SUSI2

Frequently Asked Questions

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Frequently Asked Questions

General OB preparation

Where will my object fall on the CCDs?

The optical axis of the telescope (the reference for pointing) is in CCD#45, very close to the gap. The NTT points very well, however, it is possible that your object may fall into the gap. To make sure this does not happen, you can either add/subtract 1-2 arcsec to the RA of your object (remember the cos(dec)), or start your sequence with an offset using the SUSI_img_acq_MoveToPixel template.

Can I position a given object on a particular pixel?

Yes you can. Use the SUSI_img_acq_MoveToPixel template.
You can predefine the pixel you want to position your object on (see option 1) or select it interactively (see option 2). Remember, for Service Mode observations, you need to clearly mark the object you are interested in!

Can I position a bright star in the gap between the chips?

Is coronography possible with SUSI2?

There is no real coronography mode. However, as the space between the CCDs is masked, the central gap can be used as a crude coronograph to mask a bright star. To really make sure your object goes in the gap, use the SUSI_img_acq_MoveToPixel template.

How can I minimise my overheads when observing with SUSI2?

As you can see from the table of overheads for SUSI2 , the instrument does not incur many overheads. Limiting the number of filter changes is really the only way to save on overheads, given that other factors will decide windowing and binning choices.

Are OBs for Landolt Standard fields supplied at the telescope?

Yes. The impex-stdcal directory contains a set of pre-defined OBs for selected Landolt fields. These are constructed for 1x1 and 2x2 binning, and with the rotator positioned at 0 degrees and 180 degrees (see the FAQ Do I need to photometrically calibrate both chips? to learn why the two rotator positions are defined).

Properties of SUSI2

Is SUSI2 more efficient in the red, or EMMI with the new EMMI-red chip?

Tests pending...

Is the PSF uniform across the full field of view?

Yes. The PSF is not necessarily the best looking one (tends to be slightly elongated), but it is extremely uniform over the entire field of view.

How an I achieve the best flatfields with SUSI2?

The best possible flatfields are Superflats which are obtained from the science images themselves.

Next best are Skyflats. Note however that SUSI2 has a significant baffling problem, with the largest contribution coming from the baffling of M3. This technical report describes the problem in more detail. The result is that there is a bright region in the flatfield images which rotates with the rotator. To achieve the best possible flatfield, we suggest that you take skyflats for each filter at several rotator positions (0, 90, 180, 270) so that this can be averaged out.

Domeflats should be taken as an insurance policy, but not used if there is an alternative choice. Note that domeflats are also affected by the baffling problem. We suggest that you take domeflats for each filter at several rotator positions (0, 90, 180, 270) so that this gradient can be averaged out.

I want to use a special filter that is not part of the ESO SUSI2 filter set. Can I do this?

User Supplied Filters filters can be mounted in SuSI2 upon request (and with plenty of advance notice). However, the filters must meet the following specifications:

Diameter < =100mm
Thickness < 10mm

Filters with a diameter < 100mm must be mounted in an adaptor. We have adaptors for some standard diameters, and we can manufacture adaptors for other diameters. It is critical to organize this several months in advance (email ls-susi). Be warned that it is very likely that user supplied filters will have a different optical thickness than the standard filters, causing a focus offset that will have to be measured during the first night of observations. This will cause some additional overhead during the observing run.

Do I need to photometrically calibrate both chips?

No. Several tests have shown that the chips are identical.

Is there significant fringing in the R and I bands and is it stable?

There is certainly fringing in the I#814 filter. A detailed report about the fringing pattern in this filter is available. So far we do not know how variable the pattern is. The intensity varies a lot, but the shape seems to be quite stable. We plan to do more measurements in the near future to clarify this point.

Does the geometrical properties of the SUSI2 chips affect my astrometry?

There are 2 considerations:

Due to the rotation of one chip with respect to the other, the gap between the chips is 4 pixels (0.3") wider at the top. For very accurate astrometry, the astronomer should treat the 2 chips individually.
The width of the gap is equivalent to the region of sky that is blocked. Therefore you can combine jittered images without affecting your astrometry.

WFI Reduction

Is there are pipeline to reduce SUSI2 data?

Is there a script to change the MEF SUSI image into a single, normal FITS image?

Yes. You can download a MIDAS script insusi.prg to do just this.

The long ESO headers are causing problems for IRAF. How can I fix this?

The FITS files produced by ESO instruments make extensive use of hierarchical keywords. While these are standard FITS, IRAF cannot deal with them. To convert these keywords (e.g. from HIERARCH ESO TEL AIRM START to AIRMASS) use the hierarch28 program developed by N. Devillard. This program, together with a series of other useful FITS-related programs, is available at the ESO standalone fits tools webpage. These tools are part of the ECLIPSE package, and therefore directly available at the telescope and on any scisoft machine.

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Last modified: Wed Nov 05 2003