Over the last six months, the scope I have used most heavily for my NV observing is a 180mm f/2.8 Takahashi Epsilon. While the scorching speed provides stunning images with my NV eyepiece, using a fast astrograph does have a catch - image scale. Or rather, lack thereof. For small targets, 19x just isn’t enough. So I forced myself back to my larger Newtonian to reap some details on the targets of southern Sagittarius.
Arizona suffers from an annual monsoon rain pattern, which brings frequent cloud cover to the higher elevations. Generally speaking, I lose two solid months of prime-time summer observing. So when it breaks in mid-September, the rush is on for the southern Milky Way.
When I had a clear night on the 14th, I was determined to get something done despite my 6AM flight the next morning. Sagittarius was already past the meridian and rapidly sinking towards the horizon at the end of twilight. My list was concentrated on the lower portion of the constellation, time was of the essence. This was more of a recon mission - speed dating if you will. I covered 23 objects in 90 minutes. As such my notes are very short - more impressions than details. Having separated the Wheat from the Chaff, next season I can go back for detailed observations on the top prospects.
Observations were made from my backyard 7 miles north of Prescott Arizona under Bortle 5 (yellow zone) skies. As the two softball complexes within two miles of my home were active, this sky rating may have been optimistic. The equipment was a 16” f/7 Newtonian (2800mm focal length), Mod 3c NV eyepiece, and a Leica ASPH (9mm-18mm wide field zoom) for conventional views.
IC 4673, Planetary Nebula. Smallish, dim. Very close to a tie between the Leica with a NPB filter and with NV with hydrogen alpha filter. On my 1 to 5 scale, I would score this object a 2 (below average, dim) in either eyepiece.
NGC 6565, Planetary Nebula. Small and bright with Leica/NPB or NV w/H-alpha. Score 3 (average).
ESO 522-16, Open Cluster. ESO objects are not normally on my observing menu, but AstroPlanner returned this one within my search parameters, so I was curious. It was interesting in that it showed both the encroachment of light pollution into my site, and the relative power of the NV technology. With the Leica eyepiece @ 155x perhaps five or six field stars are easily seen maybe 2 dozen very faint ones near the edge of vision. Keep in mind this cluster is rather centrally located in Sagittarius! With NV stars fill the field. Hundreds. Too many to count. The cluster? Very small and not very impressive with either view. Score 1 - Dim and disappointing.
Barnard 291, Dark Nebula. Seen in both eyepieces. Another reminder of the deterioration of my home observing site. Ten years ago I was able to capture 15th magnitude IC 4617 with a 12.5” scope. Those days are long gone!
NGC 6522, Globular Cluster. Beautiful spray of stars in NV very well resolved but still small. With the Leica I could get more image scale but it still only looked like a faint patch with little to no resolution Score 4 with NV (Above Average, makes constellation “Best Of” list). Conventional Score 2, Below Average.
NGC 6528, Globular Cluster. With the Leica resolution is hinted at as faint mottling. With NV easily 2 dozen stars resolved across a small core despite low target elevation and sky glow. Score 3.
HR 6780, Double Star. Not a NV target, this one was for the Leica only. The separation is listed at 4 arc seconds, it was the tightest double I could cleanly resolve with a well collimated 16”. Yes, the seeing was that “good”. A pretty ocher primary and blue secondary. Score 4.
NGC 6558, Globular Cluster. A very curious globular cluster. With the conventional eyepiece it was barely there just a glow. With NV it was almost like looking at a rich open cluster the spacing of the members were so wide. The innermost core wasn't dense and tight but the outer core was almost loose in character. Almost like two different objects between the eyepieces. Score 3.
NGC 6559, Globular Cluster. At the time of observation only 18° above the horizon. Barely there in the Leica eyepiece, stunning with NV. Score 4.
NGC 6624, Globular Cluster. This one had a mottled surface hinting at resolution in the Leica. With the NV the are largely resolved. One pattern is developing: These clusters appear somewhat larger with conventional eyepieces. I suspect what is causing this is the rich area of the skies they are in. The background and/or foreground stars that are below the visual threshold of conventional viewing are brightened with NV to the extent that it makes the cluster itself blends in and looks smaller. Somewhat similar to the effect you get with open clusters that are not classified as Well Detached. In any event, it was still an Above Average cluster in the Mod 3c at 108x. Score 4.
Messier 69, Globular Cluster. The first globular cluster of the night that I could resolve with the Leica. Very nice. With NV was amazing. Score 3 with the Leica, Score 4 with the Mod 3c.
NGC 6652, Globular Cluster. NV only observation very small with bright core. Score 3.
Messier 70, Globular Cluster. For a Messier cluster I was expecting something larger. Fairly small cluster but very bright well resolved. An interesting Little Dipper like asterism of field stars lays almost on top of it. Score 4.
IC 4776, Planetary Nebula. Score 3. Easily picked up with Leica or Mod 3c not much difference between the two. I seem to get a better view in NV when using the 12nm H-Alpha vs. the 640 long-pass.
Messier 54, Globular Cluster. At this point I have all but quit using the Leica. In this regime, it can’t keep up. In the Mod 3c appears somewhat larger than most of the other globular's in Sagittarius. Perhaps it is or perhaps being further off Milky Way the outer stars are not blending into a background? Very rich and well resolved outstanding. Score 5.
NGC 6723, Globular Cluster. Appears larger and looser than the other globular's I have looked at tonight. Almost like a very rich open cluster. Score 5.
Messier 55, Globular Cluster. Looks like a rich open cluster to me. Very large and loose. Score 5.
IC 4391, Elliptical Galaxy. Extremely faint, Score 1.
IC 4946, Spiral Galaxy. Extremely faint, Score 1.
NGC 6902, Spiral Galaxy. Extremely faint, Score 1.
At the end of working an observing list, I like to treat myself to a few Eye Candy objects to end the night:
M27, Planetary Nebula. A very nice view with the 640 long-pass, the dumbbell shape was sharply defined. However after viewing so many emission nebula, I was expecting it to be brighter, especially in a 16” aperture. I had observed M27 with my 8” f/9 Newtonian back in April, but did not recall which filter I used. I tried going filterless, and the view was actually better! Then I put in a 12nm H-Alpha filter and was absolutely amazed. In addition to the Dumbbell shape, extensions were seen in the hollow area. I was shaped like a football (American football) and oriented at a right angle to, and larger in size than, the dumbbell structure. Doing a search for images of M27 the next day, I found that it even some CCD images don’t show this as well as the unfilmed L3 tube does!
M57, Planetary Nebula. Hot on the success of M27 I went for the ring nebula. As with the Dumbbell, 640 long-pass was not the best filter choice. Unfiltered or 12nm H-Alpha were better. In all cases, the central star was obvious. I’m not sure, but I thought I saw a second star. Next session I will make a more concerted effort to see if a second central star is visible. M57 was very nice, but not near the detail of M27.
M13, Globular Cluster. The NV view is absolutely amazing. Best I have ever seen. As an interesting aside, my old friend 15th magnitude IC 4617 was an easy catch next to the cluster. Sure is a lot better to be pointing the scope away from the Prescott light dome!
Some Thoughts on the Night
In the July 2017 Sky & Telescope article by Ed Mihelich (CN member Chemisted) he claimed impressive magnitude gains on globular clusters. At first read, I thought he was overstating things just a bit. However after this night I had a large enough sample base to say Ed was right on the money. The pattern emerged right away of large gains. As with emission nebula, globular clusters are the strong suite of NV eyepieces. So much so that using conventional eyepieces on these targets is largely a waste of time.
OTOH, NV could not rescue dark nebula from the light dome to my immediate south. The combination of sky brightness and low elevation is too much. I either need to wait for local conditions to improve after 11pm, or go to a darker site to complete my survey of DN’s in the far south.
I was also surprised that planetary nebula seem to react so well to H-Alpha filters.
It was nice to get some image scale (compared to the 500mm Epsilon) to break these clusters open. But even with a large focal length scope (2800mm), the NV eyepiece is barely a medium powered affair at only 108x. After some experimentation and looking at the math (aperture vs. speed) I am not a big fan of barlows with my astrograph. It’s just not very efficient. There is certainly some convenience there as logistically it is much easier to pack a barlow than a second telescope! Nevertheless, I have just been marking targets for later return with a larger scope rather than barlow the Epsilon.
But when you are talking about your largest telescope and you are still only getting 108x, there is no recourse but a barlow. I did attempt to use a 2.4x Dakin on a few of the targets that night, but was not pleased with the results. I believe it outstripped the seeing, especially at the low elevations I was working.
The field of NV astronomy is still relatively new, and I am still feeling my way around. At this point it appears the path forward will be a two scope solution. Modify the large scope to use focal reducer(s) to bridge the gap between it and the Epsilon as best I can. I hope to avoid having a third scope. For more scale on the big scope I will be in the market for a barlow in the 1.5x to 1.8x range.
Edited by Jeff Morgan, 19 September 2017 - 02:07 AM.