diff --git a/ObservationSim/MockObject/SpecDisperser/SpecDisperser.py b/ObservationSim/MockObject/SpecDisperser/SpecDisperser.py
index 07d099a60bde65c74043ab03c34f0fbbf1bad585..3f3f52b01a1d9b061099491d5e49f677979411ee 100644
--- a/ObservationSim/MockObject/SpecDisperser/SpecDisperser.py
+++ b/ObservationSim/MockObject/SpecDisperser/SpecDisperser.py
@@ -122,7 +122,7 @@ class SpecDisperser(object):
                                                                  beam=beam)
 
         ### Account for pixel centering of the trace
-        yfrac_beam = ytrace_beam - floor(ytrace_beam)
+        yfrac_beam = ytrace_beam - floor(ytrace_beam+0.5)
 
         ysens = lam_beam * 0
         lam_index = argsort(lam_beam)
@@ -155,7 +155,7 @@ class SpecDisperser(object):
         sensitivity_beam = ysens
 
         len_spec_x = len(dx)
-        len_spec_y = int(ceil(yfrac_beam[-1]) - floor(yfrac_beam[0]) + 1)
+        len_spec_y = int(ceil(ytrace_beam[-1]) - floor(ytrace_beam[0]) + 1)
 
         beam_sh = (self.img_sh[0] + len_spec_y, self.img_sh[1] + len_spec_x)
         modelf = zeros(product(beam_sh), dtype=float)
@@ -165,9 +165,15 @@ class SpecDisperser(object):
 
         dxpix = dx - dx[0] + x0[1]
 
-        dyc = cast[int](ytrace_beam)
+        dyc = cast[int](np.floor(ytrace_beam+0.5))
+
+        dypix = cast[int](np.floor(ytrace_beam - dyc[0] + x0[0] + 0.5))
+
+        frac_ids =  yfrac_beam<0
+
+        dypix[frac_ids] = dypix[frac_ids] - 1
+        yfrac_beam[frac_ids] = 1+yfrac_beam[frac_ids]
 
-        dypix = dyc - dyc[0] + x0[0]
         flat_index = idx[dypix, dxpix]
 
         nonz = sensitivity_beam != 0
@@ -185,7 +191,7 @@ class SpecDisperser(object):
             origin_in[1] = self.origin[1]
             dx0_in = dx[0]
             dy0_in = dyc[0]
-        originOut_x = origin_in[1] + dx0_in
+            originOut_x = origin_in[1] + dx0_in
         originOut_y = origin_in[0] + dy0_in
 
         if self.flat_cube is None:
diff --git a/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx b/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx
index f5be0b2794ae24d94c1f17f7e50062bf0f1d26b4..41dbf1402e7aa4d756865489898cc738187a094a 100644
--- a/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx
+++ b/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx
@@ -86,12 +86,12 @@ def disperse_grism_object(np.ndarray[FTYPE_t, ndim=2] flam,
                     k1 = idxl[k]+j*shg[1]+i
                     if (k1 >= 0) & (k1 < nl):
                         flat_ids = k1*nlamb+k
-                        full[k1] += fl_ij*yfrac[k]*flat_eff_all[flat_ids]
+                        full[k1] += fl_ij*(1-yfrac[k])*flat_eff_all[flat_ids]
 
-                    k2 = idxl[k]+(j-1)*shg[1]+i
+                    k2 = idxl[k]+(j+1)*shg[1]+i
                     if (k2 >= 0) & (k2 < nl):
                         flat_ids = k2*nlamb+k
-                        full[k2] += fl_ij*(1-yfrac[k])*flat_eff_all[flat_ids]
+                        full[k2] += fl_ij*yfrac[k]*flat_eff_all[flat_ids]
 
     else:
         for i in range(0-x0[1], x0[1]):
@@ -109,11 +109,11 @@ def disperse_grism_object(np.ndarray[FTYPE_t, ndim=2] flam,
                 for k in range(nk):
                     k1 = idxl[k]+j*shg[1]+i
                     if (k1 >= 0) & (k1 < nl):
-                        full[k1] += ysens[k]*fl_ij*yfrac[k]
+                        full[k1] += ysens[k]*fl_ij*(1-yfrac[k])
 
-                    k2 = idxl[k]+(j-1)*shg[1]+i
+                    k2 = idxl[k]+(j+1)*shg[1]+i
                     if (k2 >= 0) & (k2 < nl):
-                        full[k2] += ysens[k]*fl_ij*(1-yfrac[k])
+                        full[k2] += ysens[k]*fl_ij*yfrac[k]
 
     return True