diy spider for my dob

[ponytale]

i have started making the spider /secondary mirror holder for my dob , i have the idea of a moveable spider because i want to have 2 interchangeable mirrors of differing f ratios in one tube/ota, i am thinking of a spider in a ring that can be moved up and down the ota to 2 preset places to keep close collimation , has this been done before ? or something similar ?.

i am using .5mm x 26mm sheet brass for the mirror supports and the ring see pics of spider, any ideas welcome,

[Moonshane]

I'm glad you started this thread as I'll be making one in the future! I suspect your plan will be scuppered by the focuser?

would it be better / easier to have two positions for the primaries? if you are taking the primary and cell out anyway.......

[Doc]

Design the scope with either a movable primary that adjusts up and down or adjustable truss poles.

[drummerp]

looking awesome mate, cant wait to see more of your progress with this project.

cheers

neil

[ponytale]

focuser!Doh why didn't i think of that moonshane,thats what i like about this forum every one is helpful !! ps, im liking your eq platform / dob project,i have some stepper motors ready for motorizing my dob later on, i intend using arduino motor shieldvia laptop usb. Mick/doc i like the idea of adjustable truss poles as i could have 2 mirror cells to swap over, and it will be more portable, i have some adjustable ski /walking poles in glass /carbon fibre perhaps they could be utilized as adjustable poles as they are very strong , light, and lock very sturdily? my mind is in overdrive already, must get to draw some plans , i will let you know how i get on.

[NickK]

you could put an electrical focuser (screw thread) into the center obstruction of the spider. That could move the secondary in and out nicely.

[davew]

If you haven't already got it may I suggest down loading - Dale Keller's Newt Software page

Play on the screen to your heart's content. Good luck,

Dave.

[Moonshane]

you could put an electrical focuser (screw thread) into the center obstruction of the spider. That could move the secondary in and out nicely.

good idea but if going from eg f6 to f4.5, he'd need very long secondary adjusters?

[NickK]

good idea but if going from eg f6 to f4.5, he'd need very long secondary adjusters?

Good point.

Also with switching F-numbers surely you'd need to adjust the curvature of the primary mirror and not just the secondary?

[Moonshane]

I think he's using two different primaries?

[ponytale]

hi all, just to clarify a few points , the original post said that i was thinking of an adjustable spider so that i could use 2 mirrors , i do have 2 mirrors both same diam and thickness one is around f5 for dso's the other will be around f6.5/f7 for lunar/ planetary,

the dob i have made is quite sturdy with a 600mm diam base with 6.5" alt bearings and a box type central ota/tube holder which my ota slides in and out of , the idea was to have a spider that could suit both mirrors but as moonshane said the focuser needs to stay inline with the secondary mirror, so i am looking at adjustable truss design instead which means i can swap mirrors and adjust the secondary spider ring to correct FL for either mirror, i have a 10 diam tube which i can cut to make the mirror box and the focuser ring and a piece to fill the bearing box, with adjustable poles through the bearing box corners , so it will still fit in my dob base , the estimated difference between F/L is 330mm /13" i will probably have the poles adjustable through the bearing box for balancing purposes as well as adjustable in length for focus , drawings added

scope adjust.doc

truss draft.doc

[ponytale]

added trunnion box detail to drawing in previous post