Using FCCT14 or ECFF18 astrometric debiasing in Find_Orb

• What FCCT14/ECFF18 is (and why you should use it)
• Using FCCT14/ECFF18 in Find_Orb
• Getting the debiasing data

What FCCT14/ECFF18 is (and why you should use it)

A lot of astrometry is referenced to star catalogs that have systematic biases in positions and proper motions. If you know which catalog was used to reduce a given observation, and where that observation was in the sky, it's possible to determine what those biases are, and to remove them. The resulting unbiased positions can get you a considerably better orbit solution.

As of 2019, much astrometry is being referenced to the Gaia-DR2 catalog. Such astrometry is, for practical purposes, bias-free. But that leaves you with a lot of older astrometry reduced to older catalogues, some of which had rather substantial biases.

FCCT14 is described by Farnocchia, Chesley, Chamberlin, and Tholen in Icarus 245 (2015) 94-111, Essentially, they broke the sky up into 49152 = 3 * 2^14 equal-area, almost-square "pixels", each covering a little under a square degree. Then they determined the biases for 19 different catalogs relative to a subset of stars from the PPMXL catalog, on a pixel-by-pixel basis.

ECFF18 is described by Eggl, Farnocchia, Chamberlin, Chesley, in an arXiv article. This time, the debiasing was done on 26 catalogs, relative to the Gaia-DR2 catalogue which has come out in the interim. ECFF18 should be used in preference to FCTT14. It both corrects for more catalogues and does so to a slightly better catalog.

While the debiasing tables were far from straightforward for the authors to put together, it makes debiasing rather simple for orbit determination software such as Find_Orb. It determines in which of those 49152 "pixels" a given observation falls, gets the 19 or 26 possible catalog biases, and applies the bias for the catalog used for our given observation. (The "pixels" are determined using the HEALPix method. They are about as close to being "square" as you can get, given that we are dealing with a sphere.)

• Turning FCCT14/ECFF18 on in Find_Orb

The Settings dialog contains a check-box to turn on debiasing.

For users of the console version of Find_Orb, there is no user-friendly method for turning debiasing on. You have to edit the file environ.dat in the Find_Orb folder, and change the line


to equal 1. You'll know this is working because, the next time you load up observations in Find_Orb, you'll be told that you don't have the file containing the debiasing tables. (And later, you'll see the biases in question shown in the "observation info" bit of the dialog, whenever you click on an observation for which bias data exists.) Which leads us to...

• Getting the debiasing data

If you don't already have it, Find_Orb will tell you that you need the file bias.dat. The ECFF18 version of this can be extracted from (about 9.6 MBytes, with bias.dat decompressing to about 37 MBytes). This file contains the needed bias.dat file plus two other files, README.txt and tiles.dat; you can ignore both of these. You just need bias.dat.

Once this file is in your Find_Orb directory (for Windows) or ~/.find_orb directory (for Linux, *BSD, or OS/X), you'll no longer get the warning that Find_Orb can't locate the debiasing data.