CCDs and Plate Scale

Astronomical Laboratory 29:137, Fall 2013
by Philip Kaaret with text from Steve Spangler

Reading

Sections 1.1, 1.2, 2.1 in Handbook of CCD Astronomy, second edition, by Steve B. Howell.
Santa Barbara Instruments Group ST-402ME CCD camera
SBIG CCDOps manual
Kodak KAF-0402E/ME CCD
Test pattern (a rather quick read)

Introduction

Charge-coupled devices (CCDs) are widely used in astronomy for the detection of visible, ultraviolet, and infrared photons. In this lab, you will learn to work with the CCD cameras that we will use throughout the semester and learn about some of their basic operating characteristics.

Laboratory Notebook

For this lab and all other labs, your performance will be evaluated on the basis of the record that your team keeps in its lab notebook. At the beginning of each lab, one student will be assigned to do the write up for that lab. The writing assignments will rotate sequentially through all of the students on the team. Each student should be sure that they fully understand all of the material. Student performance on the research project will be evaluated on the team's overall performance and on the individual contributions of the student. Make sure that the names of all the members of your team are written on the cover of your notebook. At the start of each class period, start on a new page in your notebook and write down the current date and name of the lab at the top of the page. If your work for the lab covers several pages, write the date and name of the lab at the top of each. When the instructions below say to record something, write down a brief description of what you did. In some cases, you will need to also record measurements, calculations, and plots in your notebook. It is fine to paste or tape plots into your notebook.

Equipment

The following are the pieces of equipment that you will use in this project. We have equipment for three setups.

An SBIG ST-402XME camera, with power adapter and USB cable.
A computer running the CCDOps software package and DS9 for image analysis
A C-mount camera lens and a C-mount to T-thread adapter for attaching it to the ST-402
A test pattern printed on a sheet of paper
A meter stick to make measurements

We have three different lenses with focal lengths of 12.5, 25, and 50 mm. Each team should take measurements with two different lenses.

You will use an ST-402ME or ST-402XME camera made by the Santa Barbara Instrumentation Group (SBIG) for this lab. One of the links above under "Reading" leads to the a description of the camera on the SBIG web site. Read the description of the chip itself, and notice the plot of the ``quantum efficiency'' as a function of wavelength. Pay attention to concepts such as the ``well capacity'' of the device (saturation charge in the semiconductor, which directly relates to the maximum number of photons which are accumulated), the number of pixels, and the physical size of the pixels.

The software we will use to control the camera and download data from the processor in the camera is called CCDOps. This software package is running on the computers in Room 655 Van Allen Hall. Download and look over the CCDOps manual; you will need it to control and analyze data from the camera. For this lab, we will use SAOImage DS9, an astronomical imaging and data visualization application, to look at our images. This software should also be on the computers in room 665.

When we start using telescopes on the roof and elsewhere for astronomical observations and measurements, each lab team will need to have the software installed on a laptop computer. All the software that we use is publicly available and there are links on the class web page. It would be good to identify a (windows) laptop in the next week or so, install the software, and try it out.

Setting up the Camera

First, you need to set up the equipment. Remember to record what you do in your lab notebook. Your notebook is used to record data and analysis results, but also to record each step of what you did so that you (or someone else) can recreate what you did at a later time. As the semester progresses, you will find yourself frequently referring back to your lab notebook to figure out what you have done when you need to write it up or do it again.

Connect the camera to the computer, and establish communication between them. Connect power to the camera, and run the USB cable from the camera to one of the front USB ports of the computer. Try the one on the far left first. Bring up CCDOps on the computer.
You now need to have the computer and the camera talk to each other. You do this by clicking on the "EstLnk" button in the CCDOps Toolbar, or in the main CCDOps window, you can select the "Camera" menu and then click on "Establish COM link". The camera should make a number of clicking sounds and flash its red LED while setting up the link. The red LED should then stay solidly on to show the camera's contentment if the link is successful. Indicate in your lab notebook if you were successful in connecting to the camera and any notes about the process that you might want to remember for the future (when these instructions are not repeated).
One of the first things you should do is to have the pleasure of seeing the CCD chip itself. This will also be your first interaction with CCDOps. On the icon toolbar, click ``GRAB'', which controls taking an image. The GRAB function is described in the CCDOps manual. Set the exposure time to several seconds, dark frame to none, image size to full, exposure delay to 0, special processing to none, then click ``OK''. This will open the shudder. You can look into the camera and see the rectangular device, several millimeters on a side. That is the chip we are using. Indicate in your lab notebook if you were gratified to see the chip.
Attach the lens and adapter to the camera. Set up the test pattern at least 1 meter away from the camera and arrange things so that the camera points toward the pattern. Make sure that the test pattern is perpendicular to a line between the camera and the pattern.
Make a sketch of the apparatus as you have set it up, including the distance between the camera and the pattern (measure from the front of the lens), in your notebook.

We now need to take some images of the test pattern. First, you need to focus the lens and find suitable operating parameter for the camera.

Loosen the set screw on the lens closer to the camera body and rotate the lens until the line or dot (depending on the lens) lines up with the number 16. This is adjusting an iris inside the lens, that you can see if you look through the front of the lens. Setting the iris to "16" means that the lens is stopped down to an "f-number" of 16 or "f/16". This means that the clear aperture of the lens is 1/16 of its focal length. The f-number is also called the focal ratio. Stopping down the lens makes it easier to focus. This also reduces the light passing through the lens (something an astronomer usually wants to avoid), but since the CCD cameras are very sensitive and the room is bright, this is OK. Tighten the set screw after making the adjustment. Don't worry is the screw is missing, but remember to check the iris after focusing if there is no set screw.
Go to CCDops running on your computer and click FOCUS on the icon toolbar in CCDOps. The description of the FOCUS operation is described in the CCDOps manual. The main control you have is the exposure time. You might have to try a range of values depending on the room brightness and distance between camera and test pattern. Note that you can use short values, down to 0.040 second. Set the other parameters as: frame = full, no dark frames, no filter warm pix, update mode = automatic, exposure delay = 0, and turbo mode = off. Click ``OK'', and the camera will keep taking images as long as the "Auto update" box is checked. You want an exposure time where you can see an image (probably blurry to start with), but the image is not saturated.
Loosen the other set screw on the lens. Adjust the focus of the lens by rotating the outer part of the lens. There are marking around the lens that show the approximate focus distance. You can get a decent initial focus by setting the lens to the distance that you measured to the test pattern. Watch the images on the screen while adjusting the focus. You might want to adjust the "Mag:" value in the "Contrast" window to magnify the image. Keep fiddling this until you have the sharpest possible image. Then tighten the set screw on the lens. Play with the exposure time after getting a decent focus to try to get really sharp images. Longer exposures can provide cleaner images. However, if you increase the exposure time too much, the image will saturate - the test pattern will just look white.
Figure out which test pattern is best to use (1 mm, 10 mm, or 100 mm). You need to be able to see the individual bars in the pattern and the pattern should fill as much of the image as possible. Move the test pattern and/or camera around as needed.
Go to the Misc/Telescope setup menu item and fill in some of the parameters for Focal Length (50, 25, or 12.5 depending on which lens you have), the Aperture Diameter (calculate from the focal length divided by the f-number), and the Aperture Area (calculate from the diameter). For Description, put in something like "50 mm lens" or you can write in the model number of the lens. For Observer, put in your name(s). Click 'OK' to save everything.
When you have everything (focus, exposure time, placement of the test pattern), save an image in FITS format. To do this, first grab an image using the "Grab" button on the CCDOps toolbar using what you determined was the best exposure time. Then do File/Save in the main CCDOps window. In the "Save as type" box, choose "FITS (*.fit)". Create a directory for your lab team and save the file with an appropriate name ("Untitled #" is not an appropriate name). For the object name in the save dialog box, put 'Test pattern'. Record the directory and file name along with which which test pattern you are used in your notebook.
Put some other object (like a pencil or your finger) on the test pattern and take a second image (we will use this second image in the next lab). Try to not block the bars. Save it with a different file name in the same directory and record the file name. For the object name, use something like 'Test pattern with pencil'.
Swap your lens with another lens with a different focal length. Focus the new lens and take another pair of images as described above.

Now you are ready to use DS9 to analyze your images. You main goal today is to determine the "plate scale" of the lens/camera, i.e. the angular size of the camera pixels.

Load one of your images into DS9. Do this by starting DS9 and then use File/Open. Use Zoom/Zoom to Fit Frame to see the whole image. Use Color/Grey to make the image black and white. You might need to play with the options under the "Scale" menu to adjust the contrast. You can adjust the color scale (mapping of pixel values to colors shown on the bottom of the DS9 display) by using the right mouse button. To move the image around, use Edit/Pan then click on the position you want centered in the display. Click on Edit/Pointer when done. (Clicking on the center mouse button should do the same, but doesn't work on all Windows machines.) When you have the image displayed to your satisfaction, print it out and put it in your notebook. Note which lens was used to obtain the image.
Measure the distance in pixels between the two bars with the largest separation visible in your image. You can do this by using the on-screen display of the cursor position (in "physical" units) or by drawing a line between the two bars using the Region/Shape/Line and then double clicking on the line to bring up a box with its parameters, including length. Record the distance in pixels and the numbers of bars. It is convenient to do this by writing on the print out in your lab notebook.
Calculate the angular separation of the two bars using the physical separation between the bars (remember if you see 5 bars separated by 1 mm, the distance between the two ends is 4 mm) and the distance between the test pattern and the lens. Note that you'll need to do a little geometry (recall that tan(θ) = θ for small θ when θ is in radians) or use the small angle formula to figure out the angular size of the distance between the bars. Record your calculations and your results.
Determine the plate scale for this CCD with this lens. That is, what is the number of seconds of arc per pixel? Record your calculations and your results.
Repeat this procedure for an image obtained using the other lens (with a different focal length). Record your calculations and results.
Compare your result with the formula in section 4.1 of the textbook. Comment on the degree of agreement or disagreement. Write all of this down.

You're now done with the lab. As time permits, you might want to explore other features of the camera, such as use of the internal filter wheel. Feel free to replace the test pattern with whatever you prefer and take as many images as you like. You might try taking images of colored objects with different filters.