Ad Astra Observatory

PHOTOGRAPHIC POLAR ALIGNMENT
by Greg Beach

The one, often elusive requirement for any
astrophotographer and telescope owner is proper polar
alignment. This allows long-exposure photographs to be taken
in order to build-up on film that dim star light that has

travelled so far. It also allows the observer to see faint
details in planets, comets, galaxies, etc., without strain and
to prevent constant searching for a runaway galaxy!

This is accomplished by aligning the POLAR AXIS of the
telescope (which is quite often just a camera platform) to the
POLAR AXIS of the Earth. Easy to explain and understand but

often difficult to perform under even the best of conditions.

Most telescope owners know of at least one method of
polar alignment at the beginning of each observing session.
I present here another, little known method. I chanced upon
it several years ago when I received my copy of TELESCOPE
MAKING MAGAZINE #22 in the mail (pages 20-25).

This case study will focus on an amateur astronomer who
has taken many astrophotos with portable equipment. He/she has
decided it is now time to set up a more permanent facility.
He will then be able to spend all his energies in selecting
and photographing a target rather than worrying about polar
alignment (misalignment) and constant guiding.

We will not discuss observatory types, telescope types,
mount types, pier etc. We are only concerned with aligning
the polar axis of the telescope to the NORTH CELESTIAL POLE
(NCP) as best and as accurate as is possible by the
individual.

Photographic Polar Alignment (PPA) will work equally as

well with a fork mount or german equatorial mount. Atmospheric
refraction will not be taken into account. Since the altitude of
POLARIS is relatively high for most of North America, the error
caused by it will be less than the accuracy we are striving for
here.

Not all amateur astronomers/astrophotographers will go

through this phase. Perhaps only a minor few will or even
should! I present it here for your information. File it away
for now, someday you may need it.

LET’S TAKE A CLOSER LOOK AT PPA

Photographic Polar Alignment can be a long process. It

won’t be completed in one night. There is no need to rush into
it, in fact it could take days, weeks, months, or a year or
more! Your telescope won’t be out of commission for this
length of time. Your camera will still be of use. You may just
have to guide a little more often and maybe be satisfied with

tiny trails on your film. Each step you take will get you a
little closer to the NCP.

As a brief overview let’s take a look at the steps
required:

o You take a series of triple exposures (ie: three photos on

one frame of film) of the polar region.

o In-between each exposure you rotate the polar axis 30
degrees in Right Ascension (RA).

o Make an 8×10 print of the resulting negative or slide.

o From this print you will determine:

A) The exact location of the NCP.
B) The axis of rotation of your telescope’s polar axis.
C) The error in ALTITUDE and AZIMUTH of the polar axis.
D) The amount of movement required (mechanically) to move
a little closer to the NCP. ie: How much of the sky and
in which direction does this screw turn the axis?

o Repeat the above steps.

Sound like an involved process? YES! I do not recommend
PPA for the beginner. For everyone interested in the best
accuracy as is possible within “normal” means, I highly

recommend consideration of this method.

HISTORY

I have photographically polar aligned three telescopes
based on PPA:

1) A Perkin’s Elmer 8” f/5 astro camera mounted in a massive

fork mount with a 48” sector RA drive.

2) A 17.5” f/4.5 newtonian on a german equatorial mount.
(see SKY & TELESCOPE, May ‘87, pg 554)

3) I assisted on a Meade 2080 8” f/10 in the standard fork
mount. The azimuth adjustment was modified and patterned
after a Meade 8” LX3.

What kind of accuracy can you expect from PPA?

After just three PPA photos the Perkin’s Elmer is 28 arc
seconds (0.0075 degrees) east of the NCP and 46 arc seconds
(0.013 degrees) below the NCP. That’s the beauty of this
method! You know EXACTLY where your telescope’s polar axis is
in relation to the NCP. No second guessing.

In actual practice, using the Perkin’s Elmer as a camera
platform, we mounted a 35mm camera with 200mm lens with
hypered 2415 film on it. We exposed a frame for 35 minutes
with absolutely no guiding required resulting in pin-point
stellar images. In this case a large and accurate drive gear

helped but good polar alignment was the key.

Now we’re going to take a look at the equipment and materials
required to get started and take the first two photos. The two
major steps of PPA are:

1) Taking the actual photograph, and,

2) Analyzing the results.

Let’s break down step number one.

Assumptions: ———–

o You have a telescope that is permanently mounted (or will
be).

o The declination axis is exactly 90 degrees to the polar
axis.

o You have the ability to adjust ALTITUDE and AZIMUTH of the
mount.

Equipment Needed:

o 35mm camera (piggybacked on telescope)
o 200mm to 450mm telephoto lens (450mm is preferred)
o Clock (synchronized to WWV or CHU)
o Stop watch/timer (a 60 minute count-down timer is fine)
o Film — You have wide latitude here. Any film will do,

colour or b/w, slow or fast, negative or slide. There are
preferences though. Hypered film is not needed and in fact
it is best not to use it. With exposures of only ten
minutes reciprocity is not great and hypered film records
too many stars which make our reference stars too difficult
to find. Our combined exposure of 30 minutes (3 exposures

X 10 minutes each) could cause sky-fogging of the film
depending on your proximity to an urban centre. Colour film
printed on b/w paper is fine as we’re not concerned with
image quality.

In the example here I’ve used Kodak PLUS-X (ISO 125) and

TRI-X (ISO 400). These films will record up to 13th magnitude
in ten minutes on a single exposure. In the multiple exposure
PPA photo you’ll get down to about magnitude 10 or so. This will
let our 9th magnitude polar reference stars stand out just right.

PHOTO #1

Our first photo will be a reference photo of the polar
region which will be a single exposure using the same
camera/lens/film combination that all the PPA photos will be
taken. You’ll be referring back to this often.

Setup your telescope pointing at the polar region, (make

sure Polaris is in the frame). Use any means you have –
“eyeballing” it will do. Mount the camera and lens in its
piggyback bracket and point it too toward Polaris. It doesn’t
have to be exactly parallel to the polar axis as our PPA
photos will show the location of the telescope’s polar axis,

not the camera’s. Now, lock firmly both axes and make sure the
altitude and azimuth adjustments are locked too (this may
require a tool). With the drive motor OFF, open the camera’s
shutter (set on “B” or “T”) and lock. Expose for five minutes

and record time of beginning of exposure. [See PPAREC.TXT in
DL6 of the Astro Forum for a sample PPA record form.] This
photo will be known here as PPA-REF.

PHOTO #2

To keep the number of variables between each PPA photo

to a minimum make a mark on a non-moveable part of your mount
near the RA circle. I used chalk but be wary of using
something that rubs off easily. It doesn’t matter where you
start the first of the triple exposure as long as it’s at the
same place for each PPA photo.

With the mount and piggybacked camera set up as in Photo
#1 (and film advanced), OPEN the shutter, record the time, and
don’t touch anything. Have the camera lens cap ready. At the
end of ten minutes carefully place the lens cap over the lens,
but leaving the shutter open. Loosen the polar axis and rotate

it 2 hours in right ascension (i.e. 30 degrees) using the RA
circle as reference. Lock the polar axis, make a mark on the
mount and carefully remove the lens cap. Expose for another
ten minutes then place the lens cap over the lens once again.
Unlock the polar axis and rotate another 2 hours right
ascension. Lock the polar axis and make a third mark on the

mount. Remove the lens cap. Expose for a final ten minutes
then CLOSE the shutter.

You are now finished for the night. You’ll notice it may
take 30 to 60 seconds between each exposure while rotating the
polar axis. Don’t worry. As you progress through each PPA

photo these steps will become second nature and speed up.
Consistency is more important, and the time you record at the
beginning of the first exposure. This frame will be known here
as PPA-1.

You now have two frames of film exposed:

1. PPA-REF is the reference photo of the polar region and,

2. PPA-1 is the first PPA, triple exposure of the polar
region.

We now have film in hand with two frames exposed — one
reference photo of the polar region and one triple exposure
of the polar region. It’s now time to develop them. If you

used colour film bring it to your favourite lab and process
normally. It can be hard to find a lab that does black & white
film. That’s why it’s so often done at home. Again, develop
normally.

Once developed you have one “normal” frame of stars

(PPA-REF), and one “weird” one (PPA-1). The weird one will
have three sets of every star. Each set will consist of three
small arcs with spaces separating them. There is also three
of something else that is not readily apparent. What is it?
Three NCPs! This is what we’re looking for. The purpose of

this exercise is to find the NCP and the axis of rotation of
the telescope. We’ve established the location of the NCP. At
what point in the sky is the scope pointing at? The set of
three NCPs lay on a arc whose centre is the axis of rotation
of the polar axis. Easy to say but a little more involved to
resolve.

The next step is to make two 8×10 prints. If you used a
450mm lens, a 35mm negative is best enlarged about 16 times.
This will show an area of sky about 1.5 degrees by 2.0 degrees
on the print. We’ll be working with this print a lot. Here
are two methods of making them:

Method 1:

Make it as you normally make any enlargement. That is
emulsion side of the negative facing DOWN in the negative
carrier toward the easel. In other words — Emulsion of
negative facing emulsion of the printing paper.

Method 2:

Make the print as a mirror image of the negative. That
is emulsion side of the negative facing UP in the negative
carrier away from the easel.

[Note: Make the print of the reference photo as you normally

do.]

In order to determine our ultimate goal we need to do an
amount of drawing. This can be difficult on the emulsion side
of the print, not to mention making it messy and hard to
compare the star patterns on the reference photo. You can use
a piece of tracing paper and place it over the print and mark

all stars with a pencil. This causes several problems as star
images magnified 16 times are fat and fuzzy and errors can
crop in when marking.

I use method #2. When working with a mirror image no
tracing paper is required — all drawing is done on the back

of the print. Using a sharp point (the end of a compass for
instance) I marked the starting of each stellar arc (including
Polaris). When you turn the print over (back facing up) all
you have are small points of each image, no confusing arcs,
and a clean white background on which to work.

Choosing Your Starting Point:

You know at what time you started your first exposure,
now look at the three bright arcs produced by Polaris. Where
is your starting point? This is not as readily apparent as one
first thinks. The following is an example using a german
equatorial mount;

1st exp. – Declination axis horizontal with scope on east

side of pier.
2nd exp. – Scope rotated 30 degrees toward the west.
3rd exp. – Scope rotated a further 30 degrees to the west.

The resultant print would have arcs similar to this;

(movement of the star)

#1 #2 #3
<—- <—- <—-

——-* ——— ———
^

starting point

Still confused? Let’s go back to the beginning of the
first triple exposure photo. To know exactly the starting
point you can modify the procedure like this. ….

With the telescope motor drive OFF, camera pointing at

Polaris, and with a new frame of film;

OPEN shutter
Record time (LMT)
1) Expose for 30 seconds to one minute.
Place lens cap over lens
Wait for two minutes

Remove lens cap