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EisXstudy
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[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
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If ''n'' is the number of positions that the slit makes an image at, Σ ''t%%sub exp%%'' is the sum of the exposure times (usually there is only one) at each position (in seconds), and ''x%%sub step%%'' is the size (in arcseconds) of the step, then it takes ''n'' × Σ ''t%%sub exp%%'' × ''x%%sub step%%'' seconds to build up a spatial image of an area that is ''n'' × ''x%%sub step%%'' arcseconds across.
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EIS makes images of different widths every time it takes an exposure (and disperses it). Slit spectra contain direct spectroscopic information, but take time to build up information over areas (see ''Raster Type'' above). For more image-based information, at the expense of line profile information, we can make slot image (scanning or sit'n'stare).
''a.k.a. SLA — SLit/slot Assembly''\\
EIS makes images of different widths every time it takes an exposure (and disperses it). Slit spectra contain direct spectroscopic information, but take time to build up information over areas (see ''Raster Type'' above). To make slit spectra, EIS uses two slits: 1" and 2" in width.
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The 1" slit has less spatial coverage (because it is narrower) so there is sharper spectral resolution; meanwhile, the advantage of the 2" slit is that it is twice as wide, and thus it can accumulate photons twice as quickly as the 1" slit. (Note that this argument does not extend linearly to the width of the slots).
For more image-based information, at the expense of line profile information, we can make slot images (scanning or sit'n'stare). In both cases the images are dispersed on the detector. The slot images are a sort of convolution of a spectrum with an image. However, the narrow slot — 40" wide — has been chosen so that essentially monochromatic images of the Sun can be made in many lines, without significant blending with images made in nearby lines.
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__Raster Title__\\
You can also get a quick idea of most of the characteristics of a raster from the single-string raster ''Title'' field in {{{eis_xstudy}}}.
The width of the wider slot — 266" — was chosen for ground calibration reasons, rather than scientific considerations. However, the Fe XV line at 284 Å is sufficiently removed from other active region lines that we can essentially make images of only Fe XV in bright coronal loops. In other lines, the images resemble the ''overlappograms'' produced by the ''Skylab'' S020 spectrograph (see Figure 2 of [http://solar.physics.montana.edu/reiser/] for a full-Sun idea of what this means), though with smaller spatial extent.
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%%information Note that since engineering studies are created by hand, rather than through the planning tool, it's not possible to view their contents (such as number of exposures, or exposure time) in the Raster panel.%%
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In order to make the process of transferring data to the ground more efficient, data from all Hinode instruments can be compressed on-board the spacecraft by the mission data processor or ''MDP''.
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The MDP allows several choices of compression:
%%sortable
||Scheme||Compression Type
|Uncompressed|None
|[DPCM|~DPCMScheme]|Lossless
|[JPEG98|~JPEG98Scheme]|lossy (Q factor 98)
|[JPEG95|~JPEG95Scheme]|lossy (Q factor 95)
|[JPEG92|~JPEG92Scheme]|lossy (Q factor 92)
|[JPEG90|~JPEG90Scheme]|lossy (Q factor 90)
|[JPEG85|~JPEG85Scheme]|lossy (Q factor 85)
|[JPEG75|~JPEG75Scheme]|lossy (Q factor 75)
|[JPEG65|~JPEG65Scheme]|lossy (Q factor 65)
|[JPEG50|~JPEG50Scheme]|lossy (Q factor 50)
%%
%%information The links in the above table each take you to a page where the estimates of the Data Compression (''DC'') factor are listed. These values vary depending on the target and SLA choice. They will be updated from time-to-time until the team are satisfied that they are adequate enough for planning purposes.%%
In addition to these webpages showing the more involved analysis results, the current values used for DC factors in making observations are stored in {{{$SSW/hinode/eis/idl/spacecraft/decompression/eis_get_compression_factor.pro}}}
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Whereas the width of each spectral window can vary (see Line Lists below), the height of the image ''y%%sub size%%'' read out from the CCDs is necessarily fixed for each window. So each raster has the same height in all spectral windows. This height is expressed in arcseconds. In EIS's design, it was decided that a 512" image height would be sufficient (1 pixel = 1"), so this is the maximum image height that can be made at any one time.
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''Scanning raster-specific''\\
This is a little confusing, but it tells you the number of times ''N'' that the mirror must move between observations to build up a scanning raster, i.e. 1 less than the number of positions at which EIS makes an exposure.
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__Working out the size of a raster__\\
If ''N+1'' is the number of positions at which the slit makes an image, Σ ''t%%sub exp%%'' is the sum of the exposure times (usually there is only one) at each position (in seconds), and ''x%%sub step%%'' is the size (in arcseconds) of the step, then it takes ''n'' × Σ ''t%%sub exp%%'' × ''x%%sub step%%'' seconds to build up a spatial image of an area that is ''x%%sub size%%'' = ( (''n'' × ''x%%sub step%%'') + SlitSize ) arcseconds across.
Putting this together with the ''x%%sub size%%'', worked out above, tells you the spatial extent of the observation that this raster produces.
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EIS allows its user to make more than one exposure at each position on the Sun, and these exposures can have independent shutter-open times. In fact, you can have up to eight such exposures at each fine mirror position.
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Delay times can be up to 655.35 seconds long (2^16 -1 × 10-ms units), and come after each exposure. The number of delay times has to be the same as the number of exposure times, but typically these delays are set to zero.
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%%information Note that since engineering studies are created by hand, rather than through the planning tool, it's not possible to view their contents (such as number of exposures, or exposure time) in the Raster panel.%%
For example, with two exposure times of 10 and 120 seconds, and delays of 100 seconds and 200 seconds, a typical ordering of events could go like this:
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||Event
|Move fine mirror to starting position
|Take an exposure for 10 seconds
|Read out exposure
|Delay for 100 seconds
|Take an exposure for 120 seconds
|Read out exposure
|Delay for 200 seconds
|
|Take an exposure for 10 seconds
|Read out exposure
|Delay for 100 seconds
|Take an exposure for 120 seconds
|Read out exposure
|Delay for 200 seconds
|
|Move fine mirror to next position
|''etc.''
__Raster Title__\\
Knowing what all the above information means, you can also get a quick idea of most of the characteristics of a raster from the single-string raster ''Title'' field in {{{eis_xstudy}}}.
This "summary", if you like, is composed of a series of square-bracketed terms in the following order:
|RA:RasterID
|Scan:NumberOfPositons-1(SizeOfStep)steps,
|ss:SLAchoice
|wH:WindowHeight,nWins:NumberOfSpectralWindows,LL:LineListID
|ExpT(Delay):ExpTimeNumberOneInSeconds(DelayNumberOneInHundredthsOfASecond), ...
and can be handy when you understand how it works, because it means you don't need to delve into the raster description to find out how large it is, which slit or slot it uses, how long the exposure times(s) is/are, ''etc.''
%%information Note that since engineering studies are created by hand, rather than through the planning tool, it's not normally possible to view their contents (such as number of exposures, or exposure time) in the Raster panel.%%
\\
\\
----
!Line List
The most critical part of the raster description that __isn't__ summarised in the ''Raster Title'' is the Line List. This is because the line list is too variable to easily summarised. So, it gets its own panel in the raster description.
Note that, like studies and rasters, line lists have and ID and acronym.
\\
__Rationale of a Line List__\\
When EIS makes a spectral image of a slit- or slot-sized portion of the Sun, it exposes the whole of both detectors, which amounts to some 2 × 2048 × 1024 pixels. At 16 bits per pixel (2 reserved for marking which half of which detector a pixel belongs to), this would create a massive 8 MB for all data from the detector, __at each position on the Sun!__
So, in order to limit telemetry need — even before the transition to the S-band antenna — EIS was designed so that only the spectral portions of interest on the detector would be telemetered to the ground for a given observation.
Line lists are described in terms of absolute wavelength, according to the standard wavelength calibration, so you don't need to work things out in terms of pixel co-ordinates.
__Title__\\
You'll see that, like the raster, a Line List has a one-line so-called ''Title'' string associated with it. Since this is truncated, it doesn't serve much purpose, and we'll skip it here.
__Author__\\
Again, fairly self-explanatory\\
:-)
One point to perhaps reiterate, though, is that you can use a line list created by anyone else in your study. All line lists, rasters, and studies in the OSDB are there to be re-used and cannibalised by anyone wanting to propose an observation.
__Number of lines__\\
There can be up to 25 spectral windows in a line list, but the minimum is normally three: the core lines. The core lines are
||Ion||Wavelength (Å)
|Fe XII |195.12
|Ca XVII|192.82
|He II |256.32
__Line List Detail__\\
Each spectral window can have a width that is an integer multiple of 8, from 8 to 1024 (half a CCD width), and there can be up to 25 windows, each with independent widths. In the example image above, you can see that most windows have a width of 24 pixels (a little over 0.5 Å), which is usually adequate. But a few windows are wider (32, 40 or 48 pixels). Wider windows often (but don't necessarily) indicate that the line list author was trying to capture other lines by using a wider spectral window.
This also helps to explain why the core lines of {{Fe XII}} (195.12 Å) and {{Ca XVII}} (192.82 Å) aren't explicitly included in this line list: they are already covered by the {{Fe XII}} window at 194.91 Å and the {{O V}} window at 192.84 Å, respectively.
This page (revision-57) was last changed on 07-Dec-2016 14:14 by David R WilliamsTop