6" H-alpha OTA: Baffles:

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Tuesday 4th Feb. 38-41F, cool, but bright start, then dark overcast, then bright again.

The difference in scale between a 180mm [7"] and a 150mm [6"] objective and their respective cells. The 7" looks opaque due to misting over after a night in the cold observatory. Then being brought out into warmer, outside air. The same sometimes occurs when I open up the dome to sunshine after a chilly night. Hence the recent investment in dew heater bands. Clear skies for observing and imaging are far too precious to be lost to dewed optics!

I have found some scrap, aluminium roof flashing which could be used to make the baffles for the new 6". Metal baffles are always advisable for a solar telescope to avoid the risk of fire. 

I clamped two planks on the portable bench jaws with a tapered gap between them. This was to represent 150mm at the wide end and 60mm at the backplate/tailpiece. 

A quick guesstimate suggested the 115cm total length should be divided into 5 baffles at 20cm spacing. The baffles would be just under 160mm in outside diameter to fit comfortably, square-on, inside the main tube. The first baffle behind the objective lens ought not to be made too narrow. This isn't a real problem with an oversized tube. 160mm tube to a 150mm lens. There is no grazing incidence on the undersized tube just behind the lens. 

I used the tapered planks to measure the gaps between them at the marked baffle positions. 135, 115, 100, 85 and 70mm in series. The numbers were then drawn as a series of circles onto the metal sheet with compasses. With outer circles of 160mm using the same, center-punched centers to ensure concentricity between the circles. 

The outer circles were cut out first with ordinary, straight cut, tin snips. Followed by drilling a short chain of holes in each inner circle to allow smaller, curved jaw, tin snips to get a start in the metal. I then cut out the inner circles to make the baffle rings while wearing protective, rubber gloves. Quite a lot of effort is needed for the curved cuts and the grippy gloves really help here.

I smoothed the outer circles with the 12", bench, disk sander. The inner circles were smoothed with a "half round" file. Finally I flattened the baffle rings with a weighted plastic hammer using a block of wood as an anvil.

The next step is to stack and then drill all the baffles identically near the periphery. These holes are for small diameter studs [threaded rods] to hold the baffles at the correct spacing. Pairs of nuts will be run down the studding to hold each baffle in place. I find a rechargeable drill to drive the threaded rod makes quick work of this. Each pair of nuts being left loose until they are all roughly in place.

 I need to buy some studs from the nearest builder's merchants. Followed by painting the completed baffle assembly with matt black paint. The friction of the baffles inside the tube is usually enough to hold the whole baffle "tree" safely in place without any fixing.  I used exactly the same baffle construction system in the 7" f/12 refractor.[Image: Right] Thought the 6" will only need three studs thanks to the shorter overall length.

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