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There still remains the
matter of optical layout. I had intended a 3-fold parallel layout with
two mirrors. A crossed or figure-of-4 layout has certain advantages for
relaxed viewing of objects at modest altitudes. Over a certain object height it is arguably best
to use a normal star diagonal for comfort. Below that point the star
diagonal become a nuisance unless the observer sits to one side of the
OTA. The more complex folding forms, mentioned above, require larger flats
or a more bulky OTA. The simple 3-fold design is compact and maximizes the
sizes of the folding mirrors by having smaller angles of reflection. The layout can also be
neatly arranged in a simple, narrow, but taller, rectangular, plywood box. The
dewshield can be added as a friction fit once the box is on its mounting.I have to admit to not really wanting a "Newtonian" folded design. Simply because I hope to be able to sit behind the OTA while observing. This allows a quick squint up the tube possibly through simple ring sights. It also provides easy access to a straight through finder. The relatively short OTA has only a small arc of movement at the eyepiece. Rather like using quite a modest refractor but far more bulky in girth only. Provided the observer can reach the eyepiece, in the middle point of its travel, then any object above or below should be reachable by bending or stretching slightly.
This
small movement can probably be accommodated from a comfortable chair. Or even an
adjustable height, padded
stool. I have an example of the latter but it needs attention to make
height adjustment far more sensible. At the moment it requires a long
bolt be withdrawn, the seat refitted as the bolt is pushed through the
seat support and a
finally, a knob tightened for security. Better, by far, to be able to
simply lift the seat off and then hook it back on over any one of a series of
rungs. This is easily achieved with a few lengths of studding and a
piece of channel section fitted to the back of the seat.Such seats are available but cost silly money which I'd much rather spend elsewhere. These seats are really designed to cope with working people of different heights. Once set they tend to remain that way. They cannot be raised far without the user's feet losing contact with the ground. Sideways stability can then become an issue on soft ground. An adjustable or folding footrest would allow greater variability of seated height.
There is little point in allowing horizontal viewing angles except for bird watching. So that reduces the eyepiece arc quite usefully to [say] 30 to 90 degrees viewing altitude. Lower objects usually suffer from poor seeing anyway. Besides, viewing objects at lower altitudes can be achieved simply by raising the observing stool. Or removing the star diagonal and looking straight through the eyepiece.The short OTA removes the problem of tube balance with changing eyepiece weight. Balance was a serious issue with the long straight tube and required a hefty counterweight.
A lower eyepiece height usually requires a lower mounting. There is no point in stretching just to be able to look at the zenith. Pointing at the zenith always expects the use of a star diagonal. This sets the minimum comfortable height of the eyepiece in use. The builder must decide in advance if that height should be reachable by nearly sitting on the ground. Or more comfortably seated at a normal office chair height. The former reduces the overall size of the instrument when mounted but my age greatly reduces its desirability. A more normal seated height makes a lot more sense.
I plan to mount the folded refactor on a Dobsonian style of mounting. Though probably not of the normal reflector style. That would demand very high sideboards meaning considerable weight to be dragged around. The rocker box would also get in the way of my knees. I am thinking more along the lines of a Berry-style, counter-weighted, offset fork. How to lift the fork to the required height needs consideration.
A tripod is a serious encumbrance for the user's legs when carrying or using it for observing. It is also difficult to make light enough while retaining enough stiffness. Nor does it readily lend itself to having wheels fitted. A four legged pier is perhaps more desirable. Four feet is far better than the usual three even if it does add some weight. The problem is a getting a stiff joint between the horizontal legs and the upright pole without adding weight.
Or, I could have a few prepared holes lined with loosely capped PVC pipe. These would allow the fork, on a pole, to be carried and simply dropped into place. There would be no need to carry the OTA from place to place until the mounting is safely set up. Nor even having the fork itself attached, if it can be easily fitted or removed from its supporting pole/pipe. The sunken PVC pipes must be capped to avoid becoming wildlife traps, tripping pedestrians or filling with muddy water. 15cm, 6" drainage pipes and fittings are readily available and I already have some useful lengths. I can't imagine a mere four foot length of 6" PVC pipe flexing much if securely mounted in the ground. My recent back pain has taught me a valuable lesson. I had completely failed to consider my expectations of weight lifting capacity into increasing old age. How I expected to lift a 40lb, 7' long OTA above my head well into my 70s I have really no idea.
Click on any image for an enlargement.
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