Going H-alpha: Improvements.

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Sunday started at 23F, -5C with bright sunshine but more 40+mph gales. I have been out there for an hour now, just trying to improve the view as more and more cloud comes over. I connected the synchronous motor drive for RA to save searching for the sun between adjustments. There is a prominence at my 3 o'clock and a small black and white spot near the lower limb. It all went downhill from there.

After convincing myself that a PST's etalon Barlow lens cannot produce parallel light with any other than an f/10 objective I did a quick drawing. It is obvious that it can do so because it rejects all outlying [converging] beams from a "faster" lens than f/10.

Placing the -20cm focal length, negative lens 20cm inside focus produces parallel light for the etalon regardless of the "speed" [f/ratio] of an objective. It simply ignores all greater angles of convergence. Only those contained within an f/10 converging beam of the same diameter as its own aperture can pass. The negative lens will effectively "stop down" any objective lens until the f/10 condition is reached. So my own 150mm f/8 objective becomes a 120mm f/10. If I demand a 6" aperture H-alpha telescope then I must find a 6" f/10 objective.

I hope the image makes this clear. Sunlight shines upon the negative lens as it converges from the entire objective. But cannot pass through the small aperture of the etalon group unless it lies inside an f/10 cone. Parallel light passes through the etalon and emerges as H-alpha light in the deep red. It is then re-converged at the same f/10 angle as before, by the matching, 20cm focal length, positive lens which lies just behind the etalon. I have not show the re-convergence for simplicity. The actual apertures involved are only put there for clarity. A 4" f/10 is a popular telescope for H-alpha modification. An 8" f/5 would be an extreme example but the drawing shows how much of the extra aperture is simply wasted. All the extra light it focuses cannot reach beyond the etalon/lens group. 

Despite lots of sunshine, today, I never enjoyed such sharp images as I have had before. Surface texture was indistinct and the one, complex prominence a struggle to focus. Later I returned to the 1.25" star diagonal but things did not improve dramatically. Perhaps the sky [or seeing] was hiding all the missing detail? The day finished with snow flurries and 29F. Tomorrow promises to be quieter [but not remotely still] with yet more sunshine. I shall battle on making adjustments as they occur to me.

I discovered an easy way to adjust the angle of the D-ERF filter on its original baffle. I removed the entire tail assembly and gave the baffle a poke up the bum with a sharp stick! A precision sharp stick, of course.

My handheld snaps at the eyepiece were horribly overexposed! Too much contrast for the camera to cope. It's too cold to break out the laptop and Celestron camera. All the cables would freeze solid. 

Sunday steadily clearing to sunshine and 25mph gusts @ 0C. Carefully re-measured the focus point through the D-ERF. Made it nearer 27cm rather than the earlier 26cm. Brought the WO 60mm extension back into play before the etalon and was rewarded with much better images than yesterday.

I really need to fix  the first extension properly to stop it wobbling up and down with the weight of all the extra 'junk' dangling off it. An aluminium backplate would help but would have to be laminated from 10mm plate. I just don't have anything thick enough and wide enough in my dwindling scrap collection to do it one piece. Nor do I have a shallow enough focuser to hold it firmly in place. A brass flange and extension tube would make good sense to help to balance the nose heavy OTA. It would need a decent clamp for the 2" AOK etalon adapter spigot.

I chose instead to turn a hefty alloy ring in the lathe to go inside the temporary plywood adapter. This fully supports the 2" spigot of the WO extender so there is no more sag. I'll replace the plywood with aluminium soon.

Now I know I'm on the right track I can turn my attention to the D-ERF tilt. I seem to be getting some flooding of the image with an asymmetric red blush. Which meant I could tilt my own view through the eyepiece to darken the background behind a prominence. Previously the looseness of the optical system meant it was difficult to be sure what was happening. With the whole set up firmly held I can narrow down the optical alignment of the D-ERF. Just to see if it helps or hinders clarity. A small tilt is desirable to avoid rings forming but the optimum degree of tilt is still an unknown.

Monday: Constant sunshine with light winds. Back out there again with a much improved tail end thanks to the internal support ring. I can see several prominences including a nice, detailed arch. A single dark streak near the limb is very low contrast. I sill don't think I have the H-alpha telescope optimized.

I am getting thermal effects on the limb from the roof and chimney which are directly to my south. I should take the telescope up onto the observatory platform to check if that helps. The problem is lifting the heavy Fullerscope's MkIV mounting let alone its massive, welded steel pier. Probably best to put the MkIV on the pyramid pier for isolation. I'll have to rig up my winch.

Instead of putting the MkIV mounting up there I decided to lift my massive DIY/ATM Goto [GEM] mounting. There followed the usual struggle to set up two ladders on the platform. I used a massive, square steel tube resting on the pier and dome base rail for the first lift. Once the mounting was safely onto the platform I had to lift the chain hoist up to the peak of the opposing ladders from yet another ladder. My habit of adding ratchet strap "guy lines" for the ladders was repeated.

All in all, it takes a coupe of hours from starting to having the big mounting safely bolted onto the pier. I was aching and tired by the time I'd finished. Just lifting the builder's step ladders bodily up to the platform is very hard work. The they have to be moved around, tied together and finally brought upright. Now I need a proper weather proof cover for the big mounting. The huge, garden waste bags I bought in a supermarket are wearing thin from constant exposure. Nothing else is remotely big enough to safely cover the mounting. I keep thinking about pond liner as a tough, long lasting cover. Welding one into a bag shape would be perfect.

Click on any image for an enlargement.

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