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WFC3 Grism Frequently Asked Questions

Welcome to the WFC3/Grism F.A.Q. Webpage.  As questions are asked and answered this page will be updated, so bookmark and check back often.


Phase 1 Proposal Preparation

  1. My targets are fairly bright in the near-IR.  What is the shortest exposure time that can be used for the WFC3 IR array?
  2. Do I need a direct image for the UVIS Grism?  The 0th order is separated from the first order spectrum, so in theory, one should be able to find the location of objects without a direct image.
  3. What is the assumed size (in pixels) of a “Resolution Element?” 
  4. Since the UVIS spectra are significantly curved, what is the wavelength range of the 1st order which is least likely to be affected by overlap from higher orders?
  5. Are there recommended filters for Grism direct imaging?

Phase 2 Proposal Preparation

  1. Which direction will a POS TARG move my target?
  2. How do I simulate roll angles in aXesim?
  3. How do I “see” the dispersion direction of the Grism in Aladin?
  4. What aperture should be selected for the Grism direct images?
  5. Are there any guidelines for using Buffer Dumps?
  6. Is it possible to generate a noiseless simulated Grism image with aXesim?

General Data Analysis Questions

  1. How does flat-fielding work for the Grism data?
  2. If I extract the spectra from the flt images on my own, how do I convert to physical units?
  3. Is it possible to combine multiple extracted spectra of a source outside of aXe?
  4. Is it possible to do any type of masking or CR rejection outside of aXe?  
  5. The Grism observations are not dithered and aXe requires dithering for cosmic-ray (CR) rejection. Can I use another method to mark bad pixels?

General aXe Related Data Analysis Questions

  1. Is aXe compatible with the newest 64-bit version of IRAF (v2.15)?
  2. My observations were obtained using a sub-array mode, but aXe crashes when I attempt to process the data.  Is there a workaround?
  3. Do I have to use SExtractor for detecting objects in the direct image?
  4. How will aXe be affected if SExtractor cannot compute a valid magnitude for an object? 
  5. How do I use the “dimension_info” parameter in the task IOLPREP for objects outside the normal field of view?
  6. Why doesn’t IOLPREP  seem to accept the SExtractor generated .cat file?
  7. How is the contamination model used?
  8. I’m a little confused about contamination model choices for aXecore.  How do they differ and what do I need for each to work properly?
  9. What values should I use for the parameters norm and gaincorr in the task aXeprep?
  10. Why is aXe is generating divide by zero errors and crashing?
  11. The task axedrizzle crashes with the error: “IOError: [Errno 2] No such file or directory:”
  12. Can aXedrizzle resample spectra with pixel sizes smaller than the native platescale (like multidrizzle can with imaging)?
  13. What parameters do I set for vertical extraction of spectra?
  14. How do I specify the exact width of the aperture to use for extraction?

G280 (UVIS) aXe Data Analysis Questions

  1. Are there any limitations for the G280 calibrations?
  2. Is there a difference in how 2-chip data (WFC3/UVIS) is processed compared to 1-chip data (WFC3/IR)?
  3. I’m working on UVIS Grism data (G280) and the task drzprep crashes and produces the error:  aXe (src/drizzle_utils.c, 40): Fatal: Order of dispersion solution: 4! At most quadratic solutions are allowed! aXeError: An error occurred in the aXe task: aXe_DRZPREP
  4. Will aXedrizzle work with the G280 (UVIS) grism?

Version 1.3 Sep 2011