As a result of my primary mirror now being 17 years old, and never having been re-coated, I decided to replace it. I could have it re-coated, but that would mean shipping it overseas plus paying for the coating and the return shipping. And then I would be without a mirror for a while...
Instead, I applied for funding from the Kingdon-Tomlinson Fund and the Auckland Astronomical Society. And now together they have generously approved to fund my purchase of a Royce 12.5" f/4 conical mirror
This is really all thanks to my Beta Pictoris image from last year. I have been very fortunate to get to know the good people of the Auckland Astronomical Society and in particular Dr. Grant Christie. This has been very rewarding and since December 2011 I have been invited to give talks on Beta Pictoris and image processing to both the Auckland Astronomical Society, Tauranga Astronomical Society and latest at Auckland University of Technology.
I also picked up my first awards recently by winning the Scientific and Deep Sky sections of the 2012 Harry Williams Astrophotography Competition. In addition I also won the Beaumont Prize for writing, for an article about... you guessed it: Beta Pictoris
12.5" was the largest I could fit in the observatory since I didn't want to go too low on focal ratio. Also, I want to keep the weight at a minimum because my current setup is tracking beautifully. For this build I'm applying some weight savings here and there and I hope the 12.5" OTA with a conical mirror will weigh more or less the same as the old 10" did with a standard mirror.
So far I have completed the main structural elements: Upper cage, central brace and mirror cell. I'm now working on the details around the secondary holder and wire spider.
The mirror is expected to be ready in January. In the meantime I aim to finish everything else and just leave the trusses to be cut to the correct length once I have the mirror and can balance the OTA properly.
Wow Rolf, you are doing some wonderful stuff mate and congratulations all round
Mike
Thank you Mike I'm really looking forward to finishing this new scope! I'm hoping for some increased resolution, but most importantly I'll look forward to the faster imaging times. Allthough that'll of course promptly be utilised to obtain deeper images, not shorter .
Quote:
Originally Posted by coldlegs
Excellent work Rolf. How much did the 10" weigh in the end?
Cheers
Stephen
Thank you Stephen. The weight data of my current 10" is as follows:
OTA without mirror and finderscope (but with finderscope brackets): 7.85 kg
OTA incl. mirror: 11.85 kg
OTA incl. mirror and finder scope: 13.15 kg
The finder is an Antares 80mm and is ridiculously heavy at 1.3 kg alone. I'm thinking of switching to a Telrad for the 12.5". I don't need the finderscope except while I do the initial star hopping to my target (I don't have goto).and so there is no reason the mount should carry it during imaging... I could also make some sort of quick release for it, provided that it'll still be aligned when I put it back on.
I've done some more work on the scope in the last few weeks.
I have made a lightweight focuser holder and fixed it to the upper cage. I could have used a simple rectangular piece of plywood mounted between the upper and lower ringe on the cage, but that would put the focuser an unnecessary 18mm further out and away from the secondary, resulting in a larger secondary required. So instead I used two pieces of 3mm thick aluminium angles.
I also received the focuser, a Moonlite CLR large format 2.5" Crayford with the shortest possible drawtube at just 1". This focuser is a thing of beauty!
I wanted to reuse my existing focuser, but given the larger and fatter light cone it just isn't big enough (12.5" f/5 instead of 10" f/5). The Moonlite will be awesome I'm sure, and the repeatability of the stepper motor will be very handy.
It is quite heavy though, but as I mentioned in the previous post I can probably save some weight on my finderscope which should offset this.
Finally, I've also made the secondary holder. My previous holder was just a piece of bent aluminium but for this bigger secondary I wanted the strings to attach further down. This gives better geometry of the wire spider and greater stability. So the simple design I used before will not work here. Therefore I had constructed a slightly different holder this time, but entirely based on the same principle. The only real difference is that I have split the metal piece where the strings attach into two, because I might need to run power to a dew heater in the future. This way I can simply run it through the spider strings.
Currently I'm keeping dew off the secondary with a 120mm fan mounted on the side of the upper cage, but since I don't know it that will cut it for the larger mirror I decided to future proof the design in this regard
Time for another progress report.
I have now finished both the mirror cell and the upper cage, complete with guitar tuners for the wire spider.
The secondary holder is also completed. It is an 'enhanced' version of my current minimalistic holder, meaning that the position of the holes for the wires provide an improved geometry of the wire spider. Also the split down the middle means I can in the future run power directly through the wires to the secondary if I'll ever need to install active dew control. Currently this has not been necessary, a fan seems to do the job, but this is a 100mm secondary mirror compared to my current 70mm so I thought I'd better future proof it.
I'm happy with how the big Moonlite focuser sits nicely just between the ply rings. Everything on it is easily accessible while the weight and dimensions of the structure is still kept to a minimum.
Finally, everything has had numerous applications of Sikkens marine grade finish.
With the 4 UTA supports, do you think they're adequate to prevent any lateral movements?
I've found that the weight of the secondary mirror exerted sufficient force on the strings to flex or move the upper part of the UTA, sufficient to show deflection with a laser.
I therefore added two rectangular ply pieces along the x and y axes. that strengthened it a lot. I'm wondering if it'll be more pronounced with yours as the focuser woud be quite heavy and when the scope is at say 45deg, the upper ring might show some deflection due to the mirror and focuser's weight.
Could the Al tube bite into the ply underneath over a period of time due to the lateral forces?
Also with the washers underneath the guitar head holders, wouldn't that allow them to swivel as the bolt is at one end?
I put washers at the junction of the Alum tubes and the ply. Tube sits on the washer giving a nice flat wide surface so no bite.
That's about the worst thing you can do.
You actually want it to bite in a fraction. If you use a good quality hardwood plywood it will only bite in less than .5mm. This is just enough to give the spreader poles grip on the plywood so there is no lateral movement in the top end at all. You should use aluminium spreader poles with a wall thickness of .9mm.
Dave Kreige from Obsession Telescopes actually cuts a 1mm deep circular recess in the plywood to seat the spreader poles with a Forstner bit and stop any movement. Peter Read at SDM Telescopes just screws the spreader poles up tight to the plywood face and lets it bite in a fraction. The reason Peter Read does not cut a recess in the plywood is because once you varnish the plywood it fills in the edge of the recess and it's then very hard to seat the spreader poles properly.
I think there is enough of a sharp edge turn down on the washers to bite in a small fraction ( I orient them on purpose ) and as you say a good coat of varnish tends to lock it all in place anyway. My rods are plugged at each end with a wooden dowel to take the screw that holds it all together.
Mine has yet to move after 18 months and good deal of being carted around from Ob to workshop etc. I'd be more concerned with dimensional accuracy, even 'hard' wood has softer spots and a wider base minimises any movement later.
I like the recess idea though. Very secure as long as you can be accurate with the depth of the recesses. That might be in the next build if I ever build another one.
With the 4 UTA supports, do you think they're adequate to prevent any lateral movements?
I've found that the weight of the secondary mirror exerted sufficient force on the strings to flex or move the upper part of the UTA, sufficient to show deflection with a laser.
I therefore added two rectangular ply pieces along the x and y axes. that strengthened it a lot. I'm wondering if it'll be more pronounced with yours as the focuser woud be quite heavy and when the scope is at say 45deg, the upper ring might show some deflection due to the mirror and focuser's weight.
Could the Al tube bite into the ply underneath over a period of time due to the lateral forces?
Also with the washers underneath the guitar head holders, wouldn't that allow them to swivel as the bolt is at one end?
Thanks Alistair I haven't noticed any movement with my current 10" which is built with the same 4 poles, but it's certainly a possibility to add some ply pieces to prevent movement if I see any. I don't have a laser collimator but plan to get one and then I'll be able to test how it holds up while moving the OTA around.
Re the guitar tuners, that was exactly why I added the washer - to make it easier to swivel them
I found that the orientation of mine are quire hard to adjust when the wood is screwed down together without a washer, you sort of have to break them loose again if you even need to adjust the direction of the strings. I had to do that once when I replaced the secondary and re-stringed the spider. Now with washers underneath I expect the holders to be easier to swivel, and no it doesn't move at all when it's tightened.
I thought about adding some mechanism to adjust this angle with some screws but that would add extra weight so I didn't.
Quote:
Originally Posted by ausastronomer
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You actually want it to bite in a fraction. If you use a good quality hardwood plywood it will only bite in less than .5mm. This is just enough to give the spreader poles grip on the plywood so there is no lateral movement in the top end at all. You should use aluminium spreader poles with a wall thickness of .9mm.
...
Cheers John, that's exactly what I did. The poles are sitting tight up against the plywood and they do indeed make a circular mark and bite a little into the surface. Should be secure I think.
So do you have a wooden dowel inside the Al tube and if so how did you secure it to the tube?
as for the guitar tuners, how about routing the hole a few mm each side rather than the swivel? that way you could actually shift the tuner head sideways which is what you'd want to line up the strings, and then tighten it.
Shouldn't move as the forces aren't line line with the groove.
yes, I'd suggest using a laser to check shift in collimation. I was really surprised at the shift when I rotated the scope cause all my joints were secure.
another test is to leave the scope standing vertically on the floor with the laser on, and just gently push the top ring of the OTA and watch if the laser moves on the primary.
after I reinforced mine, I tested with a cats eye AC and was relieved to see absolutely no shift in collimation.
When do you get your mirror and do you have any pics of your mirror cell?
cheers
So do you have a wooden dowel inside the Al tube and if so how did you secure it to the tube?
Hi,
Ideally you should insert a piece of dowel into the ends of the truss poles only, not the spreader poles. Inserting a short (25mm to 50mm or so) piece of dowel into the ends of the truss poles adds a lot of strength and structural integrity to them without adding much extra weight and the weight comes lower in the scope, where it has less effect on balance, which is importnat in the case of a dob. You normally don't insert the dowel into the ends of the spreader poles because you normally have no need to remove the spreader poles ever again from the Upper Cage Assembly (UCA) and its very unlikely they will ever get damaged on the ends whilst in place on the UCA. The truss poles on the other hand are removed and inserted all the time and can easily get damaged on the ends without the dowel. Further, inserting the dowel into the spreader poles adds a relatively large amount of extra weight (compared to the weight of the bare spreader poles) at the wrong end of the scope. This isn't a problem for an equatorially mounted truss scope like Rolf's, but it's a big design consideration for a dob. Depending of course on scope and UCA design some people will need to insert the dowel into the spreader poles as they then screw from the outside of the plywood rings into the dowel in the speader poles. Another option to attach the spreader poles to the plywood rings is to use "push fit threaded inserts" like these:-
You can also buy these direct from Dave Kriege at Obsession Telescopes.
The best way to insert the dowel into the poles is as follows:-
1) Cut your truss poles about 25mm longer than required (about 12mm each end).
2) Turn the dowel down on a lathe from a piece of decent quality timber like tassie oak and bring it down so that it is a snug fit into the end of the pole but can still be easily inserted and removed.
3) Glue it in place in the end of the pole with a high grade epoxy like Araldite Super Strength or Epiglass HT9000 epoxy
Epiglass HT9000 is a marine grade epoxy and it doesn't get much better. It makes Araldite look like Perkins Paste. If you try to separate a HT9000 joint made in 2 pieces of wood, the wood will break and splinter before the glue joint.
4) Trim the poles to the exact length required (after the glue has dried) by cutting through the aluminium and the dowel together in a drop saw with a very fine tooth combination aluminium / wood blade. I use a 10" (254mm) sliding compond drop saw with a 100 tooth Koyo combination blade. It does a beautiful fine job. You should need to dock each end of each pole by about 12mm. Then paint the end grain of the timber with some marine varnish and let it soak in.
So do you have a wooden dowel inside the Al tube and if so how did you secure it to the tube?
as for the guitar tuners, how about routing the hole a few mm each side rather than the swivel? that way you could actually shift the tuner head sideways which is what you'd want to line up the strings, and then tighten it.
Shouldn't move as the forces aren't line line with the groove.
yes, I'd suggest using a laser to check shift in collimation. I was really surprised at the shift when I rotated the scope cause all my joints were secure.
another test is to leave the scope standing vertically on the floor with the laser on, and just gently push the top ring of the OTA and watch if the laser moves on the primary.
after I reinforced mine, I tested with a cats eye AC and was relieved to see absolutely no shift in collimation.
When do you get your mirror and do you have any pics of your mirror cell?
cheers
Hi Alistair,
Yes I put dowels in the tube ends, fitted with Araldite. It's a bit dark in this photo but you can just make it out here, it's what the screws are sitting in: http://www.rolfolsenastrophotography...3sKK&lb=1&s=X3
I don't think I'm going to fit dowels in the main truss tube ends. I don't have any on my current scope and it's very rigid and stable. The OTA is also not going to be disassembled again anyway.
Re the guitar tuner blocks I figured the swivel was the simplest and most rigid approach. It's also easier to swivel in small controlled movements rather than shifting the entire piece of wood. And if the hole for the bolt had to be larger then the block of wood would have to be wider too = more weight
Mr. Royce says the mirror will be finished Feb/Mar. The mirror cell is one of the pics above, and also in the gallery: http://www.rolfolsenastrophotography...LTsG&lb=1&s=X3
The mirror is a conical and will be bolted to the triangular ply panel though a hole in the centre. There is no need for a traditional mirror cell
Time for another update:
The Royce 12.5" f/4 conical mirror has arrived! It feels surprisingly light, which is nice. I have ordered an ultra precision 1/30 wave diagonal from Antares Optics to go with it.
Now that I have the mirror I have been able to balance the parts and calculate truss lengths, and the OTA is rapidly taking shape. Finished installing the truss poles today!
I'm really looking forward to finishing this new scope! I'm hoping for some increased resolution, but most importantly I'll look forward to the faster imaging times. Allthough that'll of course promptly be utilised to obtain deeper images, not shorter 
.
