Coyote 8/27/03

by Nick Biunno

Meifong and I made a last minute OI change from MB to Coyote Lake because we had two major objectives for the night that we planned for a while and had concerns about the marine layer moving in. First objective was to compare views between an Orion XT10 and a Discovery 17.5 TD. The second objective was to view Mars at a modest 34 million, give or take a million, miles from Earth. Seeing was fair to good at Coyote with the cloud layer at bay until about 12:30 - 1:00 when we packed up to leave. Only Dennis and two others showed up with scopes at the boat ramp parking lot.

As for the first objective, the results were readily predictable, "aperture rules". But, performing the side-by-side test was still highly instructive. The observation test was performed with a 12 mm Radian EP in the XT10 (f/5, f = 1225 mm) and an 18 mm Radian EP in the 17.5 TD (f/5, f = 2222 mm). This set gave not exact but nearly equivalent FOV's. We also switched EP's at an observing point to compensate for the lost of brightness of the 12 mm in the XT10.

Our first stop was M56 and M57 high up, seeing fairly steady. On the globular M56, the XT10 gave good views with some of the brighter stars resolved. The majority of the stars though were washed out in a haze in the respectable apparent diameter of the object in the 12 mm EP. In contrast, the core of M56 and the outlying stars were fully resolved in the 17.5 TD with the 18 mm. The perceived brightness/contrast (B/C) ratio, of the 17.5 TD increased about 2 fold giving the eye-brain circuit less stress in constructing a detailed image.

M57's luminosity in plenty bright and the XT10 could fully resolve the ring in the 12 mm EP with a respectable apparent diameter. Contrasting observations of M57 in the 17.5 TD were interesting in that the increased B/C ratio allows the eye-brain circuit to conceive more detail in the nebulosity of the planetary. It can be imaged that on a very dark sky, the central star will be detectable. Other emission nebula, such as M8, M17 gave similar results in the comparison even though these objects were in the Gilroy muck. Meifong found the Veil nebula and we spend a consideration amount of time in all its regions. The XT10" would not perform well in either eyepiece only showing faint emission structure. The 17.5 TD did perform well in the Veil with either EP again due to the increased B/C.

While more apertures does physically give more resolving power, at this level of comparing DSO's in 10" and 17.5" it is our observation that the increased B/C gain in allowing the eye-brain circuit to have more information to work with is a major part of "aperture rules".

Mars on the other hand was also enjoyable giving are best views to date. After, Mars got high enough we started viewing with the 18 mm radian. The edges of the disk were sharp and the image fairly stable at that magnification and height in the sky. A little later we tried the 12mm radian and the edges of the disk were still sharp in both scopes. The disk was a respectable size and detail was popping out. A little later, Dennis lets us use his 8 mm radian with a light blue filter. The edges of the disk were still sharp and we were getting some very nice detail. A little later, we used a 6 mm radian EP with a neutral density filter in the 17.5 TD. The edges of the disk, to my surprise, still sharp and the apparent diameter was enormous. As we allowed time the study the disk much fine detail was observed. In this instance, the additional resolving power did help somewhat being offset by a larger seeing cell.

Note: I have used the term B/C with no mathematical or physical definition as a true ratio. It is a visual conception only and similar in concept to the term (s/n) signal-to-noise ratio.

The CSC looks like it may clear up enough to be good at MB this Friday so we shall stick to our plans to be there. Man the gate, keep the peace, and help out if the public shows up.