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and the diffuser; therefore, build the lamp section to allow you to place the lamp immediately behind the diffusing screen or considerably further back. Drill a hole for an on-off switch, and install clamps to hold the battery firmly in place.

The LED Diffuser. This screen is a piece of opal glass or milk plastic roughly two inches on a side. Opal glass and milk plastic are translucent materials that completely scatter light. Both materials look like milk—smooth and uniformly white. Mount the diffuser on the LED side of the LED bulkhead.

The Slide Holder. This is mounted on the aperture bulkhead so that you can insert slides from the top of the box. Make the holder from heavy cardboard or Mi-inch plywood. Paint the parts black to minimize light leakage.

The Aperture Slide. The purpose of the aperture slide is to pass the right amount of light to make well-exposed flat-frames over the range of exposure times needed. A complete set of aperture slides provides a 1:1000 range of intensity, but you need only one (the one that's the right size) to test your digital sensor.

If you want to simplify the design by making only one slide, follow the plan below but make one slide instead of eleven. Aperture A5 is right in the middle of the brightness range, and is probably the one you will need.

To make a complete set of slides, make 11 slides 2 inches wide by 6 inches long from heavy cardboard or light plywood. One inch from the bottom end of each slide, chisel a recess half the thickness of the material and VA inches square; and in the center of the recess, drill a hole 1 inch in diameter. (See Figure 8.5.) Number the slides from A1 to A11. They carry a set of holes drilled through thin aluminum or brass shim 1 XA inches on a side. Drill the holes as shown in Table 8.2, and glue each piece of thin metal into the recess in the slide. Each slide passes half the light of the next larger one in the series.

Figure 8.5 A full set of eleven L3S apertures allows you to set the light level in a completely repeatable way over a range in brightness of 1000 to 1, but you only need to make as many as you need to get good test images. The apertures are holes drilled in thin aluminum or brass shim stock.

The Midsection. Between the LED bulkhead and the CCD bulkhead, insert a cardboard or thin plywood baffle to reduce scattered light. Cut a hole 2 inches in diameter in the center of the baffle and install it in the box. The LED bulkhead and the CCD bulkhead will be approximately 10 inches apart.

The CCD Bulkhead. Cut a 2!/2-inch hole in the bulkhead. On the side of the CCD bulkhead that faces the camera, mount a second diffuser 3 inches square made of opal glass or milk plastic. The fully illuminated area of the diffuser should be at least 2lA inches in diameter.

The CCD Holder. Provide a solid mounting for your camera on the end of the L3S box. It should be held well enough that it will not wiggle or shift if you need to move the L3S. If your CCD camera accepts 2-inch O.D. accessories, for example, the mounting can be as simple as a 2-inch diameter hole lined with black felt. Try to keep the separation between the diffusing screen and the CCD as small as practical to insure even and shadow-free illumination of the chip. Line the hole with black felt so that when you insert the camera tube into the opening, you have alight-tight seal.

Assemble the L3S box with glue and wood screws. Carefully seal around the aperture bulkhead and slide holders to prevent light leaks. Paint the interior flat black. Using bright light, check that no light sneaks around the edges of the aper ture slide. When you turn on the LED with the largest aperture in place, you should just be able to see a dim glow on the second diffusing screen.

It is not necessary to adhere rigidly to these dimensions and materials. Any method you can work out for providing an adjustable, low-level, diffuse illumination will serve for advanced CCD testing.

8.3.2 Setting the Low-Level Light Source for Use

Setting the brightness of a newly built L3S usually involves making trial-and-error adjustments of the lamp placement. The goal is to place the lamp behind the first diffusing screen at a distance such that using a 100-second integration with the aperture A5 (the one in the middle of the brightness range), the CCD generates a signal of about one-fourth of the full-well capacity—about 1,000 ADUs for a 12-bit camera and 16,000 ADUs in a 16-bit camera, above the bias level. These settings mean that the L3S will be able to generate the brightest and faintest levels you will need in testing.

Install the CCD in the L3S. Swaddle the end of the box and the CCD in black cloth, then turn on the camera's electronics and cooling system and allow it to reach equilibrium—at least 15 minutes. With air-cooled CCD cameras, be sure to allow the free circulation of air around the cooling fins.

Set the software to the readout mode that you most often use to make celestial images. With aperture A5 in place and the LED turned off, make a dark-frame integration of 100 seconds, and save it as a test dark frame. Turn on the LED and make a second integration of 100 seconds; save it as a test bright frame.

Subtract the test dark image from the test bright image, then measure the average pixel value generated by the light from the L3S. Repeat the process of making and measuring images, altering the position of the lamp as necessary, until with the aperture in place, the average pixel value in a 100-second integration is 1000 ± 200 ADUs for a 12-bit camera or 16,000 ± 3,000 ADUs for a 16-bit model. Once you have set the output, you are ready to begin testing.

If you cannot get the right brightness using aperture A5, then try larger and smaller apertures until you find the one that gives you the right brightness.

• Tip: In AIP4Win, load the test bright frame and the test dark frame. Use the Multi-Image tool to subtract the test dark frame from the test bright image. Use the Pixel Tool to measure the average pixel value of a region near the center of the frame.

8.3.3 How to Make Advanced CCD Test Images

Make test images after the CCD has been running long enough to reach thermal equilibrium. If in doubt, allow the camera to run for at least 60 minutes in the L3S. The purpose of waiting this long is to minimize temperature changes during the test. For your initial runs, use the readout mode that you most often use to make celestial images. You may later wish to test your camera in readout modes that you use less often.

The full set of test images includes multiple bias frames, multiple skim frames, multiple flat-field frames at different integration times, and multiple dark frames. All of the images should be taken in a single session. Work in an area with very low ambient light levels, and cover the L3S and CCD camera with black cloth to prevent any light leaks. Give yourself two to three hours to take the complete set of images.

8.3.3.1 Make the Bias Frame Set

Bias frames are integrations made with no light falling on the CCD for the shortest integration time your camera's software allows. The only contributions to a bias frame should be the zero-point offset (i.e., the image bias), readout noise from the CCD's amplifier, and any electronic interference that is present. Make the bias frames as follows:

1. turn off the LED so no light falls on the CCD;

2. set the shortest integration your CCD allows;

3. shoot nine bias frames;

4. save the bias frames as BIAS001 through BIAS009.

8.3.3.2 Make the Skim Frame Set

To detect charge traps in the CCD, you need flat frames made at a very low light level. The total charge that accumulates during integration should be less than 300 electrons, and the integration should be sufficiently short that hot pixels do not exceed this value. If the L3S is properly set up using aperture A5, an integration time of 5 seconds with aperture A9 (!/i6 as much light) should produce the correct number of signal electrons.

2. shoot nine skim frames;

3. save the skim frames as SKIML001 through SKIML009;

4. turn off the LED;

5. shoot nine skim dark frames;

6. save the skim darks at SKIMD001 through SKIMD009.

Note: since you know the conversion factor between pixel value measured in ADUs and the number of electrons, divide 300 electrons by the conversion factor to obtain the signal level in ADUs.

8.3.3.3 Make the Flat Frame Set

To determine the conversion factor (electrons per ADU) and test the linearity of the CCD, make a series of flat frames of increasing exposure. As a check against the brightness of the LED changing during the test, make the flats in two sets, one with increasing integration times and a second set with decreasing integration times. Use aperture A5 to give the desired signal levels with integration times from 0.5 to 100 seconds.

For the increasing series, execute steps 1 through 5 below with integration times of 1, 3, 5, 9, 15, 25, 45, and 80 seconds. Name the flat frame integrations FLATxxA and FLATxxB, and name the flat darks FLATxxD, with "xx" designating the integration time. For example, call the group of 1-second integrations FLAT01A, FLAT01B, and FLAT01D.

As soon as you reach the last of the increasing series, begin the decreasing series. Execute steps 1 through 5 with integration times of 60, 30, 20, 12, 6,4,2, and 0.5 seconds. Use the same file naming system.

2. set the integration time to xx seconds;

3. take two flats;

4. save them as FLATxxA and FLATxxB;

5. turn off the LED;

6. take one dark frame;

7. save the dark frame as FLATxxD.

Remember that each set consists of three frames per integration time—two flats and one dark. It is easy to get confused when you are carrying out boring and repetitive operations. Work carefully and stay alert.

8.3.3.4 Make the Dark Frame Set

The final step is to make a set of dark frames with an integration time long enough to produce dark current in all pixels, but short enough that the hottest hot pixels do not saturate. An integration time of 500 seconds should satisfy these requirements.

1. Turn off the LED;

2. shoot three dark frames;

3. save the dark frames as DARK001, DARK002, and DARK003;

4. shoot a bias frame;

5. save the bias frame as DARKBIAS.

This completes the set of test images. You will have nine bias frames (BIAS001-BIAS009), nine skim frames (SKIML001-SKIML009), nine skim dark frames (SKIMD001-SKIMD009), 32 flats and 16 flat darks (FLATxxA, FLATxxB, FLATxxD, etc.), three long-exposure dark frames (DARK001-DARK003), and one dark bias (DARKBIAS).

8.3.4 Analyzing the Advanced Test Image Set

The most important thing to bear in mind as you analyze the test image sets is to look at them with an eye that is both analytical and quantitative. Pattern noise in the bias frame might look like a mountain range but have an amplitude of 0.05 ADU and therefore be insignificant, while a uniform doubling of the readout noise is devastating. Remember that your ultimate goal is making astronomical images.

Table 8.3 Sample Bias Frame Data

Frame

Mean PV

Minimum PV

Maximum PV

Standard Deviation

bias1

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