telescopeѲptics.net
▪
▪
▪
▪
▪▪▪▪
▪
▪
▪
▪
▪
▪
▪
▪
▪ CONTENTS
◄
10.2.2.2. Wright, Baker camera
▐
10.2.2.4. SchmidtCassegrain telescope
► 10.2.2.3. SchmidtNewton telescopeThe only difference in the optical layout of the SchmidtNewtonian vs. Schmidt camera is in the position of corrector lens. In the telescope, the corrector is typically positioned at a distance somewhat inside the focus of the spherical primary (FIG. 173). The reduced mirrortocorrector distance has practically no effect on the
FIGURE 173: SchmidtNewtonian telescope optical elements. Schmidt corrector cancels spherical aberration of the spherical primary. It also usually supports the diagonal mount, which eliminates spider diffraction effect. Due to displaced stop (normally coinciding with the corrector), coma, astigmatism and field curvature are all lower than in the comparable Newtonian with paraboloidal primary with the stop at the surface (while the benefit of lower astigmatism or field curvature can be achieved by manipulating stop position of a paraboloid as well, its coma error is independent of stop position). While the corrector is very forgiving to miscollimation, it does complicate adjusting the flat. needed corrector power to cancel spherical aberration of the mirror. Needed corrector shape is determined from Eq. 101 or Eq. 101.1 (higherorder aberration is negligible). However, as a consequence of the stop now being closer to the primary, portions of mirror's coma and astigmatism are reintroduced, which is evident on the ray spot plot. The PV wavefront error of lowerorder coma in the SchmidtNewtonian for object at infinity is given by: Wc = (1σ)αD/48F2 (110) with σ being the correctortoprimary separation in units of the primary radius of curvature (σ is numerically positive), α the field angle in radians, D the aperture diameter and F the focal ratio. With α=h/ƒ, h being the linear height in the image plane, and ƒ the mirror focal length, it can be also expressed as Wc=(1σ)h/48F3. It makes coma in the SchmidtNewtonian lower by a factor of (1σ) vs. paraboloidal Newtonian. The lowerorder astigmatism PV wavefront error is given by: Wa = (1σ)2Dα2/8F (111) or, alternatively, Wa=(1σ)2h2/8DF3. In other words, it is by a factor (1σ)2 lower than for a mirror with the stop at the surface. Change in astigmatism also changes the median (best) image surface. It is now given by:
with the sagittal surface
curvature radius 1/Rs=(2/R)[2(1σ)2/R],
and the tangential surface curvature given by With the usual value for σ of ~0.45, the SchmidtNewtonian coma is lower by a factor of ~0.55, astigmatism by a factor of ~0.3, and median field curvature by a factor of ~0.4 vs. comparable paraboloid with the stop at the surface. Of course, similar gains in the reduction of astigmatism and field curvature can be just as well obtained with a paraboloid, by moving the stop away from the surface. Elements alignment in the SchmidtNewtonian is somewhat more complicated than in the allreflecting arrangement. The two mirrors and the focuser have to be aligned as in allreflecting system, but all three also need to be aligned with the corrector. Decentered corrector will induce coma, as given by Eq. 109, while corrector tilt will induce tilted image surface. With the corrector commonly supporting diagonal mount, the limit to collimation accuracy is set by the accuracy of the corrector/diagonal alignment (unless correction is made at the focuser). On the other hand, better coma
correction of SchmidtNewtonian makes the tolerance for mirrors/focuser
misalignment nearly twice more forgiving than in a comparable
allreflecting Newtonian.
◄ 10.2.2.2. Wright, Baker camera ▐ 10.2.2.4. SchmidtCassegrain telescope ►
