RC250F10 In the Making

By RGP

Tuesday, August 01, 2006

The basic idea came from seeing and reading about the BIG telescopes such as Keck, VLT and LBT. As a result I decided I wanted something that incorporated  the same high quality, no compromise, design.  When I embarked on the project I looked at many of the scopes out there and it soon became clear that you either had an LX200 or something else!  In the something else bracket there are some really serious scopes from manufacturers such as Optical Guidance Systems and RCOS, as well as some unusual designs from small scale manufacturers, such as OfficinaStellare and Dream Scopes. The big stumbling block for all these scopes is PRICE! Not only the cost of the basic OTA but also the cost of the mounts and all the accessories soon adds up to the cost of a decent family car. Way out of my league!

My second motivation for building my own design was to have a scope that could take accessories and be developed over a period of time to perform more and different things, such as visual and photographic and maybe in the future spectography and photometry.  The BIG scopes are designed from the start to take a host of instruments and I wanted a scope like that, but one that I could afford and could fit in my shed such that my wife didn't say "....and where are you going to keep that?"

I soon realised that my design of choice was the Ritchey Chretien, which at the time I knew little about. In fact the name was not very often heard until Meade tried to hijack the name for their (then) newly released RCX400. At the launch I was in the depths of researching about RC designs and even I knew then that they were stretching it a bit with their RC claims...anyhow that's a different conversation and one that the courts have resolved. Read More?

It soon became apparent that the RC design is not simple and that a Newtonian or Dobsonian telescope would be far cheaper and easier to construct.  In fact I read several accounts which said that RC scopes are really beyond the capabilities of most ATMs (Amateur Telescope Makers). This last statement made me want to build an RC scope even more! The mirrors I originally wanted to make too but are the one area where I did concede defeat.  This was a decision taken following a visit to a local mirror maker Oldham Optical Ltd. They were kind enough to show me round their facility and Norman, the owner, gave me an insight into the making, and testing, of high quality RC mirror sets.  I soon realised that in order to make a good RC mirror set you needed both a lot of mirror making experience, along with some expensive equipment, such as a large optical flat for performing the autocollumation tests on the mirrors and the system as a whole. All of this was clearly beyond my means and skill. At that point I decided to buy the mirrors.

I would have commissioned Oldham to make the mirrors for me; only a couple of reasons stopped me. The first was that (at the time) the smallest RC mirror set they made was 300mm or 12.5" which, following a quick calculation, was simply too big. Secondly I would have had to wait a long time for them, but mainly they would cost £3000, which I couldn't afford. My proffered size was 250mm or 10", so the search began. I soon realised that there are not many companies worldwide that make Cassagrain mirror sets and that most of the main RC scope makers used mirrors from one of only a couple of makers, either (predictably) in the USA or (not so predictably) from Russia. And it seems that the Russians are pretty innovative opticians making some of the highest quality mirrors on earth...so the east is where my search moved.

The power of the Internet soon threw up a small mirror maker in the Czech Republic called Drbohlav. They had a selection of mirror sets on offer, from the usual Newtonian jobs to the more unusual Cassegrain designs, including the elusive Ritchey Chretien sets. They also started their RC mirrors at 250mm / 10". They had supplied mirrors to projects in the Czech Republic, as well as other eastern bloc countries, to institutes and research observatories. The guy who runs the company, Jiri, was very informative and gave me a number of choices in the design. I finally decided on a 250mm / 10" Pyrex mirror with a focal ratio of F10.  The really good news was that they would be completed within six weeks from order and all for 1/3 of the price of Oldham (sorry Norman!)

I have had the mirrors for some time now and the wait from Oldham wouldn't have mattered as I will be a while yet finishing the OTA. I think I was a bit optimistic with my time to build this complex scope.

By the time the mirrors arrived the tube construction was well under way and I had decided on an open tube design incorporating a mixture of aluminium, carbon fibre, titanium and stainless steel. (Lovely!) Although wood is a fantastic material, with many great properties for scope making, none of the BIG scopes (VLT, Subaru etc) contain wood in their construction! - except maybe for the packing cases used to deliver the parts in!

As I (now) have a fully equipped engineering workshop in my converted garage it made sense to use metals as the primary materials for the scope. I also wanted the challenge of finding out about new materials, such as carbon fibre and titanium.  It also became apparent that BIG scopes are not just small scopes scaled up. This sort of engineering doesn't scale (much). In the reverse a small scope cannot be made by scaling down VLT.  I did however want a design which shared not only the look and feel of a BIG scope but the flexibility and adaptability mentioned before.  There are a number of open truss scopes on the market, some of them that stick to the Surrier principle, most do not. Read more about the Surrier principle...  I took design ideas from a number of the designs commercially available and, as all ATMs, added my own flavour to the design. 

The design was made principally using Sketchup - the free 3D design tool now belonging to Google.  I have been using Sketchup for over 5 years and would highly recommend it to anyone. It's the best piece of software I own (or don't own, in fact!)

An overview of the design done in Sketchup. (Download Sketchup V7 SKP file for my RC250F10 here...(1.5mb)

 

Mirror cell calculations were done with another fine piece of software called PLOP. PLP is a FEA (Finite Element Analysis) tool dedicated to mirror supports.  If you have ever played with other FEA software you will appreciate how brilliant and simple PLOP is!  Using PLOP I designed the primary mirror support, a six point see-saw design.  It's pretty simple yet should maintain the correct figure of the mirror. Originally I was going to employ a 9-point design, however I worked out that the added complexity wasn't worth it for the gains over the 6-point design. In fact the additional flexure in a 9-point design would, I believe, have not outperformed my more sturdy, simple, 6-point cell shown. The mirror will be held to the cell using RTV or silicone (bath sealant!). This is a technique used quite widely now on medium (<2m) mirror designs. It's preferable to clamps as it seems very difficult to get a clamp to work without deforming the mirror...not good when you spent all that money to get 1/8 wave performance!

 

Calculating the C of G.  This is a fairly easy thing to do, however it does pre-suppose you know the weights and positions of every component in the design, which I didn't!.  You therefore have to weigh anything you actually have and the rest you must estimate based on the known weights of the raw materials and working out from the sketchup design as near as possible the weight of each component.  Once all the weights are known you simply multiply the weight by the distance of the art from the datum. The datum in this case is the end of the focuser tube, therefore all moments are positive. The spreadsheet below shows the results and that gives the CofG for the OTA (I hope!) by dividing the total weight by the moments*weights. or...

 

Unit   Distance from datum Weight Accesory Moments
Focuser Estimate 65 1.5 97.5
Backplate Actual+est 130 3 390
Mirror Cell Actual 164 0.7 114.8
Primary Mirror Acual 204 3.86 787.44
Spacer Plates Estimate 270 8 2160
Middle Ring Actual 404 0.9 363.6
Truss tubes Actual 638 1 638
Front assembly Actual 899 1.64 1474.36
Total 20.6 6025.7
CofG 292.5097




 

Other analysis tools used in the design process are MODAS. This is a ray tracing optical design tool and was extreamly useful for calculating the correct dimensions and placement of the mirrors and baffles. The guy who developed the software, an Austrian optician called Ivan Krastev, also publishes a really interesting and informative telescope making online periodical called ATM Letters. Definitely reccomended to those wanting to know more about optics and scope design - not just Dobs, which seems to be all that mainstream ATM articles focus on in Sky & Telescope, Astronomy and Sky at Night.  Another interesting online periodical is Astronomy Technology.

 

Some of the actual parts taking shape...

The middle ring having the lightening holes bored using a hole saw on the lathe. It's amazing what you can fit onto an old Myford ML4 31/2" lathe!

Rear and middle rings with baffle tube (not cut to length yet) The mirror suspension unit is resting on the baffle tube flange. It's a 6 point design and the plop analysis can be seen above.

Front ring & Spider with secondary mirror mount. The mirror will be RTVed (Siliconed) on to the plate. Push-pull secondary collimation screws can be seen on the triangular secondary support plate.

The milling on the secondary hub was done on the lathe with a Myford vertical slide. Now I have my milling machine I realise how limited the milling slide on the lathe is. It isn't very sturdy, nor is the lathe very powerful. A good learning experience however. If there is one thing that ATMing does, it is to make you learn stuff!

Spider vanes made from 0.5mm stainless steel sheet. Plenty of tension prevents any droop. I have yet to fit the motor drive for the secondary focusing. This will also act as a counter balance to stop the weight of the secondary mirror from twisting the spider assemply...(see diagram) The droop causes de-collimation

 

Blocks and vane ends all made prior to the milling machine's arrival. Sawing, drilling and filling - you get quite good at it! Spider vane preloading is done with the allen bolt.

By tensioning or preloading the spider vanes it will do two things. Firstly increase the resonant frequency of the assembly (helps to prevent vibrations) and secondy prevent droop, again a big cause of decollimation.

This diagram illustrates spider vane droop from incorrect preloading, and subsequent decollimation of the system.

 Primary mirror suspension showing the 6 point see-saw design. The see-saws run on plane bearings and are very free moving (hopefully enough to work ok).

It's amazing how tools seem to accumulate, all by themselves! Especially files, these are just my favourites, I also have a box full in the cupboard. I don't think you can ever have too many files!

As the materials for the scope come from anywhere I can lay my hands on them, mostly from a local monthly auto-jumble.  Don't let the name auto-jumble put you off, although they are predominantly bike and car parts, there is also lots of engineering stuff and materials there too. Keep a look out for them near you but be careful not to get hooked on them like me!  The materials come in all sorts of shapes and sizes and sometimes require redesign work to better accommodate them.  Some of the detailed parts of the design were prototyped before the final manufacturing was carried out.

The large holes were cut out using a wood router and a carbide bit. The router was screwed to a piece of wood and bolted through the centre of the sheet of ali. I then worked round until the bit broke through. It was finished with a file and emery.  I have subsequently re-done the secondary ring on my new milling machine and the finish is much better now.

The three rings including the secondary arrangement. The primary reason for completing the secondary end first is to make sure the weights are accurate as they have such a big effect on the CofG and getting the truss tubes the correct length is very important as once cut there is no going back and the carbon tube cost me £138.

Once the tube lengths were confirmed from the CofG calculations I cut them from the full lengths (2.5m) into the finished lengths.  After much research I decided to cut the tubes on my lathe by using a thin splitting saw and holding the tube in a small vice on the saddle.  This allowed me to advance the tube onto the saw perfectly perpendicular to the tube.  The finish was excellent and required almost no fettling afterwards.  I found that some PVC insulating tape around the cut stopped any splintering of the carbon mat.

Above is a prototype for the ends of the tubes. The end cap is, with its ball, is Araldited into the end of the carbon tube. The ball joint end is then located into the block which contains a socket to hold the ball. By tightening up the two screws the ball, once set at the correct position, can be clamped tight. The two caps at the left have not had the cone and ball turned yet.

Ball turning was something I hadn't done before and before I could I had to make the ball turning tool. There are many designs and techniques for turning balls and my design is based on those found here.  I opted to make the tool from Aluminium, purely because I had a lot in stock, however steel would be a more durable material.  The stresses on the tool however are not large and I am sure that for turning aluminium and plastic balls it will do me for years. The carbide tip was 50p from my local auto-jumble. It's a bit rough but it works a treat!

An end cap in the process of being turned up. Using a parting off tool to turn the shank.

(Below) The endcaps on the left have not yet had the shank turned whereas the ones on the right have.  The arbour I made was necessary to give enough clearance for the ball turning tool. Held directly in the lathe chuck didn't allow the tool near enough to work. (more tool making!) Notice the marks for chuck jaw number.  If you mark your work with a sharpie it makes re-chucking work much better and concentricity seems to be maintained.

The finished ball ends.

All that needs to be done now is to glue them into the carbon tubes. This I did using the lathe as a clamp to ensure they are held tightly until the Araldite cured.  I used the slow curing sort as this makes a much stronger joint.  I have tested the prototype and I simply cannot get the end peice out, it's in there for good! (I hope).

Using the lathe tailstock to exert pressure on the tube ends whilst the adhesive cures.

The finished tubes complete with their ball ends...all 16 of them!  A lot of repetetive work, however the results are well worthwhile and the tubes are very light and VERY strong. All I have to do now is make the 16 clamps for the balls to reside. I think I need a milling machine!

Unfortunately I have no photos of the clamp making.  It was all performed on my recently acquired CENTEC milling machine. A lovely little machine, perfectly suited to my needs. Making the clamps was a breeze with the Centec and the results are really nice.  Once made I assembled the entire cage in a matter of 1/2 an hour, which was a bit strange when I had spent the last 2 years making all the individual bits.  I might dissassemble it soon just to have the pleasure of assembling it again! In fact it will have to come apart again before the end to paint / anodize all the parts.

The clamp blocks machined on the Centec. Milling, drilling and the internal cups (done with a ball nosed endmill) were all done on the Centec.

The tubes now all mounted onto the middle and secondary rings. The back plate and the middle rings are just resting on the side plates. They have been milled but still have to have the dovetails machined and be counter bored to mount them on the rings.

I am getting ever so close to fitting the mirorrs for the first time. If I could get them in and a basic alignment done prior to Christmas I will be very happy! Let's see if I can do it!

Back to the workshop to make some more swarf! ....

27-7-2010

After a long period of no progress I am now back on with the telescope. The old Myford finaly had to go to make way for the new Harrison M300...WHAT A DIFFERENCE!!!!

Harrison M300 Gap bed centre lathe

I have had to re-arrange the whole workshop to make way for the M300 lathe wich is SIGNIFICANTLY bigger and heavier than the old Myford ML4. At 600kg it was intereting to get into place.  I also had to move lots of other stuff around and the milling machine is now in a much better position.

The M300 needed alot of cleaning up and completely re-wiring for single phase and the fitting of an invertor drive.  If you are interested in how I did it there is an article here on what I did.  There is also a copy of the M300 manual here for anyone wanting a PDF of it (and not wanting to get ripped off for one on e-bay!).

And here is the new position fo rthe Centec 2A.

Centec 2A Milling Machine

The focuser is now almost there. It is basically a version of the Jim Sapp focuser. (see http://www.atmpage.org/contrib/Sapp/Focuser/CrayFocus.htm) I have made a few things based on Jim's work. He is a really nice chap and always most helpful whenever I have asked his advice.

Click Drawing to download a PDF of the drawing.

Click for PDF version.

There are some modifications to his design (it wouldn't be right not to put my two-penneth in!)...

You may see that I have put a height adjustable bearing on the right rear bearing. That's because I found that the bearing wasn't precisely in the right place.  I came up with the adjustable bearing idea and it has worked well as it has removed every last drop of play I had originally. Getting the bearing shafts perfectly parallel to the tube is tough...especially when they are down the bottom of deep holes. 


The other point worth mentioning is the bevelled ring at the rear of the body.  This isn't fixed on yet but will locate into the recess of a back plate and will be held by 3 radial screws allowing some centring adjustment, plus the ability to rotate and remove with the use of one screw.(I think...?)



Lastly, the 'thing' on the other end of the shaft from the knob is a dual rate ball gearbox from a radio tuning knob.  When I finish it I will post the details and photos.

May I say at this point:  Mr John Wall, MANY MANY THANKS for releasing this wonderfully elegant design into the wild.

Focuser back-plate, mounted to back of scope with push-pull 120degree screws.

Drilling radial holes at 120degree spacing. Using roatary table on to top-slide of lathe.

Back of the focuser plate along with the focuser body (with tapered ring mounted to body. This is then held into the back-plate with the three radial grub screws. You can see an allen key in the top radial hole. I think I will make some brass knurled thumb bolts to replace the current grub-screws.

The focuser mounted to the focuser back-plate. (still need to fnish the fine focuser) - Allen key is for radial grub-screw that holds the focuser into the focuser back-plate.

Now the finish....

I spent along time looking at what finish I should give the scope. Paint, power coating or anodizing.  For a little time I thought I might have a go at anodizing myself, however it isn't a nice process and involves lots of corrosive substances and when you have two inquisitive cats it seemed like a disaster waiting to happen! So I started to look around for companies who could do anodizing.

Eventually, on reccomendation, I visited Moss Metal Finishers (MMF) in Halifax (Hipperholme) and spoke to Dave. He showed me around the facility and described how the process worked, as well as showed me lots of examples of their work. I soon learnt allot about the whole subject, for instance IT'S MESSY AND DANGEROUS! Dave also showed me how different alloys gave different results and that some alloys, namely the 7000 series, didn't really anodize very well and often came out blotchy...this was obviously not an exact science. It seems that colour wise different alloys take-up the dies differently and the red colour I quite fancied could come out anything from a a pale pink colour to a deep cherry red. Dave said the best colour for consistency was black and as I had a load of alli from the autojumble, which was of unknown origin and composition, then this would be the best way to ensure reliable results. He also showed me some examples of how the fininsh on the ali (prior to ben anodized) effected the final result. Clearly I would have to polish out every blemish from the parts and impart either a high polish or a consistant brushed finish.

On my return to the astronoscope workshops I stripped the scope down to every component part. Fortunately the weather was very nice so I sat outside int he sun, with a load of emery cloth, and started to rub. SOme of the parts, once the scratches and other blemeshis were all removed I imparted a brushed effect by rubbing the parts with 3M scotchbright.  It is important when you do this to keep the rubbing action as linear as possible. It is ver easy to start doing arcs (piviting about your elbow!) and this doesn't look as good as a straight brush, but is a littleharder to acheive.

Once ALL the parts were finished (which took a couple of weeks of evenings and a weekend- or two) I wrapped them all up in paper to protect them and delivered them to MMF to be finished. I then went on my holiday which gave them a couple of weeks to get all 50+ parts anodized.

On my return from holiday I couldn't wait to go and collect my finished parts and I was not dissapointed. Other than a couple of the parts the colour was pretty consistant and the black was very black. Good!  The one part, made from aircraft grade 7000 series alloy, the Crayford focuser body, was the only part that was not fully black and had a very slight blue cast...but only if you directly comared it to the other parts. Standalone it looked fab!

Here are all the telescope parts prior to their re-assembly

All nice and black!  (this is a couple of photos stiched together that's why it has funny shadows)

For those of you who like statistics there are a total of 97 parts (made by me), 201 stainless steel screws (purchased) - that's a total of 298 parts. And I have made 342 holes in the process.  So far the project has taken me over 600 hours to this stage. (that's probably conservative, and doesn't include research time).

Now the build up...

Secondary spider all back together.

Secondary focuser/back-focus adjustment knob. It didn't make it in time for the anodizers, however I now quite like the bare ali knobs. I think they compliment the black nicely.

 

Rear cage fully assembled prior to the mirror cell and secondary cage and trusses been fitted.

Cooling fans and primary baffle holder now in place.

Focuser mounting plate now fitted to rear plate.

Crayford focuser almost complete.

Circular dovetail of the Crayford focuser. Tension screw at bottom needs replacing with a nice adjustment knob..it's on the list. (Knob making is the nice bit!)

And finally, all together. Complete with mirrors. (Mirror mounting details to follow....)

A good friend has leant me his EQ6 mount in order to set it all up and try it out. This is very kind and I am pleased of it as I could not wait until I have made the mount, which realistically could take me another year to complete!  I will probably have to buy somthing to put it on until I have mine made, but in the mean-time here it is.

Losmady style side plates make it a great and easy fit straight onto the ADM plate on the EQ6 - VERY sturdy indeed!  I am glad I overan the plate as this helps balance the scope about the DEC axis.

The shroud is made from some thero-plastic bought from a local (York Plastics) plastic engineering company and is used for vacuum forming plastic parts. It needs to be blackened inside which I intend to do with Edmund Optics Ultra Black Flock, glued in place with polystyrene solvent. Dovetails on all 4 sides for lots of guide-scopes and accessories.

CLick for full image

As you can see there are still a few things to finish off prior to real astronomy(like blackening the primary baffle...and making the secondary baffle!), but I should have those complete by the time the nights start getting dark again.

I have done a quick allignment and pointed it at a distant telegraph pole and there I saw a nice bright, sharp image HURRAR! it works!!! (Phew)

The guy who lent me the EQ6 was given the peasure of been the first person to look through it. Hopefully he won't be the last person to have a go...roll on winter!

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