Help with Truss Dob!!

[Darkplague]

Hi,

I'm slowly gathering parts for my home made 16" trus dob, but need to know what length the trusses will be roughly when finished, I dont want an exact figure, just a rough figure to allow me to place the order at the steel shop.

Is 8 pieces at 2m each overkill?

Also does anyone know any shops in Oz which sell good quality focusers, ie Moonlite, Jmi and such?

Thanks in Advance!

Mark

[[1ponders]]

Hi Mark. I know Bintel sell JMI focusers.

[Darkplague]

Oh thanks ,
Didnt even think to try them............
They have the JMI dx1, looks nice, wish they would show the prices somewhere but.

[janoskiss]

Grab a copy of The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes by Kriege & Berry. Well worth the investment. You can get it from Bintel.

[Vermin]

Very rough ballpark:

16" f5 = 2.032m focal length,

Assume the focuser is at the bottom of the 18" (457mm) secondary cage, so the pole length is going to be (roughly) 2032mm - 457/2 = about 1.8m per pole.

You should probably allow for at least f5.2 however which gives about 1.9m per pole.

You will therefore get three poles from a 6m length, (order direct from Capral Aluminium @ about $35 per 6m length of 1.25" thickwall Al tube).

You will need 3x 6m lengths for 8 poles.

Edit: Oops forgot about the mirror box height (subtract from pole lenght), still it means each pole is definitely going to be less than 2m.

[Darkplague]

Already got the Kreige book from Vermin! Thanks Vermin.

So I will need to get 3 lengths at least. Plus enough for the struts of the secondary too, another 2m there.

All up 4 lengths should cover me , 24m worth. You wouldnt think there would be that much until you work it out.

Thanks Guys.

[Vermin]

There should be enough left over from 3x 6m lengths for the secondary cage struts.

3x6m = 18m total length
18m - 8x2m = 2m left over from 8 poles
2m/4 = 500mm available for each strut

[[1ponders]]

JMI dx1 - $550 from Bintel. If you go to their online store many of there products are there with prices. Just search for JMI and it will be the second from the left at the top

[Darkplague]

Dammit!

I turned down a DX1 from astromart recently,
He wanted $235 US shipped for it, which I thought was too expensive at the time!

Oh well, anyone have a spare Moonlite cr2??? :whistle:

[Starkler]

I think you'll find such focusers cheaper to buy from the US.

[Darkplague]

Yep I think you are right.

Online secondhand astronomy bits:

Gecko Optical
Astromart
Cloudynights
Ebay
IIS

Am I missing any? Australian or overseas............