this post was submitted on 02 Nov 2024
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Astrophotography

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Would this even work? (sh.itjust.works)
submitted 1 month ago* (last edited 3 weeks ago) by [email protected] to c/astrophotography
13 comments fedilink hide all child comments
 

Or would the tolerances needed in the hinged mirror make the whole thing unusable?

I was looking at modern "smart telescopes" recently and noticed some are sideways and wondered if that would be possible for a normal hobby Newtonian telescope.

Possible upsides:

  • no tripod needed for use
  • mirror is light so smaller motors can be used for movement

Possible downsides:

  • maybe mirror flatness?

EDIT/UPDATE: so i tryed it with a 75mm first surface mirror, it kinda worked, at least better than a normal mirror, but i wasnt able to get it as sharply focused as i like. I suspect the mirrior i use has micro ripples because its just 2mm thick and doesent look like its seen a polishen process....guess thats how far a budget of 25bucks gets ya

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[–] [email protected] 14 points 1 month ago (2 children)

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 :)

[–] Balthazar 5 points 1 month ago (2 children)

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.

[–] [email protected] 2 points 1 month ago (1 children)

entrance pupil isn’t well defined

oh right, i didnt thought about that, maybe i could add some sort of shroud to the light inlet

[–] Balthazar 2 points 1 month ago (1 children)

To be effective, it would have to be like a telescope tube, which is what you're trying to eliminate in the first place.

[–] [email protected] 1 points 1 month ago (1 children)

thats what i was thinking :/

Tubeless telescopes exist tho, i saw some big diy newtonians that did not have any

[–] Balthazar 4 points 1 month ago

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.

[–] [email protected] 2 points 1 month ago

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.

[–] [email protected] 2 points 1 month ago

I already ordered a first surface mirror to test, i will report my findings :D

[–] SzethFriendOfNimi 6 points 1 month ago

I don’t work with telescopes but I do know that hinges never fold perfectly along a plane.

There’s always a little twist/pivot which would probably be an issue with aligning the light in a setup like this.

[–] [email protected] 5 points 1 month ago (1 children)

Surface imperfections and play in the hinge aside, this would not work as you might imagine. This configuration (as illustrated), on a flat surface, would only completely image the zenith. Put it on a mount and it would image whatever is directly above the flat mirror. As soon as you move the hinged mirror, those light rays that eventually would lead to the camera sensor will only go flying into the tube walls. The light doesn't magically find its way to the parabolic mirror at any hinge position. A couple degrees around 45°, you would probably still get an image, but only at a 45° incline on the hinge would you get a complete image.

[–] [email protected] 1 points 1 month ago

If you rotate the hinge to angle X above the horizon, light coming in from an altitude angle of 2X (=zenith angle of 90deg-2X) will get reflected to into horizontal rays inside the tube.

So you don’t need a mount with adjustable altitude angle - the hinge accomplishes that.

[–] Plauditecives 3 points 1 month ago (1 children)

Telescopes with fixed optics and mobile mirrors have been done. Mostly used for solar observation but they are few exceptions.

Most notably This telescope from Paris’s 1900 universal Exhibit . Poor scientific instruments but great party trick with focal length of 57 METERS !

[–] [email protected] 2 points 1 month ago

Wow that quite the telescope! This is basically what i wanted to build just smaller and and with a Reflector scope instead of a Refactor scope, thank you very much for this informative comment :D