Is it possible to 3D print a mirror with todays available materials?
If so, would there be a reduction in image quality?
[Physics] Is it possible to 3D print a mirror to create a high quality telescope
astronomyopticsreflectiontelescopes
Related Solutions
It's a pretty complex topic. Bottom line: if the scope is doing okay, it's probably parabolic.
The one way to tell for sure, and even ascertain figuring errors and whatnot, is to do a proper optical test. If I was in that situation, I would just take the mirror out, put it on the Foucault / Ronchi tester, and take some measurements. A ronchigram would give a qualitative answer immediately: is it a parabola-like curve or not? Are there any gross errors or not? Then a quantitative Foucault test could actually provide some numbers.
You could google the Foucault test and the Ronchi test for telescope mirrors if you're curious.
With the mirror installed in the scope, it's a bit more complex. One thing is certain: at an f/5 focal ratio, if it's not parabolic, performance would be pretty terrible. Even if it is parabolic, this being cheap mass-produced optics, some issues may remain. Even assuming a very expensive, near-perfect parabola, at f/5 an aberration called coma will be pretty obvious (it's a defect of this geometry, still exists even with perfect optics).
One thing you could do is learn star testing. It's not trivial, but it could reveal various issues. Google this topic, it's pretty big. Some links to get you started:
http://legault.perso.sfr.fr/collim.html
http://www.astrosurf.com/altaz/startest_e.htm
One outcome of star testing is that you'll get your scope collimated really really well, which is something you should do anyway. Once that's done, start looking for spherical aberration. A good parabola will show only a residual amount of spherical aberration; a top-shelf perfect parabola will show zero spherical aberration. At f/5, a pure sphere will show horrible amounts of this aberration.
At first, you'll be liable to confuse spherical aberration with turned down edge (assuming your scope exhibits both). Also, the central obstruction (the edge of the secondary mirror) will distort the diffraction figure in ways that could be mistaken for aberration, if you're not very familiar with it. Keep experimenting and learning and you'll figure it out eventually.
Use a good quality, strong eyepiece for the star test. Keep the image in the exact center, to reduce extra issues with the eyepiece. Polaris is a good star for testing, since it doesn't move and it's bright enough. Don't do the test when seeing is bad and the diffraction figure is shaking like a bowl of jello.
The resolution of a regular photograph is limited by the size of the silver halide crystals in the emulsion, while the resolution of a computer image is limited by the number of pixels used to store it.
However, a typical telescope mirror has no structure bigger than the size of a grain boundary, and this is much smaller than the wavelength of optical light. In this respect there is no finite resolution comparable to a photograph or computer image.
The resolution of a telescope is limited by the size of the mirror rather than by any quality of the reflecting surface. The light reflected by the mirror is diffracted by the mirrors edges forming an Airy disk. This limits the angular resolution to approximately:
$$ \sin \theta \approx 1.22 \frac{\lambda}{d} $$
where $\lambda$ is the wavelength of light and $d$ is the mirror diameter.
Best Answer
The highest resolution 3d printers I know of are around 1600dpi, which is a resolution of about 15$\mu m$. Telescope mirrors have to be smooth to fractions of a wavelength of light, so the resolution of current printers is nowhere near good enough.
Whether 3D printers could one day be good enough is a different question, but given that the improvement in resolution required is at least a factor of 1,000 I think it's not likely because 3D printers are designed to address quick manufacture rather than precision manufacture. In any case, making mirrors is a well established procedure. The difficulty is making them large, and it's not obvious how 3D printers would help with this.