How to Locate and Retrieve the HST Ephemeris

Users who need the location of HST during their observations will find the information in the HST ephemeris file, also known as an ORB file or an orbit file. Each ephemeris file covers 3 days, with a resolution of 60 seconds.

The ephemeris files are archived and available to any user. First the user must locate the ephemeris file(s) of interest using StarView. Retrieval of files may be done via StarView or MASTWeb can be used.

Using StarView to Locate HST Ephemeris Files

Check the StarView website for information about StarView. This site also allows the user to download and install the latest version of StarView.

Bring up StarView.
Under the Searches item in the top menu bar select HST -> Data Files
This brings up a search screen. There are 2 parts to the screen. The top portion is where you qualify your search and the lower portion displays the search results.
Fill in the following 2 fields.
- Extension: POD
- Archive Class: ORB
- Data Start Time: a small range of dates that encompass your observations
  The syntax is YYYY-MM-DD..YYYY-MM-DD. The .. is important. It indicates the input values are a range. For example, 2007-01-01..2007-01-04.
Execute the search by clicking on the binoculars button at the top of the page.

In the Results portion of the screen you will see a list of Dataset Names in a box on the left hand side of the screen and a Data Start Ti... field to the right of the box. As you click on Dataset name, the value in the Data Start Ti... field will change. You want to select the Dataset names whose start times immediately proceed the start times of your observations.

See below for retrieval via either StarView or MASTWeb.

Using StarView to Retrieve HST Ephemeris Files

This is a continuation of the above section on locating HST ephemeris files. It assumes the results screen from the location stage is available.

In the Results portion of the data files screen, select the files to retrieve by clicking on the dataset name in the box on the left hand side of the screen, then clicking on the Mark or All button at the top of the screen. There are many ways to mark datasets for retrieval. Do not be afraid to experiment.
A StarView retrieval screen will be displayed.
In the StarView retrieval screen, confirm that all desired files are listed and have a check in the "Marked" column.
Files may be added by returning to the results screen and selecting more dataset names.
In the StarView retrieval screen, when all desired datasets are listed and marked, click on the Submit button at the bottom of the screen.
The HST Retrieval Configurations Options screen is displayed.
Fill in the fields, specifying either an archive account or anonymous (check the box)
- Check the "Show override (Expert only)" box in select data type(s) for retrieval on the middle left hand side of the screen.
- Enter POD in the Extensions box and check "Fetch only listed extensions." Both are at the bottom of the screen.
  The "Calibrated/OTFR" box should be automatically unchecked at this point. If it is not, uncheck it now.
- On the right side of the screen, Choose a method of data delivery, select a method for data delivery. NB: Anonymous users may use FTP Delivery, but will be prompted to enter a username and password.
Click the DONE button at the bottom of the screen
A Submitting Request dialog box will be displayed. After the request has been delivered, that dialog box is dismissed and another dialog box, containing the Request ID, is displayed. Click OK to dismiss the Request ID dialog box.

The standard e-mails regarding submission and completion of the request will also be sent.

Using MASTWeb to Retrieve HST Ephemeris Files

If the name of an HST dataset is known a priori, it may be retrieved without going through the standard MASTWeb search pages. This is done through the dataset lookup page. Navigate to this page from the main MAST page by clicking on Hubble on the lower right hand gutter, then, under "Search & retrieval" in the left hand gutter, select "Retrievals by dataset." Or, go directly to http://archive.stsci.edu/cgi-bin/dataset_lookup .

Go to MASTWeb, retrieve by dataset name page:

http://archive.stsci.edu/cgi-bin/dataset_lookup
Enter the Dataset Names in the box titled "Enter a list of datasets."
Click on "Enter a list of datasets" to get help on options for entering the Dataset names. You may be able to submit an already prepared list of names. Case is important here. The filenames should be in UPPER case.
After the list of datasets is entered, click on the "Submit dataset for retrieval" box at the bottom of the page.
Another form will be displayed. The list of Dataset Names will be displayed along with other information. All should have Class ORB. All Dataset Names should start with a P.
If not already checked, check the Mark box (or click on the "Mark all" button). Then Click on the "Submit marked data for retrieval from STDADS" button.
The Retrieval Options page will be displayed. Fill the top section as you would for any other HST data.
You can use anonymous as the Archive Username and your e-mail address for the Archive Password. Using anonymous and requesting ftp delivery will require the entry of a username and password on the right hand side of the page.
Unclick the "Calibrated" box under "Science Files Requested:." At the bottom of the page, enter POD in the "File Extensions Requested" box. Click on the "Send retrieval request to ST-DADS" box.

The usual "Request Sent to ST-DADS" page, listing the Dataset names to be retrieved, will be displayed. The e-mails regarding the submission and completion of your requests will also be sent.

The files you get will be named like pr150000r.pod. The file can be paged on a unix system.

Increasing the Resolution of the HST Ephemeris

As archived, the HST ephemeris has a resolution of 1 minute of time. Some users need a finer resolution. Although we can not provide the HST ephemeris at a higher resolution than 1 minute, a technique does exist to interpolate between the discrete points.

Start with the HST state vector from the data headers and do a linear interpolation between HST ephemeris points.

The HST state vector, that is the X, Y and Z components of HST's position and velocity, in the header should be the state vector at the start of the exposure. For HST data the information is in the shf file for the exposures.

For example, proposal 10500 produced products named J9CX01010, J9CX01020, J9CX02010 and J9CX02020. The association file, J9CX01010_asn.fits, shows exposures J9CX01CEQ, J9CX01CHQ, J9CX01CKQ, J9CX01CNQ, J9CX01CQQ, J9CX01CTQ, J9CX01CZQ and J9CX01D5Q comprise the product. It is the shf file (e.g., J9CX01CQQ_shf.fits) for these exposures that contain the HST state vectors at the start of each exposure that makes up J9CX01010. Depending on what was requested when the data were retrieved, the shf file may or may not be present. Request uncalibrated data to get the individual member exposures, which will include their shf files.

Continuing with the same dataset, there will be 8 state vectors, one from each exposure that comprises J9CX01010. Since the data cover about 43 minutes of time, the ephemeris file should yield about 45 data points, with the first point at a time before the start of the first exposure and the last point from a time after the last exposure. A curve can be fit to the data.

Decoding Ephemeris Filenames

The archived version of the HST ephemeris files are named to follow the engineering file conventions (ICD-19), with a first character of 'P' and an encoded time for the start time of the file. The syntax is PYMDhhmm, where P indicates an ephemeris file, Y is the year code, M is the month code, D is the day code, hh is the starting hour and mm is the starting minute. The codes start at 1 and increment through the single digits, then start into the alphabet. For Y, year 1 is 1980. For M, January = 1, December = C. For D, the first = 1, the tenth = A, the 31st = V. For example, dataset 'PNBP0000R' starts in year 'N' = 1980 + 23, month 'B' = November, day 'P' = 25, hh:mm = '0000'.

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