7" f/12 iStar refractor 9: Baffling.

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It took several hours to make and assemble five baffles neatly from thin, aluminium, flashing material. I marked all the circles out first with compasses and then cut out the circumferences with metal shears. Then I bored the centers to 12mm and spun them each on the lathe to mark out the apertures. Finally I cut around these slightly indented marks with a jeweler's saw. My hope was that the indented rings would act as knife-edge baffles leaning towards the objective. However, the thin metal was far too flexible to allow much depth of cut. In retrospect it might have been better to try and find some slightly thicker raw material. 

A little de-burring with an oval file and I was almost done. Except for drilling the pre-marked and center-punched holes for the studding. These were to hold them at the correct distance from each other and relative to the objective. Because of the fine thread it takes ages to run M4 nuts along the screwed rods! So I used a rechargeable electric drill to run some of the nuts on near the center of the rods to save time. Then it was just  matter of spacing the baffles at 40cm centers with a tape measure. I rotated duplicate holes in the middle baffle to avoid having to join the rods in line.

The problem is deciding what to to do about the first baffle behind the objective. Should there be a baffle right behind the objective to stop the baffle tree from sliding up and down inside the tube? Or a gap, with nothing at all, but requiring the baffles be fixed at some point? There isn't a lot of metal left after a 7" hole is cut out of an 8" disk. Nor does one want to lose clear aperture. Shifting baffles will risk losing the clear aperture one pays for if they move towards the lens.

The advantage of using screwed rods and nuts is the ability to place the baffles very precisely and to change them later if needed. Plywood baffles glued to wooden dowels requires far greater care in design and assembly. Otherwise they could remain in the wrong place forever.

The images show a rough mock-up and is not the final spacing. Naturally the baffles will be painted flat black to kill reflections. As will the inner surface of the telescope tube. 4mm rods was a good choice for strength and stiffness without adding too much extra weight. The flashing was the only sheet aluminium I could find locally but was ridiculously expensive.

Because the tube is slightly oval I had to make the rings undersized to avoid them jamming. Though I intended to scallop the baffle edges anyway. This is to allow any thermal air currents to flow along the inside wall of the tube. Rather than cascading over each baffle in turn, like a waterfall. Which might well affect the seeing. The scallops will have to be rotated relative to each other, on each succeeding baffle, to avoid direct leakage of stray light along the full length of the OTA.

I was determined to have an all-metal baffle system to avoid the danger of fire should I ever use a Herschel wedge for white light solar observations. The focused heat from the objective passes down the tube when it is aligned on the sun. Plywood baffles might just get a little too warm if the sun should drift off-center. Though I suppose the first baffle will shield the later ones where the most heat is concentrated closer to focus. Full aperture solar foil filters avoid this problem by reflecting the heat away before it reaches the lens.

After having some doubts about my scale drawing I found a roll of lining paper. This allowed me to draw a full size light cone using a 50mm diameter circle of illumination at the focal plane. This very quickly showed that my baffles could not be made to coincide with any likely joining place of the threaded rods. It is quite remarkable how a full scale drawing shows how far a baffle has to be shifted along to avoid vignetting. 

Now I shall have to reduce the circle of illumination to a more normal 30mm or even 25mm diameter. I had used 50mm to allow an SLR to be used at prime focus. I may still make a full aperture baffle to go behind the objective lens to tidy up the rod ends. Though I would rather not for fear of reducing the clear aperture. I wish I had made a full scale drawing first. I would then have the freedom to choose baffle spacing or the aperture of each diaphragm.

Fortunately it turned out just right using 6", 5", 4" & 3" holes in the baffles. Which reminds me I must make the holes into knife edges to avoid grazing incidence reflections off the baffle apertures. This is much more important for thick baffles but even my thin aluminium ones might as well be as perfect as they can be. Any stray reflections will lower contrast by scattering light into the field of view. The advantage of the refractor is the absence of obstructions in the light beam which can spoil other designs.

In the end I chose to use a 1.5" [38mm] diameter circle of illumination. This allowed my original, one inch stepped, baffle apertures with reasonable baffle spacing. Instead of making a thin baffle ring behind the lens I am going to slide the whole baffle assembly towards the focuser. I just need to ensure a fixing to avoid the baffles shifting about over time. Particularly if parked on its nose for compact storage. Having seen a refractor crash to the floor when it rolled off a sideboard I am squeamish about storing refractors horizontally.

I have removed the 3.5" baffle as superfluous but will probably make a 2.25" just to tidy up the rod ends. Anything smaller [like 2"] gets rather too close to the focuser. It may be more sensible to leave the choice of final baffle size and position until I know the exact focal plane with a 2" star diagonal fitted to the focuser.  I could arrange for the baffle rods to be fixed to the focuser back plate. Tiny, 4M exposed Nyloc nuts would look fine in that position. As would bronze or stainless steel, domed nuts. 

Peering into the OTA from the edge of the tube now places the edge of each baffle aperture perfectly in line. Though it is possible to see the inside wall of the tube, the reflection from the blackened surface will blocked by the next baffle along. The threaded rods now overlap considerably though this is not of much importance. Once a final decision is made with the objective lens fitted the rods can easily be shortened. As I have already mentioned, it is extremely time consuming running nuts up and down such fine threads!

Click on any image for an enlargement.

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