The title is true (approximately), on the day I built the telescope I spent less than £3.50 - but I already had convenient tubing and tape lying around.
Long story short, I decided to have a rather strong coffee on Tuesday morning and that evening, having buzzed around all day, I had built myself a 75mm aperture refracting telescope. Why? Well I have always enjoyed looking through my dad's telescope back home, pointing it at Jupiter, Venus, Mars, the moon and whatever else happened to be along my line of sight. I have been having withdrawal symptoms ever since coming to uni, but my recently completed work for the ESA "fly a rocket program" (more on that in another post) tipped me over the edge.
How did I do it? Search up refracting telescope on google and you will get all sorts of lovely diagrams, adverts, pictures and links to educational websites. Trawling through this lot left me with two impressions: 1. Whatever I built wasn't going to produce pretty images - the colours would blur away from the centre of the image, the image would be flipped and shapes may end up distorted as well. 2. Magnification is done at the eyepiece (the place where your eye goes) - a good goal for the main telescope-y bit is to get it as large as possible, both in diameter (to get as much light), and length (as this counters 1.), but these are both limited by the lenses you use.
Off I went to find a nice big lens to put at the front of my telescope, the correct name for this lens is the 'objective'. Hundreds of years of gradual improvements to telescope design started pretty much immediately by getting rid of the standard convex lens as the objective, instead using all sorts of wonderful shapes to limit colour blurring.
HA HA HA.
75p later I have myself the greatest abomination of an objective lens in the form of a wilco magnifying glass. At 75mm across it is not the biggest lens, but it is a lot bigger than my eye which is actually the important bit. Another ~£1.50 later and I have myself a pair of small binoculars from a charity store, ready to be killed and resurrected as my eyepiece. None of this is going to work fantastically, but all I want to do is not have only my eyes to look at the stars with.So having spent the entire day polishing my new objective lens I rushed home to begin construction!! The key thing I was concerned about with the focal length of each of my lenses (including the ones from my binoculars). This simply involved pointing a lamp into the lens from a long way away, and then looking to see where the image came out clearly on the other side:
As you can see I had lots of fun with little pieces of tape, the first black piece after the big lens marks the focal length of that lens while the white pieces were used to measure the distance from the target to my eye.
One of the key mistakes I made at this point was to check for magnification. This being my first recent foray into optics, I really didn't want to manage to make an incredibly convoluted way of looking through a cardboard tube, with all benefits brought by the lenses cancelled in some way. As all I wanted to do was see some of my favourite nighttime targets a bit more precisely, I didn't really care how much magnification I got, simply that it was more than none. Thus I spent ages with the white bits of tape, checking angular sizes of things with and without the lenses, only to realise later that magnification comes primarily from the eyepiece, which usually like to sit very near the focus of the scope - which simplified the design immensely.
Construction was very simple:
- Take cardboard and cut section off with a length corresponding to the focal length of your objective lens, I tried with a slightly longer tube to begin with but this did not work at all.
- Cut another much shorter section of cardboard to act as a light shield in front of your objective lens.
- I then added slightly smaller rings of cardboard inside the two sections at the ends where I was going to put my objective lens to help mount the objective lens and keep it still once the telescope was complete.
- Then tape the cardboard sections together with the objective lens between them.
- Now we need to have some sort of mounting for our eyepiece (s). Looking around the web you can see that most mountings allow for the eyepiece to move towards and away from the objective lens to help focus the output image. I took a ring of cardboard with a diameter such that a toilet paper roll could slide through it. I then took a convenient object matching the cross section of my main tube (the end of the tube for when it was a postage tube), cut a hole at its center with a diameter corresponding to my loo paper roll collar, then taped the collar into this hole.
- Bodge an eyepiece. I am still doing this. Actually looking at eyepiece layouts was a good start, I eventually opted for a Ramsden configuration as this was the best I could do with the lenses available...
My conclusions...
£5 for any telescope would be good, but building it has meant that instead of a rubbishy kids second hand novelty scope I have ended up with a passable scope with an alright aperture and plenty of potential. At the moment the telescope mainly just focuses images, without enhancing them too much, but with some more lenses (and a bit of practice) I think that new eyepieces would drastically improve the telescope's performance. The dodgy objective lens reduces the potential utility of the scope: colour bleed and image distortion are utterly pathetic, and this project is missing the crucial element of any true telescope - the mount.
I envision many happy evenings spent looking at random celestial objects with the telescope, but in reality I think my efforts in telescope building may have only just begun...
