1 | Introduction
2 | Getting Started
3 | Basic Instructions for Finding Images
4 | A Simple Example of Finding and Downloading Images
5 | A More General and Typical Example
6 | Finding Objects That Are in the Solar System
7 | How to Make a Color Image From WISE
Searching for “Fixed” Objects in the Sky
If you are looking for an object that is relatively stationary in the sky (i.e. not an asteroid, comet, or other object located in the Solar System) then follow these instructions. If you are looking for an object in the Solar System then please skip ahead to Section 6: Finding Objects That Are in the Solar System.
For this section, always keep the default settings for Image Set: All-Sky Release and Single Object.
1) You can search for astronomical objects by their name or their coordinates
Name: This can be the common name or catalog name of an astronomical object. The system will use two astronomical online databases to recognize the name: SIMBAD or NED
(Note: Both "Try NED then Simbad" and "Try Simbad then NED" will work; it doesn't matter which one you choose. SIMBAD generally contains objects located within the Milky Way Galaxy [stars and nebulae, e.g.: Rigel, Eagle Nebula, M8]. NED contains objects outside the Milky Way Galaxy [other galaxies, e.g.:M81, Triangulum galaxy]. However, most common extragalactic objects are found in both databases).
The service will automatically fill in the object's coordinates.
Coordinates: If the object name is not recognized, or you are looking for a specific spot in the sky, you may search using astronomical coordinates. You may use equatorial coordinates (commonly referred to as Right Ascension and Declination - Equinox 2000 is assumed unless otherwise stated, e.g. 'J1950') or galactic coordinates. Equatorial coordinates can be give in sexigesimal format (HH MM SS, +/-DD MM SS) or decimal degrees (DDD.DDDDD,+/-DD.DDDDD). Galactic coordinates are given in decimal degrees.
Example: (enter any one of the following to search for the same object, a galaxy):
Messier 81, m81, NGC 3031, n3031, UGC 05318 or
09 55 33.1730, 69 03 55.061
147.8637519, 69.3022964 equ b1950
148.8882208, 69.0652947 equ j2000
142.0918406, 40.9001409 gal
2) After getting the position of the object you wish to search for, you now have several options to choose from for selecting the image(s) to be retrieved. To understand these choices it is useful know a bit more about the data…
As WISE orbited Earth, it captured four simultaneous images (one per wavelength band) of each piece of sky every 11 seconds. Each orbit, the telescope scanned a circle slightly different from the last. So there was a good deal of overlap from orbit to orbit, especially near the ecliptic poles. After 6 months the mission had covered the entire sky and any given place in the sky had been covered several times. This animation shows this clearly.
The data available online are both the individual frames, (also known as single exposures) and an atlas of images that are made by combining the multiple exposures together. The advantages of the combined (atlas) images are that they achieve greater sensitivity to faint objects, have reduced instrumental noise, and have some instrumental artifacts as well as moving objects removed. The combined images are square in shape (1.5 x 1.5 degrees) and are called tiles. The tiles are quilted together to form the WISE image atlas. Long story short; 'Atlas' images are bigger, higher-qualilty, and nicer-looking than the 'Single Exposure' images.
Here is an image showing how the tiles are quilted together near the celestial poles. The colors represent the number of overlapping tiles for each section of the sky, with purple being the minimum (1) and white being the maximum (6). Note the large amount of overlap right near and at the pole:
Here is an image showing the quilting pattern of atlas tiles at the celestial equator (notice the very minimal amount of overlap—the blue lines indicate an overlap of 1 tile and white is an overlap of 4):
3) You will need to select a Search Type and Region. There are four possible search types.
The first search type is “Image contains target.” This means you are looking for images that contain the object coordinates determined in step 1. For this search type, there are also two more options (found just beneath on the same screen):
- Return Image Size: The image size is going to be a number that represents the height and width of a square image given in angular size (degrees, arc minutes, or arc seconds). Single exposure images are 47x47 Arc Minutes in size and full-sized Atlas image tiles are 1.5x1.5 Degrees. If you put in a number larger than the Atlas image tiles you will only get back images that size (1.5 degrees). If you make this field blank you will automatically get the full-sized image (single exposure or atlas tile). For beginners, it is recommended to make this field blank and keep the rest of the default options on this screen as they are (which will result in a 1.5x1.5 degree Atlas image tile with your object in it).
- Return only the most centered image containing the target?: For single exposures there will be several images containing your coordinates. In atlas tiles there can be more than one as well, depending on the quilting pattern of atlas tiles at the location of your search coordinates. Answering “Yes” to this prompt will only return one image, the one where the coordinates are the most centered in the image. Answering “No” will return all images that contain the coordinates.
The second search type is “Image covers entire search region.” In this type of search it is assumed that you are searching for a region around the coordinates, rather than only at the point of the coordinates, and that you want to find images covering that entire region. This has the same options as above, along with one more additional option:
- Search Region (Square) Size: This is the angular size of the region you are looking for in either Degrees, Arc Minutes, or Arc Seconds. If you select a region that is bigger than your Return Image Size, the image size returned will be at least as big at the region size.
The third search type is “Image is entirely enclosed by search region.” This option is useful for finding only images located fully inside of a search region. For example, if you are looking for all of the atlas tiles inside of a 7x7 degree patch of sky centered on a specific object or coordinates, and do not want any of the tiles that spill off of the edge of that region. This search type has only one option (search region size) and will return all images that completely fit inside the search region. The search region size must be at least twice the distance from the search coordinates to the furthest edge of the full sized image, or else there will be no images found. In the example of M81, the galaxy lies in the lower right corner of an atlas tile. It is farthest away from the left edge, at a distance of about 1.44 degrees. So the search region size must be at least 2.88 degrees or larger in order to completely contain one tile containing the search coordinates.
The fourth search type is “Any pixel overlaps search region.” This search type is similar to the one above, however it returns any images that contain any part of a search region. It has only one option (search region size) and will return all images that cover any portion of the search region.
4) You can select to search images from specific WISE wavelengths:
W1 = 3.4 Microns
W2 = 4.6 Microns
W3 = 12 Microns
W4 = 24 Microns
5)Finally, you can select to retrieve the single exposures or atlas tiles or both.
Single Exposure: the individual exposures taken by the telescope. These images have only been minimally processed and cover 47x47 arc minutes.
Atlas: these are images made by combining multiple single exposures together. The advantages of these highly-processed images are that they achieve greater sensitivity to faint objects, have reduced instrumental noise, and have some instrumental artifacts as well as moving objects removed. These combined images are square in shape (1.5 x 1.5 degrees) and are called Atlas images are recommended for making color images because they are higher-quality (note that your desired object may not always be centered in the middle of the atlas tile).
There are additional options for retrieving these images (in the 'Optional constraints' box), but they are not necessary to consider in order to complete the search.
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