Reflecting (Newtonian) Telescope


creator of virtual planes
This isn't RC related at all. It's a project that I fully "plan" on actually doing, but right now it's just a plan.

I haven't been using this forum at all lately, but I think it's fitting to post this project on here.

Really I'm half checking that I'm not missing something before I put time and effort into building it. And when I was doing research, I couldn't find any good articles about it.

I want to build a reflecting telescope, and all the articles I've found on how to build one all assume that I'll be buying a primary mirror, and that I'd be basing my measurements off of what's specified for that mirror. Any other article I found trying to figure out what exact shape to make the primary mirror and best distances between the primary and the secondary where far too technical.

I hope to eventually write out an article and posting it onto the Flite Test main page, which hopefully doesn't go against their guidelines as it's not RC or plane related in any way. I want to write the article as easy to follow as possible. I know most people aren't going to be able to actually make one, but it's nice to see people do it.

The large concave mirror in the back of the telescope is called the "primary" mirror, because it's the mirror that's doing all the work of shrinking the image down. The 45(degree) angled small mirror is the "secondary" and is used simply to move the shrunk image into the eye piece.

I'm assuming that the further away the secondary is from the primary, the less distorted the image will be and also the smaller the secondary mirror is, and the less the secondary mirror is blocking the image.

I have no idea as to what the best balance is. How far away is too far, how close is too close, I haven't the faintest. I'd assume the further the two mirrors are away from each other the better, but making a 20' long telescope isn't practical.

I should draw, diagram, this out, but as far as I can figure, the best way to make the shape of the mirror is with a compass. To put one end of the compass right at where I want the focal point, and then use the other end to draw the arc that will be the mirror.

I'd actually want the focal point past the secondary mirror. So I wouldn't be making the focal point at where I'd be placing the secondary. I want the focal point to hit right at the "eye piece", which I'm actually going to be putting a camera. So I'd add the distance of the primary to the secondary with the distance of the secondary to the camera, and make that the distance away from the primary to the focal point. If that makes any sense.

To determine the correct size of the secondary mirror, I'd simply have to draw a line from the ends of the primary mirror to the focal point, then draw in place where I'd be putting the secondary mirror and make sure the mirror is a little bigger than the drawing.

Maybe I'm wrong with all of this. I feel like I'm trying to re-invent the wheel because I couldn't find plain English how to design a Reflecting Telescope.

I'm sure you're wondering on how I plan on making the primary mirror. I'm currently working on building a metal foundry (I'm almost done with it), and I plan on casting a solid piece of aluminum and then polishing it into a mirror finish. It might not give me the sharpest image, but you can get a nice mirror finish with aluminum and this allows me to make a 6" primary, or maybe eventually a 24" primary. Really any size I want.

If you took the time to read this long post, please let me know if you find any flaws with my planning. Maybe I'm missing something. Maybe I'm wrong as to how to come up with the proper shape of the primary.

Any questions, certainly ask. I don't plan on actually building it until this fall.


creator of virtual planes

Image I put into the first post didn't seem to work. It's tough to know what I'm talking about with out at least one diagram of a Reflecting Telescope.


creator of virtual planes
Update: I pretty much have my foundry all built and I stress tested it and it held up to having too big of a fire in it. I have yet to melt any aluminum yet and the first thing I plan on building is a 4" - 6" reflecting telescope. I do, of course, want to practice melting aluminum first before I actually cast anything.

So progress is going slow but is being made.


Old and Bold RC PILOT
You can have your secondary reflector out of the visible line by canting the reflector slightly if the reflector is properly ground and polished.

Additionally you can get quite good results if you have the reflector spun from a sheet of Aluminium and then polished. The advantage of using a spun reflector is that you can tune it through its support base if required..

I have many reservations about casting Aluminium using an open flame furnace as molten aluminium can be very combustible. Secondary though a far more important consideration is that if the aluminium is too hot and poured into a green mold there is a very high risk of a severe explosion. Any water in the mold will want to immediately combine with the molten aluminium rather violently.

Burning aluminium is used as thermite and molten Aluminium and water can be more explosive than any commercially available explosive.

Just a little info and my fears!

Have fun!


creator of virtual planes
I'm just making a telescope to have a telescope. It would be nice to see planets and a closer look at the moon and such. Not really RC related in any way. But I'm on this forum and thought people here might find it interesting.

I've been having some issues with my foundry melting down aluminum. I've tried a couple of times but didn't quite get it hot enough. Last time I fired it up it was just shy of fully melting the aluminum but then the old Shop Vac I was using as a blower quit on me so I have to go buy a new one. Which will be good because the 2hp Shop Vac I was using just barely got it hot enough (but still hot enough). Hopefully buying a 5hp one will make the fire hot enough to really melt down the aluminum.

I appreciate the concerns with the melting process. It is dangerous. Very dangerous.

I'm not sure what I'm using as a mold to cast the reflectors yet. I might just use the same plaster that I built the foundry out of. It handles the heat fine, but not for very long. My foundry clearly isn't going to last very long, not many fires, but I used cheap stuff, not the correct stuff.

So far I've been pouring my melted down soda cans into soup cans. Which actually works great. The steel of the soup can can handle the heat of the molten aluminum but is also thin enough that I can cut and peel it off of the solid aluminum when it's cool. Best part, soup cans are free.

Hopefully I can get some funds together for a new Shop Vac so I can properly melt down aluminum.

I should probably take and then post some pictures of it at some point.


Free Flight Indoorist
As someone who once contemplated doing this, and then bought one instead...

The big issue you need to look at is that you either need a parabolic primary or a corrector plate. Increasing the focal length cannot make up for incorrect curvature of the primary. Long ago there were some pretty detailed articles available on making your own primary. It is not a trivial undertaking regardless of what level of foundry equipment you have on hand. That curvature has to be extremely precise, and since it's not spherical, you've got some calculations to run and then you need very precise measuring equipment to verify you're in spec. This is why back when I bought my 16" Dobsonian, 80% of the cost of the entire scope was the primary mirror, and it took Meade 4 months to build the scope for me.

I'm not trying to discourage you from this, merely trying to make you aware of what is involved in the process.

Something you might also consider: Schmidt-Cassegrains use a spherical mirror and a corrector plate. It's possible that you could pick up a corrector plate fairly inexpensively and then just grind a spherical primary. Sure, you'd need to cut a hole in your primary, but that's trivial compared to grinding the thing to shape, and the double bonus is you get away with a flat secondary and a monster focal length on a wonderfully short optical tube. What's not to like (assuming you can easily source the Schmidt plate)?
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Toothpick glider kid
I've been having some issues with my foundry melting down aluminum. I've tried a couple of times but didn't quite get it hot enough
That sounds kinda odd, my friend and I were able to melt down multiple pop cans in a campfire using a box fan very similar to the ones FT made a drone out of. If you have a foundry that should be able to do it no problem.


Wake up! Time to fly!
Most home made furnaces I have seen are basically propane tanks and leaf blowers.

Seems the crucible used is the important part of it all.


creator of virtual planes
That sounds kinda odd, my friend and I were able to melt down multiple pop cans in a campfire using a box fan very similar to the ones FT made a drone out of. If you have a foundry that should be able to do it no problem.

It didn't take much to melt the pop cans down, as in crumple, but to get it melted down to a liquid like mercury or molten solder takes a quite a bit more heat. I got a lot of blobs of half melted, half burnt aluminum cans.


creator of virtual planes
Most home made furnaces I have seen are basically propane tanks and leaf blowers.

Seems the crucible used is the important part of it all.

I'm using a proper graphite clay crucible. Which they do seem to take a bit to heat up first.


creator of virtual planes
I also think I might be getting too much ash blown into my crucible because of how small of a vent I have on the lid.

But I still need to buy a new blower to use so I can try again since my old blower completely quit on me.

I'm sure I'll get it figured out and working. Honestly I would have been more surprised if it worked correctly the first time I tried it.