Problems may also be related to the pipeline processing or to association errors.

Second-Order Contamination (300V, 150I)

The effects of the second order contamination have become much more pronounced with the new blue-sensitive CCD. This is especially dangerous if the spectral type of a target differs from that of the standard star observed for these data. As you can see below the response curves derived from a hot and from a cool standard star differ considerably for the 300V grism, even if order sorting filters are used. PIs should therefore check carefully if the standard star included in their package has a spectral type similar to that of their targets befor deriving a response curve. If this is not the case at least areas affected by second order contamination should be excluded from flux calibration. Of course also blue targets will show second order contamination.

300V+GG435	300V+GG375	300V+NONE

Photometric Accuracy and Flat-Fielding

The twilight images of FORS1, which are used to create master sky flats, show evidence for a feature, whose position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information see Moehler et al. (2010, PASP 122, 93).

Fringing

The new FORS1 detectors are affected by fringing in both spectroscopic (e.g. normalized flatfield for 300V) and imaging data (e.g. twilight flat for I_BESS). The fringing becomes relevant from about 700 nm redward and its amplitude increases with reduced bandwidth (i.e. narrow-band imaging and spectroscopy are stronger affected than broad-band imaging). Observers interested in the wavelength range beyond 700nm are advised to use FORS2 instead of FORS1.

Lamp Reflections

FORS1 Screen Flats (IMG mode) presently show three spots of artificial light which are due to lamp reflections on the LADC surface. If you plan to use the Screen Flats for reduction, these areas will need special treatment.

FORS1 Screen Flats (LSS mode) show structure due to lamp reflections on the ADC surface. These are well off the central part which is usually exposed to the SCIENCE object. For extended objects, the Screen Flats have to be used with care.

Photometric Accuracy and Flat-Fielding

The twilight images of FORS1, which are used to create master sky flats, show evidence for a feature, whose position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information please look at the FORS Secondary Standard and Absolute Photometry Project.

Shadowing of CCD columns

All FORS1 data taken later than 1999 November 01 have the last ~20 columns (excluding the postscan region) shadowed out. This region is useless.

CCD pre-scan regions
 
The FORS1 CCD has pre-scan regions 16 columns wide. Even in Bias frames, the pre-scan level is a few ADUs lower than the actual bias level. Moreover, the pre-scan level shows a gradient of the order of 0.3 ADU/pixel. The level grows by 3 ADUs from column 1 to column 7, while from column 10 to colum 14 it settles on a constant value, about 2 ADUs below the bias level.

A steep gradient is finally visible in column 15 and 16.

A mesh of all 1024 rows of port A in a FORS1 bias taken in April 2001 is shown  here  (click  here  for the ps file). Despite this behavior, the pre-scan has been proven to be reliable in tracing the short time scale bias fluctuations (of the order of a 1-2 ADUs during a night). The user should  exclude columns 15-16 from the statistics when scaling the bias frames during the reduction process and use the same region for statistics in both calibrations and science frames, to take into account in a proper way the pre-scan gradient.

This effect is related to the pre-amplifiers of the FIERA CCD controllers. It has not proved possible to eliminate it completely without changing the electronic components.

The pre-scan level has been shown to be also weakly dependent on the median exposure level of the image. This effect causes an increase of the order of a few ADUs for a median exposure level of 40 kADU, and hence has no practical consequences on science exposures. This effect is actuallyvisible on high exposure level flats only .

IMPORTANT NOTE: due to a feature of the FIERA controller, only full frames have pre-scan regions, windowed frames don't.
 

CCD contamination

Contamination is a well known problem of the FORS1 CCD. Since this is a time-dependent effect, its growth is routinely checked as part of the instrument trend analysis.
For more details click  here.
 

Dark Current

Dark current trend plots have shown that the Dark current is stable at the level of about 8 electrons/hr/pixel. FORS1 Darks frames show a slight gradient along the columns, being the maximum and the minimum 6 and 10 electrons/hour/pixels respectively.
For more details click  here.

Image Fringing

The FORS1 detector is affected by fringing. This is due to the multiple reflections internal to the CCD substrate or between the silicon and the supporting substrate.

Of course to produce this interference pattern one needs to have a monochromatic source, otherwise the fringe pattern is cancelled. For this reason the fringes are seen in night images only and do not appear in screen or twilight flats, where the continuum contribution is very strong. Instead, during the night, the sky lines are the dominating background source and, specially in the red, they can produce significant fringing patterns. For a more detailed description of this problem and its possible solutions, please refer to:

Electronic Imaging in Astronomy, I. S. McLean, Wiley, 1997.

In the case of imaging, fringes become relevant in the z_GUNN filter only.

A 500sec image taken with FORS1 in the z_GUNN passband. Note the fringe pattern. In this example, fringes reach peak-to-peak amplitudes of the order of 0.5%.	The combination of 5 dithered frames (including the one of left panel), 500 sec each, z_GUNN filter. This is generally called a superflat or nightflat.

A standard procedure to eliminate the fringing is to obtain several frames of the same field using the dithering technique, i.e. offsetting the telescope between one frame and the other. This works particularly well if no extended objects are present, as it is the case in the example shown above.
If extended objects are present, then the user should plan the observation of an adjacent field, just to be able to produce the proper nightflat.

Dust Contamination

The contamination by dust grains and hairs has been recently recognized to be one of the major cosmetics problems of the FORS1 CCD. To quantify the increase of the dust and hairs contamination, we have compared master SCREEN FLAT images taken from April 2001 to August 2003 (one per semester). All images have been taken through the V_BESS filter. From this comparison we see that the dust/hairs contamination underwent a sudden increase in August 2001 i.e. after the moval of the instrument from UT1 to UT3. A further increase was noted after maintenance operations. Since then, the contamination has remained substantially stable with little increase during Periods 68 through 71.
As an example, the image below (2048x2048 pixels) shows the master screen flat V_BESS image taken on August 5 2003. The image has been divided into nine quadrants to allow for an easier comparison with a corresponding image taken on April 21 2001, i.e. when the dust and hairs contamination was absent or, at least, still marginal. Clicking on each quadrant links to a comparison between the corresponding sections of the two images. The quadrants are numbered from (1,1) - upper left corner - to (3,3) - bottom right corner. The dust/hair contamination is more noticeable in quadrants (3,1) and (2.3).

Lamp Problems (March/April 2003)

Arc frames taken end of March/early April 2003 have Neon instead of Argon, therefore the wavelength calibration frames look different from usual and need different line catalogs.