From: "J&S" 
To: 
Cc: 
Reply-To: "J&S" 


Hello again Mike,

>Hello!  I've designed a coude system, with the light cone coming out the
>center of a side bearing on a dobsonian mount.  The focuser is on the side
>bearing.

     Ah! Now I understand. The tertiary mirror has thrown a bit of
complication into the mix. This is where accurate construction and accurate
centering of optical components along the mechanical axis of the tube will
pay BIG dividends!
     By the way, I hope you are going to incorporate some kind of tracking
platform. A 12 inch Cass has a LOT of magnification, and on an alt-az mount
it will keep you pushing it along continuously without a moment's rest.  :)

> with a black dot in the middle of
>the secondary...
>...since I
>would not be able to see the center of the primary

     You can't see the center of any Cassegrain primary - there is none.
Don't be thinking about centers. Think about edges. Don't put dots on your
mirrors. Use the edges of the mirrors for alignment. Think about concentric
circles, not aligned dots.

>Alternately, I can glue the primary and try and align the other optics with
>it, though field alignment in such a case would seem more difficult,

    I hope you are not suggesting that you forgo the option to adjust the
tilt of the primary! Attempting to achieve collimation by bringing all the
other elements together in line with the pimary will make you crazy in
short order. The primary tilt adjustment should always be the last
adjustment made and is by far the most critical. Never build a reflector
without the option to adjust the tilt of the primary mirror. (Do you hear
that Meade? Do you hear that Celestron?)

>Does anyone have any suggestions on aligning a primary when the tertiary
>obstructs the center of the primary so that it cannot be used during
>collimation?

     Here are the steps to take Mike:
1.   To keep alignment as simple as possible, construct a firm main tube
such that your tiltable primary mirror cell, your tiltable secondary
mirror, and your tiltable tertiary are centered in the tube as accurately
as you can. Since you are using a tertiary mirror, do not install the main
baffle tube yet. Be sure to make provision to move both the secondary and
tertiary along the axis of the tube without having to move the vanes of
their respective spiders. Do this by mounting the secondary and tertiary
holders on threaded rods that pass through the hubs of the spiders. These
can be locked in position with nuts on each side of the spider hubs.
     This longitudinal adjustment of the tertiary will give you the ability
to center it under the focuser.
     The longitudinal adjustment of the secondary will give you the ability
to adjust back-focus. (If you are planning on relying on moving the spider
vanes to collimate, then you are in for extreme frustration and failure.
Once those spiders are mechanically centered in the main tube, LEAVE THEM
THERE.)
2.   By sighting through a peep hole centered in the focuser, align the
periphery of the tertiary with the bottom edge of the focuser by moving the
tertiary mount axially along the threaded rod, twisting it around the axis
of the rod, and tilting it on it's 45* as you would with a Newtonian
secondary. (At this point the cone of the light path is F/long so you
needn't bother with the Newtonian secondary offset issue as it will be very
slight.) The three motions of tilt, twist, and longitude along the axis
will get the tertiary centered under the focuser in two axes, the third
axis may have to be adjusted by tilting or moving the focuser a tiny bit.
(Do not sacrifice the centering of the tertiary's spider in the tube to
achieve this, or you will pay a stiff price later.)
     If you previously centered the secondary spider accurately in the tube,
at this point the edge of the secondary will automatically appear centered
in the edge of the tertiary. You may see the skyward end of the tube
centered as well (assuming you have no baffle tube mounted yet and/or the
tertiary is too big).
3.     Now adjust the tilt of the secondary mirror to bring the reflection
of the edge of the primary mirror concentric with the edge of the
secondary. This adjustment will be the most difficult to judge accurately.
This is also where the Dall-Kirkham has an advantage over the classical
Cass - the secondary is a sphere and therefore getting this adjustment
close is just the same as getting it perfect.  :)
     With optic cells previously well centered in the main tube, the
tertiary and it's reflection of the focuser tube will now appear centered
in the secondary as well.
4.   Now adjust the tilt of the primary mirror to center the reflection of
the secondary. You are very close to being done now. 5.   At this point,
residual construction errors may have resulted in some
slight misalignment at some point. It is good to repeat the steps above to
clean these up.
6.   Now that the complication introducted by the addition of the tertiary
is taken care of, you can add the baffle tube between the tertiary and
secondary, though for practicality's sake you may want to skip this part
since a properly-sized tertiary will serve much of the same function of the
baffle tube. In this case, it might be a good idea to construct the
seconday holder such that it has a longer (or wider) baffle than you would
normally use, to block some sky light that the tertiary won't. These
parameters (tertiary and secondary baffle size) all depend on the size of
your desired fully illuminated field. The focuser tube length, diameter,
and baffling scheme is another parameter you can use to clean up stray
light and maximize contrast too.
     [Hint: Lay your desired optical path out on graph paper, then design
the mechanics around that.]
7.   After these steps are complete, there should be no further adjustments
other than a very slight tweaking of the tilt of the primary mirror to
clean up the symetry of the diffraction rings around Airy disks.
     The primary mirror's tilt will ALWAYS be the final step, and the ONLY
step necessary in periodic collimation checks, assuming the secondary and
tertiary cells were put together well and don't shake loose.

     I hope this helps Mike. Well made Cassegrains are wonderful
instruments. Poorly made ones can be a true heartache. The key to success
is in the mechanicals. It matters not how perfect the optics are - if the
mechanicals can't keep the optics in proper alignment and properly baffled,
then the optics are just so much wasted glass.
     Have fun with this one! It will be a project full of opportunity.  :)

     Gotta go! This is new moon weekend. I'm off for 6 nights of cold, dark,
high altitude sky. The truck is loaded and the road is calling!  :)

- Jim S.

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