What is normal?
Content
This page gives examples of 'normal' calibration frames, for quick comparison. If calibration frames look significantly different, there may be some problem. All images are for 2x2 binning mode, and are made using the same skycat programme that is used for the RTD, with automatic cuts (except for the imaging flats, where cuts between ~35k and 45k were used to show the structure in the flat field image, and in the fringing and 2nd order contamination images, where the cuts are selected to show these faint structures). The cuts though the images (click on the link below the corresponding image) are taken vertically (along the dispersion direction for spectroscopy) through the middle of the frame.
Bias
The bias frame is quite flat, any noticable pattern is not normal. Only the fast mode shows different levels between the two halves, as it uses two amplifiers to read out.
| Normal | Fast |
|---|---|
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Imaging flats
The most noticable thing is the central light condensation due to the focal reducer, and the vignetting at the corners. A horizontal line of bright points is occasionally visible, this is due to reflections from the back of the filter wheel, and will hopefully be remedied by the installation of a new wheel in the near future.
Note that these images are from dome flats, but it is recommended to take twilight sky flats for imaging. See Imaging calibrations page for details.
| U | B | V | R |
|---|---|---|---|
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| g | r | i | z |
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Spectroscopic flats
Slight variations along the x direction are due to imperfections in the slit - these are very small, but it is worth taking twilight observations to make a slit illumination correction if you need to know the differences in slit width along the slit to a high accuracy. Note that the VPHGs introduce a shift in the light path, so it appears that part of the image is 'missing'. Fringes are visible for some grisms; these will shift slightly with the instrument flexures so users requiring very good correction for fringes (those needing to identify faint lines at red wavelengths) should also take internal flats during the night at the same position as the science exposures.
| Gr#1 | Gr#2 | Gr#3 | Gr#4 |
|---|---|---|---|
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| Gr#5 | Gr#6 | Gr#7 | Gr#8 |
 cut |
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| Gr#11 | Gr#13 | Gr#14 | Gr#16 |
 cut |
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| Gr#17 | Gr#18 | Gr#19 | Gr#20 |
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He-Ar arcs
In the arc exposures the automatic cuts often hide the fainter lines when there is one very bright line. See also the arc line atlases in the manual or at the arc lamps page.
| Gr#1 | Gr#2 | Gr#3 | Gr#4 |
|---|---|---|---|
 cut |
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| Gr#5 | Gr#6 | Gr#7 | Gr#8 |
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| Gr#11 | Gr#13 | Gr#14 | Gr#16 |
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| Gr#17 | Gr#18 | Gr#19 | Gr#20 |
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Other
Other things to note are the fringe pattern in night time imaging in red filters (see fringing page) and the second order that is visible when taking spectra of bright blue objects (eg standard stars) in some grisms (see this report (pdf) and the advice on reducing this with order sorting filters).
| Fringing (i) | 2nd order (Gr#1) |
|---|---|
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