Prove all things; hold fast that which is good.
1 Thessalonians 5:21
Date: Saturday, December 12, 2015
Time of observation: 22:00-23:30 UT
Temperature at commencement: -3C
Temperature at termination: -4C
Seeing: Ant I -1.5, transparency excellent, no wind.
Instrument: Well-collimated 20cm f/6 Newtonian (22 % CO). Standard Skywatcher Dob focuser. No fans employed.
Materials & Methods: A suite of double stars observed with the Newtonian, being mindful of magnifications used, aesthetics of the image garnered, and efficacy of the splits on a range of systems of varying degrees of difficulty. Mark III Baader Hyperion Zoom and dedicated 2.25x Barlow, 1.25″ Baader Neodymium filter, Parks Gold 7.5mm ocular. Instrument sat upon a Lazy Suzan Dob mount (undriven). Instrument moved from a warm and dry domestic setting to a cold and dry unheated shed, where it remained for several hours until its deployment at 22:00 UT. Flexi-dew shield attached.
Observations: One good night can dispel a myth and expose a fallacy. This time it pertains to the notion that small refractors are better suited to winter viewing and that reflectors or compound, catadioptric instruments can only do useful work in the warmer months of spring and summer.
This is a misleading notion and simply untrue. Nor is it supported by the weight of historical evidence, which shows that large Newtonians were used to great effect by some of the finest and most admired observers of past generations. The work of the great Victorian, Reverend T.W. Webb, is just one example. As we have explored previously, Webb chose to use a 9.25” With-Berthon silver-on-glass reflector to carry out observations in all weathers, including freezing winter nights. Nor did Webb employ active cooling to his telescope as no such device was available for him to use.
Octavius, my 8-inch f/6 Newtonian (also without active cooling fans) performed flawlessly this evening when turned on a variety of double stars, the Airy disks of which were observed as tiny, round, calm and beautifully resolved. Despite the large temperature differential between my body and the surrounding air, a warm insulating coat, gloves and hat greatly reduce heat loss. Attaching a dew shield to the end of the tube increased the distance between my body and the entrance pupil. All these measures and a steady atmosphere conspired to produce arguably the finest images of double stars I have seen in any telescope.
theta Aurigae: Very easily resolved at 160x but even more compelling at 360x, both the primary (magnitude 2.6) and secondary (magnitude 7.6) separated by about 4 seconds of arc. This system can be quite tricky, owing to the large magnitude differential between the components. A magnificent sight!
Rigel: Despite its low altitude, the exceptionally calm air made seeing the feeble spark from its companion child’s play at 60 diameters or above.
The theta1 Orionis complex: The 8-inch speculum showed all six of the Trapezium stars A through F at magnifications of 360x, the magnitude +11 E and F components becoming ever more distinct as one’s eyes became better adapted to the dark.
Alnitak: The easternmost star in Orion’s belt. The 8-inch telescope at 150x showed its whitish, magnitude 3.7 companion just 2.5” away to the south southeast of the magnitude 1.9 primary.
Mintaka: Very easy at all powers. The primary shines in a soft white colour but the companion, located a decent 52” away to the north presents as a beautiful, pale blue cast in the 8-inch speculum.
eta Orionis: Much more challenging but the decent aperture made very light work of this system at 360x. The telescope showed its very tight companion a mere 1.8” east northeast of the primary. This is a fetching colour-contrast double, faithfully rendered in the perfectly achromatic reflector, with the primary appearing yellow and its companion blue.
32 Orionis: The pair was perfectly resolved at 360x.The primary (magnitude 4.4) and its secondary ( magnitude 5.7) separated by 1.8”
52 Orionis: a classic Dawes pair of 6th magnitude luminaries, brilliant white and well separated by 1” using a magnification of 360x.
42 Orionis: Very impressed with this system, which was not as difficult as I had anticipated! Though only separated by 1.1”, the difficulty here is the very large brightness differential between the primary and secondary (4.6/ 7.5), the raw resolving power of the 8-inch aperture (360x) in these excellent conditions rendering the split easily.
eta Geminorum: The icing on the cake on this frigid evening! I have given mention to this orange variable star (magnitude ~3.5) in previous communications. Although I have glimpsed (and I mean glimpsed) the very faint secondary in 4- and 5-inch refractors as well as 17cm f/16 Maksutov at very high powers, the image in the 8 inch reflector was in a completely different league! My notes from the evening of January 29, 2015, using a 17cm Maksutov Cassegrain revealed the companion using a glare-reducing variable polarising filter at 340x and at a recorded temperature of -5C. That being said, the superior resolution of the 8 inch speculum was all too obvious to my eye, for it revealed the dim (magnitude 6.5) bluish secondary just 1.6” off to the west-southwest of the ochre primary more plainly than I have ever seen before in any telescope! And this was true even though the system was still a few hours away from meridian passage! The superiority of aperture being abundantly apparent, I watched eta for many minutes, savouring the sight which I had dreamed of seeing for many years.
Concluding Comments: Despite the cold, the Newtonian telescope worked perfectly well. Laziness and wilful scaremongering have prevented many from using their larger Newtonians to good effect under these conditions. The observations made this evening reaffirm the importance of aperture-dependent resolving power working under ideal meteorological conditions. They are supported by three tiers of evidence; physics, history and personal experience. Don’t let anyone stop you from discovering the genius of Newtonian optics! If you don’t try, how will you ever know? And as to the wisdom of confining one’s observations to aperture-limited refractors in winter, that’s all well and good, but bear in mind that you’ll be missing out on golden opportunities to see more, much more!