CCDs and Plate Scale

Astronomical Laboratory ASTR:4850, Spring 2018
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.
Orion StarShoot G3 Deep Space Imaging Camera
Sony ICX419ALL Monochrome CCD

Equipment

Orion StarShoot G3 Deep Space Imaging Camera
Computer with Orion Camera Studio and DS9 for image analysis
Telescope
Graph paper used as a test pattern
Meter stick or laser range finder

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 (as is likely), 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 equipment that you will use in this project is listed above. Except for the meter sticks, we have enough equipment for six setups, so each group should have a full set of equipment. The telescopes are not all identical. This write up assumes that the telescope used is a compact refractor with 80 mm aperture and 400 mm focal length. Depending on the availability of telescopes, you may need to use a different telescope. If so, you may need to adjust the distance at which you place the graph paper/test pattern and the focal length used in your calculations.

You will use a StarShoot G3 Deep Space Monochrome Imaging Camera made by Orion Telescopes and Binoculars for this lab. One of the links above under "Reading" leads to the manual for the camera. You can also find it on the Orion web site (https://www.telescope.com). Note that we use the monochrome version of the camera.

Another link is the data sheet for the CCD (charge coupled device) image sensor contained inside the camera. A good instrumentalist enjoys reading sensor data sheets (really). Some highlights of the ICX419ALL data sheet are the number of pixels, the physical size of the pixels, the relative response as a function of wavelength, and the discussion of "cruel condition".

The software we will use to control the camera and retrieve images from it is called Orion Camera Studio. We also use ASCOM drivers that interface to the camera. ASCOM stands for the Astronomy Common Object Model. Camera Studio and the ASCOM drivers for the StarShoot should be installed on the computers in Room 655 Van Allen Hall. Download and look over the Camera Studio manual; you will need to know how to run Camera Studio to do this lab and pretty much every other lab. 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. You can either use the computer on your bench or load the software onto your own laptop. Camera Studio only runs on Windows. DS9 and Python, which will be using in later labs, runs on Windows, Macs, and Linux. All the software that we use is publicly available and there are links on the class web page.

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.

Get the camera and install the 1.25" nosepiece adapter. While you are doing this, take a moment to enjoy the pleasure of seeing the CCD chip itself which is a rectangular device, several millimeters on a side. The CCD is protected by a thin piece of glass. Indicate in your lab notebook if you were gratified to see this amazing marvel of technology.
The camera has connectors for a USB cable, a 12 V power supply, and an autoguider. The autoguider connector looks like a phone jack and we won't be using it for this lab. Connect a USB cable to the camera and then to the computer. Plug in the +12V power supply and then plug it into the camera.
Start up Orion Camera Studio on the computer. I have no idea what the icon for Camera Studio is supposed to be and am curious about others thoughts on the topic. This would be a good time to read section 3 of the StarShoot manual.
Go to the "Camera Control" tab and click on connect. Note that you can pull the tab out of the main window if you like. The CCD temperature should start updating. Since you don't have cooling turned on, it should read about room temperature (~22 C). We don't really need the cooler for this lab, but let's practice using it anyway so it is in your camera turn on procedure in your lab notebook. Click on "Cooler On". The Power should jump to 100%. If it doesn't check your +12V supply and connection. Let the cooler run a few minutes and record the lowest temperature that you read. Then set the target temperature 1 or 2C above that and click the "Set" button. Watch the temperature for a while and see how if fluctuates. Record your findings.
Now we'll try capturing an image. Since the camera isn't yet attached to a telescope or a lens, this part is really about learning how to run the software and adjust the exposure. Check on the Camera Control tab that the Offset and Gain are at their default values of 127 and 185, respectively. Now move to the "Capture" tab. Click on the "Single" button. The Status box should read "Exposing", then "Downloading", then go back to "Idle". You should see a display in the image window and also in the "Histogram" window. In the "Histogram" window, select "Low" on the drop down box on the right. Look at the values for "Black" and "White". If they are above 50,000 or so, it means that the CCD is saturating. (How many ) In this case, decrease the exposure time. The minimum exposure time is 0.010 seconds. Decrease the exposure time until the value for "White" in the "Histogram" window is below 45,000 indicating that the CCD is not saturating. Each time you adjust the exposure, you need to click Single again. If adjust the exposure while taking multiple images by clicking "Loop", the exposure won't be updated until you stop the loop. The other values in the "Capture" tab should be at their default values of Type=Light, Bin=1x1, Subframe, Auto dark, and New buffer should not be checked. Depending on the brightness of the room, you may need to partially cover the camera aperture; cloth, sheets of paper, or your hand work for this.
Once you get the exposure adjusted nicely, turn off cooling by pressing "Cooler Off", then quit out of Camera Studio and disconnect the camera from the USB and +12V cables.
You may need to adjust the exposure again after you point the telescope at the test pattern. However, now you know the procedure and how to recognize saturated CCD readings.

We now will attach the camera to the telescope and set up to take images of the test pattern.

Your telescope should be mounted on a tripod. If not, mount your telescope on a tripod or other suitable mount.
If you are using one of the compact refractors, you will need to attach a diagonal or extension tube in order to be able to focus. The reflectors don't need this. In the following, we'll assume you are using a diagonal if you are using a reflector read 'diagonal' as 'eye-piece tube'. Slide the diagonal into the telescope and tighten the set screws holding it in place.
Our test pattern is a piece of graph paper. Measure the spacing between lines on the graph paper and record the value in your notebook.
You will want to setup the test pattern so that it is perpendicular to the line of sight from the telescope to the test pattern. Usually, it is easiest to tape the test pattern to a wall and then move the telescope and tripod. The test pattern needs to be far enough away that you can focus on it. The compact refractors can focus down to about 4 or 5 meters. The minimum focusing distance for other telescopes may be longer and you may have to move the hallway. If you have one of the other telescopes, discuss the minimum focusing distance with the TA.
Put an eyepiece into your telescope and focus on the test pattern. Make sure that you can get a sharp image.
Now we'll attach the camera. Check that the nosepiece adapter is tightly screwed onto the camera. Take out the eyepiece and slide the camera into the diagonal. Tighten the set screw that holds the camera in place. Check that the set screw is tight. Reconnect the USB and +12V cables.
Start up Camera Studio (again). Turn on cooling and adjust the exposure as before.
Once you have a good exposure (White should be at least 15,000 and not more than 45,000) try focusing the telescope. In the capture window click on "Loop" so that you get a continual stream of images.
The blue and red markers in the Histogram window adjust the gray scaling in the image window. Play with them a bit. You may need to adjust them as you focus. You may even need to adjust the exposure time.
Our tripods are rather wobbly so they will shake if you leave you hand(s) on the focuser and for a fraction of a second after you take your hand off. When focusing, it is easy to confuse blurring due to telescope motion with blurring due to poor focus. So, each time you adjust the focus, wait for a few seconds for the vibrations to damp out.
Eventually, you will get a nicely focused image. Note that you can play with the red and blue markers in the histogram window to enhance the contrast in the image window. If focusing just doesn't work for you, get help from an instructor.
Make a sketch of the apparatus as you have set it up in your notebook. Include the distance between the telescope and the test pattern; measure from the front of the lens which is not the same as the front of the telescope. On the reflectors, measure to the mirror, which is quite far from the front of the telescope.

We now will take some images of the test pattern.

Verify that you have a nice sharp focus and adjust your exposure time as needed to get high, but unsaturated counts.
In order to make the analysis easier, we want to get the rows and columns of the CCD lined up with the lines of the graph paper. We can do this by rotating the camera in the diagonal. To provide a reference of the row/column directions, you can either put a crosshair in the Camera Studio image window by clicking on View/Show Crosshairs or left clicking in the image window and then drawing a rectangle (which will be in red). Do one or both of these then go into Loop Capture mode.
Carefully loosen the set screw holding the camera in place and then gently rotate the camera. Be careful that the camera does not fall off the telescope. Note that you will likely need to wait for vibrations to damp out each time you rotate the camera. Try to get the camera aligned with the lines on the graph paper as accurately as you can.
Once you are aligned, focused, and have good exposure, stop the Loop Capture mode and do a Single capture. Then save the file using File/Save As. Use a descriptive name and save as a "FIT files" (even though it is odd phrasing). FITS or Flexible Image Transport System is a standard file format used in astronomy for everything from optical CCD images to X-ray event lists. We'll do a lot with FITS files in this course.
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. Save the image with a different file name in the same directory and record the file name.

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 Fit 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/None 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.
We do the horizontal plate scale first. Measure the distance in pixels between the vertical lines with the largest separation visible in your image. (Why should you use lines with the largest separation? Record your thoughts in your lab notebook.) We'll do this by drawing a line between the two bars using DS9 in order to get some practice with DS9. To draw a line do Edit/Region to get into Region mode, then do Region/Shape/Line. Now move the cursor to where you want the line to start (an intersection between horizontal and vertical lines near the left edge of the image), click and hold, move the cursor to where you want the line to end (an intersection between horizontal and vertical lines at the same vertical position but near the right edge of the image), then release. After you draw the line, you can click on the end points and move them. The magnifier in DS9 is a good way to make precise placement. Double clicking on the line will bring up a box with its parameters, including length and angle. The angle should be close to zero (if you got the CCD aligned with the test pattern). Record the distance in pixels and the physical distance on the graph paper. It is convenient to do this by writing on the print out in your lab notebook.
Calculate the angular separation of the ends of your line using the physical separation and the distance between the test pattern and the lens. Record your calculations and your results.
Determine the horizontal plate scale for this CCD with this telescope. The plate scale is the number of seconds of arc per pixel. Record your calculations and your results. Estimate the accuracy of your result; what are the largest sources of uncertainty and the magnitude of their effect on your results?
Repeat the process using a vertical line and calculate the vertical plate scale. Record your calculations and results. Are the horizontal and vertical plate scales the same?
Compare your result with the formula in section 4.1 of the textbook. Comment on the degree of agreement or disagreement. You might try putting the test pattern at a different distance and doing the measurement again.
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 thermoelectric cooler (TEC). Feel free to replace the test pattern with whatever you prefer and take as many images as you like. A good instrumentalist enjoys spending time with her instruments.