Single Shot Color Cameras
ST-2000XCM, STL-4020CM, STL-11000CM
Descriptions, Sample Images, Color Image Processing Tips
(All of the astrophotos on this page were taken with single-shot color cameras)

ST-2000XCM	STL-4020CM	STL-11000CM

As of the date of this note, SBIG offers three models of Single-Shot Color Cameras in addition to our other full frame and interline monochrome cameras:  (1) The ST-2000XCM, 2 megapixel camera with 1600 x 1200 active pixels at 7.4 microns, (2) The STL-4020CM, 4 megapixel camera with 2048 x 2048 active pixels at 7.4 microns, (3) The STL-11000CM, 11 megapixel camera with 4008 x 2465 active pixels at 9 microns.

Note:  As of September 15, 2007, we has added a new model, the ST-4000XCM camera.  See the ST-4000XCM announcement page for details.

Customers' Sample Images from the SBIG ST-2000XCM Color Cameras:
Click a thumbnail:

All three cameras are self-guiding models, utilizing SBIG patented dual CCD design with a built-in TC-237H guiding CCD.  The TC-237H CCD has 657 x 495 pixels at 7.4 microns.  The STL-4020CM and STL-11000CM use the large format camera body whereas the ST-2000XCM uses the smaller body of the ST-7/8/9/10 series cameras.  The primary difference in performance of these cameras is the field of view based on the size of the detector used for the imaging CCD.  The graphics below show the relative sizes of the three imaging CCDs and the resulting field of view for each camera when used with a telescope of 1000mm focal length.

Each of the three color cameras has a monochrome counterpart, making the choice sometimes confusing, especially for the beginner.  This note describes some of the similarities and differences between our monochrome and color cameras and offers some guidelines to help in making the choices clearer.  First the similarities:  The Single-Shot Color version of the CCDs used in the ST-2000XCM, STL-4020CM and STL-11000CM cameras are essentially identical to the their monochrome counterparts with the exception of the addition of a color filter matrix over the pixels.

The top row of images in the box above shows the basic structure of the pixel for a monochrome CCD.  The center image in each row is an actual photograph of the surface of the CCD showing a small section of the pixel array.  The drawings at the right hand side depict a side view of an individual pixel.  As you can see from the bottom row of images, the CCD structure for the Single-Shot Color version is the same as the monochrome version except for the red, green and blue pattern of filters over the pixels.  The arrangement of colored filters over the pixels in a Single-Shot Color camera is a repeating square of RGGB known as a Bayer pattern as shown below for a KAI-2020CM CCD:   

This pattern of RGGB pixels across the entire CCD allows collection of Red, Green and Blue data in a single exposure.  In the case of a monochrome camera with an external filter wheel, instead of taking a single exposure, the user captures three or four images, each one through a different filter.  These separate images are saved to disk and later combined to create the final color picture.   

So what is the main difference?  Primarily it is a trade off between greater complexity, sensitivity and flexibility at a higher cost for the monochrome camera versus the simplicity, ease of use and lower overall cost for color imaging with the Single-Shot Color camera.  As you can see. the monochrome camera takes at least three images to make a color picture where the Single-Shot Color Camera takes one.  The difference is in the amount and quality of data recorded.  In the case of the monochrome camera, the external filters can be designed specifically for astronomical use where high transmission and color balance of emission line objects and continuum light is better controlled.  The monochrome camera is also more work, however, and with the addition of the filters it is also more expensive.  In the Single-Shot Color Camera, every fourth pixel sees red, every fourth pixel sees blue and every other pixel sees green.  The RGB color data is extrapolated for the entire frame from each of the colored pixels.  So the final image from a Single-Shot Color Camera has the same number of total pixels as a color image created by a monochrome camera and external filters, although it is created by less original data for a single exposure.   As far as sensitivity is concerned, the monochrome camera is somewhat more sensitive due mainly to the nature of the external filters compared to the micro-filters placed over each pixel in the single-shot color camera.  The QE chart below illustrates this difference using the STL-4020M camera with RGB filters vs. the STL-4020CM camera for comparison..

We can see that the effective QE of the monochrome camera with external filters is slightly higher than the single-shot color camera based on the filter transmission characteristics.  But remember, the monochrome camera must take three frames for the single-shot color camera's one frame.  So for a proper comparison, a monochrome camera taking 20 minute images through each of the three filters should be compared to a single-shot color camera taking a single 60 minute image.   In this case, the color camera compares very well to its monochrome counterpart.  Moreover, self-guiding the Single-Shot Color Camera is easier due to the fact that the built-in guider is never covered by a filter.  Where the monochrome camera shines is in the grayscale image, or in taking narrow band images of emission line objects.  But for simple color images, as the sample images below demonstrate, these single-shot color cameras are very capable.

Easy Processing of Single Shot Color Images

Single-Shot Color (SSC) cameras like the STL-11000CM, STL-4020CM and ST-2000XCM (and new ST-4000XCM) produce raw images that CCDOps displays as monochrome until they are color processed.   The Color Process command in the Single Shot Color menu of the Utility menu convents these raw images into color as shown in the dialog below.  Once you have captured the image, from the Utility drop-down menu in CCDOPS select Single Shot Color --> Color Process and you are ready to go.  Processing of Single-Shot Color Camera images is easy.  Start by clicking the Defaults button and selecting RGB or DDP for the Method (DDP works great on galaxies with bright cores).

The images below are actual screen captures of the single step process.  The first image is the B&W raw frame displayed from an ST-2000 camera.  The second frame (color) was created by clicking the "process" button in the dialog box with the settings in their default positions using DDP as the method:

Step 1:  Capture the image and save the raw data:

Step 2:  Click the "PROCESS" button to create the color image:

If you don't like the color balance of the result, simply modify the color balance pointers and click the "process" button again.  For more in-depth color image processing, you can extract the RGB color channels and perform more traditional tri-color image processing techniques (see Processing Separate RGB Images, below).

Color Balance
Any color image, even those from monochrome camera with external filter wheels or from consumer digital cameras, may require minor corrections to achieve the best color balance.  This is done in CCDOPS buy simply sliding the Color Balance Sliders at the top of the dialog box. To make the image more Red (or less Cyan) move the top slider towards Red then hit the Process button again.  If there's a star or an area of the image you know is White you can White Balance on it by positioning the Crosshair over that area then right-click the mouse and select Set White Balance.  It is a very simple process.

Brightness and Contrast
Use the Brightness and Contrast sliders to adjust the image and then hit the Process button to see the results.

Enhancements
Images of galaxies tend to have a lot of dynamic range and you may find it difficult to reveal the faint details in the arms without causing the core to saturate. Try selecting the DDP instead of RGB in the "Method" drop down box.  Hit the Process button. DDP compresses the dynamic range of the image. This may not look natural on all images but don't be afraid to try it. An example is shown below.

RGB Processed	DDP Processed

The initial color processing doesn't have to be difficult or complicated.   Once you have saved the original B&W image you can experiment over and over with different settings until you achieve the results you like. Of course, if you want more control over the image you may wish to process the R,G and B frames separately and combine them just as you would if the three frames were taken separately though RG and B filters as they are with monochrome cameras.   See below for more about extracting, processing and combining RGB frames.

Easy Processing of Separate R,G and B Images

Monochrome cameras with filter wheels are used to make color images by shooting separate red, green and blue frames through color filters.  Separate Red, Green and Blue frames may also be extracted from a Single Shot Color Camera image for more control during the processing stages.  To do this using CCDOPS select Utility --> Single Shot Color --> Extract Color Channel(s)... Once extracted, the R,G and B frames from a Single Shot Color Camera are treated the same as RGB frames shot with a monochrome camera and color filter wheel.  Processing separate RGB images to create a color image involves the following steps: 1. Co-aligning the images 2. Normalizing the Sky Background 3. Setting the White Balance.  CCDOps makes this relatively easy.  Here's a step-by-step procedure for quick and easy RGB Processing:

1. Co-align the images - Open the Red, Green and Blue images and then open the Crosshairs.Visually identify a common star or feature in the images to serve as an alignment reference. Starting with the Red image, position the Crosshair on the reference position, using the peak pixel brightness on a star for example, then right-click the mouse. Select the Set RGB Red Position item to mark the reference position. Do the same in the Green and Blue images, selecting the Set RGB Green Position and Set RGB Blue Position respectively. This tells CCDOps how it will co-align the images

2. Normalize the Sky Background - Normalizing the Sky Background means making sure it comes out a neutral gray in the final image, not having a subtle color tint. Bring the Red image to the foreground and then position the Crosshair on an area of the image that represents the Sky Background, free of any stars or faint nebulosity. Right-click the mouse and select the Set RGB Black Level. This tells CCDOps how to normalize the sky background.

3. Set the White Balance - Again, bring the Red image to the foreground and then position the Crosshair over a star or area of the image that you feel represents the White Balance. If you get it wrong it's easy to adjust so don't worry about it. Once the Crosshair is positioned, right-click and select the Set RGB White Level. This tells CCDOps two things: how to set the color balance and how to set the contrast of the RGB image such that the star you identified comes out white (neutral color) and just saturates in the RGB image.  Now that the hard work is done you can close the Red, Green and Blue images and then invoke the RGB Combine command in the Utility menu. You'll be shown the dialog at right. If the Advanced Setting section is not visible, click the green triangle to reveal it. To finish the color processing do the following:

4. Identify the images - Click the Set Name button to the right of the Red and navigate through your folders on your hard drive to find the Red image. Double-click the Red Image or select it and hit Open.

If you used CCDOps to acquire the images they will be named XXXX.r, XXXX.g and XXXX.b and at this point CCDOps will fill in the names of the Green and Blue images for you. If not then click the Set Name button to the right of the Green and Blue and identify those images.

5. Initial RGB image - Click the “Do It” button to see the results of merging the Red, Green and Blue images into a single RGB image. 6. Tweak the parameters – [a] Co-alignment - Modify the Horizontal and Vertical adjustments edit fields to the right of each image to tweak the co-alignment. The easiest way to do this is to look at the outer fringes of stars my zooming in on the RGB image. If the stars have a Red tint to the right then you would reduce the Red Horizontal item by 1. After each adjustment hit the “Do It” button to see the results. [b] Color Balance – Raise or lower the Factors column to adjust the Color Balance. For example, to make the image redder, raise the Red factor. Hit the Do It button to see the results. [c] Brightness and Contrast - Raise or lower Grey Level item to adjust the Brightness and raise or lower the Contrast Boost item to adjust the image contrast. Hit the Do It button to see the results.

6. Exporting to Other Image Processing Programs - If you have a favorite image processing program like PhotoShop that you would rather use to process and combine the RGB frames into a color image, then simply save the extracted R,G, and B images in a format of your choice such as TIFF or FITS and use any other image processing program to manipulate and combine the images.  It is recommended however that you keep your original data file in SBIG format in the event that some step along the way results in an irreversible change to your image file(s) that is not to your liking.  You can always extract the raw RGB channels again from the original.   

Special CCDOPS Functions for Single Shot Color Cameras

Filter Routines Specific to Single Shot Color Images

The raw images (monochrome, prior to conversion to color) from Single Shot Color cameras like the ST-2000XCM require special handling when it comes to filtering the images so that you don't co-mingle the color data in the filtering process. CCDOps has enhanced filtering routines in the Smooth, Sharpen, Column/Row Repair, Kill Warm Pixels and Remove Cool Pixels commands from the Filter sub-menu of the Utility menu. As shown in the dialog below, these commands allow you to check a Single Shot Color Image checkbox that applies those enhancements to Single Shot Color images.

Track & Accumulate Single Shot Color Cameras

You didn’t used to be able to do this and many people don’t realize that now you can use SBIG patented Track and Accumulate with Single Shot Color (SSC) cameras like the ST-2000XCM. CCDOps has been updated to recognize SSC cameras and Track and Accumulate will then make sure it co-aligns images correctly, taking into account the Bayer Color filter Matrix applied to these cameras.

Revised: September 16, 2007 12:13:58 PM.
Copyright © 2006 Santa Barbara Instrument Group, Inc.  All rights reserved.
Please report any problems with this page directly to the Webmaster