7" f/12 iStar folded refractor: Proof of the pudding.

The image shows the permanent, stubby dewshield on the objective bayonet plate with its protective saucepan lid in place. The short dewshield, with its firmly fitting lid in place, offers a safe housing for storing the objective in its cell. The short dewshield also provides some dew protection and blocking of stray light when the telescope is in use. The main dewshield is a much longer 10" diameter, thin aluminium tube which slides easily over the stubby dewshield when required. The cranked, solid, stainless steel handles provide safe handling of the objective when climbing ladders to slide the bayonet plates together. I am seriously considering making a concentric [rotational] bayonet fitting so that I can fit the objective to either the straight tube or folded OTAs at will. I only stopped using the straight tube due to weight issues. Had I thought of the bayonet principle back then I need not have become involved in the folded design and expense of optically flat, folding mirrors. It was the objectives own weight which pushed the OTA well over my comfort zone. Lifting the entire thing overhead into the open rings of the equatorial mounting hurt my back.

I thought it might be interesting to test the practice against theory regarding my optical folding. So, just briefly, I pointed the 7" at the sun while mounted on the MkIV.

The brilliant light cone passed safely through the baffle without restriction and lit up the dust on the 1st [125mm, 5"] mirror with about a 1cm unlit margin all around. The 2nd [100mm, 4"] mirror uses slightly more of its full aperture but still had a generous, unlit margin. Finally the light passed unhindered through the focuser baffle tube without lighting up the baffle. All this suggests that I could have baffled the folded OTA tighter but at least I am not limiting the full aperture.

When I held a piece of abrasive paper up at the focal plane it instantly produced smoke at 60mm from the fully extended focuser. I normally use a 2" star diagonal so that uses up all of the extra focal length. I did not risk sending the full heat of the sun through the star diagonal or eyepieces as well. Nor did I leave the telescope long on the sun as I had no desire to overheat the folding mirrors with the intense beam of concentrated heat. Nor to start a fire on the matt black paint anywhere! When mirrors are used to reflect the sun into a fixed solar telescope they are only experiencing normal sunshine. 

The 2m [6'6"] long straight tube, with its alloy baffles, would be safest if I really wanted to do serious solar observing with a Herschel wedge prism. When using the folded arrangement it is all completely safe thanks to the Baader Astro-Solar 5.0 film blocking all of the heat and almost all of the light. So the folding mirrors are never put at risk. I have left the 7" folded refractor out ready for this evening as the sky is still completely clear and the wind is dropping. Jupiter and the Moon will be rising in the east later.

Click on any image for an enlargement.

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