Rusted

Thank you Gina! A while later it absolutely tipped down! Given sufficient foresight I could have extinguished the fire! So really it was just a case of bad timing!

On the dangers of solar imaging: There I am, Sunday morning, in my observatory, capturing videos of the sun in H-alpha. When I start smelling burning. I immediately thought my wife had lit the woodburning stove. Very unlikely, because have had already run out of furniture to burn last winter. There not much left over for logs when there is astro equipment to buy! Now the burning smell was becoming progressively worse! Not a good time as I'm busy processing videos in Registax! Go away nasty burning smell! Be gone! Finally, I glanced up and the dome was full of sunlit smoke! Is the laptop on fire? No, the smoke is coming from the dewshield of the 6" refractor. It's smoking like a chimney and now I'm coughing and choking! The culprit was a cardboard stop ring I had fitted in the dewshield on my last imaging session in thick cloud. The internal filter had reflected the sun's heat back out in a beam and set fire to the cardboard! I grabbed the burning ring and dropped it into a rainwater collection bucket on the floor of the dome. Fortunately the 6" objective had survived the conflagration and I could continue with my imaging and processing. Let's be careful out there people!

In my quest to become a real solar imager I have added a Baader 1:1.25 GPC to my 6" f/8 PST mods. This morning [Sunday 27th] before it started raining I captured some videos with and without the GPC. Here's the result of a couple of swift bits of processing: The image scale is the clue to whether the GPC was present.

I can live with screaming hedgehogs, barking deer and contralto foxes. The flying foxes just bounce of my duvet hood over an oiled wool, bobble hat. The salopettes even take care of rabid, marauding cats of a winter's night. It's the Great White Sharks leaping clear of the pond that unnerve me. Quite honestly, that's the only reason I built a two story observatory...

Cue hundreds of pictures of spherometers? An engineer freind made mine decades ago. I added two more clamping screws for concentricity and lower friction of the plunger. I used a mechanical dial gauge for years until I found an affordable, digital one on eBay. When it comes to measuring long radii [like the rear surface on a doublet] they become increasingly innacurate. A thou' difference can mean almost anything.

For the benefit of those less experienced how would you define a "quality" glass cutter? My own experience is only with a humble [dirt cheap] wheel cutter.

Many mounts have undersized wormwheels to avoid spoiling the styling or reducing the profit margin. Lower tooth counts have larger gaps between the wormwheel "teeth" to provide increased backlash. In mechanical theory, worms should never be jammed into the wormwheel to kill backlash. There should always be some clearance to avoid high friction and wear. It also assumes that perfection exists in wormwheel concentricity with the shafts and bearings. Otherwise there will [inevitably] be loose arcs of rotation and tight arcs. Under these circumstances an "uphill" drive bias may be all one can possibly hope for.

I hope the glass is normal float glass? Or you'll have more facets than you'll ever use in a lifetime. A new tool, with flat glass facets, will need to be brought to curve. That will undo whatever curve you have now even if you mould the plaster surface to your mirror. Wear safety glasses/goggles, dust mask and leather gloves for the glass cutting. Something to catch all the tiny glass shards too. So it can be safely disposed of. Kids and pets don't go well with razor sharp, glass powder. Don't put any into the air with a grinder! Choose a suitable facet size to match the diameter of your tool for maximum coverage. Just as you would do with a pitch lap. Start with a facet corner slightly overlapping the centre of the tool and allow enough space [6mm?] for cleaning between the glass rows. The tool facets will remain viciously sharp until you have done some coarse grinding on it. So beware! And warn others not to go near it out of curiosity. Keep it covered?

The mounting must be balanced in all planes. Not only from side to side but vertically and nose to tail. The simplest balance arrangement is for a single telescope with the finder outboard and in line with the declination axis. I have three different telescopes mounted at once. 180, 150 & 90 mm refractors. Despite my best efforts these require extended sub-counterweights, on threaded stalks, below the lightest telescope of the three. These balance the vertical imbalance of the three telescopes combination. I was careful to bring all three instruments as close to the polar axis as possible to reduce the necessary Declination counterweighting. You must release the clutches or shaft locks and move the telescopes to all possible sky positions. They must not move when released at any sky pointing position. It is often easier to give the telescopes a gentle push to ensure any static friction [in the clutches] is overcome. This gives a much better idea of any slight imbalance. Any imbalance will cause the telescopes to continue to move more easily in one direction rather than the opposite way. Balancing should always take place with the heaviest accessories fitted. Large solar prisms, filter wheels and cameras, etc. To add to the confusion it is considered wise in imaging circles to have a slight imbalance which ensures the telescope must be constantly lifted "uphill." This ensures that only one side of the teeth of the drive wormwheel remain in constant contact with the worm. Were the telescopes perfectly balanced the wormwheel could rock between teeth where it meets the worm due to wind gusts.

Great advice on the oil drum. Though I never found one with my name on it. I got into making grinding and polishing machines far too early. Working by hand on a solid base is much quicker and probably much better than a machine. The random stroke lengths and rotations of hand working must be beneficial to a smoother curve. I did some hand grinding and polishing using my last machine as a 24" turntable. I had no room in my shed for a drum or "going round in circles."

We had the advantage of a curve generating "shaper" to put curves on our iron tools. It was almost as much fun as watching the machines moving the glass to-and-fro.

I'd use less than a quarter as much powder as that. Probably even less. Halve your stroke speed and spread your hands flat on the back of the mirror. If you let your hands curl over the edge in later stages you will expand the glass and grind or polish it away. A pencil might be better than a Sharpie if the ink is water soluble? Edit. My wireless connection was dropped.

Good score! The Green Cross Code has obviously worked well for you. You should definitely stay away from "The Express" click bait headlines. Every single day, something is "out to get us." Read on down and it all proves to be complete nonsense. The Express used to come up as a suggestion on Google News. They were obviously aware of my interest in astronomy. TMA! Too Much Advertising! So I stopped being taken in and now I'm almost back to what passes as normal. Ticket prices to the Ark?? If you have to ask...

Adding anything on top of a curved tool will increase its radius of curvature. It follows that grinding will remove the outer regions first. Ceramic isn't that hard under the glaze. Steel washers and Araldite used to get a good press. I cut up thick glass into squares. Then used pitch to stick them onto a 16" pre-curved, dental plaster tool. The plaster was too soft even after gentle baking and a nightmare to clean up between grades. A metal disk from a scrapyard would be my choice of tool these days if I couldn't afford a plate glass tool. Plate glass has the advantage of being softer than "Pyrex." So "boulders" will be crushed into the plate glass surface first. Use less abrasive for a sharper but shorter cutting action. Too much powder and it just grinds together and turns to mud. Be guided by the noise. If there is any with your plaster and ceramics? Glass on glass rings nicely. You could play it by ear. Even if you were tone deaf.

Not ducks as well?

Well, Ron, if that is your attitude you're definitely not getting a return ticket on my Ark. Have you noticed the subtle irony of your signature from one sporting 29,000 posts? Do as I say not as I do?

Even after 60 years of amateur astronomy I still learn something new here every single day. Mind you, at my age, every day is a surprise. SGL can be anything you want to make of it. Treat it gently, feed it regularly and you will gain lots of knowledgeable friends. The best thing is that they don't all want to come round and borrow your stuff. Most of them just want to share their obsession.

Survival of the fittest? Don't even get me started.

It's no use getting theoretical. We've already lost the plot .

A decent straight edge and feeler gauges will put you close to your desired sagitta. I just used a suitable length, stainless steel rule on edge and measure the gap in the middle of the mirror. Try two or three rules in the shop edge to edge against the light. Reverse the test edges in turn. It's a null test for straightness if several edges match.

Tell me about it! They outnumber the rest all put together.

Let's not generalise too much for the sake of our younger members. What you describe only occurs where the observatory resides at the bottom of the garden.

Mine is still a baby and I'm it's great, great grandad. 👴

It would not be difficult to fit a shaped scraper or stiff brush to sweep the channel clean with every movement. Using two different profiles is clever. Such a system has been used for ages on lathe beds to avoid friction. One way guides mercilessly while the other supports the loads. There is no sideways movement for the channel wheel to cause wear. Lathe beds are subject to constant and highly abrasive debris and use scrapers and felt pads for cleaning and lubrication of the ways. The wear from [typically] infrequent uses of an ROR, even over decades, is unlikely to cause any visible damage. On your other point: A V-profile would have higher point loading than closely matching radii. In practice the materials are usually hard and inflexible enough to ignore the difference. I would imagine twin, V-profiles would require much tighter building tolerances if subject to typical twisting forces. Since both rails are desperately trying to steer. Matching radii wheels and tracks would mean that the wheels ride up the curvature of the walls if there is the vertical freedom to do so. Though both are still trying to steer. Causing increase friction unless one set of wheels are allowed some sideways float on their rail. Or words to that effect.

Thanks Alex. You appear to have captured your very own occultation of Saturn by the Sun.