[ATM] Shroud v.s no shroud

[Jim Miller]

Tim

I too spent quite a bit of time baffling my scope. Even to the point of 
buying and mounting a variable iris diaphragm beneath my focuser. (Which 
worked nicely, BTW)

However one evening I was observing at the club site and a person walked 
behind my primary with light colored pants and I was startled by the 
increase in background light.

That prompted me to really test whether the baffling was adequately keeping 
all light out of the optical path. I was quite surprised how much of the 
area around the primary needs to be masked off. The choice was either to 
extend the mask quite a bit laterally around the primary or bring the mask 
up the poles ala a shroud. I chose the latter.

The test is quite easy to perform to see if you need to do anything further. 
Setup the scope and point it down to the horizon with the back of the 
primary facing a bright area and the normal input completely shrouded with a 
cloth. (I did it with the scope in the garage with the back looking out the 
garage door.) Look through the eyepiece. It should appear completely dark 
(or as dark as your temporary shroud.) You might be surprised as I was.

Once I decided to bring the "shroud" upwards rather than outwards from the 
primary, I used a small flashlight in a darkened room pointed upwards along 
the truss tubes and moved it around until I could no longer see it through 
the eyepiece. That plus a bit more for good measure was where I stopped the 
top of my "shroud."

I made my "shroud" out of plastic signboard which was covered in black 
velvet.

pics at www.jtmiller.com

have fun

jim


Hi,

This has probably been debated before but I'll ask anyway.  Since completing 
it several months ago, I have been enjoying using my 16'' version of Mel's 
"tridob" design ( 
http://www.iceinspace.com.au/index.php?id=63,324,0,0,1,0 ).  Like him I used 
carefully designed baffles just below the focuser, behind the secondary 
(both covered in Protosar flock) and in front of the ultra-black enclosed 
mirror box.  To make the open-opticed scope more "public night safe" I was 
thinking of either:

1.making up a simple plastic shield  velcroed around the outside of the 
lower part of the scope to avoid fingers, ice cream, car keys etc. landing 
on the primary or:

2.constructing a full shroud - using a zig zag arrangement of elastic cord 
to fill in the gaps left by the 6 poles (much more complicated to make).

Several of the "more experienced" members of my astronomy society (including 
one fellow who has used refractor-like ring baffles in his 8'' dob) insist 
that I will get even better contrast and darker background if I used a 
shroud.

Has anyone out there with baffles like mine tried a full shroud and noticed 
any difference in contrast.  Personally, I suspect that those advising me to 
use a shroud are thinking in terms of the standard Obsession design which 
does not have the same baffling arrangement, but I am interested to see what 
others think.  I generally use the telescope at our club's site about a 
hours drive from the city or sometimes in my backyard, neither of which have 
problems with lighting shining directly on the scope.

Thanks for your help,

Tim Nott
(Adelaide, Australia)