5 CFR §1620.21

1. (a)An individual covered under this subpart can make contributions to the TSP from basic pay in the amount described at 5 CFR 1600.22(a)(1). Unless stated otherwise in this subpart, he or she is covered by the same rules that apply to a CSRS participant in the TSP.
2. (b)The following amounts are not basic pay and no TSP contributions can be made from them:
   1. (1)An annuity or salary received by a justice or judge of the United States (as defined in 28 U.S.C. 451) who is retired under 28 U.S.C. 371(a) or (b), or 372(a);
   2. (2)Amounts received by a bankruptcy judge or a United States magistrate judge under a judges' annuity described at 28 U.S.C. 377;
   3. (3)An annuity or salary received by a judge of the United States Court of Federal Claims under 28 U.S.C. 178; and
   4. (4)Retired pay received by a judge of the United States Court of Veterans Appeals under 38 U.S.C. 7296.