Collimating an ASA N10

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Nov 15, 2013 (3 years and 6 months ago)

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Collimating a
n

ASA N10

Introduction

Collimation is essential to make a Newtonian telescope perform at its

best.

The aim of collimation is
to align the different optical
axes

of the telescope.

The Newtonian

The ASA N10 is a Newtonian reflector. The design i
s well know and the different elements are shown
in
Figure
1
.


Figure
1
: main elements of a Newtonian reflector

With regards to collimation a Newtonian reflector has two optical
axes
; the axis of the pri
mary mirror
and the axis of the focuser (or eyepiece, laser, Cheshire, etc). These two optical
axes

are shown in
Figure
2

below for a telescope out of collimation.


Figure
2
: The optical axis of the prim
ary mirror in green and the optical axis of the focuser in red

T
he aim for collimation is to make these two optical
axes

to coincide with each other

by tip/tilt
adjustments of the primary and secondary mirrors
.

The main idea is that if the optical axis of

the
focuser (red) “hits” the center of the primary mirror and the optical axis of the primary mirror (green)
“hits” the center of the focuser these two optical
axes

will coincide and the telescope is collimated.

When looking thru the focuser into the seco
ndary mirror
you

will
see the

different elements directly,
as reflections or both.
Figure
3
below gives a schematic image of what you should see.


Figure
3
: The elements of the Newtonian as seen in the foc
user for a telescope out of collimation

The aim of collimation is to align these elements and their reflections in such a way to make the two
optical
axes

coincide
. When the two optical
axes

coincide the

elements as seen in the focuser will
line up in a sy
mmetric way

(
Figure
4
)
, except the secondary mirror’s reflection (see next section)
.



Figure
4
: The elements
appear

symmetric in a collimated Newtonian

Secondary offset

Most internet resources tell you t
hat when the telescope is collimated you should see a perfectly
symmetric and centric image of all elements and their reflections when you look down in the focuser.
However the reflection of the secondary will not be centered which is illustrated in
Figure
4
. All
elements


secondary, primary reflection, focuser reflection, Cheshire reflections


are all centered,
but the secondary reflection is clearly not.

The reason for this is that the secondary need to be slightly offset from
the center of the OTA.


Figure
5
: In the top image the secondary is centered in the OTA. In the bottom image the secondary is offset down and
forward.

Figure
5

illustrates the difference between having th
e secondary’s center coincide with the OTA
center and having the secondary slightly offset down and forward. The top image shows how the
light cone partly misses the focuser when the secondary is placed in the center of the OTA. The
bottom image illustrate
s that by a slight offset the light cone can be made to “hit” in the center of
the focuser.

The offset needed can be calculated by

Offset = (secondary size)/(4*focal ratio).

From this one understand that the offset need to be larger for faster telescopes
and/or larger
secondary mirrors.

Astrograph Newtonians like the ASA N10 are fast (f3.8) and tend to have large secondary mirrors.
Thus the offset becomes much more prominent than for slower Newtonians with smaller secondary
mirrors.

The diameter of the ASA

N10 secondary mirror is 100 mm and the native focal ratio is f3.8, giving an
offset of 6.6 mm. In
comparison

an f6 Newtonian with 60 mm secondary gives an offset of 2.5 mm.

Depending on the size of the secondary and how fast the telescope the non
-
symmetri
c behavior of
the secondary’s reflection may be more or less prominent.

In addition to aligning the two optical axes, the aim for collimation is to position the secondary to
have the correct offset.

Tips and

tricks

Tip 1
:
Always place the OTA in a horizont
al position.

If the OTA points towards zenith anything you
drop (hex wrench, etc) will fall down and hit the primary mirror. This can be avoided by place the
OTA in a horizontal position.

When looking down into the focuser you will see something like
Fel! Hittar inte referenskälla.
. Due
to all confusing reflections it might be difficult to distinguish between the different Newtonian
elements. The image can be made less confusing by implementing tips 2
-
4 below.

Tip 2
: If
you are only interested in the

position of the secondary mirror as you look down into the
focuser, you can block the primary mirror by inserting a piece of card board (or similar) into the OTA
between the secondary
and primary mirror (
Figure
6
).

Figure
7
: It may be difficult to distinguish between
different ellements due to the many confusing
reflections.

Figure
6
: A piece of red card board is inserted between the
primary and secondary mirrors. A white piece of paper is
inserted opposite to the focuser.

Tip 3
: The sec
ondary mirror’s outline might be difficult to find if the inside of the OTA is dark. Placing
a piece of white along the inside of the OTA on the opposite side from the focuser will make the
outline more prominent

(
Figure
6
)
.

Tip
4
: If you are interested in seeing all the reflections of the Newtonian elements it may still be
confusing to look into flowers and pots or whatever the OTA is pointing at (
Figure
7
). If it is not
possible to point the OTA towards

a homogeneous surface, hang a towel (or similar) on a chair (or
similar) in front of the OTA (
Figure
8
).

Collimation tools

In this document we will discuss four collimation tools
; a Sight tube, a Cheshire

collimator
, an
Autocol
limator, a Laser collimator.

Sight tube
: This is essentially just a tube with a cap. In the center of the cap there is a small hole.
When the Sight tube is placed in the focuser one can peer down thru the small hole and see if the
different elements and th
eir reflections are centered (as in
Figure
4
) or not.

Cheshire collimator
: This works just like a Sight tube but has two important improvements. Opposite
the cap (with a small hole in it) there is a cross hair inside the tube. Thi
s cross hair can be used to
check if things are centered when peeking into the tube. Both the cross hair and its reflection will be
seen when peeking thru the hole. Also there is a reflection ring on the bottom of the cap (inside the
tube) and the reflecti
on of that will be visible when looking thru the hole.

Autocollimator
: This tool is also essentially a Sight tube but with a highly reflective surface on the
inside of the cap. The reflective surface will act as a mirror, reflecting back what is seen in th
e
secondary mirror onto the primary mirror and back again. This will cast several reflections of the
same element (e.g. the primary mirror’s central spot).

Laser collimator
: A laser collimator is basically a laser that you place into the focuser. Then you

try to
adjust the different elements so that the laser hits the center mark on the primary mirror and comes
back to the center of the focuser. Note that it is essential that the Laser itself
is
collimated so that the
laser beam does not leave the Laser co
llimator by an angle.
Use a Laser collimator that you can
collimate yourself and check collimation before using.

Figure
8
: A kitchen towel has been hung on a chair in front of the OTA

The collimation steps

The collimation is done in three main steps.

A.

Square the focuser

B.

Set the correct secondary mirror offset

C.

Adjust the focuse
rs optical axis so that it “hits” the center of the primary mirror.

D.

Adjust the primary mirror’s optical axis so that it “hits” the center of the focuser

Squaring the focuser is essential to make the aligned optical axes be parallel to the OTA. If the
focus
er is not squared the tilt of the optical axes might be so that it is not possible to get a good
collimation. Since the ASA astrographs are high end telescopes this step should already be
guaranteed. Thus this document will not cover this step, but in prin
ciple one marks the opposite side
from the focuser. Using a laser in the focuser one then adds shims under the focuser until the laser
hits the mark on the opposite side.

Except for the focuser there are 5 other adjustments that can be done.

1.

The spider van
es (position of the secondary in relation to the middle of the OTA).

2.

The position of the secondary mirror along the OTA axis (towards/away from the
primary)

3.

The rotation of the secondary mirror around the OTA (axis)

4.

The tip/tilt of the secondary mirror.

5.

Th
e tip/tilt of the primary mirror.

1 and 2 above adjusts the secondary mirror’s offset (B in the list above), 3 and 4 adjusts the focusers
optical axis in relation to the primary mirror’s center (C) and 5 adjusts the primary mirror’s optical
axis in relat
ion to the focuser’s center (D).

Figure
9
: The starting poi
nt a telescope out of collimation

For the ASA astrographs the spider vanes has already been accurately adjusted in the factory so we
can jump directly to step 2.



We start with the situation as shown in
Figure
9
, which shows a New
tonian out of collimation. It is
clearly exaggerated just to make things clear. The optical axis of the focuser (red) hits a spot far away
from the primary mirror’s center mark (the triangle) and the optical axis of the primary mirror just
barely makes it
into the focuser. We also see that the secondary mirror is not centered in the
focuser’s line of sight, which means that the secondary offset is not correct.


Figure
10
: the collimation steps

In
Figure
10

the different collimation steps (excluding the focuser and spider adjustments) are
illustrated.
In each image the optical axis of the previous step is shown
in dotted gray.

B
:

The first step
is to adjust the secondary along the OTA axis. This will secure

the correct offset of
the secondary
. The secondary is moved inside the focuser’s line of sight. Note that this will also shift
where the focuser’s optical axis hit the primary mirror. Similarly where the primary’s optical axis hits
the focuser shifts some
what.

C1
: A rotation of the secondary around the OTA axis will secure that the secondary is not tilted
right/left with respect to the focuser and that the focuser’s optical axis “hits” the primary on the
up/down center line. There are small errors in all t
he elements (primary may have a slight
tilt
, the
spider may also tilt just slightly etc) so the focuser’s optical axis may not “hit” exactly spot on the
primary’s up/down center line , but it shall be close.

C2
: By tilting the secondary mirror the focus
ers optical axis is
moved to “hit” the center spot. If the focuser’s optical axis does
not “hit” exactly on the primary

mirror’s

up/down center line
,

one may need to

tilt the secondary left/right. However

this
should be a small adjustment. Most of the seco
ndary tilt
adjustment shall be up/down.

D
: The primary mirror is tilted so that the primary mirror’s
optical axis “hits” the middle of the focuser.

Depending on how large step D was it may affect where the
focuser’s optical axis “hits”

the primary. Thus i
t may be
necessary to repeat C2 and then D again. When, after step D,
the focuser’s optical axis still “hits” the primary’s center mark
the telescope is collimated: the secondary offset is correct and
the optical axes of the focuser and the primary are ali
gned.


Adjusting the secondary mirror offset

and the focuser’s optical axis

sideways (B+C1)
.

In this step we will place a Cheshire in the focuser
and use its sight tube functionality. Position the
Cheshire so that you can see the whole secondary
plus a s
mall boundary around it

(
Figure
11
)
. Since we
are only concerned with the position of the
secondary as seen thru the Cheshire we will place a
red cardboard between the primary and the
secondary. This will block all reflections fro
m the
primary and the secondary will be filled with the
pattern on the red cardboard. Also since we want
the boundary around the secondary to be clearly
visible we place a piece of paper inside the OTA
opposite to the focuser. This is illustrated in
Figure
6
.

What we now see when looking into the Cheshire
will look something like

Figure
13

(left)
.

We now loosen the secondary mirror holder’s central screw (
Fel! Hittar inte referenskälla.
) so that
we can

rotate the secondary and move it forward/backward along the OTAs axis.

The aim is to adjust
the secondary mirro
r until the white boundary around it is equally large on all sides (
Figure
13

right).
When this is achieved the second
ary offset is correct and the focuser’s optical axis is close to the
primary’s top/down center line (step B and C1).

When you are satisfied with the result tighten the
center screw.


Figure
11
: A Cheshire is placed in the
focuser so that the secondary is just
visible when peering down thru the
sight hole.

Figure
12
: By loosening the central screw we can adjust
the secondary mirror.

Adjusting the focuser’s optical axis
up/down tilt (C2)


Figure
14
: Adjusting the tip/tilt of th
e secondary so that the focuser's optical axis "hits" the center of the primary mirror.

In this step we
use the same Cheshire collimation tool, but we now use one of its Cheshire functions


the cross hair (visible in
Figure
13

as a cross over the red secondary). The aim of this step is to tilt
the secondary in such a way that the f
ocuser’s optical axis “hits” the
center mark on the primary mirror

(
Figure
14
)
. In the case of an ASA
astrograph the center of the primary is marked with a triangle. If we
can view this triange in the middle of the Cheshire we have adjusted
the focuser’s optical axis
so that it “hits” the center of the primary
mirror. This is where the cross comes in


we simply need to adjust the
secondary until the triangle is directly under the Cheshire’s cross hair.


Since we now want to see the primary mirror w
e start by removing the
red card
board. Then we start to adjust the tilt of the secondary mirror
Figure
13
: The secondary as viewed in the Cheshire.
Left: before adjustments. Right: after adjustments.


Figure
15
: Adjusting the tip/til
t
screws on the secondary

by loosening one or two of the tip/tilt screws (
Figure
15
) just a little and then tightening the
opposite
screw(s)
.

Use small movements and do not overtighten. This is
trickier than one might expect. It is
very easy to loosen/tighten a
too much which may result in a rotation of the secondary


meaning
we have to redo step C1.


Figure
16
: Adjust the secondary until the Cheshire's cross hair is right on top of the primary's center mark (triangle).

By slowly adjusting the tip/tilt screws on the secondary we w
ill make the triangle in
Figure
16

top
move exactly under the Cheshire

s cross hair
Figure
16

bottom. Note that the Cheshire

s cross hair is
s
lightly defocused in
Figure
16
.

This step can also be made using a laser collimator.
Be careful not to look straight into the laser
beam. The laser beam will travel from the fo
cuser, reflect on the secondary,
travel
to the

primary

and reflect back to the secondary. If the telescope is way out of

collimation the laser beam may miss
the secondary on its way back
.

Before you look into the OTA
,

hold a paper
sheet (or similar)
in front

of the OTA to make sure that the laser beam does miss the secondary a
nd exits

the

OTA


with
risk of
hitting your eyes.

Insert the laser collimator in the focuser and
tighten just enough so that you still can rotate

the
laser inside the focuser. W
atch

where the laser
spot hits the primary mirror

and a
djust the
secondary mirror tilt as per the instructions above
until the laser spot hits the middle of the triangle
center mark on the primary

(
Figure
17
)
.

Now slowly rotate the
laser collimator

in the
focuser. If the laser collimator is
collimated the
laser spot will stay exactly on the same spot. If the
laser spot performs a small circle the laser
collimator need to be collimated. You may try to
adjust the secondary so that the primary

s center mark is in the exact center of the circle the laser
spot traverses as you rotate the laser collimator.


Now
we are done adjusting the s
econdary, but before we go to the next step and
adjust the primary
you need to go back and check that your secondary still appears circular and centered in the sight
tube. If you are unlucky you have accidentally managed to rotate the secondary.


Adjusting
the Primary mirror

tilt (D)


Figure
18
:
Make the primary's optical axis hit the focuser's center

Figure
17
: Laser beam hitting the primary's center mark
(lower laser dot)


In this step we still

use the Cheshire collimation tool but now we will use the reflective circle inside
the Cheshire

s cap.


Figure

19
:
Looking thru the Cheshire after secondary mirror adjustments

The aim of this step is to tilt the secondary in such a way that the
primary
’s optical axis “hits” the
center
of the focuser

(
Figure
18
)
.
The center of the focuser is visible as a reflection in the secondary



the
Cheshires reflective circle.

What we can expect to see when looking down into the Cheshire is
shown in

Figure

19
. Here we can see the triangle (primary

s center mark) and th
e Cheshire reflection
ring.

T
h
e
primary mirror

s optical axis will be hitting the
center of the focuser if the primary

s center mark
(triangle) reflection conincides with the focusers center
mark (Cheshire

s reflection ring). Thus we need to adjus
the primary so that the triangle

fits inside the reflection
ring.

For the ASA N10 it is easiest to remove the back plate
(with the fan). The construction is suc
h that for each of
the three adjustment positions there are two push
screws and one pull.

Figure
20
: Adjusting the pri
mary mirror tilt


Figure
21
: Adjusting the p
rimary mirror's tilt until the Primary mirror's center mar
k
(triangle) fits insi
de the Cheshire's
center mark (reflection ring)

Adjust the primary mirror

s push/pull screws until
the triangle fits inside the reflecti
on ring (
Figure
21
).

The end result is shown in
Figure
22

and we
can see that we are almost collimated.
The triangle
fits nicely inside the reflection ring, but having
adjusted the primary the Cheshire

s cross hai
r
(defocused in
Figure
22
) is not exactly on top of the
triangle anymore. Thus we need to go back and
adjust the secondary slightly


fine
tuning.

Also for this step a laser collimator can be used but
now it is even more
essential

that the laser is
properly collimated since the laser beam travels a
larger distance.
A
precise laser technique is to use
a barlowed laser b
y which the laser beam is
defocused. This will make a shadow of the primary

s center mark

be cast back to the focuser. For a
barlowed laser c
ollimator this shadow can be observed and be made to line up with the laser
collimator.

Fine tuning (step C2 revisited)

In this step we will use the Autocollimator tool

(
Figure
23
)
.

T
he
fine adjustment
s that need

to be done
to the secondary
are
very

small

which

may be dif
ficult
using the Cheshire
. The

Autocollimator
is

essentially
a mirror with a peephole that

is
inserted into the focuser.

The mirror

in the Autocollimator

is positioned down
wards

so
that it will reflect all the reflections in the secondary back to
the secondary and from ther
e into the primary etc. The idea is
that a number of reflections of the Primary’s center mark will
be visible and the aim is to align these reflections on top of
Figure
22
: The defocused cross hair is not exactly on top of
the triangle

Figure
23
: The Cat's eye Autocollimator

each other by making tiny adjustments to the secondary tip/tilt screws

(
Figure
24
)
. We are talking
tiny he
re


something like 1/100
th

of a turn.


Figure
24
: Adjusting the secondary so that the triangle reflections are on top of each

other
.

A final check thru the Cheshire reveals the collimated image (
figure

23)


















Figure
25
:
Collimated ASA N10

as viewed thru the Cheshire collimation tool

All in all the procedure of collimation is very time consuming and elaborate, but the good news is
that normally it is only the primary mirr
or adjustment and the fine adjustment that need to be
repeated.


A note on accuracy

The final step above (fine
adjustment) is questionable. The reason being that once the triangles are
on top of each other (and have more or less disappeared) a simple tightening of the focuser will
throw the triangles out of alignment. The same is true for just a small change of vie
wing angel thru
the Autocollimator.

There is no real guarantee that the viewing angle used to align the triangles are exactly the one
applicable when adding photographic equipment.

A note on star test

M
ost internet resources

recommend making a star test to check collimation. By defocusing a bright
star in the eyepiece one gets an image of circles with a black area in the middle. The idea is then that
all these circles
should b
e concentric on a collimated telescope.

However, this is simply not true of the secondary offset is large. The “black hole” in the middle of a
defocused bright star is nothing else but the shadow of the secondary mirror. If that mirror is slightly
offset,
the shadow will be offset as well

(see

Figure
26
)
.


Figure
26
:
A
defocus
ed bright (Deneb) star of

a collimated ASA N10. Note the spider vane effects.

Adjusting the primary mirror under the stars so that a defocused star is concen
tric will throw the
telescope out of collimation.

Of course if the telescope is slow enough and have a secondary mirror small enough the offset might
be so small that a defocused star will be very closed to concentric for a collimated telescope.



Summary

He
re follows a summary of all the collimation steps without the lengthy explanations.

Position the OTA in a near horizontal position to prevent things from falling onto the primary mirror.

Step B

&

C1



Centering the secondary in the focuser

Insert the sight tube

and adjust its insertion until the secondary plus a small “ring” around is visible.


Figure
27


Step C
2



Center the primary’s reflection in the secondary mirror

by adjusting the secondary’s tilt

Insert the Cheshire collimation tool and use the cross hair (not the
reflection of the cross hair) to
center the primary mirror reflection in the secondary.


Figure
28



Figure
29




Step D



adjust the primary’s tilt until the primary’s center mark fits exactly inside the Cheshire’s

reflection ring

Using the Cheshire collimation tool adjust

the primary mirror’s tilt.


Figure
30



Figure
31



Fine tuning



adjust secondary until triangle reflections are on top of each other

Insert the Autocollimator and adjust the secondary mirror


Figure
32

Final check


Figure
33