13th Feb. The long and the tall of it.

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It's Wednesday and my prism parcel has reached another delivery office. The post doesn't usually come until well after lunch so I shall have to hope it went on the van. They have all the equipment and the technology but can't be bothered to scan all the parcels. Which makes their website, track and trace service a matter of guesswork and unnecessarily overloads their customer services. Often with 20 minute, telephone waiting times.

I shall have to allow for the extra weight of the Herschel prism in comparison with the present 2" diagonal. I may even have to add the binoviewer if TS ever get around to sending it back.

I am very tempted to go back, cap in hand, to the local engineering company who sold me some 50mm stainless steel bar for my axes. I thought I was being generous with my measurements at the time. But  have doubled the instrument load and moved the RA wormwheel to the bottom of the PA axis since then.

I now have four counterweights on the Dec shaft with only just enough room for the retaining clamp.

The PA clamp is just a stainless steel Jubilee clip. There was no room for anything else. I could do with a minimum of another 100mm [or 4"] on both shafts just to be on the safe side. Perhaps even longer would be more sensible in the long run. 

I have a 10" f/8 mirror burning a hole in its storage box and am not sure how much longer I can put off putting it in a tube. Earlier attempts to build a lightweight OTA failed due to torsional flexure. I have a 30cm [12"] steel duct tube waiting but couldn't possibly mount it alongside the existing refractors without adding several more counterweights. 

I'm not even sure it would be a good idea to have the 10" in the observatory. Which was specifically designed for refractors. Putting a 10" f/8 on a mounting above head height is really rather silly. It would need a stepladder to reach the eyepiece when it is horizontal! I am already having to stand on a beer crate. Just to reach the eyepieces of the refractors pointing at the low, winter sun! 

Putting the long 10" on the MkIV mounting pushes me right back down to the ground. Where I can't see the sky for tall shrubbery. Not to mention pushing the MkIV's capacity to the very limit. Fullerscopes used to put 12" reflectors on the MkIV but that may have been too optimistic. 

I could build a raised platform to the west of the dome but the considerable expense and all the work involved puts me right off the idea. I could build a Dobsonian but the whole point of a long reflector is a leisurely look at the planets and Moon. An altazimuth with no drives is the absolute opposite of my needs in that capacity.

I've had another idea. Instead of using weights on the 7" telescopes rings to balance the 6" I have made a simple balance weight on a threaded stalk arrangement. This reduces the moment of these weights relative to the main Declination counterweights. As well as increasing the moment of these smaller weights to balance the offset of the 6". I'll buy another length of studding [threaded rod or all thread] to allow me to make two, matching, but longer weight stalks.

I decided earlier on not to mount the 6" outboard of the 7" refractor. The 6" rings are mounted directly to the saddle. Being inboard of the 7" reduces its moment by several ft/lbs or cm/kg.

Some large, professional German mountings have a further declination counterweight outboard from the main cwts on extension rods. This saves having to extended the declination axis shaft to accommodate further weights. The outboard weight can usually be adjusted to balance the OTA[s]. If I drill and tap one cwt. for some threaded roads then I can do the same. The outboard weight can have matching clearance holes and be held by nuts on either side of the weight. This will allow fine tuning of the OTA balance and give me added counter-weighting into the bargain.

An alternative would be a weight bar mounted on brackets on top or sides of the declination axis housing. Smaller weights could be slid along the bar and locked in place with collars. Providing smaller balancing forces, than a full sized weight, but more finely adjustable.     

Click on any image for an enlargement.

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