Using the SBIG ST-5 CCD Imaging System


This document is a simple discussion of the use of the ST-5 CCD camera system on the C14 telescope at Prices Fork Observatory. The ST-5 camera and operating computer are kept in/on the computer cart on the second floor of the observatory. The camera can be used in prime-focus on the C14 or C5 telescopes; in either case a focal reducer may also be used.
 

IMPORTANT WARNINGS!!!

To avoid damage to the CCD and/or the camera head assembly, it is essential to observe the following:

Imaging Recommendations

Because the CCD chip is quite small (3.20 x 2.40 mm) the area of sky covered is also small. However, a variety of sky coverages can be produced by using different telescope arrangements. The width of the field of view (in arcminutes) is

(180 x 60/ 3.14} (w/f)

where w is the width of the CCD chip (in mm) and f is the focal length of the telescope (in mm). The C14 has a long focal length (3910 mm) so the field of view covered by the chip is quite small. For the ST-5 attached to the C14 the field of view is 2.8 arcminutes by 2.1 arcminutes. However, the C5 has a smaller focal length. Furthermore, using the focal reducer can increase the field of view by a factor of 2.5, 3.3, or 4. (The focal reducer is the small lens mounted in a black tube which can be slide into the nose-piece of the CCD camera; it is kept in a small clear-plastic cylindrical case.  Putting both brass rings on the focal reducer tube will cause the field width to increase by a factor of 4; with no brass rings the focal reducer will increase the field width by a factor of 2.5 (or is it the other way around, try experimenting). Both the focal reducer and the VB photometry "front-end" for the CCD are kept in a small cardboard box stored in the computer cart.)

Exposure times to try are dictated by the brightness of your target. Ideally, you would "integrate'' for as long as possible to obtain the brightest possible image of your object as compared with the "noise'' level generated by the CCD itself ("integrate'' is the term used for "expose'' when discussing CCD imaging as opposed to photography). However, very long integrations will be "overexposed'' yielding junk. An "overexposure'' is possible because each CCD pixel can hold only so many electrons (produced by the incident photons) before they begin to literally overflow. Even without exposing the CCD to light from an object, the internally generated electrons (the so-called "dark current'' which makes most of the "noise'' referred to above) will eventually fill the pixels. Saturated pixels will have values of 16384, and surrounding pixels will be partially filled by the overflow; a vertical line of bright pixels stretching away from a star image is another sign of a near saturation situation for that star (this is called "blooming'').

Typically, your integration times will not be excessively long, so saturation of the object of interest will not occur. You may be imaging a faint nebula, for example, and its intensity will not saturate the CCD pixels during practical integration times (however, a bright star or stars in the field may saturate, but if they are not the prime objects of interest you can ignore that defect in the image). So, instead of avoiding saturation by keeping your integration from being too long, you will probably want to simply integrate long enough to obtain a usable image.

For bright objects (Moon and planets), simply detecting them in an image is not a problem. Even the shortest exposure times will collect enough light to make a good image. The Moon may even be overexposed at the shortest integration times unless you insert a filter into the optical path. Try very short exposures for these objects (0.01 to a second or so). The shortest exposures may eliminate some of the blurring due to "seeing''!
 

General Procedure

(For more info, see the "CCD Camera Operating Manual'' kept in the dome with the CCD/computer.)

Before you do anything with the CCD take the time now to make sure the C14, its finder, and telrad are all aligned with each other (and also with the C5, if you have attached it to the C14) --- this will help later when you are trying to find and center the object you wish to image.

Setting Up

Preliminaries

The computer runs DOS. Once the computer is up an running, while at the top most directory, enter the command mkdir bob, where bob, for example, is your name. This directory ("folder") will be where you store your images as you take them.

Start the CCDOPS program by changing directory into the CCDOPS directory (cd ccdops) and entering the command ccdops. The software should show that a communications link has been set up between the computer and the CCD camera. Select the File menu's Set Path command, and enter the name of the directory you just made (e.g., enter c:\bob). Use the Camera menu's Setup command to choose a setpoint temperature about 40 degrees Celsius below the ambient temperature (which is what the software initially shows as the temperature of the CCD). The CCD system will cool the camera to the temperature you have selected. You may need to wait a bit before that temperature is reached before doing any serious imaging.

Focus the Telescope

Focusing can be tedious. Always start out focusing on a bright object. Once you have achieved focus on a bright object, the focus will the be the same for all objects (as long as you don't change anything!). One trick, detailed in the manual, is to remove the CCD head from the telescope, and point the telescope at the Moon, a bright planet, or star, or distant street light. Then remove any eyepiece you might have used to point the telescope and hold a "diffusing screen'' (e.g., a piece of scotch tape) 0.05 inches behind the eyepiece holder tube; focus the telescope until you can see a focused image of the object centered on the diffusing screen. Now insert the CCD camera head; the image will be approximately focused on the CCD chip.

Even if you use the above trick, you will still need to make fine adjustments to the focus. Do this by selecting the Camera menu's Focus command. The Focus mode displays a succession of CCD images on the computer screen and the peak brightness value of the brightest object (hopefully a star) in the image. An Exposure time (integration time) of 1 to 3 seconds should work fine. Selecting a Frame size of Full will utilize the entire chip when displaying the succession of Focus images --- this can take up time. Selecting a Frame size of Planet enables you to selecte a small portion of the full frame (on a star) and only utilizes that portion of the chip --- making things faster. Adjust the telescope focus until the image focus is best (stars are smallest) and the peak brightness level is largest. Avoid saturated stars in attempting to find fine focus.

Find and Center the Object You Want to Image

Use the Focus mode with a Frame size of Dim mode to help find and center your object. Dim Focus mode utilizes the entire chip, but with lower resolution. You may need to remove the camera and insert an eyepiece to help center an object, but don't adjust the telescope focus during this process (instead slide the eyepiece in or out if necessary). If the telrad, finder, and C14 are all aligned with each other, finding your object may be easier.

Take an Image

Before you actually take your first mage, you might want to enter in the correct information about the telescope you are using for the image, so when this information is stored with the image you will have an accurate record of what you did. Select the Misc menu's Telescope Setup command. Enter the appropriate information.

Use the Camera menu's Grab command to take an image of your object. Try different values for Exposure time (the range of possibilities is from 0.01 seconds to 3600 seconds). Use Dark Frame set to None to record only the object image for now (or Dark Frame set to Yes to take the required dark frame just before the object image --- see below). Once the image is collected by the computer, use the Display menu's Image selection to display your image. You can also select Histogram to see a histogram of the image; then under Image (once again) set the levels for display based on the histogram (in order to see more of the details in your image).

A comment on exposure times: even deep-sky objects may only require images of 10s of seconds up to minutes at most. However, on long exposures you may find you will need to "guide the telescope'' --- that's an art utilizing the C5 riding piggyback on the C14, for example, along with the slow motions to keep a star centered in the C5 (and therefore the CCD, which would be attached to the C14). The necessity for guiding is apparent if stars appear as streaks. Making your guiding accurate is a difficult thing that requires practice. It is discussed in many books/articles on astronomical photography.

When you are satisfied you have a useful image, it's time to save it to the hard disk. Use the File menu's Save command, and specify Compressed (to save space). A standard name for the file might be something like "M51-01.ST5"' if you just made an image of M51 (perhaps number 1 of a potential 99 such images!), and if you have chosen the ST5 file format (you can also choose to save images in FITS format). Make sure you are saving the image file into the directory you set up for your images (e.g., C:\\bob), not in the CCDOPS directory!

As you save images keep a record of when you took them, and what they are (including the exposure time!) --- it's very easy to forget what image is what after the fact in the absence of records.

Correction Images

You will also need to record a "dark image'' and "flat field image'' for correcting your object image(s). The dark image is taken with the telescope covered (so no light gets in) and for a duration exactly equal to that of the object image. Do this by using the Camera menu's Grab command with Dark Frame set to Only --- it will only take a dark frame. Make sure the Exposure time is exactly that of the object image. You may need a few different dark images if you took object images with different exposure times: you need one dark image for each different exposure time. Actually, the program, when run in its default mode, will take a dark image just before each object image (prompting you to cover the telescope, etc.).

The flat field image is taken as if you were taking an object image, and using the same telescope setup (don't change anything, even focus!) as for the object image, but while pointing at a uniformly illuminated, smooth surface (e.g., the surface of a large, white cardboard sheet, illuminated by the ambient light inside the dome, or stronger, diffuse illumination). The flat field image should be fairly short but the surface should be bright enough that a histogram of the image shows the values are about half way toward saturation.

The dark and flat field images are used to properly process the object images. Consult the manual or articles on CCD imaging for further details on such processing.

Transfer Your Images to Diskette

When you are done for the evening, and have exited the CCDOPS program, you will need to transfer your saved images from your directory on the hard drive of the computer to a 3.5-inch diskette you brought with you for this purpose. (If you have lots of images you may need more than one diskette). Each image has a size of about 100 kbyte. Get into your directory (e.g., enter the DOS command cd \bob), and copy the image files to your diskette (e.g., copy *.* a:). The command dir a: will show you what files are on the diskette. When you are satisfied that you have all our images on diskette, delete them from the hard drive of the computer del c:\bob\*.*), and remove the directory you set up on the computer (rmdir c:\bob). Deleting files from the hard drive of the computer must be done very carefully. We don't want to accidently delete any of the files necessary to run the CCDOPS program!! Do not regard any images left on the computer as safe. Others may delete them to make room for their own images!

Further processing can be done anywhere. SBIG (Santa Barbara Instrument Group) has a web site providing downloadable copies of the imaging/processing software.

Packing Up the Equipment

When you are finished taking images and transferring them to your diskette(s):

A Word on Photometry

I suggest you start out using the CCD to take images. But you may eventually become interested in doing stellar photometry. This work can be of scientific value. If you are interested check out the website for the American Association of Variable Star Observers (AAVSO). The procedures for doing photometry with a CCD are somewhat involved, and I won't discuss them here. Try checking out How to Get Started in CCD Photometry.

I have supplied a photometry "front end" for the ST-5 nosepiece which holds a slide containing a V and B filter. The ST-5 nosepiece will slip into the front-end and a set screw will hold the camera in place; the front end is then slipped into the telescope eyepiece tube and secured with a set screw.
 

Useful ST-5 CCD Specs


Updated 23 November 1999