From wpk@saao.ac.zaFri Jul 17 22:50:06 1998 Date: Fri, 17 Jul 1998 20:06:27 +0200 (GMT+0200) From: Willie Koorts To: harald.ringsevjen@umoe.no Subject: Re: Split-ring - ref. your web-site Hi Harald Thanks for the nice comments on my telescope. I will try and answer your questions as best I can. Unfortunately I am at Sutherland (our observing outstation) at the moment and cannot measure the pipes right now. The OD is close to 3/4" (20mm) but is not critical since it would be dictated by the size of the rubber anyway - see below. I bought the rubber plugs from a firm called Cape Rubber Company who deal in rubber in all shapes and sizes. It is important to get the rubber to be a good as possible fit inside the pipe and also over the bolt going through the centrehole of the rubber - it is therefore almost best to look at rubber sizes first and then get pipes to fit. The clamps I made myself by bending steel plate and cutting and grinding the "cams" by hand. It would be neat if one could get something ready made - the handmade ones do differ a bit - luckely the differences can be taken out by adjusting the top nylock nuts - these I adjust by frequently fitting the pipe while tightening until the pipe does not slide over any more and then slack off the nut a little, thereby getting maximum expansion and grip. I used the plywood from an old packing crate which is normally not the best quality but it seems OK- good plywood is almost unaffordable here too. The positions of the support points for the 9-point floatation system was obtained with a simple BASIC program which I got from a back-issue of "Telescope Making" magazine. I was asked about this program by someone else as well and want to still check out my old floppies to see if I can find it and put it on my website. There is a site with something similar linked from my ATM links - have a look there. The three triangular plates are sitting on the ends of three M8 bolts coming up from the bottom of the mirror cell. These ends are rounded and also drilled and tapped for a small M2 axial screw, preventing the plates from falling off. The centre hole in every triagular plate is countersunk (from the bottom as you see them in the pictures) and drilled with good clearence to the M2 retaining screw but not too big for its head to fall through. The rounded end of the M8 screw now forms a nice socket with the cone shaped countersunk hole in the triangular plate to pivot on and the small M2 retaining screw going through prevents the plate from falling off. To prevent the triangular plates rotating (mainly when the mirror is not in the cell), three bits of string ties thier inner ends together towards the cell's centre and make them all face inwards - small springs would also work here. I must make a drawing of this and add it to my site - maybe tonight still. The auto-collimator is also easier to understand from a drawing than trying to desribe it. It is basically an eyepiece with all the lenses removed. A small round mirror with a centre hole (or just the silvering removed in the centre) which can fit inside the eyepiece is stuck to the inside wall of the eye end (where the exit pupil is). To line up this mirror, while the glue is still soft, put the eyepiece in a lathe on a slow speed and look into the mirror at the reflection of your eye from the open end (opposite way you would be looking through on the telescope). If you don't have access to a lathe , just put the eyepiece in something forming a "V" and rotate it by hand. By watching the reflection of your eye as the eyepiece turns, this reflection would be stationary when the mirror is square onto the axis of rotation (optical axis) and would be wobbling in sync. with the rotation if mis-alighned. Rectify any mis-alignment before the glue cures. To mark the centre of your primary mirror, the best is to put the mirror on an old fasioned record player turntable - you would need some packing underneath the mirror because of the turntable pin sticking up from the centre. Centre the mirror onto the turntable by making it spin (33rpm is OK) and move it around until the edge of the mirror is running smooth (not wobbling sideways any more). Now take a soft filt-tipped pen and with the mirror spinning at 33rpm, steady your hand and lightly toutch the mirror with the tip of the pen as close to its estimated centre as possible. It is almost impossible to judge the mirror's exact centre but this is OK since the offset would result in a small circle being drawn round the centre - the closer you got to the centre, the smaller this circle. Hopefully you were at least within the diamater of your secondary and would therefore cause no harm being obstructed anyway. Using this mark as a guide (or draw another concentric circle of the desired diameter), stop the turntable and stick a self adhesive (doughnut shaped) ring binder strengthening sticker in the mirror's centre - some people use a square sticker to identify it better amounght all the reflections - see below. This sticker could be made more visible by coloring it with a bright colored pen. To use the auto-collimator, first collimate the mirrors in the traditional way without anything in the eyepiece drawtube using your naked eye and the telescope pointed at the daytime sky (or a brightly illuminated ceiling at night) - the centre marking will already be of help here. Now put the auto-collimating eyepiece in the drawtube and look through it - you are now looking from behind a mirror which is square onto the optical axis of the telescope and you will see multiple reflections from the three mirrors in the optical system. If you don't see the reflection of the centre marking at all, the initial collimation is too far off and needs to be repeated. Normally it is visible but probably not very bright and slightly off centre. By adjusting the primary mirror only, the reflection of the centre marking will brighten dramaticaly when (getting) near perfect alignment. Any remaining mis-alignment is now adjusted out by a final tweak of the primary mirror until the (brightened) reflection of the centre mark is concentric. As a final check, turn the eyepiece in its drawtube and what you see through it, should not change. The telescope would now be collimated as good as is possible in daytime and can only be improved by using the traditional out-of-focus method through a high powered eyepiece on a bright star at night. I find it mostly not even necessary. A split ring for 60 degrees lattitude would be very similar to my 33 degrees - 45 degrees is the ideal and 0 degrees (or 90) near impossible. Hope this makes sence. If I feel energetic tonight I might just add a bit to my website since I had similar questions before and have been meaning to do this for a long time anyway. Regards Willie Willie Koorts wpk@saao.ac.za Cape Town, Observatory 33d 56' 03"S 18d 28' 36"E GMT + 2h Wellington, South Africa 33d 38' 56"S 19d 00' 52"E GMT + 2h For - Amateur Astronomy - Telescope Making - Satellite Tracking - Visit .... http://www.saao.ac.za/~wpk/ ****** New! .... Gallery and Astronomical History pages added! ******