For 4.5" Mirror, to see around ½ degree of view you need an oval flat secondary mirror from 22 x 33 mm to 25 x 38mm without champhering. It is recommended that this flat also should be of the order of 1/8 wave. Some experts say ¼ and even ½ wave may also do. Sometimes, people also put ordinary front coated flats by which performance may be degraded keeping in view certain other factors. It is very difficult to make a flat of 1/8 wave without testing facilities. Certified 1/8 wave, 1/4 or even 1/2 wave flat may cost in hundreds of rupees. If you get this cheaper without certificate then quality may be doubtful. Some makers claim of ¼ wave accuracy, but they will not certify it. They know there is no way for an ordinary person to test it. The process of making flat is that you have to take 3 round pieces - a, b and c and grind these alternatively over each other i.e. a over b, a over c, b over c and finally see which one is closer to a good flat. You have to polish that piece and get 1/4 or 1/8 wave flatness. Then you have also to make this as oval. This process is also a bit difficult, as you have to shape/grind it in such a manner that when tilted in 45 degrees, it should look perfect circular disc. It is, therefore, better to procure it. I have a test plate and upto 52mm diameter, it can test ¼ wave flats. If anyone so wish I can test his flat. For making diagonals/oval secondary flats see following sites : http://www.starlightoptical.com/diagonal.htm |
