First light - the importance of thermal managementFirst light was June 20, the day I retired the scope. The sky was very good that night (mag ~5.2) for my home skies. I began at 9.30 pm and retired at 3.30 am. The Meade LX90 8" SC was soon setup aside, to have some reference point from a scope that was well known to me. Setting up was indeed faster for the dob (the potential slight advantage of the SC was cancelled by the alignment procedure).
The first point I noticed was that the scope cools down very very very slowly!! But I did not immediately realized how important the thermal management is for scopes of this size.
(...)
I did a first rapid star test and the rings looked what I, at first but incorrectly, considered to be a very symmetric behavior. But the rings were boiling: anything between a few waves in and out of focus were not discernable. Toward late night things seemed to improve, but not much.
At the beginning of the night stars were pinpoint only below 130x, at the end they were pinpoint at 200x but not above. Needless to say that I was a little worried of this, because (not yet knowing the importance of thermal management) I was convinced that by 3.30 am the scope should have been perfectly cooled (but it was not enough, nor was the pavement). The following night things seemed slightly better, but not that much (...) inside focus I could see a sort of "
moving spider net", which now I know
to be the convective cells forming on the front of the mirror. Where the net ends on the mirror edge, there depart spikes that follow the net motion along the edge. Extrafocal the spider net is less evident, because high atmospheric turbulence (mostly the Jet Stream) may be close to its own focus, and draw attention, showing a sort of
waterfall effect. I insist on the description of these features because, in the following months, I have learned that as soon as they are seen, the mirror either is not perfectly cooled (spider net) or there is bad atmospheric seeing (the Jet Stream).
In both cases anything but poor views may be expected. An american amateur astronomer, Steve Koehler, based of my descriptions, was able to successfully explain and reproduce the effect of mirror convective cells:
www.visi.com/~mkoehler/spike6/spike6.html . I find that his analysis well represents what I see. Since then, before any attempt to push magnification, I examine the status of convective cells (intrafocal) and of the atmosphere (extrafocal). As soon as I see de focused images like Steve's cases 0.06-0.07 I know that the cooling of the mirror is very poor. Only when I see cases resembling 0.03-0.04 I know that the mirror is approaching ambient temperature enough to allow good views (provided there are no other adverse conditions, e.g.: no Jet Stream, no hot pavement, no chimneys on the sight line etc.).
(...)
High resolution and under the dark skyIt took me nearly two months to fully understand how to manage thermal issues and get the maximum performance in terms of resolution. (...)
On August 3 –I had abandoned the pavement for the much better grass in front of my house since a month– the night was calm, but, above all,
it happened that the temperature of my garage, where the dobson is stored, and outside air temperature were matching. As soon as the scope was setup I saw little signs of convective cells (like cases 0.04-0.02 of Koehler's simulations). The mirror was at the correct temperature
. That night I was able to clearly see the airy disk of the Double Double, and also on Vega the disk was spotted, although surrounded by a much brighter soup of rings. On the DD the first ring was broken and dancing but seen. Also the second and further rings were seen very faint and broken in many dancing fragments. A lot of dark space was seen in between (in fact the separation of the DD is 2 arc seconds, and the disks measure 0.3 arc second in diameter and that was what I saw).
I was not able to assess very reliably the brightness of the rings compared to the disk because I simply run out of magnification (at that time my set of eyepieces and barlow maximum magnification was limited to ~400x). However the rings were surely dimmer than what I had seen on the SC in other occasions, and that was definitely a good sign for me. That same night
I had the best ever view of M13: at 400x it was resolved into a swarm of very tiny points of light. The word "resolution" here must be intended in a very different meaning: it was like seeing the cluster in X ray (as like being able to see further behind). In no other occasion, even from dark skies, I have been able to obtain that kind of view. Despite the naked eye limiting magnitude that night from my backyard was only ~4.5, the excellent seeing and thermal stabilization were making the difference.
In September, after vacations, I began
further investigating thermal issues, as it was clear to me that in that way I could
make the scope work as it should and as I had the chance to see on August 3. I found that aggressive cooling, by means of a big fan (house current powered) set on front of the mirror for an hour or more before observing was able to approach the perfect conditions found on August 3 (at least for the mirror thermal aspects; of course nothing can be done for the atmospheric seeing). In my cooling strategy there are two keypoints: a) the use of a fan much bigger than those usually placed on the back of most dobsonians, and b) the direct cooling of the front face, which is the face causing the warm boundary layer. On the subject I posted an analys on the Big Dob Yahoo user group. I will not go into further details here (see yahoo group for details), except to say that with that method I was able to obtain very good observing conditions in many occasions. For all September, Mars showed a lot of details, including minor features like the various tiny markings surrounding Solis Lacus, the clouds, with their bluish colors near terminator and northern pole, and vivid orange, brown, green colors. I found myself using ~400x very often, and still getting a crisp and steady image.
More recently, in December and January, I turned my attention to Saturn: the views, in some occasion were even crisper than Mars in September, which I think depends on the higher altitude reached by the planet.
In fact, once the mirror is cooled, and provided there is no Jet Stream or other adverse atmospheric conditions, I can use ~400x on a regular basis: Saturn shows orange-yellow pastels colors on the banded globe and brownish polar region; the rings appears white-gray and the Cassini division thickness is seen as a large black strip all around; variations of the ring brightness is obvious: the inner, semitransparent C ring is seen and the A ring shows a less brighter middle part that I believe to be the "Encke minimum". I have not seen the Encke division, but I think I have been very close to see it and perhaps I could see it in better atmospheric conditions (after all winter is not the best season). Very recently I bought a Klee 2.8 Barlow, with which I was able to obtain magnifications ranging from 490x to ~600x (the latter by simply keeping the eyepiece only partially inserted in the barlow, so that the magnification factor is pushed to ~3.5x). In no occasion the image really broke down, but recently the seeing has not yet been as good as the couple of December nights above, not enough to see more details than those seen that time. Thus in January and February my preferred, and most used magnification for high resolution, has been 360x (2.8x Klee + Pentax XL14). Worth to be mentioned is also the Eskimo nebula, which at 360x clearly shows its structure (the face inside the Eskimo) even from my backyard at lm 5- (with the help of a IDAS LPS filter). No such structure is seen at lower magnification, nor I had ever seen it in smaller scopes from darker skies. I have not yet seen Jupiter high enough.
The Moon too, at ~400x is a completely different (and interesting object). It is like flying above it. I have seen the Aristarchus Plateau in a couple of occasions (December and January) and I can say that the views exceeded what can be seen in many photographs like, for example, this one. For example, not only Vallis Schroteri is seen, but even smaller rilles, that can only be guessed in the photographs (near the semi submerged crater on the low right) and tinier craterlest.
