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Old 23-04-2011, 11:47 PM
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SkyViking (Rolf)
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First ATM project done: An equatorially mounted 10" Serrurier truss Newtonian OTA

I have lately been working on my first ATM project: An equatorially mounted Serrurier truss Newtonian OTA. I earlier received some great advice in this thread: http://www.iceinspace.com.au/forum/s...ad.php?t=67713, and as a result I settled for the Serrurier truss design. Thanks to all for your previous inputs.
My major inspiration came from this beautifully constructed Newtonian: http://www.aao.gov.au/local/www/sl/sl-tels.html#ni-tel

For many more images of the project see the gallery at http://www.pbase.com/rolfolsen/10_in...tube_newtonian

This project was intended to be a cheap upgrade I could work on while saving for a new CCD cam. To give the upcoming new cam justice I didn't want to mount it on my existing OTA which is a 17 year old simple sonotube, squeezed out of shape by the secondary spider supports and generally emitting a slight mouldy odour - probably due to the many years of dew and moisture absorbtion I guess. It was definitely time for something new.

While I intend to purchase a better mirror (a conical Royce mirror seems tempting) I decided to build a 10" to keep costs down, since my current telescope is of this size and so I could simply reuse the optics for the time being. Also, I was not sure how much load I could safely put on the G-11 mount in practice while still maintaining smooth tracking for photography purposes. While a 12" would be great I felt I'd rather have a stable tracking 10" scope than a 12", or larger, which may cause problems for the mount. Furthermore, the 10" is just about the right size for my observatory, any larger and I think it might start to become too bulky to maneuver around in the dark - otherwise I would have to build a larger observatory...

The design consists of three parts: The mirror cell, the central box/brace and the secondary cage.
The central brace was the first part I built, since this was the most critical component as it must be very strong and rigid in order to carry the telescope and eliminate flexure. I made this out of aluminium square tubing and some pieces of 40x40x3mm aluminium angle extrusions. I cut 12 pieces of square tubing at 45 degree anges and riveted them to the angles. The protuding angles would then later function as truss connectors. This structure alone was surprisingly rigid and ensured that everything stayed in place, but as this component would hold the entire weight of the OTA I asked a local metal smith to weld all the joints on it. The final resulting central brace was very light, beautifully simple, and yet extremely sturdy.

Next thing to make was the mirror cell and secondary cage. I decided to make these from plywood as it would be much easier for me than using aluminium, and it seemed to be the long accepted tried and tested solution of ATM's. For the secondary cage I cut two identical rings with an inner diameter of 30cm from the plywood. For the mirror cell I realised the ring could be smaller (saving a bit of weight), since being close to the mirror it would not interfere with the optical path, so I cut this ring with an inner diameter of 27cm to acommodate the 25cm mirror (10 inches) plus a bit of wriggle room for adjustments. The adjustable mirror cell was then made by cutting a trifoil shape of the plywood and mounting this on the ring with three spring loaded bolts.
The two rings for the secondary cage were connected with 4 round 1" alu tube pieces. I thought about how to connect the hollow tubes for a while before deciding to simply slip them over a wooden round and mounting them with a large screw at each end (see photos for details). The cage turned out to be very rigid, better than I expected.
To attach the focuser I turned to some 3mm aluminium plate I had lying around. I bent this to follow the inner OTA radius and made holes for attaching the focuser. I then chiseled out groowes on each cage ring and screwed the plate onto these, then mounted the focuser onto the plate.

Now I had to weigh the components and make sure the central brace would be placed at the balance point which is the purpose of the Serrurier truss design.
I hanged a board, of the same length as the OTA would be, from the ceiling and then hanged the mirror cell, with mirror attached, to one end. Then I hanged the secondary cage and the secondary holder + mirror to the other end. To make up for the weight of the CCD camera I plan to purchase I added a bag with a book of similar weight to the secondary end. I then shifted the attachement of the board around until the two ends were in perfect balance, and I could then simply measure the distances on the board to know how to place the components when assembling the trusses.

I laid out the components in the correct positions and measured the required length of the first truss tube for the upper half. I then cut that tube plus the other 7 tubes to the same length. Then I put the first tube in place and drilled a hole all the way through both the tube and the alu angle extrusion on the central brace which would be its attachment point. The tube was then fastened with a bolt with washers and lock nut. I repeated this for each tube, carefully measuring that the central brace and the secondary cage were aligned at all times. This whole process was then repeated when attaching the mirror cell.

For the spider I considered a number of different options. My old spider had 4 vanes made of thin metal bands. Even though the vanes are very thin the reality is that they are more often than not slightly twisted and so produce rather large 'fat' diffraction spikes. Also, no matter how thin this kind of spider vane is, it will only look thin to a light ray that enters exactly along the optical axis of the telescope. From any other angle it will inevitably look much fatter than a thin steel wire and thus produce much more prominent diffraction spikes. So I decided to instead make a offset wire spider. I got some inspiration from Mark Cowan's minimalist wire spider design (http://www.raddobs.com/atm/wire%20spider/wirespider.htm) but I decided to go for an absolute minimalist design where the entire positioning and collimation of the secondary is controlled only by the wires.
For the wire itself I used 9 gauge (0.23 mm) guitar steel strings. After toying with various ideas for fastening the strings to the secondary cage I simply went for guitar machine heads. I did this to ensure I would obtain smooth and exact control of the spider tensioning and positioning, and because it was just easier and more elegant than any home made solution I could think of. I went for 4 wire vanes rather than 3 simply because I like the cross-shaped diffraction pattern with 4 spikes better than the 6 I would get from having 3 vanes. Another reason was that there for a cross is more space between each of the 4 spikes which could be handy in case I will be imaging something that lies very close to a bright star.
I attached the machine heads to small wooden blocks that can pivot around a screw. This way I got full control of the angle of each individual string, which is important in order to line up the strings completely and minimize the diffraction.

The mirror cell was completed by re-using part of the cell from my old telescope. The reason for this was that it was straight forward and would provide a stable and secure platform for the mirror without me having to make any additional parts. The old cell consisted of an aluminium plate with a 1cm high edge around the mirror. At the bottom of the plate were three cork pads to support the mirror, and on the sides were attached three clips to hold the mirror in place. These clips extended over the front of the mirror so to minimise diffraction effects I decided to get rid of them and instead simply mount the mirror on the plate with silicone. I removed the cork pads and put three blobs of silicone on the plate plus some spacers and then I lowered the mirror onto the blobs and let it rest on the spacers. Once the silicone had cured I then removed the spacers so the mirror was entirely held in place by only the silicone. I also added three blobs through the existing screw holes on the edge which were left from the clips. This fixated the mirror horizontally. Finally I wrapped black electrical tape around the outside of the plate right up to the edge of the mirror. I did this to further fix the mirror in place but also to enable my mirror dust cover to easily slip onto the aluminium plate, which has a slightly larger diameter than the mirror.
The dust cover was made from a black styrene sheet. I cut out a circle to fit the mirror holder and a long strip to wrap around in order to make a cap for the mirror. I welded the two parts together using some liquid plastic glue I had left from my model building days. This glue was some 20 years old, but still worked perfectly, leaving the resulting dust cap very strong and durable.

Yesterday I got the first chance to test the performance of this OTA. The results are excellent, and more than I hopped for in my wildest dreams! I guess this just shows how well the original Serrurier truss design works.
Primarily I was looking to minimise focus drift and flexure with this project, and both have been accomplished to the utmost degree. Over the course of 2 hours I saw absolutely no shift in focus while imaging the Jewel Box cluster and Spica continually. This has always plagued me with my previous sonotube OTA but I'm happy to conclude that so far this has been completely eleminated. It also appears that the OTA holds collimation extremely well in all orientations. I have used the scope for two nights now and experienced absolutely no degration in collimation since completion of the project.
Another major improvement is the diffraction pattern which was pretty bad in my old OTA. To illustrate the difference here is first an image of Betelgeuse taken with the old OTA:
http://www.pbase.com/rolfolsen/image/134141847/original
Notice the fat diffraction spikes and just how much of the light is diverted away from the star image itself. Then for comparison this is Spica imaged with the new OTA and wire spider:
http://www.pbase.com/rolfolsen/image/134141853/original
The diffraction spikes are much thinner and fainter! The starlight is clearly more concentrated in a single point, as it should be.
Finally, I took a quick test image of the Jewel Box cluster, just to see what it would look like now:
http://www.pbase.com/rolfolsen/image/134141855/original
There are practically no diffraction spikes visible. While the presence of spikes is a matter of taste, I personally prefer to know that the precious light gathered by the optics is not spread out all over the field, since this decrades contrast and resolution and could make the difference in capturing a faint object or not.

So overall I could not be more pleased, and I look forward to be imaging with this new OTA.
Again, thanks to all who gave me advice regarding this project.

For many more images of the project see the gallery at http://www.pbase.com/rolfolsen/10_in...tube_newtonian
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Last edited by SkyViking; 26-04-2011 at 05:28 AM.
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  #2  
Old 24-04-2011, 07:21 AM
morls (Stephen)
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Hi Rolf,

This looks really great, just a beautiful scope!

I'm very interested in the secondary spider setup. How does the collimation compare to the traditional three screws above the secondary method? The use of machine heads is a beautiful touch!

Cheers

Stephen

Last edited by morls; 24-04-2011 at 07:33 AM.
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Old 24-04-2011, 09:31 AM
Rod
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I admire your workmanship. Looks great!

Rod.
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Old 24-04-2011, 09:47 AM
Mighty_oz (Marcus)
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Looks beautiful A real work of art, you'll get some nice images from that scope.
How much does the ota weigh if i can ask ?

Marcus.
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Old 24-04-2011, 01:35 PM
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DavidU (Dave)
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Fantastic effort Rolf, you are very skilled.
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Old 24-04-2011, 02:33 PM
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Astroman (Andrew Wall)
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Beautiful telescope, love the design. How does the weight compare between the old and new OTA's?
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Old 24-04-2011, 08:01 PM
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ZeroID (Brent)
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Very inspring Rolf, an elegant solution well executed. I'd also be interested in the weight as I am contemplating doing a similar project eventually.

Next morning EDIT: Just went through the pix of the build etc on your site. It's a work of art in it's construction. I love the machine head spider wire controls, brilliant ! Given me a lot of food for thought in the whole design concept.

Many thanks for sharing

Last edited by ZeroID; 25-04-2011 at 07:34 AM. Reason: Next morning epiphany ...
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Old 25-04-2011, 11:49 PM
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scopemankit (Chris)
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Beautiful work Rolf.
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Old 27-04-2011, 10:23 AM
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Some more observations

Thank you all for your kind comments. I have so far had two clear nights of testing with the new scope and then it has been cloudy since... (of course)
My observations regarding the wire spider design is that this system is actually a lot easier to collimate than having the traditional three screws on the back of the secondary. And I really mean a lot easier! To make an all-wire design was a bit of an experiment, but I must say that I'm very positively surprised by its performance. In the past I had to loosen and tighten the screws on the secondary in order to adjust collimation and this would inevitably disturb the spider wanes. Just the mere action of loosening/tightening the screws would cause a slight rotation of the secondary from the force applied, so it was always a bit of a battle to get the mirror perfectly aligned. Not that I had to do that very often though. But with the wires I now simply turn the machine heads as required and the adjustment happens very smoothly and there is no loosening/tightening involved, it's very similar to tuning a guitar. I can adjust the secondary's angle extremely precisely with minimal effort. Also, the wires don't have to be under particularly high tension. They are definitely under less load than they would be on an actual guitar. The geometry of the wire placement is what keeps the secondary in place, and the geometry I have chosen does not allow movement or rotation in any direction.

Regarding the weight of the truss tube I'm also happy to conclude that it weighs a good deal less than my old sonotube OTA, which was surprising. But this is also in part due to the solid Parallax rings that held the sonotube, they are quite heavy! Instead I have simply bolted the dovetail plate directly to the central brace, as can be seen in the photos. I don't know the actual weights of the old and new OTA, but I will try to weigh them soon just for fun.
I found that the standard Losmandy counterweight (can't remember its weight) that comes with the mount is more than sufficient to balance the entire setup. You may notice a different white counterweight in some of the photos; this is a homemade one I made earlier because I was trying to balance the old OTA better. However, this second counterweight is no longer necessary at all and I'm going to remove it next time I'm out in the obs.

I've attached a couple of photos here which shows the parts I'm talking about.

It's looking great so far and I'll post some more details and observations as soon as I get some clear nights here.
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Old 27-04-2011, 01:14 PM
Dennis
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Hi Rolf

Hmm, your talents know no bounds!

Great to hear the good news from your follow up post, describing the results from the 2 nights testing. No doubt, we will be seeing some more images of infernally obscure, deep and difficult targets once this system is fully commissioned!

Cheers

Dennis
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Old 27-04-2011, 03:10 PM
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Rolf

Looks really great - well done.

Just a thought - would your mount be able to work ok if you rotated the dovetail by 90 degrees? It might be more secure that way.
James
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Old 27-04-2011, 04:55 PM
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Nice job
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Old 28-04-2011, 06:42 AM
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Rolf,
this looks great.

I only have a concern about how you fixed the doveplate to the central brace of your construction.. same as James.
Quote:
Originally Posted by Moon View Post
Just a thought - would your mount be able to work ok if you rotated the dovetail by 90 degrees? It might be more secure that way.
James
I would use additional bars or plates, fixed to the trusses, or my central brace would have to be longer (wider).
I will have the similar issue with my own mount (EQ6 and fiberglass tube - the whole OTA weights 13 kg) and I intend to machine the conical insert (the one that is held in place by 3 grab screws on the EQ6 mount) out of steel - because this is the place where the components of the mount are stressed the most - possibly resulting in variable flexures, that may be the problem with autoguiding (depending on how the guiding scope is mounted, of course).
The balance I intend to adjust by existing additional counter-weight, sliding along the tube of my OTA.

Last edited by bojan; 28-04-2011 at 09:38 AM.
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Old 28-04-2011, 09:14 AM
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SkyViking (Rolf)
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Thanks again all for your comments. Some good points about the dovetail, and I did consider mounting the dovetail rotated 90 degrees so as to fit better with the central brace. I also think the mount would work just fine that way too. But in this case I would not be able to adjust the balance on the declination axis. I much prefer to be able to quickly move the OTA up or down a little, if required when using different equipment, rather than having to add additional counterweights here and there to balance out things. It would also add to the load the mount has to carry.
Fixing the dovetail to the trusses for further stability is not possible as it would directly interfere with the very purpose of the design. The point of the original Serrurier truss design is that sagging/flexure is unavoidable in any OTA, but the two ends of the scope can be made to sag equally when they are both balanced and mounted on respective trusses from a central brace. This then precisely maintains the optical axis of the instrument.
Note that this is different from the common truss tube design of large dobs, which are often incorrectly referred to as Serrurier trusses. The wikipedia page has some additional details: http://en.wikipedia.org/wiki/Serrurier_truss

I'm aware that if I had to move the OTA because of an introduced imbalance, then it would also mean that the properties of the Serrurier truss design are no longer optimal for the particular equipment that causes an imbalance. But I think this effect would in fact be rather small and I much prefer this trade-off over fastening anything to the trusses for the reasons stated above.
In fact I built the OTA purposely with a slight imbalance so that it will be perfectly balanced with the CCD cam I plan to purchase (QSI583wsg). This means that imbalance will occur only when I'm using the OTA for visual work, with just an eyepiece, and here the exact collimation would matter less so some differential sagging could be tolerated. Again this may be rather theoretical for just a 10" sized OTA, but at least it is accounted for.

In any case the current attachment of the dovetail is actually very sturdy, more so than one might think. It also helps that the OTA is quite light. I could always rotate the dovetail later if I notice any problems. Again, thanks for your thoughts everyone, they are all good valid points that need to be considered when undertaking these things.
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Old 28-04-2011, 09:36 AM
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Fantastic looking scope Rolf!! I've just finished building a truss 14" but it's nowhere near the level of finess of this fine scope. Mine works though, & that's what counts at the end of the day for me.
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Old 28-04-2011, 09:53 AM
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How did you go with flexure calculations and how did you implement it in your design?
Obviously, for Serrurier truss to be what it is meant to be, the amount of sagging of primary mirror cell and the secondary-eyepiece assembly must be of the same amount.
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Old 28-04-2011, 10:29 AM
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Quote:
I did consider mounting the dovetail rotated 90 degrees so as to fit better with the central brace. I also think the mount would work just fine that way too. But in this case I would not be able to adjust the balance on the declination axis.
Good point - I didn't think of that.
You really have done a fantastic job - both design and execution. Thanks for sharing.
James
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Old 28-04-2011, 10:51 AM
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Quote:
Originally Posted by bojan View Post
How did you go with flexure calculations and how did you implement it in your design?
Obviously, for Serrurier truss to be what it is meant to be, the amount of sagging of primary mirror cell and the secondary-eyepiece assembly must be of the same amount.
The idea of the design is that the two ends of the telescope are suspended from a center pivot, and thus the amount of sagging of the two sections can be made equal under the influence of gravity if the trusses are balanced. The heavier mirror sits in a shorter truss, but it also puts more load on the truss tubes and vice versa - so the sagging becomes equal and the Serrurier design does its job automatically as long as the central brace coincides with the point of balance.
Though, there may be more to it than that as I think the big observatories also use truss tubes of varying diameter, so it's probably a bit more than a simple balancing act - but for a 10 inch it should be close enough.

In comparison, a traditional truss tube dob is effectively only a half Serrurier. The disadvantage is that the triangles become very long with narrow angles (the key to truss strength is the triangle base-to-height ratio). Of course if the telescope is to be regularly dismantled and transported one would then have double the amount of truss tubes and connectors to deal with in the Serrurier design, but for a permanent setup I think the original Serrurier design is superior in every aspect.
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Old 30-04-2011, 10:24 PM
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An excellent scope Rolf - very innovative! I'll be interested to see more images and hear how the wire vane spider performs for longer subs. The offset vanes are a good idea for torsional stability (based on a design principle detailed in Jean Texereau's classic book). I'm actually planning a similar spider design (except not using wires) on a 12.5" scope that I'm currently building.
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Old 01-05-2011, 08:54 PM
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Quote:
Originally Posted by David Fitz-Henr View Post
An excellent scope Rolf - very innovative! I'll be interested to see more images and hear how the wire vane spider performs for longer subs. The offset vanes are a good idea for torsional stability (based on a design principle detailed in Jean Texereau's classic book). I'm actually planning a similar spider design (except not using wires) on a 12.5" scope that I'm currently building.
Thank you David I've now posted the official first light image over here: http://www.iceinspace.com.au/forum/s...d.php?p=715258
The results are excellent so far.
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