COLLIMATION METHODS
First, read
this: Collimating
a Newt It will explain everything you need to know about
collimating a newt. In addition, this site has a
wonderfully complete explanation of collimation and collimation
techniques. This site is so complete as to make my site redundant.
But I didn't find it until today, so I'll leave my stuff here as well.
As with most new
telescope owners, I was very intimidated by collimation when I received my
XT4.5. That carried on well into the early days of my XT10. I read
and read and re-read everything I could find on the internet. But every
time I put that darn peep hole eyepiece in the focuser (the one Orion
provides), I just got lost in all the reflections.
The thing that cleared
everything up for me was the document that Gene Baraff
has on the Skyquest group. I must have read 20
different documents on how to collimate a newtonian
reflector and still didn't get it. But for some reason, as I read through
Gene's document, it all clicked. I'm not sure if I was already on the
cusp of enlightenment and just needed any sort of nudge, but Gene's description
just made sense to me. You can read a copy of his document here.
From that point on, it
was just a matter of fine tuning my technique. It was no longer
acceptable to be close, collimation had to be perfect. It was at this
point that I figured out the inherent inaccuracies of each method of
collimation. Mind you, I had not learned how to do a proper star test to
check collimation, so I did not know if my collimation technique was actually
inaccurate. I just assumed it must be because I could not get repeatable
results with my laser. The Cheshire was a viable option, but I couldn't use
it at night and that's kinda when I needed it the
most.
Over the past year, I
have had a chance to use several different methods of collimating my newtonian telescopes. I will try to list the goods
and others for each type. First, here is a list of methods I have tried:
1. Collimation
Cap: this is the little round cap with a hole in it.
2. Orion
Cheshire/sight tube
3. Orion Lasermate
4. EZ Telescope
1.25" Laser (just like the Orion Deluxe Lasermate)
5. Barlowed laser technique
6. Catseye cheshire and auto
collimator
7. Star
collimation (you should always do this)
All of these methods
will work to some degree of accuracy or another. My favorites are the barlowed laser and the Catseye cheshire/autocollimator. These have proven to be the
most repeatable and are very, very accurate.
Regardless of the
methods chosen, the article
listed above covers all the aspects that are involved. You can pick a
different method of accomplishing all of these elements, but they all must be
done.
COLLIMATION
CAP
This is the little
round cap that comes with most Orion reflectors. Here's a picture of one.
Its
purpose is to provide a good reference for you to use when collimating your
telescope. If you just look down the focuser without the collimating cap,
it's hard to tell where to put your eyeball. This little cap keeps your
eye close to the center and makes alignment easier.
CHESHIRE
/ SIGHT TUBE
The most complete
collimating device ever invented.
The article
I have referenced several times uses a combination sight tube / cheshire to illustrate how to collimate a reflector.
It really is the only tool I have used that can do every step that collimation
requires. I'll let you refer to the article for all the gucci pictures and most excellent explantion
of how and why it works. Suffice to say it can do everything and more
that the fancy lasers and such do.
The drawback that
keeps me from using this as my only method of collimation is that I cannot use
it at night. You need some sort of light shining in the little 45 deg
mirror to make it work. You can do this at night by shining a flashlight
into it, but that's not for me. This is the same reason I don't use the Catseye method at night. First time I drop that
flashlight down the tube onto my mirror will be a horrifying moment
indeed. No flashlights for me. For night time
collimation, I use the laser and barlowed laser
method.
A bit more on the
sight tube.
The sight tube part is
just that. Without the cheshire, a sight tube
is simply a hollow tube with a cap on the end that has a tiny hole in it.
This is the tool required to center the secondary under the focuser. The
sight tube should have a ratio of length to diameter close to the focal ratio
of your telescope. For instance, a sight tube for an f/5 telescope should
be 5 times longer than it is around. This makes it easier to align the
secondary under the focuser. If you use a sight tube that is too short,
the secondary will look a lot smaller than the hole in the bottom of the sight
tube. If you use a sight tube that is too long, you won't be able to see
the edges of the secondary.
You can rack the sight
tube in and out a bit to get the right spacing around the secondary.
LASER
COLLIMATORS
I am not a big fan of
a straight laser type collimation. Either a regular laser or the EZTelescope or Orion Deluxe lasermate
lasers. Both suffer from the same weakness. They rely on the
accurate and repeatable placement of laser in the focuser every time in order
to be useful. You also need to collimate the laser, which might be hard
or not so hard. As an engineer, it gives me the willies to see that I can
effect the accuracy of my
alignment by just lightly touching the laser in the focuser. I abandoned
this method long ago.
I still use the
straight laser to align the secondary after dark. This is not the most
accurate, but good enough for after dark alignment.
BARLOWED
LASER
The barlowed laser method takes out all of the variables
mentioned above. Here is a most excellent description of Barlowed Laser Collimation.
Gene posted a message
on the Skyquest group about
this new method of collimating a newtonian reflector
with a barlowed laser. As with all things
related to tinkering, it immediately piqued my interest. I had the laser,
I had a barlow, all I needed was a target and I would
be set. So, I made a MEBLSPTT [mebblespit]
(Most Excellent Barlowed Laser Spot Target
Thingy). You can read about my advanced construction techniques for the
MEBLSPTT (and follow on NIDPMEBLSPTT) MEBLSPTT.
I am a big fan of the barlowed laser method, but it does not collimate the entire
optical train. It only adjusts the tilt of the primary. But it does
that very, very well.
For me, this is the defacto method for aligning the primary at night. As
accurate as any other tool I have used. For aligning the secondary at
night, I use the straight laser method.
CATSEYE
CHESHIRE AND AUTOCOLLIMATOR
STAR
COLLIMATON
A star test is
required as a verification of whatever method you use. That means a star
test for collimation. The best site I have found that explains how to
star collimate is here.
If the collimation method you use proves accurate via a star test, then you
should stick with that. One word of caution: a precise star collimation
check must be done in the best of seeing conditions. Simply defocusing on
a star and evaluating the concentricity of the diffraction rings is good enough
to get in the ballpark, but won't tell you if you're
gnat's ass on. That requires an in focus star
with a steady airy disc and that requires very good seeing. Don't be
lulled into a false sense of security by those apparently perfect out of focus
diffraction rings. They are not precise enough to get that final tweak
for perfection.
WHY
BOTHER?
So, really, how
important is collimation? In my experience with my XT10, it is
critical. With a slower f/6 or f/8 telescope, it is still critical, you
just don't have to be as accurate as with the f/4.7 XT10. I can say for
certain that 1/30th of a turn on one of the collimation knobs will mean the
difference between seeing 4 craterletes in Plato and
9 craterletes. It could make the difference in
spying the Enke division in Saturn's rings.
More often than not, though, poor seeing will mask any errors that may exist in
your collimation. If the seeing is bad, you might not see any craterletes in Plato or even the Cassini division in
Saturn's rings no matter how accurately you are collimated. But why miss
an opportunity to see such detail because of something you have complete
control over?