An exacto knife under flowing water will do the trick just fine. Once the pitch is hard enough, we need to cut grooves on the surface to increase the flow of cerium. The most commonly used material is pitch. Therefore, pressure no longer influences the speed of polishing.ĭuring polishing we are trading the ceramic tile tool for a softer material capable of holding the cerium onto its surface. Instead of creating fractures in the glass, the cerium oxide will act like shears to remove peaks on the surface. The strokes stay the same but the difference happens at a microscopic scale. The goal here is to remove the rough surface left during fine grinding and obtain a smooth reflective surface.ĭuring this step we’ll switch from mechanical grinding (silicon carbide and aluminium oxide) to chemical grinding (cerium oxide). When the abrasive action becomes too small to be observable with the naked eye or with the help of a magnifier, you should consider moving to the third step: polishing. Once you reach the finer grains (at this point you might be using aluminium oxide instead of silicon carbide), the surface should start to shine a little when viewed from the side. A common practice is to mark the bigger holes with a Sharpie and grind the surface until all of them are gone. Moving from one grain size to the next should only be done once all the holes from the previous grain have disappeared. To achieve that, we will work from coarser to finer abrasive. This step has only one purpose: removing the scratches and holes left by the rough grinding while keeping the shape of the surface. A tile not ground yetĪfter a while, when the final shape has been reached, it is time for the second step: fine grinding. This is done by changing from chordal to normal stroke (1/3 centre over centre). Once the desired sagitta has been reached, it is time to make the surface more spherical (rough grinding usually creates a slightly conical surface). On the other hand, for a fast scope, it can take you a lot more time. If you were shooting for a high focal ratio telescope, you won’t have to grind for too long. After a few hours using coarse abrasive, you will end up with a face approximately spherical. They consist of long chordal strokes with mirror on top. Nowadays, this tool is most of the time made of ceramic tiles glued to a plaster or plywood disk. In the old days, ATMs were using another glass blank of lesser quality. It should be about the same size and weight as the glass. To be effective, the abrasive needs to be rubbed against the glass with a lot of pressure. It is cheap and comes in various size (can range from #36 to #500). Telescope makers use silicon carbide powders (a.k.a. The theory behind grinding is that you can remove glass as long as you use a material with a higher hardness. F/D = 8) will be performing very well on planets and the moon. F/D = 4) well suited for deep sky observations. If you make the centre deep, you will end up with a fast telescope (e.g. It will give your mirror its overall focal ratio. This step is about making one side of the glass concave. Let’s break down the process into 5 steps. And this, only by pushing a piece of glass by hand. Starting from a rough glass blank, you can obtain an almost perfect parabola with surface errors in the range of 20 to 60 nm. In this post, we’ll talk about the first one which is the most time consuming but also the most interesting.Įven though some builders prefer to buy commercial mirrors and fit them in a DIY structure, I believe that making a telescope mirror is probably the most rewarding part of the whole built. Another surprising advantage is that hand made optics (if guidelines are followed) tend to have a higher surface quality than the mirrors polished by industrial machines.īuilding a reflector telescope can be decomposed in 2 steps: make the mirror and build the tube/mount. However, most of the amateur telescope makers (ATMs) who decide to build their own instrument do it because they want to understand the optical phenomenon involved in a reflector. And I’m not talking about the countless hours spent making it. Instead, if you consider getting a 10″ instrument or less, you’re very likely to spend more money building it than buying a commercial one. This question is not always about money saving. That’s a question that often comes to the mind of amateur astronomers.
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