BRUCE MILLS-'scope stuff

The Eyes Have It!

20" F4 TELESCOPE

Finished near the end of 2004 after about 18 months in the building.

Heck!..how could anything that looks so starved take so long to build?  Well,most of the time was spent thinking how to to make it look so desperately famished.  Weeks , sometimes months , would pass with nothing at all cut as other things distracted me . And,  as ever,the devil is in the detail.

When aperture fever got ahold of me I found the websites red highlighted inspirational.  If you should go to Bruce Sayre's site,within you would discover that his 22" design was a great influence upon the design of my big 'scope.  I particularly admired the way his main mirror was totally exposed to the air,and his rejection of the mirror in a box convention.  Virtually every aspect of his design displays original thinking.  Additionally he's an advocate of metal construction which in this day and age of wood 'Obsession' cloned designs for large'ish amateur built 'scopes is unusual.  Metal telescopes take longer to build, and if fit and finish is an important consideration , the opportunities for 'winging it' dramatically reduced.


I can't claim to have been possessed of an abundance of forethought and planning,scrupulously following a set of wonderful CAD generated drawings.  This telescope was built ad hoc,mostly from rough sketches and thought bubbles,and likely accounts for the fact it took over a year to create.  I did a careful scale drawing on an A4 sheet of the cell triangles which was my datum.

Another lad who influenced me greatly,in fact got the ball rolling when I saw his pioneering 24" design,was Greg Babcock. His 'scope looked so cool I was hooked and had to make one myself. 

In the photo above,six carbon graphite truss poles seperate the top secondary end from the bottom mirror cell end,and come apart as a single unit to concertina together for travel and storage.  I have only three Allen screws into studs on the top ring and another three into studs on each of the three apices of the main mirror cell.  Without rushing it might take as long as three to four minutes to lug everything outside from my backdoor,or back of the car,and be set for observing.  The whole lot including the 20mm Type2 Nagler eyepiece,which is nearly a fixture,weighs just under 100lbs.  The mirror is 2" thick and,at 50lbs,accounts for a little over half of that total.

There are some more pictures of the whole structure in the "Random Pictures" lower down the page.

20" GROUND SIDE

Here,below,is a closer look at the whole of the bottom end.  The main mirror in its cell riding within the 'C rings' which in turn rotate upon the rocker frame by way of four needle roller bearings for altitude.  The axes of those four bearings are directly on top of the axes of the azimuth bearings.It all moves nicely.

I had the 'C rings' made out of 1"x 5/8ths"aluminium stock at the White Cross Ring Co.Ltd. of Bradford, who rolled them to the radius I'd worked out for the C of G about 8" above the mirror's surface.  It could have been lower but I wanted to use the T2 20mm Nagler as my main eyepiece(100X) without compensatory weighting. 

The 'C rings' were the only engineered items I didn't make myself.  I had them rolled then united them with 1/2" rd.stock as spacers. The two struts that you can see spearing back from the outboard end of the 'C rings' to the two groundside apices of the cell have spherical bearing Rose joints, one threaded left and the other right at each end, therefore by turning the rod by way of a key through a centrally bored hole it acts as a turnbuckle and imparts further stiffness to the 'C' rings.

TOP VIEW OF 18 PT.MIRROR CELL

 



This is a picture looking down onto the mirror cell from above before it was anodized at Colour Anodising Ltd and before the rocker and base was finished.

 

 What you are looking at,above,are the eighteen quarter by one inch round rubber pads which are fixed atop inch high turned studs screwed into the apices of the six lightened,(..in actual fact to promote air flow) flat triangular plates that each float independently upon a stud that protrudes out of a spherical bearing let into the top side of one of the three rocker arms that pivot on a 1/2" C'sunk Allen screw at mid span of the main inside triangle.  -Draw breath!  -The main inside triangle "floats" within the main outside triangle by way of three 3/8ths" cap head Allen screws through clearance holes in the outboard rectangular plates at the apices of the outside triangle. These spring loaded screws whose heads are on the sky side are the collimating points and with the aid of a ball headed Allen key make collimation easy and not a chore.  A joy.

This picture,at left,of one of the three outside support plates fixed onto the main triangle illustrate the inch diameter upright mirror keeper which I constructed as a stopgap,as I wanted to come up with something more elegant than a sling support.  This keeper is eccentrically drilled vertically so the post pivots as a cam and centres the mirror.  In the event I've found it works so well that it's the elegant solution I desired.  It stays.  The brass stud is screwed into the plate and is itself internally threaded to receive the 1/4" cap head Allen screw that makes captive the truss tube blocks.  Between the brass stud and the tall mirror keeper can be glimpsed the head of one of the collimating screws upon which is suspended the whole of the inside triangle and glass.

The cell's main triangle,and the floating inner,was constructed out of 2"X5/8ths" drawn aluminium stock.  Each bar length was roughly cut to the appropriate angle and length with a mitre saw,then precisely finished on the milling machine.  Not being an enthusiast of welding, (I can't do it) I joined the bars at each of the apices with the aid of three countersunk Allen screws,the clearance and tapping holes for which were drilled with the aid of an optical chuck and the X and Y traverses of the miller.  I'm confident of the accuracy of this job and was pleased with the result.

At bottom of the above picture is part of the square flex rocker frame and base azimuth circle which sits on the ground.

This is another photo,below,of the same area but after colour anodizing and with a couple of the six SuperSpars 22mm.O.Dia.carbon truss poles in situ.Note the paleness of the black steel screw heads compared to the black of the anodizing and the carbon.  In actuality they're next to invisible but under the harsh scrutiny of flash they look pretty washed out.

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This last one,below,is one of the four corners of the flex rocker frame and azimuth base circle,which was made from the disc of aluminium that remained from the piece of plate that made the secondary ring at the eyepiece end.  The block holding the needle roller was milled out of solid.  The axis of the roller is directly above the axis of the azimuth roller beneath it,and out of sight.  It all moves very nicely.  The flex rocker frame,a ringer for Bruce Sayre's in so far as I could make it, is very light and made up out of 1/2"x3/8ths" bar. I had thought of slavishly copying Sayre's frame and making it driven with a motor each to an alt'/az' roller,and a Bartel's like controller but,as a confessed computer dummy I scared myself off that path.  Cheaper too!  Another construct using mitre saw,miller and screws.

RANDOM PICTURES OF 20"F4

  Mid March 2007.

After a couple of years I finally got around to making a baffle for opposite the focuser.  This was an event !

Putting these pictures up has reminded me that the mirror hasn't been described yet. An ATM purist of the old school where the crafting of the optics was the prerequisite to describing oneself "telescope maker",it took a long time for me to accept the modern idea that it would be a heck of a lot quicker to purchase the optics and devote my energies to the nuisance of building the structure.  So, I copped out on scratching a mirror out of solid by myself. I near enough emptied my pockets instead and bought it finished from Markus Ludes at APM in Germany.  It's the reason I built a 20" and not anything larger or smaller.  Gripped by the condition known as aperture fever, I'd decided that it would be better to have the main mirror to begin with and to build the 'scope around that than to start building and ordering a mirror to whatever prescription I decided upon and trust to the fates that I'd get what was ordered. Astro' mirror makers are,like me,dreamers and fickle beings. So,this twenty incher became available at exactly the same time I was thinking "I need a mirror"and it came with a good write up. Furthermore,at F4 it also allowed me the chance,I thought,to keep my feet planted on the ground.  In fact in the happy hunting ground around the zenith I have to stand on an instrument case,(for example) as the focuser axis is 74"above ground.  At 6'something tall my eyes are at 69".  So,..just a little step up.

Markus had put it up for sale on Astromart as a cancelled order,describing it as "a rare chance to procure a real high end paraboloid".  It came with a Twymann-Green Interferometric Test report,...which sounds very grand,and indeed clinched it for me to purchase, but in the event turned out to be a tatty third hand A4 copy of a 'Roneo' produced sheet.(...remember those hand cranked copiers?) whereon the graphs and 3D plots were barely discernable as such and,furthermore,utterly illegible.  Not the 'printed on parchment,bound in vellum with silk ties' testimony of merit I had fancifully pictured.  In response to my muted yelp of dismay Markus replied,"..Russians not too good with the paper."  Oh,well.That was that. I kept the mirror,after all,I'd started the mission.  Through the haze of overprint I could make out that the "paper" nonetheless awarded the mirror a Strehl of 0.966,a P.T.V. of 0.167 and R.M.S. of 0.027. A Strehl of 0.966. is 'up there' in the first rank of mirrors,and were it so a very rare thing indeed.  It was made according to Markus by the former."Master of Aspherics"at Intes in Moscow,"Optikmeister Sankowitsch".  It came with a 91% reflective aluminium coating.  I bought a Bryan Geer made ProtoStar 3.5" secondary with a 96% coating and a Starlight Instruments Feather Touch focuser shortly afterwards from Markus also.

 

MARISSA