Anatomy of a plate

There are many different markings and types of holes in your plate. The poster below gives you some information about what their purpose is. This poster shows a generic plate, but yours will be similar.

SDSS Science Survey Archive

To find out about all the objects on your specific plate, you can access the SDSS Science Archive Survey (SAS) site by clicking the button below. Enter your plate number in the box under PlateID(s) to find your plate. On the table below the search box, an entry should appear with information about your plate.

The relevant columns are explained below:

• PlateID: unique integer used to identify the plate.
• MJD: integer denoting the night of observation.
• RA and Dec: coordinates on the sky corresponding to the centre of the plate.
• Survey and Program: name of the survey and program the plate is associated with within the SDSS
• Quality: sometimes searching a plateID might output more than one result, look for the row with good quality
• Spectra: click on the eye symbol below Spectra to output a list of all the objects on your plate.

You will be taken to the page below which shows a list of all 1000 objects on your plate:

Here we have some of the same columns as above such as PlateID, MJD, RA and Dec. In addition to those, some other important categories include:

• FiberID: number of the optical fibre connected to that hole
• specobj_id: unique number that identifies the object observed
• z: redshift of the object
• Class: type of object – QSO (quasar), STAR or GALAXY

To learn more about a specific object on your plate click on the icons in the column called Plot. This should take you to another page with a plot of the spectrum as well as some information including the coordinates (RA and Dec) and type of the object selected.

DESI Legacy Imaging Survey Sky Browser

We can also use the web viewer of a different astronomy survey, the DESI Legacy Imaging Survey, to quickly visualise all the objects on your plate superimposed on a photograph of the sky. There are two methods to do this.

Method 1

This is the simplest method, but might not work for all plates. In this method you simply modify the following URL to replace the XXX with your plate number:

https://www.legacysurvey.org/viewer-dev?plate=XXXX&spectra&sdss-plates

E.g. for plate 9365, I can do: https://www.legacysurvey.org/viewer-dev?plate=9365&spectra&sdss-plates. Stars will appear circled with red circles, quasars with blue circles, and galaxies with white circles.

The background images, by default, are from the DESI Legacy Survey – taken using a different telescope. To see your objects superimposed on SDSS imaging, locate the “-Images/+More surveys” menu on the top right of the screen, and choose SDSS.

Method 2

This method is more involved, but should work for all plates and you can control which objects appeared circled on the image. We use an SQL search to obtain a CSV file with all the objects on your plate. You can learn more about SQL searches and how to use them in the Finding Information About an Object page. Then, we will upload this file to the DESI navigator to obtain a map of all the objects on your plate.

1. Access the SQL Search by clicking on the button below. It will take you to a webpage with a SQL Search box.

2. Delete all the text in the box and copy and past the text below. replacing XXXX with the 4 digits of your plate number and select CSV for the output format. The query will return information of up to 1000 objects, ordered by brightness (brightest ones on top). You can change the query to only return the brightest 20 objects – if, for example, you want to visualise your plate’s constellation – by replacing “1000” with “20” on the top line.

select top 1000 spa.ra, spa.dec, spa.dered_r as magnitude, spa.class, spa.z, soa.xfocal as x, soa.yfocal as y 
from specphotoAll as spa
join specObjAll as soa on soa.specObjID = spa.specObjID
where spa.plate = XXXX and spa.dered_r > 0
order by spa.dered_r asc

3. Click “Submit”, and your web browser will automatically download a csv file with one row per object on your plate.

4. Next, access the Legacy navigator by clicking the button below.

5. On the bottom left of the screen, you will see a small box for custom catalogue uploads. Click “choose file” on the bottom left of the screen and select the CSV file you downloaded in step 3, then press upload. You should end up with something similar to the image below. The objects highlighted in green are the 1000 objects on your plate.

The background images, by default, are from the DESI Legacy Survey – taken using a different telescope. To see your objects superimposed on SDSS imaging, locate the “-Images/+More surveys” menu on the top right of the screen, and choose SDSS.