7" f/12 iStar folded refractor 8: Thermal effects of the folding mirrors.

*

I have been considering the thermal mass of the folding mirror blanks as a potential problem. Perhaps one which would require forced ventilation or skeletonizing of the mirror cells and retaining shells. Then it occurred to me that I usually keep the OTA in unheated accommodation. The temperature of which hardly varies by more than a degree or two from inside to outside during the cooler half of the year.


Slitting the shell went smoothly and rather quickly using a fine blade in the jeweller's saw. Marking the slightly tapered tin needed a postcard to draw a line from both sides and then midway between the two with a sharp pencil.  Having to drag the workbench outside into better light did my back no good at all! A folding chair allowed me to get closer to the work without too much pain.

In summer the very short nights of semi-darkness at 55N do not make observation very practical anyway. Moreover, the OTA will be constructed from birch plywood. Which provides easily enough  insulation, in a well closed box, to slow any heat loss or gain from or to the air or glass within the [sealed] OTA. Since the flat mirror blanks are already near ambient they have little need to radiate any stored heat away. Nor are they exposed or enclosed in a [super-cooled] metal OTA. So need not radiate to the cold night sky.

The mirror blanks are of Zerodur so there should be almost zero thermal effect on the mirror's optical flatness under any normal circumstances. If the telescope were to be used for solar observation, without a full aperture objective filter, then the mirrors might slowly warm up. The likelihood of my using the telescope like this is very small. The cost of a few square inches of solar foil is as nothing compared with a Herschel wedge. Any likely improvements in clarity over Baader foil are very unlikely to sway me to invest in a costly Baader solar wedge. So the foil filter will keep all of the sun's heat out of the telescope when I do choose to look at the Sun.

Adding hefty lumps of metal to the OTA, other than the inevitable objective cell, makes little sense. It would add unwanted thermal mass. So the cells are best made of plywood too. Which has low thermal mass and low thermal capacity into the bargain. Common sense suggests that the supporting plywood disks for the cells should still be well ventilated. Which is easily achieved in plywood by means of wood drills and/or augers. Leaving behind the supporting ring and the mirror contact pads.

The 5" mirror propped up in its newly slit shell indoors. The gap (at 5 o'clock) is almost non-existent. I'm rather pleased how well it turned out as I was expecting a much larger gap. A better image taken outdoors will follow.

I have come to the conclusion there is really no need to retain the full 3" depth of the [baking tin] mirror shells. So I shall turn [or rout] closely fitting disks to go behind the mirror banks when they are up against their retaining lips. I shall use felt support pads behind my 10" mirror at the calculated points. These pads are meant to protect furniture from ornaments or plant pots. The pads should be ideal to support the mirror blanks without local pressure. Distortion of the blanks must still be avoided regardless of OTA orientation or temperature.

A hinge will be fixed between the cell backing disk and the supporting structure for collimation. It is fortunate that the mirror cells do not need to be minimized in size nor cleared of protrusions to reduce diffraction effects. The cells do not obstruct the light path so cannot cause such problems. The rims of the shells will reduce the clear aperture of the mirrors by only 2mm. Many argue that the extreme edge of flat mirrors should not be utilized to avoid potential turned down edge from polishing. Many mirrors would probably have a far better figure if the edge is ignored. The flat, optical folding mirrors can be moved away from the objective slightly to avoid any vignetting. Which might add a little to the overall length of the OTA but will not reduce the clear aperture of the objective nor the folding mirrors. 

Would you believe it!?! The moon is high overhead in a clear sky and I can hardly lift myself out of the chair, let lone lift a telescope. I was going to cut the disks for the cells with the router but couldn't manage its weight with my bad back. This is confirming my need for a compact telescope which can be rolled out, ready for use, preferably already on its mounting.

Progress has been non-existent due to a combination of a painful back, constant sub-zero temperatures and now snow. The forecast is for rising temperatures so I may be able to play in the workshop soon.

A new week and a completely new temperature. 43F @ 11.30am! Last week it was -8C, 17F.

I was going to rout the backing disk for the mirror retaining shells. This would leave only a 1/8" 3mm hole in the middle offering some flexibility in layout and mirror support. The problem then is working out of doors in drizzle. It involves carrying a folding workbench outside which is never kind to my back. The old B&D benches weigh a ton and are an awkward carry. Using electric tools out of doors in the wet is neither safe nor sensible.
 
I'll just have to drill out the center hole and spin the disks on a threaded rod mandrel in the lathe. This also provides fine tuning of disk size and fit in the shells. The router is more hit and miss and can leave a ragged edge which when sanded can cause rounding. Which doesn't provide quite the same support as a nice true parallel/cylindrical edge. Choice of material thickness of the disk suggests extra weight should be avoided. Though hole saws can considerably reduce the volume of heavy Birch plywood. Or even use a saw to remove material in larger segments.

I'm still undecided as to retaining the shell's rolled rims. The baking tins were 3" deep because I couldn't find any 2" deep in the sizes I wanted. The extra inch adds another inch to the OTA's length unless the collimating cells project. Not a serious problem except that it adds unwanted vulnerability. The OTA cannot be simply put down on one end without considerable care. Better, surely, to sink the collimation screws beneath the OTA's end faces?  Simple hex socket head screws should suffice to allow occasional, mirror collimation. If it needs collimating every time it is moved I shan't have done a very good job of supporting the tertiary mirrors.

There still remains the matter of optical layout. I had intended a 3-fold parallel layout with two mirrors.  A cross or figure-of-4 layout has certain advantages for objects at modest altitudes. Over a certain 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, maximizes the sizes of mirror by having smaller angles of reflection. It can also be neatly arranged in a simple, narrow, but tall, plywood box. The dewshield can be added once the box is mounted.

*