1. 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

2. 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

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

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

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

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

7. and placing them far apart. A. Kepley

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

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

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

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

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

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

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

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

16. 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

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

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

19. The spatial scales measured by the single dish and interferometer must overlap. single-dish interferometer single-dish + interferometer Stanimirovic+ 2002 A. Kepley

20. 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

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

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

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

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

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

26. 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

27. 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

28. 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

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

30. 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

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

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

33. 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

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

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

36. Step 3. Load in your images. A. Kepley

37. Step 4. Press feather. A. Kepley

38. Step 5. Tweak parameters as necessary. A. Kepley

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

40. 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

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