Telescope Reviews: LCROSS Project

LCROSS impact imaging - banked cooled CCD imaging strategy

I have suggested that the best imaging strategy for the LCROSS impact is lucky imaging using fast subsec captures with mid- to high- range planetary cameras (e.g. ImageSource and Luminera). This strategy seeks to capture at 15 fps and get maybe 8 to 10 crisp images across the 40 second key duration of the impact event. Lucky imaging involves taking many images at sub-second intervals to capture about 8 to 10 crisp images during still air moments in atmospheric turbulence.

But cooled DSO CCD cameras also are widely dispersed in the amateur community. In a discussion with a New Mexico amateur, Dave Dockery, he suggests that traditional cooled CCDs have a performance advantage that favors their use - a deeper dynamic range.

This note summarizes my thinking from that discussion and concludes that clubs with many cooled CCD imagers and few planetary cameras may want to try a banked cooled CCD imaging strategy.

An SBIG10 has a minimum readout time of about 4 seconds. Over 40 seconds, it would collect about 8 to 10 images. This is inconsistent with a lucky imaging strategy that relies on subsecond exposure capture rates to increase the probability of capturing crisp images in still air. While CCDs with long-read out times can be used, the probability of have a seeing distorted image increases. In contrast, a low-end and lower resolution, uncooled Meade DSI can read out at essentially video capture frame speeds.

An planetary camera can capture AVI frames quickly, e.g. - 30 to 15 frames per second - but at a lower dynamic range. The dynamic range of an astronomy camera refers to the signal strength of its lowest detectable signal as compared to the signal strength of its highest detectable signal. Dynamic range can be expressed using the decibal scale. The decibal scale is familar to most people with respect to terresterial noises. The dynamic range of a low-end 99 USD LPI camera might be 16db. The dynamic range of a high-end CCD, with a low-readout speed, might have a dynamic range of 80db (Apogee Instruments (CCD) Alta U32 CCD Camera). Specialty low light video cameras will take AVI frames quickly, but also have a higher db range. Modern uncooled lunar imaging cameras (e.g. Image Source) will capture fast frames (15 fps) and have a db maximum of 36db.

What this means for practical LCROSS plume imaging is that if you calibrate your exposure time so the lunar mountain M1 is not overexposed but you use enough gain so that the shadowed region of west Cabeus is not pegged below 0 pixel values, an LPI might not have enough dynamic range to pick up an unexpectedly fainter plume, although it will capture many crisp frames. Conversely, a cooled CCD may see an unexpectedly fainter plume due to its higher dynamic range, but since it captures frames at only 1 for every four seconds, the brightest part of the impact plume may begin and end between the camera's capture frames and not be seen at all.

One cooperative imaging strategy for local clubs that have several interested cooled CCD owners is banked imaging. In this strategy, CCD owners could self-organize themselves and set up in observing banks of three or four CCD imagers. The CCD imagers would need to determine the read-out and auto-repeat time of their cameras. On a voice "go" signal, each imager would start their CCD cameras with a time offset of let's say half-second intervals.

With a CCD banked strategy, three or four CCD imagers would increase their chances that would of their cameras would capture a "deep dynamic" range image of the shadowed portion of west Cabeus at the critical moment. Everyone may not capture an image showing a plume, but at least one of you might.

The final selection of a plume imaging strategy is a camera user judgment call that should be made considering the inherent camera response characteristic tradeoffs discussed above. There is not necessarily a right or best answer because there is uncertainty as to exactly how bright the plume will get.

Clear Skies - Kurt