**3.5. Future prospect for space-based
Ly surveys**

Because of the existence of only two medium-band filters in WFPC2
suitable for finding
compact emission-line objects (F410M and F467M), we are limited in the
number of redshifts we could search (*z*
2.4 and *z*
2.85). The Cycle 5 data
offered a unique opportunity to examine the *z*
2.4 epoch, but similar
observations with the F467M filter on HST can be carried out at *z*
2.85. The scientific return
from a modest investment of
exposure time in the HDF in the F410M and F467M filters would be
considerable, since all
the broadband *UBVI* data is in hand. This would be an excellent
direct determination of the space density of *z*
2.4 and *z*
2.85 subgalactic clumps. A
more extensive
medium-band filter set on the Next Generation Space Telescope will be
needed to find
these objects at *z*
2-7. Indeed, more random observations with WFPC2 in F410M
are necessary to confirm these initial findings at *z*
2.4, and to better determine the
luminosity function of the *z*
2.4 clumps. We also note that
these studies can no longer
be done with HST if the WFPC2 gets turned off after installation of the
ACS in 1999,
since the ACS does not have the required medium-band filter set, or
ramp-filters with a large enough FOV to do the exercise.