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February 21, 2018

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Out with the old, in with the new. JOST got a new camera that is brand-spanking-new!

Previously, JOST has been using a SBIG monochrome CCD camera, but the exposure times and readout times for it are so long that running any experiment on JOST took a long time: a defocused image needed an exposure time of 10 seconds, even when the laser power was cranked up to the maximum, and the readout and data saving took around 10 seconds too.

Apart from making the experiments very long, the CCD is also super sensitive to background light, and we always needed full darkness in the lab when running JOST, which rendered our main room unusable for other work.

Both of these issues were solved by replacing our detector with a monochrome CMOS camera from ZWO (ASI1600MM), which was put on the JOST translation rail (see Fig. 1). The detector size is only marginally smaller than the CCD, so we will still be able to test wavefront sensing and control on wide fields.

Figure 1: The new JOST imaging camera is a ZWO ASI1600MM CMOS camera (red, to the right). The shorter exposure and readout times will allow for faster experiments, while we also don’t need to darken the room anymore, the enclosure is enough to keep unwanted light out. Note how the pupil imaging lens on the flip mount is not in the beam in this image.

Since the CCD camera was considerably bigger, it was relatively easy to mount the new camera on the translation rail, as there was more than enough space. By running our autofocus python codes we were able to focus the setup quickly. Since HiCAT also uses this camera (among others), this was the opportunity to further unify the software between the two testbeds. Ultimately, JOST should be able to run by directly using the hicat python package, and by integrating the camera codes into the JOST software interface, we are now one step closer to this goal.

Figure 2: New ZWO camera on translation rail, back side. Note how the pupil imaging lens is down in the light beam in this image.

A significant difference between the SBIG and the ZWO camera are their pixel sizes. While we used pixels of 9 microns with the SBIG, the pixels of the ZWO are 3.8 microns small. This means that while we were just about Nyquist sampled with the CCD, we are now imaging highly oversampled PSFs. This results in us taking images that are twice as big (1024 x 1024 pixels instead of 512 x 512 pixels) in order to still capture the full PSF, but we are binning them back to 512 x 512 pixels to save computation time in the image processing and application of phase retrieval algorithms.

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