converting 12" gso dob to truss dob

[monoxide]

Hi guys,

i've been looking into making up a truss dob to drop the optics from my 12" into and i thought id ask a few questions.
would it be a good idea to make a new mirror cell from scratch or adapt the one from the dob?
the moonlite connectors seem a bit expensive, are they really worth it?
i also plan on making my own curved spider or is there a reason i shouldnt?

any input is appreciated :thumbsup:

[stephenb]

Hi Monoxide,

With the truss Dob you are thinking about, are you planning to build a Obsession-style truss or your own unique design? i.e. square mirror box or a round mirror box?

I may not be able to answer your question outright, but I am doing this same 'conversion' with my project here (http://www.iceinspace.com.au/forum/showthread.php?t=36105). I have a BinTel 12" on order and I will be dropping in its GSO optics in my own truss Dob. Now I haven't seen a lot of the primary mirror cell in the GSO-style Dob's but I have taken the dimensions of the mirror and I made my own mirror cell. Fortunately I am able to weld so that helped. I chose to effectively start from scratch with the dimensioning and I used Kriege's Book as a reference. My mirror cell, and indeed the whole scope, will end up being a smaller version of the 'big Dobs' described in the book. Remember, when it comes to 'over-kill' and 'over-engineering' I am the King! The cell doesn't have to be as complicated as my one is.

Although I haven't yet reached half way, I am happy with my progress, and I can offer you measurements and dimensions of my work. I can also point you in the right direction regarding some of the 'custom made' items.

[monoxide]

Hi Stephen,

i just noticed your thread after posting this, the cell looks great.
i've been reading Kriege's book also, some great stuff in there.
im still sketching but i like the idea of the whole dob stacking away in the rocker box, ie. mirror box turns and fits inside the rocker box, UTA slides into the mirror box with a lid over everything, still need to work out how exactly to do that.

going with the advice given in your thread i think ill make a 6 point cell
i have an idea that will replace the split blocks and will hopefully work just as good as the moonlite connectors, depends what size aluminum tube i can get though.

ill be keeping an eye on your progress :)

[stephenb]

oh and check out Stringscope's scope here (http://www.iceinspace.com.au/forum/showthread.php?t=35756) for a compact truss Dob.

[CoombellKid]

Interesting, you have ordered a Bintel 12" I guess you want to use the
rest of the bits for your truss dob?

Did you consider buying just a 12" mirror set? Or you could of (for a few
more dollars than a complete 12") you could of brought a 16" mirror set
direct from GSO. I think my one cost me AUS$872.00 but the US$ has
slipped a bit since then.

regards,CS

[GrahamL]

good luck with the project :)...as for the moonlite connecters .. i think they are great .. very simple and I rarely have to do much in the way of collimation with them .. are they worth it ?.. they do run out to $300 or so dollars so .. maybe .. maybe not;) depending on you budget i guess.

[CoombellKid]

I can vouch for Grahams moonlite connectors, they work a treat but are
not cheap.

For my scope I went with making split blocks and got lazy and brought
upper connector from SDM for $120.00 which you then have to sand
and stain so you can match the stain scheme of your scope. Splitblocks
take a bit to setup porperly so you can just drop the SC straight onto
the poles, There is a simple trick to do this and if you go with splitblocks
PM me and I'll let you know how to get it just right, it is worth it. I think
in the last dozen or so outings I've had to tweak the collmnation twice.

With spltblocks I used laminated ply 9mm hoop pine, it has 7 layers per
9mm. The more layers the better, less prone to crack. Also make a couple
extra at the same time in case one does crack. I used 25mm poles with a 1"
spade bit for the seats. This will give you 0.4mm clearance between pole
and seats.

And I agree with others, making your own DEFINITELY improves the over-
all quality of the scope IMO. Grahams 12" is a cracker!!!

regards,CS

[tnott]

I just bought a 10" GSO optics set for a similar project. Didn't buy the whole scope so I could use my choice of spider, focuser etc. Going with the Highe design with three thick poles used by Plettstone.
http://pw1.netcom.com/~ahighe/12_5ultra.html

This guy also sells similar kits as a retrofit for GSO dobs. Not as cheap since the dollar has gone down but some of the ideas might be useful for you:
http://www.dobstuff.com/

The connectors can either be threaded inserts/knobs or simple clamps.

Have fun:)

[stringscope]

G'day Monoxide,

From what I have seen, curved spiders are :thumbsup: I have seen a really rigid one made from a couple of steel rulers ;)

I reckon you are better off making your own primary mirror cell. The GSO one is a 9 point design but if you run the data through PLOP it appears for a normal 12" mirror, a 6 pointer is better. it also will allow you improve on the edge supports and retaining clips.

Re Moonlite connectors, I was going to use them on my 16" but I ended up making my own. I used the same design on the 8" for Phil Hart and I will be using it on the 10" scope. Works well and cheap as...... I have attached a couple of pics of the 8" scope that show the general arrangement.

Go luck, I will be following your thread with interest.

Cheers,

[monoxide]

thanks for the input everyone, much appreciated :thumbsup:

thats the best thing about building your own scope it seems, searching around the internet checking out everyone elses and picking the bits you like the most from each to build the scope thats exactly what your after.

another thing i've been keeping in mind is that since i have the Argo Navis, i have to think of a tidy way of installing the encoders.. the azimuth encoder will be no problems but still looking into the altitude one..

[tnott]

Yes, either make sure the side bearings contain the central pivot point or, if your side bearings are U-shaped or crescent-shaped, you can make up a double tangent arm. If you go the second option it needs to be very precise.

[PhilW]

You might already have a good way to do this, but the Astrosystems pivot kit is worth considering:

http://www.astrosystems.biz/pivot.htm

You should definitely build your own cell. I don't have PLOP at work for some strange reason, but a full-thickness 12" mirror would be well supported with a 6-point cell.

It's a great idea btw.

[monoxide]

i like the way this guy did it http://www.darkerview.com/atm/dob12.php

the only problem i see with that is getting the exact axis of rotation

[gbeal]

A great idea TJ, and one which I did a few months ago, with my old but nice 10" optics. Truss was all from ply, with C/F poles and struts between the top and bottom sections. Probably no lighter, but looks nice, and was cost effective as well.
Good luck,
Gary

[monoxide]

looks good Gary, how much does the scope weigh?
also where did you get the CF struts?

[gbeal]

TJ,
weight? It was about 11 kilos to start with, and really, is no lighter, probably a smidgen more. The whole concept was to make it lighter, but really it isn't.
The C/F poles cost about $2.50 each from memory, a crowd here in NZ. I can help you source them if you need. Click on the "round tube". http://www.carbon-tube.com/
Gary

[PhilW]

Those are both beautiful scopes.

Here is my old (& far more primitive) 10" tube to truss conversion. Alas I never weighed the OTA, but it was light as a feather. Having a light scope is great, you use it so much more.

[monoxide]

i dont know if im overly worried about the weight aslong as i can lift it.
i guess i can start listing what id like to see in the scope and work from that..

1) the mirror box and UTA must be able to stack away inside the rocker box so all there is to transport is 1 box and the truss poles.

2) quick repeatable tool-less setup

3) argo navis install without hardware sticking out all over the place

4) mirror box and UTA oversized to fit a 12.5" mirror if i decide to buy one (i know i'd need new trusses)

5) power at the UTA, ill probably run wiring up some of the trusses

not sure if cooling would be better from behind the cell or a fan blowing over the face of the mirror.

[CoombellKid]

Use two truss poles as wire :thumbsup:

regards,CS

[stephenb]

Before you begin your rocker and mirror box, get in touch with Wildcard Innovations, I understand he is very helpful with choosing a set of encoders for a one-off project like yours and mine. He should be able to give you some advice on how to construct the rocker and mirror box to accept Argo Navis gear.




Here's an idea which I would do myself: Wire one of the truss pole + (12v) and another one - (ground).

If you build a wood truss clamp similar to Kriege's design, you could have a small brass shim (similar to to the brass ring inside a focuser assembly which clamps onto the eyepiece). So when you clamp the truss clamp, the brass shim makes contact with the aluminium pole.

If you use this method, you can still disconnect the poles with no wires in the way - the poles become the wires!

If you had the Moonlite connectors, you could not do this however.

Just a thought.

[GrahamL]

I gave a bit of thought to this of late and to be honest its not a big task to power up your trusspoles with the moonlites .. keeping in mind all your poles can rotate through 360 ~ and drawing a contact at a common point top and bottom isn't a big issue .

[Starkler]

Thats exactly whats done with SDM scopes.

[monoxide]

thanks for the tips guys, any way of getting the job done without wires everywhere sounds good to me.

i have the encoders/AN installed on my 12" at the moment so i guess its just a matter of working it into the design rather than 'banging it on' as an after thought.

this is going to be very slow to take shape for a while since im starting work again in a few weeks and when i get home i dont feel like doing anything but catching up on sleep.

[stephenb]

oh :thumbsup:I didn't even know. Great minds.... no really it's just a common sense alternative I guess.

Stick with it mono!

[monoxide]

i wont be starting anything until i have everything on paper, im notorious for making stuff up as i go along but i want this dob to end up in working order :P

once im happy with the way my sketches are going i'll scan them and see what you guys think.