Like many, I live under suburban skies. And like many, I lack the inclination and means to drive long distances or install an observatory. Yet I have a burning interest in galactic nebulae. What to do? My solution: make full use of binoculars and make full use of filters.
By "handheld binoculars" I mean those which I can hold without any particular support for enough time to get a good view of the object. This extends for me to a 10x70 weighing 2 kilos and a 15x70 weighing less than 1.5 kilos, and naturally includes stabilised binoculars.
By "suburban skies" I mean those in which NELM at zenith is around 5m0. I would define 5m5 as "very good suburban at transition to rural", and 4m5 as "poor suburban". Away from zenith NELM declines, and is regularly 4m-something near the horizon. What can be achieved under such skies?
Last night I was out with the 6x24 binoculars after having found a simple way to fit a threaded 1.25-inch adapter to the front barrels. Sky quality was exceptional for my circumstances, with 5m5 in zenith and the bifurcation of the Milky Way between Cygnus and Aquila distinct. For first-light with the filtered 6x24, I chose the three largest nebula complexes in Cygnus: NGC 7000 (North America) with IC 5067 (Pelican); the IC 1318 complex around Sadr; and the Veil Nebula.
As the binoculars have 12° FOV, I had high hopes of seeing the whole Veil Nebula floating in wide surroundings, and even of catching the IC 1318 complex in its broader context. Too high hopes, it emerged, but read on:
NGC 7000 (North America)
Unfiltered: Nothing doing
Astronomik UHC: Full shape of North America emerges, incl. Florida and Mexico. No sign of Pelican.
Lumicon UHC old-type: Sky background darkened, but no real gain over Astronomik UHC.
Astronomik OIII 12nm: North America now set off better from surroundings. Still no sign of Pelican.
Baader OIII 8.5nm: Best sight! North America floating against truly dark sky background. Eastern margin, where transition to dense star field was still a little unclear in OIII 12nm, is now quite clear. A vague hint of the Pelican.
Astronomik H-Beta 12nm: North America still there in full north-south extent, but reduced in east-west extent and amorphous compared to the view in OIII, more like a long potato than the continent. No sign of Pelican.
After each filter change I panned over to the big 68-Cygni Nebula (Sh 2-119), in the wild hope of seeing, with such tiny aperture, the brightest eastern section. No luck, and no surprise!
However, I noticed during each such excursion that North America remained strong and distinct right up to the edge of the 12° field of view. This was a very pleasing effect. It bears out the advantage of fitting filters in front of the objectives rather than behind the eyepieces.
Next up: The Veil Nebula with its brightest elements, the western Witch’s Broom NGC 6960 and the eastern Network Nebula NGC 6995.
This was a disappointment. Only with the Astronomik OIII 12nm filters did I get a vague impression of the position and extent of the Witch’s Broom, and a very faint notion of the Network Nebula directly at 52 Cygni.
Now for the most ambitious project of the night: The IC 1318 complex around Sadr, the brightest component of which is IC 1318A (DWB 82, also known as Dolphin Nebula), a triangular patch on the northern fringe of the complex.
Nothing seen in any of the filters. Here I was not really disappointed, for my only hope was for IC 1318A in H-Beta (it is my consistent experience in larger binoculars that all elements of the complex respond best to H-Beta). My clear sighting of North America in H-Beta suggests that these binoculars, although 50 years old, have decent transmission at that wavelength. So I think the non-sighting is due to a mixture of insufficient exit pupil and too small image scale.
I strained my eyeballs out to get a sighting of the Crescent Nebula NGC 6888, pinning particular hope on the 8.5nm OIII filters, but to no avail. Here, again, this is surely due to a mixture of insufficient exit pupil and too small image scale, probably compounded by a lack of modern coatings on the old binoculars.
Those of you who have read on up to here may interject that, overall, no great results were achieved. And yet I found the night strangely satisfying and relaxing. Before packing up I looked around without filters through the binoculars, now with heightened sensitivity for and appreciation of the stars and colours that the filters had swallowed.
The view with the 8.5nm OIII filters of the North America Nebula floating with 5 degrees of space to all sides of it will remain with me for long.
I aim to continue occasional reports of this kind in this thread. I welcome those of others. I beg you only to gauge and report NELM as accurately as you can, for reports without that information would be impossible to compare to others.
Edited by C.Hay, 10 August 2020 - 02:22 AM.