This report derives the throughput of the LRIS in the imaging mode.
The throughput of the LRIS is 20% at B, 33% at V, 31% at R, and 43% at I, assuming filters with 100% transmission which are 1000, 1000, 1500, and 1500 A wide respectively. The two telescope mirrors (assumed to have 90% reflectivity each) and the atmospheric transmission losses have already been taken into account. Thus this is the throughput of the instrument (including a peak filter transmission below that cited above) and the detector.
Measuring the filter transmissions from the curves in the LRIS Manual of Operations, we find for the LRIS + CCD detector, B = 26%, V = 49%, R = 45%, and I = 32%. These, except for B, which is low, are very close to those predicted prior to construction of the LRIS.
We want to derive the throughput of the LRIS in the imaging mode. The standard field PG 0918+029 with photometry in Landolt (1992, A.J. 104, 340) was observed in B, V, R and I filters on the nights of Jan. 18, 1994 and also Jan. 19, 1994. Both nights were believed to be photometric. The detailed measurements are given in the appendix.
For each color we obtain C = 2.5*log(DN/sec) at airmass = 1.0 for a star with magnitude 0 in that color. The values of C measured on the two nights are given below:
| Filter | 1/18/94 | 1/19/94 | delta[(1/19)-(1/18)] |
|---|---|---|---|
| B | 27.33 | 27.35 | 0.02 |
| V | 27.50 | 27.52 | 0.02 |
| R | 27.50 | 27.54 | 0.04 |
| I | 27.37 | 27.43 | 0.06 |
We use the mean value for the two nights.
The electronics was adjusted to 1.72 electrons/DN during the January 1994 LRIS run.
We assume the mirror has an area of 0.97*(pi)*1.0E6/4 square cm, where the 0.97 is for the central hole. We assume that each of the two telescope mirrors has a relectivity of 90%. We assume atmospheric transmissions of 0.85 for B, 0.90 for V, and 0.95 for R and I.
To make further progress we need to know the transmission of the LRIS filters. We measured the effective width of the B, V, and R filters by integrating under the transmission curves given in the LRIS Manual of Operations. We obtain W(B) = 770 A, W(V) = 615 A, W(R) = 964 A. For I we use W(I) = 1600 A. This is a guess, as the long wavelength cutoff for this filter is not defined by the filter characteristics but by the CCD quantum efficiency red falloff.
We can now calculate the flux expected in each filter bandpass above the earth's atmosphere from a 0 magnitude star using Oke's absolute calibration measurements. A summary of the input values is below:
| Filter | Flux [photons/sec/cm*cm/A] |
|---|---|
| B | 1420 |
| V | 1000 |
| R | 643 |
| I | 413 |
This leads to the following throughputs, where the atmosphere and telescope have already been removed, as has a filter whose characteristics are described above.
| Filter | Throughput |
|---|---|
| B | 26% |
| V | 51% |
| R | 48% |
| I | 40%* |
Note that this is the throughput of the instrument plus detector plus any deviation from the assumed filter width (at 100% transmission) which was given above.
The numbers given in Table I of the LRIS Manual of Operations for the predicted throughput are computed from 0.66 (the predicted instrumental throughput) * CCD efficiency,or
| Filter | Throughput |
|---|---|
| B | 43% |
| V | 51% |
| R | 53% |
| I | 33%* |
Except for B, the agreement between the observed and predicted throughput is quite reasonable. Presumably some optical element (or the CCD detector) has poorer transmission than expected at the extreme blue end of its range. Since the red side of the LRIS will not be used at B permanently (i.e., one will presumably use the blue side of LRIS), this is not a serious issue.
file /scr3/jlc/pulsar/standards
Standard field of PG 0918+029, Keck LRIS direct images
Flat fields normalized over region x=900-1100, y=900-1100. Jan. 18, 1994 UT (first night of Jan.94 commissioning run)
All are identical, star B, PG object, star A, then star C, then sometimes star D, then 4 sky areas.
Magnitudes of these objects:
| V | B-V | V-R | R-I | |
|---|---|---|---|---|
| Star B | 13.963 | 0.765 | 0.417 | 0.370 |
| PG 0918+029 | 13.327 | -0.271 | -0.129 | -0.159 |
| Star A | 14.490 | 0.536 | 0.325 | 0.336 |
| Star C | 13.537 | 0.631 | 0.367 | 0.357 |
| Star D | 12.272 | 1.044 | 0.575 | 0.535 |
Note: Star D was always within a few pixels of the right edge of the field of view of the image. It was always off from the others by about 0.1 mag, and was not included in the averages.
The observed mag are -2.5*log(DN/sec). Extinction corrections were 0.174*0.8 for B, 0.080 for V, 0.050 for R, and 0.030 mag/airmass. These are the nominal Palomar values multipled by 80% at B only. No color terms were used. They did not appear necessary.
The offsets (true mag - observed mag) are:
| B | 27.276 | (sigma = 0.055) | (BAD SIGMA) |
| V | 27.463 | (sigma = 0.029) |
| R | 27.474 | (sigma = 0.044) |
| I | 27.361 | (sigma = 0.033) |
When corrected for atmospheric extinction to 1.0 airmass, these are as given below. The values derived from the same field observed the next night are also given. The agreement is very good.
| 1/18/94 | 1/19/94 (UT) | delta[(1/19)-(1/18)] | |
|---|---|---|---|
| B | 27.33 | 27.35 | 0.02 |
| V | 27.50 | 27.52 | 0.02 |
| R | 27.50 | 27.54 | 0.04 |
| I | 27.37 | 27.43 | 0.06 |
B frame - 2 sec. airmass=1.523
| X | Y | Box | Mean | Sigma | Max | Min | B(true) | B(obs) |
|---|---|---|---|---|---|---|---|---|
| (lower left corner) | size | -B(obs) | ||||||
| 1014 | 1207 | 45 | 111.84 | 199.08 | 1394.35 | -12.86 | -12.575 | 27.303 |
| 1344 | 662 | 45 | 456.22 | 816.06 | 5205.34 | -28.15 | -14.147 | 27.203 |
| 863 | 742 | 45 | 84.00 | 143.98 | 885.49 | -13.42 | -12.243 | 27.269 |
| 360 | 822 | 45 | 187.57 | 329.82 | 1762.90 | -17.43 | -13.161 | 27.329 |
| 1787 | 1065 | 45 | 444.17 | 754.83 | 5038.84 | -32.75 | -14.118 | 27.434 |
| 1220 | 1500 | 45 | 5.95 | 11.28 | 41.77 | -29.09 | Mean of 4 = 27.276 Sigma = 0.055 | |
| 1300 | 1280 | 45 | 6.13 | 11.30 | 46.81 | -28.84 | ||
| 830 | 1290 | 45 | 6.32 | 6.36 | 31.99 | -17.85 | ||
| 520 | 1100 | 45 | 5.65 | 6.60 | 28.54 | -17.06 | ||
| 1080 | 940 | 45 | 6.13 | 11.12 | 40.90 | -30.07 | ||
X,Y(Lower left corner), Size of box, Mean, Sigma, Max, Min
1017 1205 45 259.80 469.47 3580.45 -4.07 -13.471 27.434
1344 657 45 467.44 806.49 5380.68 -18.97 -14.145 27.472
864 738 45 168.40 283.31 1825.56 -6.80 -12.955 27.445
360 818 45 397.77 701.67 3766.52 -4.93 -13.962 27.499
1787 1061 45 1313.24 2190.04 13372.07 -17.43 -15.294 27.566
1220 1500 45 18.58 12.16 54.92 -22.26
1300 1280 45 18.24 11.49 68.68 -21.48 Mean of 4 =
830 1290 45 17.76 6.99 44.38 -4.04 27.463
520 1100 45 18.81 7.40 42.19 -7.20 Sigma = 0.029
1080 940 45 17.71 11.43 63.88 -15.30 Sky = 18.22
---------------------------------------------------------- DN/pixel
R frame, 2 sec, airmass=1.471
real1376.dst
X,Y(Lower left corner), Size of box, Mean, Sigma, Max, Min
1018 1205 45 408.57 758.71 5274.89 30.29 -13.885 27.431
1346 657 45 474.54 832.61 5562.17 21.84 -14.070 27.526
866 738 45 257.38 431.49 3378.18 24.71 -13.279 27.444
361 818 45 585.72 1084.41 7860.90 34.64 -14.326 27.496
1220 1500 45 55.57 12.86 102.75 12.46
1300 1280 45 55.42 12.73 101.11 6.98 Mean of 4 =
830 1290 45 53.80 8.34 83.85 24.95 27.474
520 1100 45 54.84 9.10 91.38 20.37 Sigma = 0.044
1080 940 45 55.39 12.91 100.27 7.32 Sky = 55.00
---------------------------------------- DN/pixel
I frame, 2 sec, airmass=1.444
real1374.dst
X,Y(Lower left corner), Size of box, Mean, Sigma, Max, Min
1015 1200 45 575.10 919.73 7443.92 90.68 -14.162 27.338
1346 656 45 440.89 613.16 4520.89 89.11 -13.783 27.398
865 736 45 366.96 523.66 4187.31 84.25 -13.500 27.329
360 817 45 781.61 1288.23 9285.61 94.50 -14.567 27.380
1786 1055 45 3516.87 5764.92 37085.61 148.29 -16.342 27.504
1220 1500 45 121.71 14.21 167.39 68.56
1300 1280 45 122.00 14.35 172.78 76.42 Mean of 4 =
830 1290 45 114.71 10.24 152.19 75.41 27.361
520 1100 45 115.47 13.89 192.69 77.16 Sima = 0.033
1080 940 45 120.27 14.28 163.62 72.47 Sky = 118.83
DN/pixel
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