In this paper we have discussed the effects that severe image crowding has on stellar photometry. The investigation covered two fields of the high-surface brightness elliptical galaxy M 32 observed with the HST's PC (archive data, ID 5233 and 5464).
We have carried out a large number of artificial-star experiments, that is, addition and retrieval of artificial stars. Our experiments clearly show the presence of "binjumping": The faint stars added to the programme frames are recovered too bright; the fainter the star and the denser the field, the bigger is the effect of "binjumping". The traditional indicator of (in)completeness of the photometry, m1/2, merely provides a detection probability but not the quality of the photometry.
For the less dense field at 
from the centre of M 32, the effects
of image crowding show up around the limiting magnitude I1/2
(
).
Stars fainter than that are found too bright by several tens of a magnitude
as compared to their true magnitude,
whereas their colour is mainly unaffected.
For the far more dense field at 
from the centre,
"binjumping" affects stars that are even brighter than I1/2 (
).
As our Fig. 9 (click here) demonstrates, only the
very brightest stars with
may be considered to
represent single objects.
Thus, we do measure single
stars of, e.g., 
(corresponding to 
,
the expected tip of the red-giant branch
for metal-poor stars) in the off-centre field at ,
but based on the present data
it is not possible to claim that for the central field at ,
where the density is 40 times higher.
Finally, we would like to emphasize that the data analysis, which has been introduced and described in the present paper in relation to HST imaging, should be carried out for essentially all imaging of severely crowded fields.
Acknowledgements
We wish to thank P.B. Stetson for the benefits of using his photometry package DAOPHOT II.