Hi all, for the past 4 months or so I've been working on my 10inch Dobsonian and I thought I'd share my progress and also ask a few questions as well.
The scope is an 8 truss design based on Jerry Oltion's scope in the Jul/Aug 2022 AS&T magazine. Although I'm a woodworker I'm going with minimal use of wood because I don't want things changing shape with moisture.
The primary mirror cell and ground boards were cut from a piece of HDPE from AGC catering equipment, which was sold as a chopping board. It was probably 5 times cheaper that way rather than getting a sheet from a plastic engineering company. Just for reference I bought a sheet 450mm X 600mm X 13mm but it came as 456mm X 607mm X 11mm. I cut it with the jigsaw and a guide to give me straight cuts. I'd appreciate feedback and questions. https://www.agcequipment.com.au/kitc...ories/?pgnum=2
Will also be following this thread with interest. Meant to reply to your previous thread about the design problems about the secondary and focuser placements as they seemed conflicted.
Also just checked the article from Sky & Telescope you referenced, looks like an inventive design, with 3D printed parts, light weight and portable.
As to the design constraints, an 1 1/4" focuser drawtube is going to vignette the light cone entering the drawtube. The governing factor is the height of your focuser, if a standard low profile focuser you still have to allow approx 80mm to the eyepiece focal plane. If you have some other design such as a low profile helical focuser that will help but from what NEWT shows, even allowing for a focuser height of 50mm and a tube ID of 300mm you will need a 63mm diagonal mirror, otherwise you are just throwing light away ( not reaching the eyepiece focal plane ).
Thanks Jeff for the info about the focal plane and focusers I'll keep that in mind. I also don't have a 3D printer so my design is slightly different, he also said in the article it take 20 minutes so set up so I'm trying to make it as easy as possible to set up. I was able to attach the truss connectors to the primary mirror today so here's some photos of that.
#1 The mirror cells and ground boards. I'm using the offcuts from the corners to make random things like blocks to center the mirror and also make a lot of practice attachments and glues and things like that.
#2-4 Making and attaching the truss connectors for the lower end. They're made from 10mm aluminium tube which I cut to length and flattened in the little vice. Originally I used a sandwich maker to heat them first, but when I broke one and didn't have spares I didn't bother heating it, so I don't think heat was necessary. To attach them I drilled a 4mm hole sideways into the mirror cell and then put an 8mm hole vertically to so they met, that way I can tighten them as much as I like.
The connectors stay permanently on the mirror cell in transport but the trusses will come off.
Sorry about the lateness of this post, I had to reduce the file size of all my photos.
I've now almost finished all eight trusses and I've put together the OTA now. The trusses are 10mm aluminium tube drilled out to 8mm internal diameter so they can take an 8mm rod. The rod is so the truss can come apart like tent poles, and each truss packs down into three roughly 30cm lengths. I don't have a picture of the OTA yet but here's some of the trusses.
#1 Each insert for each truss section is marked so I know what goes with what. The seam between sections is almost invisible.
#2 All the truss sections cut to length.
I've also basically finished the upper end of the scope so here it is.
#1 Made out of 25X25X1.6mm aluminium angle, it's 340mm wide on the outside and 290mm wide on the inside. I simply cut one side of it, drew some lines on the table so I knew where to stop and bent it by hand. It took a lot of maths to work out the length of each side but they're meant to be 128mm each. #2 The secondary holder is fully adjustable, similar to Gary Seronik's design.
The truss connectors for the top are glued on four sides and I made them by heating and then gluing two pieces of plastic together. They're simply holes drilled at a 7˚ angle into the plastic and tension (not sure where from) holds them in.
So now to really iron out the diagonal and focuser questions.
My tube diameter at the top end of 340mm and I'm not going to make it smaller, but that means I can have a really low profile focuser. The focuser will be Jerry Oltion's design on his trackball scope and I was thinking of mounting the focuser upside-down from the focuser board. I can do that as long as it's smaller than 45mm which I can also do, then in theory I can focus really really close to the edge of the light path, or still focus at the normal low-profile distance. The only problem I can see would be the focusing knobs but that should be easily fixed once it's all assembled. If I did all that I think I could get away with a 52mm diagonal mirror. The reason I was wanting to do that was because I really don't want to change the upper ring and it lets me put the focal plane close to the light path. This is my first go at ATM so what are the thoughts of those with experience at ATM?
Thanks for all your help in advance and also to those who have already replied.
From what I can glean from running your figures into Newt, an 50mm secondary and 1 1/4" focuser just doesn't work. The secondary misses some of the reflected rays from the primary and you wont get a fully illuminated field at the eyepiece focal plane . Basically throwing away the the light reflected from the primary to the secondary so that the light gathering to the eyepiece would be more akin to that from an 8" / 200mm mirror , or maybe less , a waste of the full 10" diameter mirror .
To get the full benefit of the 10" , you really need to bite the bullet and use a 2" focuser, low profile is better obviously, and at least an 62mm secondary mirror. Also how do you intend to be able to collimate the secondary mirror.
Thanks for all your help with this Jeff I really appreciate it.
I understand the issue with a 52mm diagonal is it's too small but when I run the numbers in Newt it works for me. I'm probably completely wrong but its been working for me in Newt so I'm just wondering if it's possible in real life. I 99% sure I can achieve focus with 10mm spare focus if when I mount the focuser inside the tube and have it move out, I've just never seen it done before, thats all.
I'm certainly intrigued with your design of the placement of the focuser. Would like to see some drawings or sketches of your design to get a better picture of what you're trying to achieve.
From what I can picture the placement shortens the distance from the secondary to the focuser, which means the light cone will be narrower and you will not lose as much reflected light and the secondary will be further from the primary. Placing the focuser where you propose will mean extra obstruction of the incoming light to the primary though.
Hi again, sorry I haven't been able to keep you up to date the past few weeks. Now that schools back I'm much busier and work has slowed to the weekends and morning/afternoon either side of school. Jeff here's one of my drawings of the focuser.
The edge of the paper represents the edge of the light path, the long line represents the side of the OTA, the rectangle is the drawtube and the shorter lines are where my eyepieces come into focus. The thick line is where the draw tube ends, and the three circles are the three pairs of bearings. (sorry there's so many lines Since the shorter lines (where the eyepieces come into focus) over lap when I move the rectangle along I take it there is enough travel to fully focus and I can put the focal plane 16mm out from the OTA.
Here's some photos of my progress on the mount. I'm not doing altitude rings partly because I'm keeping it compact and also I don't want to use too much wood because dew might make it warp. So I'm going with two A-frames instead.
#1 One of two A-frames using 20X20X3mm Aluminium angle.
#2 I had to cut a recess into the bottom part because otherwise it hits the bearing between the ground boards and the corners of the lower ground board. I used the drop-saw (for the straight bits) and the jigsaw (for the curves).
Progress is coming along nicely just a lot of drilling left on the mount and a few small things like edge supports on the OTA.
Looking good ...F4.. is tight even when everything looks good on paper.
ATM though is all about the little things .. some work some dont .. the best part is about the understanding to embrace what works and to be able to reject and to fix what dosnt.
Thanks Graham, in the past few weeks I've continued work on the mount, and now I'm almost finished it. The A-Frames don't have too much wobble, which is good, I think it definitely helps that I tapped the top so it the pivoting bolt also holds it together. The altitude friction isn't heaps though, so I'll try wingnuts and springs and I also bought two of these: https://www.amazon.com.au/Clamp-Ligh...s%2C293&sr=8-9. But if you've got any good ideas on slowing down the altitude movement I'm all ears. I already tried washers with various materials so I'll see how we go with springs.
Last edited by PJO; 03-06-2023 at 01:02 PM.
Reason: Punctuation.
Last week I was able to put everything together, the trusses were sagging a bit, and the ones on top of the OTA were pulling out of the blocks at the top, which made everything come out of place and flop down. So dad suggested shock cord and that worked perfectly - it doesn't sag a bit.
We didn't know what to hook them onto at the top but after looking around at bunnings we found these: https://www.bunnings.com.au/hangman-...-pack_p3961739
I had to cut them shorter but they work great.
By the way, motion is perfectly - even 'buttery' - smooth in both axes.
Does anyone know where I can get, either by making or buying a 60mm long 1.25" or 32mm drawtube? I can't seem to find anything suitable. Either its too short or the wrong inside diameter or both. At the moment I'm using two PVC connectors joined together, but they're too wide internally, and the join in the middle makes it bump slightly when it goes of the bearings. I thought about 3D printing, but I don't have a 3D printer so and my uncle just gave his away , so that wouldn't work.
If anyone knows where I can get a suitable drawtube that would be much appreciated.
