CCD Imaging: Q&A
John A. Blackwell
johnb@regulusastro.com
Introduction:
This document is permanently under construction, since new
information will be constantly added.
Every now and then an amateur astronomer asks me
what equipment they will need to do good CCD imaging,
or what the techniques are for good imaging. The
questions lead to more questions, more answers, and a lot
of typing! Compiled below are the most frequently asked
questions I hear when it comes to the topics of CCD
astronomy. The hope is that this article will get read,
and that the reader is prompted to continue research into
this rewarding field of astronomy. Many of the images in this
paper have been reduced in size. To see the full size image with
details, just click on the image.
These articles set the stage for the rest of this
discussion. Be sure that you want to get involved with
CCD imaging. It is a time-consuming and patience-
testing activity. It does not become easy overnight and
can lead to a lot of frustration. Plan on imaging one
object per night. Plan on a long night. Plan on
spending most of the evening focusing! If you
are the type to get frustrated easily, then take a few
steps back and do some wide field astrophotography.
The results are lovely, and require minimal equipment
and effort. If you love checklists, details, and
have a way with electronics when it's cold outside and the
bugs are biting, then this might be a hobby for you!
What is a CCD?
The term CCD is an acronym for "Charged Couple Device".
What does this mean? Now it is time to talk about a little
theory. A CCD chip is a thin piece of silicon in a wafer
format. These wafers are sensitive to light in an interesting
way. When light falls upon them, the "Photoelectric Effect"
takes hold; electrons are knocked free from the crystalline
structure of the silicon and deposited into small units or
wells. There is one well per pixel. When the image is complete,
the electrons are sent into a holding register where they are
counted. The more electrons in a given register, the brighter
that pixel will be. That is the simplified version. Those
looking for a more complete theoretical discussion should
see one of the many fine books about CCD theory now available.
One good one is: CCD Astronomy : Construction and
Use of an Astronomical CCD Camera by Christian Buil,
Emmanuel Davoast (Translator), January 1991, Willmann-Bell;
ISBN: 0943396298.
What is Quantum Efficiency?
This is a measure of the CCD chip's sensitivity to the
various wavelengths of light in the spectrum. In an ideal world,
a CCD would have 100% Quantum Efficiency across the visible
wavelengths of light, assuming one wishes to study just those
wavelengths. This is an imperfect world, and chips have varying
sensitivities across the spectrum as can be seen in the graph
below (source: SBIG):
Signals and Noises:
Every CCD imager detects both signal and noise. Signals are easily
dealt with, and are desirable. Noise is a random component of an image
and is difficult to reduce once it is in an image. The best way to
deal with it is to prevent it from happening.
Sources of Signal Information:
- Image: This is what you want to get the most of! This is the
signal incoming from the object(s) you are trying to image.
- Thermal: This is the CCD's dark current, the slow and steady
growth of signal caused by the heat inside the CCD itself. This is lowered by
cooling the chip (usually thermoelectric cooling) and can be removed by
subtracting a dark frame from the final image.
- Bias: This is signal information that is on the CCD chip before
an image is even taken. This signal can be removed by subtracting a
bias frame (0 second exposure) from the completed image.
Sources of Noise Information:
- Thermal Noise: This is heat noise caused by inconsistencies in
the rate at which thermal signals are generated in
the chip. This can also come from components within the
imager such as an amplifier circuit.
- Reception Variation: This is caused by the inconsistent
reception of photons from the source. Examples would
be clouds, dew, ice, haze, an airplane, etc.
- Read Noise: This is caused by errors in the amplifier circuit.
- Quantization Noise: This is noise created in the analog-to-
digital conversion of the data.
- Sensitivity Variations: CCD's do not have the same sensitivity
from pixel to pixel nor across the whole
chip.
- Light Leaks! Yes, sometimes unwanted signals come from light
leaking into your optical path. This is more common with open tubed telescopes,
but also a bright LED or PC screen can get its light into your CCD. It should
be noted that LEDs are bright in the IR, and CCDs are very sensitive to IR.
What is a Light Frame?
This is the image one is trying so hard to obtain. The light frame is
the data from the actual imaging of the object. This is taken through
a lens system or telescope with the shutter open and lens cap off. A light frame
also contains noise: thermal, light leaks, radio interference, read noisse, etc. All
of this has to be removed to get your final image.
What is a Dark Frame?
A dark frame is an image of the same duration and temperature as the light
frame taken with the CCD imager with the lens cap on. This image contains a
"map" of the dark signal across the surface of the chip. During processing,
the dark frame is subtracted from the light frame using software to remove
the dark current signal. This is one of the easiest ways to improve your
images. If there is one piece of advice to follow: take and subtract dark frames!
Your images will show the improvement. To improve your images, and especially
if you are doing photometry, you should take many darks and then average them
together. An additional suggestion: Take the dark frame integration just before or
after your imaging run so that the temperature conditions of the imager are close
to the conditions used for the image itself. If you have a CCD which has thermal
control, then you cna take many dark frames at a variety of temperatures: Make a
library! I have taken 10 dark frames for each temperature (C): 10, 5, 0, -5, -10, -20,
down to -50C. That is cold. Used in conjunction with bias frames (a 0-second integration),
the darks can then be averaged together then scaled to ANY integration time that you
have made. Take 10 bias frames as well... at each temperature.
What is a Bias Frame?
A bias frame is an image, taken again with the lens cap on, for a 0 second
duration and at the same temperature as the light frame. This is a "map"
or record of the bias signal and can be removed by subtracting it from the
final light frame image and its dark frame during processing. This is a
very subtle image maipulation and really only becomes necessary when
doing high precision photometry. Note that the bias is also used to scale
your darks for different integration times.
What is a Flat Field?
This is an image taken through the optical system at an evenly illuminated
neutral white light panel or source. This information is then divided out
of the final light frame to remove the effects of uneven chip sensitivity
often caused by such things as the chip itself, optical path vignetting,
and the like. If you do photometry, flats are essential. Take 30 or more
flats which will then be averaged together to improve results.
Some software, like MaxImDL, will allow you to use the same library
of darks and biases to process the flats. Yes, flats also need to have
darks subtracted! Also, if you use filters, you must take a series of
flats through each filter you intend to use that evening. Also... you
need to take your flats at the same temperature as the imaging session.
What is Binning?
Some cameras allow the user to be able to combine the information from
a group of pixels (2x2 or 4x4) instead of just single pixels. This makes
the effective pixel size larger. The net effects include: a decrease in
image download times (good for quick focus); smaller image size; greater light
sensitivity; a loss in resolution. It is possible to bin in order to
match pixel size with an optical system's focal length.
What is Gain?
Gain is a measure of a CCD's efficiency. Each pixel element collects
electrons. This number of electrons is then counted and converted
into a digital value via an analog-to-digital conversion process.
The minimum unit of counting is called an ADU (Analog to Digital Unit).
Gain is measured in the number of electrons per ADU. In general, the
lower the gain, the better the system. The point to remember is that
the CCD's efficiency also depends upon the total well depth of the
system. This is the total amount of electrons that a pixel can hold before
becoming full. If the well depth is greater than the A-to-D conversion
gain, then a loss of a chip's usable sensitivity will occur. For example,
if a chip's pixels can hold 90,000 electrons (well depth), and the chip
gets 1.0 electrons/ADU in gain, if it is a 16-bit A-to-D conversion, the
imager can count only 216/1.0 electrons per pixel. This is a
loss of 90,000-65,536 electrons per pixel. Most modern CCD imagers
account for this and also balance between higher noise and a suitable
gain.
What is Correlated Double Sampling?
One may read that a particular imager uses correlated double sampling
to reduce readout errors (noise) in the system. This method involves
injecting a sum of electrons (charge) into the readout register on the
CCD chip so that it reaches a known level. The image electrons are then
transferred into this register and read out by the system. The known level
(reference charge) is then subtracted from the count to give the actual
pixel value.
What is Dynamic Range?
This is the ratio between useable ADU to the read noise of the system.
An example: If a CCD has a well depth of 100,000 electrons and a readout
noise of 13 electrons, the dynamic range would be 10 log(100000/13)
which is 39dB or 13 bits. It is easy to see that even a 16-bit imager can
not readily use all 16 bits worth of A-to-D conversion if there is a lot of
noise.
What camera is best?
The single most important aspect of imaging is getting a good match between pixel size
and the focal length of your telescope. This can easily lead to
a deep arithmetic treatise, but let us just say these words...
Your average seeing conditions will allow you to get between
1.3-4.0 arc-second wide star images. Average nights in my area
rarely allow for those smaller values. This information allows
one to guess that a good imager should use a couple of pixels
to cover one star's image. This would be a value of 2" per pixel.
This would allow you to use the chip to its maximum ability while
still getting good-looking stars in your images. Using too many
pixels to cover a star's image will surely get you a finer "grain",
but would be a waste of your imager's light gathering ability.
If you are imaging the Moon or the planets, then using as many pixels
as possible across the planet's image would be in your best interest.
In this instance a sky coverage of 0.5" (or less) per pixel would be best.
News as of July 2000:
Starlight Xpress has created a new software/hardware product
called the STAR2000 which allows their series od imager to
"self-guide" for long exposures. That is their cameras do not
need a second guide chip (like ST-7/8) to guide themselves. The
results are quite good. The camera can also be used as an autoguider
for those doing film astrophotography. This make the product quite
versatile. The self guiding also works for the one-shot color camera,
the MX-5c. For my "First Light" images using this technique, see:
The Veil Nebula
Cameras for science? Cameras for pictures?
Choosing a CCD imager requires that you now what you want to get into
imaging for! Do you want to take images like those seen in magazines?
Do you want to get into supernova hunts or asteroid searches?
Maybe stellar photometry interests you? Each of these specializations
could benefit from using a different type of imager.
Pretty Pictures:
You have seen those magnificent images in Sky & Telescope,
Astronomy and online. What is the best equipment for being
able to take images like that?
Immediately recommended is the anti-blooming (ABG), thermally controlled CCD
imager. Imagers with ABG allow one to take images of objects that have
wide ranges of brightnesses without getting those ugly-looking streaks
of light caused by too big a buildup of electrons on a photosite on the chip.
These electrons then spill over to the next pixel and so on causing streaks
like those seen below:
Another option to avoid blooming is to take a series of shorter
integrations of the desired object then later add them together into
a single image. This allows the faint details to be brought out and keeps
the bright stars from blooming and ruining the image. An example of this
technique is the below image of M-77. It is an addition of 10 one minute
integrations with ST7, C-8 at f/6.3. A Dark frame was subtracted from each
image before addition and Digital Development (in MaxImDL) was used to
bring out details.
Thermal control of an imager's chip allows one to significantly reduce the
amount of dark signal (thermal signal) produced within the CCD chip itself.
In the chip the actual substrate materials actually vibrate due to heat,
and create electrons in the photosites. This is often called "noise" but
really it is a signal form that is readily removed by subtracting a
"dark frame" from your images during processing. A dark frame is just an image
taken for the same time duration as your "light frame" (the image of the
desired object) and taken at the same temperature. Processing softwares
provided with the imager then allows you to subtract this unwanted signal
from your image. Below is a graph to demonstrate the worth of a CCD cooling
system. The author made this with an ST-7 by taking a series of five-second dark frame
integrations, each at a different temperature on the chip. The average
pixel brightness value was then calculated in MaxImDL's Information Screen.
It is obvious that the dark current values decrease rapidly as the temperature decreases.
The value in being able to control the temperature on the chip is that one
can save a series of dark frames at known temperatures for later subtraction
in the comfort of the home the next day. Some popular cameras do not have temperature
regulation, but do have thermoelectric cooling. This is better than no cooling,
but the user must take dark frames while outside at the telescope to insure that
the temperatures of the image and the dark frame are equal. This can become
tedious when one is tired. This can be difficult if the local ambient
air temperatures are changing rapidly too.
For those imaging nebulosity, the analog to digital conversion method should
also be mentioned here. You will see references to 8-bit, 12-bit and 16-bit
imagers on the market. Go for the larger value. This determines the amount
of separate levels of grey that your imager's A-D converter can separate the
output into. Thus an 8-bit converter will give you 28 grey
levels possible, where a 16-bit imager will give you 216
grey levels. The 16-bit CCD imager will thus show a finer gradient between
grey levels, which is more pleasing to the eye.
Photometry:
Those interested in doing photometric work (the precise measurement of
an object's brightness) will want to avoid cameras that have anti-blooming
gates installed. The reason for this is that as the charge builds up
on a photosite the response becomes non-linear the closer it gets to
blooming in a ABG CCD chip. That makes measuring the brightness very
difficult by introducing non-linear equations. It is much easier to measure
magnitudes with software if the CCD's response to light is linear throughout
the length of the integration. One still can not allow the object's light to
bloom across the pixels. Another benefit of non-ABG systems is that they
are more than 10% more sensitive to light than ABG system making the
time for an integration shorter.
Those wishing to do photometry will also want to invest in
special filters and a filter wheel to change between them. These
filters are standardized research grade filters that allow only
certain regions of the light spectrum to pass through. More information
about this can be gotten from the
American Association of Variable Star Observers or the
Center for Backyard Astrophysics.
For those just starting out, a single V photometric filter (V = visual) will
get you started on your way and will provide valuable scientific data.
Asteroid and Supernova Hunting:
The same basic rules apply here as they do for Photometric-capable
CCD imagers, only one would be looking for a wide field of view. You can
either get a large chip (very expensive), or get a chip with small
pixels and use it on a short focal length lens. Supernovae hunters can
use a 9-micron pixel CCD chip with a 50" focal length system and be able
to image thousands of galaxies: enough for a lifetime of study! Since measuring
the object's brightness is important, non-ABG CCD's should be used.
Square pixel systems are also very helpful when it comes to
astrometry, the precise measurement of an object's location in the
sky. Software such as Guide and Astrometrica are also important for
astrometry. Those interested should visit
IAU: Central Bureau for Astronomical Telegrams and read on the
process for reporting "discoveries".
Color:
Making color images is no longer as difficult as it used to be. There are
two options available to the astronomer now: color filter wheels and
color CCD chips. The latter of the two options is available today through
Starlight Xpress and their unique color CCD imager, the MX-5c. It can take
a single image that is later combined with its color values to create a
full color image like this:
For those wishing to use monochrome CCD's to take color images there
are a variety of filter wheels and sliders available that allow one to
image through clear, red, green, and blue filters for later combination
into a color image (tricolor). These filter systems are integrated into
the CCD control softwares allowing for easy remote and automatic operation.
The issue is time: For tricolor CCD imaging one must image the same object
three or four times (depending on your technique) then later combine them.
The following images were taken using these techniques:
Helpful Devices (and additional cost):
Buying a telescope and a CCD imager is not the end of the
road as far as equipment is concerned. It is really just
the beginning. Here are some more items that are quite
helpful.
Flip Mirrors & Slide Mirrors:
Many amateurs start out by trying to find an object
in an eyepiece, then replacing the eyepiece with the
imager. This is one of the most difficult methods
of finding and centering an object on a CCD chip's
tiny surface. Many companies make flip mirrors or
slider mirror assemblies which help to both find
and focus on an object for imaging. Several on the
market also act as off-axis guider ports for guiding.
This one device is guaranteed to save one hours of
frustration. For information, pictures and a
discussion of techniques used with a Taurus Tracker III
unit see:
Taurus Tracker III Product Review.
Filter Wheels/Sliders:
Interested in color imaging or photometry? Filter wheels
are available that work in concert with popular imagers and
their software. They hold the necessary color filters
for doing tri-color imaging or photometric work. Be aware that
these devices, like flip mirrors and guiders, all require a
longer backfocus from your telescope optics. Some telescopes
will not be able to reach focus given the use of a focal
reducer and a flip mirror. Contact the telescope manufacturer
for details on their backfocus. Contact the equipment
maker for details about their backfocus requirements.
Off Axis Guiders:
The off-axis guider is necessary if you are not using
a self-guiding CCD system like an ST-7 or ST-8. Those cameras
have second smaller CCD chips that act as guiding chips while the
larger main chip takes the image. Other CCD makers are
using various techniques to allow the imaging chip to
guide the telescope while imaging at the same time.
Be aware that guiding is very important in CCD imaging.
One is trying to keep a star centered on a couple of pixels.
That is a fine requirement. Typical off-axis guiders
allow one to place a guiding eyepiece or an autoguider
like an ST-4 into a port which picks off a small amount of
the telescope's light cone for tracking on a star. Autoguiders
are by far a lot easier to use than guiding manually, but it
is suggested that manual guiding be done at least a few times
just to get familiar with the mount's errors. When selecting
off-axis guiders, also investigate flip/slide mirror devices
as they can often be used as an off-axis guider.
Guide Scopes:
Guide scopes are another option when looking into
guiding solutions. They allow one to be a bit more flexible
in finding a suitable guidestar. They are also
useful if the main optical assembly doesn't have enough
backfocus to accomodate a flip mirror assembly, or if
a Fastar lens assembly is being used. Most guidescopes
are refractors of a focal length equal to or longer than
the main optics. This allow for guiding par with the
resolution in the main scope. Using a shorter focal length
would cause star images to be larger and less pinpoint.
Another issue to remember is if one intends to do long
exposure imaging with a Schmidt-Cas telescope, the
main mirror might flop inside its cell as the scope guides
(especially through the meridian). Off-axis guiders are
the only real solution to this problem unless you install
lock down bolts on your main mirror, a task not intended for
the faint of heart!
Mounts:
This is another key to successful imaging. Many of the commercial
mounts these days are not really up to the task of guiding,
but they can be used with a few precautions. Make sure it is
polar aligned well: do not jsut aim the polar axis at Polaris
and hope to guide out the errors. This will just produce
field rotation issues.
Make sure the mount is as sturdy as possible. Check for loose bolts
and wires that could cause vibration. Use the counterweights
to accurately offset the optical tube weight. Make sure all
electrical connections are solid and supported with tape. There
have been many frustrating nights trying to figure out why a
certain axis would not track in one direction: a broken connector
was the problem.
If you are looking to buy a new mount, this is one area where
more money spent equals more quality product. Get a solid and
reputable piece of equipment that is built to hold more than
twice your current optical setup's weight. You will not be
disappointed. Get something with built in dual axis drive controls
with an autoguider input. These are rapidly becoming standardized for
most autoguiders and CCD imagers with guider outputs. If you
ever intend to make the telescope permanently sited, check to see if
the mount's equatorial head can be placed on a permanent pier without
too much machining.
A common question is: Can I use an alt-azimuth mount to do imaging?
The answer depends on the type of imaging you wish to do. If you are
going to do planetary or Lunar imaging, then, sure, you can do it. The
integration times will be under one second most of the time. Long
duration integrations will not be possible unless one obtains a
field derotator, an electric device that attaches to the focus of
the scope to rotate the imager opposite the direction of the field's
rotation through the integration time. Most people find that the
cost and added possiblity of all these electric motors failing
in the middle of the night is not worth it. It is much simpler just to
have an equatorially mounted scope.
The Fastar Setup:
This is a unique optical setup made by Celestron. It has created a
whole slew of questions by the amateur community. The scope is an f/10
Celestron Schmidt-Cas (SCT) that has a removable secondary mirror which
can be replaced with a lens assembly to which a CCD imager is inserted.
The optics then have a new focal ratio of F/1.95, which is very fast.
The concept was created to allow amateurs the ability to image
very rapidly, often in minutes without too much worry about guiding
and such. It was created in conjunction with an SBIG CCD imager which
is no longer made, but there are still imagers capable of
being used with the system, Starlight Xpress' MX series being
but one. The problem becomes guiding. What is interesting is that
there are also a bunch of short tube 80mm and 90mm scopes on the
market that are perfect for this job. Just mount the short tube scope
on top of the Celestron Fastar and use the short tube for an autoguider.
The results are worth it, as can be seen in the M-45 image above.
Software:
There are bunches of options in this catagory. All commercial CCD
imagers come with their own software which allow for the basics
of processing. Eventually better software will be desired. Choices
range from freeware and shareware on the net to more expensive
solutions like MaxImDL/CCD from Cyanogen. If you are looking
for options, investigate the processes like Lucy-Richardson
Algorythms and Fast Fourier Transforms. There are many things
that a good image processing package can do for one's images.
Some packages also handle the taking of the exposure automatically
in series with filter wheels, dark frames, and are even scriptable!
Methods:
Finding the Object:
One of the most frustrating processes is to find
the object you are looking to image, and then getting
that object centered in the field of a small CCD chip.
Many people start out by tring to center the object in
the view of an eyepiece, then replace the eyepiece with
a CCD imager. This is a poor method as the weight of the
imager often causes the view to shift. It is highly
recommended that a flip mirror or slider mirror device
is used. These have a moving mirror that either allows
the starlight to get to the imager chip or be sent through
an eyepiece for focus and centering. They are made so that
the CCD chip and the eyepiece come into focus together.
That makes the tough job of focusing a bit easier to
do. Some models also have a third port that is used for
guiding. These have a small pick-off mirror or prism
like an off-axis guider that grabs the light from the
edge of the scope's field and sends it to either
a guiding eyepiece or an autoguider.
Either way, one still has to find the object to image.
The use of software is a great way to accomplish this
end, since a computer is available anyway for the
control of the CCD imager! Applications like
theSky and
Guide
even allow one to place a box representing the CCD
field of view on the star chart. Programs also have
integration with many popular digital setting circles.
These handy, though expensive, devices act as a digital
representation of mechanical setting circles on one's
mount. The nice thing is that they also contain whole
searchable catalogs of objects (NGC, Messier, etc).
One can also star-hop using an atlas, which is an art form
in its own right and is a lot of fun.
Focusing:
Just when finding an object is getting frustrating,
focusing on it can also be as patience testing.
Spend a lot of time focusing. It can really boost
the quality of an image. There are a lot of ways to focus:
- Full-width-half-maximum
- Visual
- Parfocal eyepiece
- Diffraction method
- Maximum brightness
Of these, the simple visual method is the least precise.
With modern SCT's, a very popular scope design, 1/500
of a turn on the focus knob will throw the image out of focus
enough to degrade a high resolution image.
Full-width-half-maximum (FWHM) is a method found in
some imaging software that is very precise. It is the
measure of the width of star image across the pixels
where the pixel levels are half the maximum brightness
of the pixels for that star. For a clearer description of this, see:
FWHM explained
Other softwares allow the user to view the brightness
of the star images. By achieving the highest brightness value,
one can state that they have achieved best focus.
A parfocal eyepiece is simply an eyepiece that is in focus
at the same time as the CCD chip itself. This is usually in
some sort of flip mirror device. The use of a parfocal
eyepiece is very helpful in obtaining near focus
conditions. It is by no means the way to achieve
best focus. It is great for finding a rough focus on an
object, but that is all. Do not be misled by some advertising
claims that state that one can focus and be imaging by use of
a parfocal eyepiece. The human eye readily accomodates to slightly
out of focus images in an eyepiece. What appears to be in focus
to the eye may not be in focus on a CCD chip. If you have to use
this method, then use a very high power eyepiece (4mm focal
length). The eye has a more difficult time accomodating to these
than, say, a 25mm eyepiece.
The diffraction method of CCD focus is another fine way of
reaching accurate focus. It is somewhat more time-consuming,
and requires a piece of home-made equipment for some scope types,
but is not difficult. In short, some sort of rods or diffraction
spike-causing object is placed in front of the scope's objective.
This causes diffraction spikes to be produced around brighter stars
in a CCD image. Warren Offutt has written an article about
these devices and the method for Sky Publishing and can be seen at:
Diffraction Focuser for CCDs.
Another similar method is the use of the Hartmann Mask.
This is a dark mask or disk with a few holes cut in it. This
is then placed over the front of the scope causing multiple
images of a star when the star is out of focus. When in focus,
the star's images become one. See:
How to focus a CCD camera for more details. Another article
about the use of Hartmann masks can be seen at:
How to focus a CCD Camera.
Guiding:
Guiding is done in a couple of ways: autoguider, self guider, or
manually. It is not recommended that manual guiding be done, though
it is possible with the relatively short integration times of modern
CCD imagers. The accuracy of manual guiding is the problem.
SBIG and
Meade make autoguiders. The SBIG ST-4 is a full-featured autoguider
and imager, allowing one to see the star for focusing and centering. The
Meade 201xt is just an autoguider with no imaging ability. This
makes it difficult to find, focus, and center a guidestar.
Both hook up to the guide port in modern mount eletronics.
This tells the scope's mount which way to correct as stars drift
due to improper polar alignment or periodic error. It is just as
important to focus the autoguider as well as the main imaging camera.
Some CCD imagers have built in secondary guider chips, like the
SBIG ST-7 and ST-8. These chips are at the same focal plane as
the imaging chip, so focusing one automatically focuses the other.
That is very time-saving.
Either way, the use of an autoguider is pretty much the same.
Find and center a guide star on the guider chip. Make sure
everything is focused, then train the guider as to how it has to react
to fix various directional guide errors. This is done via a process
called calibration. It involves the guider moving the mount
in each of the four directions (+/- RA and +/- Dec) and taking
an image at each position. It then looks for the same star in each image
and calculates its correction values for the mount, the
declination, and the guider's orientation on the field of view.
It's pretty neat mathematics and a real time-saver. Some very
good articles about the use of an ST-4 may be found at:
Astro-Articles
by Chuck Vaughn.
The latest in self-guiding technology is from the Starlight Xpress
company in the United Kingdom. They have developed a process
using their chips which captures signal information on even rows
of pixels while reading the odd rows for guiding. Halfway through the
exposure, the rows are switched so that the even rows are used for
guiding while the odd rows are then used to capture image signal.
The result is longer required integration times, but with self-guiding!
This is called the STAR 2000 system. I have written a review of it, with tips here:
The Starlight Xpress STAR 2000 Self-Guiding System.
Taking the Image:
Once the telescope is centered on the object and guiding, it
is time to actually take the image! This is the easy part!
Tell the software to start imaging for the amount of time that
is desired. If you are using a non-ABG imager, then you may want to
limit the integration time to just under where blooming begins
with the field's brightest stars. Also be aware of the imager's
temperature. If it has no control over the cooling, let it
stabilize for at least 10 to 15 minutes before imaging to get the
best results. Be sure to take a dark frame as soon as the image is
complete. For those with controlled cooling, just be sure that
the imager has reached its set-point (the temperature you
have set the cooler to obtain) and image away!
You may also wish to read:
Methods I have used at my observatory.
Online Resources:
Here are some more online resources for you to investigate.
Invest some time into studying the many available options
before spending a lot of money. Remember that you can also
start with low- to medium-end equipment before upgrading to
professional level imagers and telescopes. Resale value
of CCD's is pretty good at this time, so trading up is
a possibility.
Yahoo's Astronomical CCD Imaging Club
Yahoo's CCD Imaging Club
Sky & Telescope Online
Apogee CCD University
SBIG
Last Modified: 3/19/03 8:55p
This page:© Copyright 2005 by John A. Blackwell