As explained in earlier blogs, I have discovered, through extensive experience in the field, that Newtonian reflectors can render beautiful, colour free images of a wide variety of celestial objects, from the Moon and bright planets, to a wealth of bodies beyond the solar system. In my own niche area of double star observing, I found that a simple 130mm f/5 Newtonian reflector resolved tighter pairs than a 90mm ED refractor and indeed, as deduced from my previous field notes, also exceeded the performance of a very fine 102mm f/15 classical achromat on good nights of seeing.
Refractors have a well earned reputation for garnering very stable images at the eyepiece, a consequence of the decent height of the entrance pupil above the ground, glass properties (crown & flint doublet objectives in particular), less intense tube currents and relatively small apertures which are quite often immune to the vagaries of the atmosphere. In comparison, reflectors can be rather temperamental. With their need for precise collimation and greater tendency to manifest thermal effects coupled to the (often) larger apertures employed in the field, Newtonians typically (but not always) serve up images significantly more unstable from moment to moment.
As I also explained before, this is not really a big issue for a seasoned observer, who has more than enough patience to remain observing long enough to ignore or wait for the disappearance of these various bugs that attend the use of a good Newtonian telescope. That having been said, I elected to investigate the effects of some simple modifications that could potentially ameliorate the effects of tube currents in the 130mm f/5 Newtonian, so as to stabilise its images as much as possible.
This led me to investigate the effects of insulating the inside of the thin, rolled aluminium tube that houses the optics of this small Newtonian. My researches led me to explore the properties of cork; one of the best, natural and renewable insulators from the Creation.
I heard many a yarn recounted by veteran observers, that lining the inner tube with cork could dramatically dampen the effects of tube currents in Newtonian and compound telescopes. It seems to have originated sometime in the late 19th century or early 20th century, but in more recent years, some amateurs, whose work I trust, have also recommended cork as a suitable insulator for their telescope tubes.
The theory is fairly simple; the thin aluminium tube is an excellent conductor and radiator of heat. Indeed, under a clear sky, the temperature of a metal tube rarely tracks the ambient air temperature perfectly but instead can often fall off to a few degrees lower than the surrounding air during radiative cooling in the field. But by lining the inside of the tube with some kind of insulating material, one can keep this temperature differential between the aluminium tube and ambient air to a minimum. This should create more stable images, especially at the highest powers, which would in turn make their visual study more profitable, as well as increasing their aesthetic appeal.
Materials & Methods
I checked out what types of cork were available and settled on the purchase of self adhesing cork sheet, which arrived promptly from the seller. Next, both the primary and secondary optics as well as the focuser were completely removed from the aluminium tube, which was then lined with the cork sheet.
Initially, I had just intended simply to paint the cork a flat black colour but was unconvinced that it was really dark enough to compare with regular flocking material. I therefore elected to cover the cork with the flocking material, which, in effect, would act like a double layer of insulation. After lining the tube with the cork overlaid with the flocking material, I also lined the drawtube of the focuser with more flocking material before putting the telescope back together again. I was very pleased at the light dampening properties of the instrument during daylight hours and noted that the images served up by the telescope were a little bit more contrasty than before the flocking material was added. The result is seen below:
Finally, I was now ready to study the images of a variety of high resolution targets to see whether or not this tube insulation worked in practice. My tests were carried out over a number of winter evenings, where the ambient temperatures sometimes fell to −10C. Most of my observations were conducted on the evenings of January 18, 19, 20 and 24, but also included some shorter vigils during more unsettled spells. The telescope was given time to cool off to near ambient before commencement of observations.
The targets included some tricky double and multiple stars; theta Aurigae, delta Geminorum, iota Cassiopeiae, as well as easier subjects like Castor A & B. In each case, I charged the telescope with a power of beween 260x and 406x diameters (so between 52 and 80x per inch of aperture) and the images studied as they moved across the field of view.
The carefully focused stellar images were very impressively presented in the telescope and appeared significantly calmer (read less susceptible to thermal degradation) as they moved across the field, their forms morphing significantly less than I had previously noted in the uninsulated tube. Indeed, during these vigils I enjoyed some of the finest images yet garnered from this modest telescope. Specifically, the stellar Airy disks were much more in keeping with those I have enjoyed during the milder months of Spring and Summer. I was able to pick off faint and close companions much more easily and efficiently than I can remember when using the same optics in previous winters. The activity of insulating the inner metal tube most definitely improved the images from moment to moment, allowing me to enjoy their perfect forms for longer.
My correspondence with some highly experienced observers also alerted me to other ways tube currents could be minimised or even completely abated in Newtonians, including housing the optics in an oversized tube, constructing non cylindrical tubes (think hexagonal designs) and using active mirror cooling. A combination of all these strategies have been shown to improve image stability in reflectors and are well worth investigating in their own right. They will surely make an already good telescope into an excellent one.
I intend to insulate my larger Newtonians in the same way, and in due course.
The author would like to thank Martin Mobberley and Garyth64 for interesting discussions on cork.