Hello all,
I have tested a few TeleVue Ethos eyepieces with a Zygo interferometer at a very fast f/2.8 light cone (tested with a f/1.9 transmission sphere, and aperture limited in software to 67.9%).
Three tested eyepieces have quite different amounts of spherical aberration: 21mm Ethos Z8 = 0.80 waves (undercorrected)
13mm Ethos Z8 = 0.33 waves (undercorrected)
8mm Ethos Z8 = -0.48 waves (overcorrected)
Now let's compare two cases:
Case 1: A f/2.8 paraboloid mirror is perfect, and the eyepiece has 1 wave of spherical aberration.
Case 2: A paraboloid mirror has 1 wave of spherical aberration, and the eyepiece is perfect.
When we make a star test with these two telescopes, will we see the same error in both cases? Or can we argue that the error in the eyepiece is less severe, because it occurs closer to the observer's eye?
The backgound of my question is that a friend of me has figured a 14" f/2.8 mirror, and we think the mirror is quite good, verified by two independant tests.
However when making the star test the mirror isn't as good as expected. We are trying to figure out what's going on.
know if the spherical aberration originates near the focal plane or near the exit pupil.The pupil of the eye will be the limiting aperture if the system is
designed for comfort, but it may or may not be in a test setup.
Let's assume that in a star test we use high magnification, so that the exit pupil is smaller than the eye pupil. No vignetting.
In the absence of vignetting, I think they'll be pretty similar since
both are basically occurring in the pupil (of the telescope). If the
eyepiece were optically closer to the image, the effect would be less,
because the phase errors would have mainly a local effect, and of course
right at the image, it would hardly do anything except change the FOV.
(It would be a field lens and not an eyepiece.)
That makes things complicated, because the Ethos eyepiece is a complex design and at least one (field-)lens is in front of the image plane. As far as I know, the optical layout hasn't yet been published. I tested the eyepiece as a black box and I don't
Some pictures of the 21mm Ethos test setup: https://www.facebook.com/astroelectronic/posts/428917360903364
Michael
The pupil of the eye will be the limiting aperture if the system is
designed for comfort, but it may or may not be in a test setup.
In the absence of vignetting, I think they'll be pretty similar since
both are basically occurring in the pupil (of the telescope). If the eyepiece were optically closer to the image, the effect would be less, because the phase errors would have mainly a local effect, and of course right at the image, it would hardly do anything except change the FOV.
(It would be a field lens and not an eyepiece.)
Well, you could do a Foucault knife-edge test. Not as easy as a star
test, of course, but you don't need additional optics.
Well, you could do a Foucault knife-edge test. Not as easy as a star
test, of course, but you don't need additional optics.
My friend tested the mirror with a Bath interferometer from the center of curvature. Spherical aberration was subtracted in software. I don't remember if he also made a Foucault test.
I tested the same mirror with a Zygo interferometer in autocollimation against a calibrated reference flat. Both results are in good agreement. We are quite sure that the mirror is good.
The problem is that in the star test some eyepieces show overcorrection and some show undercorrection.
Michael
Hadn't heard of a Bath interferometer before.
It's quite popular among amateur mirror makers. Many informations about the Bath interferometer can be found here:
https://groups.io/g/Interferometry/wiki/home
Michael
Hadn't heard of a Bath interferometer before.
Phil,
I'm not sure if I have understood all this correctly. Let's use another example:
We are star testing a lens with 10cm diameter and 100cm focal length.
Case 1: The lens has 10 waves of astigmatism (over the full 10cm aperture). The spot size in the best focal plane will be much larger than the Airy disk.
Case 2: The lens is perfect, but we put an astigmatic window exactly in the middle between the lens and the focal plane. The window introduces 10 waves of astigmatism (over 5cm aperture) into the light cone.
In both cases the light cone has 10 waves of astigmatism when it arrives at the focal plane.
But I think case 2 will have a smaller spot size than case 1. Do you agree?
Case 3: Let's assume the astigmatic window is placed at distance x in front of the perfect lens. In this case the spot size is independant of distance x, right?
Are there any general rules how to estimate the spot size, depending on the position along the light path where the aberration is introduced?
On 06/18/2018 11:57 AM, Michael Koch wrote:
Phil,
I'm not sure if I have understood all this correctly. Let's use another example:
We are star testing a lens with 10cm diameter and 100cm focal length.
Case 1: The lens has 10 waves of astigmatism (over the full 10cm aperture). The spot size in the best focal plane will be much larger than the Airy disk.
Case 2: The lens is perfect, but we put an astigmatic window exactly in the middle between the lens and the focal plane. The window introduces 10 waves of astigmatism (over 5cm aperture) into the light cone.
In both cases the light cone has 10 waves of astigmatism when it arrives at the focal plane.
But I think case 2 will have a smaller spot size than case 1. Do you agree?
Sure. If you put it right at the image, it wouldn't aberrate the spot
at all. The spatial frequency of the ripples corresponds to the angle
at which the aberrated component is spreading out, so the less distance
it has to propagate, the less the spot spreads out.
In this example, the aberrator isn't in the pupil, so the two cases
aren't equivalent.
Case 3: Let's assume the astigmatic window is placed at distance x in front of the perfect lens. In this case the spot size is independant of distance x, right?
Not exactly. If it's far enough away, some of the aberrated components
will get vignetted, so the spot will actually improve. And if it's far
from the pupil as it was in Case 2, the effects will be different.
Right at the lens it doesn't matter which side you put the aberrator, provided that both are sufficiently thin. (It would matter for, say, a
100x microscope objective, because the working distance is much smaller
than the focal length.)
Phil,as one wave of spherical aberration in the telescope mirror. One wave of astigmatism produced near the secondary in a Newton telescope gives the same result as one wave of astigmatism in the primary mirror.
I did make some raytracing simulations and came to the conclusion that it doesn't care on which surface an aberration is produced. The result in the focal plane is always the same. One wave of spherical aberration in the eyepiece gives the same result
Under the assumption that the Zernike polynomial is defined over the acutal diameter of the light cone at that surface.
http://www.astro-electronic.de/Zernike_in_Systems.pdf
Michael
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