Re: [ATM] Minimum Inner Tube Diameter

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

ntent wrote:
> If I'm not mistaken, the rule of thumb for a tube is two (2) inches
>  greater than your primary mirror diameter. Wouldn't this be
> applicable to a truss scope? Although, using the principle below,
> the "2-inch principle" is indeed larger:

I'd call it the "2-inch theory" rather than principle, and the theory 
does not take into account the size of the mirror.

The theory is that an inch of space around a mirror in a closed tube
will help keep tube currents away from the mirror.  In truth, I'd like
to see more like 2" on each side of the mirror for anything over 8 
inches aperture.  But in reality that just lets more stray light into 
the tube, and makes the tube bulkier and heavier due to larger diameter.

In my view, it is not applicable in a truss scope unless the mirror
box is fairly high (making it act like a short tube) and there is no 
other venting.  Vents/fans in the top of the box will allow the warm 
air to escape, and the "theory" is irrelevant.  If air can move around 
the mirror, the extra space then could be viewed as causing extra 
weight and bulk, just as with the tube.

> Thinking on it, a larger solid tube diameter would be preferred so
> as to allow for better cooling and possibly less convection.

Yes.  Oversizing the tube with a ventilated mirror also allows you to 
neglect giving air currents a path up the tube, and then you can use 
some internal ring baffles to make the tube even darker.  The last 
baffle at the top of the tube is carefully sized to limit the largest 
illuminated field to exactly the size you want.

	Mike Lockwood

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