Re: ATM Unsquare focusers

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Seems this thread is pursued by at least three distinguished list members (Dan, Doug, 
Dwight) - it is not easy to grasp all the points and my apologies if I bring up 
accidentally a point that was made off list, or mix up who said what. I find myself in 
the not entirely disagreeable situation of diasgreeing with these even where they do not 
disagree with me <!>

To start somewhere:

I think Dan said:
>>>If the focuser is not square to a tangent of the tube, you have a big
problem; the axis of the drawtube will not align with the center of the
diagonal (optical axis).  This would give you the tilted focal plane
addressed in earlier notes.>>>

{provocative mode ON} Since I made my first OTA 7 yrs ago using my hacksaw spider design, 
I seem to have forgotten the awkwardness not to say PAIN of using the pre-WWII orthodox 
center-adjusted spiders that most others have to make do with {provocative mode OFF, grin 
ON} - I just slide my diagonal mirror in line with my cave-humanoid sighting tube and 
that's that. But if you can't move your diagonal, you need to move your focuser. But if 
you can't, and despite this tilt your secondary to center the primary in it, you will be 
in trouble with the focal plane tilted. Is THIS perhaps the core of the problem?

I disagree with anyone claiming the tube is involved in the collimation - it will be if 
it ends up crooked enough to obscure the light, otherwise not.

Dan again: 
>>>but in order to have collimation,
the focal plane MUST be square to the plane of the drawtube and
subsequantly to the eyepiece (exception : the standard Schiefspiegler). 
If the focuser is tilted tangentially ( I mean with the OTA horizontal,
that the focuser is tilted up or down)  then the axis of the drawtube is
pointing away from the primary OTA optical axis.>>>

I think this is an *effect* of collimation (in telescopes where the focal plane is truly 
perpendicular to the optical axis, Schiefs excepted), not a prerequisite. Collimation is 
- as I see it - the act of defining the optical axis. If you start from the focuser, you 
let its axis define the first part of the optical axis, and the second part is then aimed 
at the center of the main mirror by tilting the secondary. 

Doug mentions the Orion "caveman tool" - I haven't seen one, but I use something like it 
but cut to size to fit my f/ ratio. First I center the secondary, then the primary, 
inside the tube opening - there is no way that an incorrectly tilted optical axis would 
escape attention. But I can't imagine how others do. I guess this is equivalent to what 
Dwight describes as "a sight tube or a Cheshire with cross hairs or stop ring ( I like 
the stop ring because it doesn't require any marks for locating the optical center of the 
secondary )".

Doug also answered my comment:

NO:>>> the reason is that a small and otherwise insignificant error in the
centering of the laser beam at the primary's center spot will cause an
error that is likely quite significant in collimating the primary. The
Cheshire does not have this problem. Of course, if the center spot isn't
at the *optical* center, there will >be an error using either.

Doug:>>Absolutely! that was what I meant, in part, with the comment about people
usually don't check their lasers.

Sorry, the problem has nothing to do with misalignment of the laser, it is inherent in 
the method. Suppose you have a squarely monted focuser and a perfectly aligned laser 
device. Then you align the secondary - suppose the spot misses the center of the main 
mirror by half a millimeter - this could be about one arc-minute and it may not be easy 
to do better, looking from the mouth of the tube. The returning beam will be half a 
millimeter off at the focuser, and if you adjust the main mirror to center it exactly, 
collimation will be half a millimeter off. Using a Cheshire and a center spot not much 
smaller than the Cheshire face, there is no way you could miscollimate so bad without 
noticing.

>Apparently good alignment with the caveman tool will produce a condition
>where the optical axis of the eyepiece is not perpendicular to the focal
>plane, 

I don't know what Orion recommends, but if you tilt the secondary to center the main 
mirror in the tube opening, I can't see how this could happen. If you can't center it, 
there is something fundamentally wrong.

Dwight:
> On fast
>mirrors, this offset can be quite large but depending on
>the size of the opening at the top of the telescope, may
>not be a large issue.

Even for a fast mirror, this misalinment at the opening is not more than some 1.5% of the 
aperture! BTW I fully agree with the rest of his analysis - when it comes back to the 
rotating cage we could yet sort out a thing or two {G still on}.

Doug:
>too bad we can't all get together, show examples of what
>we're talking about, fix the collimation, and then figure out how to
>describe both the problem and the solution. ...........

> There are certainly a lot of instruments, both
>commercial and ATM, that aren't properly collimated.

Amen. 

Nils Olof