The extragalactic background light (EBL) is the accumulated and reprocessed radiation of all the starlight produced over the life of the Universe. As such, measurements of the intensity and evolution of the EBL offer a critical test of cosmology and structure formation. An accurate knowledge of this light density also plays an important role in the determination of the intergalactic magnetic field (IGMF) strength. The direct measurement of the EBL radiation field is hampered by the existence of foreground sources within our own solar system and Galaxy. This complication can be overcome by use of very high energy (VHE) emitting blazars. These sources produce gamma rays which interact with the EBL photons through pair production, altering the observed VHE spectrum in a distance-dependent manner. The most promising blazar for such studies is 3C 66A, which has a putative redshift of z=0.44, a measurement based on a single line observed in a 40 year old spectrum. If confirmed, the observed VHE spectrum would negate the validity of one of the most comprehensive EBL models to date. Recent efforts to confirm the redshift with 10m-class telescopes have proven unsuccessful. We propose far-UV spectroscopy with HST/COS for 3C 66A to search for the signature of the z~0 Lyman alpha forest, to thereby confirm or refute the putative redshift. With 3 orbits of G160M spectra, we will firmly establish it to have z>0.2 (or not) and if within z=0.2-0.45 establish its redshift to a precision of 0.05 (90% confidence limit). If the redshift of 3C 66A is found to be greater than 0.53, all EBL models produced thus far will be nullified. The three orbits granted will comprise of a single visit with initial target acquisition followed by three target exposures (FP_POS positions 1, 2 and 3, respectively). The target aqcuisition involves a 120 second PEAKXD exposure followed by a series of five 80 second PEAKD exposures with 0.9 arcsecond steps, each onto the G160M grating at 1623 angstroms. The first target exposure will immediately follow the target aqcuisition and cover the remainder of the first orbit (1779s in FP_POS 1). The second and third target exposures will use the remaining two orbits. This target is a variable target, found to vary by +/- an optical magnitude in the R-band (http://users.utu.fi/kani/1m/3C_66A.html). GALEX measured a FUV magnitude of 17.7. In order to ensure that our measurement will be successful independent of state, we have calculated the proposed observing plan assuming that the target is in a faint state (FUV ~18.7 magnitude). With the safety of the instrument in mind, a check was made on each of the proposed exposures on the chance that the target will be in the brightest state during observation (FUV ~ 16.7). The exposures requested are found to produce no instrumental risk (see below for ETC run numbers). This visit sequence is requested independent of source magnitude at time of observation. Exposure Time Calculator run with FUV magnitude of 16.7: Exposure 1 ETC Run #: 413339 Exposure 2 ETC Run #: 413340 Exposure 3 ETC Run #: 413341 Exposure 4 ETC Run #: 413342 Exposure 4 ETC Run #: 413342 COSMOLOGY 12 false 0.0 0.0 false false false false SUPPORTED GALAXY BL LAC LYMAN ALPHA CLOUD This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 COS/FUV Spectroscopic No No G160M 1623 The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. This target is a variable target, found to vary by +/- an optical magnitude in the R-band (http://users.utu.fi/kani/1m/3C_66A.html). In order to ensure that our measurement will be successful independent of state, we have calculated the proposed observing plan assuming that the target is in the faintest state (FUV ~18.7 magnitude). With the safety of the instrument in mind, a check was made on proposed target acquisition exposures on the chance that the target will be in the brightest state during observation (FUV ~ 16.7). The exposures requested are found to produce no instrumental risk (see below for ETC run numbers). This visit sequence is requested independent of source magnitude at time of observation. Exposure Time Calculator run with FUV magnitude of 16.7: Exposure 1 ETC Run #: 413339 Exposure 2 ETC Run #: 413340 (Tic-Visit :proposal-id 12863 :visit-id "01" :proposal-version -1 :trans-version "20120604" :date "25-Sep-2012 09:16:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 217504 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12863 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12863 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2677 :al-overhead-at-end 0 :internal-dur-at-end 95 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3010 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12863 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2788 :al-overhead-at-end 0 :internal-dur-at-end 114 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3010 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12863 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2788 :al-overhead-at-end 0 :internal-dur-at-end 442 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3010 :pcs-gap 0 ) true 3 0 Three of the four FP-POS positions will be sufficient to account for detector imperfections. Three of the four FP-POS positions will be sufficient to account for detector imperfections. Three of the four FP-POS positions will be sufficient to account for detector imperfections.