A Simple Guide to Collimating a Newtonian Reflector
One of the
toughest jobs a beginner has to learn is collimation. This needs to be carried
out relatively frequently on Newtonian Reflector type telescopes and it can be
a very scary and/or frustrating experience for a beginner.
This guide
will take you through the various steps towards perfect collimation. I have
included both diagrams to show you the ideal as well as pictures showing how it
will actually look as beginners are often confused between the idealised views
of a well collimated telescope and the views they are seeing with their own
equipment.
The scope being collimated in these pictures is a Sky-Watcher
200P with an aperture of 200mm and a focal length of 1000mm (Focal Ratio f 5).
This is a common enough fast reflector and the collimation process is
essentially the same for any Newtonian telescope. Where there are differences
between a fast scope and a slow (high f number) scope I have provided text and
pictures to highlight the differences.
Fast telescopes are typically of
around f5 to f3 or even faster. You can calculate the focal ratio of your own
telescope by the simply dividing the focal length by the diameter of the
primary mirror. The Sky-Watcher 200P has a focal length of 1000mm and a primary
mirror diameter of 200mm so 1000 divided by 200 = 5 ( focal ratio ). Slow
telescopes are usually at f6 - f7 or slower.
To accurately collimate
with this guide you will need a Cheshire collimator and a collimation cap. My
own collimation cap is made from a focuser blanking cover with a small hole
drilled through the exact centre. You can make a collimation cap from a 35mm
film container or buy them ready made.
I have not covered collimation with
a laser because my own experience has been lasers seem to create more problems
than they solve as they also require collimation which isn't always possible
with some low cost lasers. Lasers generally only work well in telescopes which
have a high quality focuser with minimal 'slop'. The average mass produced
telescope very often has a focuser that is too imprecise to make use of a
laser.
This guide
may look scary - if you take it one step at a time its not that hard. In order
to cover off all of the issues the guide is somewhat larger than some others on
the web - many guides omit various details which can lead to confusion and
frustration in the beginner.
To collimate
the telescope it should be aimed towards a bright but not glaring surface such
as a well lit pale painted wall or fabric in order to get a good image in the
telescope and the Chesire should be arranged so that its shiny angled surface
gets a good light.
A Cheshire Collimator
A simple collimation cap
Tips before you
start ........
Rule 1 - Before
starting collimation don't assume that anything is out of kilter and check each
element before adjusting anything.
About 90% of collimation errors are down solely to the primary mirror so don't
jump in and start meddling with the secondary unless you are sure that there is
a problem. Each stage of this guide will show you how to check the various
elements - only if they are out of alignment will you need to make adjustments.
Rule 2 - Make sure you have all of
the tools you require ready to hand. Some telescopes will require Allen keys
and a screwdriver. You don't want to be looking for tools when you halfway
through collimation.
Rule 3 - Make
sure you have budgeted for enough time. Collimation can be a very time
intensive process for a beginner. It can be carried out over successive
sessions if you need to but don't plan on collimating on a night when you want
to be observing. It will lead to a rush job and frustration - TAKE YOUR TIME and you will be rewarded with good
collimation and better viewing through your telescope.
Rule 4 - Do not over-tighten nuts and adjusters.
They need to be firm and tight - they DON'T
need to be tightened until the threads pop.
Rule 5 - Whenever you are working on a telescope
ALWAYS work with the telescope tube in a
horizontal position. If you drop things into the tube you don't want them to
impact on the primary mirror. I also wear cotton gloves when I am working to
make sure I don't damage anything with sweat from my hands.
Collimation Step 1 - The Spider Vanes and
Secondary Mirror Holder
Before
attempting to collimate the secondary mirror it's essential to get the mirrors
holder central to the tube and the primary mirror. Some scopes allow adjustment
of the spider vanes. If your scope has this facility you need to check whether
the secondary mirror holder is held perfectly central by the spider vanes.
Note: Some scopes require the secondary mirror to be offset
from the centre of the tube. This in itself raises questions. Some telescopes
will have the secondary offset on its holder in which case the centre of the
vanes should be centred to the tube while other manufacturers will opt for the
vanes centre point being off centre. The offset on most scopes will be in
millimetres and wont affect collimation by much. You should check whether your
telescope requires offsetting with the manufacturer or supplier. If you cannot
get an answer assume that the mirror holder is to be central to the telescope
tube.
There are two
methods to accomplish getting the mirror holder central to the tube. The first
is relatively simply which is to measure the distance between the tube and the
secondary mirror holder along each of the vanes. The distance should be
identical for each vane. If the vanes are not equal than adjust them using the
vanes attachment / adjustment screws. The cross head on the central screw makes
an ideal reference point.
Measuring the spider vanes
Adjusting the spider vanes
The second
method for aligning the mirror holder is a little more complex but more
accurate and easier once the initial work is done. Cut a piece of card out which is the same diameter as the
telescope tube and drill a small hole in its centre. To find centre on the
circular piece of card fold the card into quarters. The point where the folds
cross is the centre. Now look through the hole and the central screw on the
secondary mirror holder should be exactly underneath. If it isn't adjust the
spider vanes until it is. This provides a simple and reliable method of getting
the mirror holder central and once you have made the piece of card future
centering of the vanes will be easier. To adjust
the vanes you will need to slacken the screws and then slowly tighten them in
turn to pull the mirror holder to the centre.
The hardest
stage of collimation is the secondary mirror alignment. This is almost always
very time consuming but once done should not need readjustment unless the scope
is severely shaken or dropped or the vanes and the secondary are removed for
any reason.
The easiest method to align the secondary mirror is with
either a collimation cap or a sight tube. With my Sky-Watcher 200P I find a
collimation cap is the better solution as a sight tube or my Cheshire do not
allow me to see the very edges of the secondary mirror through them.
The first object of
secondary mirror alignment is to get the secondary mirror directly under the
focus tube and appearing as a circle. The secondary mirror is an elliptical
shape but when presented to the focuser at the correct angle will appear as a
perfect circle.
The diagram to the right shows
the idealised view through the focus tube.
Many people find
the reflections in the mirrors confusing and in truth it can make this stage of
the process quite hard as your eyes can be confused by the multiple reflections
between the secondary and primary mirrors which can create a kind of optical
illusion which can make it hard to assess whether the view is circular or
elliptical.
A simple solution is to use a piece of white card or paper
to block the primary mirrors reflections. Simply insert a piece of white
paper into the telescope tube between the secondary and the primary mirrors.
This will create a view of the secondary as just a white circle. Another tip
here is that it's often quite hard to see whether the secondary is presenting
as a circle against the black background of the tube. I insert a piece of
coloured card or paper behind the secondary which allows my eyes to more
readily detect whether the secondary is appearing as a circle and is centred to
the dark circle created by the edge of the focuser.
This is shown in the pictures
below.
White card inserted into telescope tube to block reflections in the secondary.
Coloured card applied to scope tube to allow the secondary to
be shown more
clearly.
View through focuser/collimation cap.
If the secondary
is not centred under the eyepiece you will need to adjust it using the centre
screw of the secondary holder.
This is shown below. You are advised to
have the telescope tube near the horizontal position while you carry out this
work in order to ensure that if there is an accident the secondary doesn't fall
down the tube and into the primary mirror. Keep a hand holding or supporting
the secondary mirror holder without touching the mirror surface itself. Proceed
with great care and caution and take your time. The adjuster screw is quite
coarse and the mirror will move quite a lot with each turn. Be careful not to
loosen the screw too much which will cause the secondary to disengage from its
holder and fall off.
Adjusting the secondary mirror centre
screw.
The centre
screw adjusts the secondary mirror both up ( away from the primary mirror ) and
down ( towards the primary mirror.
To adjust this you may have to loosen
the screws or Allen keys which control the tilt of the secondary mirror. These
are also visible in the picture to the left.
Be careful not to apply too much
force and go slowly.
Simple collimation cap on focus tube.
Use a
collimation cap in the focuser to force your eye central to the focus tube and
rack the focuser tube as far out as possible.
Now move the mirror up or
down the tube using the central adjusting screw on the mirror holder until you
have the secondary mirror centred.
You will also need to rotate the
mirror until it presents as good a circle in the focuser as possible and adjust
the tilt screws to to acheive a good circular shape on the
secondary.
Aligning the secondary to the focuser -
the view. You can see that the secondary mirror presents as a circle which is
concentric to the circle created by the coloured paper and the focus
tube.
You should
eventually get to a view which is similar to that shown on the left - this view
is not perfect as it was taken with a small digital camera but it should serve
as a rough guide as to how things should be looking if you have adjusted the
secondary correctly.
Once the mirror is aligned correctly under the
focuser lightly tighten the tilt screws.
You can now proceed to the
second stage of the secondary alignment which is aligning the secondary to the
primary mirror.
Collimation Step 3 - The Secondary
Mirror/Primary Mirror Alignment
Aligning the
secondary mirror to the primary is easily the hardest task. Still using the
collimation cap in the focuser you now align the secondary using its tilt
adjusters and occasionally a slight amount of rotation on its main centre screw
( you may need to loosen the screw tightly and twist the secondary mirror round
by grasping its holder - DO NOT TOUCH THE MIRROR ITSELF ) to get the secondary
centred onto the primary. This process is covered in the pictures
below.
Adjusting the secondary mirrors 'tilt'.
First remove the coloured
card and the white 'baffle' card in the telescope tube. Now look into the
collimation cap and using the tilt screws get all of the primary mirrors
retaining clips into view and at the edge of the secondary mirror.
IGNORE ALL OTHER VISUAL INFORMATION !
All you are trying
to do at this stage is get the secondary mirror centred on the primary mirror.
The diagram to the left shows the
classic view of the mirror clips showing in the secondary mirror which are
equally spaced and equally proud. Not all scopes
will have three mirror clips - larger scopes maye have 6 or even
eight.
It may be that to achieve this view the
tilt screws alone may not be enough and you may have to apply a small amount of
rotation to the secondary mirror by slackening slightly the centre screw and
applying a small amount of rotation to the mirror.
Don't worry about
this as it may just show that your visual check was 'off' by a small amount.
In the diagram
above there is a circle marked in yellow. This is the likely extent of your
view if you are using a Cheshire eyepiece at this stage. I personally prefer a
collimation cap which provides a slightly wider view of the
secondary.
A view of
a perfectly aligned secondary mirror.
The photograph to the
left shows what the actual view should be like at this stage of the process.
Note the three mirror clips are all visible
and are all equally close to the edge of the secondary mirror and the secondary
is showing round and centred to the focuser tube ( the dark area at the edges
of the picture)
You may need to move your focuser in or out to achieve a
good view but the image should be very similar to this photograph.
When all three
clips are in view (and some scopes may have more clips) and shown at the edge
of the secondary mirror with equal space around them this part of the
collimation process is complete. Tighten down the tilt adjusters and take a
final check.
The tilt adjusters can be set quite tight but be careful
not to apply too much force and bend the spider vanes. As with the previous
step its best to have the telescope horizontal during this step in case an
Allen key is dropped into the tube.
Once the primary mirror clips are
shown reflected in the secondary and the tilt screws have been tightened take a
final check using the procedures in stage 2 that the secondary still shows as
centred and circular. If the secondary has
shifted then you will need to realign using the procedures in stage 2 and then
run through the procedures in stage 3 again. Its not uncommon for some rotation
error to creep into the mirrors position while the tilt screws are adjusted.
Take your time, be patient and continuously go back over your work and make
sure all is well at each stage before continuing.
When you are satisfied
that the mirror is centred and true as in step 2 AND all the mirror clips from
the primary are in view as per step 3 you can now tighten the adjusters down,
these need to be firm but do not apply so much force that you strip threads. A
standard Allen key is best rather than an Allen headed screwdriver or a multi
tool as it will limit the force you can apply. Take care also that you do not
apply too much force and cause the spider vanes to bend.
Now you should
check using a small mirror that the secondary adjustment is more or less equal
and that no one screw is causing the mirror to be canted too far over as per
the diagrams below.
Checking the secondary mirror tilt with a small mirror.
Secondary Mirror close to square on its holder. There will be
some small tilt shown on close inspection - OK
Secondary mirror excessively tilted. Retry alignment and start
again.
Collimation Step 4 - Aligning the Primary
Mirror
The final step in collimation is the primary mirror. This is
relatively simple to carry out and the cause of most 'in the field' collimation
errors.
At the rear
of the telescope you will find three pairs of screws. These are the primary
mirror tilt adjusters and their respective lock nuts.
Note: Some telescope
manufacturers (Sky-Watcher for instance) cover the collimation adjusters with a
steel plate. This will need to be removed before collimation can
proceed.
Removing the primary mirror cover.
Collimation screws. Lock screw to left, adjuster to right ( differs between
telescope manufacturers ).
For this stage you will need the Cheshire collimator. Insert this
into the focuser and take a look through the collimator. You should see a
pattern similar to the one shown below right - this is from a telescope that is
in collimation.
Cheshire collimator in focuser.
View through the Cheshire collimator
It may be
that that the pattern is offset and will show the cross hairs as being adrift
from the central circle created by the Primary Mirrors centre spot and that the
centre spots are adrift. If this is so then you will need to adjust the primary
mirror.
Collimation error - the centre spots are
not aligned and not
centred under the
crosshair of the Cheshire.
Close up of the central area of the picture to the left
showing the
centre
spots
misalignment.
If collimation of the primary mirror
is required loosen the lock screws off and adjust the primary mirror using its
adjuster screws until you see a pattern as per the diagrams and pictures below.
Take care when
using the adjuster screws that you do not allow the mirror to come too far
forward on adjustment as the screws may disengage and also take care that the
collimation screws and lock screws aren't tightened down to a point which may
cause the primary mirror to become 'pinched' or distorted. They should be
finger tight only.
Bear in mind the collimation pattern may be slightly
different depending on the f ratio of your scope. For more information see
below.
A note
on spider vanes in the view............ When collimating the spider vanes, apart from the inistial stage
of making sure they are holding the secondary mirror central, are not relevant
to the collimation procedure. Ignore them - you should only be looking at the
Cheshires cross hairs and making sure that the cross hairs intersect the
primary mirror centre spot.
Fast / Slow Telescopes You should note that fast
scopes of f5 and above are less tolerant of collimation errors and they also show slightly offset collimation patterns. These are
shown below as a guide.
Slow scope collimation - the classic collimation
pattern
Fast scope (f5 and above ) collimation pattern note the
offset
Above
illustrations courtesy of Jason Khadder
In the fast scope patterns above and below the circles are in fact
concentric and true (ie. totally circular) but the offset of the secondary
mirror creates an illusion that the reflection is elliptical and/or off centre.
These differences are shown with diagram plus an actual photograph of a fast
scope collimation view to give you an idea what the final view through the
Cheshire should look like.
This
is an actual photograph taken through a Cheshire and shows a near perfect
collimation pattern for a fast (f5) Newtonian. It is in fact my own Sky-Watcher
200P
Note that the centremost circle is made up of the primary mirrors
centre ring and its associated reflection. The cross hairs exactly
intersect the centre circle.
Note also the offset typical of a fast
Newtonian.
This
is a view taken through a Cheshire collimator of a 'slow' telescope when
collimated. It has been heavily retouched to give a realistic
view.
You can see that the circles are now perfectly concentric unlike the fast
telescope above which has a natural offset to its secondary
mirror.
When
collimation of the primary has been achieved and the Cheshire pattern is
correct tighten down the locking nuts and recheck. Do not over-tighten the lock
nuts. They should be firm but do not require massive amounts of pressure. I
usually feel for them coming into contact with the mirror cell and then add a
small amount of pressure.
Replace any protective plates and remove the Cheshire
collimator.
When this step is completed and all of the alignment
patterns shown in this guide are true then the telescope is collimated to
within the limits of what you can achieve with a Cheshire and you should find
the telescope is perfectly well collimated for use.
You may wish to
carry out a 'star test' to verify this and/or to make any fine fine tuning
adjustments. Star testing is simple enough to carry out and there is a section
below which will explain this and show the most common
problems.
Note: Bear in mind that even fast scopes with their lowered
tolerance for bad collimation have a little 'slack' in their collimation and
small collimation errors will not be visible in actual use to even the most
demanding observer.
When adjusting your
telescope bear this in mind and try not to become obsessed with having every
element exactly perfect. Many things will ultimately upset the 'perfect'
collimation. These include focuser 'slop' - this is the looseness of all but
the very best quality focusers, slight imperfections in the shape of the
secondary mirror, temperature variations and even the anglular position of the
telescope. Primary mirrors are quite heavy and most telescope tubes are
quite light. As a result a telescope tube will flex in normal usage and this
alone will upset collimation.
So - get it as good as you can but do not overly worry if there is
some small element which isn't perfect. My own collimation via a Cheshire is
always a little 'off' - star testing confirms the scope as being spot on so
bear this in mind.
Star Testing Your Telescope
When collimation is complete you may wish to carry out a star test
which will confirm the collimation or show small errors which can be fine tuned
out.
To star test you will need a bright star and good seeing
conditions. Polaris is the best star to test on but almost any bright star will
do. You will need your telescope to have cooled down to the ambient temperature
and also to be away from obstructions such as houses which may cause air
thermals.
Get the star centred in your view using a low power eyepiece.
Now use a very high power eyepiece ( or the highest powered you have available) and defocus VERY
slightly. You should see a classic Airey disk as shown in the pictures below.
Ideally the Airey disk will be perfectly formed for both sides of the perfect
focus (Extrafocus and Intrafocus) but in reality most telescopes wont do this
perfectly owing to slack within the focus mechanism or atmospheric turbulence
but you should see an Airey disk on at least one side of the perfect focus
position.
The
optimum magnification for star testing is at 25x per inch of aperture to start
with and then fine tune at 50x per inch if seeing permits - you will need a
very stable sky to get maximum magnification.
Note: When you adjust the
primary mirror collimation screws while star testing you may find the star
jumps out of your field of view. This is because moving the mirror shifts the
aiming point of the telescope slightly and is quite normal. Re-centre the star
and continue. Most Newtonian scopes will also create slight distortions towards
the edge of the field of view - again this is perfectly normal. The star you
are using for collimation should be centred in your eyepiece for this
reason.
If you are not seeing an Airey disk this then there are
several reasons why. This may be due to faulty collimation or other conditions
which may not be correctable.
A list of common conditions and their
corrective responses are shown in the tables below.
Be aware that in UK skies
atmopsheric turbulence will be very often the pattern you will see.
Unfortunately the typical turbulence pattern is very similar to the pattern you
will see with a badly damaged mirror. Unless you have reason to believe the
mirror IS damaged then there is no need to panic.
Even a turbulent atmopshere pattern
will allow you to see whether the corcles are concetric or whether they are off
centred and non-concentric.
Collimation perfect. Airey disk shows neat
concentric circles. The
innermost are dark due to the telescopes central obstruction caused by the
secondary mirror.
Collimation is out of adjustment - Results such
as these suggest the
primary mirror is not well aligned. This is simple to correct in the field by
adjusting the primary mirror collimation screws. Small adjustments only are
needed.
Atmospheric turbulence - The air around the
telescope and in its
line of sight is being disrupted by rising heat or by higher altitude turbulence. You cannot star
test with this present.
Tube currents - the telescope has not cooled
sufficiently to the outside air temperature. Allow the telescope more time to
cool down.
Pinched Mirror - this is the classic 'heart'
shape indicating the primary mirror has become stressed. Initially check that
the collimation adjusters are not over tight.
If the problem persists you will need to remove the primary mirror
cell and make sure the clips holding the mirror to the cell have
not been over
tightened.
Another
common cause of 'pinching' is often over tight tube rings.
Astigmatism can be caused by poor collimation but
also by poor quality optics. Secondary mirrors which are not flat either
because of bad collimation or bad polishing are typical causes.
Usually caused by incorrect figuring of the primary
mirror. This cannot be corrected by the user. It indicates faulty optics in the
mirrors design or production.
These show typical patterns for mirrors with poor
optical surfaces. This may be caused by roughness of the mirror caused by poor
polishing or damage from poor cleaning. This is not correctable.
