Not Quite First Light

Why is this such a scary moment? I’ve taken a newly silvered or new mirror out of it’s lint free cotton packaging and tissue paper a dozen times yet it still scares the c%^p out of me.

Looking into it’s freshly illuminised surface with not a microscopic blemish in sight is like looking into another bizarrely warped dimension. Knowing that it’s virgin surface will soon be covered in bug poo, dust, pollen and the mouth froth of an overexcited observer is heart-wrenching (for a few moments only).

At 12in diameter this 7.5kg F3.3 1/8wv P-V mirror was no mean feat when it was made 35 years ago so I approach with no small amount of trepidation!!

With the 75mm secondary installed EXACTLY as per the CAD but with temporary 3D printed spider vanes and a quick and dirty 3D printed focuser draw-tube (admittedly turned down a bit on my lathe for a bit better precision) the top end was approximately aligned with an old toy I purchased from Laser Max years ago…..

This 5mw laser with reticule does a nice quick job of approximately helping centring the secondary. A point to note is this setup INCLUDES the 5.6mm offset away and down from the eyepiece to centre the FOV. Another small point (literally); there’s only a small 7.5mm hole drilled in the side of the OTA art this point because if the position is not correct I can still drill the main draw-tube hole above or below the “test point”. That is however enough light for a star test.

So this is it. First light or more appropriately “First focus”. I don’t like calling this first light as all i’m trying to do here is get to the exact focal plane and make sure my CAD is all correct. The mount i’m using for the OTA is called the Camping seat non- equatorial tilter. It’s capable of not tracking anything and only pointing in one direction. It needs some work. Never the less I did manage to find a star!!!! and shove my phone camera up to the eyepiece.

So after a quick star test with a few different eyepieces I got the barrel stop position marked on my “focuser”. As it turns out I was only a few mm off the CAD meaning the stated F.L. of the mirror was very close to the actual. I’m about 60-70mm off the tube sides which is ample for use with a low profile Crayford and having enough back-focus for a coma corrector.

So that’s it. Drama over. It all gets stripped down now ready for it’s real components being installed, OTA blacked out inside. I can finalise the CAD now knowing where everything is exactly.

It’s amazing to think that those were the first photons of light from a star ever to be bounced off it’s surface in over 35 years as this mirror was actually made for a military laser firing test chamber on an alignment bed. It was never intended for use in a telescope. It of course has a paraboloidal figure. (it so happens, fortunately for me, that it’s original intended purpose needed to have a parabolic mirror).

Another way of looking at it is when this mirror was being ground and figured, the light leaving the Star Arcturus then (alpha bootis) has just arrived now so maybe i’ll catch some of that. The mirrors own birth light.

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