Starlight Xpress TRIUS PRO-25C User manual

Handbook for the TRIUS PRO-25C Issue 1 April 2019
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The TRIUS PRO-25C Colour
CCD camera user manual
Thank you for purchasing a Starlight Xpress CCD camera. We hope that you will be very satisfied with
the results.
The TRIUS PRO-25C is an advanced, high-resolution one-shot colour, cooled CCD camera, especially
designed for astronomical imaging. It uses a version of the very popular Sony ‘SuperHAD’ CCDs that
offer very high QE and extremely low thermal noise. It is a substantial upgrade on the SXVR camera
range and incorporates several new features, such as an internal USB hub with 3 external ports and a
dry argon CCD chamber fill. The USB hub permits several other devices to share the single USB
connection and greatly reduces the number of cables required in a typical set-up. For example, a
Lodestar or UltraStar guide camera and an SX filter wheel could use two of the USB ports and the
third might connect to an electric focuser, or similar peripheral. The argon fill, along with other
improvements to the cooler stack, has improved the delta T to about -40 degrees C.
As per the TRIUS range, this camera also includes a CCD temperature monitoring circuit that provides
regulated set-point cooling of the chip, adjustable chip alignment and a very compact overall size.
The TRIUS PRO-25C uses a Sony ICX453AQ ‘SuperHAD’ progressive scan CCD, with 3024 x 2016 x
7.8uM pixels in a 23.4 x 15.6mm active area. This device has an excellent quantum efficiency, with a
broad spectral response peaking at around 60% in yellow light, and an extremely low dark current,
well below that of any comparable CCD currently available. While this device also has an excellent
blue light sensitivity, it has a strong infra-red response, which makes it ideal for all aspects of both
planetary and deep-sky imaging, especially with an H-alpha filter. The H-alpha QE is about 55%,
considerably better than other interline chips.

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The full-frame download time is approximately 3.5 seconds and a binned 4x4 download takes only 0.5
seconds, so finding and centring are very quick and easy in this mode.
Here is a diagram of the connectors to be found on the rear panel of the camera:
Please take a few minutes to study the contents of this manual, which will help you to get the camera
into operation quickly and without problems. I am sure that you want to see some results as soon as
possible, so please move on to the ‘Quick Start’ section, which follows. A more detailed description of
imaging techniques will be found in a later part of this manual.
‘Quick Starting’ your TRIUS PRO-25C system
In the shipping container you will find the following items:
1) The TRIUS PRO-25C camera head with external cooling fan assembly.
2) A universal AC power supply module.
3) A USB camera cable.
4) An adaptor for 2” drawtubes and M42 Pentax thread lenses.
5) A guider output to guider port RJ12 lead.
6) Two Mini to Mini USB 0.5m Cables.
7) Two Mini to Male B USB 0.5m Cables.
8) A 'Y' power lead for connecting the cooling fan.
9) A USB Stick with the TRIUS PRO-25C control software and this manual.
You will also need a PC computer with Windows XP, Vista, Windows 7 or Windows 8 installed. This
machine must have at least one USB 2.0 port available and at least 256 Mbytes of memory. If you
intend to view the finished images on its screen, then you will also need a graphics card capable of
displaying an image with a minimum of 1024 x 768 pixels and 24 bit colour. A medium specification
Pentium with between 1GHz and 4GHz processor speed is ideal, but I recommend avoiding some of
the lower specification ‘Netbook’ computers, as they can’t handle the fast data stream from the
camera. Please note that USB 2.0 operates at a very high speed and cannot operate over very long

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cables. Five metres of good quality cable is the maximum normally possible without boosters or extra
powered hubs, although you can sometimes get good results at longer distances with very high
quality cables.
Installing the USB system:
First, find a free USB socket on your PC and plug in the USB cable (do not connect the camera at this
time). If you do not have a USB2 capable computer, it is normally possible to install a USB 2 card into
an expansion slot.
The next operation is to run the software installer from the CD ROM provided. Insert the CD into the
computer and wait for Windows Explorer to open with the list of folders on the ROM. Now run the
SETUP.EXE file that it contains – this will initiate the self-install software which will guide you through
the process of installing the SX camera software onto your computer.
Now connect the USB cable to the socket on the camera rear panel.
Windows will report ‘Found new hardware’ and will ask for the location of the drivers. Point the
installer at the appropriate driver folder on your CD ROM (32 bit or 64 bit) and the driver installation
should proceed smoothly. (Ignore any warnings about the driver having not been tested by
Microsoft).

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At the end of this process, the USB interface will be installed. The XP drivers will show up as a
‘BlockIOClass device’ and the camera software will be able to access it. You can confirm that the
installation is complete by checking the status of the USB devices in the Windows ‘Device Manager’
(see above). Start up the Windows ‘Control Panel’ and select ‘System’. Now click on the tab labelled
‘Device Manager’, ‘Hardware’, and all of the system devices will be displayed in a list (see above). If
the installation is successful, there will be a diamond shaped symbol labelled ‘BlockIOClass’ and
clicking on the ‘+’ sign will reveal it to be a ‘Starlight Xpress USB 2.0 SXV-SX25C BlockIO camera
driver’. If this device shows as faulty, try clicking on it and selecting ‘properties’ and then ‘update
driver’. Following the on screen instructions will allow you to re-select the correct inf file from the CD
(SXVIO_M25_398.inf) and driver files (SXVIO.sys and generic.sys), which should fix the problem.
The newer ‘Signed’ drivers create a device in the ‘USB’ section of the devices list and this will be found
at the bottom of the device manager window.

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Now connect up the power supply and switch it on. The supply is a very efficient ‘switch mode’ unit,
which can operate from either 110v or 220v AC via an appropriate mains power cable (supplied). You
can now start the software by double clicking on the icon, when you should see the main menu and
image panel appear. If this is the first time that it has been run, you will receive a warning about the
lack of an ‘ini’ file – just click on ‘OK’ and then open ‘Set program defaults’ from the ‘File’ menu. In the
bottom right hand corner of this box, select SXV-SX25C. Now click on ‘Save’ and the ini file will be
created and the software set for your camera.
Now click on the camera icon at the top of the screen. If the USB connection is OK, a message box will
inform you of the ‘Handle’ number for the SXVIO interface and various other version details etc. Click
‘OK’ and the main camera control panel will now be seen.

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As can be seen above, there is a CCD temperature monitoring window at the right hand side of the
panel. At switch-on, this will default to full power cooling with an end point of -40C (actual
temperature) and, needless to say, this is rather extreme. I recommend changing the set point to
about -10C for normal use, but you can go much colder if you are imaging during the winter months.
Under indoor conditions, the low airflow will limit the cooling capability, and you should use a set
point of no lower than -5C for stable cooling. You can determine the optimum settings for your
camera and ambient conditions when you have some experience of using the system, but do not try
to operate at extreme cooling when the air temperature is high. Connecting and using the external
fan unit will improve the cooling by at least -5C and can give as much as -10C extra delta T. It is
connected by using the 'Y' lead provided in the camera component set, as below:
Recording your first image:
We now have the camera and computer set up to take pictures, but an optical system is needed to
project an image onto the CCD surface. You could use your telescope, but this introduces additional

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complications, which are best avoided at this early stage. There are two simple options, one of which
is available to everyone with a sheet of aluminium baking foil:
1) Attach a standard ‘M42’ SLR camera lens to the TRIUS-SX25C, using the 26mm spacer to achieve
the correct focal distance.
Or
2) Create a ‘Pin hole’ lens by sticking a sheet of aluminium baking foil over the end of the adaptor
and pricking its centre with a small pin. Make the hole as small as possible.
If you use a normal lens, then stop it down to the smallest aperture number possible, (usually F22), as
this will minimise focus problems and keep the light level reasonable for daytime testing. The pin hole
needs no such adjustments and will work immediately, although somewhat fuzzily!
Point the camera + lens or pinhole towards a well-lit and clearly defined object some distance away.
Now enter the ‘File’ menu in the SX25C software and click on ‘SX camera interface’. Select an
exposure time of 0.1 seconds and press ‘Take Photo’.

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After the exposure and download have completed (about 4 seconds) an image of some kind will
appear on the computer monitor. It will probably be poorly focused and incorrectly exposed, but any
sort of image is good! In the case of the pinhole, all that you can experiment with is the exposure
time, but a camera lens can be adjusted for good focus and so you might want to try this to judge the
high image quality that it is possible to achieve.
Various other exposure options are available, as can be seen in the picture above. For example, you
can ‘Bin’ the download 2x2, or more, to achieve greater sensitivity and faster download, or enable
‘Continuous mode’ to see a steady stream of images. ‘Focus mode’ downloads a 128 x 128 segment of
the image at high speed. The initial position of the segment is central to the frame, but can be moved
by selecting ‘Focus frame centre’ in the ‘File’ menu and clicking the desired point in a full size image
with the mouse. The focus window has an adjustable ‘contrast stretch’, controlled by the 12-16 bit
slider. The image will be ‘normal’ if 16 bits is selected, while setting lower values will increase the
image brightness in inverse proportion.
If you cannot record any kind of image, please check the following points:
1) Ensure that the power indicator lamp is on and that the cables are properly home in their sockets.
2) If the screen is completely white, the camera may be greatly overexposed. Try a shorter exposure
time, or stop down your lens. See if covering the lens causes the image to darken.
3) If the USB did not initialise properly, the camera start-up screen will tell you that the connection is
defective. Try switching off the power supply and unplugging the USB cable. Now turn the power
supply on and plug in the USB cable. This will re-load the USB software and may fix the problem
after restarting the SXV_hmf_usb program. Otherwise, check the device driver status, as
previously described, and re-install any drivers which appear to be defective.
4) If you cannot find any way of making the camera work, please try using it with another computer.
This will confirm that the camera is OK, or faulty, and you can then decide how to proceed. Our
guarantee ensures that any electrical faults are corrected quickly and at no cost to the customer.
As the SX25C is a ‘Single-shot’ colour camera, the full resolution raw image will show a fine grid
pattern, due to the RGB colour filters on the CCD chip surface. This is normal and will disappear once
the image has been converted to colour by the software. Unlike commercial digital cameras,
astronomical colour cameras do not normally output a colour image directly. This is so that various
image corrections can be performed on the raw data (e.g. dark frame subtraction), as these are
impractical after conversion to colour. Also note that binning the raw image at anything other than
1x1, will destroy any colour information and so output only monochrome images.
Converting your images to colour:
The images from your camera are monochrome at the moment and they need to be converted into
true colour before enhancing them. Any processing of the image, other than calibration with dark
frames etc., will distort the colour rendering of the result and so the conversion needs to be done
before proceeding. Important! You cannot convert a binned image to colour, as binning sums the
different colour pixels together and averages out the colour information. All images that you intend to
colour convert must be taken in binned 1x1 mode. Also please note that you cannot sum frames
together before colour conversion. This must be done after the images have been colour converted.
Here is a small enlarged section of a daylight image which may be similar to your test image taken
with a 50mm camera lens

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The visible grid encodes the colour information within the image data and will now be used to decode
the red, green and blue image planes into a colour picture. To do this, open the ‘Colour’ header tag
where you will see the ‘Synthesize colour image’ option. Click on this to open the colour synthesis
control panel.
The synthesis control panel contains several options for correcting colour bias and noise.
1) Daylight Image? – If an unfiltered lens is used, the infra-red content of the light will tend to
produce a Green – Blue shift in the balance. A partial correction of this bias is provided by
selecting this option, but the best option is to use an infra-red blocking filter when taking
daylight shots (see 5 below).
2) Light Pollution Correction: - This option is for deep sky colour images, where a strong colour
bias is often present in the sky background. This option attempts to return the background
colour to a neutral grey.
3) Colour Smoothing Filter: - Applies a low pass filter to the colour data to smooth out colour
noise.
4) Apply Anti-Alias: - Runs a special filter over the colour data to remove coloured artefacts
around sharp edges. This is especially useful for cleaning up erratically coloured star images.
5) IR Filter Used? – Sets the colour balance to allow for the loss of infra-red content when using
an IR blocking filter.
6) Stretching: - A selection of contrast-boosting options which are preset for various subjects.
7) Remove Background: - This option will adjust the sky background brightness to give an
optimum background level.
8) High Pass filters: - Automatically sharpen the luminance data to emphasise fine details. Most
useful for sharpening planetary images but will also increase the noise content.

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I suggest that you switch on the Colour smoothing filter and the Anti-alias filter – you can experiment
with the other options as you gain more experience. Now click on ‘Create image’ and you should see
that the grid pattern disappears and is replaced by colour image data.
If the colour is roughly correct, then all is well. However, the colour decoding is reliant on the filter
pattern being properly registered with the processing software and this can vary. Colour conversion
involves running a software routine which uses the brightness data from each pixel to generate colour
values. As the brightness of each pixel is related to the colour of light falling on it and the colour filter
integrated onto the pixel during CCD manufacture, we can translate this into true colour data. It is
very important that the software knows the colour filter positions, or the colours will be translated
incorrectly, so the software allows the user to define X (pixel) and Y (line) offsets to correct this. If the
colour is badly wrong, try opening the ‘Set program defaults’ dialog box and altering the ‘Pixel’ and
‘Line’ offsets. Reload the raw file and try synthesis again. If it’s still not right, experiment with just a
pixel offset or just a line offset until the colour is good.
There is every reason to expect that the image will be reasonably well colour balanced, but if it is not,
you can adjust the colour in the ‘Set Colour Balance’ dialog box (Under ‘Colour’ in the main menu).

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The colour balance controls seem complex, but are really quite easy to use. The most useful controls
are the ‘Start’ and ‘Saturation Factor’ settings. Saturation factor will simply vary the colour intensity,
without any alteration of the colour balance, but the ‘Start’ settings for each colour will alter the
colour rendering of the dimmer parts of the image. As most astronomical images are badly affected
by light pollution, which affects the dimmer background colours in particular, the start point settings
are particularly helpful for correcting this. If you move the start point of the Red histogram a few
points to the right and click on ‘Apply’, the new image will have LESS red in the background and will
appear more Cyan. Similarly, if you move the Green start point to the right, the image will appear
more Magenta and moving the blue start point will turn it more Yellow. Do not move the start points
beyond the start of the main burst of histogram data, or you will introduce colour errors into low
saturation parts of the image. Just move the sliders by small amounts in the clear area below the main
peak, until the background is nicely balanced.
In some cases, the histograms may all start a long way above zero (usually in astro. images with a lot
of light pollution). In this case, slide the start point settings for all colours until they are equally distant
from the beginning of the colour data, but quite close to it. This will both correct the colour balance
(apart from any fine tweaks) and increase the colour saturation.
In some images, the colour data may be balanced differently in the highlights and shadows. For
example, the dark areas may be quite neutral in colour, but the highlights may be bluish in tint. This
indicates a ‘slope error’ in one or more colours and, in this example it is caused by the blue data
increasing too quickly when compared with the red and green data. To correct for this, the other
histogram sliders may be used as slope adjusters. In the case of the excess blue, try increasing the top
end (Max) value to greater than 255 (300 max is allowed). This will reduce the blue slope and result in
a more yellowish rendition. Alternatively, you can increase both the red and green slopes by setting
the upper red and green sliders below 255.
Image enhancements:
Your first image may be satisfactory, but it is unlikely to be as clear and sharp as it could be. Improved
focusing and exposure selection may correct these shortcomings, and you may like to try them before
applying any image enhancement with the software. However, there will come a point when you say,
“That’s the best that I can get” and you will want to experiment with the effects of image processing.
In the case of daylight images, the processing options are many, but there are few that will improve
the picture in a useful way. The most useful of these are the ‘Normal Contrast Stretch’ and the ‘High
Pass Low Power’ filter. The high pass filter gives a moderate improvement in the image sharpness,
and this can be very effective on daylight images.

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Too much high pass filtering results in dark borders around well-defined features and will increase the
noise in an image to unacceptable levels, but the Low Power filter is close to optimum and gives a
nicely sharpened picture.
The ‘Contrast’ routines are used to brighten (or dull) the image highlights and shadows. A ‘Normal’
stretch is a simple linear operation, where two pointers (the ‘black’ and ‘white’ limits) can be set at
either side of the image histogram and used to define new start and end points. The image data is
then mathematically modified so that any pixels that are to the left of the ‘black’ pointer are set to
black and any pixels to the right of the ‘white’ pointer are set to white. The pixels with values
between the pointers are modified to fit the new brightness distribution. Try experimenting with the
pointer positions until the image has a pleasing brightness and ‘crispness’.
At this point, you will have a working knowledge of how to take and process an TRIUS PRO-25C image.
It is time to move on to astronomical imaging, which has its own, unique, set of problems!
*********************************************************************
Astronomical Imaging with the TRIUS-SX25C
1) Getting the image onto the CCD:
It is fairly easy to find the correct focus setting for the camera when using a standard SLR lens, but
quite a different matter when the TRIUS PRO-25C is attached to a telescope! The problem is that most
telescopes have a large range of focus adjustment and the CCD needs to be quite close to the correct
position before you can discern details well enough to optimise the focus setting. An additional
complication is the need to add various accessories between the camera and telescope in order that
the image scale is suitable for the subject being imaged and (sometimes) to include a ‘flip mirror’
finder unit for visual object location.
A simple, but invaluable device, is the ‘par-focal eyepiece’. This is an eyepiece in which the field stop
is located at the same distance from the barrel end, as the CCD is from the camera barrel end.
When the par-focal eyepiece is fitted into the telescope drawtube, you can adjust the focus until the
view is sharply defined and the object of interest is close to the field centre. On removing the
eyepiece and fitting the CCD camera, the CCD will be very close to the focal plane of the telescope
and should record the stars etc. well enough for the focus to be trimmed to its optimum setting
Several astronomical stores sell adjustable par-focal eyepieces, but you can also make your own with
a minimum of materials and an unwanted Kellner or Plossl ocular.
Just measure a distance of 17mm from the field stop of the eyepiece (equivalent to the CCD to
adaptor flange distance of the camera) and make an extension tube to set the field stop at this
distance from the drawtube end. Cut-down 35mm film cassette containers are a convenient diameter
for making the spacer tube and may be split to adjust their diameter to fit the drawtube.

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It is necessary to set up a good optical match between your camera and the telescope. Most SCTs
have a focal ratio of around F10, which is too high for most deep sky objects and too low for the
planets! This problem is quite easy to overcome if you have access to a focal reducer (for deep sky)
and a Barlow lens for planetary work. The Meade F6.3 focal reducer is very useful for CCD imaging
and I can recommend it from personal experience. It does not require a yellow filter for aberration
correction, unlike some other designs, so it can also be used for tri-colour imaging. If you use a focal
reducer, using it at maximum reduction may cause the relatively large chip of the TRIUS PRO-25C to
suffer from considerable ‘vignetting’ (dimming towards the corners) and this will be difficult to
remove from your images. Experiment with the distance between the reducer and the camera to
optimise the results. The longer the extension tube used, the greater the focal reduction will be. As a
guide, most CCD astronomers try to maintain an image scale of about 2 arc seconds per pixel for deep
sky images. This matches the telescope resolution to the CCD resolution and avoids ‘undersampling’
the image, which can result in square stars and other unwanted effects. To calculate the focal length
required for this condition to exist, you can use the following simple equation:
F = Pixel size * 205920 / Resolution (in arc seconds)
In the case of the TRIUS PRO-25C and a 2 arc seconds per pixel resolution, we get
F = 0.00369 * 205920 / 2 = 380mm
For a 200mm SCT, this is an F ratio of 380 / 200 = F1.9, which is much less than can be achieved with
the Meade converter and appropriate extension tube. However, moderate deviations from this focal
length will not have a drastic effect and so any F ratio from about F4.5 to F6.3 will give good results,
especially if you operate the camera in binned 2x2 mode. It is clear from this result that the ‘Starizona
Hyperstar’ adaptor is very well suited to use with the SX25C, as it operates at around F1.95, so you
might be interested in getting one of these.
The same equation can be used to calculate the amplification required for good planetary images.
However, in this case, the shorter exposures allow us to assume a much better telescope resolution
and 0.25 arc seconds per pixel is a good value to use. The calculation now gives the following result:
F = 0.00369 * 205920 / 0.25 = 3040mm
This is approximately F15 when used with a 200mm SCT and so we will need a 2 or 3x Barlow lens.
Barlow lenses are less critical than focal reducers and most types can be used with good results.
However, if you are buying one especially for CCD imaging, I recommend getting a 3x or even 5x
amplifier, or the planets will still be rather small in your images.
Achieving a good focus:
Your starting point will depend on the focus aids, if any, which you are using. With the par-focal
eyepiece, you should slip the eyepiece into the drawtube and focus visually on a moderately bright
star (about 3rd magnitude). Now withdraw the eyepiece and carefully insert the camera nosepiece,
until it is bottomed against the drawtube end, and then lock it in place.
SXV_hmf_usb.exe has a focus routine that will repeatedly download and display a 128 x 128 pixel
segment of the image at relatively high speed. This focus window may be positioned anywhere in the
camera field and can be displayed with an adjustable degree of automatic contrast stretching (for
focusing on faint stars). To use this mode, start up the software and select the SXV camera interface
(File menu). Set the camera mode to Binned 1x1 and select an exposure time of 1 second. Press ‘Take
Picture’ and wait for the image to download. There is a good chance that your selected star will
appear somewhere within the image frame and it should be close to a sharp focus. If the focus is still
poor, then it may appear as a pale disk of light, often with a dark centre (the secondary mirror
shadow in an SCT, or Newtonian). Now select the ‘File’ menu again and click on ‘Focus frame centre’;

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you can now use the mouse pointer to click on the star image and the new focus frame co-ordinates
will be displayed. Now return to the camera interface window and click on ‘Start’ in the Focus frame.
The computer will now display a continuous series of 128 x 128 pixel images in the focus window and
you should see your selected star appear somewhere close to the centre. A ‘peak value’ (the value of
the brightest pixel) will also be shown in the adjacent text box and this can be used as an indication of
the focus accuracy. Although the peak value is sensitive to vibration and seeing, it tends towards a
maximum as the focus is optimised. Carefully adjust the focus control on your telescope until the
image is as sharp as possible and the peak value reaches a maximum. Wait for any vibration to die
down before accepting the reading as reliable and watch out for bursts of bad seeing, which reduce
the apparent focus quality. Quite often, the peak value will increase to the point where it is ‘off scale’
at 4095 and in this case you must halt the focus sequence and select a shorter exposure if you wish to
use the peak value as an indicator. Once you are happy with the focus quality achieved, you might like
to trim the settings of your par-focal or flip mirror eyepiece to match the current camera position.
Although you can reach a good focus by the above method, many observers prefer to use additional
aids, such as Hartmann or Bahnitov masks (an objective cover with several spaced holes) or diffraction
bars (narrow parallel rods across the telescope aperture). These make the point of precise focus
easier to determine by creating ‘double images’ or bright diffraction spikes around stars, which merge
at the setting of exact focus. The 12-16 bit slider control allows you to adjust the contrast of the focus
frame for best visibility of the star image. It defaults to maximum stretch (12 bits), which is generally
ideal for stars, but a lower stretch value is better for focusing on planets.
Taking your first astronomical image:
I will assume that you are now set up with a focused camera attached to a telescope with an
operating sidereal drive. If so, you are now in a position to take a moderately long exposure of some
interesting deep-sky astronomical object (I will deal with planets later). As most drives are not very
accurate beyond a minute or two of exposure time, I suggest that you find a fairly bright object to
image, such as M42, M13, M27 or M57. There are many others to choose from, but these are good
examples.
Use the finder to align on your chosen object and then centre accurately by using the focus frame and
a short exposure of between 1 and 5 seconds. The ’12-16 bit’ slider in the focus frame allows you to
adjust the image contrast if you find that the object is too faint with a short exposure. Once properly
centred and focused, take an exposure of about 60 seconds, and observe the result. Initially, the
image may appear rather barren and show only a few stars, however, there is a great deal of data
hidden from view. You can get to see a lot of this, without affecting the image data, if you go to the
‘View’ menu and select ‘Auto Contrast Stretch Image’. The faint image data will then appear in
considerable detail and I think that you will be impressed by the result!
If you are happy with the image, go to the ‘File’ menu and save it in a convenient directory.

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A 1200 second exposure of the Rosette through an IDAS filter (non-linear stretched)
Most competitive brands of CCD camera require a ‘dark frame’ to be subtracted from your images to
achieve the best results. A dark frame is simply a picture which was taken with the same exposure as
your ‘light frame’, but with the telescope objective covered, so that no light can enter. It records only
the ‘hot pixels’ and thermal gradients of your CCD, so that these defects are largely removed when
the dark frame is subtracted from the light frame. However, the TRIUS PRO-25C CCD is quite different
from those used in other brands of camera and generates an extremely low level of dark noise.
Indeed, it is so low that subtracting a dark frame can actually INCREASE the noise in your images! This
is because the statistical noise of the dark frame can exceed the ‘pattern noise’ from warm pixels and
hence add to that of the subtracted result. If your test pictures have an exposure time of less than
about 10 minutes (as above), then don’t bother with a dark frame, just ‘kill’ any hot pixels with your
processing software. In TRIUS-SX25C, the ‘Median filter’ can do this, but other software (e.g. Maxim
DL) will provide a ‘hot pixel killer’ that can be mapped to specific locations in the image, or methods
such as ‘Sigma combine’ may be used.
In the unlikely event that you feel that dark frame really is necessary, please proceed as follows:
To take a dark frame, just cover the telescope objective with the lens cap and take another exposure
with the same length as that of the light frame. This image will be a picture of the dark signal
generated during your exposure and it should be saved with your image for use in processing the
picture. If many such darks are recorded and averaged together, the statistical noise will be reduced,
but the gains to be had are rather small compared with the effort involved.
As variations in ambient temperature will affect the dark signal, it is best to take the dark frames
within a few minutes of capturing your images. For the same reason, it is not wise to use ‘old’ dark
frames if you want the best possible results, however, some software allows you to scale library dark
frames to match the image (e.g. AstroArt) and this can be useful as a time saver.

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‘Bias frames’ are somewhat more useful than dark frames when using the TRIUS-SX25C. A bias frame
is essentially a zero exposure dark frame and records any minor readout defects that the CCD may
possess, so a ‘bias frame subtraction’ can clean up any ‘warm columns’ or shadings that are created
during readout. To record a bias frame, cover the camera aperture and take a 1000th of a second
exposure. If you take at least 10 such frames and average them together, the resulting ‘master bias’
can be used to clean up readout defects for many months before CCD ageing changes require another
set to be recorded.
‘Flat fields’ are often recommended for optimising the results from your CCD camera, but these are
generally less important than dark frames, especially if you make sure that the optical window of the
camera is kept dust-free. The purpose of a flat field is to compensate for uneven illumination and
sensitivity of the CCD and it is better to avoid the need for one by keeping the optics clean and
unvignetted. I will ignore flat fielding for current purposes and describe the process in detail at a later
stage.
Processing a deep-sky image:
1) Make sure the ‘Auto Contrast Stretch’ is switched off and load your image into SXV_hmf_usb. If you
intend to subtract a dark frame, select ‘Merge’ and then ‘Subtract Dark Frame’. Pick the appropriate
dark frame and the software will then remove the dark signal from your image, leaving it somewhat
darker and slightly smoother than before.
M42 – 30 seconds exposure at F2 with a C8 Hyperstar
2) Once you have subtracted any dark frame, you can convert the raw image to colour.

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3) The resulting image will probably look faint and dull, possibly with a pale yellowish background,
due to light pollution so it is now time to process the ‘luminance’ (brightness and contrast) of the
image to get the best visual appearance. First, use the ‘Normal’ contrast stretch to darken the
background by setting the ‘Black’ slider just below the main peak of the histogram. Alternatively, you
can use the ‘Remove Background’ option to let the software decide on the best setting. This will
greatly reduce the background brightness and the image will begin to look rather more attractive,
although dark. You can now try brightening the highlights with another ‘Normal’ stretch, in which you
bring down the ‘White’ slider to just above the main image peak. The best setting for this is rather
more difficult to guess and you may need several attempts before the result is ideal. Just use the
‘Undo last filter’ function, if necessary, to correct a mistake. Normal (linear) stretches can give a nice
result on many objects, but you may find that bright areas ‘burn-out’ badly with this function. It is
often much better to use a ‘Non-linear’ stretch to compress the brighter regions, while expanding the
faint data. Here is the result of a non-linear stretch on the M42 image:

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The image now looks quite impressive and I hope that you like the result from this simple processing.
Further small refinements are usually possible and you will become expert at judging the best way to
achieve these as your experience increases. As a rough guide, the ‘Filters’ menu can be used to
sharpen, soften or noise reduce the image. Strong ‘High Pass’ filters are usually not a good idea with
deep sky images, as the noise will be strongly increased and dark rings will appear around the stars,
but a ‘Median’ filter can remove odd speckles and a mild ‘Unsharp Mask’ (Radius 3, Power 1) will
sharpen without too much increase in noise.
Other things to try, include summing several images for a better signal to noise ratio. Summing can be
done in the ‘Merge’ menu and involves loading the first processed image, selecting a reference point
(a star) then loading the second image and finding the same star with the mouse. Once the reference
is selected, you can either add directly, or average the images together. Averaging is generally better,
as you are less likely to saturate the highlights of the picture. The signal-to-noise ratio will improve at
a rate proportional to the square root of the number of summations (summing 4 images will double
the signal-to-noise), but different exposures must be used. Summing an image with itself will not
improve the S/N ratio. Also note that you cannot sum images before colour conversion, or the colour
data will be destroyed!
Although I have concentrated on the use of a telescope for deep-sky imaging, do not forget that you
have the option of using an ordinary camera lens for impressive wide-field shots! A good quality
200mm F3.5 lens with an infrared blocking filter will yield very nice images of large objects, such as
M31, M42, M45 etc. If you cannot obtain a large IR blocker for the front of the lens, it is quite
acceptable to place a small one behind the lens, inside the adaptor tube. You can even try using a
hydrogen-alpha filter to bring out nebulae, reduce light pollution and sharpen the star images to pin-
points.

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Taking pictures of the planets:
Planetary imaging is in many ways quite different from deep sky imaging. Most deep sky objects are
faint and relatively large, so a short focal length and a long exposure are needed, while planets are
bright and very small, needing long focal lengths and short exposures. High resolution is critical to
achieving good results and I have already shown how a suitable focal length can be calculated and
produced, using a Barlow lens.
Many camera users comment on the difficulty of finding the correct focus when taking pictures of
Jupiter etc. This is usually due to poor seeing conditions, which are only too common, but may also be
due in part to poor collimation of your telescope. Please ensure that the optics are properly aligned as
shown by star testing, or by using one of the patent collimation aids that are widely available. It is also
better to use a star for initial focusing, as planetary detail is difficult to judge in bad seeing. Although
the star will also suffer from blurring, the eye can more easily gauge when the most compact blur has
been achieved!
You could begin by imaging lunar craters, or the planets, Jupiter, Saturn or Mars. The rapid variations
of seeing which accompany planetary imaging, will ruin the definition of about 95% of your images
and so I recommend setting the camera to run in ‘Autosave’ mode. This will automatically take a
sequence of images and save them with sequential file names in your ‘Autosave’ directory. Dozens of
images will be saved, but only one or two will be satisfactory for further processing. The ‘Subframe’
mode of the SXV may be found useful for limiting the wasted area and reducing the download time of
small planetary images.
To start the Autosave process, call up the SXV Camera Interface and select the ‘Continuous Mode’
check box at the top (make sure the rest are unchecked). Now check the ‘Autosave Image’ checkbox
near the bottom of the window. If you now click on ‘Take Picture’ the automatic sequence will begin
and will not stop until you press a computer key. The images will be saved in FITs format with
sequential names such as ‘Img23, Img24….’ and will be found in the ‘Autosave’ directory (or a sub-
directory of Autosave, set up in the program defaults menu).
The exposure time needed for good planetary images is such that the image histogram has a peak
value at around 200 and does not extend much above 220 (Ignore the major peak near zero, due to
the dark background). If you use too short an exposure time, the image noise level will be increased,
and if too long a time is used you will saturate the highlights and cause white patches on the image.
With the recommended focal length, Jupiter and Mars will both need an exposure time of between
0.1 and 1 seconds and Saturn will need between 0.5 and 2 seconds.
Processing a planetary image:
Planetary images have one major advantage over deep sky images, when you come to process them –
they are MUCH brighter, with a correspondingly better signal to noise ratio. This means that
aggressive sharpening filters may be used without making the result look very noisy and so some of
the effects of poor seeing can be neutralised.

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A raw image
Try applying an ‘Unsharp Mask’ filter with a radius of 5 and a power of 5. This will greatly increase the
visibility of any detail on the planet, but the optimum radius and power will have to be determined by
experiment.
After conversion and the application of an ‘Unsharp mask’
In general terms, the larger the image and the worse the seeing, then the wider the radius for best
results. My Jupiter shots are usually about one third the height of the CCD frame and I find that the
‘radius 5, power 5’ values are good for most average seeing conditions. If you have exceptionally good
conditions, then a reduction to R=3, P=3 will probably give a more natural look to the image, as too
large a radius and power tends to outline edges with dark or bright borders.
As a finishing touch, the application of a Median filter or a Weighted Mean Low Pass filter can be
useful to smooth out the high frequency noise after a strong Unsharp Mask.
As with deep-sky images, it is advantageous to sum planetary images together to improve the signal
to noise ratio. In this case, the ‘averaging’ option should always be used, or the result is likely to
exceed the dynamic range of the software and saturate the highlights. Aligning the images is always
something of a problem, as there are rarely any stars to use when imaging the planets, but Jupiter’s
satellites can be useful reference points. Otherwise, you will have to find a well-defined feature on
the planet, or estimate where the centre of the disk is located. Some more sophisticated software can
automatically align planetary images and you may find these programs (e.g. ‘Registax’) to be very
useful.