Below is a comparison of the Point Spread Function of each design and each of the three focal ratios: f8, f10 and f12. Again, this is at the edge of a 2.0" diameter field. RMS spot size (in microns) at the 2.0" diameter edge is given for each PSF just below the image.

Please note that the below PSF's are not an "actual size" representation of the star's size at the edge of the 2.0" field. It is merely a tool in analyzing the star's shape and overall size versus other PSF sizes for other designs or focal ratio systems. 


f8 Classical Cassegrain

f10 Classical Cassegrain

f12 Classical Cassegrain




f8 Ritchey-Chretien Cassegrain

f10 Ritchey-Chretien Cassegrain

f12 Ritchey-Chretien Cassegrain




In reality the above differences in spot sizes from say the f8 compared to the f12 are mostly buried in seeing. The exception would be for those who own land on a Peruvian mountaintop... The difference between two systems that sit next to each other above are definitely buried in the seeing.

You can see from above that as you go faster with the focal ratio of the system the RMS spot sizes suffer a greater spread of the star image as you jump from each progressively faster focal ratio. The difference in RMS spot size from the f10 to f12 is very slight, while the jump from f10 to f8 is relatively large in sizes. If planetary and/or very narrow fields are desired, then the f12 with it's 36.7% obstruction (5.50" secondary with 7.3" baffle diameter) will yield slightly higher contrast images (at least visually) than the f10 variation that uses a 41.0% obstruction (6.3" secondary with 8.2" baffle diameter). The f8 variation has a 46.9% obstruction (7.5" secondary with 9.375" baffle diameter) and would not be an ideal instrument for planetary work, at least visually.

Even a fast parabolic mirror, say f3.0 like the above Classical Cassegrains use, will work for planetary use. As long as the object is placed on-axis and the optics are very well collimated. Most optics will yield the best optical quality directly down the center of the optical path (on-axis), no matter what their design. Cassegrains can be designed to offer wider fields at the cost of increasing the size of the on-axis star sizes but this is typically not how commercial optics are produced. Professional sky survey type work that is using 10" CDD arrays needs this type of design change to the optical perscription but otherwise almost all instruments will have their best optical quality on-axis.

You can also see from above that as the focal ratio slows from f10 to f12, the general shape of the star for the Classical Cassegrain begins to become more round in nature. At f12 the difference between CC and RC is very, very slight.

All of the above off-axis PSF star images are shown with the optical system focused on a star at the middle of the optical axis (on-axis). If the optics are focused in (toward the primary) slightly, about 0.3mm in the case of the f10, the PSF at the edge of the field is reduced. The PSF on-axis increases but as long as those are kept at or below the airy diameter, they will not experience any measureable difference in their size. Again, this is a function of seeing limiting what can and cannot be detected, even with a CCD camera. Continue one page forward to see graphical illustrations of this defocus method.

With any of the above designs, whether it be CC or RC, f8, 10 or 12, a field flattener can greatly reduce the PSF of the stars in the outer 2/3rds of the field. Even without such flatteners though the above spot sizes are very small. A 1" square CCD chip will give stunning images with even the f8 version of the RC.