Advanced Telescope Supplies
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Technical TipsInterfacing Losmandy Mounts and SBIG Cameras and Autoguiders
Both the Losmandy stepper drive and Gemini GoTo system may need to be isolated from the output of some SBIG cameras. Not doing so may result in damage to your telescope mount control electronics. The table below summarises the compatibility of SBIG and Losmandy equipment:
Camera Losmandy Stepper Drive Losmandy Gemini System SBIG ST4 *plug compatible *plug compatible SBIG STV, STV deluxe *plug compatible *plug compatible SBIG ST237 and ST237a requires SBIG relay box requires SBIG relay box or Losmandy Optocoupler SBIG ST7,ST7e requires SBIG relay box requires SBIG relay box or Losmandy Optocoupler SBIG ST8, ST8e requires SBIG relay box requires SBIG relay box or Losmandy Optocoupler SBIG ST9, ST9e requires SBIG relay box requires SBIG relay box or Losmandy Optocoupler SBIG ST10, ST10e, ST10ME requires SBIG relay box requires SBIG relay box or Losmandy Optocoupler SBIG 1001 requires SBIG relay box requires SBIG relay box or Losmandy Optocoupler SBIG ST5c requires SBIG relay box requires SBIG relay box or Losmandy Optocoupler *plug compatible i.e. you can plug the auto-guider cable directly into the Hand Control or Auto-guider jack of the Losmandy electronics panel using a 6 pin RJ 11 cable. Note: SBIG model ST 237/7/8/9/10 cameras with .TTL output can be plugged directly into the Losmandy stepper drive, provided they are powered from an isolated power source. Use of DSC's connected to a RS232 of a PC that is also connected via its parallel port to a SBIG camera can cause an earth loop and failure of the stepper drive.
Notes
SBIG Relay Box and Losmandy Optocoupler Cable
You will need a cable to connect some SBIG cameras to the autoguider port of your Losmandy electronics controller. A cable diagram is available here in adobe .pdf format, and may also be downloaded from the SBIG web site.
This cable plugs into the 9 pin connector of the SBIG camera (if fitted) and the RJ-11 input of the Losmandy Optocoupler or SBIG relay box.
Pictured left is a Losmandy optocoupler and relay interface cable.
The optocoupler then plugs directly into the autoguider port of the Gemini system, or hand control port of the stepper drive.
SBIG ST237, ST237a and ST5c
These cameras have a RJ11 style (or "telephone jack" connector) for the output of the auto-guider signals. You will need to set a jumper pin, located within the SBIG camera control electronics case, to energise one of the output pins to +12VDC. Please refer to your SBIG camera manual for the location of this jumper. If the jumper is not set, the SBIG relay box or Losmandy Optocoupler will not work. With the jumper set, simply run a RJ11 cable from the camera autoguider socket to the Relay box or Optocoupler RJ11 input socket. You will need second RJ11 cable if you are using a SBIG relay box to connect it to the Losmandy autoguider port.
SBIG ST 7/8/9/10/1001 Cameras
These cameras have a 9 pin D-connector located next to the parallel port connector on the camera head.
SBIG also supply a 9 pin to RJ11converter, on their CFW8 filter wheel, which does not have 12VDC power output on the RJ11 side. To power either the SBIG relay box, Losmandy Optocoupler and SBIG CFW8, you need to plug a jumper cable directly into the 9 Pin connector on the camera head.
You can make up a jumper cable quite simply by using 9 pin computer connectors and ribbon cable as pictured left. A relay box/optocoupler cable can then be connected to one of the jumper outputs, along with the CFW8 filter wheel connector.
Basic CCD Image Calibration Guide
Many novice CCD users are disappointed by their initial results, as their first CCD images are often blurred, noisy and have a limited range of brightness values within the image. The image at left is a typical example of this. The image is covered with a "salt and pepper" noise pattern, and does not show much of the information which it contains.
Apart from taking an image like this object through your telescope (this is called a data or light frame), you will also need two other CCD images in order to process your "light frame".
The first is called a Dark Frame (an example is pictured left), which is literally a CCD picture taken with the lens cap on (you can dispense with the lens cap if your CCD camera has a shutter)
Why would we need to do such a thing? Because the Dark frame contains a map of the thermal noise of the CCD detector. This noise will vary with the CCD temperature and exposure time, however the map will remain unchanged for any particular exposure time an temperature, hence you will be able to use it for different light frames, provided they also have the same exposure time AND were taken with the CCD camera set to the same temperature.
Many experienced CCD users build up a library of Dark Frames, so they do not have to waste precious time at the telescope taking pictures of thermal noise.
To remove the thermal noise from your "light frame", all that is required is a Dark Subtraction. This process is easily handled by most CCD camera control or Image processing software. The software subtracts the thermal noise mapped by your Dark Frame from the thermal noise and data in your light frame (thermal noise also builds up in light frames too!). The result? We have removed the noise leaving data only!....well almost. This leads us on to the next step.
The second type of calibration image is called a Flat Field ( an example is pictured left). Flat fields are literally pictures taken through your telescope of an illuminated, but otherwise blank screen (or field). Again, why would we do such a thing?
The reason is, microscopic dust particles can fall on your telescope optics, CCD window or CCD itself, will cause small "dust doughnuts" on the image in combination uneven illumination from the telescope or optical system (i.e vignetting) with the centre of our image being brighter than the edges.
To remove these effects, we use the image of a blank surface to profile the dust motes and uneven illumination seen by our CCD detector.
Important note: the Flat Field also needs to have a Dark frame subtracted from it prior to its being used for calibration.
Once we have our dark calibrated flat field, we use this image by dividing it into out light frame. Why divide it? Well, consider flat fielding a picture of a flat field. Each pixel value in the image, say "X" would be divided into itself, i.e X/X and produce an image full of pixels with a constant value of 1, i.e. quite featureless, which is how a blank card should look! Again, most CCD image processing software will allow you to easily perform this operation simply by asking you to nominate an image to use as your "flat", and will use this to calibrate your light frame.
Some Further Notes: Flat fields need not be taken at the same temperature or duration of your light field. Simply adjust the exposure time so that the CCD is about 30% saturated (e.g. If the full capacity of your CCD pixels is 65,000 counts try for around 20,000). Doing so will minimise any noise introduced to your light frame by the flat field process.
Some suitable flat field targets are:
- A large, (i.e. bigger than the telescope's aperture) evenly illuminated sheet of white card
- The evening twilight sky...just make sure you don't catch any bright stars as well (very easy to do with a CCD!)
- A translucent plastic "lightbox"
Once you have "calibrated" your light frame (i.e. subtracted out the thermal noise, and and compensated for uneven illumination of the CCD) you can further process the image with your image processing software.
Many programs allow you to "grey scale" or brighten the dark portions of an image, and darken the brightest parts into something which reveals much of the information contained within your original data or "light frame".
There are other techniques such as unsharp-masking, image de-convolution, digital development etc, but these all require that you first calibrate your light frame. Not doing so will exaggerate the noise and vignetting in your light frame when you apply these advanced techniques.
