Transit telescope query from newbie

[Tempus]

Hi - bit of an usual one this (perhaps), but I wonder if anyone can help with amateur transit telescope suggestions? In times past I've dabbled a little in astronomy, but after a recent visit to the Royal Observatory at Greenwich I feel inspired to put together a "back yard" transit telescope to make solar/bright star measurements. By way of background, I also have an interest in the history of precision timekeeping & navigation...

But nothing too grand planned here! I gather that in Victorian times there were commercially available small refractors of only, say, 30 mm OD designed to give sufficient precision to allow accurate clock setting in conjunction with tables.

Stability must be paramount & I should be able to put together a concrete mounting post in the garden - but what to do for a (simple as possible) measurement telescope? In producing something, I'm looking to keep the cost low & see if the measurement interest lasts! If it does, I'd perhaps take things a stage further later on.

I guess the mount could be an ordinary altizimuth but how best to measure the altitude above the horizon? And the optical axis of the telescope is going to have to be exactly at right angles (or adjustably so) to the shaft around which the telescope rotates.

Are there any commercial amateur transit measurement 'scopes? Could an old theodolite be used? Or sextant? Or maybe a modified inexpensive "Go To" telescope (I have no experience of these)? I guess that mount would have to have to be pretty high precison/adjustable to achieve accurate tracking...

Sorry about the long spiel, but at a bit of a loss as to where to go next before committting myself...! Any thoughts very much appreciated. Many thanks & seasons greetings.

[brianb]

The technology is obsolete, but ...

As you say, a transit scope was a simple long focus refractor mounted on a special mount which allowed for adjustment in altitude only (after installation). The tube of the scope was either fastened to a large protractor (about the same diameter as the length of the tube) or a scale of similar size was fixed to the mount. The angle could be read with the aid of a vernier scale to a very fine tolerance, certainly less than a minute of arc.

The scope is used at high power with a crosshair eyepiece, usually with a horizontal wire & two or more vertical wires, one in the exact centre & the other(s) used for calibration & practice. The altitude was set to the appropriate value for the star a minute or so before the meridian crossing, when the star intersected the crosswires a chronograph key was pressed to register the exact time (a chronograph was a pen recorder with two pens, one recording second ticks from an accurate local clock, one recording presses of a morse type signalling key).

Just about any scope with a stable optical system could be used for a modern equivalent but the mount & chronograph would have to be custom made. If you have good engineering skills & a modicum of 19th century electrical knowledge the mount & chronograph could be self constructed reasonably easily. Transit instruments were usually mounted in a building using the walls as the pillars for the bearings; rigidity is everything; I can't see that any currently available commercial mounting would be suitable.

These days astrometry (position measurement) is done by measuring CCD images; exact pointing precision of the mount is unneccessary & measurements can be carried out in any part of the sky, not just in the line of the meridian.