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.
Nick and Meifong