I had my first oppertunity to make a direct comparison of light grasp
between bino and mono modes in a telescope. Basically, this was my setup
last night:
SV102ABV with SV BV1 binoviewers (Norin made) and SV23mm FMC eyepieces.
NHII (80mm) with 14mm Orion Epic ED2 eyepieces.
Both combinations yield the same power, 34.3x. Also, by coincidence, both
setups yielded almost the exact same true field of view. The only real
unknown variable is the exact through-put of each eyepiece, but I think
since they are both "budget" eyepieces this should be close enough not to
be a factor. Now, the question is, which will yield a brighter image; the
SV102ABV with BV1 binos or the 80mm Nighthawk with single eyepiece? I evaluated
four open clusters in Auriga, the "Sword" of Orion, and various dim
(magnitude ~10 stars). Visual limiting magnitude was ~4 with no Moon but
lots of Christmas lights across the neighborhood creating more than
average light pollution.
I could easily see magnitude 9.5 stars with direct vision with both
scopes. Around mag 10 the stars entered averted vision and were again
about equal in both scopes. It should be noted that very careful alignment
and focusing of the binoviewers was neccessary to approach the performance
of the NHII on star targets. Going back and forth numerous times between
the scopes, there were occasions where I could see a slight edge in the
SV102ABV, but this could be due to changing atmospheric conditions.
On open clusters M37, M36, and M38 (moving eastward across Auriga), all of
these clusters appeared to have equal brightness. The dim open cluster NGC
1907 was a different story, however. With the light pollution this open
cluster appeared nebulous, that is, I could not resolve it into individual
stars. The apparent brightness of this cluster was slightly greater in the
NHII. But due to the fact that images appear larger in binoviewers than
they do through a single eyepiece at the same power, 1907 looked bigger
through the SV102 even though it was the same magnification as the NHII.
(So did nearby M38.) As a result, although slightly dimmer, 1907 was not
really harder to spot and view through the SV102ABV. I needed averted
vision to see this cluster in both scopes.
Then I went to M42. If I simply concentrated on the image brightness of
M42, it was about even. But, the SV102ABV through the binos showed a
larger apparent image scale, better contrast, and more comfortable viewing
using two eyes. As a result, it was much easier to pull detail from the
nebula. Oddly enough, M43 was actually a bit brighter and easier to see
through the binos, as was the dim reflection nebula around Nair al Saif.
This was probably due to better contrast. (Optical quality has to be taken
into account too, the SV102ABV is a doublet apo and the NHII is an 80mm
achromat.) I could see the entire Sword in the field of view of both
scopes.
Bottom line, if I had a choice studying these targets it would be the
SV102ABV in bino mode, but overall image brightness was exceedingly close,
any differences being very slight. Certainly, even with a reduced image
brightness, aperture still rules, and details of the Sword of Orion were
more easily seen as a result. Hopefully, I will get another chance to view
other deep space objects with better skies to see if these results
continue to hold up. (I tried to be as objective as I could, but it's hard
to be objective when you are looking at the Great Orion Nebula through
binoviewers!)
David E
