THE CARDBOARD DIFFRACTION MICROMETER
----------------------------------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2001 August 1
Introduction
----------
To follow a binary star's motions over the years you need a means
of actually assessing the orientation and distance of the pair. It is
out of the realm of home astronomy to build or buy the traditional
filar micrometer. This is an expensive device and requires a permanent
scope with stable mount, accurate clockdrive, and excellent optics.
This micrometer also demands quite clear and keen eyesight, patience,
and endurance. And then some maths to make sense of the measures.
After asking around among double stars observers, Dr Martin
Gaskell, University of Nebraska, recalled a technique I actually
played with some thirty years ago. He pointed me back to the
diffraction micrometer, made from a hunk of cardboard!
Constructing the micrometer
-------------------------
Yes, cardboard. You can make it in an afternoon and observe binary
stars with it on that very night.
The method does involve some maths, but that's what your
calculette is for. As long as it's a scientific or engineering model,
with the trigonometric functions on it, you're set.
Cut a round disc of stiff smooth thin opaque cardboard to
completely cover the front end of your telescope, like a cap. Paper is
too flimsy and board is too heavy. Cardboard from gift boxes is about
perfect. If the front end has a well or seat, like a very short dew
shield or a bezel for the corrector plate, make the disc fit smoothly
within in. If not, you can tape a low collar of cardboard around the
tube for the disc to sit in. Be sure the disc can be turned smoothly
and easily without binding or sticking, yet tight enough so it doesn't
blow or shake loose.
For noncircular tubes you'll have to build a carriage or bed for
the disc, which itself is seated on the front end of the tube. This
may be of any sturdy material, even of cardboard. A bit of ingenuity
is handy in this situation.
In this disc cut carefully, using a hobby knife, a grid of
parallel slits. Leave enough land all around to keep the disc rigid.
The solid and open strip of each slit are equal in width so the 'fill'
ratio is quite 50%. The width of each slit (either the open or solid
zone) is from 1 to 2 centimeters. Use which ever width gives you the
most slits across the aperture of your scope while maintaining
structural integrity of the disc. (If this shtick of really getting
data out of binaries grabs you, you'll later make several discs for
different width of slits.)
Your scope may have a central obstruction. Make the solid zone of
a slit sit over this obstruction to minimize its intrusion into the
open area of the slit. If this obstruction has a spindle, you may cut
a hole for it so the disc can rotate on this spindle.
Testing the micrometer
--------------------
Let's test this thing.
Aim the scope a known and resolvable double star. Then plop this
slotted disc -- the diffraction micrometer -- on the scope. First off,
you'll see the stars are much dimmer, by about a full magnitude.
That's partly due to the blockage of the solid strips in the disc.
Closer inspection reveals that the stars, all of them in the field
except for the dimmer ones, have 'satellites'! These, one on each side
of the main image, are tiny spectra of the stars. Very crude and
likely not colored, but they are true spectra. You're seeing the two
'1st order' spectra. There are 2nd, 3rd, and higher order spectra
farther away from the central image but they are for sure far too
faint to notice.
The double star will, then, consist of six dots, the primary
star's central image, its two attendant spectra, the secondary star's
central image, and its spectra. Note that the three images of all of the
stars are parallel.
To see these images more plainly do pump up the magnification a
bit. Use the highest power the scope, eye, and sky will permit. This
is more than the working maximum power of 1D (D is the scope aperture
in mm). Get the stars well apart even if they are a little swollen.
You may go to 2D or even 2.5D.
Now try rotating the disc. Gently turn it in its well by nudging
it around. You may want to glue cardboard ears on the disc to
facilitate this chore. The images in the eyepiece rotate, too. That's
the secret. You can align all six binary star images in a straight line
or move them to sit exactly side by side.
Some maths
--------
The little spectra, seen as dots next to the central image stand
angularly away from that image by delta arcseconds, where delta is
(delta) = (lambda/width)*(206265 arcsec/radian)
width is the pitch between the slits, in millimeters, spanning one
open and one solid zone. You know this from the construction of the disc.
Ideally you should have a slit width that gives a delta comparable to
the actual angular separation of the binary star, but for now go with
what you have. Later you can make extra discs.
lambda is the wavelength of the star's light. This is NOT a single
value due to the continuum spectrum given out by the stars. However,
the eye-brain perception and the fact that stars have their maximum
emission within the visual sensitivity range, conspire to let us take
this as 570 nanometers.
If you are concerned about such an assumption, you may tape over
the slits green gelatin filters, like Wratten 58, to barricade out the
far blue and red rays and let in just the green ones around the peak
of the eye's spectrosensitivity. This is a wide bandpass filter, not a
real monochromatic one. Can you use a monochromatic, hydrogen-alpha,
filter? Yes but the stars will so dim, from turning away so much of
their light, that it's just unworkable.
delta is in arcseconds due to that extra factor to shift radians
into arcseconds. This delta is a property of the disc and can be
written on it permanently. Make sure you properly juggle the metric
prefixes! milli- means 1/thousand and nano- means 1/billion.
For a specific example suppose we made the slits 18 mm apart. Then
delta, the angular separation of the satellites spectra from the main
image, is
(delta) = (570e-9 m)/(18e-3 m)
= 3.167e-5 rad
= 6.525 arcsec
You can label this particular disc 'delta = 6.525 arcsec' for good.
Now you need an index mark on the diffraction micrometer disc.
Any place on the circumference is fine, but it's easier to find at
night if you put it at one end of the slit that passes thru the center
of the disc. Make it good and bold for easy reading at night.
Refinements
---------
After this first test you have to add a compass scale to the
scope. This is easiest done on a paper collar marked off in degrees
all around the circumference of the tube. You can lay this out by
geometry on a large sheet of paper. Or plot out the markings with a
computer in BASIC or PostScript and a LaserJet printer. If lazy, get a
giant protractor, the open circle kind, from an art supply store and
snip out the inner struts so it fits snugly over the tube. Please
accent the degree marks on the collar so you can read them at night.
On the collar the degrees must run clockwise as you look down into
the tube from the eye of a star. If the degrees run anticlockwise,
flip the collar over. Or make a correct new one.
Examine the star in the scope with the clockdrive shut off. Rotate
the disc so the three images of each star line up exactly with the
diurnal drift. After several runs you'll get surprisingly good
alignment. Note the angle of the disc; write this down. It should be
270 degrees, the direction to west.
It sure will not be 270 due to the arbitrary way you attached the
collar. Leave the collar alone and just record that this actual degree
reading stands for 270 degrees. In general you'll have to go thru this
process for each session. The telescope tube may be rotatable in its
rings, you may have an altazimuth instrument, you may not precisely
polar align the mount. All these (plus a few more) will cause field
rotation during the session and require you to establish the east-west
offset for each double star.
You could make the collar a slipring; slide it around so 270 deg
lines up with the index. I find that for a degree ring made of paper,
the extra handling will soon loosen and tear it. Leave it fixed in place
and just note the offset.
Note that life is far easier if you seat the disc so the index is
closer to the top of the tube, nearer to its north point, so you read
off of the upper edge of the collar. This immensely reduces the chance
of reading an angle from the under side of the scale.
First measurements
----------------
Now we're ready for our first measurement of of a binary star.
Examine the star under the highest tolerable power. Let's call the
primary star and its satellite spectra a', a, and a". Those for the
secondary are b', b, and b" a and b are the central images, Please
remember that as you rotate the disc the central images of the stars a
and b remain fixed while the satellites a', a", b', and b" rotate in
step with the disc. They align perpendicular to the slits as seen from
the focal plane.
Pick, it doesn't matter which, one of the satellite images, say,
a". Rotate the disc so the angle formed by line a-b and line b-a" is
as exact a right angle as you can tell. The right angle is at point b.
You will find two orientations of the disc that give this aspect.
Ve sure you line up with the SAME satellite! Note the angle of each
orientation on the degree collar, say they are alpha1 and alpha2.
Double check this setup. The leg between the central image a and
its satellite a" must be the hypotenuse. If it's one of the legs of
the right angle your setting is all wrong. It may be more comfortable
to use a crosshair eyepiece to line the images against. Or you may
find the hairs too distracting and prefer a clear eyepiece.
The sketches here show how the images must line up in the two
orientations. I show them nice and big and spread out here. In the
eyepiece they'll be packed into the center of the field. You do need
that high magnification.
+ | +
a' | a'
|
|
a * + | + * a
/ | b' | b' | \
/ | | | \
/ | | | \
+-------* | *-------+
a" b | b a"
|
|
+ b" | b" +
orientation #1 * = central image orientation #2
-------------- + - satellite image --------------
You may find that delta, by the construction of your disc, just
will not make that right triangle. It is too small or too large. Now
you see why it is well to have several discs with different delta.
Separation and position angle
---------------------------
The position angle of the double, theta, is
(theta) = (alpha1 + alpha2)/2
which is the average of the two angles.
There will be the ambiguity of 180 deg; this is resolved by a
quick look at an orbit chart of the star. Because we did not zero the
collar, be sure to add or subtract the offset, as found bt the drift
method above.
The separation of the two stars, rho, is
(rho) = (delta)*cos((alpha1 - alpha2)/2)
First the alphas are subtracted, then divided by two, then the cosine
is taken. Leave the algebraic signs alone! Don't try to 'fix' them.
The calculette will sort things out for you.
Of course, errors will creep in. Do the same exercise for each of
the other three satellites in turn, yielding four sets of data. they
should agree in position angle and separation within, believe it or
not, a fraction of one percent.
Why this works
------------
It's a lot easier to see what's happening by joining the two
diagrams into one along the line a-b:
a
*
/ | \
/ | \
/ | \
+-------*-------+
a"(1) b a"(2)
We constructed, in two steps, the isoceles triangle of a,
a"(1), a"(2). The altitude of this triangle is the separation
between a and b. The two equal sides are each delta. The purpose of
building this triangle in two steps is to get at the apex angle, the
one at a. angle a is that included between alpha1 and alpha2, or is
(alpha1 - alpha2). Then, for either of the two right triangles within
this big triangle the angle a/2 is then (alpha1 - alpha2)/2.
Simplified method
---------------
Do you really have to got thru all this? Well, if you just want to
casually see a double star go thru its rounds you can bypass the
maths. Rotate the micrometer disc until all six images are exactly in
a straight line. Read the degree collar. This, with the offset, is the
very position angle. Proportion the separation between a and b against
the delta along this line of images. That's the separation.
History
-----
This is really a cunning and simple method of assessing binary
star aspects! This method of diffraction micrometry was developed
apparently first by Schwarzschild (the blackhole guy) in 1896. He used
a variation of this method to measure many binaries. In his device,
the width of the slits was adjustable, and thus delta, to make the
images stand exactly as far apart as the components of the binary.
Thus the delta for this aspect was the very separation of the
components.
He varied the slit size by oblique perspective. The disc was a
'tent' of two slitted plates set on a carriage atop the telescope. By
raising or lowering the peak of the tent, the telescope 'saw' narrow
or wide slits, there by adjusting the delta. Guess what? he built his
contraption out of, erm, cardboard.
Limitations
---------
The diffraction micrometer has limitations. First off, as you saw
when you first tried it out, the images are far dimmer than with no
disc. This comes from the obstruction by the solid parts of the slits
and the dispersion of light into several images for each star. You'll
probably be limited to studying double stars only as faint as two or
three magnitudes above your normal threshold for the scope.
If the double star components are too different in brightness the
images of the dimmer will be overwhelmed by those of the brighter. It
seems that the two should be within two magnitudes of each other for
easiest work. This limitation does depend on keenness of your vision.
You do need at least a good size aperture. I tried the micrometer
on small scopes, 75mm to 100mm aperture, the ones you see with me at
our starviewing meets. Apart from the frail disc that remains after
cutting the slits, the images are just too weak. When I played with
the diffraction micrometer in the 1960s and 1970s I used a 150mm
Newton and then a 200mm Schmidt-Cassegrain. Both gave very pleasing
results.
A small scope also lacks the raw resolution to split the closer
binaries. Based on the rule of (res) = (120 arcsec.mm)/(aperture in
mm), an 80mm scope resolves 1.5 arcsec at best. The stars's images
touch and are barely distinquishable under the best of seeing. On the
whole the larger the scope, the better for increased number of slits,
tighter resolution, and brighter images.
Atmospheric seeing, blessedly, is not that much of an obstacle.
The images may be soft or swollen but as long as they are actually
round you can get good fixes on them. Never the less, the steadier is
the air, the more certain is the measurement.
Cardboard is not all that durable. It works superbly for gentle
use and care of handling. If after a while you find yourself hooked by
binary star observing, consider making a micrometer of sheet metal
with the aid of a mechanic or metal shop.
Each disc has one geometry of slit and one delta value. Being that
binary stars have a diversity of separations, you'll find that for
a given star your disc has too small or too large a delta. You then
have to make a set of discs for a stepped range of delta. Don't even
think of building a Schwarzschild micrometer! Or any other kind with
slides, rails, hinges, gears, &c to vary the delta.
The micrometer does NOT, as some authors may tell you, bring out
more details in planets or nebulae. It does NOT resolve stars into
discs. It does NOT vastly increase the resolving power of your
telescope. It does enable you to consciously measure very small
separations of point sources, the double stars.
Homebased binary star work
------------------------
Yet, after all is said and done, why isn't the diffraction
micrometer in every observer's toolkit? It is, to be fair, very seldom
described, even in litterature for double stars. The material I read
dates from the 1980s at the latest to all the way back to Schwarzschild's
own paper in 1896. There seems to be no recent papers on it for the
home astronomer, while there is lots and lots on diffraction (and
other) micrometers for observatories.
There's also the matter of disincentivity for double stars. Look
in any observing book, What's emphasized? What gets the awards and
attention? Not double stars. The data in such books is often
incompetent, copied from ancient sources heedless of the real motion
of the stars. And let's not think about the color descriptions applied
to the stars! Lipstick manufacturers could take lessons from them.
On the other hand, binary star observing is one of the regions
where home astronomers can, like for variable stars, make genuine
worthy contributions. The sheer simplicity of the equipment and
technique should appeal to just about anyone who is otherwise disposed
to do some useful work in astronomy. The combination of many
observer's reports, like for variable stars, will smooth out the
inevitable random dispersion of measurements.
Are binary star observations really needed? Here's the rub. Yes
and no. Yes in that with so many stars as binaries -- about 1/3 of all
the stars you see in the sky are binaries! -- there's plenty of work
for everyone. So much so that most binaries are observed infrequently,
some were observed only once at discovery.
No, in that there is no credible bona-fide solicitation for double
star measures, setting observing standards, collecting and compiling
data, encouraging observers, perfecting techniques, distributing data
to other astronomers, and so on.
So, this is a new century (get used to it, OK?). Can our profession
kick off a collaborative effort for home and campus astronomers to
mainstream double star observing?