Re: [ATM] Draw Tube Length

[Mike Lockwood <melockwo@uiuc.edu>]

Doug,

Doug Williams wrote:
> I understand that the draw tube extending into the OTA a little bit is not a
> big deal, but is there any reason why I shouldn't opt for the short draw
> tube to reduce the distance that the draw tube intrudes?  I should also save
> some weight with the short draw tube (though not much).  As long as all the
> eyepieces can focus, is there any optical advantage/disadvantage associated
> with draw tube length?

It is nice to use a baffle at the bottom of the focuser tube to 
improve contrast.  The bottom end of a focuser tube will act more and 
more like a baffle the closer to the edge of the light cone it gets, 
and this will improve stray light rejection.  However, if it intrudes 
on the light cone, it causes vignetting.

On an open-tubed telescope, I think there is a contrast benefit to 
having the slightly longer tube, but not long enough to cause 
vignetting or to make an appreciable increase the size of the 
secondary necessary.

I have two of the 2"-travel Moonlite focusers, and they work well. 
However, I can say from experience that the inside of the barrel will 
VERY likely need flocking or flat black paint.

On my 12.5" Cassegrain contrast was improved and stray reflections 
from the moon and planets were GREATLY reduced by adding flocking.  Of 
course this causes problems if you use a 2" laser collimator - it 
simply won't go in the focuser!  My solution was to use a 1.25" 
collimator, and rotate it in the barrel to make sure the 1.25" adapter 
was true.

On my 10" F/8.8 I use the same focuser, but I haven't blackened it 
yet, though I probably should.  Of course I have seen the Horsehead 
and glimpsed the central star in M57 with that scope, so it can't be 
that bad.  Of course I use a huge light shield on the secondary cage 
of that scope.

	Mike Lockwood




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