Looks like you have done some research! My original 8" binocular used silicone squeezed fairly thin to mount all my optics and I eventually found them all to have been seriously warped by the thinly spread silicone causing quite a bit of astigmatism , which is why I suggested a minimum of 3mm.
If you had more than three blobs of silicone or a single large area then that would constrain the glass and likely cause astigmatism. By having just three blobs this would allow any of them to shrink / swell without straining the glass (assuming the blobs are not too large as then you would get differential pressure across each blob).
No one has any suggestions for the type of silicone? Then I will probably test Selleys Glass Silicone - it appears to be low modulus (high flexibility) so I will try a thickness of 1mm for three points. My great fear is that the silicone gives out and the secondary falls off! Selleys Glass Silicone appears to provide the most secure bond for glass (and hopefully carbon fibre / epoxy resin). I had originally thought that I should use a neutral cure, but I don't think this is necessary since it is not coming into contact with metal.
I don't know about mounting a mirror but in the contracting game most use Polyurethane sealants rather than silicone. Sikaflex is a popular one that is available from Bunnings. There are various types from Sika so read the tube about what they are used for.
But overall I would use a polyurethane over a silicone.
Thanks Greg, that's interesting. I actually picked up a Sikaflex brochure from the local hardware a few days ago but haven't really considered it as I haven't seen anyone else use it for a mirror. Do you have any specific reason why you think it would be better? Looking at the TDS on the website, I see that Sikaflex 291 has similar properties to Selleys Glass Silicone, except that it has 5% shrinkage whereas Selleys Glass Silicone indicates no shrinkage. There is a Sikaflex 291I that has 2% shrinkage (although the site says this is for "experienced professional users only" ??).
Last edited by David Fitz-Henr; 09-10-2011 at 07:18 PM.
Well, time for another update; I'm getting down to the business end now ... and as far as the weather goes, it's been great for ducks and telescope building, hasn't it!?!
Attached images below:
Stainless extension shaft that I turned on my lathe. I calculated that I need it to extend the shaft on the Paramount to achieve balance (otherwise I would have to purchase a 4th counterweight which would be overkill). Cost from Bisque would have been approx $300 becuase of high shipping costs; cost of stainless stock for me to make it: $40.
Shows the extension shaft in place.
I always wanted a generous finderscope to be more than just a finder and plan to use a WO ED 70 for that rich field experience; however I became worried when I tested the scope in the WO rings I bought as I needed to tighten it quite hard to stop slip. I decided to replace the delrin tipped screws with some large nylon faced stainless inserts that I made to provide better friction but also do not rotate as you tighten the screws (non-marring). I got the idea from the Alpine Astro site.
The finder / rings attached to the tube.
I needed to drill 4 holes (2 widely separated pairs) in good alignment on the tube (which is not perfectly round) so I used this jig, which is self aligning on the tube.
I've seen a couple of horror stories regarding rodents setting up house inside a Paramount (due to the open RA shaft that has holes at the back end to permit through the mount custom wiring). Here is my solution: an end cap I made out of fibreglass that attaches to the shaft end and has 3 electrical glands that pass the wires through.
I'm currently working on the electrical / USB systems and then I will be ready for a test mount of the mirrors to determine exactly where to drill the holes for the primary. Work has been slow as I have had a lot of back pain over the last few months and looks like I will need a back operation, but hopefully I will complete the scope before that happens.
I'm also building a kit shed that I will use as an area in which I can spray paint the tube; I planted the idea with my wife that we need extra storage in order to tidy up our garage (for mower, etc) so I am not only getting brownie points for doing it but am scoring my spray painting booth as well
The weather finally cleared for me to be able to achieve first (test) light!
Here are the results from some initial focusing tests that I did last night, mainly to determine the optimal position to drill the holes for the main miror cell. I clamped the main mirror / cell temporarily in the end of the tube, but I still did a reasonable job of collimation. Just the very basic setup at this point; I still also have to paint the tube / some fittings and mount the guidescope / finder / dewcap / filters / fans. I am quite pleased with the results though - a very auspicious start!
I must admit (after a few years designing / building this thing) that I was full of trepidation at the thought of actually performing a real star test
Attached Images:
First image is the basic scope. I run all 12VDC (13.8VDC to be precise) power through the mount and distribute the power to each individual device via a RigRunner (inside the black enclosure). Similarly, I run a single USB cable through the mount to a powered USB hub inside another enclosure on the other side, which then feeds to each USB device.
Another image of the scope.
After finding reasonable focus (trial and error by taking a series of images) I took a series of 9 x 3min luminance of the Horsehead region. I applied dark frames and stacked them, and then just some basic stretching in Photoshop. It should also be noted that I did not turn CCD cooling on - the frames were taken at ambient temperature last night (about 22C). The star images look good right to the corners; the only slight negative I have noticed is some brightening in vane spikes distant but belonging to a couple of bright stars (one off-frame). I believe this is an artefact caused by the unpainted stainless vanes - I have sanded them with wetanddry ready for painting, but they are still highly reflective (especially at grazing angles). I also realised after the tests that I inadvertently used the (slightly) wrong focus position - I was 20 steps (50 microns) off the best position I found. Not much diffierence though, just ever so slightly softer stars.
A CCDInspector analysis of one of the raw 3 min subs.
I have also decided on a name for it; follow my process:
Firstly, some relevant terms: Stellar AStrographic High QUAlity Telescope CCD
Selecting some letters to form an acronym:
SASHQUATC --- apply anagram --> SASQUATCH
But that's a northern hemisphere beast, and the analogous equivalent down under would be a BUNYIP perhaps? So, an analogy to an anagram of an acronym A creature not much mentioned nowadays, so possibly one could say that it is also a bit of an anachronism as well
Congratulation David on a really superb piece of engineering.
Regarding diffraction spikes I have heard anecdotally that another way to suppress them is to insulate the vanes with royal black velvet. The thermal boundary layer you get on the surface of bare metal vanes actually creates an optical thickness much thicker than their mechanical dimension. The insulation while thickening the mechanical profile reduces the optical profile by removing the thermal boundary layer.
Another approach is to use a curved spider milled out of solid aluminium.
Congrats David - I know how long and hard you've worked on this project. You should be pleased with such a good preliminary result.
Thanks Marcus, yes I'm ecstatic having such a good result on my first test images! I'm also very pleased with the way some of my design features have worked out; for instance, collimating the secondary mirror is an absolute breeze! No wrestling with a central nut for rotation, etc.
Quote:
Originally Posted by marc4darkskies
Don't like your name for it though. Why not just call it something simple and unassuming - like the The Great Fitzy Newtonian.
Hmmm, yes well I guess I just don't have your turn of phrase, Marcus! Though it could have been worse, what if it had been the Great Fizzer
Quote:
Originally Posted by gregbradley
Super result. You must be very happy with that.
Greg.
Yep, thanks Greg - very happy indeed. It has taken a few years so it is a very exciting time for me at the moment!
Quote:
Originally Posted by Satchmo
Congratulation David on a really superb piece of engineering.
Regarding diffraction spikes I have heard anecdotally that another way to suppress them is to insulate the vanes with royal black velvet. The thermal boundary layer you get on the surface of bare metal vanes actually creates an optical thickness much thicker than their mechanical dimension. The insulation while thickening the mechanical profile reduces the optical profile by removing the thermal boundary layer.
Another approach is to use a curved spider milled out of solid aluminium.
Thanks Mark! Yes, I actually have some black flocking material from Protostar that I will use in key places. I have thought of using it for the vanes but I feel it will add too much to the thickness. Interesting though what you suggest about the thermal layer, but this won't be a concern as I will have three maglev fans mounted beneath the main mirror which will pump sufficient air up the tube to prevent any thermal layers forming.
David
It's great to see you get to this stage - well done and thanks for sharing your progress.
James
Thanks James. I plan to drill / reinforce the holes for the main mirror this weekend, and also put up a kit shed that I plan to use as the painting booth. I've ordered some spray cans of Krylon Camouflage flat black which is supposed to be very good (back-ordered - eta late March).
I was hoping to finish painting the tube today; apart from final assembly it's the last thing left to do. I did manage to fill, sand and prime the tube, and then sanded that ready for the white enamel. Unfortunately, that will now be delayed for a few weeks as I am having a spinal fusion tomorrow , but at least I've completed the (physically) hardest parts.
I'm just a beginner spray painter, so I made a couple of mistakes:
The automotive lightweight fillers that are around cure very quickly (less than 5 minutes), so I found it extremely difficult to squeegee it around the tube (to fill small voids in the surface of the CF). I had to mix small (~20ml) batches, and trying to measure the hardening agent for that (~0.4ml) is very difficult, even using a syringe (it is a paste). Adding to my difficulties was the fact that it gets quite hot in a tin shed! I wonder if there is a better filler that I should have used for this purpose? I found that I probably under-measured the hardener, which made sanding very difficult as it wasn't as hard as it should be and would tend to clog up the sandpaper) Trouble was, more hardener would set it off extremely quickly.
When spraying the etch primer, I had the nozzle set to too wide a spray pattern, so it had a very textured surface (due to thin overspray) which needed more sanding. Anyway, the results don't look too bad, just a lot more work than necessary!
Some images below:
The raw tube in the new kit shed I built for my wife; just a coincidence it was the perfect size to use as a spray booth
The tube after being filled and sanded.
The tube and some fittings (light shield, bottom fan plate and cap) after priming.
The system looks to be delivering superb image quality that would otherwise be extremely expensive.
I've had some discussions in the past with Peter Ceravolo (Canadian master-optician mate of mine ) about secondaries.
Yes, as Mark suggested you can run a curved secondary but instead of the energy going into the diffraction spikes, Ceravolo says it goes around the stars and uniformly and hence drops the contrast.
If you really want the whole shebang, an optical window is the best (albeit not cheap) way to go.
Yes, as Mark suggested you can run a curved secondary but instead of the energy going into the diffraction spikes, Ceravolo says it goes around the stars and uniformly and hence drops the contrast.
Peter, I think the energy dissipated out of the Airy disc by spider vanes is pretty infinitesimal. `Contrast ' is a pretty emotive term in optics particularly when anyone talks about loss of it. I'd certainly like to see some hard numbers some day in relation to overall field contrast of curved spiders versus windows.
Most of the energy scattered by a curved vain if spread out evenly would be orders of magnitude less in brightness than a diffraction spike from a straight vane. Windows also have their issues - scatter from dust , condensation , optical surface smoothness and transmissive properties of the glass .
On another note, very few curved spiders I see are correctly designed for even distribution of diffracted energy, which takes the form of 3 vanes , each sub tending a 120 degree arc of a circle intersecting the edge of the secondary and the edge of the primary mirror.
There's a good project for some dedicated imager- to interchange a spider and an optical window and see which wins the shoot-out !
David, that's very impressive. Lots of care, ingenuity and patience. Some very useful fabricating techniques. The carbon fibre casting is excellent. I will be using it to improve on some of my own creations.
Thanks Marcus / Peter / Mark / Rowland. I'm back from hospital now and realise that it will be a few weeks before I can think about tackling the final tasks such as spray painting the enamel coat. I've also just found that my order for Krylon Black Camouflage spray cans is delayed until end of May, so no rush at the moment.
Once it is finished, I'll post the fully completed scope images.
Love the cf casting for the primary, I have to add some back focus to a gso 10" and this method looks a shizza. Did you roll the ss sattle straps? And is that 1.2mm ss your using?
Looking good so far David, inspirational...only wish I had just a little of your skill, patience and tenacity
Thanks Mike!
Quote:
Originally Posted by 2stroke
Love the cf casting for the primary, I have to add some back focus to a gso 10" and this method looks a shizza. Did you roll the ss sattle straps? And is that 1.2mm ss your using?
I had the Stainless straps cut and rolled by a local stainless sheet metal fabricator (Jezline at North Richmond). The straps are 0.9mm thick and 50mm wide. I used 2 stainless roll pins in each end of each strap to secure the stainless threaded attachments that I turned on my lathe.
Selleys Glass Silicone appears to provide the most secure bond for glass (and hopefully carbon fibre / epoxy resin). I had originally thought that I should use a neutral cure, but I don't think this is necessary since it is not coming into contact with metal.
Trouble was, more hardener would set it off extremely quickly.
