Yesterday I constructed my very own refractor telescope (above) which I built completely from scratch. It is the second telescope I have built and frankly, puts my first attempt to shame. Granted, my first attempt (below) wasn’t hard to out do…it was a humble 10cm long x 5.5cm diameter spyglass. Have you ever seen Robin Hood, Prince of Thieves with Morgan Freeman? He pulled out a “telescope” much like my first attempt. It was really exciting when I first saw a magnified image through the lenses. It was riddled with chromatic and spherical aberrations but I achieved a 7.6x magnification! The problem was, while the image was bigger, it was no clearer. It couldn’t resolve the larger image. I had Danielle write a sentence on a piece of paper and tape it to the furthest wall in our apartment so that it was just out of readability. I tried with all my might, but couldn’t read it any better than with my own two eyes. It’s one advantage was it’s light collecting ability, or f-number. Divide the focal length by the objective lens (large lens) and you get the f-number which, for my little scope, equaled f/1.8.
I learned a lot from my first telescope so that, when it came time to build my second, I was well prepared. I purchased my lenses through the same online supplier that carried the lenses for my fist telescope. There are other sites that sell telescope lenses but they were either too high priced or didn’t have the focal lengths I wanted. Anchoroptics.com carries a plethora of “experimental grade” lenses that didn’t pass quality inspection and cost half the price. The 4.1cm objective I purchased had one tiny scratch on the surface which is completely unnoticeable when looking through it. This time I did my homework and got a lens with a decent focal length, 39.5cm compared to the first lens I bought with a 10cm focal length. The eyepiece I chose had a 12cm focal length. Aside from the focal length, the other big improvement these lenses had over the previous were their achromatic elements. The objective has a lens element that corrects for chromatic aberrations which basically means it’s sharper. The eyepiece has three elements in total. This vastly improved the quality of the image over my previous scope which was only sharp in the very center of the lens.
When the lenses arrived at my door I couldn’t wait to open them up and experiment with them. I always have an urge to just hold them up to my eye and see if I can produce a magnified image without any supports. I know I won’t be able to do it; even if I did it would be a fleeting glimpse of an image. I decided to wait till tomorrow to set them up and calculate the spacing needed between the lenses. For my last telescope, I created the most ghetto rail system to hold my lenses and determine the spacing. It consisted of a yard stick (you would not believe how hard it is to find a meter stick. I still haven’t found one), balsa wood bases, wooden sticks, two dollar clamps, duct tape and rubber bands. It worked for the first telescope so I had all confidence that it should work for my second. I spent the better part of my day off trying to produce an image to no avail. I started thinking that there was something about achromatic lenses that I wasn’t aware of. The focus didn’t seem to behave like that of a standard lens. I became frustrated and started posting on forums, asking what I was doing wrong. I gave up and decided to take a break till next week.
After receiving some feedback and regaining confidence that there was nothing strange about achromats, I decided to put my faith in science. I didn’t care if I couldn’t produce an image with the lenses before constructing a telescope. I had done the math, I understood the principles; two double convex achromatic lenses should produce a magnified image if aligned and spaced appropriately. This time around I wanted to upgrade the materials I used. Instead of a paper towel cardboard tube I stepped it up to PVC pipe. I must have spent an hour and a half in Home Depot searching for the right combination of tubes. I wanted to use the black plumbing pipes but I couldn’t make the right combination of sizes. I ended up getting two white tubes, one that could snugly fit inside the other, and a few attachments for the lens hood and eyepiece holder. I picked up some bolts to hold the lenses in place and a thumb screw to lock the focus in place. I spent probably three hours when I got home carefully constructing my telescope, the whole time not even knowing if it was going to work. Once I got the lenses mounted (by far the hardest part) I took it outside, put it to my eye, slid the tubes away from each other and focused in on a beautifully magnified image, 40 times its actual size. Science had conquered the day!
The f-number is a little dismal; the smaller diameter and longer focal length means not as much light reaches your eye, but observing bright objects like the moon and Jupiter are no problem with an f/13. I determined the exact magnification (not through calculations but through observation, there’s always a discrepancy) to be 42.5x. This is really cool because it is thought that Galileo’s best scope had around a 40x magnification. So there you have it. The greatest accomplishment of my life. Where do I go from here? I really don’t know. I could build another, more powerful refractor telescope. I could grind my own lenses. Or I could try my luck with reflector telescopes. That is the route most amateur astronomers take. I am told they are much more practical to build and are also much more forgiving in the construction process. Whichever path I take, all I know is that I will definitely build a third.