Imaging Equipment Setup
After countless nights of experimentation, many equipment changes, and
lots of new technique
additions and improvements; this article was written to describe a
acquiring and processing astronomical images. My methods
can certianly be improved upon but currently result in acceptable image quality.
Many budding astroimagers begin their journey by holding a digital
camera over a telescope's eyepiece, hoping to hold it still long enough
to snap a picture. This actually works and some decent shots have
emerged from the uncountable bad ones.
It quickly becomes apparent that better equpiment means better
images. Then, not so quickly apparent, is learning how to
most out of the equipment. Excellent polar alignment and focus
are absolutely essential. The best equipment can produce crappy
images if not tweaked to accurate calibrations and focus.
Finally, processing all of those raw red, green, blue and luminance
images took a while to get right. Well, actually I don' t think
that I have them quite right yet. still working on that
As in any complicated process, the basics have to be right before the
final result is right. While there is always bigger and
better, my current mix of equipment is satisfactory in its ability
to produce fine images of the cosmos.
Losmandy G11mount with Gemini, on a concrete pier
6" Astro-Phyiscs Starfire refractor telescope
2.8" Televue Pronto refractor telescope
SBIG ST2000XM CCD camera
SBIG ST5C CCD camera for autoguiding
RoboFocus on the Starfire
Kendrick dew heaters on the scopes
An old PC or two
Too many cables and wires
CCDops - camera control & imaging software, usualy used for the
CCDSoft - camera & robofocus control and imaging software, usualy
for primary imaging camera
RoboFocus - smart hardware, pc control for focusing
TheSky - map of the sky, can control scope position
Gemini Control Lite - excellent utility for running the Gemini
Telescope, Mount and Camera Setup
The following setup areas are all important to taking good images, and
all must be done as accurately as is reasonably
possible. A well polar aligned scope will produce images
stars for up to a few minutes without guiding.:
- Polar Alignment - without it, your stars will be worms.
Close isn't good enough for imaging, although it is fine for visual
observing. Do a good drift alignment. When done well
allows pictures of one to a few minutes without autoguiding. With
addition of autoguiding,the telescope can follow the target almost
exactly, and for hours.
- Build a good object model in the Gemini - The mount's controller can
build a 3D image of the pointing characteristics of the mount by
accurately centering on one object at a time and calibrating an
additional point on it. The more objects you add to the model, the more
accurate telescope slewing becomes. When it's set up
well, the scope will point just about anywhere and put the target in
the center the field of view. Put 8 or a dozen eastern
it and another set to the west.
- Target Selection - choose a target that fits your telescope and
field of view. Many targets will be either too large
fit in the image, too small to be more than noticable, or too faint to
get much detail in a short exposure. TheSky can show your FOV
relative the part of the sky being imaged.
- Autoguiding - Use a seperate guide scope and camera and rigidly
to the main telescope to avoid flexture. Using the second CCD
chip on the ST2K camera works fine as a guider, but 1) has a small FOV
that might not contain a good star, and 2) you will often loose the
star when the blue filter is in place. I have read a
recommendation that at least 1/2 of the mm focal length of the main
scope should used as a
guide scope for accuracy. I don't disagree.
- Focus - the word says it all, it's criticaly important and can be
quite a challenge to zero in
on. Small stars should resolve to a pinpoint, it's easy to get
close, but close isn't good enough. The RoboFocus absolutely
helps to tweak it up.
CCDSoft's @focus routine often does a good job of finding where it is,
but I often tweak it by eye anyway.
- Imaging - finaly, shoot some images! Take a number of the same
shot for each color component you take. They are: red, green,
blue, luminance, hydrogen alpha. The exposure length of each
image depends on the target. Planets and moon are very bright and
a short exposure time is used, everything else is faint and exposures
of 5 or 10 minutes are typical.
- Dark Images - a dark represents the electronic noise in the
camera. They should be done with the same temperature and
duration as the light exposures they will fix. Take 9 darks
and average them into a master dark, then bulid a library of tempature
and time combinations.
- Flat Field Images - should be taken after each imaging session but
are optional. Experience suggests that they're more apt to be
needed if the moon is bright and/or it's a little hazy out. They
eliminate the little dark dust donuts that show up in your
shots and reduce vignetting. Lazy people like me don't take
them as often as we
should. To make a master flat, average 9 flats after dark
subtracting them. The flat is valid for all light images until
the camera is loosened. This is because the flat corresponds to
dust or other stuff in the optical path.
Processing all the raw images is discussed on the next page.