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▪ CONTENTS ◄ 14.1. Early telescopes ▐ 14.3. Observatory telescopes ► 14.2. ATM TELESCOPES
Most ATM telescopes are of known designs, but their makers can customize them to their liking. That makes them sort of unique, and adds to the satisfaction of accomplishing, sometimes very challenging, creative projects. With more complex systems, it all starts with a design created and optimized through raytracing, as the basis for elements' fabrication. And some of ATM telescopes are truly unique systems, out of common knowledge of the amateurs and professionalsalike.
Marc Baril's 7" f/8 modified SiglerMaksutovCassegrainMaking MaksutovCassegrain of any kind is not for the faint of heart, primarily due to the full aperture Maksutov lens corrector, whose "easy" to make spherical surfaces have to remain within tight tolerances for the surface itself, the two surfaces relative to each other, and both relative to the meniscus thickness. A good example of it is MaksutovCassegrain made by Marc Baril, an amateur astronomer with professional background. Main modification to the Sigler's design was shortening the tube length which, according to Baril, did somewhat lower system's axial correction, but still allowed a highlevel performance. Here is raytrace of the prescription given in the article.
Design shows great field correction, with nearzero coma and astigmatism (it could, in fact, have 23 inches  or even more  smaller corrector separation without perceptibly affecting coma correction). Correction barely changes going from the field center to 0.5 degrees off axis. One thing to notice is that axial monochromatic correction of the starting design is a bit off (top left). It looks like undercorrection, but it is suboptimally balanced 6th/4th order spherical, with the lower order (undercorrection) being nearly twice larger than the (overcorrected) higher order. With the RMS wavefront error of 0.043, it is at the level of just a tad stronger than 1/7 wave PV of lower order spherical. Baril mentions having a slight undercorection when arriving at the testing stage, which he removed by working the first corrector radius with a petal lap. While he thinks that what it did was slight aspherising, even just a slight strengthening of the first radius  going from 10.3507" to 10.344"  would do the trick. After taking care of undercorrection, we come to a design with 0.982 Strehl in the eline, 0.927 in the F, and 0.926 in C. The polychromatic (430670nm photopic) Strehl is 0.964. If we minimize the chromatism by bringing the colors tightly together strengthening corrector radii to 10.05" and 10.481", the two lines go to 0.972 and 0.976, respectively, with the eline at 0.98. The polychromatic Strehl goes to 0.977. The Strehl figures are actually somewhat improved due to the 38% central obstruction, which took out the central deformation of the wavefront caused by spherical aberration. But the central obstruction did, of course, more damage than good at the level of system's PSF. Still, it's negative effect is commonly exaggerated. Baril states in his article that the biggest disadvantage of this design is its large central obstruction. How big a disadvantage it really is? The fact is that it reduces the wavefront error in all wavelengths  the larger wavefront error, the more so. If we would take it out, these same optical surfaces would produce wavefronts of lower quality. The central line, for instance, would go from 0.183 wave RMS and 0.982 Strehl, to 0.0225 wave RMS and 0.972 Strehl. At the same time, obstruction causes 0.86 drop in the relative central intensity of the diffraction pattern, with the second 0.86 reduction factor for the energy contained in central maxima  0.73 reduction in total  coming from the reduced central maxima size. Due to the size reduction, central maxima is not only smaller, allowing higher limiting resolution, but also significantly brighter than what 0.73 degradation factor implies. In this case, central maxima is only 7% less bright than in a perfect aperture, in this respect corresponding to 0.93 degradation factor. So, while the nominal degradation factor is 0.74 (rounded from 0.73*0.982/0.972), it is also accompanied by 13% higher limiting resolution, and higher contrast transfer in the high frequency range. MTF compilation below illustrates the magnitude of both, loses and gains.
Here we see that the worst part of the central obstruction effect is lowering contrast transfer to the level of 33% smaller aperture in approximately 00.3 range of frequencies. In the 0.71 range the transfer increases to the level of 16% larger aperture, while over the 0.30.7 range we have transition from lower to higher transfer, extending all the way to the resolution limit of 16% larger aperture. Overall, nominal view favors obstructed aperture, which has contrast transfer advantage in nearly as much of the frequency range as the unobstructed, with extra high frequencies added beyond the resolution limit of unobstructed aperture. However, to better understand the implications, MTF frequencies should be associated with magnification. With the limiting resolution at cutoff frequency λ/D in radians (times 206,265 in arc seconds), magnification at which a general detail is magnified to 5 arc minutes (likely more near threshold of detection), needed for the average eye to recognize its shape is 300D/206,265λ, or 2.64D for the aperture diameter D in mm and λ=0.00055mm (67D for D in inches). For details of Airy disc size, 2.44λ/D, corresponding to 0.41 frequency, it is 1.1D, and half as much for twice larger detail (0.2 frequency). So we could say that high magnifications start, quite approximately, at 0.3 frequency. If so, the bad part of central obstruction effect falls to the range of mid and low magnifications, while the good part covers most of the high magnification range. That said, should be noted that the largest contrast drop for this obstruction, some 25%, is at about 0.4 frequency  somewhat deeper into the high magnification range  while diminishes toward the midtolow magnification range (<0.3 frequencies), down to 12% at 0.2, and 5% at 0.1 frequency. There are direct implications of different contrast levels on the limiting resolution. Taking approximate minimum contrast requirements for different object types from Rutten/Venrooij, we get that the obstructed aperture will have appreciably higher resolution limit not only for the conventional MTF limit (two bright lines, nearly identical to that for two near equally bright stars), but also for bright contrasty objects, like the surface of Moon (1 and 2, respectively). On the other hand, unobstructed aperture has higher resolution limit for bright lowcontrast objects (3) and dim lowcontrast objects (4). Obviously, we can't compare apples and oranges, so whether the effect of central obstruction is good, bad, or insignificant, will depend on the specific use. In all, the effect of central obstruction is a mixed bag, and certainly is not only bad. For photography, considerations are somewhat different, but the effect is generally considered less of a factor than for visual use. Summing it up, while it is not possible to execute the actual optics to its theoretical optimum, it is not even necessary. No mortal soul can see the difference of one or two 1/100s in the Strehl ratio. From Baril's article, it is very likely that his commitment to making this telescope as close to its design marks as he could, did bring it to the level where the difference vs. theoretical optimum didn't matter.
Sure, performance level would have been even better with a system with
slower primary, such as the original Aries' Maksutov Cassegrain, without
significant difference in the tube length. But if one wants a faster
system, there is a price to pay. Clyde Bone Jr. 30inch f/5 MersenneIn an S&T article from 1999 Clyde Bone (San Angelo, Texas) describes his 30inch f/5 Mersennetype telescope, for which he used Tele Vue's 140mm f/5 Petzval apo at the eye end (there is also an article on the Tele Vue site). The telescope was built with the highest standards, using, for instance, astroSital mirrors with 96% reflectivity  1/16 wave primary, 1/10 wave secondary  and Pyrex flats with dielectric coatings (98.5% reflectivity). The two mirrors are confocal paraboloids, producing collimated beams free of aberrations (except Petzval image curvature, which is here 836mm). An imageforming system is required and, if the completed system is to be a visual telescope, it has to consist of an objective and eyepiece  i.e. telescope. Image below shows raytrace of such a system. Since no information on the Tele Vue refractor was published, a Petzval system that should be similar to it in its optical output is used to illustrate the combined performance level. It is a borderline "true apo" based on the errors in the four unoptimized wavelengths (F and C balance out at about 0.80 Strehl, and only the violet gline is little over the required 1/2 wave PV), but not according to its 0.917 polychromatic Strehl (photopic). This is mainly caused by keeping the violet gline error nearly as low as possible, causing the blue and red  particularly the former  to be suboptimally corrected. If the Fline error is minimized, its Strehl increases to 0.856 (w/0.898 Cline), gline drops to 0.183, but the polyStrehl is up to 0.945 (all needed to accomplish this is to change R5 i.e. 9 in this prescriptionto 520mm, and R8 i.e. 11 to 237.7mm; that produces f/5.15 system, rescaling it to f/5 would lower the polyStrehl to 0.934). The actual secondary is six inch in diameter, but in order to avoid stopping down the aperture, the reflected beam width has to be no more than 140mm (here, it is slightly less, which produces an effective f/5.1 system). Note that refractor alone is evaluated at f/5, and the setup used only stop at a distance of the Mersenne system exit pupil (located at k+k^{2} from the secondary, toward primary's focus, in units of primary's focal length, with k being the marginal ray height of the axial pencil at the secondary, in units of aperture radius). Prescription with the complete system given with the image is a compilation of two prescriptions (OSLO Edu has 10surface limit). This means that the nearly flat field here in the actual telescope corresponds (approximately) to the Petzval image surface, somewhat lessened by astigmatism induced by lens objective processing diverging pencils from the secondary (shown on astigmatism plots). Also, the 0.5° field radius in a 700mm f.l. refractor corresponds to 0.09° in the 3889mm f.l. system (will be explained ahead).
The field is limited to 0.5° i.e. 0.09° radius, since offaxis aberrations are already significant. It is a consequence of the widely separated aperture stop (also, as the box right from prescription shows, vignetting at the objective lens becomes significant). Part of it is that the incident ray angle is magnified after reflection from the secondary. With infinite secondary magnification, incident angle is magnified by a factor 1/k; with k=0.18 here, the incident angle magnification is 5.56, i.e. 1° angle at the primary becomes 5.56° at the lens objective. Hence, 0.5° at the lens objective is 0.09° in the actual system (linear field height is identical in both, since the two respective focal lengths differ by the same ratio). The central obstruction is 20% linearly, or little more. Since its effect on offaxis aberrations here is negligible, it is omitted to keep interferogram patterns intact. Also, to show the original polychromatic Strehl of the refractor, which would be superficially increased in the presence of obstruction (specifically, it would increase the polyStrehl to 0.934  and to over 0.95 with the Fline error minimized  as a result of small wavefront error decreases in all wavelengths  the larger error, the more  hence only in part reflecting better chromatic correction; on the other hand, that small gain is paid for with a 0.96 drop in central intensity  to the actual 0.897  plus another 0.96 ratio energy loss due to the smaller central maxima, for the final 0.87 central maxima energy ratio on axis). Wavefont map for the 0.5° point shows that most of the PV error comes from raised edge on one side. Because of that, both PV and RMS error are somewhat misleading, implying larger than the actual error. The main wavefront component is in the blue area, less than 1/3 of the entire 8 wave PV error. Zernike analysis gives as dominant terms primary coma (#6), astigmatism (#4) and defocus (#3), followed by secondary astigmatism (#11), coma (#13), primary spherical aberration (#8) and trefoil (#9). However, the primary spherical aberration term indicates 0.32 wave RMS, much more than what is present on axis; this means that most of this term is actually lateral spherical aberration, a secondary Schwartzschild aberration of the same form as primary spherical, but increasing with the square of field height. Also, raytrace shows that comalike ray spot plot grows significantly faster than in proportion to field radius, indicating presence of other "odd" secondary aberrations, such as lateral coma (secondary Schwatzschild aberration, of the same form as primary coma, but increasing with the cube of field radius), and probably some others. Diffraction simulation for the 0.5° point (eline) shows some elongation, but still well formed central maxima, embeded in a tight, bright ring (mostly due to 0.01 normalization); however, 80% energy radius is 0.0286mm (large black circle), much larger than the Airy disc (not sure how adequate is this number, since it could be based on PSF centroid location, which is here by some four Airy disc diameters  or 0.025mm  from the PSF peak, indicated by the dense ray area above the Airy disc circle on the ray spot plot). The bright central oval is similar in area to the Airy disc, but the peak intensity is only 0.40, indicating energy comparable to that with 1/2 wave PV of spherical aberration (note that the peak intensity is much higher than central intensity, due to intensity shift from the PSF center). Box bottom right shows corresponding ray spot plots for the refractor alone, for the same 0.5° radius field; evidently, all off axis aberrations are a consequence of the large stop separation. There is a discrepancy between the 0.5° wavefront error magnitude and the corresponding PSF. Even if limited to the blue area on the wavefront map, it is as much as 23 waves PV, not agreeing with still quite compact central maxima. Taking a closer look, we see that OSLO default wavefront map is for "minimum RMS OPD" (image below, left).
The corresponding interferogram shows that most of the blue area is tilted, with a small depression going into the raised upper edge. With the fringetofringe separation being 1/2 wave OPD (optical path difference), the tilt of the blue area is little over two waves. Since tilt itself doesn't degrade point image, merely moves it in the direction of tilt, all needed to find the wavefront corresponding to the best image is to center it on that point (the raised edge adds another 56 waves, but it cannot be optimized tiltwise simultaneously with the remaining, larger portion of the wavefront; its energy is, therefore, widely spread, contributing very little to the central diffraction maxima). The second OSLO option, "best focus" displays wavefront that combines tilt with a significant defocus over the blue area (middle). It actually has lower both, PV and RMS error, with only a slight difference in the vertical shift (Y; the defocus shift along Z axis is not shown). In order to correct for the tilt alone, the third option, "reference ray intersection" needs to be used. Entering 0.002 Yshift to the default coordinates, brings out the wavefront shape over the blue area for the best image proximity (right). It shows that most of this area is a lowaberration region, outlining a twocircleintersection shape, oriented horizontally (the nominally higher overall PV and RMS are the consequence of a larger error in the raised area, which is of little importance since it was already mostly wasted with respect to its energy contribution). This explains both, vertical elongation of the PSF central maxima and its integrity. Unfortunately, OSLO Edu has no option to calibrate PSF output with other than default data (it produces the best diffraction image decentered, in the process of generating PSF simulation for a wider area). However, entering 0.044mm defocus produces interferogram similar to that on the left, only with somewhat less of tilt in the blue area. For this point, PSF peaks at 0.52, suggesting it is the best focus (image below). Note that the actual PSF peaks are lower by 0.96 factor, or little more, due to the effect of central obstruction.
Note that even with OSLO making vertical shift adjustment, it is not the wavefront centered on the PSF peak, since it was looking for the "minimum RMS" error, and found it at an image point with still significant tilt error in the main, lower portion of the wavefront. If we manually enter 0.0002 shift  which should be equivalent of +0.0078 shift with respect to the default  we see interferogram nearly corrected for tilt and other aberrations (except a roughly 1/4 wave zone) in an area somewhat larger than with the "minimum RMS" focus corrected for tilt, which should be the reason for the higher PSF peak. But the PV and RMS wavefront error are again larger for this point. This illustrates why both, nominal PV and RMS wavefront error become less reliable indicators at large error levels. The "best focus" option is supposed to find what it promises, but it evidently doesn't always work as planned. Contrast transfer plots below show the defocused image at 0.5° as having appreciable higher contrast transfer than the "minimum RMS" focus. Note that these plots do include the effect of 20% linear central obstruction, omitted above.
The contrast advantage results in a significantly better limiting resolution of the defocused "minimum RMS" image for bright lowcontrast (BLC) details. For comparison, dashed plot shows contrast transfer for 1/2 wave of defocus (unobstructed aperture). Tolerance to miscollimation between the mirror and telescope axis is comfortable: 5mm decenter will lower the central eline Strehl to just below 0.95, while 0.5° tilt at the objective  implying 4.4mm axistoaxis deviation 0.5m behind  lowers the Strehl to 0.96. However, 1° tilt already induces over 0.4 wave PV of allfield (even magnitude across entire image) astigmatism, with the Strehl dropping to 0.73. Miscollimated secondary would induce combination of tilt and decenter. Considering large separation, it is the most sensitive part of the system. As little as 0.1° of tilt would translate to 5.5mm decenter combined with 0.2° tilt.
Replacing the above Petzval refractor with two others (FPL53/ZKN7 and
FPL53/BK7; the former has 0.999 central line Strehl, similar error magnitude in the gline,
but tighter rest of them, resulting in 0.954 polychromatic Strehl,
while the latter has the central line Strehl already below 0.95,
due to more of higher order spherical residual), all with different
limits to spherical aberration correction
show that off axis aberrations in this setup mainly result from
uncorrected spherical aberration in the refractor: the more of it, the
larger off axis aberrations induced (uncorrected off axis aberrations also are a
factor when present). Since in this demonstration the refractor limit is at
0.018 wave RMS, or less than 1/16 wave of primary spherical, the actual units
would produce more of off axis aberrations, and significantly so. For instance,
the FPL53/ZKN7 combination has only 2.4 wave PV wavefront error of
the same form at 0.5°, and we are talking 1/51 vs. 1/16 wave PV
of primary spherical aberration (the FPL53/BK7 combination, with 1/8 wave
PV of primary spherical equivalent, has as many as 22.3 waves PV
at 0.5°). Considering
that this field size (0.18° diameter in the actual telescope) is
already within its mediumhigh magnification range, there is no room to absorb further field
definition worsening. It is wise not to push too far with the
objective speed: all else equal,
an f/6, or f/6.5 could do noticeably better in this respect than f/5. Terry Platt's 318mm f/21 Buchroeder "QuadSchiefspiegler"Described in detail in Tonkin's "Amateur Telescope Making", this tiltedmirror system stems directly from Kutter's Schiefspiegler, with the third mirror added to cancel out astigmatism once the primary and secondary are arranged to cancel out coma (in the book it is called "quadschiefspiegler" but since reflecting flat does not influence system output, it is a threeschiefspiegler). Reflecting flat is added in order to create two alternative focus locations, as indicated below. However, since it doesn't influence raytracing, and the telescope is fully operational without it, it was omitted from consideration. Primary mirror is aspherized (0.55 conic) in order to minimize axial aberration, with spherical secondary and tertiary. Field angle is chosen to create image that fills out 1.25inch barrel.
Astigmatism plot reveals significant image tilt. At the field edge, defocus due to the tilt is over 2mm, or over 1 wave PV. However, what matters is how much of accommodation it will require in the image formed by the eyepiece. Axial correction is much better than in the Schiefspiegler (considering three times larger aperture), with the design limit better than 0.95 Strehl. According to Zernike wavefront decomposition, nearly 90% of the axial aberration is trefoil, with some residual coma. Longitudinal astigmatism of 0.85mm at the top edge (somewhat less at the bottom) implies 0.44 wave PV wavefront error of astigmatism, which makes it dominant aberration in the outer field, with small contributions from trefoil and coma. Field aberration is not radially symmetrical, but it is relatively insignificant since at the very edge of the field central diffraction disc remains well defined. The maker of this telescope states that the image tilt  which is over 9°  does not significantly affect observing. It shouldn't be a surprise, considering the slow fratio, but it can be informative to assess it specifically with raytracing. Using 30mm f.l. 2+1 Koenig design, ray spot plots are shown for a perfect telescope (bottom left) and the actual telescope (bottom right). Note that the field angle is too large for this particular eyepiece design, requiring lenses larger than 1.25 inch barrel, and resulting in a sharp increase in the field edge aberrations (the corresponding undistorted field of view is over 53 degrees which, with 15% positive distortions produces 61° AFOV; a bit too much for a 50° AFOV design). Except the perifferal field, telescope aberrations dominate, which was to expect with this slow fratio. The effect of image tilt only shows toward field edge, increasing eyepiece aberrations on the bottom, and decreasing them on the top (the two field sides should have edge astigmatism at the level of the eyepiece in a perfect system). Accommodation required for the field edge is less than 1.5mm which, for 30mm f.l. eyepiece i.e. diopter equaling 0.9mm, translates to nearly 1.5 diopter of accommodation (infinity to 0.67m), positive on the field top, negative on the bottom. Without excessive edge astigmatism of the eyepiece, accommodation required would have been yet smaller.
In all raytrace confirms that the telescope, as a design, offers
well corrected field despite nominally significant image tilt.
That, however, assumes well collimated system. The 0.80 axial Strehl
tolerance for tilt is 0.025° (less than 1/14mm deviation
at the edge) for the primary, 0.05° for the
secondary, and 0.035° for the tertiary (for the aberration
induced by tilt added to the existing design minimum). Since this is
with the "tilt and bend" choice "on"  meaning that change in tilt
of the primary automatically adjusts axial position of the
secondary and tertiary (and change in the secondary's tilt adjusts
position of the tertiary)  the actual tilt tolerances should be
tighter. On the other hand, secondary and tertiary
are relatively insensitive to decenter (up to a few mm still leaves
the Strehl well above 0.80). Bratislav Churchich's 146mm f/3.9 Wright cameraMuch less well known than its older cousin Schmidt camera, the Wright camera is not as well corrected, but has features that can make it preferred choice for some applications. In relatively slow systems, correction difference becomes less important, and its better compactness and inherently flat field can overcome the disadvantage of having to make an oblate ellipsoid for the primary and twice stronger Schmidttype corrector. The latter apparently wasn't an obstacle for Churchich, who in order to make the camera more compact decided to go with even more strongly aspherized primary (nearly 1.5 conic), which also required as much stronger Schmidt corrector. As he describes in the same Tonkin's book, he stopped a bit short of the optimum primary conic (1.5) preferring to keep an exceptionally smooth figure. All this makes his Wright camera different from the standard. The corrector was made with 70.7% neutral zone, being the easiest shape to fabricate, but it also has the lowest spherochromatism (86.6% neutral zone has the smallest blur, but over 2.5 times larger wavefront error). Table below describes the perfect Schmidt profile needed for this instrument. Any deviation from it generates wavefront error by a 0.517 ratio (eline), since the light speed in the BK7 glass vs. air is 1/1.517:1.
It shows that its maximum depth is less than 10 waves of 550nm light, i.e. that corrects mirror wavefront error 1.517 times smaller, around 6 waves PV. The diagonal mirror is 55mm in diameter, taken as the size of central obstruction. Raytrace of this system below assumes perfect surfaces.
Due to the relatively slow focal ratio, chromatic correction is excellent, comfortably within the "true apo" requirements. The ray spot plots show residual coma (about 0.80 Strehl large at 1° off), but it is low enough not to significantly alter the shape of diffraction image (for fully canceling coma, either primary conic of 1.5, or corrector separation of 460mm are needed). The diagonal is not quite large enough to fully illuminate 2° field (it is raytraced w/o offset, but in the actual telescope is offsetted to be centered under the focuser). At the field edge, vignetting is about 10% (taken as that on the horizontal ends of the diagonal, the 3rd from left), which is within acceptable. Note that 711 out of 917 rays is what remains after central obscuration.
Since the main purpose of the instrument is imaging, what really
matters is size of the circle of energy. In this case, the camera
places 80% of energy into 8micron circle on axis, 10micron
circle at 0.7° and 14.8micron circle at 1°. Angularly,
it corresponds to 2.9, 3.6 and 5.4 arc seconds. The actual star
image, as the diffraction images implies, is a bit larger, some
17 microns 1° off (of course, before seeingcaused bloating). The
Airy disc for an f/3.9 system is 5.2 microns in green light (546nm)
but these numbers are for the five wavelength from 436nm to 706nm
with even sensitivity. Since neither photo emulsion nor CCD have
flat sensitivity in that range, the actual circle sizes are somewhat
smaller. In this case, they were smaller than the typical photographic
blur, despite the unplanned, not negligible spherical aberration
residual. If the system was built as the standard Wright, with
a weaker primary and corrector, the 80% energy circle 1° off
axis would have been 13 microns, not much smaller than 14.8
microns in Churchich's customized camera (if optimized by
going for 1.5 primary conic, the circle would measure 13.6 microns). Dan Cassaro's 11.4inch f/4.3 SlevogtSchmidt cameraAlthough still in making, this ATM camera posted at the CloudyNights forum is both, interesting and challenging. In order to obtain flat field with the Schmidt corrector and two spherical mirrors, corrector needs to be widely separated from the mirrors. The alternative is the WriteSchmidt, less than half as long, but with significantly more field astigmatism, and requiring a hard to make oblate ellipsoid as the main mirror. Its advantage is significantly smaller central obstruction, but if field quality is main concern, SlevogtSchmidt is the way to go.
Design data posted at the forum describe a 11.4inch f/4.33
SlevogtSchmidt with f/2.294 spherical primary. It implies
secondary magnification of 1.89 which, for given back focus
distance, supplies necessary data to calculate the rest of
needed parameters: mirror separation, i.e. hight of the
marginal ray of the axial beam at the secondary, secondary
radius of curvature, corrector separation and aspheric
coefficients (mirror and corrector separations are already
given with the system, as well as secondary radius of curvature,
but the design usually starts with the primary, final fratio
and back focal length as the basis for calculating remaining
parameters). In general, in order to have flat field
SlevogtSchmidt needs to have secondary radius of curvature
about 0.96 of the primary's, and secondary magnification about
1.8. Denoting needed parameters as
Raytrace shows excellent color correction, with as litle as 0.26/0.06 PV/RMS wavefront error in the violet gline. Astigmatism is very low, reaching 0.22/0.05 PV/RMS error 1° off axis on flat field; central diffraction maxima is well defined all the way to the field edge. To fully cover the 2° field the secondary would have to be 6.48 inch in diameter, or 57% of the aperture diameter clear area only. Somewhat smaller secondary would be acceptable: shown is vignetting with the secondary clear area 6 inch in diameter: it is still only 6.1% at 1° off axis (circular red area is vignetting by the central obstruction). Considering that the reduced central obstruction would recover nearly 5% of light over entire field, including edge, field illumination actually improved with smaller secondary. Both, encircled energy plot and MTF show nearnegligible drop to the very edge of the field (note that polychromatic refers to the five wavelengths plotted, for even sensitivity; it indicates that the actual CCD performance is little better). While given prescription indicates neutral zone near 0.707 radius height, which generates over two times smaller wavefront error than neutral zone at the 0.866 height, raytrace show that the latter would have actually a slightly smaller 80% encircled energy radius. This is due to the effect of large central obstruction, reshaping the wavefront to practically cancel much larger error. Still, the 0.707 neutral zone corrector is significantly easier to make: its maximum depth at the neutral zone is 2.25 times smaller. Note that the actual corrector here is with split profile, i.e. each side gets half of it, but the effect is the same as with a single side profile. Table below shows surface profile as the sag values at each 1/16 inch of aperture radius. At right is exaggerated profile with a quarter inch scale.
Would the image quality be better going with ρ=0.96 and m=1.8? It would result in somewhat smaller secondary, but the back f.l. would be reduced to 5 inch, with the RMS WFE 1° off axis slightly higher. Extending back f.l. to 7.2 inches by placing secondary closer to primary would change the value of k and make best image surface slightly convex toward mirrors (R~2500), only slightly worsening the RMS WFE 1° off axis (0.055 vs. 0.054). Extending back f.l. by strengthening the secondary radius would make best image surface concave toward mirrors (R=166), with the error 1° off axis cut in half on the best surface, but increased to 0.15 wave RMS flat. Hence, when significantly changing back f.l. determined by m~1.8 and ρ~0.96, using equality m=ρ/(ρk), balk of the change in secondary magnification should come from the change (increase for extending back f.l.) in k, with the change in ρ only needed to offset the induced field curvature. In other words, to make β=(m+1)k1 by 46% longer (7.3 vs. 5 inch), most of the increase is to be contributed by a change in k. Looking at the astigmatism aberration coefficient for flat astigmatic field and using starting values ρ~0.96, m~1.8, a simple relationship approximating the needed change in ρ to compensate for the change in k is ρ'=0.990.07k'. In this particular case, k changes from 0.425 to 0.444, and ρ from 0.96 to 0.956 (minor adjustments to the aspheric coefficients and radius term are also needed).
The above raytrace is design limit; the actual output will be
negatively affected by alignment errors and thermal imbalances.
Tolerance for the corectortoprimary separation is fairly
loose, with the error due to coma at 1° off axis increasing
from 0.05 to ~0.07 wave RMS with about an inch off (little more
away from, little less than inch toward primary). Error of 0.1" in
mirror separation induces ~0.2 wave PV of spherical aberration.
Secondary decenter of 0.1mm up induces over one wave PV of mainly
positive allfield coma, with that error more than doubling at 1° off
due to the image tilt. Secondary tilt of 0.1° counterclockwise
(top away from the primary) induces nearly
one wave PV of negative allfield coma, along with somewhat more
than half as much of image tilt as the decenter. Since the secondary is spherical,
there is an offsetting combination of tilt and decenter; in this case,
offsetting tilt for the 0.1mm decenter is 0.116°, but the error
1° off axis would double due to the residual image tilt.
◄ 14.1. Early telescopes ▐ 14.3. Observatory telescopes ►
