I have read Kriege and Berrys book and have now produced some drawings that I was hoping could be critiqued by the IIS gurus. I would like to build a 16Ē truss dob, with a GSO/Meade primary mirror from bintel, maybe GSO direct. While I value the quality and performance of a premium mirror, the mirror alone would blow out my budget for the whole scope. So for now, Iíll use a standard mirror, and upgrade it later.
I used Web Newt for getting the dimensions for the telescope and have attached the .newt file in a zip file. I think it is more or less right, but I do have some concern in regards to my focuser. It is a Moonlite CR with 2Ē draw tube. It is currently installed on a 12Ē GSO solid tube, but I intend on transferring it over to the new scope. So at the moment it is fitted using a universal fitting kit (curve adaptor and spacer), which raises the base of the focuser roughly 45mm (2Ē) from the tube. According to newt and K&B, it should be as low as possible, so I would forego the spacers. But this would put 45mm of focuser tube into the UTA and Iím afraid that the draw tube would block the light path when fully racked in. Iíll be using the Astrosystems spider and 2.8Ē secondary with their dew heater too.
In regards to the mirror box, the current size is somewhat arbitrary. Iím aiming to keep the mirror box under 400mm in height (indeed all other components except trusses of course) so that they can fit in the boot of my sedan. So Iím aiming on making the mirror box nice and heavy, with a lot of corner support, heavy primary mirror and cell with fans and heavy altitude bearings. But the exact size wonít be known until I have made the UTA. I intend on buying the mirror cell from Company 7 but Iím still deciding what to do with the truss connectors. Iíve seen many diy solutions, but the Moonlite ones look so strong and pretty. Maybe a better long term solution. Thoughts? Contrary to K&Bís suggestions, I want to put the truss connectors inside the mirror box. The alt bearings will probably be hardwood like the rest of the scope... I know a furniture (chairs/stools) manufacturer that could probably help out with some welding but Iím a bit iffy about the quality of their work to be honest; otherwise Iíd ask them to make the mirror cell too.
While Iím getting all this stuff from Astrosystems, I might as well get their pivot kit and Teflon bearing kit. Iíll try to find some laminate locally, for the bearings and also UTA. I will also try to make a powered ground board...Iíve seen some good ideas around.
So thatís my plan! Shoot it down, tear it to shreds. In regards to the suppliers of the various components, Iím not really set on anything or anyone yet. Heck, if I can find a bigger mirror and cell for the right price, that will keep the EP no higher than 1740mm and mirror box <400mm, ill change the plan completely! The only thing I really want is the Astrosystems secondary heater, so I figure I may as well get their spiders and holder. And given that they also do pivots and bearings, I may as well get them from there too, to make shipping easier and cheaper. True, AS do truss connectors but Moonlites look more appealing to me. If you have suggestions for other suppliers or better places to get everything from, let me know. I realise Iím buying components that can be diyíd but I donít really have the tools or knowledge to make the fiddley bits work well...maybe Iíll make them on my second attempt, when Iím more confident. Might have a confidence boost after the next trip to the hardware store!
Thanks a million
I've nearly abandoned the traditional design altogether. Taking Ian's and Alex's words into careful consideration, I realised that the traditional obsession style dob, in all its glory, will not achieve what i want it to do. I want ultra compact, minimalist, fit in the boot of my Magnarrrr sedan.
The nail in the coffin was when a work colleague asked if i would be 'star gazing' tonight. I replied I dont bother taking my scope out in the back yard anymore, since I started going to dark sites and seeing the huge difference light pollution makes. So there it is.
As a result, ill be pulling down the attached files. There are out dated now, and had a few flaws anyway, so they wouldnt really help anyone out.
Looks like a good start to me. I wasn't able to open your newt file as I don't have that program on my laptop.
I have a few questions:
1. Have you calculated the centre of gravity for the completed telescope tube? Using the K&B model, this needs to be calculated to establish the height of the mirror box and the altitude trunion location.
2. What is the inside diameter of your secondary cage?
3. What tools/fabrication capability do you have at home?
4. Yes, I would be a little concerned re using a 2" travel focuser drawtube as it may partially obstruct the light path with larger field stop eyepieces. You would needto raytrace this though.
5. To me, if your budget is limited, the Moonlite connectors seem rather expensive. Nice to use though.
I will send you a PM with my phone number if you would like to discuss over the phone.
Yes, I see in Newt the 2" travel drawtube does vignett the 75% ray.
I have a few years experience of regularly transporting a 16" dob to dark sites in a variety of vehicles. At the end of the day I decided the 16" mirror box was just too heavy for me to consistently lift into and out of the back of my vehicle with minimal risk to my back. I went for a van, wheels and a loading ramp. I reckon the minimum weight you could get a classic design 16" mirror box for would be around 20kg - the mirror will be about 12 kg. It might be worthwhile considering the dimensions of the mirror box and lifting it into and out of a car boot with regard to your specific circumstances.
Have you considered any other truss design other than the orthodox "Obsession" one?
While the Obsession design is well tried and tested, there are other designs that have the same length of pedigree.
I too read the Kreige book from cover to cover, but the design I chose suited my requirements, tools and abilties.
All the dobsonian principles espoused in the "Good Book" are fulfilled in mine: rigidity, bearing sizes, tube selection, ease of movement, material considerations, etc.
What I then did different was come up with a design that suited ME. I made the design fit my car, my storeage facilities, and my understanding of the mechanical principles of other designs.
I was never going to make an Obsession style scope just because everyone has one. I could see certain design elements that could be improved upon, and was willing to put faith in my understanding.
In the end, the design stage took about one year, with the final design having nothing in common with my original one. The construction took four months over weekends, and with one month where nothing was done.
The whole thing weighs about the same as the mirror box of a similar sized Obsession. And it fits in one side of the rear compartment of my Mazda Tribute (other than the poles).
To have someone comment that my "SDM" is brilliant was a fabulous case of mistaken identity, .
Just consider the alternative designs that exist. They are just as good, and can be made to better suit YOUR requirements.
Yes i've looked at a few different designs, particularly Reiner Vogels: http://www.reinervogel.net/index_e.html it was these that made me want to build my own in the first place. But i really feel uncomfortable with how exposed the mirror is, not only in regards to dew, but dust and its general integrity. And that you cant put a shroud around it.
But now after a chat with Ian (stringscope), I'm thinking that i may be better off rebuilding my current solid tube 12" GSO dob. There are a number of factors that have changed my mind. Firstly the difficulty is significantly reduced, purely by the reduced mirror size. It will be easier to achieve the 400mm max height. It will be much cheaper, given that i already have the primary mirror. One thing that really stuck me was that John (astronut) said that you don't really get the wow factor when going from 12 to 16 inches. Sure its there, but perhaps not quite as much as one might think. In fact, the one time i did look through a Meade 16" i was not as blown away as i might have expected. Maybe if i used a premium mirror, but not the one i intend on using.
In other words....bugger it. LOL
I'll have another poke around the internet for dob designs, but i think ill stick with the traditional design...unless a forum member can persuade me again lol.
FYI I need a full bearing so i have somewhere to attach an alt encoder for my DSC's...this was the reason i left the low, open profile mirror box design.