ATM Collimation is driving me crazy!

[Bill Brady <wmbrady@olg.com>]

Hi folks,

I have been lurking on ATM for a while, lotsa good traffic here. I hope 
no-one minds my asking for help.

I, and my very intelligent better half are going nuts collimating my 6" 
spherical mirror newtonian. I am using a cheshire-type eyepiece (Orion 
Collimating Eyepiece).

I have read about a half dozen instructions from the net, with graphics, 
the Orion instructions, and the 'scope instructions and I am still 
confused. They all contain statements that blow the whole instruction 
like: "the image of the blivit must be concentric". Well, nothing can be 
concentric by itself! what must the blivit be concentric *with*. <grr...>

My problem is this: when I get the primary reflection, the black hole in 
the EP, the red circle in the center of the primary, the X formed by the 
spider, and the X formed by the EP cross hairs (both images) all 
concentric, the reflection of the EP polished flat is offset against the 
black image (that nobody mentions) of the secondary HOLDER. I can see 
about 20% of the side of the secondary holder nearest the focuser. I can 
see three of the four screw heads of the screws that attach the spider 
vanes to the base of the secondary holder.

I believe this de centering is caused by an offset of the diagonal 
assembly towards the primary. If I look through the focuser with the EP 
holder removed, I can see about 1/4 inch of the edge of the actual 
secondary mirror. In other words, looking through the focuser, the 
diagonal flat is not "concentric" with the focuser hole.

Sooooo... is this a problem? Should the outline (if any) of the secondary 
holder be completely behind the image of the secondary (cheshire flat 
image)? Should the two be concentric? Or does having everything else 
concentric do it?

note: I've done a little star testing with this setup, and I get a nice 
concentric donut both sides of focus. But I don't really know what that 
means either. (am I supposed to have a dot in the middle? another ring 
outside of the donut?)

Any help would be greatly appreciated.



Wm. "Bill" Brady, Harwood MD - Merry HoHo to all!