worth spending a little time considering your options.
Where you buy is also appropriate. Again, this is not an absolute rule but you are more likely to get a quality starter scope at a Telescope specialist outlet or a camera shop or even second hand. Many people upgrade their telescope and will sell their existing one and you can get some very good bargains for a tried and trusted scope. You tend to get the plastic toy versions in toy and craft shops and super markets.
Personally I shop with Ktec Telescopes which is an Irish on-line telescope specialist company. They sell everything from the simple starter to the more complex machines. I should add, I have no connection with Ktec and I am not sponsored by them, this is just an honest assessment from a customer!
Finally I thought an example of a toy that is being sold as a real telescope in one of the larger chain stores in Ireland and Britain would best demonstrate the pit falls in looking for a telescope.
The advertising on this box is one of the worst cases of misleading and false information you could find. The first red flag is the description title. A 700mm Telescope. The standard way to refer to a telescope is to state the diameter of it’s objective lens. So if you say you have a 700mm telescope that means the diameter of the objective lens is 700 mm or about 27.5 inches. That is a massive telescope! But if we read the text below we see the 700mm refers to the focal length of the lens.
Now stating the focal length is a useful piece of information but to do so with no mention of the diameter of the lens is very unusual. I would estimate at best it is 2.5, possibly 3 inches wide, which we now know would allow an absolute magnification of 3 x 50 or 150X. Given that the lenses in this scope will not be very high quality, the best you could maybe hope for is probably 50X.
The box advertises 525X which is an out and out false claim. And given the low quality lenses, apart from perhaps a clear Moon, nothing will be seen through this scope that isn’t too blurry or faint to detect any detail.
And even if you could see anything given the cheap, loose plastic mount, it would never be stable long enough in your field of view to look at it.
€64.99 may seem a good price for a telescope but it’s a bit expensive for the toy that it is!
Viewing the skies is a fantastic hobby for young and old alike and can lead to a lifelong passion for astronomy and an appreciation for proper science, which in these days of climate change deniers, anti-vaccers and flat earthers is needed now more than ever. So please do consider your loved one a good telescope.