300F6 “LIGHT”: Mission (equilibration) impossible?….A solution is to equip it with a disc brake!
From years, in his beautiful box taped ZAOT (the brand of my favorite aluminizer) I had that mirror Ø305 F 1830 unused.
I had bought it for a short time before because it was "gray", i.e. with a good parabolic surface, but happened “among head and neck" (like we say in Italy meaning "by surpise")”, to a “young mirror maker” inexperienced, before he have completed the polishing of the surface. Mirror that I had polished, finished and then parabolized from myself, and then tested at its "first light" within the structure of another my homebuilt dobson 14 ' F5 type Obsession, exploiting the fact that the focal length of the 360F5 is almost identical to that of 300F6, and then trial and star test could be made in the same structure, without changes truss frame.
I in fact for a long time feed the idea putting that mirror in the construction of a fourth and final dobson type "light", lighter than 14 ", but also a little more robust and easier to collimate in comparison the other my homebuilt ULTRALIGHT Strock 250 F5, as a little luggage “"cabin" model for carry-on plane.”, weighing only 9 kg complete and ready to observation.
The dual interesting perspective was preset that: If at any time after construction, the experiment "300F6 light" could not be satisfactory, It would start an option 2, with the refiguration of the mirror with carborundum grit 120, that with a bit of gym (better in winter!!) would lead to , deepening of additional arrow 1,5 mm shortening his focal to the F5 ratio.
The advantage of this drudgery would see the recovery of entire light structire, with simple shortening of truss tubes of about 30 centimeters, and a further reduction of the base diameter of about fifteen centimeters.
As a truly "light" telescope with such a damned long focal length, It seems very difficult to balance, I needed to prepare a robust structure on which to apply my experimental constructive variants, among which the number one:
The "parking disc brake".
Brake that have the function of keeping blocked the vertical movement of the telescope pointing in any direction, almost regardless of the weight of the secondary cage.
And then You have to move the instrument vertically, It is expected that You must push a brake lever type for bicycle handlebars, BUT applyed to the truss tube of the telescope, which opens a gripper biting on a half metal-disk applied to one of the “side bearings” (in form semicircular), freeing the azimuth instrument movement, otherwise blocked.
The liberation movement from the brake is made by easy and instinctive implementation, because is obtained “"taking in one hand"” one tube of the truss-frame to move the telescope, at the same time with the same hand You push on the bicycle brake lever applyed on this tube, and tightening the two objects together (truss tube and little brake lever in the same hand) make the shifting of the dobson telescope.
The pictures in the gallery that follow, are more eloquent than any word, and display, moreover, the type of lever with fastening ring openable, for Bicycle handlebar tube diameter 22 mm (..and off course, for truss tubes of same diameter).
However, some more information on the object is not bad:
- I went to look for the necessary friction using "classic" bearings , with the usual Teflon coupling-HDPE having sandy surface (equal worldwide to the american Ebony star type).
- The primary cell is standard, and made with steel , with sling for mirror support belt, and collimation screws operable from the back. That is, on the classic mode Kriege & Berry.
- In order to use also on this telescope, my passive pointing computer Jimi NGC MICRO (already in use on my 14 "F5), I reserved the space to House on the two axes, the two optical US-Digital encoders by 10 thousand "tics" every turn, between the mirror box and the underlying rocker box, and on a side crescent bearing extended to its center of curvature.
- The Focuser is from 1 "1/4 , and has been extracted with a late from a black colour nylon (rilsan) cylindric bar . Its weight finished is 40 grams (costs less of 30 euro, including a rod of black rilsan), and is made by turning on a spartan design, with a focusing thread on its diameter 40 mm, having step 3 mm, but with spiral having "three principles" .
In practice it is simply three parallel spirals step 3 mm moving the fok of 3×3 = 9 mm each turn.
- The secondary mirror is diameter 63 mm, purchased GSO (china) premounted on its plastic support.
It was taken a little bigger than it should be, in order to eventually extract more focus 90 mm, useful to adopt a (possibol future) binocular vision. - The square framework of secondary support is well as the Focuser, in light nylon (rilsan) black, with M6 bolts which are also lightweight because made by classic wite nylon (rilsan).
- The truss frame is realized in aluminium tubes 22 diameter×1.5mm thick, suitable for mounting auxiliary brake lever bike type for the telescope release.
They are mechanically paired in 4 groups of two canes (but with “hindsight” they could very well have been transformed into three groups by two pipes, with saving a third of their weight).
The mechanical coupling of each pair of tubes is obtained at the top of the truss by means of a short stretch of aluminum angle, so that by installing each pair of pipes, the angle provides automatically to the secondary case, a simple, safe and especially self-levelling and self-centering support.
Further benefit of this choice is that the eight lower attacks the truss, can be simple threaded studs outgoing from the primary case , Thet means the truss tubes have no more need for clamps inclinated to guide the mounting of the telescope.
- The brake caliper is made with a gripper with a robust car charger spring, within which JAWS I glued two blocks of wulkollan what organs of grip and friction on the disk.
- The disc-brake is actually a steel half-disk applied to the Crescent side bearing, I have cut it with manual electric jigsaw, from steel plate thick 2 mm.
- The outer wooden structure of the base is designed to accept the same wheelbarrow poles already in use for my other dobson 14 "F5, useful to move the whole telescope mounted and in use, during the bservation evening. Also this wooden external structure would allow the possible addition of lateral friction pads, in balancing..
- And the more 3 big feets adjustables (to easy level your telescope, improving highly the PUSH-TO computer use) they are my "deja vues" at the cost of 2 euro each, described in another article (See here in this same bolog the article titled "INCREASE THE PRECISION OF DIGITAL CIRCLES POINTING, THE DOBSON, WITH THREE ADJUSTABLE LARGE FEET ... AT 5 EURO".) .
CONCLUSIONS :
This telescope optically goes very well. Mechanically it is docile and fluid in all movements;
- The complete base weighs 17 kg ;
- The secondary cage complete with quickfinder and optical finder 6×30, weighs 1700 grams;
- The truss tube 8 tubes Ø22mm x 1480 mm, weighs 2320 grams,
For a total weight of 21,3 kg complete of my heavier eyepiece, Explore Scientific 14mm 82°FOV.
For now uses a bag of lead pellets as a counterweight 2 kg (replaceable with elastic as dynamic counterweight in future), in addition to weight (1300 grams) the portable lamp Beghelli Maya type, modified with dual battery to provide two speed at the fan making turbulent "boundary layer" extraction, with regulation achieved using electronic (as described in another article (See in this same blog the article titled " 0-6-POWER SUPPLY 12V DC FOR SUCTION CONTROL DOBSON") Here).
The lead shot bags are convenient and much less painful if they fall on the feets, compared to the impact of equal monolithic weight.
This telescope had the honor of publication in the November issue 2016 of the American magazine “Sky & Telescope”, under the columns of: “Astronomers Workbench”, on pages 66-67, (see the article in the gallery) With title “Braking Bad Habits, How to make your lightweight scope behave” (In Italian: “Frenare le cattive abitudini, insite nel comportamento dei telescopi dobson di tipo leggero”), by posting also the following link to other works: here
THE MOST ORIGINAL DEFECT of this telescope was the height of 181 cm from the ground to the zenith eyepiece, so to observe at the zenith I needed al little stool high 15 or 20 cm.
For this reason, I then modified the telescope in a version “LOW-RIDING” that I present in another article here.
A gallery with some photos is certainly the best technical testimony “multi-Language”, a comparison with thousand words otherwise needed for a sufficiently comprehensive explanation.
