Ipcs Data Acquisition Path

Contrary to classical imaging, photon counting needs to be fast to avoid the stacking of events as much as possible. Two photons (or more) falling on the same pixel during a single frame are usually counted as a single event, which leads directly to a severe loss in QE. The mean fraction of missed photons, M, can be evaluated assuming a Poissonian process for the photon emission with Eq. 1:

where X is the mean number of photons expected during the resolved period of the detector. To counteract that phenomenon it is necessary to use a fast CCD as shown in Fig.1.

Illumination (ph/s/pixel)

Figure 1. Photoresponse of an IPCS pixel with respect to illumination.

Illumination (ph/s/pixel)

Figure 1. Photoresponse of an IPCS pixel with respect to illumination.

This need for speed leads to naturally high data rates. The previous camera we used was a 1K*1K, 40 images per second (fps) CCD. Sampled at 8 bits per pixel this sustains a flat 40MB per second datarate. Moving this amount of data from the telescope to the host computer was not completely trivial as LVDS signaling only allows for 5 m cables with our data rate. This problem was solved by designing a point-to-point dedicated 1.5 Gbits/s transmission over optic fibers. Signal from the camera was transferred over the fiber link, then back into LVDS before acquisition by the host computer using a DSP-based frame grabber.

The new system uses a 1K*1K camera at 60 fps (operation at 512^512, 100 fps is also possible) CameraLink standard. As the connectivity changed, this led to a redesign of the linking between camera and host, as well as a 50% increase in data transmission rates.

In the last couple of years we have seen gigabit ethernet over twisted pair become more and more mainstream, up to the point where it is now included in almost every motherboard on the market. With data rates up to 100 MB/s, Gb Ethernet is a viable solution to carry image data up to about 80 MB/s (to take network overhead into account), provided the data moves on a dedicated line that has no other traffic.

We replaced the dedicated point-to-point Fiberoptic link by a CameraLink / GbEth. translator that connects to an ubiquitous Cat. 5e or 6 twisted pair cable, allowing for very easy maintenance and replacement of failing parts from anywhere in the world.

Image acquisition in itself is also simplified. Whereas previous systems had to use frame grabbers to obtain image data, the new one simply uses the network adapter from the motherboard, making again for a much easier to replace acquisition computer should the need arise. It even allows for operation from a laptop computer.

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