I have decided to take the plunge and go ahead with a 10" (252mm) f/5 imaging newt build. Why not just buy one you say, well after my very nice 127mm iStar strut refractor build earlier this year I am fairly confident I can do a good job on the newt, and the prices for the completed scope just went up again due to the AUD falling, and I could pick up a nice 10" mirror at a reasonable price.
The goals of this build will be:
Lighter weight than any of the 10" production scopes (which can weigh 15KG easily for just the oTA), so when I add the guidescope, finder scope, camera (DSLR) etc the total load is within the imaging limit for my NEQ6Pro (which is advertised as 18kg but I'd like to limit it to 15KG max) .
Stiffness and flexure control - so this will be a boxed truss design, no conventional tube. The advantages of the boxed truss are twofold, one is the stiffness, and second is the ease of the build. The truss box will be built of plywood (like my refractor rings), as a retangular box section with front and rear plates and intermeadiate frames - all bonded using boat building epoxy adhesives.
Use well established Dob build practices, including mirror cell design and mirror support and retention systems.
Use Moonlight flat mount focuser for load carrying capacity and ability to rotate the load (not to forget it can be collimated itself).
Primary mirror has been ordered, more to follow as things progress.
Update. Did the initial scope tube design drawing and made a slight change in concept, have decided to go with carbon fibre tubes as the struts that link circular plywood bulkheads together and to provide the necessary stiffness to the whole structure. I have gone ahead and ordered the carbon fibre tubes today (from Carbon Fibre AUstralia). The design will utilise six 24mm longitudinal carbon strut tubes running in parallel arranged at 60 degree spacing around (and through) the ply bulkheads, front and back plate - all bonded with boat bulding epoxy system products.
Update: The mirror arrived today: GSO 10" f/5 (34mm thick) already centre spotted and I like that they include the plastic raised mirror cover that rests on the bevel edge. The marks on the side are from the 'fingers' used to hold the mirror during machining and do not affect the mirror surface area or coating - they only involve the beveled edge. Now that the mirror is here I can start work on the carrier.
The marks on the side are from the 'fingers' used to hold the mirror during machining and do not affect the mirror surface area or coating
They were only used to hold the mirror during coating(hence the 'shadows') but yes, they will not cause problems. You might like to blacken the bevelled edge tho.
Update: The mirror arrived today: GSO 10" f/5 (34mm thick) already centre spotted and I like that they include the plastic raised mirror cover that rests on the bevel edge....
With the Serrurier build being so open I use that as the cover when not in use. Just slides down inside the light shroud.
With the Serrurier build being so open I use that as the cover when not in use. Just slides down inside the light shroud.
My plan exactly, I still use the cover that came with my 16" strut dob and they are great dust covers, which being raised prevent mirror damage if anything is dropped in there.
Made some good progress today. All the frames are cut and routed. I have set them up on the old ladder jig that I used for my long refractor build. I received the carbon fibre tubes from Carbon Fibre Australia. I checked the telescoping of the tubes (so that I can create joints).
Some photos are attached of where I am up to at this point. The carbon tubes are shown in their notches on the frames, there are six parallel carbon tubes. I will put the plastic sleeves back on the tubes when I epoxy the tubes to the frames, to keep the nice carbon fabric finish clean.
The back plate has been cut for the fan, but futher lightening holes will probably be added once I get the mirror cell base complete and installed on the back plate.
When I glue it all up I will setup my laser on the back and fire through to a target on each frame (to insure everything is centred). I have increased the diametre opening on each frame going forward to allow for a 1 degree increase in aperture going forward.
First photo is a side shot of the ladder jig with the frames setup (at least the last three), and the last shot is the look down the 'tube' front.
Still deciding whether to make the front section removable, which is why the front frame is not setup in the photo. I also want the front to be able to slide so that I can set up the focuser and secondary properly before I fix it in place.
Update: I have completed all the carbon fibre tube bonding for the back of the scope the front section is to wait until I do a star test (I want it and the focuser to be able to slide on the assembled tube rails). Once I am happy with the camera focus I will mark the front on the tubes and bond it in place. The photo shows the tube is still in its raw state - ie the epoxy cleanup has not been done yet. I want to complete all the epoxy work before I get into the messy and tedious cleanup phase. Trust me it will look much better when that is finished (anyone familiar with my refractor build would have seem the end result).
I have considered a couple of solutions for the secondary mount, a curved vane design and the normal four vane. I looked at DIY builds of both and I thought the curved vane probably does not provide the support that the four vane does and it (the curved vane) would have to be thicker. Typically people use stainless steel rulers bent to the shape of the curve, in the four vane there are plenty of hacksaw blade vane solutions around. I will be building the four vane using hacksaw blades (deburred of course). The secondary vanes will be attached to the front ring with four timber posts, a bit hard to visualise without a drawing I know.
I have completed the fabrication of the mirror cell and tested it on the tube today, (alignment photo attached). It uses a Push - Pull bolt set with springs on the Pull bolts, pretty much exactly the way the Stellafane one is made. All the hardware for the cell is from Bunnings. I have two springs sets, a light one and a heavy one, the photo shows the light ones (the heavy ones are the same springs I used on my old 12" dob). The side support L brackets are not installed yet, I need to do the final finish of the cell before I attach the side supports.
Next step will be to put on the tube extensions at the front (for the focuser and secondary section to slide on), and the complete cleanup of the epoxy on the tube assembly.
I bought a GSO 2" Linear Focuser to use, as they are strong, and relatively cheap, I also have received the fan for the rear plate.
More soon. I am aiming to have this done, or at least useable, by the next new moon so I can take it out to Bretti for field trials. I may not have the shroud ready by then but Bretti is dark enough that I may not need it.
Update: I have completed the spider and secondary holder, it's pretty much a standard four vane design with the 3*120 degree spaced collimation screws. The materials are: hacksaw blades for the vanes (teeth ground down), Ply centre hub, and the mirror holder is made of model aircraft balsa wood to keep the weight as light as possible (notice that I used 5 cent coins as push plates on the secondary holder). Photo are attached. Yes the screws will be painted black.
The carbon fibre tubes have been covered with plastic sleeving to protect them and the tube joints and frame members cleaned up in prep for sealing the frame members with clear coat epoxy.
The primary cell is nearly complete, I just need to spray it.
Next step is to mount the mirrors on their cell/holder and install in the tube for a test. I have all the spacings worked out in Newt 2.5 but it is a visual design app so I need to do a real life test with my camera on the focuser to check focal plane distance and adjust if necessary by moving the front frame station forward or backward on the tubes.
Ordered some Nylon/Lycra for the shroud, but I don't have a sewing machine so it will be interesting to see what I can come up with to make it into a shroud. Any ideas would be appreciate.
Update: Finished all the assembly so that I can test to confirm the secondary and focuser placement. The various Newtonian design apps around on the Net calculate on visual model so they are only rough guides at best for an imaging scope - which has to fully illuminate a sensor. Imaging newts also have larger secondaries than visual scopes, so the app calcs related to the secondary size, placement, offset, etc can't be relied on, and placement has to be confirmed by actual testing. Use of cable ties to hold the front ring and focuser are to allow the movement of these components during testing to fine tune the placement of the focal plane on the camera sensor. Once placement is confirmed those components will be epoxied in place and the plastic sleeving on the tubes removed.
Well I can answer that question now. I had my 'first light' test last night and it's a very nice scope. The concerns I had about the setup of the front frame and secondary and focuser evaporated as soon as I put the DSLR in the focuser, slewed to Canopus and put the mask on to focus the camera. The focal length was spot on and I had enough in-travel to achieve focus on the camera and go beyond to defocus on the other side (so I can move back and forth from In and Out of focus) - but it was close - I think I measured about 5-10mm of available additional In-travel on the focuser barrel when the camera was in focus. I ran through a number of EPs in visual mode and it achieved focus on the focuser travel alone for most of them but I had to add my 35mm extension tube for a couple (Televue's of course) - and it's a very nice visual scope. I did some test subs with the camera to check that I was filling the sensor completely and they all came up great. Very happy with the two mirrors.
For those that would laugh at the cable ties, they are only there for confirmation and testing - allowing adjustment of the front frame/secondary and focuser on the tubes if needed.
So to the question of the mount on the NEQ6, yes I added a dovetail mount board for this first test, and it will be attached permanently now that I have confirmed the configuration ( I have a carbon fibre sheet coming and I hope to make the guide scope mount platform and dovetail board out of that carbon fibre sheet. I have attached some photos from last night. Note that it has most of the weight in the rear at the moment (hence the mounted position) but that will improve once I get the guidescope and guide camera installed, and the finderscope. An interesting discovery was that I could use the hollow carbon fibre tubes as a default finderscope for locating my alignment stars last night.
I weighed the scope prior to the test (without the camera) and it came in just under 10kg. Once I finalise the structure, add the guidescope/camera/guide mount, and the finderscope, I expect it will top out at around 13kg (including the shroud and light shield). This compares very favourably to a production 10" GSO imaging newt which weighs 15kg for just the OTA. My goal was to have it under 15kg all up.
The carbon tubes protruding from the front frame will be cut off once the frame is epoxied in place. I will probably use a slightly smaller diametre carbon tube insert to hold the light shield - that way it can simply slip in place in the hollow tubes. It will need a shroud (for backyard use, not so much for dark site use) and I have the nylon/lycra for that already. I also need to install the primary mirror fan. Plenty left to do but from a design point of view it is now proven to be good scope and that's a relief.
