Calibration Details
Long-Slit Spectroscopy Calibrations
| MODS1 | MODS2 | |
|---|---|---|
| Biases | Telescope configuration: Telescope stationary. Dark chamber Notes: The prescan columns in full-frame, unbinned and binned, 8Kx3K, images are used by the modsCCDRed routines to remove the bias level, and these work well in removing the level differences that result from the 4 amplifier, 2 channel-per-amplifier, readout architecture. Therefore, for most purposes, 8Kx3K biases are not needed, however scripts are available to take them. There is no prescan or overecan for all other readout regions of interest (3Kx3K, 1Kx1K, 4Kx3K) and it is important to take biases in these modes. Because the bias levels vary, take the biases on the same night as the data for which they will be used. For most purposes, 10 biases provide sufficient s/n when median combined. |
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| Darks | Telescope configuration: Telescope stationary. Dark chamber.Notes:Darks are not typically taken. The measured dark current on the blue channel CCD is 0.5±0.2 e/pixel/hr, and the Red channel CCD is 0.4±0.1 e/pixel/hr. With this low dark current, taking explicit dark frames is not an indicated calibration step with MODS1/2. |
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| Spectral Slit Flats | Telescope configuration: Spectral Slit flats are taken with the chamber dark, the telescope at zenith, instrument in CALMODE, using internal calibration source.Notes: These are used by the modsIDL and other pipelines to map the trace in 2D spectra. These may also be used for fringe correction in the red, although the red channel detectors are deep-depletion devices and the amplitude of fringes is low, ~2%. Recommend acquiring 3 each of lamp flats for long-slits. Dual scripts are setup to take Spectral Slit Flats in both channels. Note that for prism mode because of the wavelength dependence in prisms there exists no red+blue combos of filter and lamps and filters that allow for the channels to be taken simultaneously. As a result, in this mode blue and red are taken successively, leaving one or other channel idle. |
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| Pixel To Pixel Slitless Flats | Telescope Configuration: Pixel to Pixel spectral flats are taken with the chamber dark, the telescope at zenith, instrument in CALMODE, using internal calibration source. Notes: These slitless lamp flats are used to remove pixel to pixel variations including gain difference between quadrants of the device. Prism slitless flats saturate and are not useful. It is not recommended to take slitless twilight flats in any mode. |
|
Twilight Sky Flats |
Telescope Configuration: Twilight flats are taken the telescope setup in TRACKMODE, at standard position at sunset/sunrise. A table of coordinates of blank sky fields for sky flats is available from the NOT web site. Notes: We do not recommend taking slitless twilight flats in any mode. Twilight slitless flats are not demonstrably better than internal lamp flats and are not worth the pain of chasing the twilight exposure times. |
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| Wavelength Calibration | Telescope configuration: The wavelength calibrations are taken with the telescope at zenith, instrument in CALMODE, using internal calibration source. Unlike flats the chamber lights do not matter. Standard wavelength calibration are taken through the 0.6-arcsec longslit mask for grating wavelength calibrations.Notes: It is recommended to take comparison lamps through the 0.6 arc second slit for grating mode and 0.3 arcsecond slit for prism mode. For the grating, the 0.6-arcsec will yield 80-100 reasonably bright and unblended lines to work with in the red, and maybe 50 in the blue. For the prism, the 0.3-arcsec slit is the smallest slit that will be imaged cleanly by the camera optics, and gives the least blended spectra. Only one exposure per lamp or lamp combination is needed. Wavelength calibrations are very stable, modulo a small flexure shift of order a few pixels at most which is readily measured and removed using night sky lines. Note that because the red and blue channels are mechanically different relative to gravity below the common focal plane, any amount of shift in wavelength due to absolute instrument flexure that is not corrected for by the IMCS is expected to be different between the red and blue channels. |
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| Spectrophotometric Standards | Telescope configuration: The LSx60x5 (60×5-arcsec spectrophotometric “fat slit”) mask is used to get full wavelength coverage with minimal losses due to atmospheric dispersion and seeing. |
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Multi-Slit Spectroscopy Calibrations
| MODS1 | MODS2 | |
|---|---|---|
| Biases | Telescope configuration: Telescope stationary. Dark chamber Notes: The prescan columns in full-frame, unbinned and binned, 8Kx3K, images are used by the modsCCDRed routines to remove the bias level, and these work well in removing the level differences that result from the 4 amplifier, 2 channel-per-amplifier, readout architecture. Therefore, for most purposes, 8Kx3K biases are not needed, however scripts are available to take them. There is no prescan or overecan for all other readout regions of interest (3Kx3K, 1Kx1K, 4Kx3K) and it is important to take biases in these modes. Because the bias levels vary, take the biases on the same night as the data for which they will be used. For most purposes, 10 biases provide sufficient s/n when median combined. |
|
| Darks | Telescope configuration: Telescope stationary. Dark chamber.Notes:Darks are not typically taken. The measured dark current on the blue channel CCD is 0.5±0.2 e/pixel/hr, and the Red channel CCD is 0.4±0.1 e/pixel/hr. With this low dark current, taking explicit dark frames is not an indicated calibration step with MODS1/2. |
|
| Spectral Slit Flats | Telescope configuration: Spectral Slit flats are taken with the chamber dark, the telescope at zenith, instrument in CALMODE, using internal calibration source.Notes: These are used by the modsIDL and other pipelines to map the trace in 2D spectra. These may also be used for fringe correction in the red, although the red channel detectors are deep-depletion devices and the amplitude of fringes is low, ~2%. Recommend acquiring 3 each of lamp flats for long-slits.Dual scripts are setup to take Spectral Slit Flats in both channels. Note that for prism mode because of the wavelength dependence in prisms there exists no red+blue combos of filter and lamps and filters that allow for the channels to be taken simultaneously. As a result, in this mode blue and red are taken successively, leaving one or other channel idle. In prism mode, saturation is an issue because of the large spectral pixels, must tune exposure times and use ND filters. |
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| Pixel To Pixel Slitless Flats | Telescope Configuration: Pixel to Pixel spectral flats are taken with the chamber dark, the telescope at zenith, instrument in CALMODE, using internal calibration source. Notes:These slitless lamp flats used to remove pixel to pixel variations. Prism slitless flats saturate and are not useful. While there has been marginal success taking twilight slitless flats, there was no obvious benefit over the internal lamp flats. It is therefore not recommended to take slitless twilight flats in any mode. |
|
Twilight Sky Flats |
Telescope Configuration: Twilight flats are taken the telescope setup in TRACKMODE, at standard position at sunset/sunrise. A table of coordinates of blank sky fields for sky flats is available from the Nordic Optical Telescope’s web site. Notes: We do not recommend taking slitless twilight flats in any mode. One or 2 twilight sky flats taken through the MOS mask can be useful for determining the Multi-Slit Mask Intercalibration Correction. Although, to first order (and perhaps better than that), we expect that slit-to-slit on a given mask the relative photometric calibration is a gray-shift in intensity because of small variations in slit width. Twilight slitless flats are not demonstrably better than internal lamp flats and are not worth the pain of chasing the twilight exposure times. |
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| Wavelength Calibration | Telescope configuration: The wavelength calibrations are taken with the telescope at zenith, instrument in CALMODE, using internal calibration source. Unlike flats the chamber lights do not matter. Wavelength calibration are taken with the user custom mask in place.Notes: Through the 0.6 arcsec slit in grating mode the lamps yield 80-100 unblended lines in red, and 50 in blue, and with the 0.3 arcsec slit for prism mode wavelength calibrations. Wider slits may yield some blending. Only one exposure per lamp or lamp combination is needed. Wavelength calibrations are very stable, modulo a small flexure shift of order a few pixels at most which is readily measured and removed using night sky lines. Note that because the red and blue channels are mechanically different relative to gravity below the common focal plane, any amount of shift in wavelength due to absolute instrument flexure that is not corrected for by the IMCS is expected to be different between the red and blue channels. |
|
| Spectrophotometric Standards | Telescope configuration: The LSx60x5 (60×5-arcsec spectrophotometric “fat slit”) mask is used to get full wavelength coverage with minimal losses due to atmospheric dispersion and seeing. |
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| Focal Plane Mask Image | Notes: Mask Images are taken regularly as part of instrument check out, but are not distributed as part of the standard calibration set. A focal plane image of a mask may be made available upon request. |
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Imaging Calibrations
| MODS1 | MODS2 | |
|---|---|---|
| Biases | Telescope configuration: Telescope stationary. Dark chamber.Notes: Overscan on subframe readouts, such as imaging mode 3Kx3K, does not function. For most purposes, 10 biases provide sufficient s/n when median combined. |
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| Darks | Telescope configuration: Telescope stationary. Dark chamber.Notes:Darks are not typically taken. The measured dark current on the blue channel CCD is 0.5±0.2 e/pixel/hr, and the Red channel CCD is 0.4±0.1 e/pixel/hr. With this low dark current, taking explicit dark frames is not an indicated calibration step with MODS1/2. |
|
| Pixel To Pixel Flats | Telescope Configuration: Pixel to Pixel spectral flats are taken with the chamber dark, the telescope at zenith, instrument in CALMODE, using internal calibration source.Notes: Internal lamp flats used to remove pixel to pixel variations including gain difference between quadrants of the device. Imaging flats taken with the “vflat” lamp work well, but have a 2% top-to-bottom gradient from the calibration illumination system in MODS1. This gradient is larger, ~20%, in MODS2. |
|
Twilight Sky Flats |
Telescope Configuration: It is recommended that twilight flats be taken on a blank sky field, with the telescope in TRACK mode and making small dithers between images. The flats may be taken as soon as the sky brightness reaches about 11th magnitude. A table of coordinates of blank sky fields for sky flats is available from the NOT web site. Morning twilight fields should be selected in the west to avoid pointing the telescope towards sunrise. Taking twilight flats will require some user interaction.Notes: For MODS1, twilight sky flats are typically not taken unless taken as illumination correction frames to take out the 2% top-to bottom gradient noted in the internal flats. For MODS2 the gradient in the internal calibration lamps is on the order of 20% and as such it is recommended to obtain twilight flats either from the archive (twilight flats are stable) or during the run. |
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Bad Pixel Masks
Bad pixel masks are provided below for the full-frame 8Kx3K readout mode as well as for the 4Kx3K mode used for spectroscopic observations using the prism and the 3Kx3K and 1Kx1K modes used for imaging and imaging acquisition. The full-frame 8Kx3K masks assume that the image is unbinned and the prescan regions have been trimmed. These are the masks provided with modsCCDRed. Bad pixel masks for the other readout regions of interest were derived from the 8K x 3K masks and are provided for convenience.
| MODS1B | MODS1R | MODS2B | MODS2R | |
| 8Kx3K | mods1b.bpl | mods1r.bpl | mods2b.bpl | mods2r.bpl |
| 4Kx3K | mods1b_prism.bpl | mods1r_prism.bpl | mods2b_prism.bpl | mods2r_prism.bpl |
| 3Kx3K | mods1b_3Kim.bpl | mods1r_3Kim.bpl | mods2b_3Kim.bpl | mods2r_3Kim.bpl |
| 1Kx1K | mods1b_1Kim.bpl | mods1r_1Kim.bpl | mods2b_1Kim.bpl | mods2r_1Kim.bpl |
