Rusted

Thanks. The sheer scale and technical level of your project is all a bit mind boggling. I came close to making a [relatively tiny] 16" F/5 until I realised it wouldn't fit in any storage space I owned. I couldn't even get my arms around the solid PVC tube to lift it! That was decades ago before truss tube Dobs were visible on the horizon. A 12" f/5 Dobsonian, again with a plate glass mirror, went much more quickly. The ease of movement of PVC and Formica on PTFE was an absolute delight. Just testing the bigger mirror at C of C was a struggle to find enough linear space. Re-figuring the mirror to F/4 proved to be far more prolonged than I'd hoped. Plate glass is very slow to cool after polishing in a cold shed. Particularly when bringing the mirror into a much warmer room to test it. Hopeless in fact! In perfect hindsight I just didn't have the facilities to complete such a project. I 'm sure you'll have more luck. As long as you have enough room for storage! This was back in the 1980s when I first heard of Dobsonians.

Thanks. My GEM was built back in 2017 with later improvements. I'm afraid my build diary blogs are horribly "inaccessible." I realised early on that I had no access to suitably rigid box sections. So I made them rigid by constraining the boxes with the massive, tensioned, internal studs. Anyway, I am going seriously off topic for your fascinating build thread. Perhaps I should explain my design in a fresh DIY thread.

Hi, No mention of my build on FLO. I usually blog and photograph all my projects to death. The details are all there including my endless discussion of the design as it evolves in my mind. I rarely bother with drawings and just make it up as I go along. I have a lathe and power tools. The GEM uses multiple, tensioned, threaded rods [studs] in all three planes, to reinforce each other. Only the longitudinal and cross studs are shown here. A similar set of studs run towards the camera. They not only resist each other in compression of the flanged bearing boxes, but all are carefully placed to contact each other to further resist bending. These studs are all hidden within the bearing housing's 10mm thick, stressed aluminium plates. The whole thing weighs "a ton" but was intended for permanent mounting on a solid pier. I need a chain hoist to lift it as one unit. It has AWR Goto drives. With multiple drive rates and full planetarium support via ASCOM. I am primarily a solar imager, these days, with a fast ZWO camera, so I don't need long exposures. My long focus refractors 180/12 & 150/10 aren't very suitable for imaging the night sky anyway. Other than the moon and planets of course.

Don't ignore the option of "furniture nuts." I use them all the time in my constructions. A couple of dozen on my big GEM alone. These are of galvanized steel to avoid rust. A "gold" finish is optional but I wanted to match the aluminium. The large, flat heads provide a tidy solution without needing load spreading washers and "Nyloc" nuts. The hex socket doesn't mar easily no matter how often they are driven and removed. You also have the option of putting a hex driver in a power tool for speedily applied torque.

Indeed Pete. I wish I could have done it better justice. I was a martyr to "the thermals." Though I do my best to equalise the temperatures inside the dome with those outside. The sun is too weak to warm anything at this time of year. It being so low is a major handicap for any celestial object. Normally it is a complete waste of time to look at anything much below 30-35° local altitude. That doesn't mean I won't try. There isn't much choice in the matter at 55N.

Disagree. Both the "plug" and "socket" are well defined, parallel cylinders with very close tolerances. Parallelism is assured with the first screw to be tightened. The further two screws merely secure the item more firmly in the socket. The secured item would remain safely in place regardless of load and equatorial mounting orientation causing rotation of the receptacle provided three thumbscrews are used. No taper, nor variation in seating diameters is likely, given mass production using massive CNC machines with very stiff boring bars and external bit holders. The side loads involved during cutting are absolutely minuscule with modern, ceramic, or other hard, cutting bits in free cutting aluminium using free flowing, cutting fluid. Both items remain chucked for both internal and external turning. There is no removal midway as might occur with amateur hand turning. Optical Concentricity is a function of the differences in diameter of the two cylinders involved and can never be worse than one screw. Given the minute errors involved one would be very unlikely to see, nor detect, any optical defect in the image regardless of the number of thumbscrews. A simple test would involve mounting both items, once tightened together, in a precision, self centering lathe chuck or, even better a precision collet. A lever, dial indicator will cheerfully monitor concentricity and parallelism while turning the chuck, or collet holder, by hand. A finder has wide tolerances over widely spaced constraints along the axis. It is designed as such to allow completely independent adjustment to allow alignment. Or words to that effect.

Thanks Stu.

Quite a sunny day here but with really awful thermal seeing and constant thin cloud. 6"f/8, internal D-ERF, 1.125GPC, Baader 35nm H-a, motorized PST etalon, ASI174. Poor images but I managed to capture surface and prom simultaneously. SharpCap, AS!2, PhotoFiltre7.

I still have two "simple" [one lens] eyepieces in turned brass, with RAS threads. Which I bought in my early teens back in about 1960. I still have the heavy, brass, push-pull focuser to match. Which I used with a 60mm, 1/2 diopter, simple objective [spectacle lens] of 2m focus. Used as an aerial telescope hanging from the washing line post. To see a huge and colourful Saturn low in the sky. All by the same company but the name now escapes me. Irving? Probably listed in Exchange & Mart back then.

I routinely drill and tap for more thumbscrews until there are three. It still isn't enough for the extenders on the long filter stack of a PST mod. The Baader Twist-lock mechanism is very secure for holding heavy loads like the Lacerta 2" solar prism + binoviewers. It also has the advantage that you don't need to reach tiny thumbscrews tight up against its large, flat surface. For the worst, ever socket design and construction I offer the PST eyepiece receptacle. Or just the entire PST itself, if you like. Until it is turned into something much more useful. No, not a desktop pen and pencil holder!

Thank you Martin. For another excellent tutorial!

They look strong enough if you remember to set them without fail. The two legged vermin would struggle to lift the roof off too. I prefer permanent hooks or plates riding over the moving parts. Though it may not always be possible with some roof designs.

Thanks Merlin. I thought it was a bit too easy. Hope all is well in your area Down Under? The fires have been dominating the global headlines.

Well done! Like the recycling, design and workmanship. Make sure you have plenty of strong roof restraints in case of gales. Preferably something which works without needing to be set every time you close up. An arched roof may produce lift if the wind is in the right direction.

There seems to be some variation in user's experiences here. Can I try to expand the discussion slightly? Are there alternatives to these "dedicated" diagonals + blocking filters to obtain considerably more, clear filter aperture? I'm thinking of 1.25" or 2" filters which can be placed further upstream. A Baader D-ERF energy rejection filter is assumed as standard up front. UV and IR blockers like the Baader H-alpha 35nm and Beloptik's IR KG3? About 250 Euros for the pair in 2" ahead of the etalon. Could use multiples of each and stacked in series for greater reduction outside the nominal passband? How much more blocking is needed for visual [or imaging] H-alpha beyond these three filters?

Aha! But think of the advantages of not having to strain one's last few remaining brain cells when an app is available at the mere brush of a key? There was a time I would write software to calculate and depict the spherical, chromatic and other aberrations of five colours of light at the focus of doublets and triplets. Back then, offences against the Sin condition would roll off my tongue like a snake oil salesman at a village fairground. Now they have OSLO. So those new-fangled floppy disks and thermal printers are as out of date as a horse a buggy. Pick a card. Any punched card! I'll get my [white] coat.

Hej Lars, Yes that is the correct filter. It is used to remove much of the sun's energy before it reaches the etalon and later filters. I am sorry for the loss of your friend. He seemed very dedicated. I can read some Swedish thanks to being able to read quite a lot Danish. The advantage of eyepiece projection was the precision of spot placement on the solar disk drawing. Most people use solar foil for visual solar observing now. So they would need to transfer what they see onto a scale drawing of the disk. The PolCor2 Debris Finder seems similar to a folded, mechanical spectrohelioscope [or rather spectroheliograph?] in some respects. Very interesting but looks horribly complex to make and align such a thing and then get it to work as expected.

Hi Lars, Your app does indeed work with longer telescopes and much more distant filters. Using manual overwrite entries. I use a 90mm internal D-ERF filter half way inside the main tube of a 150mm f/8 modified PST, H-alpha, solar telescope with a ASI174MM camera. [~11x7 sensor] I haven't checked the actual distance to the filter with a tape measure [yet] but your app's figures look close enough. Many thanks for your useful resources.

Thanks Lars. May I ask why you limited the distance between the sensor and filter to 150mm? For some solar H-alpha applications, energy rejection filters are sometimes placed up to midway between the objective and camera sensor. Where these distances might well be up to 1000mm, or even more, with a longer focal length lens. I have used the bright, tapering light cone to decide on best filter placement to avoid vignetting but this is very fiddly in a long, closed tube.

I managed 16" at f/5 on a full thickness plate glass blank but when I decided to regrind to F/4 I ran out of patience. The F/5 went surprisingly quickly. A low expansion blank is far easier to work with than plate or float glass. No endless waiting for testing. A thin mirror probably needs to be mounted on a multi-point supporting cell and rotated regularly to avoid astigmatism. That's how I worked mine on a 24" lathe faceplate, face up and driven on its edge by a long v-belt from a gearbox motor. I almost always worked by DIY reciprocating machines. Hand work is far quicker and arguably provides a smoother surface. Machines can cause rings and zones from repeating exactly the same length of stroke without care. With handwork, no two strokes are the same but you have to avoid the heat of your hands affecting the glass. Machines have the advantage that you can provide more power and relax during wets but are not compulsory. Mike Lockwood suggests that thicker blanks take such a long time to cool that they should be avoided in all sizes. Not just for the change of figure, when cooling, but the thermal currents rising from the surface. Fans can break up thermal currents and force more rapid cooling or equalization. But need power and may cause vibration.

Cons: Sheer size, weight of literally everything involved, sheer bulk, viewing height, danger of falling, bad back, covering the beast, recovering the beast after another storm, moving it about, number of clear nights, planning permission for a barn conversion, etc. etc. Ready made? Don't forget to add the cost of the air fair for the forklift driver's seat for the primary alone. Or cost of borrowing one at each end. Making a 24" mirror starting from zero? Nah. Several years minimum. Even if you are a "natural." Practice at the dinky sizes doesn't scale well, IME. Mr Lockwood himself recently mentioned lots of interest in using "light amplification" even on on his larger apertures. Bingo!

Some chainsaw suppliers will sharpen any saw chain for you. Only takes a couple of minutes with the right machine. Handy for saws with odd-sized chains or discontinued models. Small, narrow chains don't have the stamina and soon blunt. I spent years sawing and splitting logs for home firewood. Until I started valuing my own labour and limited free time. It's a mug's game.

The DeWalt 54V 9A rechargeable chainsaw is ridiculously powerful without the motor racket. I used it to clear huge birch stumps left by the lazy tree feller. Which meant cutting into very dirty wood and gritty soil on a bank. Like butter! It needs the same respect for safety as any powerful, motor saw. But doesn't exhaust you with the weight, constant noise and endless restarting.

A bit harsh? Texas sunshine? Probably a bit fiercer than sun-baked Surrey? I think Stu's eye shields do offer rather more than mere light exclusion. They also provide some useful, distance-related, eye relief control. I had similar excess eye-relief problems with my 40mm Meade 4000 Plossls on my TE binoviewers for H-a. Never used them again! Had I really wanted to, I'd probably just add a couple of black "funky foam" eye tubes. The 40mm "tunnel vision" view was very disappointing when I was hoping for a 32mm but with extra low power on steroids.

I've been using barbel weights for years on my old Fullerscopes mountings. Aldi did a nice set in a box with loads of smaller weights up to 1.5kg. Very handy! I used the cheapest and smallest diameter 5kg weights I could find online on my big mounting. At 9" Ø they only just fitted in my lathe to smooth the rim on the rather rough castings. These weights were sold as "Olympic standard" bore but were rather loose on my 50mm stainless steel shafts. I eventually painted them white to avoid them attacking me in the dark. It didn't work. They weren't properly "house trained" before I got them.