this post was submitted on 02 Nov 2024
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Astrophotography
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Every interface (mirror or lens surface) adds error, and that error is multiplicative. The question is whether that error is worth the convenience in form factor, and that isn't something that can be easily answered. Sometimes you need to build it and use it :)
Flat mirrors are pretty easy to get right (no aspheres involved). Of course, you want a mirror reflecting from the front of the glass, not from the back like those you see every day.
I think your main problems are going to be practical: it now needs to be mounted differently, and it would be much harder to mount equatorial, so tracking is more difficult. You would probably have a problem with scattered light, as the entrance pupil isn't well defined, and there may be direct paths for light to get to the focal plane without going the route you've highlighted; that will make it much more difficult to do faint work, and I think it's the main problem you'll have.
Technically getting one surface flat is easy. Hell, it's one of the first thing you learn in measurement science (three plates and perfect smoothness). However a mirror isn't just about being flat, it is also about light reflection. And that makes it more interesting. In a perfect vacuum, you could do a silver mirror without the glass and have it be perfectly flat and not worry about oxidation. But the reality of making that mirror stay perfectly reflective means that glass or similar is usually involved. And then you move away from the perfect flatness problem (relatively easy) to perfectly parallel planes (significantly harder).
Furthermore, keeping a plane or surface perfectly.flat after manufacture requires uniform temperatures, which are rarely present in amateur telescopes.
The end result is almost always the introduction of additional error.
oh right, i didnt thought about that, maybe i could add some sort of shroud to the light inlet
To be effective, it would have to be like a telescope tube, which is what you're trying to eliminate in the first place.
thats what i was thinking :/
Tubeless telescopes exist tho, i saw some big diy newtonians that did not have any
There are good reasons to not use a tube: tubes limit airflow over the mirror, increasing "mirror seeing", and they add weight. But then you need an alternative way of rejecting off-axis light. One way of doing that is a dome or similar enclosure.
I already ordered a first surface mirror to test, i will report my findings :D