A question about the ID format in the desi_dr1.x1p5__zpix__sdss_dr17__specobj cross-match table

Question

I have a question concerning the details of the desi_dr1.x1p5__zpix__sdss_dr17__specobj table.

This table appears to be the result of a cross-match between SDSS DR17 and DESI DR1, providing parameters that are crucial for matching these datasets. I truly appreciate the work that went into creating this resource; however, I am having trouble understanding the format of the id1 column.

When comparing id1 with most of the ID fields under the desi_dr1 catalog, I found that they do not match. In fact, there is a difference in the number of digits between them. As a result, I am unable to use the IDs from this table to retrieve subsequent DESI data.

Does anyone know how this id1 is constructed or which specific DESI identifier it corresponds to?

Answer 1

Thank you for your question. I ran a simple test query:

SELECT TOP 10 x.id1, x.id2, z.z, s.z
FROM desi_dr1.x1p5__zpix__sdss_dr17__specobj AS x
JOIN desi_dr1.zpix AS z ON z.id = x.id1
JOIN sdss_dr17.specobj AS s ON s.specobjid = x.id2

This returned 10 results as expected; in this case id1 is exactly desi_dr1.zpix.id, and you can use that to obtain any data from the corresponding rows of desi_dr1.zpix. However, desi_dr1.zpix.id is specific to that table and is intended for queries like the one above, rather than being a universal identifier of DESI objects.

To help you further, could you please describe in more detail the sort of query you are attempting to construct?

Comments

Thank you! Actually, you’ve already solved my problem. I previously thought that zpix didn't correspond to id1 because the id in the first column of the zpix data I downloaded had a different number of digits than id1. In reality, I had overlooked the fact that this length can be variable.

My original intention was to use targetid which is the unique identifier within the DESI project (now  I know it) to obtain the photometry data corresponding to id1 for later use. I have now successfully implemented this process. Thank you very much for your help!