Wide angle compound telescope ?
Posted 06 October 2012 - 08:55 PM
Posted 06 October 2012 - 10:24 PM
Posted 07 October 2012 - 02:06 PM
Come on, Mikey- show us P-A-L-E-A-S-E-!!!
Posted 07 October 2012 - 08:25 PM
Posted 07 October 2012 - 10:23 PM
I put a 5" aperture refractor with 1 meter focal length viewing into the 4X Mersenne, after two fold flats. If you pull the second flat out of the path, the refractor is able to view the sky directly. Pushing the second flat back in front of the refractor multiplies the refractor focal length by 4, and reduces the FOV to 1/4 that of the refractor by itself (for the same eyepiece).
The Mersenne thus acts as a 4X Barlow. Over this small FOV, the aberrations are almost non-existent.
Just a thought for your consideration - seems like this is one possible answer to your OP.
Posted 08 October 2012 - 12:09 AM
What should be the relation between the diameter of the secondary parabola on the Mersenne amplifier, and the aperture on the refractor?
The naive answer is - make them equal. But then the secondary parabola would act as a diaphragm for the refractor, clipping the incoming cone of light when observing at a wide TFoV.
If the refractor's aperture is bigger, part of it is wasted. If it's smaller, then some incoming light from the Mersenne amplifier is wasted, effectively wasting some of the primary aperture.
There doesn't seem to be a simple answer.
I am not sure how to define aberrations for an afocal system, but I wonder if the Mersenne amplifier has some aberrations of its own. If the secondary parabola is negative (convex), would that compensate the aberrations (coma-like) introduced by the primary?
How do you define and analyze aberrations in an afocal system?
Posted 08 October 2012 - 12:49 AM
Posted 08 October 2012 - 02:20 PM
One thing that always bothered me about the Mersenne design (at least with a convex secondary) is that the virtual pupil image that is made by the secondary mirror is far from the refractor's objective lens. Ideally, one would want a real pupil image on the refractor's objective lens. Does having the pupil far from the refractor's objective lens make any significant difference in image quality?
You could made a Gregorian secondary, yielding a real pupil image, and put the refractor lens there. However, there are practical issues with the layout which makes that option difficult.
Posted 08 October 2012 - 02:57 PM
Here, the 4X exit pupil radius (6.25/4) = 1.5625 in. = 39.6875mm. The Mersenne exit pupil is 13.5" behind the secondary (toward the stars). The total propagation path from exit pupil to refractor is 13.5+48+13+6 = 80.5 in. = 2044.7mm. Using a wavelength of 0.55µm = 0.00055mm, the Fresnel number is thus
F = (39.6875^2)/(0.00055 x 2044.7) = 1400.6 (deep near field)
Thus the refractor is still well within the near field of the exit pupil, and almost no edge diffraction will be visible. So, at these pupil diameters and wavelengths there's no real effect of having a remote Mersenne pupil other than vignetting by the refractor. A Fresnel number of 1400 isn't even an issue for adaptive optics. If this was a 5 or 10mm diameter pupil system, or it was being used at 10.6µm, relaying the pupil image would be necessary.
Posted 08 October 2012 - 03:08 PM
A refractor that images the afocal light has to have a larger aperture than the on-axis beam if one expects to have any appreciable FOV. The extra refractor aperture is not wasted; it is used to intercept and focus off-axis light from the Mersenne. The trade in Mersenne design is magnification versus reimaging optics aperture. I used 4X in the drawing because it's a good trade value.
The focal length of the primary mirror is unimportant, only the amplification ratio. But, field curvature increases with decreasing primary focal ratio, as does difficulty in alignment. That's why I chose roughly f/6 for "wavefront"'s mirror - that's a relatively easy paraboloid to make, lots easier than say an f/4 or f/3.5.
Posted 09 October 2012 - 12:32 AM
The extra refractor aperture is not wasted; it is used to intercept and focus off-axis light from the Mersenne. The trade in Mersenne design is magnification versus reimaging optics aperture.
Okay, got it. You just design it with a refractor aperture large enough to get the off-axis light. Now that might be a problem with a very large primary - you'd have to either use a pretty large refractor, or ratchet up the amplification factor in the Mersenne system.
Now, the primary and the secondary parabolas - shouldn't they have the exact same focal ratio? Or is it the same principle - you need to design the secondary a bit larger to capture the off-axis stuff? If they have the same focal ratio, a 4x Mersenne would have a 25% central obstruction. In your diagram, it's a bit larger than that.
Posted 09 October 2012 - 07:30 AM
Posted 09 October 2012 - 10:04 AM
Posted 09 October 2012 - 09:12 PM
I guess I'm asking because I've no idea how to even define aberrations for an afocal system. Couple it with a known entity (like the objective of an apo) and see how the aberrations of the apo's objective change?
Very illuminating thread, thanks Mike!
Posted 10 October 2012 - 09:34 AM
Posted 22 November 2013 - 10:45 PM
Posted 22 November 2013 - 11:30 PM
At the small fields used here, the aberrations of the Mersenne are essentially zero.
One of the criteria mentioned by the OP was the he does "not like the narrow field types" and from what I am seeing here, it seems like this design results in a relatively narrow field of view, at least by my standards. 12.5 inch with a 1.6+ degree TFoV.
Just kind a wondering, interesting stuff.
Posted 25 November 2013 - 09:53 AM