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Manually Adding a Camera for use with SkyTools 3

     

SkyTools ships with a list of cameras to choose from, but the list is not exhaustive.  We have included all of the cameras that we could find complete data for and are always adding more.  Unfortunately some manufacturers are less forthcoming with their data than others.  This guide is intended to help you find and enter the data you need to set up your camera manually for use with SkyTools.

 

First determine what type of camera you have.  SkyTools supports traditional CCD cameras, consumer digital cameras, and video/webcams.  It is important to make the correct selection when you enter the data because each type of camera has its own specialized capabilities and limitations.

 

A camera is made up of two basic parts: the CCD detector (or sensor or "chip") and the electronics in the camera itself that read, process and store the images.  Two cameras that share the same detector will also share many of the same specs, so finding out the model number of the CCD detector is a very good start.  The detector itself usually determines the following:

  • Pixel size

  • Read noise

  • Dark signal

  • Maximum Quantum Efficiency

In many cases the following parameters are also determined by the detector, but they are sometimes modified by the camera design:

  • Number of effective pixels 

  • Gain 

  • Spectral response (quantum efficiency at different wavelengths)

Basic Camera Data

 

Start with the specs for the camera from the manufacturer.  You can often find this information on the manufacturer's web site.  If not, try web searches on the camera model number plus "specifications", "specs", "sensor", "pixel size", "read noise", etc.  One really important piece information to find is the make and model number of the detector (or sensor) used in the camera.  With that you can often find much of what you need from the detector spec sheet from the maker of the detector.  As a last resort you might contact the camera manufacturer.  Remember, some of the camera specs may differ from those of the detector alone.  Always use the camera specs if they differ from those of the detector.

 

CCD Detector/Sensor Data

 

Once you determine the model number of the detector (sensor, "chip") used by your camera you can often find a spec sheet from the manufacturer of the detector by searching on the model number.  An example model number is "ICX418AKL".   Sometimes there is a dash or space (e.g. it may be "ICX-418AKL") .  Make sure and try different combinations in your search.  Google will often see a dash as a wild card character so it doesn't hurt to try putting in some dashes.  The trailing letters (AKL in our example) often indicate variations in the model, such as color vs. mono, so try to match the exact model number used in your camera.  

 

If you are lucky we will already have the detector/sensor in SkyTools.  If so, you will be able to select the detector from the pull-down "Detector" menu on the camera data dialog.  This will set the spectral response (quantum efficiency at different wavelengths).  Be aware that the presence of a glass window in front of the detector will change the spectral response, so be careful to match the model number exactly.  In some cases the camera may place a glass window or filter in front of the detector.  If that happens this too will affect the spectral response.  In this case you will need to know the spectral response of the camera/detector system (together) rather than the detector alone.

 

Camera Data in Detail

 

Below we will look at each of the specifications you must enter for your camera in detail.

  • Pixel size -- although often supplied in the specs for the camera the pixel size is determined by the detector, so you may use the values given in the specs for either the camera or detector.  In some cases the pixels are square so only one size is given.  In this case enter the same value in both boxes.

  • Number of Effective Pixels -- this together with the pixel size is going to be used to compute the physical dimensions of the detector, which in turn will determine your Field of View (FOV).  Take care to get the "effective" number of pixels.  A detector is often manufactured with pixels that are masked and thus don't contribute to the final image.  In some cases the camera design may also reduce the number of "effective" pixels returned by the camera.  The best way to find the number of effective pixels is to simply measure the usable width and height (in pixels) of an image obtained with the camera.  Note: the width of the detector in mm is the number of pixels in width multiplied by the pixel size (in microns) divided by 1000.  If you cannot find the pixel size, but know the physical dimensions of  the detector, it is possible to use this formula to compute the pixel size: width (in mm) / (number of pixels in width) * 1000.  But use this as a last resort because it is not as accurate as knowing the exact pixel size.  

  • Read Noise -- this is a property of the detector.  It is often cited in the specs for the camera but can also be found in the specs for the detector.  Together with the dark signal the read noise is used to estimate the noise in the final image.  This in turn affects the SNR computed.  

  • Dark Signal -- this is a property of the detector.  It is often cited in the specs for the camera but can also be found in the specs for the detector.  Note that the dark signal depends on the temperature of the detector, so this value will change if the camera is cooled (it will be lower).  Make sure to note the temperature conditions for the quoted dark signal, if available. Together with the read noise the dark signal is used to estimate the noise in the final image.  This in turn effects the SNR. computed.  This value is important for digital cameras because the optimum sub-exposure time is usually determined by the dark signal rather than the sky.  Unfortunately it can be difficult to obtain the dark signal for many digital cameras.  Typical values for modern Prosumer digital cameras fall between 2 and 20 electrons/sec.

  • Gain -- this is the amplification of the signal by the electronics in the camera expressed in e/ADU.  It is particularly tricky, especially for digital and video cameras where the gain may be varied by the user.  For digital cameras the gain is set via the ISO (or film speed) selection.  If possible, find out the ISO setting used for the quoted gain.  Larger ISO settings will decrease the gain.  If all else fails, use 1.0.  For some astronomical CCD cameras the gain will vary with binning.  Thus there are two gain entries: unbinned and binned.  Most cameras will use the unbinned value so you can leave the binned value blank (or set it to be the same as unbinned).  The Gain is used for one thing only by SkyTools: the calculation of the suggested sub-exposure time when stacking images.

  • Bit Depth -- this is often a property of the camera rather than the detector.  The camera may return images in several formats, where each pixel is represented by an 8-bit, 16-bit, or 32-bit integer.  For 8-bit images the value of each pixel can only have a value between 0 and 255.  For 16-bit cameras the range is 0-65535.  For 32 bits the range is 0-4294967295 (a very large number).  This is a minor variable used to estimate the quantization noise and will have only a small effect on the model.  For 12-bit cameras or if you don't know what to enter use 16 bits.

  • Built-in Lens -- the focal length of a non-removable lens.  This parameter applies to video/web cams only.  In most cases this field will be left blank; even for video cameras the lens is usually removed for astronomical imaging.  For a digital camera without a removable lens add the built-in lens via the Assign Lenses button on the Select Cameras to use with Telescope dialog.  

Quantum Efficiency (QE or DQE)

The quantum efficiency (QE) describes how sensitive the detector used by the camera is over the light spectrum.  Hopefully the detector your camera uses will be available for preselection from the list.  But watch out for subtle differences between detectors.  Some may have microlenses added and some will have a window.  The QE values will differ in these cases.

The spectral response determines how much signal is recorded by a pixel in general and how sensitive it is to different colors of light.  The higher the QE the more sensitive the detector is and the greater the recorded signal.  if the QE is comparably low in the blue to that in the red, blue sources will require longer exposures than red ones.

The QE determines how much signal to expect from astronomical objects and the sky background,  The signal will depend on the color of the object (or sky) and which filter (if any) is in use.  Ultimately the model sky signal will be used to compute a suggested sub-exposure time and both model signals together will be used to compute the SNR.  The QE is most important for traditional CCD cameras meant for long exposures.  For video cameras, which are limited to excellent lunar, solar, and planetary photography, the QE is much less important.

Usually the quantum efficiency will appear in graphical form in the detector spec sheet.  It will often be called "spectral response".  The quantum efficiency (in percent) will be graphed vs. wavelength.  You will need to read the values off the graph to enter into the Camera Data dialog (set the Detector to Manual Entry).  Read the height of the graph at each of the wavelengths required by the program.  SkyTools uses wavelengths in nm. You may need to convert to other units depending on those used by the graph.  Note that:  1 nm = 10 () Angstroms = 0.001 microns

A color camera may have more than one line on the graph.  These lines may overlap (see example below).  If so always read the value from the highest line on the graph.

One final but important note: the values entered into SkyTools must be absolute quantum efficiencies (expressed in percent) as opposed to relative quantum efficiencies.  Many camera spec sheets will graph relative QE instead.  To convert your relative QE values obtained from such a graph you must multiply each one by the maximum QE of the detector.  Unfortunately in these cases it is often difficult to find the maximum QE of the detector!  This is usually the biggest stumbling block in entering the data for a given camera.  If the detector spec sheet fails to indicate the maximum QE then you may need to contact the manufacturer (of the camera or the detector).  In some cases we have had success using online search engines.  Search for the model number of the camera or the model number of the detector plus "quantum efficiency" or "QE".  Often times the maximum QE will simply be represented as "the" QE for the detector.

If you find the data but have trouble interpreting it, please feel feel to contact me for help.

Example 1: the SBIG ST-402ME

This is your basic monochromatic astronomical CCD camera.  The graph below is from the SBIG camera spec sheet. It graphs absolute QE vs. wavelength in nm. 

To read values off the graph we first find the wavelength at the bottom, in this case 500 nm.  Follow the vertical line up to where it crosses the QE response curve (red line).  Now find the corresponding value on the scale to the left (52%).  In the box marked 500 nm on the SkyTools camera dialog you would enter "52".  

But sometimes the graphs don't extend far enough in wavelength.  For instance SkyTools wants a QE value for 350 nm, but that's off the left side of the graph.  Sometimes you have to make due with what you've got.  You could enter the same value as we get for 400 nm (45%).  Or extend the graph yourself to the left by estimating the curve based on the trend in the data.  In this way we can estimate a value of around 35-40%.  We'll call it 38%.

Example 2: MallinCam Color CCD

Our first example was fairly straight forward.  This next one is much more complicated.  Below is the graph found in the technical specs for the Sony ICX418AKL detector, which is used in this camera.  There are two complications here compared to the previous example: as a color camera the spectral response is drawn with separate lines indicating the response for a given internal "filter" so there is more than one line.  In addition, the relative response (or relative QE) is plotted on the vertical axis rather than the absolute QE that we require.  You can spot these because they always reach all the way to the top of the graph.  The top of the graph (at 1.0) is the maximum QE for the detector. 

We read the graph much in the same way as in the first example, except that we always read the value from the highest curve. So at 450 nm we skip right past the Ye and G curves all the way up to the Cy curve to read the value. At this intersection we read a value of 0.62 (this is equivalent to 62%). 

But now we have to know the maximum QE for the detector to convert this value (0.62) to the absolute QE.  Sony really doesn't like to share their QE data so it can be difficult to find the value.  In fact, I was unable to find the source that I used when I created the spectral response for this camera!  Someone may have sent it to me or I found it in a really obscure location.  Regardless, the maximum QE I have for this camera is 52%.  These video-type cameras often have much lower maximum QE than standard astronomical cameras such as the one above, which was approximately 83%.

So we take our value from the graph at 450 nm of 0.62 and multiply it by 52% to get 32%.  In the box marked 450 nm on the SkyTools camera dialog you would enter "32". 

There are some assumptions used going in for color cameras which may limit the reliability of the results, and for a video camera like this one many variables are, shall we say, "untamed."  The SkyTools exposure model works best for traditional monochromatic astronomical CCD cameras, but the good news is that it is these cameras that require the most accuracy from the model to begin with.  Ballpark estimates are usually good enough for digital camera and video imaging.