The Dark Sky window for June is closed. Despite losing three nights to unseasonable weather right at New Moon, I did get in four nights and over 20 hours of time, primarily using my 16" f/7 Newtonian both prime focus and afocal. A few observations from this window.
1) 50mm SLR Camera Lens
Start with the fun stuff first: 50mm (1.8x magnification) is an awesome addition to the NV kit! The 22 degree field frames small and medium constellations nicely, and of course with the depth only NV provides. Using a 7nm h-alpha filter I was able to pick up the Zeta Ophiuchi nebulosity plainly. 50mm is a fun niche!
These lenses can be had at very fast speeds for moderate prices. The Canon FD lens I acquired is f/1.4 and it only cost $105 shipped from Japan. It’s a little bulkier than the native ENVIS lens, but still balances wonderfully in the hand. It’s a real treat at the end of a telescope session to flop down the in the Zero Gravity recliner and just wander for 20 minutes.
In the end I suspect I will add 85mm and 185-200mm telephotos to supplement my 50mm and 135mm lenses and cover the “sub-telescopic” framing options. Yes, low power NV viewing is that good.
2) Galaxy Summary
I used much of this window to finish out my Ursa Major, Leo, Leo Minor, and Virgo galaxy lists. Over 120 galaxies in this window. At last I have found the overall best strategy for delivering detail on galaxies! It is the same as with conventional eyepieces - use a bigger scope!
Magnification vs. Speed
Early on, I noted that star clusters are much less affected by slower f/ratios, whether it be from the scope, or an added barlow. The reason is that they are point sources. Since galaxies are made up largely of stars, I hoped they would also take magnification well. And by dumb luck, the first galaxy I tried this on (M82) had the correct morphology, surface brightness, and significant detail due to it’s proximity.
Unfortunately, that was more the exception than the rule. For the large majority of galaxies, just going with the fastest speed option gives the best result. Especially face-on spirals. The reason being that the intensifier craves the speed and truth be told, most galaxies just don’t have intricate detail to offer (at 16” aperture) so there is little upside to more image scale.
It’s a big sky and there are exceptions of course. If the galaxy type is Elliptical it will take magnification well, usually. But again, not much detail to offer (note - Centaurus A is already large, so the barlow added little).
Edge-on spirals and irregulars with high surface brightness also do well with barlowed intensifiers. In addition to a size that is more pleasing to the eye, sometime's these types offer gains in detail too. But sadly, by the numbers there are just not many M82’s out there. My galaxy technique is to start with either the 55 afocal or prime focus + 0.7x reducer first. If it has some promise, it's easy to remove the reducer and go native f/7 (103x in my scope) and evaluate the potential for further magnification.
I have been using a DGM GCE filter throughout my spring galaxy observing, with over 200 galaxies logged. For those not familiar, this is a filter intended for use in glass eyepieces that suppresses wavelengths centered around common light pollution wavelengths. Overall, it would be classified as a Broadband type. Results were best with face-on spirals, but even in these cases it was only a subtle enhancement. OTOH, I never felt the filter was harming the image either. All things considered, I don’t see this filter gaining widespread use with for NV galaxy work.
When I started with NV the Conventional Wisdom was you needed to have a long pass filter, so I bought the 640 based on my light pollution levels. Later I acquired the 685. The 685 makes the field quite dark. Initial results were not impressive, but more trials are needed.
However the trend is the more I observe, the less I use long pass filters. Best usage is for targets low in a light pollution dome. I almost never use them anymore at high target elevations, here “raw” is best. Inasmuch as there are some cases where long pass filters are useful and the cost is low, I’ll keep them around.
These days my advice to aspiring NV astronomers would be to get two h-alpha filters and save the long pass filter for your second round of accessory purchases. Part of a complete kit yes, essential, not so much ...
3) Open Clusters, Dark Nebula, and NV
One of the problems with NV on open clusters (and to a lesser extent dark nebula) is that these targets lie along the galactic plane. NV brings up so many foreground and/or background stars the target is overwhelmed.
In the past, I have used the Detachment rating from the Uranometria Field Guide to determine which clusters would respond best to NV with “well detached” being the best. For Not Well Detached, I had been using my glass eyepieces.
Previously I had been adjusting manual gain upwards until scintillation became noticeable, and pretty much leaving it there. Well guess what? Use manual gain control to slowly ramp that down and viola! The cluster (or dark nebula) emerges. Trumpler 30 is an amazing example of this. Sometimes it’s the simple things you forget first ...
Credit here to Alan Green (alanjgreen) who mentioned the technique as a method to “pop” the arms on galaxies where high gain settings emphasize galactic cores. I merely tried his idea on different targets. And it delivers!