Almost all beginners want a colour camera and the attraction of colour images is obvious. Unfortunately, colour imaging comes at a cost in CCD performance, and you should be aware of this before making a decision. There are two common techniques used to create true-colour images. One of these is the classic ‘Tri-Colour’ technique of taking three images through red, green and blue filters and then combining the results into a single colour frame, using ‘Photoshop’ or a similar program. This method is used with a mono camera and so you can swap between colour and mono imaging at will, but you need a set of good quality colour filters and a filter wheel or holder is essential. You also must use at least three sequential exposures to capture the data and so it may be difficult to create a good image of moving objects, such as comets and planets. On the positive side, this technique will not reduce the resolution of your images and you have maximum flexibility to use any filters that you choose – or none at all.
The second option is to use a ‘One-shot’ colour camera, such as the SXVF-H9C or SXVF-M25C. This is quite a lot easier than ‘tri-colour’, as you capture all three colours in a single exposure and there is no need to register the RGB images together to create the final picture. A filter wheel is not necessary, although an ‘infrared blocking filter’ might be found to improve the colour rendering of the results. On the negative side, your CCD is permanently filtered and so there is some loss of sensitivity (longer exposure times will be needed, compared with an unfiltered mono CCD) and there is a small loss of image resolution. Despite the filter matrix, it is still possible to take narrow-band (H-alpha, OIII etc.) images with a one-shot camera, so the limitations are not as severe as some experts might try to tell you.
Please note that our ‘One-shot’ colour cameras do NOT download a colour picture directly. The filter data is encoded in a mono image and this is extracted by software after image capture. This gives the best possible results, as the mono image can be fully ‘calibrated’ before colour extraction, unlike colour video cameras etc.
An important fact that you should note is that light pollution can cause major colour balance and gradient issues with one-shot colour cameras. If you image from a light polluted location, then you should seriously consider following the mono camera option. You can then use narrow band filters to combat the light pollution and achieve results that would be very difficult to obtain with a colour imager. A light pollution blocking filter (e.g. IDAS) will help a lot, but a truly dark sky is the best for colour imaging.
The attraction of multi-megapixel chips is great, but many common telescope designs cannot give a high quality image over the entire area of a large CCD. This is especially true of the popular ‘SCT’ instruments and many camera users have been disillusioned by the poor star images that tend to appear at the edges of their images. A normal SCT has a high focal ratio of around F10 and this is generally too large for deep sky imaging, unless you intend to specialise in small, bright objects like planetary nebulae. Because of this, I recommend that a focal reducer should be used, but avoid reducing below about F5 if you want to keep a reasonably flat field across your CCD. At F5, acceptable results can be achieved with CCDs up to about 2/3 inch format (such as the SXVF-H9), but this is very much the limit. To cover a larger chip, you must consider purchasing an Astrograph of some description, such as a well-corrected refractor or a flat-field reflecting design. One of the best available is the Takahashi FSQ106 and this will cover any of our CCDs with pinpoint stars, but there are many cheaper options with very good performance. Another popular option is to convert your SCT into a very fast (F2) imaging platform by replacing the secondary mirror with a ‘Hyperstar’ lens from ‘Starizona’. These lenses will cover an SXVF-M25C chip and offer an extremely fast imaging system. The low profile and symmetrical body design of the SXVF cameras is ideal for these on-axis optical systems. Here is a summary of suggested cameras for different telescopes:
- Standard SCT with focal reducer (F5): SXVF-M5, M5C, H5, M7, M7C – possibly SXVF-H9, H9C and M9. Limited by field curvature and vignetting.
- SCT with Hyperstar (F2): All cameras up to SXVF-M25C – it is well suited to small pixel cameras (M8C, H9, H9C)
- Highly corrected refractor (F5): All SXVF cameras. Some types may need extra optics to cover the largest chips (e.g. Sky90)
- ED glass refractor (F5): Most SXVF cameras, but limited by UV and IR colour error (fringe filters may be needed). Field curvature can limit the CCD size, so check the specifications.
- Astrographs (e.g. R/C and Epsilon): Most SXVF cameras (some may not cover the H35 or H36).
The latest drivers (versions 188.8.131.52 and 184.108.40.206) are new ‘user mode’ drivers and will not cause ‘blue screen’ crashes on modern machines. They are also compatible with both 32 and 64 bit computers and are easy to install. Simply download the zip file package, unzip it into a convenient folder and then run the 32 or 64 bit installer exe file. All of our current cameras are supported by these drivers.
The introduction of Windows Vista and Windows 7, with their enhanced security features, means that getting the SXV drivers to install can be a bit of a challenge.
Fortunately, it isn’t really all that difficult, if you follow these simple instructions (N.B. You need to be logged in as the Administrator before starting!):
- Download the driver package required from the downloads page.
- Create a folder on the root of drive C – e.g. ‘SXV drivers’ and unzip the driver package into it.
- Plug the USB cable into your camera (no need for power) and wait for the ‘New hardware found’ message to appear.
- Select the ‘Locate and install driver software’ option. Select ‘Continue’ if Vista asks for permission.
- Select ‘Don’t search online’.
- Select ‘I don’t have a disk – show me other options’.
- Select ‘Browse my computer for driver software (advanced)’.
- Browse for your folder on drive C and select it.
- Windows will find the driver but will ask you for permission to install it – select ‘Install this driver software anyway’.
- After a short delay you should see the message that ‘The software for this device has been successfully installed’.
- If you are using the supplied SX software, you will need to assign ‘Administrator’ rights to it, or disable ‘UAC’ in the Windows security settings. Otherwise it cannot write a temporary file to drive C and you will get the message ‘Unable to create bitmap file’ or similar, when running it.
- Celebrate and start using your camera!
Most telescope mounts will not track well for more than one or two minutes and so a guide camera may be a valuable accessory for you. The problem may be overcome by simply taking many short exposures and stacking the results together, but this is not practical at long focal lengths. Some SX cameras permit you to use ‘STAR2000’ guiding, where the CCD image is composed of two successive exposures, one for odd-numbered lines and a second for the even-numbered ones. During these exposures, the other lines are used for guiding the telescope and so you can guide on the object being imaged, without a guide camera being required. This works well, but you will effectively halve the camera sensitivity and so you may not want to make this sacrifice. If this is the case, then the SXV guide camera offers you a good solution. This tiny, but sensitive camera, as small as a 1.25″ eyepiece, connects directly to the main camera for control and will work with any small guide ‘scope or off-axis assembly. In cases where you want to guide another camera, such as a DSLR, then our ‘Lodestar‘ USB2 controlled guider offers you a tiny but powerful option.
If you want to guide at a long focal length, then a simple guide camera may not be enough for good results. In this case, you might consider purchasing one of our AO units for high-speed precision guiding.
Starlight Xpress recommends Warren Keller’s video tutorials ‘IP4AP’ at: www.ip4ap.com
These tutorials will take you through all the stages of processing, from the basics to advanced image enhancement. Highly recommended!