I began my research on CCD cameras a few years ago after seeing the wonderful astrophotographs so popular on astronomy sites on the web. I also wanted to poke through the light problems of my area.
The human eye is at a significant disadvantage when it comes to light pollution. To begin with, background light obscures dim objects be decreasing contrast (the difference in light between the object and the sky background). The eye needs about one magnitude difference to be able to distinguish something from its background. The second disadvantage is that the brighter the sky is, the less dark-adapted the eye gets. a truly dark adapted eye is far far more sensitive to light than a non-dark adapted eye. This is the curse of light pollution. It's a double curse.
The CCD camera requires no dark-adaptation, and it needs nowhere near one magnitude difference between background and object to distinguish an object. If a camera is capable of collecting counts of 65000, it is theoretically possible to detect an object if it is 1/65000th more intense than background. Still,, it is very difficult to detect objecgts that are dimmer than background. A CCD camera is very helpful in light polluted sites.
At the time, my telescope was a Celestar-8. The scope cost about $1000, and had a
wedge-pod. I had a battery-powered clock drive and was ready to take on the imaging world.
The path I took is presented in the subject of this page.
Polar Alignment
Locating the target
Focusing
Polar Alignment
I had been told that in order to do astro-photography, I would need to be well polar
aligned. I decided that a good polar alignment could be achieved using the drift method.
That was the method presented in my Celestron instruction manual.
My first purchase toward that end was the Celestron Micro Guider eyepiece. This eyepiece
gives a high precision bulls eye with precise distances between the circles. Using this
eyepiece, I could center the star in the bulls eye, and watch for drift. Boy did I see
drift. Checking the drift method instructions, I noticed that if the star drifted one way
in the eyepiece, I should move the azimuth direction to the right, and if it drifted the
other, I should move it to the left. How do you make azimuth corrections with a wedge-pod?
Well after asking around, you just kick the northmost leg and that will move the azimuth
direction one way or the other depending on which side you kick. As you can imagine, the
wedge-pod adjustment method just wouldn't work. I couldn't get very good results doing the
kicking method. The altitude adjustment was even worse. You loosen a couple of set screws
with an Allen wrench and push the scope either up, or let it drop down (free fall). I
needed a better mount.
Purchasing the adjustable mount
My second purchase was a mount which could give me control over altitude and azimuth. I
bought the Field Tripod from Celestron. I also bought the JMI wheeley bars to move the
scope as a unit between the garage and the driveway where I did my observations.
The azimuth control
The azimuth adjustment consisted of two screws, one to move the alignment toward the west,
and another to move the alignment toward the east. The adjustment mechanism rotated in
azimuth using a 4" radius circular plate. The screw making the turn has 20 threads
per inch. Therefore a complete turn of the adjustment screw produces an azimuth rotation
of
atan(0.05/4)=0.716 degrees, or 46 arc-minutes.
Note 0.05 represents 1/20 of an inch which corresponds to the 20 threads per inch.
The altitude control
How good an alignment can we get with this kind of control? In Azimuth, I can easily get
an adjustment within an error circle of about 5 arc-minutes, which when using the formula for polar
alignment error T=23636.23M/FX, gives a 60 second maximum exposure.
What I discovered
After spending several evenings doing alignments using this mechanism, I noticed that if I
achieved alignment in the south, I would throw off alignment in the east, and could not
get convergence. After doing some research, I began to suspect that the altitude and
azimuth axis were not at 90 degrees, in other words the planes were not orthogonal. To
test this premise, I performed the following procedure:
1) use a level to make my kitchen table level in all directions. I used shims to
accomplish this.
2) place the C8 on the table.
3) place a line on one of the fork arms exactly down the center of the arm that goes
through the center of the bearing pivot of the OTA.
4) Use a carpenter square to see if the line up the fork arm is at right angles to the
table top. If it is, go to step 6
5) Loosen the bolts holding the fork arms to the base, and adjust until the line up the
fork arm is at right angles to the table top. Then tighten, and re-measure
6) use a level and adjust the OTA so that it's top is level with respect to the table.
7) set declination circle to 90 degrees. (this procedure is needed to align digital
setting circles later).
After completing this procedure, I could achieve a level of polar alignment.
Another thing I discovered
Although I could get a pretty good polar alignment now, I noticed a lot of movement in the
direction of right ascension. This movement could not remove this motion, and finally
identified it as periodic error. I decided that the fault lay in the drive mechanism
itself, and after a little analysis found that the drive motor was a simple gear which had
a bit of backlash in it. I decided to replace the drive mechanism including the forks. I
purchased a used Byers Drive over Astromart. When I did this, almost all the periodic
error was removed.
Purchasing the Moto-track IV with PEC
When I upgraded to the Byers Drive, I lost the declination drive power port. To provide
this port, and to improve drive even further, I acquired the JMI Mototrack IV drive
corrector.
At the end of all this, I was achieving sufficient quality of polar alignment that I could
use to make images. By this time, I could achieve a quality 60 second tracking.
The thing I discovered
Polar Alignment takes a long time. To minimize the time, I purchased the JMI Polar
Alignment Scope. The intent of the scope is to record an accurate polar alignment once it
has been achieved by centering certain stars in an observing scope using alignment screws.
Then when polar alignment is to be achieved the next time, the user simply reacquires the
stars in the scope to recapture the alignment.
Locating the target
I use Nagler eyepieces for visual work. These eyepieces have incredible fields of view,
usually something on the order of 1 degree or so. A CCD camera has a field of view of
something on the order of 10x15 arc-minutes, or about 5-6 percent of the eyepiece field of
view. Targeting must be done with higher power eyepieces to improve the chances of
acquiring the field to be photographed. To acquire the general field, I use Digital
setting circles. This usually places the object in the large eyepiece view. I then center
the object, and switch to a higher power eyepiece, and re-center. A 200x magnification is
usually sufficient to give a reasonable field of view. One of the problems I had with the
C8 is that when the camera head is placed on the scope, it causes a flexure of several
arc-minutes. Compensation usually requires a clockwise rotation of the fine declination
knob of about 1 turn, indicating a displacement of something on the order of 10
arc-minutes. This is roughly the vertical dimension of the imaging area for the camera.
Achieving focus
getting a good focus is a significant issue. The focus plain is quite narrow, something on
the order of a small fraction of a millimeter. To see anything at all, focus has got to be
quite close from the outset. This can be achieved by visually focusing using an eyepiece
of approximately 200x which is parfocal with the camera. Such an eyepiece can be made by
using trial and error go get a first focus, then marking an eyepiece at the focal position
that has the image come into focus when the camera is removed. This arrangement allows for
a reasonable place to start.
Focus accuracy can be achieved by moving the camera in and out from this location using
very small changes in focus. Highly accurate focus is not achievable using the standard C8
focusing knob. To accomplish very accurate focusing, I purchased the JMI NGFS-DRO focuser.
Using the camera maximum pixel intensity reading, I could make small changes in focus. At
some point during focus, light intensity would strongly peak. This is the place where
focus is perfect. It should be noted that the focus peaking is non-linear. Focusing light
obeys the inverse square law, so a spike is what is observed.
The image can then be taken.