Round or rectangular mirror for the secondary mirror

[legswilly]

Could I use a round or a rectangular or square mirror for the secondary mirror in a reflector? If not, why not?


Thank you for replying
Werner

[multiweb]

Any corner in the light path will create diffraction patterns and light scattering. I would not recommend it if your mirror doesn't fit in the shadow of the round holder.

[mental4astro]

Werner,

As Marc has said, the sharp straight corners of a square or rectangular shape in a telescope's light path will create a diffraction pattern, and genrate additional scattering of light which reduces contrast.

The other thing about telescope mirrors is they are "first surface mirrors". In other words they are reflective from the first surface the light touches. Everyday mirrors, such as your shaving mirror, are second surface mirrors as the main reflection comes from the silvered surface below the glass. These mirrors are no good for telescopes. Not only because of the glass that the light goes through, but they are not smooth/flat enough as needed for telescopes. Flat mirrors for telescopes are much more expensive than ordinary everyday mirrors because the manufacturing techniques used are much more expensive.

The ellipse shape of these mirrors is both to reduce diffraction patterns, but the shape is geometrically optimised for the optical path. When you look down into a Newt the secondary mirror looks perfectly round.

Alex.

[legswilly]

If the secondary mirror is larger then the refection of the primary, then there would not be a corner.

The reason I am trying to find out is that I would like to make a mirror for a autocollimation test stand, the mirror would be a perfectly flat one with a hole in the centre.

To make the mirrors flat, I need 3 pieces of glass. If I am successful and I will try to be, I would have 3 flats and could use one for the autocollimation rig. That leaves me with 2 flats. To cut the oval at a 45deg angle would be more difficult again. So I could use one for a secondary.
If one would get a secondary twice as large as required, would it matter other than the weight and the cost?

[multiweb]

You still have a sharp angled obstruction in your circular scope aperture so that statement is incorrect. The reflection from the primary fitting within the secondary surface only means you have no vignetting.

I don't know if you can or how to grind square or rectangular surfaces flat. Someone else can chime in.

[legswilly]

[QUOTE=multiweb;1519244]You still have a sharp angled obstruction in your circular scope aperture so that statement is incorrect.



I did not think of that and you are right. I found some more information on this site https://skyandtelescope.org/wp-content/uploads/GS-Adler-Secondary.pdf.


To make a flat and to test it for flatness. It is similar to scraping a cast iron table flat. It needs 3 pieces and altenative scraping with prussian blue as marker. Testing for the glass flat is with an optical flat and monochrome light.