Intro to XRT Obs (Click each item to expand its description)
This document, "An Introduction to XRT Observations", is designed to give a brief overview of the parameters that must be considered when planning XRT observations. The primary target audience is the space scientist who is interested in proposing observations with Hinode and XRT, but who is not an expert on the technical details of Hinode and XRT operations.
After reviewing this document, the target audience should be able to formulate a proposal for XRT observations that will be adequate to achieve their scientific aims and be sufficiently detailed such that the Hinode request committee (called the Scientific Schedule Coordinators) can reasonably evaluate the merit of proposal.
In a later section of this document (Where to Learn More About XRT), information and links are provided for how to prepare a Hinode observation request, as well as where to find more detailed information about Hinode and XRT.
Hinode[1] is a satellite observatory for studying the Sun and its atmospheric layers. Hinode (called Solar-B before launch in 2006) is operated by the Japanese space agency (JAXA). It is comprised of three instruments: the Solar Optical Telescope (SOT; [2]), the Extreme Ultraviolet Imaging Spectrometer (EIS; [3]), and the X-Ray Telescope (XRT; [4]). These instruments are international collaborations among JAXA, NAOJ, NASA, PPARC (now STFC), and ESA (collectively speaking).
The instrument teams operate together in order to optimize the value of the Hinode observatory. All requests for observations, even for just one instrument, are properly addressed to the Hinode team. This document will help one understand and prepare the XRT section of such a request, but it does not represent the entirety of a request.
This document introduces two complementary aspects of planning XRT observations: the daily cycle of operations, and the structure of a single observing block. A rudimentary understanding of these two levels will help one a lot with preparing a strong observing proposal.
+ - References
First, a proposer formulates a description of their scientific purpose and their need for Hinode data. From this, they should decide which Hinode instruments are necessary, and which of the others would provide helpful support. For each of the necessary (or primary) instruments, the proposer is expected to provide a detailed plan of observations. (The set of details required depends upon the instrument. This document describes those details for XRT.) For a supporting instrument, it is sufficient to name the instrument and request "appropriate support".
The instrument requests, plus some other information, are compiled into a single request. (Instructions for preparing a Hinode request are provided at http://www.isas.jaxa.jp/home/solar/guidance/index.html .) One then submits the full request to the Hinode request review committee, who are called the Scientific Schedule Coordinators. They meet to evaluate observation requests on a monthly basis, so one should not expect an immediate response to a submission. A full Hinode request needs to be made, even if only a single instrument is desired--- it is easiest for the Hinode team if all observation proposals follow the same path.
After acceptance, the proposer is notified. About one or two weeks before the observations are scheduled to happen, the instrument teams will contact the proposer to further refine the observation details. Therefore, a proposer should not worry about getting their proposal "exactly right" on the first attempt. There will be opportunity to work with the instrument teams to optimize the plan with their expert knowledge.
The proposer, or their representative, is expected to be available for planning and discussion during the days when the observations are scheduled. (This is discussed a bit more in the following section, Overview of XRT Operations.)
By one to two weeks after the observations, all of the scientific data will typically have become available to the proposer. (This delay is due to the nature of the download schedule and the nature of the on-ground telemetry collection and assembly.) Any presentations or publications using Hinode data should include proper accreditation[5]. (The XRT team also requests that all publications and news reporting about XRT be brought to our attention.)
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In this section, an overview of XRT operations is provided. Strictly speaking, this information is not necessary for one to prepare the XRT section of a Hinode observation request. However, this information is helpful for understanding the context and scheduling of one's requested observations.
Note that this section describes nominal operations, which is the proper assumption to make when preparing an observation request.
Three times a week, new observation and spacecraft instructions are uploaded over the course of a few hours. These instructions control the spacecraft and instruments until the next scheduled upload. The instrument teams all coordinate their observation plans, but the technical details of the observations are designed by each instrument team independently.
Obviously, in order for the XRT Team to prepare instructions for some requested observations, all of the technical details need to be negotiated sufficiently early before the upload such that the XRT Chief Observer has time to implement those instructions. The lead time will typically need to be 2-5 days, depending upon the time of week and the nature of the changes. (The lead time to change the target pointing is shorter, but is an absolute minimum of half a day.)
The observation proposer (or their identified representative) will typically be contacted by the XRT team one to two weeks before their observation dates, in order to start negotiating the details and preparing the observation plan. The proposer is expected to be available by email contact from then on, until the end of the observation dates.
An important item to note here is how this arrangement impacts the interaction of the observation proposer with the XRT team (as well as with the other instruments). The proposer MAY NOT expect to alter the upcoming observation plan less than a day or so before it runs (except for the pointing). Such alterations are simply not possible due to the preparation schedule.
[[It would be nice to have a table here that splits the week into the three OP periods, and suggests corresponding "due times" for changes to be submitted to XRT.]]
Another important thing for the proposer to remember is that XRT has regularly scheduled engineering and scientific activities, which places some constraints on the negotiation of the observing windows.
After the data are taken, it eventually is downloaded to a ground station and forwarded to the operations center at ISAS, Japan. This data stream is a mixture of packets from the spacecraft and all of the instruments, so it is a non-trivial process to collect and re-assemble the data. The data for a specific observation block may not all be reformatted consecutively. There is typically a wait of a week or two for all of the data to become available. Before that, the XRT team has access to "Quicklook" data, so that they can evaluate within a day or two how good the observations were. The Quicklook data are NOT adequate for scientific analysis.
The data will show up in the Hinode and XRT data-servers as it becomes reformatted. At that point the data are available for scientific analysis
In this section, XRT observing plans are described sufficiently for a non-expert to design an observation plan. Many complexities are not addressed here, and it is assumed that the proposer will have a future opportunity to refine the observation plan with the assistance of XRT team members. The objective of this overview is to help a proposer prepare an XRT request that can be reasonably evaluated on its scientific merit by the Hinode Scientific Schedule Coordinators.
Note that this section only addresses the XRT section of a Hinode observation request. The other instruments have some similarities and some differences to what is described here.
XRT observation planning starts from the premise that a specific scientific objective is pursued by taking a set of image exposures. The particular combination and ordering of images is arranged to obtain useful data while optimizing the usage of limited instrument resources. Usually, this set of exposures is taken repeatedly, or "looped", for the duration of the observing window. A particular set of observations of this sort is called an "XOB", and represents the unit of design which the proposer must describe in their request.
So what are the components of a complete XOB? First, there needs to be a description of the science objective and target. This is not merely a justification for the observations--- it also provides guidance on setting priorities on the limited instrument resources, and ensures that the XRT planners (who are all scientists) understand the purpose of the observations as they apply their judgement. Second, there needs to be a list of the types of exposures to be taken. XRT is a versatile instrument and has many exposure parameters, which will be described shortly. The cadence of the image set must be specified, and furthermore, it is possible to nest or to separate sets of image with differing cadences. Also, XRT can switch to a different XOB for flare events, which provides even more flexibility when designing flare observations. And a specific implementation of an XOB has an associated estimate of the necessary telemetry footprint. Each of these will be discussed below. Lastly, the observing times and target pointing must be specified; however, these are not just XRT-level but also Hinode-level details, because they affect the other instruments directly.
Items 5.1--5.5 apply to the entire Hinode request, because they affect all of the observatory.
+ - 5.1 Contact Info
Who is the proposer and what is their email address? If someone different will be the campaign point of contact during the observations, provide their contact information, too.
+ - 5.2 Brief Description of scientific objective
A paragraph or two describing the scientific objectives of this proposal. Indicate any other observatories that will be coordinating on these observations. This section will be used to evaluate all of the following parameters for consistency.
+ - 5.3 Observing window (schedule and times)
What dates and times of day are the observations requested for? This is usually constrained by coordination with other observatories. Also, it is helpful to give some indication of how firm these times are. Another possibility is the "Target of Opportunity", where the appearance of a particular solar feature will determine the observing time. For example, "follow any active regions between Oct.10 to 23."
+ - 5.4 Target type
What solar features are to be aimed at? Examples of typical targets: active regions, quiet Sun, filaments, the solar limb, x-ray bright points, coronal holes, et cetera. This should be consistent with the scientific objectives, of course.
+ - 5.5 Pointing (target location on the solar disk)
All three instruments are aligned to the pointing of the Hinode spacecraft. SOT and EIS have very small fields of view; XRT is capable of Sun-sized fields of view, but these can be wasteful of precious telemetry, so accurate pointing is still important. Some scientific objectives dictate fixed pointing at the solar limb, poles, or Sun-center. Others simply need to track the appropriate target. Yet another possibility is that the proposer will need to make daily updates to the pointing, but this should be justified by the scientific objectives. To prepare the observation request, this is the level of detail that is useful. Actual pointing coordinates will be determined later, when the instrument Observers are directly communicating with the proposer.
Items 5.6--5.18 are XRT-specific details.
Because of telemetry constraints, it is often necessary to prioritize the following parameters. The proposer needs to prepare a request that asks for a reasonable amount of XRT's daily telemetry allocation (between 0% and 100% is a good start). Further refinement of these details will be possible if the proposal is accepted.
+ - 5.6 Scientific priority (XRT)
In order to help set parameter priorities, the XRT Team typically assigns each observation plan to one of the following categories: Topology/Structure, Dynamics, Thermal Study, or Flares. The proposer should indicate which one of these best fits the scientific objectives. This will communicate a lot to the XRT Team about how to design the XOB to achieve the objectives. (Note that these categories are not mutually exclusive, but they do imply competing priorities on instrument resources. The input of an XRT expert might be required for complex objectives.)
A Topology/Structure plan suits observations of coronal structure, whether it is the large-scale orientation of large features, large areas to collect statistical distributions, or high resolution details of coronal loops. These plans give priority to the size of the field of view, the level of resolution, and possibly the level of compression.
A Dynamics plan suits observations of rapidly changing features. (Examples would be waves, flows, loop evolution, and transient events.) These plans give priority to the image cadence, in order to sufficiently sample the changes.
Thermal Studies are appropriate for doing temperature analysis of coronal plasmas. These plans give priority to a wide selection of analysis filters, which is how XRT performs temperature discrimination.
Flare studies are concerned with collecting lots of flare-time data. Most of the time, the Sun is not exhibiting a large flare. So these plans will usually take a lot of data in a short interval, and almost no data at other times. If requesting a Flare-type XOB, the flare-time observations should be described, but some description should also be provided of the non-flare-time observations (these are usually designed as secondary context data).
It is possible to construct an XOB as sets of images that loop at different frequencies. To make an analogy to computer programs, images can be arranged in nested and sequential loops (but there are not really any conditionals). The simplest way to use this flexibility is to design the XOB to have two sets of images. One set happens more frequently at a specified cadence, and it is referred to as the "Main loop". The second set then happens at a slower cadence, and this is referred to as the "Context loop". The principal observations necessary for the scientific objective should be put in the Main loop. Any auxillary observations, or images which may be allowed to be taken at a slower pace, are put in the Context loop. For example, a "Dynamics" XOB to study loop oscillations may operate its Main loop at a fast cadence (say, every 60 seconds) with images that have a small field of view. The Context loop might then have larger images to see the surrounding active region environment, but only recur once every 30 minutes. Another way to think of this: the Context loop is used to achieve secondary scientific objectives.
Items 5.7--5.12 are used to describe the Main loop, and then items 5.13--5.18 are repeated to describe the Context loop with the same parameters.
+ - 5.7 Cadence [sec] (Main loop)
What is the frequency (in seconds) at which the Main set of images should be repeated. This parameter has a large impact on the telemetry, so it is better to request the lowest frequency that will achieve the scientific objectives.
+ - 5.8 Continuity [min] (Main loop)
Longest allowable gap in observing runs (in minutes). For instance, during eclipse season, there are enforced gaps in viewing that may be several minutes long. Would such gaps prevent achieving the scientific objectives at this time?
+ - 5.9 Field of view [arcsec] (Main loop)
The XRT CCD has 2048x2048 pixels, at about 1 arcsecond each. So the full XRT is about 2048x2048 arcsec, which is a bit larger than the apparent solar disk. The proposer is encouraged to select a FOV from the following list that best suits the scientific objectives. This parameter has a large impact on the telemetry, so it is better to request a well-matched FOV. (The proposer may expect the XRT Team to help them further optimize the FOV after the request is accepted.)
full-Sun [Sun-centered]: Sees the entire solar disk
Sun-sized [target centered]: A FOV that is about the size of the solar disk, but is centered on some target other than the center of the Sun. In this option, because the XRT full FOV is only a little larger then the Sun, the entire solar disk is usually not within the FOV.
Active Region-sized: self-explanatory. Also an appropriate size for observing coronal holes.
EIS-sized: A small FOV which matches well the FOV of the EIS and SOT instruments. This option is useful for observing plans where XRT is in a supporting role to those instruments.
limb-strip: A long, thin image strip. This option is useful for observing the solar limb or poles
+ - 5.10 Resolution / Compression (Main loop)
There are two parameters which affect the spatial resolution of XRT: image binning level ("resolution"), and image compression. Binning combines CCD electrons into larger effective pixels, in groupings of 2x2, 4x4, or 8x8 CCD pixels. Compression does not change the effective area of a pixel, but the more aggressive levels of compression are lossy (i.e., information is lost), and sometimes image artifacts may be produced. The XRT Team recommends that either binning or compression be used, but not both together. Each of these options has a large impact on the telemetry. The proposer should choose one of the following options.
Compression, lossy: This is the default option. It is used with the 1x1 (i.e., non-binned) resolution level, and is appropriate for studying bright features, such as active regions.
Binning, 2"x2" pixels: No compression. This option is good for scientific objectives which emphasize large FOV, thermal studies, or coronal holes.
No binning AND no compression: This is a scientifically desirable option, but is VERY expensive in telemetry. This option is only feasible for a short burst of observations (e.g., less than 30 minutes). The possible achievable duration is heavily dependent upon the selection of the other features that also impact telemetry.
+ - 5.11 Filters (Main loop)
XRT has a set of x-ray analysis filters with different energy responses. In effect, observations with a set of filters can be inverted for information about the plasma temperatures and emission measures. For scientific priorities like Topology/Structure or Dynamics, it is usually better to sacrifice thermal diagnostics for speed of cadence or resolution. For Thermal Studies, a large set of filters should be requested. After the observation request is accepted, the XRT Team will help optimize the selection of filters to meet the scientific objectives. The proposer should choose from the following list of options.
single filter, broad response: Will usually be one of the broad response filters that see the widest range of coronal temperatures, starting at around 1-2 MK. Good option for high cadence or support-role observations.
two complementary filters: Good overall option, with some minimal temperature information.
single filter, high temperature: The response starts at 3 MK or higher. Good for Flare studies.
several filters: Good for Thermal Studies.
all filters: Excellent option for Thermal Studies and DEM analysis. However, will require a slow cadence, so best for short observations.
+ - 5.12 Exposure (Main loop)
XRT has on-board dynamic exposure control ("AEC"). This facilitates optimized exposure times for the target, and helps to protect the CCD against quick changes in solar intensity (i.e., flares). However, some scientific objectives require deep exposures. The proposer should choose from the following list of options.
dynamic, moderated: This is the default option. Images exposures are automatically adjusted to optimize for the target brightness.
dynamic, longer: It is possible to request deeper exposures from the AEC.
fixed exposures: This option does not use the AEC--- instead, the exposure durations are completely determined in advance. This option makes sense for a few scientific objectives, but is also dangerous. The proposer should present a strong argument for requesting fixed exposures.
Items 5.13--5.18 are repeats of items 5.7--5.12, but are used to define the Context loop of observations.
These parameters can be evaluated or chosen differently for the Context loop than they were for the Main loop. In fact, they SHOULD be chosen differently, such that the Main and Context loops are complementary and jointly effective for achieving the scientific objectives.
5.13 Cadence (Context loop)
5.14 Continuity (Context loop)
5.15 Field of view (Context loop)
5.16 Resolution/compression (Context loop)
5.17 Filters (Context loop)
5.18 Exposure (Context loop)
The Hinode website to help one prepare an observation request: http://www.isas.jaxa.jp/home/solar/guidance/index.html
The XRT website to help prepare the XRT section of an observation request: http://solar.physics.montana.edu/HINODE/XRT/request_obs.html
The general XRT website is at: http://xrt.cfa.harvard.edu
The XRT Analysis Guide is at: http://xrt.cfa.harvard.edu/resources/documents/XAG/XAG.pdf