Staring @ the Sun, 56
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I machined an adapter from PVC pipe to connect the two tubes. A piece of aluminum channel and
For comparison, here's the view in the native Lunt in its usual configuration (barlow on snout):
What works and what didn't with the 90mm F10 achromat serving as the Lunt's objective:
I machined the adapter to connect the two tubes from PVC to produce something very close to what is said to be the optimal spacing (see thread linked below, the flange of the Lunt should be 71mm inside prime focus of the new objective). Maybe it's good enough; maybe it's a prototype for an eventual aluminum adapter. I lined it with Protostar flocking material. I could not quite reach focus with the original adapter, so I brought the objective 20mm or so closer (so the spacing is now something more than 71mm inside focus). I think the results are excellent! It's possible to experiment with the spacing because there's nothing about the bracketry that precludes moving the 90mm objetive further away from the etalon. Just be sure you can focus, 'cause if you can't focus a tight bandwidth means nothing.
Here's what my Frankenscope looks like riding the AT10RC just after shooting the picture at the top of this page:
There's a long-duration M-flare building in the first photo and in its full glory in the lower frame made about an hour later using the stock Lunt.
Shortly before 3 PM EDT, AR1520 began its second act of the day. I'd kept the solar scope's view feeding onto my second monitor where I saw a loop prominence taking shape in the magnetic canopy of AR1520, so I hustled the 90mm OTA back into place and lined up for more photos. The C/CS adapter was in place and the barlow was inside the 1.25-inch snout:
Loop prominences above AR1520
Late addition: best 50 out of 1940 here
Soon after I started using the Lunt, I experimented with mating a 100mm objective to its internal etalon. The notion held promise. But it is critically dependent on a few key variables (the spacing of the etalon assembly in the converging light cone of the objective, primarily). In the months since I messed around wtih it, a dedicated group of solar enthusiasts nailed down the numbers.
See here for a fascinating thread including measurements, trials, errors, examples, thoughts. I benefitted from months of this forum's dedicated experimentors. The critical bit is that the flange of the Lunt 60THa hydrogen-alpha assembly must be 71mm inside the prime focus of the new objective. I am not sure how critical this is. I failed to find focus in my first experiments with that spacing and I increased it by several mm, and that is where things were (I believe) when I made the photos on this page. I'll mess around some more before it is over and let you know what I find out. There's a little bit of flex in the current outfit -- maybe the channel aluminum bends, maybe a full square tube would fix that, maybe I need to build a more elaborate support structure. Anyway, it works, and it works well. If I can make it work better, better.
This opens the door (eventually) to putting the Lunt HA module behind the 5-inch A-P or the 6-inch Kunming refractor. Both are shorter focus, and if they're too short, that's what Barlows are for. I believe the Kunming is identical to the telescope used as the brawn in Lunt's 152mm solar telescope, so I am squinting at its tailpiece with metalwork in mind. The 90mm performs spectacularly, but it may just be a testbed. This could be an exciting path.
7/19/2012. I went to Lowe's for a 1" square AL tube, 36" long, some 1.25" 1/4x20 socket head bolts, washers, and some PVC cement (just in case) all in an effort to make the modified Lunt a more rigid 'scope. The square tube seems to have done the trick, that and moving one of the four attachment rings over the looser PVC / aluminum connection. This thing is ready to go. Clear skies soon, please.
7/21/2012. I can't keep my hands off the data from that arcade display on July 17. No harm in that, I suppose. It's cloudy and lightningy lately. So I combined a clip captured with a lower gamma setting for better foreground detail with the best loop data above. The two clips were made about three minutes apart, which is just about negligible, and combined them with better, or at least more dramatic, color mapping:
Same tech specs as before, except foreground detail
I've taken advantage of down time to gather parts for the 152mm solar project. If there is not enough backfocus behind the Astro-Tech 152mm achromat, I may turn to the 127mm A-P instead. In either event, these parts are either in hand or on the way:
The ERF may be redundant since I have the two-piece ERF from the old Lumicon prominence filter. But I bet Lunt's works better (fewer reflections, single element, etc) and I'll have more confidence that it's appropriate for the Lunt's etalon. In any case, both the 90mm and prospective 152mm solar 'scopes could use an additional ERF, so neither filter will look for work for long. The ERF is bare glass, and dreaming up ways of mounting it has involved lots of half-baked schemes, at least three of which will likely work. A simple and "obvious" solution finally dawned on me and is described a few lines below.
The heavy aluminum square tubes will form a framework on which I can mount the tube rings to hold the 152mm achromat and a Losmandy dovetail to hold the 60mm Lunt "h-a module." Since ordering these tubes, I've found a fellow on YouTube showing how to make Losmandy dovetails from flat aluminum stock and a carbide-bladed table saw. I could make long plates of arbitrary width. We'll see how the tubes work out first.
I've spent considerable time standing in the shower late at night doing design work on how to mount the ERF. Retaining rings of PVC segments, O-rings, picture frame bumper stickies, set screws, machined notches and rolled up ProtoStar material... The obvious solution finally slapped me in the face this morning: use step up and step down rings to make a filter-holder; machine space in PVC and either glue one side of the holder in place or use set screws on the metal rim. This should provide protection for the slightly delicate ERF and plenty of mounting options at the same time. I got excited and ordered a bunch of rings from Hong Kong (eBay) before noticing that delivery might take a month. So I ordered a couple more from New Jersey (also eBay). I'm thinking there may be several ways to use threaded rings, so the more the merrier. Lord knows I'll do something with them.
The Losmandy blocks and 3-point rings will be useful for deepsky guiding or (perhaps) for holding the "H-a module" at the proper height and alignment behind the larger-aperture solar telescope. The big rings I am using now for guiding and focuser support are really a bit too large for either duty (but they may work great with the 90m solar kit). The way my projects seem to go, you can't have too many dovetail D-clamps, and the rings were a bonus. A good deal on Astromart made adding them to the pile o' project parts irresistable.
8/05/2012. I moved the Robofocus bracket and Lunt clamshell from an ADM G11 saddle clamp to a much smaller G8 saddle clamp. In the process, I decided to save more space by using a shorter timing belt to connect the Robofocus to the FeatherTouch focusing knob (the belt is a 50XL037 to replace the 90XL037 supplied with the Mandel setup). Polytech Design offered a reasonable minimum order ($15) and had the shorter timing belt I wanted in stock. They also offered bar-stock of timing belt pulley material. I bought two belts and some bar-stock to meet their minimums. USPS Priority mail added a little more than I'd like, but less than anyone else's order forms did. All the pieces will be coming to my door soon at a decent all-up price. Why would I complain?
I'll be able to mount the RoboFocus much closer to the focuser it's driving and replace a knob on the focuser(s) with a proper pulley to engage the timing belt. The outfit should be smaller and lighter and more precise and responsive into the bargain. I can also make the oft-threatened idler wheels to adjust belt slack should the need arise again. (Note about sizing timing belts: "50" and "90"
[8/8: the 50 belt is way too short; the "50" didn't mean what I thought it meant. More are on order; I'll sort out the measurement and the belt and get back to you. Order by tooth-count, knowing the spacing is 0.2 inches and you can't go too wrong. I hope. 8/10. Wow! Delivered today: a handful of timing belts ranging from still too short to longer than needed by way of just right. Focuser remounted, nice and compact, ready for more 90mm sungazing.]
8/06/2012. Parts on order to make my own D-plates of whatever size is needed to mount and balance these ungainly solar "Frankenscopes." About time: carbide blade, metal stock, proper bolts for mating metal, and centerpunch for the price of one D-plate. Hey y'all, watch this: YouTube.
8/09/2012. Man, wouldn't you think a Lunt 75mm ERF would be 75mm? Well it's not. It's 78 or 79mm in diameter. So the 72-77mm step up and step down rings I've ordered from New Jersey, China, and some other foreign country are useless for the work intended. Furthermore, I don't think I'd be able to use the next larger standard size (82mm) because the 77mm rings are 79mm in outside diameter. That implies that 82mm rings would be about 84mm, which is 1mm larger than the space available for the solar kit. So, you know what? I have a lathe, and I generally enjoy using it, so here's an excuse to get down to it. I'll use PVC for the main structure and some of these otherwise useless 77mm pieces and parts as inserts.
The raw 3 and 4 inch aluminum stock is here. As is the carbide blade and many bolts. Also a decent center punch (to supplement the one I made in Q. D. Greene's machine shop class about 150 moons ago (really)).
8/12/2012. As promised in the YouTube video above, a carbide blade on the tablesaw makes quick work of cutting 30-degree bevels in 1/4-inch aluminum. I bevelled a 14-inch-long piece and attached it to the square aluminum tubing that braces the 90mm solar telescope.
I know this is going to be incomprehensible without pictures (just wait), but I'm noting what I've done so I can be reminded just why I did it this whenever I think about screwing around with the arrangement. There's a GM8 saddle attached to the clamshell that holds the Lunt OTA. Attached to that saddle is a short bracket for the RoboFocus motor. This way the motor stays with the Lunt no matter how it's mounted or in what mode it's used. The new aluminum plate is attached to the othropaedic-looking brace that holds the Orion and Lunt tubes in line and properly spaced. It adds very little weight, but it allows the Frankenscope to be mounted in two ways. It can ride the AT10 piggyback (as always, with the GM8 saddle gripping the D-plate on top of the AT10) or it by itself on the G11 or LXD55 via the new aluminum plate and the Losmandy saddles on those two mounts.
I don't think the new mounting options will come into play until the Sun moves too far south to clear the pines in the backyard.
Also, the old two-element Lumicon ERF is now sandwiched between the Lunt and Orion tubes where it fits loosely but does not seem to be in any danger of being damaged by rattling around. If for some reason this is not satisfactory, then the Lunt ERF could be used and swapped between this scope and the next one. Bwahahahaaaa.
8/14/2012. That photo of the loop prominence was the "Editor's choice" on S&T's gallery from the day after I made it until yesterday or today. That's July 18 - August 13. Nice run. Should be in the October issue, too. I dry-fitted the pieces for the 152mm solar telescope today, but for some reason was reluctant to cut metal. I just don't want to be frustrated, which is no attitude for a tinkerer. Maybe tomorrow.
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