Euclid DDF Dithering

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We’re starting to look at the best way to observe the Euclid deep drilling fields. For the other fields, we typically dither by shifting up to 0.75 degrees in a random direction. Since the Euclid field is a double field, we’re now trying out preferentially dithering in certain directions to better fill the planned Euclid footprint. Here’s an initial attempt where each field is dithered between -0.25 and 1 degrees towards the other field and between -0.25 and 0.25 degrees perpendicular to the other field.

As always, we’re looking for metrics that could help us tune these parameters so we maximize the scientific usefulness of the observations. Is it better to increase the dither size to get more overlap with the full footprint, or decrease the dither size to increase depth?

euclid_coadd Final coadded depth in g. Euclid footprint in red.

euclid_nvis Total number of visits after 10 years, Euclid footprint in red.

dither_positions The dither positions used.

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We are currently making deep field observations of EDFS with DECam (ugriz). Last week we observed hex_1, hex_2, and hex_4. We hope to expand coverage working outwards to eventually cover most of EDFS. We started with the central part of EDFS to maximize overlap with two planned VISTA/VIRcam pointings. One of the goals of these DECam observations is to have deep external reference data if EDFS is selected as one of the LSST DDFs. Note the EDFS is located with the DES wide survey footprint.

We can provide an update on the DECam observations in ~2 weeks when the next observing run is completed.

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If you have ideas on how to decide what the ‘best’ observing strategy and dithering pattern for these Euclid DDFs would be, please reach out to us. We have a family of simulations that uses a variety of these dithering options - ‘euclid_dither’ in the v1.7 release - but very little in the way of metrics that is would help us choose the pattern.

Some things to consider:
At a time before the end of the survey: Is it preferred to reach full depth over a limited area, or limited depth over the full area (given that the Euclid field requires two separate pointing ‘centers’ to cover the whole area)? What ‘order’ do we do things? This is also a question about cadence, since uniform coverage over the entire area means a slower cadence, given that we have a limited number of visits per year to spend on the field. Cadence is also being driven by general DDF considerations, given that detection of SNe1a and AGN variability were subjects of specific white papers requesting particular DDF cadences.

When evaluating the dither patterns, should we look for uniformity of depth across the field? Uniformity of number of visits? Uniformity of (# visits or depth) across the field? Adherence to a more frequent cadence?

Is there something good or bad about how the coverage rolls off toward the edges? With any dither pattern, there will be edge effects.