[APML] Knife Edge Focusers was RE: September images

["Koebel, Alen" <Alen.Koebel@christiedigital.com>]

Frank Barrett wrote:
 
> I also use the STI knife edge focuser on an SCT (C8).  I was 
> getting soft focus on many of my shots and for a long time 
> thought it was operator error <g>.  
...
> To make a long post short (oops, too late!), don't trust the 
> factory setting!  

Thanks for the warning. Myself, I was saved from experiencing 
this kind of problem by genetics. Being part German and part 
Scottish (and a few parts something else) I am naturally tight
with money (g). So, I decided I would not buy a focusing device
but make one instead. I followed the general advice in Chris 
Schur's July 1993 ASTRONOMY article "How to Get Razor Sharp 
Astrophotos." This article is also referenced by Jerry Lodriguss
on his fine web site. (BTW, how DO you pronounce "Lodriguss"?)
(http://www.astropix.com/HTML/I_ASTROP/FOCUS/FOCUS07.HTM)

As you can read on Jerry's site, this is really two devices. The 
first is used only once (ideally) to calibrate the second device,
which you use to do the actual focusing. Since I am using a 
Lumicon 2" Newtonian Easyguider, I ended up making the second 
device with a carefully-selected plumbing fixture made of ABS 
plastic that fits over the bayonet mounting on the guider. It 
seems to work effectively on an f/5 Schmidt-Newtonian. When I am 
careful with the mechanical setup (all screws tightened, etc.) 
my smallest stars are definitely below 0.04mm and probably closer
to 0.03mm, but I won't be able to tell for sure until I get them 
scanned. (I don't have a scanner.) Nebula details look sharp,
so I think I am darn close to best focus, if not right there.

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