Review of 16 inches Dobson "Ariete" by Germano Marcon

Fig.1 - The telescope ready for use (click to enlarge)

Foreword. I have no undisclosed interest in reviewing this telescope. I review it because –after 8 months of ownership and use– I feel the scope is well worth a positive judgment, and that a review might be useful, especially for European customers. This review is also on Cloudy Nights (but with less and poorer pictures).

Why a 16" Dobson?

This is my sixth scope. Like many people, I have been investing money and time in the endless search of the "perfect" scope. Although it is clear that a "perfect" scope does not exist, when I look back to all my previous scopes I definitely see that they satisfied better and better my personal needs; and that happened according to my growing awareness of what my needs actually were. Indeed the choice of a telescope is a careful trade-off between competing requirements, basically of two kinds: portability and power (not to mention cost). But "portability" and "power" are concept rather fuzzy, which need to be clarified for a satisfactory trade-off. One aim of this review is thus also to clarify these concepts by revisiting my previously owned scopes and showing why they were better and better suited to my needs. I hope this can help other people to better analyze their need and find their "near-perfect" scope(s).

Portability basically means how easy it is to move and setup a scope. Factors like size, weight (of both parts and assembled scope), need of help from other people, setup time etc. come into play with different relative importance depending if the observing site is the home balcony, the backyard, a remote dark site, the place where one is going to spend a short or long vacation, etc. A scope can be portable for certain uses but not for others: e.g.: a scope that barely fit into the car boot may be suited for remote dark sky use, but not as a companion for long vacations, where the boot space must be shared with a lot of other stuff. To make another example, a scope with wheelbarrows or a transport cart may be well suited for backyard use if there are no stairs between the storage place and the backyard, but, in case of stairs, handlebars are useless and the heaviest part and time demanded to setup become the bottleneck. Thus analysis of how one is going to use a scope is mandatory to be able to evaluate if it is suited for the use.
Power, on the other hand, is a property that exhibit a more continuous behavior: the larger the scope the more things it will show. However a distinction between scopes for visual use and scopes for photography must be made here, because factors that improve "power" for visual use are different from those that improve photographic use (large diameter for the former, stability and precision of mount and sensitivity of detectors for the latter). A scope that must be optimized for both portability and power for visual use is going to be rather different than one optimized for photography. This review is about visual use and scopes optimized for it.

My first scope was a Meade ETX 90 Astro (the non-EC 1996 version, which had a RA drive; a picture of mine, standing on a Manfrotto tripod, may still be found here: http://www.weasner.com/etx/techtips/tripod_manfrotto.html ). I bought it in summer 1999. As my first scope I did not want to invest much money. The scope soon turned out to far exceed my portability requirements: I was able to pick the whole scope-tripod assembly with one hand, and place it on my terrace in a matter of seconds. But the views were far below my expectations. On Deep Sky it needs a lot of what I would call "adverted imagination" to see anything but the most basic features (for example: M13 looks like a fuzzy blob, and on M57 it is very difficult to spot the donut shape). On the planets it shows some basic features (like the A and B rings of Saturn, some banding on Jupiter etc.) which may be certainly interesting for a newbie, but again not enough –for me– to sustain interest for long times. A feature the scope is also completely missing –in my opinion– is rich field (with a maximum TFV of about 1° @ 2 mm exit pupil the views indeed are not properly "rich fields").

Only a few months later I decided to upgrade to a Celestar 8" SC. It was a great jump in viewing experience (M13 shows stars, M57 is a donut, Jupiter shows the GRS etc.). For balcony use it was almost as portable as the little mak. In fact I used to keep the scope assembled in my living room (my wife was not very happy, though). The scope is light enough to be transportable fully assembled, on the terrace. It served me for a longer time, nearly a year and a half, during which I discovered other types of use and related portability issues. First of all, to transport the scope to dark sites disassembly and reassembly is necessary. This unfortunately needs tools and thus is not straightforward: first the base of the fork mount uses three standard bolts to attach to the tripod and requires a wrench for assembly/disassembly; second, and far more problematic, the finder is not quick detachable and needs to be removed by means of a screwdriver, to place the scope in its box for long transportation. Remounting the finder calls for realignment. These two facts strongly discourage frequent assembly and reassembly (and in fact for balcony use I preferred to keep the scope assembled in the living room). In addition the OTA carton box (which is larger than strictly necessary) plus tripod used almost all the space available in my car boot. Because of this, I was not able to carry the scope with me in my summer 2000 vacations that I spent in a pretty dark sky in Central Italy. So, I regretted having sold the little mak, that I could have carried in vacations. As for what concerns performance limitations - besides the obvious limits that depends on being "only" 8" in diameter- I found that the small size of the finder (6x30) made finding DSOs very difficult in light polluted skies (i.e.: at home).

In February 2001 I bought a Konus Vista 80 (a kind of Short Tube 80). Its intended use was twofold: a) as a scope to carry with me in summer vacations, b) as a helper scope for star hopping. The first time I carried it to a dark sky, I discovered the real meaning of "rich field". In fact, at low magnifications the views were so bright and rich of stars that we (me and friends) spent all that night touring open clusters and the winter Milky Way. At the end of that night I pointed M51 which was high in the sky and to my surprise, at 28x (Pentax XL 14 eyepiece), the double galaxy disclosed it spiral structure, which I had never seen before so bright (I think the reason was dark sky combined with perfect dark adaptation). I carried the scope with me in 2001 summer vacations (see section "Il Forno Antico" on my other article: Astronomy and Farm Holydays in Central Italy). The scope is well suited for travel. Its limit is that, being a fast achromat, it can only provide rich-field low-magnification views. I found that 40x was a magnification at which stars begin to loose their pinpoint appearance and higher magnifications, even if possible, are little appealing. In a certain sense the little achro could be complementary to the little mak. They should be kept both, but of course carrying both in travel (which is their primary use and justification) would void the same portability requirement they should serve.

Thus, the following year, in preparation for summer vacations 2002, I began considering a little apo as my travel scope. I ultimately bought a Borg 76 ED. As many reviews already pointed out - and I agree- this little apo gives good performance for its price. The resolution is not a the very top of its size class, as its star test indeed reveals a combination of residual aberrations (unavoidable trade-off in a two-lens design that has photo aims too). Three inches apos of other brands can be a little better here, but at much higher cost. On the other hand, however, extreme performance for planetary use was not what I was seeking for a travel scope: after all planets can be viewed the remaining part of the year with the C8 from my home light-polluted sky. The apo turned out to be better than a Vista-ETX combo (beating both on their terrain), anyway. Same rich field than the Vista. Pinpoint stars and views that remain sharp well in excess of 100x. Brighter views and greater limiting magnitude than the ETX (the 13.1 star near M57 was easily seen). No color fringes at all at the moon limb. Planetary views begins to soften above 150x (here is where other above mentioned first-class costly three-lenses/exotic-glass designs can do a little better, I think). On the deep sky the advantage of medium and high power views (~100x and above) compared to the 40x limit of the Vista put this scope in another league. Placed on a Manfrotto tripod with a fluid balanced head, and being equipped with a 7x50 finder this scope made me love star hopping and changed my opinion about alt-azimuth mounts. A photo of the scope, mounted on the fluid, balanced, head can be found on a friend website here. I think that having experienced how easy it is to use an alt-azimuth fluid mount, and how easy it is star hopping when aided by a good finder, prepared me to accept the dobson style. The only limit of this little apo is indeed the aperture itself. 3" makes the scope well suited for travel (especially air travel - which I have not yet done however) but for most travelling a larger aperture is still feasible and could give deeper views.

If the ETX-Vista/Borg class clearly emerged as the scope type suited for travel and vacations, the C8 remained my scope for home use as well as for one-night dark sky exits until spring 2001, when I upgraded to a Meade LX90. The motivation was twofold: a) to fix the assembly/disassembly limitations of the Celestar 8, and b) to solve, by means of the computerized GOTO, the pointing problems in light polluted sky plaguing the 6x30 finder. Incidentally the LX90 optics turned out to be better than those of the C8 (I do not mean that all LX90 are better than C8, only that mines were, maybe I had a bottom line C8 and a top line LX90). Anyway the new optics were really good, showing a nearly perfect star test and more color and contrast on planets; and that was an unexpected plus in the upgrade. For the LX90, mounting the fork/OTA on the tripod was much easier, needing only the manual turning of a big knob. The finder is quick detachable. The whole assembly of the scope needs little time, and I began (with my wife's relief) to store the scope disassembled in its original carton in the garage, which also made me more inclined to observe from the garden rather than terrace (with better seeing) and more willing to load the scope into the car for driving to dark sites in the week-ends.

The choice

By the end of 2002 I had clearly understood three types of use for a scope:

  1. At home, from terrace or garden/backyard (about ~30-50 times a year).
  2. At dark sites in the week-ends for one-night (about a dozen times a year or fewer).
  3. During summer vacations (once-twice a year for a total of ~20 nights a year).

Most important: I had a clear picture of how I was going to actually use the scope in the three cases (e.g.: transport needs for each case; that I wanted to be able to assemble and move the scope by myself alone; that I was interested only in visual astronomy and thus wanted the largest possible diameter; etc.).

Other people might have a different list/ways of use (e.g.: want to do photography; do not mind if they need a companion to handle the scope; leave the scope fully assembled in a garage and roll it out; etc.). What is important however is that, before engaging in an endless search of a perfect scope one tries to list and describe his own types of use (a good exercise might be writing the list of uses, and for each writing how the use is carried out).

I began to reason about reorganizing my scopes system. At the beginning I thought that a big dobson, in the 16" size could be the maximum compatible with uses 1 and 2 but not for 3. For use 3, a scope slightly larger that the little apo (maybe like a Portaball 8 or a Teleport 7) perhaps could have been feasible, providing the maximum performance for that case too. In spring 2003 it happened that I had to replace my aging car. The new car was thus chosen with a larger boot, which solved the problem for use 3, namely that of transporting a large scope together with all other stuff in summer vacations. Now a 16" size dobson, if carefully selected, could be compatible with case 3 too (the little apo could be kept for possible air travel, whereas the LX90 was going to be sold).

I considered many dobsonian telescopes of the American market and all those that I was able to find in the EC. One manufacturer draw my attention because living only 30 km from my home. A phone call and, at the beginning of march I visited his workshop. This was determinant (I have to admit that in other cases I was a little worried by some difficulties, like long backlogs, EC custom regulations, shipping concerns, etc.).

His name is Germano Marcon and he lives in Paese di Treviso (30 km from Venice in the NE of Italy). He does not have a website yet and can be contacted only by phone or e-mail (gercomi@libero.it). He plans indeed to setup a website (which is ready but not online) but, as he explained to me, he fears he would be overwhelmed of orders and no longer be able to produce enough scopes. In fact he is a craft man who was a furniture maker. Being interested in astronomy he learned mirror making many years ago, and now the only kind of "furniture" he makes is the structural part of his scopes. But he also makes mirrors and he used his mirrors for many years in his personal scope(s). So he makes the whole scope system, indeed..

A visit to his workshop, in early march 2003, swept all my remaining doubts. We discussed of many aspects of his telescopes, how he makes them, my options, etc. I was very interested in seeing a 20 inch Sitall Ritchey Chretien primary in the workbench. Then we assembled, disassembled and moved a 16" structure with a 16" glass blank in place of the primary mirror. That helped me to get a precise feeling of the difficulty of moving and setting up the scope. I verified that I was able to lift the mirror box by myself alone, place it into the car boot, etc. Having solved all my doubts, about my ability of being really able to move and setup the scope by myself, and being satisfied of the scope and with the kind answers received, a few days later I placed my order.

The scope price was 3750 EUR (to which Italian –and I think also EC– customers must add VAT). I got a little discount of 200 EUR, since the price was inclusive of crating (but not shipping) which I saved because I will have retired the scope in person. The price also included a light shroud and a 9x50 Orion finder.

I picked my new dob from Germano's workshop three months later, on Friday, June 20, 2003. As always happens to me when I buy a new scope, that night was very clear :-) (I know... you all have to wait two weeks... :-P). That night sky from my home backyard was a rather unusual mag 5+.

Mechanical construction

The scope adopts some unusual solutions, which give it a different look than the "classical" dobson.

The first difference is that it has four beams instead of the typical eight struts (Fig.2). This gives a completely different look to it (aesthetically, however, I would prefer the nicer 8 struts), but also impacts on performance and setup. I believe that setting up the scope is somewhat simplified and faster (see steps in Fig.2). The four beams are clamped at UTA and mirror box ends by means of threaded knobs. Proper seats allow repeatable positioning, and the beams cannot be misplaced or exchanged because the seats/knobs are made different. Repeatability of parts positions is good enough that I fine re collimate only for magnifications above 400x for planetary viewing; for most of my observing sessions I do not re collimate at all.

When I first saw the four beams design I was a little concerned with possible lateral compliance and inability of maintaining collimation. At first I though that, in case of need, I could have fixed any compliance easily, by means of wire diagonals. Fortunately it turned out to be not necessary. As I have long experimented, the beams are stiff enough, and collimation is maintained perfectly as the scope moves, pointing to different positions in the sky, from the Zenith to near the Horizon. In fact the beams have sufficient cross section for that purpose. They are made of hard wood, a material (Germano - consistent with his origin as woodworker - seems to be biased towards wood) which also provides excellent damping of vibrations.

Fig.2 - Telescope design is four beams. Depicted is the setup procedure (click to enlarge).

The steps shown in Fig.2 needs only a couple of minutes. The total time needed to be ready to view (from car boot to view) is less than five minutes. For comparison it is about the same setup time needed by my Meade LX90 (actually mounting this scope is a little longer, but then the SC needs to go through the alignment procedure, thus the total time is the same).

The second unusual solution of this scope is that it has bearings on both axes: the altitude axis runs on two journal bearings at the sides of the rocker box; the azimuth axis is made by three ball transfer units placed in the ground board, which ride on a circular raceway cut on a metal sheet placed under the bottom of the rocker box. Two clutches allow to control axes friction: altitude clutch is controlled by the two knobs at the sides of the journal bearings; azimuth clutch is controlled by a lever on bottom of the rocker box (Fig.3).

Fig.3 - Knobs (left) and lever (right) which control clutch friction on altitude and azimuth bearings (click to enlarge).

The advantage of bearing/clutch assemblies is that friction can be tailored to personal preferences: from stiff to virtually zero. For example one may release the clutches to the point that the scope moves under its own residual imbalance and that a blow in the horizontal direction makes it rotate about azimuth axis for a complete turn! My scope was built balanced for normal eyepieces (or maybe, I chose a heavy finder) but I use Pentax XL eyepieces that are heavier. Thus I eventually bought some rubber coated weights, which I place into the mirror box to obtain a perfect balance. I discovered that I generally prefer the minimum amount of friction. Once the scope is perfectly balanced for the eyepiece I use (and Pentaxes all weight the same), I release the clutches to the point that friction is barely the minimum needed to prevent motion during eyepiece exchange. The motion in these conditions is so soft that I can drive the scope with a feather push. Last summer I found myself to instinctively track Mars continuosly. However, such a soft setting is not liked by most of the users that happened to look into my scope, because "too responsive" for them. I have to admit that, in many occasions, I use stiffer setting too, especially when I do not want to pay too much attention to careful handling. For soft setting the azimuth motion occasionally presents hints of jamming. This must not be confused with stick-slip phenomena however. It is due to the fact that the ball transfer units used for azimuth axis rest on a bed of recirculating spheres, the motion of which however sometimes jam. In these case the scope needs to be gently shaked. This is a limit of the ball transfer units. Germano told me that new units, which uses alternated steel and Teflon recirculating spheres should eliminate this little defect.

Other features of the scope are shown in Fig.4.

Fig.4 - Details of the scope (click to enlarge).

Consistent with the four beams design, the UTA is square, and partially fits into the mirror box (see also Fig. 5 below). The spider uses 4 tangential vanes. The mirror is 2" thick, made of Pyrex. The mirror cell is 18 points (not shown) and is made mostly of wood (again Germano's bias towards wood), which, to provide enough stiffness, have parts of unusual thickness (compared to the thickness of metal rockers and triangles). Besides this, it works fine, as I have never seen any trace of miscollimation induced by flexure of the cell structure. In addition wood provides a good feeling of safety when travelling on bumpy roads: you never hear shocks due to the mirror hitting cell supports. Lateral support is guaranteed but a sort of simplified whiffle tree design (four wooden posts 90° apart). Again I have never seen signs of astigmatism due to them, except when I re collimate with the scope pointing low (in which case the friction induced by posts and acting normal to the mirror plane may induce flexure): In this case it is good practice to point the scope briefly to the Zenith, to seat the mirror on the new positions of the 18 points and release posts friction (that is enough to get rid of astigmatism). During last Mars opposition, with Mars low on the horizon, the above procedure yielded astigmatism-free images. As a finder my scope sports a 9x50 Orion finder, which I is very useful for finding DSOs, especially when comparing finder views with Uranometria maps. A little more difficult, with this finder, is aiming the scope, because of the relatively narrow field of view. However, looking with both eyes is a trick that works for the purpose of gross pointing. As a focuser the scope has a helical Borg unit (model 7427). There is no special reason for this choice, except that it was promptly available (maybe one reason Germano suggested me the above focuser might be that it uses an internal telescopic structure which combines low profile with enough focus travel and without needing to enter inside the optical tube of the main instrument). Lastly there is a light shroud included in the price (not shown in pictures).

Portability

The telescope is very compact. The part sizes are carefully selected. Fig.5 compares the scope with my Meade LX90 8" SC, both are shown packed and ready for transport. As seen the amount of space needed is almost the same. The dobson does not need a box: it is its own box, whereas the original carton of the SC is used for the purpose. The overall size of the packed scope as shown in Fig.5, with the UTA partly inside the mirror box, is 525 x 525 x 770 mm (20.7" x 20.7" x 30.3"). The height (30.3") is relative to the focuser (the finder is not removable in my scope, but a simple dovetail would allow it to be quick detachable). In Fig.5 left, on the SC box stands a suitcase, needed to carry the eyepieces, LX90 finder, and also parts that are not needed by the dob, like the Autostar, spare batteries, diagonal, F/6.3 reducer etc. Conversely, inside the UTA of the dob (right) there remains enough space to put the eyepieces, filters and other accessories needed by the dobson, and still remains free space.

Fig.5 - Compact design, compared to a Meade LX90 8"SC, both ready for transport (click to enlarge).

If the volume needed for transport is similar, the weight however is a lot higher. In fact, the total weight of the dobson is 49 kg (108 lb). The weight of the mirror box plus rocker plus ground board is 38 kg (84 lb). In fact, a drawback of the bearing/clutch arrangement shown in Fig.3 is that the mirror box, the rocker and the ground box cannot be easily split, and must be carried together. If the mirror is left in place, the above assembly weights, as said, nearly 38 kg. I am able to lift it into the boot of my car (but I am 80 kg, 1.8 meters tall), or to travel say 10-20 meters, including a few stair steps, by resting the assembly on my stomach (this was one of the first things I wanted to check during my first visit to Germano workshop before ordering the scope). Nevertheless, I prefer, when possible, to roll the scope out by means of a wheeled table (seen in pictures above - wheelbarrow could be an alternative here). After all, I feel that the above assembly stretches my carrying ability, and that women, or men less robust than me, could have serious difficulties in carrying the assembly. In these cases the mirror (15 kg) may be easily removed (it is sufficient to release two post knobs) leaving an empty 23 kg assembly, much easier to carry (If you plan to follow regularly this procedure my suggestion is to have a mirror case fabricated for the purpose and to keep the mirror in it). Of course two people will have no problem at all to move the scope, even in the assembled state. I have noticed that there is a 4 kg weight in the box under the mirror (Germano told me it is an adjustable weight that serves to balance the scopes to meet different focal lengths requested by different users and/or different types of finders). I plan to make this counterweight removable, i.e.: attached to the bottom of the mirror box but outside, so that I can remove it before moving the assembly (this, in place of removing the mirror, should be enough for me to feel more comfortable, removing both the counterweight and the mirror leaves of course an empty 19 kg mirror box assembly).
If somebody is going to consider this scope however, I recommend him to carefully evaluate the weight issue, and possibly to consider a scope with a slightly reduced diameter (e.g.; 14 inches - ask Germano) which would be much lighter.

Optical Performance

Here is the point that everybody wants to know. There are two aspects: a) what to expect from a good quality 16" and b) how good is this specific scope/brand. The first part of the question is easy, and will be addressed first; the latter more difficult, will be addressed further below.

First light - the importance of thermal management

First 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. As I learned in the following days and months I did a mistake that very first night: I set up on a concrete pavement - see photos- that was hot all the day. This turned out to affect the dobson, whose mirror and optical path is closer to the pavement, a lot more than my reference point, the SC, smaller and atop of its tripod. In addition I had carried the dob in a hot car only a few hours earlier, and the thick glass mirror had accumulated a lot of heat. I was not aware yet of how long it takes for large mirrors to cool down perfectly.
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 (the mirror had not the starting heat accumulated during the travel by car of the day before, but I was still setting up on the hot pavement). In addition a more careful examination of the star test revealed that, contrary to the first day impression, there were differences between intra and extra focal images, which I will discuss below. Except for those differences, 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: http://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.).

The first days of use, because of imperfect cooling and setup on concrete pavement my observing conditions were thus far less than ideal. As a consequence in focus star light was diffused for many many rings around stars, forming blobs made of a swarm of points as seen in Koehler's simulations. The brighter the star the larger the swarm size. For example, the Double-Double looked like two touching or overlapping blobs with no clear dark space in between (which means 2+ arc seconds wide for magnitude 5 stars).

A fact that I soon noticed was that the light distribution caused by turbulence and imperfect cooling often peaks noticeably at the center of blobs and traces of the airy disk can be seen through the swarm of dots (as again seen in the pictures by Koehler). Smaller instruments produce a more uniform distribution - not peaking the same amount at the center - and, even if stars may appear as blobs of roughly the same diameter and, for example, the splitting of the DD may look similar or even better, in the large dob there happens two important facts that beat the bad seeing: a) dimmer and dimmer stars are smaller and smaller blobs in diameter and b) extended objects like the moon and planets show resolution far better than the blob sizes (details the size of the brighter center part of the blobs can be seen).
In fact, even the first night I was able to go to 380x on M13, (despite magnitude 5 stars were 2 arc seconds blobs, mag 13 were not) and the moon, before set, was the best ever image I had of it, at only 170x, showing much better color and details than ever in smaller scopes.

The deep sky of course is where a large dob literally "shines". M13, for example, is a complete different thing, even from light polluted backyards. From 85x to 380x its stars are always seen and the streaming effect is striking. Contrary to common belief the large diameter allows to work around light pollution and do some deep sky observing even from light polluted skies. There are many factors for this: first, brighter images permit to better perceive DSOs despite lower contrast due to light pollution; second, thanks to more light collection, nebular filters (especially OIII and the like) can be better exploited; third, the scope goes deeper in magnitudes showing stars that in smaller instruments need perfect dark skies; fourth, in light polluted skies often one is never dark adapted, and the more light collected by the larger instruments compensates for the imperfect dark adaptation.

The night of the first light (June 20) I checked the limiting mag on M57 field (http://c3po.cochise.cc.az.us/astro/deepsky02.htm#M57, http://c3po.cochise.cc.az.us/astro/images/M57!dss2_3.jpg) and found that the 14.7 star was easily seen (the LX90 side by side was showing only the 13.0 with difficulty). The central hole of the nebula was clearly brighter that the background.
Another striking difference with smaller diameter instruments is that stars are striking CoLOReD! I have been able to see clear colors on magnitude 10 stars and above. In many open star clusters red giants stands out and are easily distinguished by the other bluish stars. Actually the ability to see colors must have something to do not only with instrument size but also with other aspects like optical quality, smoothness, quality of polishing, reduced obstruction. Saturation of colors, indeed, is a feature that I have been able to notice on the moon (delicate hues of blue, brown etc.) as well as on the planets (e.g.: vivid orange, green, brown and blue on Mars) and which were not seen on the other instruments (except perhaps in the little apo).
Colors are so nice that bright stars like Vega (white), Arcturus (orange) etc. become interesting objects by themselves and occasional curious observers looking at them were really fascinated (not so in smaller instruments).
Small globulars like M56 and M71 were resolved easily even under light polluted skies. M71 in the milky way field reminded me of the rich field experience that I only had with my Borg apo under dark sky of Tuscany the year before. Rich field experience from the backyard!!

High resolution and under the dark sky

It took me nearly two months to fully understand how to manage thermal issues and get the maximum performance in terms of resolution. In the meanwhile I went to a dark site a weekend in late July and had my summer vacations (21 nights in a row) in a fairly dark Farm Holyday in August. A description of what can be seen under magnitude 6+ dark skies may be found in the last section (Borgo Poggiolo) of my article concerning Farm Holydays. Similar views were achieved during the July week end in the mountains (e.g.: the 16.1 star in M57 field was seen from the mountain too).

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.

Rating the optics

The above performance holds probably for any fair 16" optics, provided the scope is properly setup (collimated, in thermal equilibrium, far from local seeing disturbances and without Jet Stream and other natural causes of bad seeing). What emerges from the description above is the main reason for me for choosing a 16" dobson (the largest size suited for all the three uses above). Namely power. The power to see fine details that cannot be seen by smaller instruments, and the power of going deeper on DSO and being able to do some Deep Sky observing even from mildly light polluted skies (also compensating for imperfect dark adaptation, a fact very common in light polluted places).

But, how good are the optics of my scopes? I spent a lot of time trying to get a precise idea of the real optical figure and quality of the mirror. Germano –who figures optics by means of null tests in autocollimation with a quality optical flat– does not release any "numerical certification". Besides figuring for the optical null at the bench, he also assembles the scope and carries star tests and Ronchi tests under the sky. I received two ronchigrams he took during the testing of mine, based on a grid spacing of 6 lines per millimeter. They are shown in Fig.6.

Fig.6 - Intra (left) and extrafocal (right) ronchigrams taken with 6 lpmm (150 lpi).

I did the same tests with a Ronchi grating by Orion, and found the same overall appearance. The little undulations shown in Fig.6 is due to seeing, as in the live experiments they were moving and reversing signs at moments. I was not able to see any hooking reversing sign between intra and extra focal positions at the edge, thus I think the increase of brightness in the image above is probably edge diffraction.

According to the Ronchi test the mirror should thus be of outstanding quality. But the Ronchi test, (especially at "only" 150 lpi) is not that very sensitive. For this reason I extensively carried out star tests in many occasions and found that the mirror indeed does NOT show a perfect symmetric behavior. There are fairly obvious differences between inside and outside focus. I have been even able to reproduce the observed star test, by using Steve Koehler's Diffract.r, and by that way to obtain some idea of both the profile and amount of deviations. I will however not make any statement here of what I believe to be the "numbers" of my mirror. I rather prefer to rate it in terms of its performance.

According to Mel Bartels Rating Mirrors page, I will use two of his criteria:

a) The Bratislav's scale (Mel says 99% of all scopes he has seen seen fall into '4-10' bracket), which is as follow:

  1. Can't find anything wrong with it, absolutely perfect: '<expletive>' Yet to see one after ~25 yrs
  2. Defects visible only in extrafocal images, and only after extensive star testing in best seeing conditions ( << 1/10 wf): 'You lucky b@$#@rd!' Can count these on fingers of one hand
  3. Extrafocal defects readily visible,  but really minor ( < 1/10 wf): 'Excellent' Best examples of  best commercial telescopes (Zeiss,AP,Tak etc) Best examples of homemade optics
  4. Extrafocal defects fairly obvious, but in focus image still essentially perfect ( 1/10 - 1/6 wf): 'Very good' Majority of current 'best commercial telescopes'; best examples of mass produced scopes
  5. Large defects visible on extrafocal images, in focus image suffers only slightly ( 1/6 - 1/4 wf): 'Good' selected examples of mass produced telescopes, most well made amateur optics; some examples of 'best commercial scopes' can still be found here
  6. In focus image visibly suffers ( ~1/4 wf): 'Acceptable' good mass produced scope, most good large/fast mirrors I've seen
  7. Image deterioration serious, clearly beyond 1/4 wavefront: 'Light bucket' majority of older generation mass produced scopes, special purpose telescopes (astrographs)
  8. It's difficult to determine when scope is in focus at all ( 1/2 - 1 wf): 'If you're happy with it ...' unfortunately, not that difficult to find !
  9. Usable only at very low magnification  ( ~1 wf): 'I don't want to have anything to do with this one'
  10. Absolutely useless: '<expletive>' unlike 1, I've seen these :-)

b) The roll-off magnification criterion:

  1. Mirrors that can sustain 35x to 50x per inch of aperture I rate as excellent.
  2. Mirrors that sustain 25x to 35x per inch of aperture I rate as good.
  3. Mirrors that sustain 25x per inch of aperture are acceptable.
  4. Mirrors that sustain 15x to 25x per inch of aperture are poor, usable only at lower powers.
  5. Mirrors that fail at 12x per inch of aperture are plain just not finished.

According to the Bratislav's scale, I would rate my mirror as grade 4, maybe 4 but tending to grade 5. In other words, extrafocal defects are obvious but in focus image should be near perfect or suffering very very slight. This rating considers the appearance of out of focus star test images, the low brightness of the rings as seen on the night of August 3 (and a few others, when the rings were also spotted), the kind of details that I was able so see in good seeing conditions at high magnification (e.g., recall the "X-ray" views of M13), and ultimately the estimated figure that I obtained by simulating the star test with Diffract.r.

According to the roll-off criterion I would rate my mirror as grade 2 at least, because it is clear it can easily sustain ~500-600x. But it might also be grade 1: simply I run out of magnification at ~600x and cannot tell which is the real roll-off magnification. Also, I consider the "roll-off" magnification as the magnification which still provides views that are essentially sharp. In other words the magnification at which the image "starts" to soften. Of course higher magnification may be used with softer image, but I do not consider the ultimate magnification before image break-down as the "roll-off" limit.

Besides overall figure, I think however there is another factor that should be considered in evaluating mirrors: namely the contrast that results from good polishing and smoothness of the surfaces. This factor is perhaps never considered enough, and has nothing to do with the large scale profile. A reading of these pages 1 and 2 may be useful in this light. What I mean with term "contrast" is the contrast of the MTF curve in the high-contrast low-spatial-frequency region. High contrast here means no halos, probably color saturation, and an overall pleasant image. Indeed I think this kind of contrast is paramount to see dim stars near the moon limb, the dark side of a 5-days moon, the translucent Crepe ring of Saturn, faint nebulosity in rich starry fields etc. (and these are all features that this scope impressed me for having since the beginning).

Conclusions

The choice of a scope is a difficult compromise between power and usability. For visual use I believe that a dobsonian the largest diameter that still is compatible with planned ways of using it, may be the optimized solution. For me it was a ~16" dobsonian, that I can transport in vacations (the most demanding of all my ways of using it).

I bough a scope of which I am happy. Of course it might be further improved: for example improved ball transfer units on the azimuth motion, pre-built active cooling system, a lighter design (removable mirror, with mirror box, removable counterweight, etc.). The overall performance and value of this scope in any case is good, and I recommend it as a valid option.

The manufacturer, Germano Marcon, does not yet have a website (to my knowledge it is planned to be www.arietetelescopi.it) and can be contacted by email at gercomi@libero.it (or alternatively at arietetelescopi@libero.it or by phone at +39 0422958670).