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Handbook for the SXV-M25C     Issue 1 June 2004 

 

18

To use your flat fields, they must first have a dark frame subtracted. Although this 
may appear to be unimportant with such brightly lit and short exposures, there is the 
‘bias offset’ of the camera in each image and this can produce an error in the final 
correction. As we are mainly interested in the bias, any very short exposure dark 
frame will give a good result. The dark subtracted images should then be averaged 
together before use.  
 
After the above procedures have been executed, the flat field will be ready for use. 
Load up your image for processing, subtract the dark frame and then select ‘Apply 
flat field’ in the ‘Merge’ menu. The result should be an image with very few signs of 
the original artefacts and you can then process it in the normal way 

 
Processing a deep-sky image: 
 

The following instructions include the subtraction of a dark frame, but this may be 
regarded as optional. 

 

1) Make sure the ‘Auto Contrast Stretch’ is switched off and load your image into the 
SXV_M25C program. 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. 
 
2) The next step is to process the image into colour, but you may find that the 
application of a gentle contrast stretch to the image before synthesis will improve the 
colour result. Don’t overdo the stretch, as the colour range can then exceed the 
dynamic range available, leading to un-correctable colour errors. Keep a copy of the 
original file – just in case! Set the colour synthesiser to apply the anti-alias filter and 
then run it. 

 

3) The resulting image will probably still look faint and dull, with a pale orange or 
green background due to light pollution. Try using the ‘Start point’ adjustment in the 
Colour balance menu to get something close to a neutral background colour. 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’ icon button 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, if 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. In many cases, a ‘Normal’ 
contrast stretch will give a good result, but may ‘burn out’ the bright regions and 
leave the faint parts of the image rather lacking in brightness. To combat this, many 
imagers will use a combination of ‘Normal’ and ‘Non-linear’ contrast stretches. The 
best settings are different for different objects, but performing a non-linear or power 
law stretch, followed by normalising the background to black with a normal stretch, is 
the usual procedure. 
 

Summary of Contents for SXV-M25C

Page 1: ...optional add on autoguider output and integrated dual serial ports for filter wheel and telescope control The SXV M25C uses a Sony ICX413AQ a very large SuperHAD interline CCD with 3040 x 2016 x 7 8u...

Page 2: ...to the mount 7 A USB2 camera cable 8 An adaptor for 2 drawtubes and M42 Pentax thread lenses 9 A disk with the SXV M25C control software 10 This manual Optional extra items include 1 A serial port sp...

Page 3: ...ot see the directories C Windows Inf and Windows System32 Drivers this will be due to the setup of your Windows Explorer software In this case go to the Tools menu followed by Folder Options and selec...

Page 4: ...elled BlockIOClass and clicking on the sign will reveal it to be a Starlight Xpress USB 2 0 SXV M25C camera driver or similar If this device is faulty try clicking on it and selecting properties and t...

Page 5: ...r the SXV M25C Issue 1 June 2004 5 The camera rear panel Connect up the power supply and switch it on You can start the SXV_M25C software by double clicking on the icon and you should see the main men...

Page 6: ...defaults from the File menu and then saving the defaults window by pressing the Save changes button Once the camera control panel is seen you are all set to take your first images Recording your first...

Page 7: ...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 towa...

Page 8: ...d the image using the sliders at the right and bottom edges or select Bin high res display as found in the View menu This reduces the displayed image to half size and most of it will now be visible th...

Page 9: ...er This will re load the USB software and may fix the problem after restarting the SXV_M25C program Otherwise check the device driver status as previously described and re install any drivers which ap...

Page 10: ...our 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...

Page 11: ...h the processing software and this can vary 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 a...

Page 12: ...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...

Page 13: ...rpness and this can be very effective on daylight images Too much high pass filtering results in dark borders around well defined features and will increase the noise in an image to unacceptable level...

Page 14: ...25C and a 2 arc seconds per pixel resolution we get F 0 0074 205920 2 761mm This is a very rough guide and so the 500mm FSQ is well suited as are most other short focus refractors Achieving a good foc...

Page 15: ...ction 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...

Page 16: ...minutes as above then don t bother with a dark frame just kill any hot pixels with your processing software In SXV M25C the Median filter can do this but other software e g Maxim DL will provide a hot...

Page 17: ...ture onto the CCD chip where they appear as shadow do nuts Most optical systems show some vignetting at the edges of the field especially when focal reducers are used This causes a brighter centre to...

Page 18: ...nge available leading to un correctable colour errors Keep a copy of the original file just in case Set the colour synthesiser to apply the anti alias filter and then run it 3 The resulting image will...

Page 19: ...strongly increased and dark rings will appear around the stars but a Median filter can remove odd speckles hot pixels and a mild Unsharp Mask Radius 3 Power 1 will sharpen without too much increase i...

Page 20: ...ent the creation of a proper colour image 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 find...

Page 21: ...tube You can even try using a light pollution filter to bring out nebulae reduce light pollution and sharpen the star images The Hutech IDAS types are particularly effective with a colour camera Adju...

Page 22: ...he 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 you...

Page 23: ...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...

Page 24: ...routine will learn the angle of the head and compensate for it but it is easier to understand the motion of the guide star if the guider frame is aligned with the RA and Dec axes Now connect the head...

Page 25: ...5 The autoguider installed on a 80mm refractor guide scope in the author s garden To use the autoguider please proceed as follows 1 Having started the SXV M25C software open the autoguider control pan...

Page 26: ...guiding rate is selected on your mount typically 2x sidereal Adjust this if necessary 5 Move the mount until the required object for imaging is properly framed in the main CCD image leave the guider m...

Page 27: ...ge button again 11 Once guiding is taking place without problems the main exposure can be allowed to finish and if all is well you should see an image with tiny circular stars If the stars are not cir...

Page 28: ...hort exposure time as the read out noise will become dominant About 30 seconds is a reasonable minimum Now go to the Multiple Exposure Options and select a number of exposures to take You can also sel...

Page 29: ...with the standard autoguider input of most telescope mounts It provides 4 active low opto isolator outputs and a common return line capable of sinking a minimum of 5mA per output This socket may be u...

Page 30: ...ohol A very suitable cloth is the Micro Fibre type marketed by PENTAX etc and suitable alcohol is available from Tandy Radio Shack etc as tape head cleaning fluid A bright light and a strong watchmake...

Page 31: ...periods between uses The cold CCD will collect ice by slow diffusion through any small leaks and this will become corrosive water on the cooler and CCD pins when the power is removed If substantial am...

Page 32: ...tion to any statutory Consumer Rights of Purchasers CONDITIONS OF GUARANTEE 1 The equipment shall only be used for normal purposes described in the standard operating instructions and within the relev...

Page 33: ...Handbook for the SXV M25C Issue 1 June 2004 33 Web site http www starlight xpress co uk...

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