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Flat field images (N and S)

 

An example flat taken with the GMOS-N Hamamatsu detector array (installed in February/March 2017) is shown below. The image highlights a number of features characteristic of the Hamamatsu array: 

i) "Scalloping" (result of the CCD manufacturing process): The "scalloping" features close to the edges of the chips are stationary, so that they can be removed from science data by the flat field correction.

ii) Bright lines at the borders of the chips: Due to these lines, the effective inter-chip region, not usable for science, is larger than the physical size of the chip gaps. Recommendations for the spatial or spectral dither sizes that are needed to achieve a full coverage of the inter-chip region (including the area affected by the bright lines) are listed under "Observing Strategies" for the respective observing mode.

iii) "Ragged line" feature of yet unknown origin: The "ragged line" feature extends over the top part of the three chips. As the feature moves downward with increasing illumination level and cannot be corrected via flat fielding, it is recommended to use a combination of dithered exposures.

Single GMOS-N Hamamatsu twilight flat
Single GMOS-N twilight flat frame taken with the Hamamatsu detector array through the r'-fitler in 2x2 binning and slow/low readout mode. The 12 amplifiers have been mosaiced without any bias subtraction. "Scalloping" can be seen as the circular patterns close to the chip edges. The "ragged line" feature is obvous at the top of the illuminated filed. (See text for further details.)

Note that the flat shown above exhibits an attenuation in the bottom part of the field (in this case most prominantly in the bottom left corner). This is due to the long-standing issue with air bubbles in the index-matching oil in the GMOS lens interfaces. 

 


Example imaging flat fields for GMOS-N with the red EEV CCDs are shown below. The flats are normalized and have been displayed with a greyscale from 0.8 to 1.2. The data are from the commissioning and have been released. The g', r' and i' imaging flats are from dataset GN-CAL20010910, while the z' imaging flat is from dataset GN-CAL20010830.

In 2006, a feature appeared in GMOS-N flat fields. More details, and example images, can be found on the GMOS imaging pages.

g'-filter twilight flat field.

r'-filter twilight flat field.

i'-filter twilight flat field.

z'-filter dome flat field.





Example imaging flat fields for GMOS-S with the E2V CCDs are shown below. The flats are normalized. The data are from the commissioning. The g', r' and i' imaging flats are from dataset GS-CAL20030524, while the u' and z' imaging flat are from dataset GS-CAL20030717 and GS-CAL20030720  (data not released).

g'-filter twilight flat field.

r'-filter twilight flat field.

i'-filter twilight flat field.

z'-filter twilight  flat field.


 
u'-filter twilight flat field