TITLE:   GCN CIRCULAR
NUMBER:  30731
SUBJECT: GRB 210820A: CAHA 2.2m Optical Detection
DATE:    21/08/27 18:03:36 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), C. Thoene, M. Blazek, J. F. Agui Fernandez (all 
HETH/IAA-CSIC), A. Fernandez, and I. Hermelo (all CAHA) report:

We observed GRB 210820A (Swift detection: Beardmore et al., GCN #30664) 
with CAFOS mounted on the 2.2m CAHA telescope at Calar Alto, Alemria, 
Spain, under inclement conditions (bright moonlight, passing clouds, low 
transparency). We obtained 10 x 90 s each in filters B, V, Rc. Stacks 
reveal no detection of the afterglow in B (very shallow because of 
moonlight) and Rc (shallow because of incoming clouds which also 
prevented further observations). However, the afterglow (Beardmore et 
al., GCN #30664; Butler et al., GCN #30672; LaPorte & Beardmore, GCN 
#30686; Nicuesa Guelbenzu et al., GCN #30695) is clearly detected in V, 
somewhat north of a source also seen in Pan-STARRS and in SDSS.

We find the afterglow to be V = 22.43 +/- 0.15 mag (AB mag, derived from 
two SDSS field stars transformed to V via the equations of Lupton 2005) 
at 0.43868 days after the GRB.