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Frequently Asked Questions

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Frequently Asked Questions

General OB preparation

Can I position a given star on a particular pixel in the mosaic?

Yes you can. Use the WFI_img_acq_MoveToGap template.
You can figure out the x,y of your pixel using the diagram and table of the WFI mosaic.
Remember, for Service Mode observations, you need to clearly mark the star you are talking about and give us the coordinates where you want it!

Can I position a bright star in one of the inter-chip gaps?

Yes you can. Use the WFI_img_acq_MoveToGap template.
You can figure out the x,y of your pixel using the diagram and table of the WFI mosaic.
Remember, for Service Mode observations, you need to clearly indicate the star you are talking about. It will be moved to the closest gap (unless specified otherwise).

The pointing of the telescope is centred at what pixel on the mosaic?

The pointing is currently centred such that your target object will fall on pixel (4150,3950). This is in the top right-hand corner of chip 7. The pointing of the telescope is good to within 10 arcsec.

I want to make large offsets between exposures. Will that cause problems?

This really depends on how big your offsets are and whether you choose Combined Offsets = True. If you choose Combined Offsets = True, the success of the combined offset depends on exactly where we have managed to find a guide star on the tracker chip, and exactly how big your offset is.
I would suggest you set Combined Offsets = False if you plan to do large offsets. This means we will have to choose a new guide star after each offset (overhead

1 minute each time), but inevitably we will save time doing things this way, rather than having to abort and restart your OB again when the guiding fails.

How can I minimise my overheads when observing with WFI?

As you can see from the table of overheads for WFI , by far the biggest overhead which can be avoided is that involved in changing filters. Therefore, the easiest way to minimise overheads is to minimise filter changes.

Can I make my OBs longer than 1 hour?

Yes you can (Note: for Service Mode observations, you MUST get a Phase II Waiver approved first). However, we don't encourage this practice since the weather conditions at La Silla are not stable over long periods of time. In particular, the seeing can vary widely over the space of an hour. For examples of how the seeing can vary during the night, look at some of the recent plots from the La Silla DIMM.
So if you request good seeing (<1 arcsec), your OBs have a better chance of being executed fully within constraints if you keep your OBs short!

Is there a standard OB to do a sequence where a standard field is placed on each of the 8 chips?

Yes there is. You can pick up this OB from MAKE OB AND PUT SOMEWHERE. You can import this directly into your P2PP and change as you need, but this gives you the offsets required.

Service Mode Preparation

What filters are included in the Standard Calibration Plan?

Starting in Period 71, only the following filters form part of the Standard Calibration Plan:

U/50
B/123
V/89
Rc
I/203

For more information on the Standard Calibration Plan for Service Mode observations, please click here.

I have to submit calibration OBs for non-standard filters. Will I be charged time to execute these OBs?

For non-standard Skyflat and Domeflat calibrations, you will not be charged for time (provided you don't send us a million of them to do!)

For photometric and spectrophotometric standard fields, you WILL be charged the time

Properties of WFI

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

We do not normally mount special filters (or even filters from other ESO sets) in the ESO WFI. However, you may state your case and ask ls-wfi@eso.org whether it would be possible. If they agree, the filters must be provided to La Silla at least 2 months before the start of the observations.

What was the reason to upgrade the B/99, Ic/Iwp and U38 filters? What is the improvement?

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

There is certainly fringing in the Ic/Iwp filter. A detailed report about the fringing patter in this filter is available here. 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.

Are the photometric zero points of each chip known?

Are the colour terms of each chip known?

How good is the astrometry solution in the FITS header?

Is there a significant wavelength shift between the centre and edge of the mosaic when using the narrow-band filters?

WFI Reduction

Is there are pipeline to reduce WFI data?

I'm sure there are many piplines floating around to reduce WFI data, however, none of them are publicly available. That being said, there are many packages that exist to reduce mosaic data. At the very least there is:

IRAF: mscred
The Flips reduction package at the CFHT

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.

Alternatively, you can download the ESOWFI package for IRAF to help you out.

Send comments to : ls-wfi
Last modified: Thu Jul 31 2003

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