[ATM] balancing a serrurier truss

["Andreas Derwahl" <andreas@duda-derwahl.de>]

Hello list,

after having finished a 10" f4.9 mirror (well, at least I hope I have -
startest still pending...) I am currently building the dobsonian
travelscope.

I plan a Serrurier truss design made up of three major rings connected with
trusses.

A rendered picture of the rough design can be found here:
http://www.duda-derwahl.de/ATM/aluscope2.jpg

The altitude rings will be attached to the middle ring, I plan the rockerbox
to be collapsible. It will be made out of one wooden ring with four vertical
aluminium tubes (3 to 4 cm diameter), which are horizontally interconnected
by three tubes of the same diameter to form the rockerbox. This will sit on
a base triangle to form the azimuth bearing. I am not sure if this will be
strong and rigid enough, I guess I will try and see how I go...

With this kind of design I can freely choose the centre of gravity/altitude
axis offset of the OTA.  To calculate the truss lenght and to correctly
place the alt axis I need some advice (comments on the design and possible
flaws are also very welcome):

My plan so far is:
- assemble the upper ring including finder/telrad, focusser and secondary
but excluding eyepieces and then weigh it out
- assemble the lower ring with mirror cell and mirror - take the weight
- choose the maximum weight difference my scope will have to cope with (i.e.
if I find I cannot be without a 2" Nagler eyepiece this will be close to 1
Kg)
- calculate the shift in COG for this weight difference

If I do a rough calculation I come to a COG-shift of about 120 mm for a mass
difference of 1 Kg. Does that sound about right?

Is it a good strategy to place the altitude axis between the two extreme
positions of the COG?
This means with the heaviest eyepiece I have an COG/alt-axis offset of 60
mm. Do you think that will be to much for the scope to stay put when I
change the eyepiece?
Or should I rather choose the alt axis position to coincide with the COG
when using the favourite eyepiece? Which one that will be I don't know yet,
as I am still to buy eyepieces for the scope, once it's finished.

Since this is my first real telescope and my first ATM project I really
appreciate your comments.

Andreas Derwahl


_______________________________________________
ATM mailing list http://www.atmlist.net/