Finding the Target
Finding and centering an object on a small CCD chip can be a tricky business if you do
not have a telescope with a computerised high precision GOTO function like the Meade
LX200. Even with such a scope it is helpful to visually check that the GOTO
successfully centered the object before taking the image. What is needed is a device
that allows you to look through the telescope while the camera is attached and
A flip mirror finder is just such a device. The finder is placed in
the optical path of the OTA, directly in front of the CCD camera. The front of the
finder attaches to either the telescope visual back, focal reducer
or telenegative and the camera attaches to the rear of the finder. The finder
contains a small mirror which can be rotated from a horizontal to 45 degree
position. When the mirror is horizontal, light passes over it and into the
camera. When in the 45 degree position, light is reflected up through a standard
eyepiece allowing you to see what is centered in the telescope FOV and thus on
camera. Finding and centering objects for the camera is now easy. Just set the
finder mirror to the 45 degree position and place the object in the finder eyepiece FOV
using your normal methods (star hopping, finderscope, analogue setting circles, digital
setting circles, GOTO etc..). Now adjust the RA and DEC of the OTA to center the
object in the finder eyepiece, move the mirror to the horizontal position and you are
ready to take your image! I have successfully used flip mirrors from
Meade and True
Taking the Image
Once you have focused your camera and
centered an object there are two key things left to do to get that
image. The first is deciding how long an exposure to take and the second is to keep
the object steady on the CCD Camera by guiding the telescope for the duration of the
With all exposures, the aim is not to expose the image to the point where any pixels
become saturated. This can be checked by looking at the histogram of the image at
the end of the exposure. If there are fully saturated pixels then reduce the
exposure time. You also need to check the histogram to ensure that you have used a
long enough exposure to have captured an image with enough information for image
processing to yield good results. The histogram should start off at a high peak
(which is the sky background noise) and then tail off in an exponential curve towards 0.
If you just have the high peak and little or no exponential curve then you need to
increase your exposure duration. Typical exposure times are:
Less than 1/1000th of a second for
solar and lunar images
A few tenths of a second for
5-10 mins for each sub exposure for deep sky objects.
This can seem contradictory as to see faint details around nebulae and
galaxies you need a total exposure time extending into multiple hours.
However, it is impractical to take a single image that long as it could
be ruined by a guide error of just a few seconds and some areas of the
image would be over exposed (e.g. bright stars or the core of a galaxy).
The best technique is to take multiple exposures or subframes (say
24-36) of 5-10 min duration and then average them in image processing
software. This increases the signal to noise ratio by the square
root of the number of subframes averaged. The resulting image is
now equivalent to a single exposure of multiple hours but avoids
overexposing the bright areas and any guide errors will not result in
all the image data being affected.
Errors in polar alignment or in the RA tracking of the telescope drive can cause the
object to shift position on the CCD chip during a an exposure lasting
several minutes. If these errors go uncorrected the result is a blurred image and star
To correct the errors as they occur you first need to be able to see them happen and
then have a means of altering the position of the OTA to compensate for the tracking error
without touching the telescope. Touching the telescope to operate the manual RA or
DEC controls would introduce a lot of vibration in the image and ruin it.
Compensating for tracking errors in this way is known as guiding and there
are two methods of doing it.
LX10 the handcontroller and DEC motor
allow for small corrections in RA and DEC to be made without touching the telescope.
On the LX200 or RCX400 you can use its hand controller to make small corrections in RA and DEC
without touching the telescope. So now we have a method of correcting the errors,
how do we see them happen? For this we can use an off-axis guider (OAG) or a
An off-axis guider is a unit placed in the optical path of the OTA. It connects
to the visual back of the OTA or the focal reducer. The
flip mirror and CCD camera are then connected to the rear of the OAG. The OAG has a
small prism which intercepts part of the light around the edge of the field of view of the
telescope. The prism is small enough so that it does not intercept any of the light
destined for the CCD camera. The intercepted light is diverted into an
eyepiece. This allows the user to view a small portion of the sky at the same time
as the camera is taking an image. The eyepiece used with the OAG needs to have
an illuminated scale and is known as a reticle. To guide the telescope with the OAG
you need to locate a star in the reticle eyepiece and watch its movement against the
scale. If it moves in RA (left/right) then adjust the speed of the RA drive with the
handcontoller until the star returns to its original position. If it moves in DEC
(up down) then adjust the DEC of the OTA using the handcontroller. Most of the
corrections should be in RA as precise polar alignment should have
eliminated nearly all the DEC tracking error. Most OAGs will let you
rotate the prism around the edge of the field of view of the imaging chip to
make it easier to find a guidestar. I have used the Celestron Radial Off-Axis
Guider as it has several features which make it much easier to locate a star in the
reticle. I have used the Celestron
Illuminated Microguide Eyepiece as the reticle.
A guidescope is a separate telescope (usually a refractor) mounted on the main
telescope OTA. You use the reticle as the eyepiece for the guidescope. As with
the OAG, you locate a star in the reticle and correct any movement against the scale
using the hand controller.
There are many pros and cons you should consider when deciding whether to use an OAG or
guidescope for guiding. These are discussed in depth in an article by Philip Perkins.
As a summary, the advantages of an OAG are that it does not suffer from flexure
(movement of the guidescope against the the OTA) and it can correct for image shift
(movement of the image due to movements in the OTA mirror as the telescope tracks across
the sky). The main disadvantage of the OAG is the difficulty in locating a guidestar
in the small field of view afforded by the OAG prism. The advantage of the
guidescope is that it is relatively easy to find a bright guidestar and the disadvantages
are the possibility of flexure and an inability to correct for image shift.
With a telescope mount that has an autoguider port (such
as the LX200, RCX400 and Takahashi EM-200 mount) it is possible to use a CCD
camera to automatically make the guide
corrections to the RA and DEC during an exposure. Essentially, the CCD
camera is put in the place of the reticle eyepiece on the OAG or
guidescope. Using the camera control software the user will select the brightest star in its FOV as the guidestar and
set the camera to take an image every
few seconds. The camera control software looks for movement in the guidestar from one image to
the next and sends commands to the RA and DEC motors of the telescope to compensate for
the movement. Clearly the main advantage of this guiding technique is that you are freed up
from the work of making the guide corrections yourself. Manual guiding
is very demanding physically and I was only ever able to take images of 20
minutes before I needed to rest. Autoguiding lets you take as many
images of as long a duration as you want. The CCD camera never gets
tired! The other advantage
is the CCD camera can 'see' stars that are often too faint to be used for visual
guiding. Many of the SBIG cameras have two CCD chips, a main imaging
chip and a smaller chip that can be used as the CCD camera for autoguiding.
As with everything there are pros and cons with this design. Pros:
You don't need two CCD cameras (one for taking the image and one for
guiding). You don't need a separate guidescope or need to use an OAG.
Cons: If you are using color or narrowband filters then you
will be imaging the guide star through the filter which means you need to
select a brighter guide star and there may not be a bright enough star near
your target. Also, the guide chip is in a fixed position relative to
the imaging chip and so the camera must be rotated and positioned such that
a suitable guide star is on the guide chip. This may mean the framing
of the target in the imaging chip isn't exactly how you would like it.
I have always found that the pros of this design out way the cons,
especially as you can use the guide chip in the SBIG cameras to control
adaptive optics for guiding (see following discussion).
Autoguiding an LX200 or RCX400
can produce good results (round
sharp stars) at lower focal lengths (e.g. 1000mm or below). However,
the accuracy of the mount on these telescopes even after PEC training does
not yield very good results a the full focal length of the scope (2000mm
or above). This is because the mount position can only be corrected
every few seconds at best and any error in the tracking of the mount
during the correction periods shows up in the images taken at longer focal
lengths as large of elongated stars. It is not just the stars that
are affected though. The fine detail in the object being imaged is
also lost as it is smeared out over the CCD chip during the
exposure. A very good solution to this problem is to use the AO-8
adaptive optics system from SBIG. This unit attaches between the CCD
camera and the telescope. It has a lens which control where the
light from the telescope falls on the CCD chip. Very short exposures of a
guide star (usually around 10 images per second) are taken using the guide chip in
the camera and the AO-8 tilts its lens between each exposure to
compensate for any movement caused by poor tracking or atmospheric
disturbance. If the guide star drifts too far over time for the
lens tilt to compensate, the AO-8 sends a pulse to the mount to
adjust the guide star position back towards the centre of the
lens. The frequent and very accurate correction of the tracking
that is possible with the AO-8 means inaccuracies in the
tracking of mount are corrected as well as some atmospheric
disturbance effects. You can see the difference between autoguiding with
adaptive optics and just the guide chip in these images of M1.
Once you have taken multiple 10 minute subframes of your
target they will need to be combined into one long duration exposure using
image processing software. The normal way of doing that is to first
register the images so that all the stars and the target are aligned in
exactly the same pixel locations. Then you average all the images to
get a single image with a much higher signal to noise ratio than is in any
one of the subframes. In general the signal to noise ratio increases
as the square root of the number of subframes averaged. However, if we
just average each pixel location using pixels from each of the subframes we
are missing an important opportunity to remove artifacts from the image.
An artifact could be a satellite trail or a hot/cold pixel in the CCD
camera. Most image processing software will let you average only those
subframe pixels that fall a within a certain range of the average value (the
range is known as standard deviations from the mean or average). Using
this combine method will avoid including outlier pixel values like a
satellite trail in the average. Those pixels will be rejected allowing
all the other good pixels in the image with the trail to be used.
However, this technique won't reject hot or cold pixels from the image
unless we also use a technique called dithering. Dithering simply
means that the software controlling the CCD guide chip will randomly move
the mount a few pixels up, down, left or right between each image.
That means that each image falls on a different set of pixels in the chip.
Once the images are registered all the stars and the target are aligned but
the hot/cold pixels in each image will now be in slightly different
locations due to the dithering. Using the standard deviation rejection
technique will now remove these hot/cold pixels from the average.
As well as taking images of the target you will need to
take special images that characterize dark current, variations in the CCD
chip sensitivity and dust in the optical system. These images can then
be used during image processing to correct for these artefacts.
Image exposures greater than a few seconds will be affected by noise, know as dark
current, generated within the CCD camera itself. To remove this noise from your
final image you need to take what is called a dark frame. This is an image made at the same
ambient temperature and exposure duration of the original image but with the telescope
aperture covered or camera shutter closed. You then subtract this dark frame from your image as
part of the calibration step in image processing and the dark current noise
will be removed. A flip mirror finder is useful for
taking dark frames as you can achieve the same effect as covering the telescope aperture
or closing the camera shutter simply by rotating the finder mirror to the 45 degree position. With the mirror in
this position all light is blocked from entering the camera.
For cameras that can't be cooled to a set temperature you
will need to take dark frames during each imaging session so they are taken
as close as possible to the temperature the images were taken at.
Cameras like the SBIG ST range can be cooled to a set temperature and that
allows you to take dark frames at any time which match the temperature the
images were taken at. You can build a library of dark frames for
different temperatures, durations and binning that can be used to calibrate
images across many sessions. The library should be refreshed
this library at regular intervals (e.g. every 6 months) as the camera
behavior can change over time.
To reduce the noise in your dark frame I recommend taking
several dark images at the same temperature, duration and binning and then
average them to create a single Master Dark you can use to calibrate your
images. I take 9 dark frames and average them to create a Master Dark.
These are images taken against a field of even illumination (e.g. the evening or morning
sky when it is light enough not to show stars but dark enough not to saturate the pixels
quickly). These images can be used to correct for variations in
sensitivity across the CCD chip or for specs of dust on the chip, filters
or optics that can appear as dark doughnut
shaped shadows on the image after processing.
is not always possible or easy to take a
flat field of the evening or morning sky during an imaging session.
If that is the case it is best to use an artificial light source.
The easiest way to do this is build yourself a light box.
This is a device which fits over the end of your telescope and uses a
standard lightbulb to provide the even field of illumination required for
taking flat fields. The advantage of using a lightbox is that you
can take the flat fields at any time during the imaging session and you do
not have to worry about stars appearing in the image and ruining your
flat. I constructed a lightbox for the LX200 and your can see the
On my Takhashi
TOA-130 I use a Flip Flat from
Alnitak Astrosystems. This is a circular paddle that contains an
evenly illuminated light panel which covers the OTA opening in place of the
dust cover. It is motorized and so can be opened for imaging and
closed for flat fielding under the control of automated imaging software
You will need to take a set of flat field frames during
each imaging session. A flat will need to be taken through each filter
and at each camera angle you used. To reduce the noise in your flat
field frames I recommend taking several images through each filter and then
averaging them to create a single Master Flat. I take 4 flat frames through
each filter and then average them to create a Master Flat.
All the techniques described thus far can be put together
in a manual imaging sequence:
Focus the telescope
Find the object
Program the camera control software to take a series
Refocus the telescope every hour to compensate for
focus shifts due to temperature changes.
Take dark and flat field frames
What would be really great is if we could have software
do all these steps for us so it can run all night and we can get some sleep!
Well there are several software packages available that will let us do just
that. I use CCD Autopilot from
CCDware. Here's how you can use CCD autopilot to automate your
Point the telescope at a well known star using the
Telrad. Establish a link to the telescope in TheSky and sync on
the star you pointed the telescope at. This tells TheSky
roughly where the telescope is pointed.
Use the field of view indicators in TheSky to select
your target and camera position angle. TheSky can show you the actual
field of view that will be captured by your imaging and guide chips .
This lets you select the camera position and angle to best frame the
object and have a suitable guide star on the guide chip. Alternatively
you could use CCD Navigator from
CCDWare to select a target and camera angle which can then be saved
and imported into Autopilot.
Import the target position and camera angle into
Autopilot from the TheSky or one saved by CCD Navigator
Tell Autopilot how you want to image the target (e.g.
number and duration of images to be taken through each filter), how
often you want to focus etc... and then start the imaging session
Autopilot will now take a short image and pass
it through plate solving software to find the exact position of the
telescope. It will then select a close by star to focus on and
move the telescope to that position. It will then instruct
FocusMax to autofocus on that star.
Once focused the telescope will be moved to the
target and the camera rotated to the correct angle (if a rotator is
installed). It is likely that the telescope won't have moved to
exactly the right position due to polar alignment inaccuracies. So
once at the target another short image will be taken and plate solved to
find out precisely where the telescope is pointing. The telescope
position will then be corrected as necessary.
Autopilot will now control the imaging and
autoguiding using the camera's control software.
If you tell Autopilot to focus at regular intervals
during the imaging session it will stop imaging at the specified time
and move the telescope to a suitable focus star and autofocus again
using FocusMax. It will then move back to the target and resume
the imaging sequence.
Once all the required images have been taken
Autopilot can take dark frames and flat fields as required
This lets you image all night without any involvement.
It's the key to taking really deep exposures that may total 10 or more