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There are several things you should be aware of before you begin reducing your NIRI data. The NIRI detector has several problems/features and all frames should be carefully examined before including them in the final reduction, noting that some of these effects can only been seen after sky subtraction.
The first exposure of every new sequence will show poor background subtraction and should be rejected (see detector array for an explanation). These bad first images ARE included in the distributed data, so care should be taken to review and reject them from all sequences (science, flats and darks) as necessary.
Some NIRI frames contain an electronic pattern that manifests itself as vertical striping with a period of eight columns. The intensity of the striping is usually different in each quadrant and may not be present in all quadrants. The striping, which is most noticeable in the low read noise mode, is much less severe and less frequent than in the past, but should still be removed before including the affected frames in your final data set. The recommended utility is a stand-alone python routine called cleanir which almost perfectly removes the striping.
The saturation level in a single coadd and low-noise read pair is about 16,000 ADU (although the exact well depth varies with position on the detector and intensity of the incident radiation), and saturated pixels should be flagged by NPREPARE in the data quality plane. Because of the way the array is operated (reset-read-read), progressively brighter sources will approach saturation and then begin to get FAINTER as the array begins to saturate in the time between the reset and the first read. Saturated stars often show a central hole, sometimes even becoming negative in the core. Such heavily saturated objects may not be flagged by NPREPARE if they are below the set saturation threshold. Because the array reads along rows from the corners to the center, this effect may be different at different places in the array. In particular, when the whole array is saturated, values in the center will pass the saturation value and start approaching zero again. A vertical gradient in the background with saturation near the top and bottom edges and lower levels in the middle indicates a highly saturated array!
The NIRI detector response is a function of exposure time, radiation intensity, and the number of photons collected. The most noticeable non-linearity is seen at short exposure times (<1s) and near full well (>10k ADU). We are currently working on a stand-alone python routine to correct for this problem. More information about obtaining and using this script may be found on the Detector Linearization web page.
Taking spectra of bright stars results in a reflection spectrum which varies in position as the primary star is offset along the slit. Typically the bright star is the target (i.e. a Telluric standard) and this reflection may be ignored. However, if the bright star is not the target, it's reflection may contaminate the target spectrum. It is therefore recommended to avoid position angles which put unnecessary bright stars in the slit.
Ghosts with Altair at wavelengths > 3 microns
Ghosts (reflections) of bright stars are visible when using NIRI with Altair at wavelengths longer than 3 microns. The ghosts are present with both the f/32 and f/14 cameras with separations of dx=+0.09" dy=-2.43" for the first ghost, and dx=+0.18" dy=-4.86" for the second ghost (they are below and to the right of the primary). The ghosts do not rotate with position angle and they have the same spatial profile as the primary star. The amplitudes of the ghosts are given in the following table:
|Filter||Wavelength||1st Ghost||2nd Ghost|
|Kprime||2.20 um||< 0.2%||< 0.2%|
|H2O ice||3.05 um||15.6%||2.2%|
Pixels in columns 534 and 535 in the bottom right quadrant (rows 1 to 512) always have very similar values. Stars which fall on these two columns will therefore appear slightly elongated or boxy.
Bad pixels in the 256 Subarray
Additional unusable pixels appear when using the 256x256 subarray. These pixels are the last 8 pixels read out in each quadrant, yielding a bad block of 32 pixels (16 pixels wide x 2 pixels high) located exactly in the center of the detector. The pixels appear hot with random values of 1e5 - 1e6 which will not be removed via sky subtraction, and therefore the pixels must be added to the bad pixel mask during data reduction.
Vignetting by the peripheral wavefront sensors
The peripheral wavefront sensors are used for tip-tilt correction and guiding with NIRI. The peripheral wavefront sensor probe arms will vignette the f/6 field of view when the separation from the science target is less than approximately 5.5 arcmin. Sometimes, however, the only or best available guide stars are near this limit and some positions in the dither pattern may be affected. Care must be taken to exclude vignetted images when constructing the sky image if the other unvignetted images are not to be corrupted during sky subtraction.