Construction of a homemade telescope
The making of a telescope
Purchased the plywood for the mount today, as well as the assorted items I’ll required for building most of the rest of the telescope, including the curtain-rod for making the central piece of the spider, and the cedar shingle-shims to be used as the vanes of the spider. At the lumber store, I asked them to make the cross cuts to the plywood, so all I now have to do is cut out the individual pieces.
At 11:00 p.m., I started to feel doubts about my ability to make the spider, and I contemplated ordering one online. Then I got hold of myself and made myself try it. For my first scope, I’d rather make everything (with the exception of the optics) by hand. Besides, what’s the worst that could happen? I screw it up and have to buy a commercial spider. On the other hand, if I do it right, I’ve got one I can use for a fraction of the cost. And so, with a minimum of fanfare, I began the actual production of the telescope tonight, beginning with one of the three piece I’m worried about. The spider.
All went well. It took me three attempts to cut what I thought was a good enough 45 degree angle on the dowel for the secondary mirror, and did so only after abandoning what the plans suggested (wrapping paper with 45% cuts around the dowel) and tried my own method of marking lines the equivalent of the diameter of the dowel back its length, then drawing lines to the front edge. It seemed to go well, and after putting the two cut pieces together, they seemed an almost perfect 90 degree join to me, at least to the naked eye, and I think that may be good enough for this.
After that, I marked the grooves on the dowel for the vanes, and used a carpet knife to cut them out, finishing them off with the end of a file. Again, that worked nicely, and I lodged the shingle-shim vanes into the spider without too much trouble. And so, after one night, staying up late and spending about 2 hours, I have mostly completed the spider assembly – a part I was worried about because of its complexity.
Did a little shopping today for more parts. At Staples I picked up a 1-1/2 inch cardboard mailing tube. At Home Depot I picked up a 1-1/2 inch diameter PVC pipe to fit inside the cardboard tube, to be used as a focuser, as well as a six-inch long version of the same PVC to be used for aligning the optics. As well, I picked up a piece of Masonite to be cut and used to mount the focuser on the tube. Total costs for all of the above, approximately $10.
The plans actually call for chrome plated brass, not PVC, for the internal tube of the focuser, but I thought this might work nicely, and it had a lip on it to stop it sliding past the end of the cardboard tube. I tried it for fit when I got home, and it is perfect. At it’s deepest insertion, it becomes quite snug, but can still move. I think this will work nicely.
When I got home, I was in a mood to start doing some work, and with the help of the kids managed to do the final cutting on the Plywood to get all the parts ready. That took about 30 minutes, plus five minutes to clean up. After dinner, still feeling eager to work at this project, I went back into the basement and put together most of the cut pieces to make the Telescope stand and mount. If I’d had a LP record and some Teflon the job would have been finished tonight! But I’ll probably finish it tomorrow. I figure I’ve got less than an hour left on finishing the mount, before painting and finishing it off. I can’t believe how easy it has been so far.
Let’s face it, I am as far from a handy man as you can get, but the plans from the Sidewalk Astronomers are easy to follow, very straightforward, and accurate. Just follow the directions, and everything seems to work out. I’ll be sending off the money tomorrow for the primary mirror, secondary mirror, and eyepiece, and will hopefully have delivery within a week or two. I think the challenging part of the project, the part where detail and exactness will count, is putting together the optical tube assembly. I can hardly wait!
Went out to Home Depot looking for rounds of wood I could use as the side bearings on the tube box, but could not find any. Instead, I picked up a couple of PVC caps. These look pretty good. The are about 4-1/2 inches in diameter, made of strong white plastic. I measured for position on the side of the tube-box, then glued and screwed them into place. They worked fabulously. The cradle now sits in the cradle arms and moves nicely, even without Teflon. I placed the tube in the cradle, just for show, and it looked amazing,. The scope is coming together. It sits in the basement, partially constructed, tube in cradle, and I can see that I’m making a real telescope. Cool.
Tackled a couple of the little jobs that I can still do before the mirror gets here. Cut the Masonite for the backing of the eyepiece tube. Cut the four strips of plywood that will be used to support the mirror in the tube. Built the tailgate and mirror cell, installing the three machine bolts, attaching cardboard, little squares of Masonite. I also painted the spider.
I would have done more, but my drill is broken! I’ll try to pick Tail gate assembly up a small hand drill today so that I can finish these little jobs. It would be nice to get them done before the optics get here. If the optics do get here this week, I’m really going to try to complete the scope by the weekend, which would make it a one-week project, working in the evening.
So far, tailgate and spider are the most complex parts I’ve made. The eyepiece will be about the same. In other words, the optic tube itself is going to be the part that requires really careful work. No more slapping things together.
The optics for the telescope arrived today: six inch F8 primary, secondary, eyepiece and some Teflon. I am a bit disappointed in the quality of the glass on both primary and secondary. The primary is streaked with swirled lines as if it has been cleaned by a rough cloth. There are similar lines on the objective lenses of a pair of old binoculars I own. There a small black speck about halfway between centre and the edge of the mirror where it appears there’s no coating on the glass, or a flaw in the coating. The mirror itself is quite dirty, but I’m reluctant to clean it given the existing lines and swirls. Now, it could be that I’m expecting too much from the optics. This is the first mirror I have ever purchased.
I really have no experience at this, but I had presumed, because amateur astronomers always talk of optical perfection, that a purchased mirror would be pristine. The secondary, too, was a bit out of shape, with a chip on one edge of the coated surface, approximately 1/8 inch across. Could these defects have happened during transit? Again, I Completed mount. don’t know. They were very well packed and wrapped. I bought both from Peek’r Technologies, where the service was amazingly good. The great service, in fact, was the reason I purchased the optics there, despite the price being higher than at other suppliers. Will these imperfections have an effect on the scope’s performance? I don’t know. I’m not an optical expert. I do know that if I’d been purchasing these locally, and had inspected the mirrors before taking delivery, I would not have taken them. Be that as it may, this whole exercise is a lesson in many ways, and I intend to continue with the optics I’ve got to finish my scope, and to use it to the best of my abilities.
On with the show…
With some spare time tonight and the house empty, I tackled a number of items on the telescope’s to-do list. First I confirmed the focal length of the primary. Using the method suggested by the plans, I determined the focal length at 46 inches, a couple of inches less than the stated. Within the expected deviation, I think. I also pealed the plastic from the inside of the tube. The plans said it was there, but I didn’t believe it until I pushed at the edge of the tube and some of it came off.
Using the focal length figure I determined where to cut the eyepiece holes, etc. I cut a hole for the eyepiece, installed the support pieces for the cell inside the tube, installed the mirror and tailgate to test fit, and all that went nicely. I also put together the eyepiece assembly (mailing tube and Masonite), and attached the secondary mirror to the spider using pieces of leather and glue.
I completed the stand, attached the base to the rocker, and that was easy enough. The Teflon pieces underneath an old LP really works.
After all that, I painted the inside of the tube with flat black spray paint, painted the spider, and installed and attached the eyepiece assembly into the tube with screws. All went well, and tonight I’ll continue and hopefully get the spider installed and the optics aligned.
After getting home from work, I was eager to do some more work on the scope. I measured the spider for fit, then trimmed each of the vanes. After bevelling the ends of each vane, I pushed it into the tube. A nice, tight fit. Looking through the scope, eyepiece and framing the open end of the scope in the secondary mirror, it didn’t take long to get that fitted nicely, after which I glued it in place. Note: the spider was a snug fit, and probably did not need glue, but the glue adds a measure of safety.
Next, I popped in the primary mirror, and attempted collimation. The goal here is to centre the the little mark (sticker) at the centre of the mirror the centre of your eye as it’s reflected in the secondary. Took a little experimenting, until I realized that the turn of the collimation bolt had the effect of “pushing” or “pulling” the reflection of the eyeball across the surface of the mirror. After that idea crystallized in my head, it took literally a few seconds to “push” and “pull” with the collimation bolts to get the decal perfectly centred over the reflection of my eye.
Purchased paint to finish the scope today. Total, including brush, for primer and top coat, came to about $40. One of the more expensive items so far. I took the scope apart, took the stand apart, removed the primary and tailgate and the eyepiece, then put on a coat of primer. It went on very nicely. The guy at the paint store mixed a dark grey primer to make it easier to put on the navy top coat. Using some of Val’s indoor/outdoor craft paint, I painted the top of the spider fire-engine red, which gives it a very nice look when looking into the tube. I’ll give the primer a few hours to dry and will apply the top coat after supper tonight. I’m hoping to get the scope finished today.
Finished scope After mostly finishing painting, and attaching a rudimentary finder (two pieces of 1-1/2 inch PVC mounted at end of scope and on tube-box, aligned like a gun sight), I took the scope out tonight. Hazy sky, very few stars visible at all through the haze, but Jupiter bright and alone almost directly overhead and to the south. Used the finder (sort of) to get the planet into the field of view and had a look. Again, I need to let the scope cool down, but even in this awful haze, I easily saw four moons, three on one side and one very close on the other side. No detail on the disk. Not sure if this was the haze or just because the disk is so bright. I will try again in about twenty minutes after the scope has had a chance to cool down.
Couple of things to note. After painting and installing the tube firmly in the tube-box, it no longer has the balance it had. It is slightly bottom heavy, which means it does not stay put. This could be because of the Teflon. I don’t know. I may have to move the tube in the box, or add a little weight to the top end.
Also, the finder is next to useless. I will need a Telrad sight or something similar if I am to find objects without a struggle with this scope. It really will make the difference between having a useful scope and one that I won’t use much.
Final touches. Painted the base of the finder this morning, and that’s it. The scope, version 1.0, is now complete. All changes after this will be modifications of one sort or another to improve performance. They say that building a dob never ends. I guess I’m about to find out. In the mean time, I’m happy to say I’ve successfully built my first homemade telescope!
Made first serious modification today. After researching “ATM finder scopes” on the Internet, I learned that a lot of amateurs were using red dot BB gun and air gun sights. These are relatively inexpensive, and apparently quite effective. I took a trip to Army Surplus at lunch, and, lo and behold, they had them in stock. Crossman Red Dot airgun sights. The price was $29.95 plus taxes, for a total of $34.14. To purchase a Telrad sight or a Rigel Quickfinder would cost me $69.99 plus $20 shipping plus taxes, for a total of $94.88. I may still get a Telrad in the long run, but for now the Crossman Red Dot will do. And do it does.
Installing the Crossman Red Dot was a bit of a chore. I had to remove the PVC, scrape off the glue, then do some cutting on the plywood to allow the sight to mount properly on the scope. A little drilling was also required for the mounting screws,. It went on alright and seemed pretty solid.
After a couple of adjustments to get it set right, the aim was near perfect. Testing outside stars and planets were instantly in the field of view, usually in the centre of it, at 57x magnification. That’s a lot better than I was managing with the PVC tube, which got me in the general vicinity and no nearer. The lens of the sight is slightly dark, to allow aiming during bright daylight, which makes it a pain for faint stars, but by keeping both eyes open I managed to get a superimposed image and aiming was really no problem. The best part was that I no longer have to drop to my knees in the snow to aim. Just bend over the scope for a peak. Very nice.
Note: Original construction occurred between February 1, 2003 and February 9, 2003. A couple of modifications were made in the week following.