The 4 inch mirror after White Aluminum Oxide 5 micron. Notice how it reflects light. The surface is smooth, gray, and satiny smooth. I can read regular print through it. There are absolutely no scratches/pits. The surface is flawless. Pitch pouring and pressing process coming soon
As I progress through building my telescope, it is important to know and understand the focal length, or distance where all of the light from the star forms into an actual image at one point. There is a simple yet effective way to do this. You don’t need much, but a checklist is helpful.
When I perform this test, I use:
a bucket halfway full with clean water
a tape measure that extends to about 100 inches
the mirror (duh)
a piece of chalk or card to mark the distance
and a white poster board (optional but it helps)
Here is the setup:
So here’s the steps from Building a Reflecting Telescope:
As you can see, the process is so simple and straight to the point that I will agree not to explain what is being depicted. My results came out well as expected. I was aiming for an f/12 telescope, which would yield a 48 inch focal length, and it came in at 47 inches. No harm done, as an inch will not effect the performance in the least bit.
As I was grinding on the weekend, I am on the last 2 abrasives; aluminum oxide 9 and 5 micron. It is said that when grinding has been successfully completed, the mirror will feel like the finest satin to the touch, and newsprint can be read through it. It’s already unbelievably smooth, and not quite able to read through, but nevermind.
I caught a snag while finishing my abrasive: I scratched the mirror’s surface near the edge. I’m not quite sure how it happened, but my best guess is that somehow, dirt of some sort got between the glasses, and scratched. If this furrow was near the center, I wouldn’t be stressing over it as much. It takes about 3 times as long to remove such a thing at the edge than one near the center, as the glass naturally grinds out fastest at the center. Oh well. I knew that I would get a scratch at some point, but I’m happy it wasn’t during polishing, or I would have been in some serious trouble.
Hope to be finishing this project soon, because I’m itchy to look through this telescope!
When I first got the idea of actually building a telescope in my mind, the first book I read was Amateur Telescope Making. To many this represents the pinnacle of telescope making books/guides. However, there are more out there than I once thought.
For example, here is the book on mirror making that my grandpa wrote back in the 60s. It was typed with a typewriter and the drawing were all made by him, which were really great drawings. This book is the simple way on how to build a 3 inch F-12 reflector. But don’t search for this book, because I have the only one.
Another good book for telescope making is Building Your Own Telescope, by Allen Thompson. I have not read it myself but many have said it is a very good reading, and can had for around 10 dollars (here). Look around, you’ll find countless books on this subject, they are not very recent, but still have the same valuable information to build a telescope from the ground up.
So, the question is, which book to buy? This depends on how far you want to go as far as precision for the telescope. I reccomend a simple technique that does not require the foucault test, because it just makes telescope making a whole lot simpler. Plus, a mirror’s figure does not need to be perfect in order to observe celestial objects with satisfactory results. It has been said time and again that a considerably less than perfect mirror will perform as well as a perfected mirror, it is just a matter of conscience.
Before I got really serious about building my telescope, I was demanding for perfection. I had to do the focault test, had to have a parabolic surface, and I wanted at least a 6 inch diameter mirror. It turns out that I did not do/get any of those, and that is fine with me based on what it said in Amateur Telescope Making. Sometime in the near future, I will be putting my grandpa’s book on this website as a page, along with all of the pictures, sketches, and references.
This weekend, I had been pushing some glass myself- only 2 more grades of abraisive left to go! I ground through the 12 micron today, and and hour was enough. Last weekend (and the previous post), I had thought of something that would be simple and effective in cleaning house for the next finer grade of abraisive.
A simple idea really, all I did was cover the top of the grinding stand in clear plastic wrap, then set the tool in place as normal. I tried it, and it really works! Cleanup was easy as ever, all I had to do was simply pull up the tape, and lift the wrap off. Then the new wrap can be laid on and taped, ready for action.
As I am progressing through these finer grades, the glass has started to lighten up a bit, and it is not as frosted as it previously was when I ground the first grade (#80 silicon carbide). I have been told that when the final grade has been finished, the glass should almost be transparent, though not quite, as polishing will remove the final grade of pits.
While progressing through the project, I’ve been planning ahead for gathering the finishing materials (diagonal, diagonal mirror, mirror cell, etc) and found a place to get mirrors aluminized cheap, as well as pretty inexpensive secondary mirrors, and other components.
On Friday morning, I was able to take out my 70mm refractor for a good hour and a half. I was able to get an extraordinary view of the Orion Nebula, which is saying something, because I have seen this diffuse cloud through a 14 inch reflector, so I know what I am talking about. I’d say that the view at 56 power is pretty close to the view through a 6 inch reflector. I hope to have this sketch on Astronomy Sketch of the Day, just like when I had my M31 sketch on there.
Here is my sketch of M42 :
All I can say is, I hope to be sketching this through my 4 inch soon.
Grinding is wrapping up for my telescope mirror project. I only have 3 more abrasives left to grind through, and the on to polishing. The project is going very well, and the abrasive is like a powder. It has been rainy the past few days, so that ruled out most of the observing, but before I get into the project, I had a spectacular night with binoculars.
I have a decent pair of Barska 10×50 binoculars, and there were clearings from the clouds and the rain late at night. Cassiopeia is getting pretty high up in the sky now, so that means prime observing time for the Double Cluster, NGC 869. Through the binos, it is large, bright, and resolves approximately 40 stars with an unresolved background of stars. This large bright grouping could even be sen with my unaided eye, as 2 faint small patches of light.
My next target was another Cassiopeia cluster- NGC 7789. It was faint, but there as an irregular glow of unresolved stars. It was nice to see a non-Messier object with binoculars.
Next was M31, Andromeda galaxy. It is easily seen in the binoculars, but not naked eye. The core was bright and obvious and was very large. I could not detect M32 nor M110.
My last target was M33, Triangulum. If you observe in skies similar to mine, you know how difficult of an object this is. I started scanning the area several times, and I detected a faint oval patch of light. I had finally found M33! It was not an impressive view, but It was certainly visible.
It turned out to be a great night, and was good enough for me. Sometimes, small optical devices are all you need to enjoy the night sky. The clouds came and went as the night wore on, but I was satisfied to have my look at the heavens.
In the last post I set up a grinding schedule to accomplish one grade of abrasive every week. It looks like that plan is going to work. I set up my equipment and got to work. I always find it interesting to feel and study the surface of the glass closely and then grind the next finer abrasive for 30 minutes or so and then look and feel the difference. This was apparent after transitioning from Silicon Carbide #80 to #120. These are very coarse grains, but there is a big difference.
It seems impossible that there are still 3 more grades left, because the surface is so smooth to the touch. I opened up the 5 micron container and sampled its size. I detected no difference in texture from the 5 micron to the 15 micron. As I was cleaning up shop for the day, I thought about just putting a layer of plastic wrap over the top of the workstand, then removing it when it is time to change abrasive sizes. That way, I don’t have to clean it outside and re-level the stand each time which would save a huge amount of time.
The continuation of my telescope making project is making headway. I just got all of the abrasives necessary to finish all for the grinding . I ‘m making good progress, but the going is slow. I used my grandpa’s book that he made in the 1960’s- Building a Reflecting Telescope. Don’t try to look for this book, because it was never published. It does have some very good drawings and explains a simplified way to build a telescope.
Well, I’m doing an even more simplified version of his book. I found in the last chapter of Amateur Telescope Making. Here’s a quote:
“For the first mirror we had to abandon Pyrex, the mirror handle, the pitch polishing lap, and the paraboloid. We teach the beginner some of the rudiments of the Foucault test but only because we are present to coach the worker in its use. Although pitch laps give finer polish and fewer zones, they are so difficult for the average beginner to make and alter that we regard them as the principal bottleneck in mirror-making. They have discouraged more beginners than any one thing, or any ten things. Although we get fewer fine mirrors with honeycomb foundation, it best suits our purpose, which is to finish the first mirror while the maker’s enthusiasm lasts.”
I pretty much agree with everything that they are stating here, and this “group” is the telescope making group from the Cleveland Museum of Natural History. I checked out their website, and I’m pretty sure that they don’t make telescopes there anymore. Apparently, there used to be a free program to build your own telescope from scratch, using a honeycomb foundation instead of regular pitch. The only thing I don’t agree with is the part for a beginner not being able to make a successful pitch lap.
I guess that that this does not apply in my case as I have telescope making in my blood (thanks grandpa). I’ve set up a grinding schedule and I plan to follow it quite closely. I plan to grind through one grade every weekend. I have 5 more grades to grind through, so I guess I won’t finish it by my birthday, but I’m not devastated. Good mirrors take time, and that’s fine by me.
On Friday, I was really excited so i made some good progress through Saturday as well. I ground my way through Silicon Carbide #320 and then on to White Aluminum Oxide 25micron. It is very strange going from this muddy grey sludge to a white sludge. The abrasive is becoming so fine that it doesn’t really feel gritty. The only similar substance I can use to describe it is powdered sugar.
I have also thought about starting a new WordPress blog. I want to do a blog that is like a sketch reference website. But it has to be with at least 1 small telescope, as well as with a bigger telescope. Of course, I have to have a lot of objects sketched in order to that, and I haven’t done that. I have made many sketches, but not 110, yet.
Also, they would probably need to be black paper/background to really get noticed. I have heard of running the white paper sketch through a scanner and then altering it to a negative image, but I don’t have a scanner, so I guess that means I have to use real black paper. But that’s fine with me, I’ve been eager to use a different media for sketching. Hopefully, sometime in the near future I can have a sketch on Astronomy Sketch of the Day.
Someday, I will make larger, more complicated telescopes as well.