Re: Laser versus Non-Laser Collimation

From: canopus56 (canopus56_at_yahoo.com)
Date: 02/28/05 

Date: 28 Feb 2005 12:08:39 -0800

Previously you wrote:
>> My problem is that when I am dead-on with the laser, I cant see an
>> even ring of space around the primary when I check it with the
>> eyepiece cap. Conversly, when I use the eyepiece cap, and get a nice

>> even ring around the primary, the laser doesn't point to the center
>> of the primary. Why aren't the two tools showing the same thing?
See - http://web.telia.com/~u41105032/kolli/Kolli_9.gif in
http://web.telia.com/~u41105032/kolli/kolli.html

I had a similar problem and frustrating experience with a Chesire,
laser colliminator and eyepiece cap, when first learning how to
collimate. The diagnosis was an out-of-round focuser tube. It's
satisfying to have your scope squared up, so you can quickly check the
collimation before each session.

I trust a Cheshire and a laser collimator in combination over an
eyepiece cap (unless the scope is new and out of the box) because a
laser collimator and Cheshire have longer barrels that better align
along the optical axis of a used and worn focuser tube (as suggested by
others in this thread). Most focus tubes on a Newt get worn
out-of-round after a few years. My previous experience (and as
suggested elsewhere in this thread) is that a Chesire, laser
colliminator and eyecap have differing sensitivity to the optical axis
of the focuser. If your focus tube is particularly out of round, I find
it helpful to wrap a piece of paper around the Cheshire or collimator
tube. (If your focuser tube is grossly out of round, the best solution
is to spend $40 and replace it.) This may eliminate any gross
differences between a Cheshire and the laser collimator, but not the
eyepiece cap.

I ended up abandoning the eyepiece cap. There was no way to
consistently get the eyepiece cap to work in an even slightly
out-of-round barrel. A Cheshire also has a narrower field of view than
the eyepiece cap. This improves its accuracy over the eyepiece cap,
when trying to do a gross visual alignment.

My personal practice in collimation is not to use one, but all three
methods - 1) Cheshire collimate (gross align in a comfortable, light,
heated room at home every fourth observing sessions before driving to
the observing site, shining a red flashlight into the Cheshire to get a
clear mirror reflection), 2) laser collimate (each session, medium tune
either before leaving home and/or at the observing site) and 3) star
test collimate (each session, tweak to fine tune the collimation after
the tube has cooled down for a half-hour). I usually fit-in the star
test collimation after fine aligning the finder scope. I accept small
errors in collimation revealed by the star test unless my purpose is
astrophotography. Some scopes simply will not align perfectly or are
too much trouble to align perfectly, particularly if my purpose is
pleasure observing.

Sirius is presently well-positioned at 9pm to star test your
collimation, even from urban light-polluted or Moon washed out skies.
http://web.telia.com/~u41105032/kolli/comacomp.JPG in
http://web.telia.com/~u41105032/kolli/kolli.html
http://www.skywatchertelescope.com/EducationST.html
Star test collimation is simply a matter of focusing on Sirius using a
high-magnification lens and/or Barlow. Then defocus inside (away from
the point of focus toward the tube body) and/or outside (away from the
point of focus away from the tube body) until the star turns into a
series of concentric rings. This usually happens within the first
360° rotation of the focuser tube.

I know my collimation practice sounds like a bit of effort. After the
initial learning curve, it becomes second nature and is easily
incorporated into relaxed pleasure observing.

> Do I NEED to offset my secondary somehow, . . .

Before changing the secondary position, I would resolve whether the
tool is wrong - as suggested by previous posters. My recommendation is
that you align using a Cheshire and a laser collimator. Then star test
on Sirius. Look at some craters on the Moon at high magnification to
get a sense of the sharpness of linear detail.

Repeat the star test after realigning with the eyepiece cap.

Mostly likely you'll conclude that the eyepiece cap gives a false
signal.

BTW, I did end up repositioning the secondary mirror in a Newt after
the first time that I used the above procedures. Before trying to
change the horizontal offset of the secondary (the distance between the
primary and the secondary), I changed the more easily adjusted vertical
offset of the secondary mirror (the distance between the secondary and
the bottom of the focuser.) First, I marked with the original vertical
setting of secondary mirror stalk with a dab of white-out. This minor
adjustment gave good image results - both the star test looked good and
a linear feature test on the Moon looked sharper than before the
adjustment.

If, after just changing the vertical setting, the resolution looks okay
to your viewing requirements, I'd stop there.

Only later, did I mess with the trickier horizontal offset of the
secondary mirror - but that was mostly for self-education.

Since then, my Cheshire and the laser collimator give reasonably
consistent results.

In conclusion, what were you results using the star test of collimation
- a test suggested by others in this thread? How did it change after
you aligned with each tool. Sirius is well positioned at 9pm to do the
test. Working through the collimation tests in your living room is an
enjoyable way to spend an evening, with some quick pops outside to do
the star test collimation.

- Peace - Canopus56

Relevant Pages

  • Re: Laser versus Non-Laser Collimation
    ... You can not do a fine star collimation without seeing the secondary's ... the optical axis tilted relative to the tube. ... this offset does not effect the reflection. ... wouldn't even bother playing with a spot laser type. ...
    (sci.astro.amateur)
  • Re: Newtonian Sonotubes
    ... > weaken at all and hold their collimation just fine. ... out moisture) from much use over the years, that it's tube is as healthy ... such as around the focuser or mirror cell, ... This is NOT a fault of the material. ...
    (sci.astro.amateur)
  • Re: collimation query
    ... What is important is that the optical axes are co-linear. ... If the axis of the focuser is not co-linear with the optical axis of the OTA, you will introduce aberrations, notably astigmatism and coma. ... The only simple way to be certain is to perform a star test. ... If the diffraction rings of a defocused on-axis star are nice and concentric, your collimation is fine. ...
    (uk.sci.astronomy)
  • Re: Laser versus Non-Laser Collimation
    ... > no matter what the tilt ... don't have a scope handy here to test it. ... collimation tool (Cheshire, laser, star test) to show mis-collimation. ...
    (sci.astro.amateur)
  • Re: Collimating an old Celestron C8?
    ... making sure that they are all snug, and then only adjust two out of ... collimation screws aren't the only things holding the secondary to the ... I know when I need to do an additional star test, ... can be too rough to detect this amount of miscollimation. ...
    (sci.astro.amateur)