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By Ben the Ignorant
I wouldn't want to miss observing the Sun in a full-size 300mm filter, the detail compared to smaller instruments is such a joy to see. I do have an off-axis mask for my 300mm dob but not all the time. If daytime seeing is too bad for such a large scope, I'll switch to a smaller one, but the extra resolution and ease of spotting micro-detail thanks to the brightness is something I can't do without.
A sheet of 500mm wide Baader fillm allowed me to craft this. I had enough left to cut several smaller filters.
To protect it I made a storage box from cardboard of obvious origin. The box had to be custom-made to save space in my dwelling, and it also hosts a bahtinov mask for nighttime. At f/5 fine focus is not too hard with the smooth single-speed focuser, but in order to enjoy moments of clarity immediately at high power, I pre-focus with the bahtinov, which sees through turbulence, what a nice invention, and simply wait for calmer air. No need to rush to fine focus and waste precious moments. This mask needs to be copied in some sort of plastic, the cardboard trial proved the number and width of the slits are fine.
The rear of the filter shows the four clips mating the four button screw heads outside the tube.
And this lateral view shows the sandwiching. A rigid backing plus two layers of lightweight neoprene foam cut from a cheap mat in tiger-stripe deco, its poor taste matching its 7€ price. The bottom sheet used to be a shower booth panel, my neighbor had in its garage. Hey, do you need that? No? Okay, thanks!
The Baader film is held between the neoprene sheets by staples and many strips of tape. The neoprene is held to the shower booth sheet by little collars, and its edges are sewn together for a finished look. Oversize 10mm stitches didn't take too long to do, and the foam layers can be separated for replacing the film, something glue would prevent. Holes in the neoprene allow to check that the clips' bolts are always tight.
Close-up of a clip. They are made of a material whose name I forgot, I only remember it is made in the UK (thanks, guys!). This is a PVC foam that's much easier to cut, drill and sand than full-density PVC, and it's much lighter (thanks, air bubbles!), which is essential for an item that's at the front of a scope where leverage is maximal.
My GSO dob has four of these screw heads, one for each spider vane. They proved very convenient for attaching the filter. The black knob is an add-on to move the scope without having to grab the large tube with both hands.
Pretty obvious what we're seeing here. Note the clip is flush with the tube, and pressed hard against the scope's metal rim for a very secure and tight fit. Only four clips but eight attachment points, really. Building this with tight tolerances was more lengthy but more reassuring.
It is impossible to disengage a clip by accident because tolerances are too narrow, and the shower booth plastic tough spring action doesn't allow it. But should it happen three clips would hold the filter safely. All three layers of frame material are flexible, so, to remove the filter from the tube you just bend it at a corner while you lift the clip, and the clip disengages.
I already rebalanced the dob by pulling the 7-kilo mirror to the rear thanks to shorter and tougher cell springs but I'm working on a sliding counterweight made of pliable steel ribbon. Adhesive felt pads protect the paint. Thanks to the large hole the 300mm filter is not too heavy (and it's used without the finder) but the off-axis mask has only a 115mm hole, so it's heavier and may require this. While I study the combinations of heavy or lightweight eyepieces, finder/no finder, mask, filter, I can change the experimental counterweight by cutting off or adding pieces of steel ribbon. Little mounting nuts and bolts through the many holes, there's always one at the right place.
Ever hated to feel the Sun while loving to watch it? If you have no sunshield you did. Heat is a discomfort on the head, and light kills off contrast. So I cut a plastic folder in this weird pattern to allow sliding it between the shower booth plastic and the neoprene mat.
When it's stuck there it makes quite a large area of shadow, that's such a relief. But that's also only a prototype that wants to be made again with a better material. Has to be opaque, quite thin, very lightweight, but rigid enough. I'll stumble upon the right material sooner or later.
Recently I purchased one of these dobsons from ES following the good critics I read in several forums.
The Gen II includes some improved features with respect to Gen I, 2 Nylon pad brakes, Virgin Nylon alt and azimuth support bearings (instead of plastic), counterweight bar included with a couple of 1kg counterweights, a classical ES dovetail for the finder.
Some of these improvements were demanded by Gen I users, and are highly appreciated, but there is still some minor improvements that the final user has to apply. I will describe them lately.
The set up is very fast, less than 15 minutes, though the assembly of the upper cage needs some patience the first times is done. Regarding the optics, It is superb, though in my opinion the 75mm minor axis secondary is too big (other similar modes in that range use a 63mm one), it provides full illuminated views on my ES 100 degree eyepieces, the stars in the background seen in clusters like the NGC457 are astonishing.
The main mirror cell support is a mix of support concepts, a nine point mirror cell support with a couple of rubber wheels at 90 degrees, and 3 glued slings to the mirror laterals screwed to the back of the mirror cell support, provide lateral support for the mirror. The collimation as in Gen I is done from above in less than 5 minutes.
In the first light in semirural skies the views confirmed the good optics, as I said specially in star clusters the amount of stars which could be seen in the background increased with respect to my 200PDS, the globular clusters were literally smashed, and small planetary nebulae like M76 showed internal details.
Coming back to the cons, as in any Chinese made product there are quality problems not solved in this Gen II, the paint coatings from the secondary cage just jumped when I changed the position of the truss brackets.
When I tried to open the main mirror box for the first time, the scratched the yellow paint from the altitude bearings, a tolerances problem which I solved adding 2mm nylon washers between the altitude bearings and the mirror box.
More improvements to make on your own are to make a plywood cover to prevent moisture on the main mirror while waiting for the night, to put foam pipe covers around all the tubes, they make the winter usage of the scope more comfortable and at the same time help with the vibrations damping (which even with a heavy ES 20mm 100d EP where almost insignificant).
One more thing to add is a PWM motor controller, the fan only work at full speed, and there are so many nights when the temperature gradient does not fall fast and the fan speed can be left at a lower regime to save batteries.
In brief, the ES12'' Gen II, it is the Dobson truss scope with the lowest price in that aperture range, the optics is damn good, the mechanical improvement done in the GEN II, improve a lot the vibrations, and the movements of the scope, though a better control on the tolerances and the paint process should be done.
I attach some pictures taken at home and while the first light.
Given that assembling a skeleton tube dob in the field requires collimation checking, what models of say F7 to 8 upwards, 8 to 10 inches aperture are available, which would not exceed an OTA vertical height of 72inches/6 feet?
I like Portaball scopes, do they come with shrouds, or can shrouds be bought separately for them?