OSIRIS Hieroglyph The OSIRIS User's Manual
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Known OSIRIS Problems (or "Features")

[Scales of Judgement]
This page will list known problems with the instrument and (hopefully) ways to fix/recover from these problems. Some potential problems may also be listed in the main OSIRIS User's Manual.


Darren DePoy (depoy@astronomy.ohio-state.edu)
Bob Blum (rblum@noao.edu)

Vignetting at the corners of the array (November 1998)

OSIRIS was not originally designed to illuminate a full 1024x1024 array (with 18.5 micron pixels) and thus this vignetting is unavoidable without a costly modification of the optical system. This is not planned for the forseeable future, unless you care to write us a big check.

Shading at the array quadrant boundaries (March 1999)

The HAWAII HgCdTe array in OSIRIS exhibits a time variable "shading," or ramping of higher than average bias level at the boundaries of the upper and lower quadrants. This feature generally subtracts cleanly to a high degree between frames (e.g. between succesive object and sky images), but may show residual counts sometimes (particularly between the first image and subsequent images in a series). The unilluminated portion of the OSIRIS images can be used to make a row-by-row correction for this. Ask the support astronomer for details.

20 July 2000. The new array installed in December 1999 does not exhibit the unstable shading of the previous array. Therefore, we recommand that no line-by line bias subtraction be done.

Bias discontinuity across quadrant boundary (April 1999)

The HAWAII HgCdTe array in OSIRIS exhibits a bias discontinuity across the quadrant boundaries (primarily across the boundary between the upper and lower quadrants) which appears to be flux (ADU/second) dependent. The source of the problem is not known. The effect results in a discontinuity in the count level which does not subtract out between similar images taken with and without illumination (e.g dome lights on - dome lights off) when the count rate on the array is above a threshold value of approximately 1000 ADU/second. The flux should be kept below about 1000 ADU/second in order to insure properly calibrated data. This is particularly important for flat fields. Check with the support astronomer for details/status.

20 July 2000. A new array was installed in OSIRIS in 1999 December. The bias discontinuity has been greatly diminished.

Fringing in spectroscopic modes (May 1999)

As with the original NICMOS III detector in OSIRIS, the new HAWAII array exhibits fringing in the spectroscopic modes. Spectra should be taken at multiple (7 to 10) positions along the slit to average out the effect of fringes.

Slit position changes on array (15 January 2001)

The apparrent y pixel position of the slit appears to change on the array when the telescope is moved. It is not clear if this is just a worse than normal flexure or if the acquistion mirror is not repeating its position well (something may be loose). Tests with the telescope in one position and repeated flips of the acquisition mirror indicate it reproduces its position to +/- 0.1 pixel. We will derive a new flexure map on the next engineering run. Until this problem is resolved, observers should pay more attention to the slit position (we recommend checking it on each new object). The full range of movement reported by observers is about 4 pixels.

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Updated: 1999 February 16 [osiris@astronomy.ohio-state.edu]