The Demands on Image Acquisition

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Now that we have optimized our setup we enter the most enjoyable part: taking the images! Over the last 5 years one trend has emerged. Good images require much longer exposures than anyone at first thought necessary for CCD imaging. In the "early" days, which are only within the last 10 years, most image exposures were measured in tens of seconds and taken in groups (sequences) of maybe 10. Today, to obtain enough data to produce publishable-quality images, total exposure times can exceed 6 hours!

A topic of much discussion is the individual exposure time necessary to obtain good data, depending on whether you image from a suburban or dark sky. In general, the longer the exposure the better your image will be. However, in practice exposure times vary considerably, depending on the brightness of the target, the seeing, the size of bloomed stars, how much nighttime you have, the sky background and your mount's performance. Most highresolution deep-sky color images taken today have minimum total LRGB exposure times of 60:20:20:25 minutes, respectively, with 2- to 3-minute individual exposure times (see Figure 8.12). Maximums can go up to staggering exposures of 240:60:60:80 minutes, respectively, with 10-minute individual exposure times.

Two hardware features greatly assist in acquisition of "keeper" images in highresolution work. One is the off-axis guider, which is a separate CCD camera that "looks" down the same optical path as the main imaging camera and tracks the relative motion errors of both the optical system and mount. This is much better than using a guide scope, which, due to differential flexure between the guide scope and main imaging scope, makes it almost impossible to obtain consistently round star images with long exposures. SBIG manufactures CCD cameras with a guider chip built in to the main camera body, which overcomes the differential flexure problem. This has many merits but, because the guide camera images through the color filters, light to the guiding chip is much reduced. This is especially a problem when narrowband filters are in use. At subarcsecond/pixel scales it is much harder to find suitable guide stars; therefore, many imagers are beginning to use a separate off-axis guide camera in front of the filter wheel. The most important benefit is that the guide star is unattenuated no matter what filter is in use. In addition, there is a bigger area to "sweep" for a guide star when using a free-rotating device. The down side is the need for more back focal length to insert the off-axis pick-off tube. Maintaining focus at the autoguider with nonparfocal filters can also be a problem, but some scopes even have a remote autoguider focus to remedy this.

Figure 8.12. Emission Nebula, NGC6820. (Ha)(Ha/R,G,B) image of 96:30:30:30 minutes total exposures, respectively (3-minute individual exposures on the RGB, 5-minute for Ha) using a 3nm Ha Custom Scientific Ha filter for the luminance frames and Custom Scientific RGB filters. The 96-minute Ha image was combined 50/50 with the R frame in Maxim DL. Image calibration and color combination using MAXIM DL (RC Console for Sigma Combine and pixel cleanup within Maxim DL), image registration using Registar, AIP for Lucy Richardson deconvolution on the luminance, luminance layering in Photoshop for final color and star shaping processing. Equipment: RCOS 20-inch f/8 RC and Finger Lakes IMG6303E CCD camera with all images acquired in 2 x 2 bin mode for an image scale of .92 arcsecond/pixel in 3.8 arcsecond seeing and magnitude 4.9 suburban/rural skies.

Figure 8.12. Emission Nebula, NGC6820. (Ha)(Ha/R,G,B) image of 96:30:30:30 minutes total exposures, respectively (3-minute individual exposures on the RGB, 5-minute for Ha) using a 3nm Ha Custom Scientific Ha filter for the luminance frames and Custom Scientific RGB filters. The 96-minute Ha image was combined 50/50 with the R frame in Maxim DL. Image calibration and color combination using MAXIM DL (RC Console for Sigma Combine and pixel cleanup within Maxim DL), image registration using Registar, AIP for Lucy Richardson deconvolution on the luminance, luminance layering in Photoshop for final color and star shaping processing. Equipment: RCOS 20-inch f/8 RC and Finger Lakes IMG6303E CCD camera with all images acquired in 2 x 2 bin mode for an image scale of .92 arcsecond/pixel in 3.8 arcsecond seeing and magnitude 4.9 suburban/rural skies.

Another useful tool is a high-speed tip/tilt mirror ahead of the main imaging camera to eliminate the short time scale disturbances to the image. These can be caused by seeing, wind buffet or telescope drive anomalies. This device counteracts object wander and produces a stationary image on the CCD. These devices do not discriminate between seeing effects, mechanical bumps or hiccups in the drive so the net result is much tighter star shapes. SBIG manufactures a device called the AO7, which does this job, and it is an important tool for advanced highresolution imaging.

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