Coma Correction

The SC-2438 telescope uses a parabolic mirror 24 inches in diameter with a focal length of 73.5 inches (f/3.1). A mirror this fast suffers from both coma and a stigmatism off axis as well as field curvature. The spot diagram shown below illustrates this. Figure 1 shows how the incoming starlight is spread out on the focal plane based on the distance off-axis that the light comes from. The color wavelength is listed in microns and the scale of the diagram is also in microns. Note the scale at the left edge. For comparison, astronomical CCD's have pixel sizes that range from 7 to 24 microns. Notice that on-axis (upper left) shows just a dot, meaning that all of the light coming in on-axis strikes the focal plane at one spot. As we move farther off-axis to 0.15 degrees (upper right) the spot not only gets bigger but also develops the characteristic “coma” shape. Now moving to 0.3 degrees off-axis (lower center) the coma grows considerably and the size also grows. To put some numbers to it, on-axis the spot is < 6 microns, at 0.15 degrees off-axis it grows to about 100 microns, and at 0.3 degrees off-axis it is now a whopping 190 microns. Clearly this optical system is only usable very near the optical axis.

Uncorrected parabolic reflector spot diagram

Figure 1 Un-corrected parabolic reflector spot diagram

In order to improve the off-axis performance, the SC-2438 telescope uses a corrector. The required corrector is dependent on the size of the CCD camera chip. The Paracorr® corrector by TeleVue works very well for those chips where the active CCD area has a diagonal measure of less than 17 millimeters which corresponds to a 0.33 degree field of view. Such chips include the KAF 1602E, KAF 0401E, KAF 0261E, and the KAF 1401E made by Kodak. Other chips that have a diagonal measure of less than 17 millimeters will also be useable with this corrector without significant vignetting.
For larger CCD's which will have a corresponding larger field of view, it is necessary to have the corrector custom built. ScopeCraft has had a custom corrector of the Wynne type designed which will give corrected views out to 0.6 degrees off-axis. The effects of this custom corrector on the spot diagrams are shown in Figure 2. Again, the upper left is on-axis, upper right is 0.15 degrees off-axis, and the lower center is 0.3 degrees off-axis. Note the scale change of the diagrams from 400 microns in the un-corrected diagram to 20 microns in the corrected diagram. In this case the spots range from 6 microns on axis to 10 microns at the field edge. This corrector will now provide a field of view that is over 0.6 degrees with virtually pinpoint star images.  Whereas the un-corrected parabolic mirror is purely reflective and has no color effects on the image, the corrector has refractive elements and thus the color alteration on the image must be controlled. As you can see from the spot diagram, even though the different colors come to focus at different points the overall spot of all colors is still extremely small.
Spot Diagram with Coma Correction
 Figure 2 Spot diagram with Coma Correction

The layout of the lenses of a Wynne type corrector is shown below. It consists of three elements which are mounted in a housing and placed in front of the camera. In order to produce such great images, the corrector causes a slight increase in effective focal length.
Coma Corrector lens layout
Figure 3 Model CC-1 Corrector Optical Layout

ScopeCraft offers both the Paracorr and the custom Wynne type corrector for the SC-2438 telescope.  The custom corrector, Model CC-1 is suitable for CCD's up to 40 mm diagonal measure. This corrector is 4 inches in diameter and about 8 inches long. Due to its size and weight it must be additionally supported in the telescope and is a function of which CCD camera is chosen. CCD's of this size will have a field of view of over 1 degree! The CC-1 changes the effective focal ratio of the SC-2438 to f/3.48.