Guidelines for designing EIS studies

v2, 2009 March 23, Peter Young

Raster & Studies

A study is a collection of one or more rasters. When it comes to the timeline planning (i.e., when a day's observations are planned), it is studies that are inserted onto the timeline, not rasters. It is thus the study definition that is exported to MSSL for testing, not the raster definition.

A raster can either be a raster scan (i.e., scanning a 2D area of the Sun by moving the EIS mirror), or a sit-and-stare (i.e., keeping the slit fixed on an area of the Sun.

Rasters are much more complicated to design than studies, since studies are simply a combination of pre-defined rasters. All the information about spatial area, slit numbers, emission lines, etc., are contained in the raster.

Getting started

First of all you must have an idea of what type of observation you want to make. Questions you may want to ask are:

  • Do I want spectroscopic information, or are images OK? This determines whether you want to use one of the slots (40", 266") or one of the narrow slits (1", 2").
  • Raster or sit-and-stare?
  • Do I want just a few lines for basic temperature discrimination? or many lines for detailed diagnostics?

Compression & data rate

Early on in the mission the average EIS data rate was around 50 kbit/s. However in 2007 December the Hinode X-band antenna failed, and so science data had to be transmitted using the S-band antenna which offers a much lower data rate. For example, during 2009 February the EIS data rate averaged around 7 kbits/s.

Before 2007 December it was typical for the Hinode instruments to observe for 24 hours/day at fairly steady data rates. The situation is much more flexible now and a detailed description is given on the EIS wiki. For example EIS can still run studies at the previous high data rate, but it will only be for a few hours and no other studies can be run during an OP period (2-3 days).
For this reason study designers should think in terms of the data volume their studies produce and compare this with typical data recorder (DR) allocations given on the Hinode Daily Events webpage.

Previous to the loss of the X-band antenna, most users chose DPCM compression for their studies which gives around a factor 2 lossless compression. Using JPEG compression is now much more common and general guidelines are that Q-factors of 92 or higher are reasonable for narrow slit data, while lower Q-factors can be used for slot data. Compression factors are discussed further on the EIS wiki.

Trade-offs: window width vs. slit length vs. number of lines vs. exposure time

The varieties of rasters are almost endless, and developing a perfect raster is something of an art form. The ultimate limiting factor is the EIS data rate and the user must trade off four parameters against each other in order to keep the data rate down to 50 kbits/s. These are:

  • The wavelength window width.
  • The number of pixels along the slit.
  • The number of wavelength windows.
  • Exposure time.

CDS study designers will be experienced in performing this sort of trade-off, for others a certain amount of trial-and-error will be necessary.

One recommendation from CDS experience: you may design a study to observe a loop, or bright point, or some other solar feature, but then find that you actually catch an interesting flare or transition region brightening, for example. You will then curse yourself for not including Fe XXIV, O V, or some other ion. Thus, unless you really want big spatial coverage or high cadence, I would recommend always trying to have a good selection of emission lines.

Wavelength window width

For EIS, the size of wavelength windows can be set in blocks of 8 pixels (i.e., 8, 16, 24, etc.), and the windows can have different sizes. (This contrasts with CDS, where each window had to have the same size, but you could set the size to any value.)

Before the loss of the X-band antenna, choosing window widths of 32 pixels was recommended. However, users are now strongly constrained by the data volume of their studies and so it may be preferable to go to 24 or 16 pixels. Bear in mind that high velocity events may be missed if 16 pixel windows are used.

Given the high instrument sensitivity in the Fe XII 195 line, users should consider having a larger width for this line. Similarly, since flare lines can show large blueshifts (400 km/s, for example) then broader windows should be considered for Fe XXIII and Fe XXIV. For example a 400 km/s blueshifted component for Fe XXIV 192.0 will be 11 pixels from the rest wavelength of the line.

Note that there are regions of the spectrum where you may want to choose a much larger window in order to pick up several useful lines in a single window. An example is the region around 256.3 to 258.3 angstroms where around 10-12 lines are found.

Choosing your exposure time

There are no hard-and-fast rules due to the wide variation of the EIS sensitivity with wavelength, but the following guidelines can be taken into account:

  • For quiet Sun, good line profiles for Fe XII 195 can be obtained in as little as 10 second exposures. To get good profiles in a number of different lines requires 50-60 seconds.
  • For active regions, 1 second exposures give useful profiles for Fe XII 195. For other strong lines (e.g., the Fe XIII 203/202 density diagnostic) 5-10 second exposures are fine. Some of the strong lines are likely to saturate if you go to exposure times beyond 30 seconds, but these exposure times are needed to pick up weak lines such as the transition region lines.

Remember that if you use the 2" slit, you will get twice as many counts so exposure times should be adjusted accordingly.

Why use the 2" slit?

As well as allowing you to use shorter exposure times (see above), the 2" slit also allows you to raster a region more quickly (since you need half as many steps to do it).

The downsides are that you lose spatial resolution in the X-direction, and that the spectral resolution is also degraded.

Choosing emission lines

Please refer to the separate document Choosing Emission Lines for EIS Studies.

This page mantained by Dr.P.R.Young, last revised on 23-Aug-2013