Atacama Large Millimeter/submillimeter Array Karl G. Jansky Very Large Array Robert C. Byrd Green Bank Telescope Very Long Baseline Array Short Spacing Corrections from a Single-Dish Perspective Amanda Kepley North American ALMA Science Center

Higher resolution allows us to study details of objects like galaxies. CREDIT: Minchin et al., NRAO/AUI/NSF (HSA); Travis Rector, Gemini Observatory, AURA (optical). A. Kepley

Bigger dishes have higher resolution. Θ [rad] ≈ λ / D A. Kepley D

Telescopes with high resolutions are needed to study distant objects. A. Kepley Local Group Local Volume HII region MW 1 kpc 20cm

Very large dishes have to be supported against gravity/wind. A. Kepley

You can get higher resolution by splitting up a single dish into smaller dishes… A. Kepley

and placing them far apart. A. Kepley

Interferometry 101 Why Short Spacing is Necessary Interferometer + Single Dish Combination Methods Tools for Combining Data A. Kepley

Interferometry 101 Why Short Spacing is Necessary Interferometer + Single Dish Combination Methods Tools for Combining Data A. Kepley

The signals between the different dishes are correlated together. A. Kepley d = baseline d sin θ θ Visibilities Fourier Transform Images

Interferometers are analogous to double slits! A. Kepley λ / D λ / d D λ / D d = baseline

The resolution goes as the distance between the dishes, not the dish size. A. Kepley d Short baselines = low resolution = large spatial scales

The resolution goes as the distance between the dishes, not the dish size. A. Kepley d Long baselines = high resolution = small spatial scales

Interferometers are only sensitive to a range of spatial scales. A. Kepley Resolution Largest Angular Scale

Interferometry 101 Why Short Spacing is Necessary Interferometer + Single Dish Combination Methods Tools for Combining Data A. Kepley

What happens when you have missing short spacings? Braun & Walterbos 1985 Ideal Minus Short Spacing Effect of missing short spacing Observed Spatial Frequencies Instrumental Response A. Kepley

Missing short spacings result in negative bowls around your emission. Oh no! Negative bowls! A. Kepley

Single dish data can provide the missing short spacing information. A. Kepley

The spatial scales measured by the single dish and interferometer must overlap. single-dish interferometer single-dish + interferometer Stanimirovic A. Kepley

Single dish observations can be requested during the NRAO proposal process. Two proposals with same scientific justification. Joint proposal indicated on cover sheet. Added to your proposal automatically based on your requested angular resolution and largest angular scale. VLA GBT ALMA A. Kepley

Interferometry 101 Why Short Spacing is Necessary Interferometer + Single Dish Combination Methods Tools for Combining Data A. Kepley

Correcting for short spacings starts with fully calibrated data. A. Kepley

There are three basic options for data combination. Image DomainFourier DomainDeconvolution A. Kepley Stanimirovic 2002

Feather combines data in the UV plane. FT Single Dish Interferometer Stanimirovic 2002 FT Scaling factor A. Kepley

Overlap Region = fluxes should be equal = 100 meter dish = 43 meters for shortest baseline A. Kepley

26 T D (x,y) Slide shamelessly stolen from D. Wilner’s Synthesis School Lecture Deconvolution corrects your data for the discrete sampling of the interferometer. b(x,y) restored image CC model

Devolution extrapolates inner flux. Amplitude uv-distance ? ? ? FFT of “Dirty” Interferometer map CLEAN model Deconvolution is done via clean, but MEM can provide similar results. A. Kepley

You can use the single dish data as a model for the deconvolution. Amplitude uv-distance ! FFT of “Dirty” Interferometer map CLEAN model A. Kepley

Interferometry 101 Why Short Spacing is Necessary Interferometer + Single Dish Combination Methods Tools for Combining Data A. Kepley

We’re going to focus on feathering tools, since this is the way that ALMA will combine data. Image DomainFourier DomainDeconvolution Stanimirovic 2002 A. Kepley

Most (all?) interferometeric data reduction packages support feather. IMERGimmergefeather A. Kepley

Standalone casafeather is gui interface to feather task in casa. A. Kepley

Let’s combine interferometric and single dish images using feather in CASA. Interferometer Single Dish Images taken from forthcoming M100 ALMA Casaguide by Crystal Brogan, Jennifer Donovan Meyer, and Tsuyoshi Sawada. A. Kepley

Step 1: Regrid your data so that all your images have the same size, etc. A. Kepley

Step 2: Correct single dish data for the primary beam response of the interferometer. A. Kepley

Step 3. Load in your images. A. Kepley

Step 4. Press feather. A. Kepley

Step 5. Tweak parameters as necessary. A. Kepley

Step 6. Science! Interferometer Only Interferometer+Single Dish A. Kepley

If you remember nothing else, remember this! Interferometers can produce higher resolution images than single dish telescopes, but are not sensitive to emission on size scales greater than the largest angular scale. The characteristic sign of missing diffuse flux in an interferometer image are negative bowls surrounding your main emission region. You can combine single dish and interferometer data to get high resolution images that are also sensitive to diffuse emission. Several techniques to combine data: – Image domain – Fourier domain (feathering) – Deconvolution Feathering will be the default image combination method for ALMA A. Kepley

Thanks! Megan Johnson Snezana Stanimirovic Crystal Brogan Juergen Ott A. Kepley