Pier height and dome clearance. Pt.2

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My plan is to see how much actual dome clearance there is above the long dewshield. Then raise the pier height slightly if possible. Or  just raise the tube in the rings. Though this is not the same thing since only pier height affects the OTA's height over the sill. This will give me the maximum eyepiece floor clearance. The dome will have internal reinforcing angle profiles at intervals so it may not provide quite the clearance I hope for. If it becomes desperate I can use flat [or flatter] sections instead. Hence the choice of 3m semi-diameter for the "dome" in the hope of some reserve clearance. This should allow some leeway in height and positioning.

Or, not. The long tube OTA is balancing nose heavy. I shall have to add another sliding balance weight at the focuser end. Though where I'm going to find an identical stainless steel towel rail I have no idea. The one I have is absolutely perfect! It came from a charity shop for only couple of quid equivalent.  Famous last words. It is an Ikea product. Flat rectangular SS plates to fit on the 8"Ø main tube after gentle curling in the soft jaw vice. Stainless steel screws in the ends of the tube so the whole thing can be dismantled with ease to slide the home made brass weight on and off without effort. Only a fiver equivalent plus postage. Whoopee! 😊

Initially, I had to space the bar supports to allow the brass weight to slide over the Orion UK rolled tube rings. Then I swapped to the heavier rings for greater rigidity and the spacers shown became redundant. I doubt anybody but an ATM would need such a large sliding weight support but it is an ideal and rigid solution. he standoff of the rail/bar/tube sets the maximum diameter of weight allowed. Any larger and it rubs on the tube, or the end brackets.

Even the refractor's internal baffles affect the bracket fixings. Long screws, pointing inwards, would hinder the baffle 'tree' from sliding into the refractor's main tube. So I reversed the stainless steel screws to bring the nuts and lock washers external. Forcing my hand through the razor sharp, smallest baffle nearest the focuser was painful. Getting it back out after pushing my forearm through to the limit was even more unpleasant. No blood but a few red marks for my trouble.

Eventually I decided to scallop the edges of the last two baffles to let it slide over the screw heads unhindered. Edge scallops are supposed to aid the refractor's internal 'seeing.' By avoiding thermal currents flowing through the central apertures in the baffles. The scallops are supposed to let any airflow move up or down the tube wall outside the baffles. Where they will not inflict themselves on the light path. Some rotation of the scallops, from baffle to baffle, would seem appropriate to avoid stray light taking a short cut between the objective and the eyepiece.

Rather than make the trip into town I decided to turn a second identical weight out of scrap brass to add to the existing weight rail. A second bar can be bought next time I am in the Ikea area. I needed to obtain a better balance point to avoid ground-scraping eyepiece positions. Each brass weight is 3.5lb. The second one should be more than enough to re-balance the heavy 7" iStar objective in its cell. This will allow me to raise the OTA in its clamping rings but adds more weight to the OTA. Weight adds to the OTA's moment as well as making it more difficult to carry and fit to the mounting.

I didn't have a spare clamping knob handy and the second weight will be fixed for all intents and purposes. It just ought to have  a matching, plastic knob to look quite right. One does have one's pride even as an ATM committed to scrap metal, charity shop saucepans and flea market finds.

Those burdened with pedantry [like myself in a former life] will notice the hole in the weight looks slightly too large. This is a deliberate mistake born of the need for the weight to go over the slightly expanded ends of the tube. I made it as close to 16mm as I could but it is still [rubber hammer] tight at the ends. Being hollow, I had no desire to reduce the diameter of the tube just there. They obviously used a press to fix the screwed end plugs rather than weld them. WYSIWYG. It was ever thus.

To ensure I had the OTA's middle, balance point I had to fit the lens to the long tube. It looked so filthy with collected dust and dried dew that I gave it a clean. A rubber bulb, dust blower first, followed by a gentle brushing with a large, natural bristle make-up brush. More blowing followed by a brushing with lens cleaner. Then a wipe with a lens tissue followed by gentle drying with a well washed microfiber lens cloth. It looked fine after another blowing with the lens bulb to evaporate the remaining film of lens cleaning solution.

The balance point is now within a centimeter of the center of the tube when rolled back and forth on a thin steel rod to act as a fulcrum on the workbench. I discovered, years ago, that a larger diameter rod would tip the OTA too much each way and make the balancing too clumsy and slow. Twirling the rod slowly with the fingers ensures the telescope tube isn't joggled as it gently finds its own balance point.

I use a folding B&D workbench with four widely spaced, large, plastic press-in blocks to stop the tube from rolling off. If an OTA did roll off it would be a disaster with the objective likely to be cracked across by the impact with the floor or ground. Never lay any OTA, of any size, on a flat surface and expect it to stay there!

Click on any image for an enlargement.

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