Image Noise Problem Cured

12/15/2004 - Flagstaff, Arizona

The Mimir team has cured an image noise problem after an intense investigation, allowing Mimir to proceed to being mounted onto the Perkins telescope and new observations to proceed. The noise problem seem to have started when the detector array control electronics were moved from their former location within the larger Mimir electronics box to outside the box to accomodate the new configuration Mimir has while on the Perkins telescope (see pictures of old and new Mimir telescope configurations here).

"Herringbone" noise with a strength some 20 times the normal readout noise covered the entire detector array, as shown in the large-scale and zoomed images below. This type of noise is caused by injection of a high-frequency, low-voltage signal usually resulting from a ground-loop condition. Believing this resulted from grounding problems on the detector array signal output lines, the Mimir team modified a large number of grounding circuits outside the Mimir cryostat. Unfortunately, if the problem was in the cryostat, little help could be found as the cryostat containing the Mimir optics and detector array is currently under hard vacuum and cryogenic cold conditions.

Herringbone Noise in image of spectroscopic slit. Click on the image to see the full sized picture.

Zoom of the herringbone noise in the center of the image above. Click on the image to see the full sized picture

After much effort, and not a little consternation, the ground lines (AGND) on the analog-to-digital converter boards in the array readout electronics were tied to the case of these readout electronics and the herringbone pattern was suppressed, leaving normal read noise images (see below).

Same slit image after grounding problems cured. Click on the image to see the full picture.

Zoom of same central region after herringbone noise gone. Click on the image to see the full picture.

The Mimir team next proceeded to cure the "Ghosting" problem (click here for that link).