High-cadence & high time-resolution observation
Science case requiring high-cadence and high-resolution observations, such for instance lunar and KBO occultations, transits of extrasolar planet, exo-planetary transits, etc., can be conducted by using the BURST or FAST PHOTOMETRY modes recently offered with HAWK-I. In these modes the fraction of time spent integrating is typically ~80% of the execution time and the minimum DIT is in the range ~0.001 - 0.1 sec. This is achieved by windowing down the detectors to speed up the observations and to decrease the overheads. In what follows we will show an example of fast photometry to timely monitor the transit of an exoplanet. Usually in these cases, the observer is interested in performing relative photometry, hence what matters is the differences between the magnitude of the target and of a reference source. Being both generally quite bright, such observing strategy does not involve any sky background measurements. However, even if in the following example we will stare on target to acquire many frames without moving away from it, the template we will use, i.e.
Click here to go back to the HAWK-I main tutorial page where you can find other examples for HAWK-I observations.
In this tutorial we will prepare an OB that perform fast photometry of a sparse field in the Ks filter. Your goal here is to observe the transit of an exo-planet around a bright star whose coordinates are: RA(2000) = 03 15 00.31, DEC(2000) = -07 24 25.4. For convenience hereafter we will call the target ESO-1 (Note: This is a fictional exo-planet).
The sample OB will illustrate the use of a variety of features of P2PP (version 3) and illustrate the kind of decisions to be taken at the time of preparing in advance an observing run, as well as some aspects that are specific to the preparation of OBs for HAWK-I.
The Phase 2 process begins when you receive an email from the ESO Observing Programs Office (OPO) telling you that the allocation of time for the coming observing period has finalized and that you can view the results by logging into the UserPortal and clicking on "Check the webletters." Note that the username and password that you need to use for the User Portal are the same as those you will use to prepare your OBs.
You follow the instructions given by ESO and find that time was allocated to your run with HAWK-I. Therefore, you decide to start preparing your Phase 2 material. First, you collect all the necessary documentation:
- The HAWK-I User Manual
- The HAWK-I Template Manual
- The Service Mode instructions for VLT/VLTI and VISTA
- The P2PP Documentation
and you proceed with the installation of P2PP (version 3) on your machine if necessary.
Now you start with the definition of an OB for your science target.
For the sake of this tutorial, we will hereafter use the following P2PP information:
- P2PP ID: 52052
- password: tutorial
This is a special account that ESO has set up so that users who do not have their own P2PP login data can still use P2PP and prepare example OBs. You cannot use this account to prepare actual OBs intended to be executed.
After starting P2PP and logging in using the tutorial account, the P2PP main GUI will appear as follows:
Runs for a number of instruments appear in the
Select the folder corresponding to the HAWK-I Tutorial run, 60.A-9253(L). In this tutorial we assume that time was allocated in Service Mode. This is indicated by the SM letters that appear next to the RunID of the HAWK-I run.
You can now start defining your OB.
Click on the OB blue icon on the upper left side of the P2PP main GUI. This creates an entry under the Observing Runs area. The red dot next to the OB name means that the OB fails to pass some fundamental verification criteria, as may be expected from the fact that no template has been attached to the OB yet.
Now the next step should be naming the OB, however because in some special cases a particular prefix must be given to an observing OB, you check first on the Service Mode Guidelines webpage if a special naming convention must be given to your HAWK-I imaging OB. Consulting that page you find that your observations do indeed need a particular OB naming prefix. Because you will use the Fast Jitter mode your OB must be named with the prefix 'FAST'. Hence, you decide to name your OB: FAST_ESO1-Ks. Select the OB, press Enter and you type FAST_ESO1-Ks in the No Name field.
The P2PP main GUI should then appear as follows:
Double click on the OB name to have access to the main OB window where you will define the contents of your OB. At this point the OB main GUI should appear as follows:
It may be useful in many cases to have an easy way of identifying an OD (Observing Description), like when having observations of a number of targets performed with an identical instrument configuration and observation template parameters. The OD Name field in the View OB window allows you to define names for the ODs. The OD name appears in turn in the Obs/Calib Blocks area of the P2PP main GUI, thus allowing the identification at a glance of all OBs sharing ODs with the same name. In this tutorial the observation description will define the imaging observations in Ks-band. Thus, we enter the name Kfast-imaging in the OD Name field.
The User Comments field can be used for any information you wish, to keep further track of the characteristics of the OB, to alert the staff on Paranal to special requirements, etc. Here, we do not have any special information or requirements and leave this field blank.
The HAWK-I specific field refers to the Magnitude of the brightest object in the field hence it must be used to insert the J, H, and K magnitude of the brightest star present in the 7.5' x 7.5' HAWK-I field. In order to retrieve this information you query the 2MASS catalogue. When querying the catalogue you can provide a search box size of 7.5' x 7.5', that is already a quite big area given that you will later window the detector. The query result tells you that the brightest object in the whole field has the magnitudes: J=8.1, H=7.3, K=7.7mag. You find out on the HAWK-I service mode specific webpage that this is too bright for HAWK-I observations. However, since you want to use the Fast Photometry mode with DIT much shorter than 1 sec (i.e. 0.15sec) and the detector windowed, you consult the Exposure Time Calculator to check the saturation level when using such a short DIT. Since you will not exceed the limit of 5 time the saturation level you continue with typing J=8.1 H=7.3 K=7.7 into the Instrument Comments field (Note: imaging observations leading to strong detector saturation are not scheduled in service mode).
The first template that must be part of any science OB is the acquisition template, so let us define it next. In the Template Type list, make sure that the acquisition entry is selected. This will list all the acquisition templates available for HAWK-I in the Template list below to it.
After reading the description of the templates in the HAWK-I template Manual, you have noticed that the HAWKI_img_acq_FastPhot template is the acquisition template suited for your purpose. Therefore you click on the template named HAWKI_img_acq_FastPhot from the list of possible templates in the Template list. Then, you click on the Add button next to it. The window should now look like this:
Now you need to decide on the acquisition parameters, and if necessary, modify the default values given in the acquisition template.
The first fields to fill out in the acquisition template are the DIT (detector integration time for a single read) and NDIT (number of DITs to co-average) settings. Your decision for the values of DIT and NDIT depends on the brightness of the target. Your target is very bright, it is easily identifiable in a short exposure of 0.15 seconds at Ks-band, moreover since it is located in a sparse stellar field there is no need to take an initial sky frame to be subtracted from the science field hence, you then enter:
- DIT (secs): 0.15
- NDIT: 1
- Take initial sky frame? (T/F): F (i.e. not-checked, the telescope will not move off target to take a sky frame).
The next two parameters of the acquisition template refer to the offsets in arcsec in case one decides to take an initial sky frame. Here there is no need for such offset so you enter:
- RA offset to sky: 0
- DEC offset to sky: 0
Next step is to set the parameters which will define the dimension and position of the windows within the 4 detectors. The detectors are read in 16 vertical stripes, spanning 128 x 2048 px each. A given window can be defined in each of the stripes but the location of the windows in the 4 detectors are not independent. This implies that you will only have to set up the windows in one detector, then it will be replicated in the other 3 chips.
The choice of the dimension and geometry of the windowing is related to the selected DIT. The smaller is the window and the shorter is the allowed minimum DIT. To properly sample the transit with high time-resolution you figured out that you need to use a DIT=0.15 sec. After checking on the HAWK-I User Manual the allowed minimum DIT for a given windowed configuration, you decide to use contiguous regions (i.e. the windows on the individual strips are as wide as the stripes themselves, so within a given detector there are no gap along the X-axis). Along the Y-axis you instead decide to go for a 64px wide, hence the actual dimension of the windowed region on each detector will be 2048 x 64 px. To do so you enter:
- Number of columns for each window stripe: 128
- Number of rows for each window stripe: 64
- First column of window within a stripe: 1
Note that because you are only interested in two sources (i.e. science target and reference star) which will fall in 2 different detectors, what will matter to you are only 2 windowed regions (although the system will setup 4, one on each detector).
Now you need to decide where to place the window along the Y-axis. Such choice is tight to the distance of the reference star from your science target because you need to observed both sources to perform relative photometry. In this respect, the rotation of the field, which is defined by the parameter Position Angle on Sky (deg), is also very important for us because it will help us to properly place the science target and the reference source within the two windowed regions. Given the separation between the target and the reference star you measure that by rotating the field by 87 degree and placing the windowed region along the Y-axis between px 1020 and px 1084 both sources will fall in the regions. So, you enter:
- First row of window within a stripe: 1020
- Position Angle on Sky (deg): 87
Next, you have to decide about the telescope guide star. It may happen that one wants to observe a region in a large dark cloud where no optically bright star is present that can be used for the telescope active optics guiding. In such a case the parameter Telescope Guide star selection must be set to NONE. However, in the large majority of cases, and also in our tutorial example, there is no problem for the telescope operator to automatically find a telescope guide star from the catalogue. Therefore, in most cases and so for our example here, this field can be left at its default value:
- Telescope Guide Star Selection: CATALOGUE (Default)
Only in case the Telescope Guide star selection is set to SETUPFILE the values for "RA/DEC of telescope guide star" (the next acquisition template keywords) are used.
Finally, you have to enter the filter that should be used for acquisition. It is recommendable to choose the same filter as you will use in your first science template to minimize the overheads, and because you decide that this will be Ks-band, you choose from the filter drop-down menu:
- Filter Name: Ks
The OB main window should look like this:
To access the view window where you should provide the Target information click on the Target icon in the top left of the main OB window. The new OB GUI should appear as follows:
Here insert the target name, which could be the same name as used for the OB itself. In other words, insert Name: FAST_ESO1-Ks. Also, you need to provide the right ascension and declination for ESO-1. However if you enter the exact ESO1 coordinates then the target will end up within the detectors gap. Moreover, you need to remember that both target and reference star should fall in the windowed regions. To do so you calculate that you must apply a shift of 80" and 95" E-S to the target coordinates. So you enter:
- Right Ascension: 03:15:05.9
- Declination: -07:26:0.4
Furthermore, edit the entry in the target-tabbed subpanel Class. In our case, choose Planet for exo-planet. All other target related fields can be left at their default values.
The Target view window should now look like this:
As stated in Section 1, we assume for the purpose of this tutorial that the program has been allocated time in Service Mode. You thus need to specify a set of constraints, which indicate under which conditions your OB can be executed. You can do this by clicking on the Constraint Set icon which will give you access to the view window that looks like this:
Now you should fill all the entries specific to HAWK-I observations (i.e. only the first 6 entries)
Name: give a descriptive name to the constraint set about to be defined. Since you have decided that this constraint set will be applied to all Ks imaging observations, you type Ks_constraintsin the Name field.
Sky transparency: since you wish to perform relative photometry, you request Clear conditions in the Sky Transparency entry.
Seeing: since your target and reference source are both bright and you are interested in relative photometry you can run your observations with moderately poor seeing condition, you specify 1.2 as the value of the Seeing field.
Airmass: set the airmass to 1.6 to ensure that your observations are not carried out at too low elevation.
Lunar illumination and Moon Angular Distance: since you are doing broad band observations in the near-infrared, the lunar illumination has very little influence. You can thus leave the default values of 1.0 and 30 degrees for the Lunar Illumination and Moon Angular Distance fields.
Twilight (min): Because HAWK-I is an infrared imager, observations of bright objects may be carried out in twilight. From P91 onwards, there is a new constraint in P2PP3 called twilight constraint. This constraint can be used to define the earliest time with respect to the end of the astronomical twilight when the execution of the OB can be started. While the relation between the time difference from the evening twilight end and sun elevation varies during the year, for Paranal due to its low latitude this difference is small. Therefore the constraint is given in minutes as a difference in time with respect to the end of astronomical twilight (i.e. the time when the solar elevation is -18 degrees). The default value of twilight constraint is 0. A negative number means that it is allowed to start the observation before the end of the astronomical twilight. The twilight constraint can take values between -45 and 0 minutes. In this specific case, your observation can be done during twilight time, as soon as the Telescope Operator manages to acquire a suitable guide star (typically about 30 min before the end of twilight) hence you specify -30 as the value for the Twilight constraint.
NOTE: In your Phase I proposal you already specified some of these constraints (lunar phase, seeing, and transparency). At Phase II, you can relax your constraints to improve the chances of execution of your programme, but you cannot specify more stringent constraints.
The Constraint Set view window should now appear as follows:
We will assume now that your HAWK-I observations need to be executed in a particular time window, for instance between October 02-10-2012 and 05-10-2012. You can specify this in the Time Intervals view window, which is accessible by clicking the Time Intervals icon. See below
To provide the time window for the observation click the New TI button. A pop up window will appear giving you the option to select the date and time of the start and end of the observation.
Select 2012-10-02 and 2012-10-05 for Start and End date, respectively, and then press ok
The Time Intervals view window should look like this:
When you have done filling with the time interval constraints, the view window should appear as follows:
Once the view window Target, Constraint Set and Time Intervals are completed, the science template(s) can be inserted. To do so go back to the Obs. Description view window. In this example OB, we will insert only one science template as we wish to define an observing sequence of exposures with the Ks filter.
On Template Type, select now science. The existing HAWK-I science templates will appear.
Now you have to make a choice between the different HAWK-I imaging templates and to do so you consult the Template Reference manual. You then realize that to run high time resolution imaging (i.e. Fast Photometry) you can only use the HAWKI_img_obs_FastPhot template. So you select it from the template list. Click on the Add button to attach the template to the grid below next to the acquisition template selected and filled previously.
The OB view window should look like this now:
You have already decided to use a DIT of 0.15 and then, after consulting the Exposure Time Calculator to check the derired S/N, you decide to use a NDIT=200 and to take 13 exposures. The SATLEVEL keyword is set per default to 25,000 and you leave it like this. Because your target and reference star are bright and you will be performing relative photometry you don't need to spend time monitoring the sky background (by jittering or going off target). Your main goal is to constantly monitoring the transit hence you decide to stare on the same position and acquire many frames as possible (considering the 1hour rule for the length of a single OB). The HAWKI_img_obs_FastPhot template thus has the following parameters:
- DIT (secs): 0.15
- NDIT (secs): 200
- Toggle Burst Mode (T/F): not-checked (i.e. False)
- SATLEVEL: 25000 (Default)
- Number of exposures per offset: 13
- Return to Origin ? (T/F): checked (i.e., True) (the telescope will return to the initial position after the execution of the template)
- Observation Category: SCIENCE (Default)
- Absolute date of future event: 0 (Default)
- Absolute time of future event: 0 (Default)
- Jitter Box Width (arcsec): 0
- Number of offsets within a box: 1
- Filter Name: Ks
You can already check the execution time of this OB by clicking the Recalculate button on the right side of the Execution Time field.
... and the template section in your OB window should look like this now:
You can now close the OB window and go back to the P2PP main GUI. In there, under the Obs/Calibs Blocks subfolder you should see an entry with the following contents:
- Name: FAST_ESO1-Ks
- Status: (P)artiallyDefined
- Target: ESO-1
- OD: Kfast-imaging
- CS: Ks_constraints
- Acquisition: HAWKI_img_acq_Preset
- FindingCharts: (0)
You can reshape the columns as indicated in the P2PP User Manual to view the full contents of each entry. See below
Now to verify that your OB is free from error you can select Verify option from the Reports pulldown menu in the upper bar of the P2PP main GUI. This will produce a log message report listing possible warnings and/or errors that you should take care of. If you have followed step by step this tutorial then the log message should not report neither errors nor warnings.
By selecting the Execution Time option under the Reports pulldown menu you can check both the Execution and Exposure time of your OB, where the latter refers to the time free from overheads
Under the Schedule subfolder you should then see an entry with the following contents:
- Name: Fast_ESO1-Ks
- Priority: 1
- Contrib. to group:
- Abs. Time Intervals: 1
- Earliest After Prev.:
- Latest After Prev.
In case your run contains more than one OB (very likely), you can select a priority for this OB which is currently set to 1 as default. The priority is a number ranging from 1 to 10, where 1 corresponds to the highest priority. You can assign to your OB a priority by clicking on the number diplayed for your OB in the P2PP main GUI under the Priority column.
The next thing to do is to attach the respective Finding Chart(s) to the OB. The Finding Charts must be prepared as jpeg or jpg files and must fulfill all general and HAWK-I specific requirements for finding charts. You can use any tool of your choice to create the Finding Charts in jpeg, jpg format. P2PP, however, does not contain such option.
Let's assume you have prepared a jpg-Finding Chart for this tutorial run [remember: run ID 60.A-9253(L)], which you called 60.A-9253L.ESO-1.jpg, and which is saved in a sub-directory of your home directory.
Now, in the P2PP main GUI click on the OB which you want to associate with this finding chart, then select Finding Charts from the top menu bar, which opens a drop-down menu as you see here below:
From the drop-down menu select Attach Finding Charts, which will open up a new window that allows you to enter path and filename of the Finding Chart you wish to attach to the selected OB. In our example you choose 60.A-9253L.ESO-1.jpg
Finally click on the Attach Finding Charts button (you could select more than one Finding Chart). The pop-up window will close and in the P2PP main GUI, Obs/Calib Blocks under the Finding Chart column, you will see the entry
- Finding Charts: (1) 60.A-9253L.ESO-1.jpg
If you are interested in a more comprehensive explanation on how to create and attach or detach finding charts, you should have a look at this page.
With the completion of the OB, we consider the example developed in this tutorial to be finished. The P2PP main GUI displays the OB that we have prepared:
We will now submit the OB to the ESO Database: select the OB, go to the File menu in the P2PP main GUI, and select the Check-in option. A dialog box will appear asking for confirmation and, if you click on OK, it will be saved in the ESO Database.
Our tutorial with this example of creating and submitting the HAWK-I OB for Ks imaging using the fast photometry mode ends here. For the preparation of the Phase II material for a whole run, more OBs may have to be created. Furthermore, the complete Phase 2 package lincludes also the README file. As of P75, the README file is submitted along with the OBs by using the P2PP tool. A tutorial for the README file is available here . When all the OBs and the README file for a given run are submitted, the Phase II submission is finalized by pressing the Whistle icon.
As a courtesy to the next user who follows this tutorial, we would like to ask you to finish these exercises by removing the OBs form the ESO Database. The P2PP User Manual gives you detailed directions on how to do this. In short,
- Select Check-out from the File menu in P2PP.
- In the Database Browser window that opens, type 60.A-9253(L) in the Prog ID selection criterion
- Click on the Query button on the lower left.
- Select all the OBs that appear in the display area after the query. Normally there should be your submitted OB only, but if another user has submitted other OBs from this same account without removing them afterward you will see them as well.
- Under the File menu in the OB window, select Check-out.
In this way the OBs will be removed from the ESO Database and will be left in your Local Cache only. From there you can delete them if you like by selecting them and clicking the red X icon in the upper bar of the P2PP main GUI.
Click here to return to the HAWK-I main tutorial page.
HAWK-I P2PP tutorial
- 0: Goal of the Run
- 1: Getting Started
- 2: Your First OB
- 2.1: Define an OB with P2PP
- 3:Attaching Finding Charts and ReadMe File
- 4:Finishing the preparation and submitting the OBs