Dominant off-axis aberration in the Newtonian is coma; astigmatism is low in comparison. Image surface deformation is field curvature, with distortion being zero with the stop at mirror surface. All three aberration are independent of mirror conic, which means that they are equally affecting both types of the Newtonian telescope, those with spherical and those with paraboloidal primary. Following are relations specifying the aberrations.

● lower-order coma, from Eq. 12-13 and 15.1, after substitutions, as the P-V and RMS wavefront error at diffraction focus for object at infinity is given as:

respectively, with α being the field angle (α=h/ƒ, h being the linear height in the image plane, and ƒ the mirror focal length). Note that this is double the error given by Eq. 12, which expresses only the peak aberration. The RMS wavefront error is related to the P-V error as:

In units of the λ=0.00055mm wavelength, the coma wavefront errors are, slightly rounded off:

for D and h in mm (or with the numerical factor 38 and 6.7 replaced by 965 and 170, respectively, for D in inches).

For the 0.80 Strehl ratio, ωλ=1/√180, which corresponds to the field angle α=F2/90D in radians, for D in mm (α=F2/2286D for D in inches), or the linear height in the image plane h=ƒα=F3/90. Actual Strehl ratio at this field point is somewhat higher than 0.80, due to the presence of astigmatism, which causes edges of the wavefront deviated by coma to fold back slightly, reducing both, P-V and RMS wavefront error. The combined RMS error at this field angle level is approximated by subtracting the astigmatism RMS from the coma RMS, or ω(C+A)~[1-(7h/D)]ωC. Substituting diffraction limited image height h from above, for coma alone, gives approximation for diffraction limited field height for combined coma and astigmatism as h~F3/[90-(7F3/D)], or h~1/[(90/F3)-(7/D)].

As Eq. 70 shows, coma of the concave mirror is independent of its conic. It, however, changes with the stop position and object distance, as described in 2.2. Coma (it also contains specifics on geometric, or ray aberration). In terms of object distance o and mirror radius of curvature R, the coma wavefront error changes in proportion to (o+R)/o, with R being, according to the sign convention, negative. Thus, mirror coma diminishes with the object distance, falling to zero for object at the center of curvature (o=-R).

What is usually seen of the coma in the eyepiece, with sufficiently bright stars, roughly corresponds to the dense 1/3 of its geometric blur (sagittal coma). Its angular size needs to be ~5 arc minutes to be clearly recognized by an average eye. Since the angular size of sagittal coma in the eyepiece (neglecting eyepiece distortion) is 3.75ε/F2, in arc minutes, with ε being the eyepiece apparent viewing semi-angle in degrees, the viewing angle in the eyepiece at which the geometric sagittal coma would become obvious to most people is given by ε~1.25F2. This means that one could see it at the edge of a 40° AFOV eyepiece at ƒ/4, regardless of its focal length.

 However, what we see in the eyepiece is not a ray spot, but an actual diffraction image deformed by the aberration. The above "geometric" angle at which sagittal coma becomes visible is noticeably reduced in conventional eyepieces, due to significant additional blurring caused by eyepiece astigmatism. Also, in order for the comatic deformation to be seen in the actual star image, this image has to be noticeably deformed by the aberration. For coma, that occurs as the aberration decidedly exceeds ~1 wave P-V on the wavefront (or, as the full size blur exceeds four times the Airy disc diameter, and the sagittal coma is nearly 1.5 times the Airy disc diameter). At this aberration level, and beyond, the actual visible blur of relatively bright stars roughly resembles the form and size of geometric sagittal coma. As the error diminishes bellow ~1 wave P-V, the effect gradually transforms into one-sided intensity distribution asymmetry, with the spurious disc still well defined. 

From Eq. 70, the field height at which the coma error becomes large enough (~1.25 wave P-V) to begin to resemble sagittal coma in the actual star image can be approximated by h~F3/30, in mm. For an ƒ/4 mirror, that corresponds to ~2.1mm (FIG. 73). For the magnification level needed to enlarge the actual comatic image of a bright star with this amount of coma to 5 arc minutes apparent size, the eyepiece focal length is approximately f~1.3F. Hence an average observer, with a 5mm eyepiece and an ƒ/4 mirror, can expect to notice coma on bright stars from ~2mm off-axis out. With an ƒ/6 mirror, it will take an 8mm eyepiece to show coma becoming apparent some 7mm off-axis (which would require 80+ degrees AFOV); higher magnification will reveal little of the characteristic comatic deformation inside 7mm off-axis, due to the deformation becoming less apparent with the decreasing wavefront error. Compromised image sharpness, however, will be noticeable well within the 7mm-radius field circle.

Due to eyepiece astigmatism, which increases with the square of off-axis distance, comatic-like deformation in the eyepiece grows rapidly toward the field edge. Eyepiece astigmatism increases with the square of the focal ratio; combining it with the coma wavefront error increasing with the cube of the focal ratio, it becomes understandable why off-axis performance of the paraboloidal mirror drops so quickly with the increase in relative aperture beyond ~ƒ/6. Field curvature, if present to a significant degree, has similar effect, although it is unusual for it to be strong enough to affect visual observing. 

● lower-order astigmatism, from Eq. 18, after substitutions (for θ=0), as the P-V and RMS wavefront error at best focus, for object at infinity, are:

with ƒ being the mirror focal length.

In units of the l=0.00055mm wavelength, the errors are:

for D in mm (or with the numerical factor 227 replaced by 5766 for D in inches).

The field angle at which astigmatism reaches 0.80 Strehl is α=√F/622D in radians, or at the off-axis height in the image plane of h=αƒ. It has no significance, with the coma error being absolutely dominant at this field angle. The two aberrations' RMS wavefront error equalizes for the field angle α=1/6F in radians, or for the h=D/6 height in the image plane, after which the astigmatism quickly becomes the dominant aberration.

The wavefront error doesn't change with object distance. However, it does change with the stop position. More details on this, as well as on the geometric (ray) aberration in 2.3. Astigmatism.

● field curvature: Petzval curvature of a concave mirror is Rp=R/2, R being the mirror radius. Due to the presence of astigmatism, actual best image curvature is different from the Petzval. Also, it varies with the stop position. For the stop at the surface, best, or "median" image surface equals the negative of mirror's focal length:

which makes it positive (convex toward mirror) for mirror oriented to the left. Position of the aperture stop influences mirror astigmatism, which in turn causes changes in the median image curvature.

Miscollimation sensitivity of a Newtonian is determined by the diameter and F-number of the primary mirror. Sources of miscollimation are: (1) primary tilt/decenter, (2) flat tilt/decenter/despace, (3) focuser tilt/decenter and (4) tube/structural flow resulting in any of the former. As a result, axial image point is shifted away from the field center, and replaced with certain amount of off-axis coma.

Expressing the linear center field shift as 2τƒ/(60x57.3), for τ the mirror tilt angle in arc minutes and ƒ the focal length, and substituting it for h in Eq. 70.1 (right), gives the RMS wavefront error of coma shifted to the field center (in units of 0.00055mm wavelength) as:

for the aperture diameter D in mm (for D in inches, ωt = τD/10F2).

Assigning H to the diagonal-to-focus separation, and H' to the focus height in the focuser (measured from the focuser base), the miscollimation sensitivity per arc minute of tilt is smaller by a factor H/ƒ for the diagonal, and by a factor of H'/2ƒ for the focuser, respectively, with ƒ being the mirror focal length.

Sensitivity to decenter - also in units of 550nm wavelength - is identical for all three, mirror, flat and focuser, and given by:

for the decenter ∆ in mm. It is also the despace sensitivity of the flat for despace in mm.

More details on collimating Newtonian on the following page.