1、 Evans Visual Sky Photometer User Manual Revision 1.4 3 / 1 / 2004 2 Table of Contents Overview3 Setup Procedure4 Operation Procedure.5 Technical Details7 Alignment Procedure8 Calibration Procedure10 Theory of Calibration14 Polar Alignment.23 VSP Calibration Data.25 3Overview The Evans Visual Sky Ph
2、otometer uses dual optical paths to compare the sky brightness of the solar disc through a calibrated neutral density wedge. The Sun is occulted in one path and has a field of view of about four and a half solar diameters as viewed through the eyepiece. A neutral density wedge is moved in front of t
3、he disc to obtain equal shade of color for both the Sun and background sky images. The values are read directly from the wedge and referenced to a calibration curve to obtain sky brightness values in millionths of the solar disc. 4Setup Procedure 1. Remove the Visual Sky Photometer from the shipping
4、 case. The VSP was shipped assembled as a unit to insure the same alignment is maintained. 2. Assemble the Losmandy GM-8 mount and tripod as per the manufacturers instructions. Refer to for additional detailed information on this mount and its operation. 3. Attach the assembled Visual Sky Photomete
5、r to the Losmandy GM-8 mount by tightening the setscrews on the custom mounting block. 4. Align the GM-8 to celestial north using the mounts built in polar scope. Refer to the Polar Alignment section of this manual for detailed information on this procedure. 5. Place the Desert Storm protective stor
6、age cover over the instrument and mount to protect them from the elements when not in use. 5 Operation Procedure 1. Remove the protective cover from the Visual Sky Photometer and mounting. 2. Point the Visual Sky Photometer at the Sun by observing the shadow of the beam arms occulting disc. This sha
7、dow position should be adjusted until it is concentric with the Main Beam aperture A1. (reference Fig.1) 3. Insert the wedge (Wf) into the S2 Comparison Beam photometer slide mount. (reference Fig. 1) 4. Look into aperture A2. Observe and note the brightness of the central disc compared to the surro
8、unding sky. 5. Slide the wedge inward or outward to vary the brightness of the central disc. 6. Adjust the Losmandy mounts RA or Dec if necessary so that the sun is completely occulted by the central disc. 7. Carefully adjust the wedge position so that the central disc brightness closely matches the
9、 sky background brightness in the area just outside of the central disc. 8. Read the position of the wedge by observing the numerical scale value with respect to the datum line. Interpolate this position to the nearest tenth. 9. The sky brightness value is obtained from a calibrated table of scale v
10、alues and corresponding sky brightness values. 6 10. The wedge must not be exposed to direct sunlight for extended periods or it will be damaged. Remove the wedge when the observation is completed and place it in the storage container. 11. Replace the protective cover over the sky brightness instrum
11、ent and mount. Figure 1 7 Technical Details Figure 1 illustrates the essential optical elements of the Visual Sky Photometer (VSP). D1 is an occulting disc that shades aperture A1 from direct sunlight. Lens L1 images a location between D1 and infinity onto a second occulting disc D2, by reflection f
12、rom the black-glass mirror M. The white upper surface of D2 is illuminated by direct sunlight through density wedge Wf and lens L3 (the sun is not focused on D2). Disk D2 is at the focus of lens L2, which images aperture A1 onto exit aperture A2 through a green filter F (Wratten 58). The filter perm
13、its more consistent visual brightness judgments than are possible without it. By this arrangement, direct photospheric light diffracted at D1, is occulted at D2, and that portion of diffracted light that is further diffracted into the system at A1 is occulted at A2, which is slightly smaller than th
14、e image of A1. Thus the sky field around the sun, free of diffracted light, is viewed trough A2 as an illuminated field around the disk D2, which itself is illuminated directly through wedge Wf and lens L3. Adjustment of Wf matches the brightness of D2 to that of the sky field around it. The positio
15、n of Wf is read from its attached scale, and converted by a calibration chart to sky brightness in millionths of the solar disk. 8 Alignment Procedure In use, the VSP is pointed at the sun and visually aligned so that the image of the sun seen through the exit aperture A2 is occulted approximately c
16、entrally by disk D2. Wedge Wf is then adjusted for brightness match between D2 and the field around it. No other routine adjustment is necessary. However, the central alignment of the image of A1 on A2 is quite critical to insure that the edge of the image is occulted. This alignment should be check
17、ed occasionally by the procedure described below (2). The long exit tube is vulnerable to bumps and other stresses that may disturb its alignment. It should never be used as a handle. Adjustment of L1 and L2 is quite painful due to the poor mechanical design of the lens mounts. This instrument shoul
18、d be handled with great care to avoid shocks that may lead to misalignment. Ordinarily, no other adjustment should be necessary unless the instrument has been damaged. However the full procedure for aligning is given here. 1. Remove D1, D2, L1 and L2, leaving in place the threaded holes that support
19、 D1 and D2. See that the D2 support is centered in the aperture surrounding it, which defines the sky field. View the sky or other bright field through A2 (filter F may be removed for better visibility). Adjust the M and D1 mounts so that the D1 mounting hole, aperture A1 and D2 mounting holes appea
20、r concentric. This adjustment generally requires both tilt and longitudinal adjustment of M. If the axes of D1A1 and D2A2 do not intersect, a lateral correction can be made by lightly rapping sidewise on the D1 mounting stem. Do not move the mirror M after the initial adjustment. This alignment is n
21、ot optically essential, but it avoids excessive de- centering of L1 and L2 to bring the system into final alignment. 92. Replace L2, with F and A2 removed, check whether the image of A1 is located in the plane of A2. This is best done by pointing A1 towards a bright field and viewing its image throu
22、gh a magnifier. No appreciable discrepancy is likely to appear unless L2 is changed. Use the lateral adjustments on L2 to approximately center the image of A1 in the exit tube. Then replace A2 and F and carefully center the image in the aperture so that its edge is uniformly occulted. This is best d
23、one by placing a centering reference over aperture A1( a hole punched with a divider point in Mylar film, for example). This is centered in A1 while the image is viewed through a magnifier, with A2 removed. Then A2 and F are replaced and tightened. L2 is now adjusted to locate the centering referenc
24、e in A2. This step is very critical. Even a slight misalignment of A2, D2, M and A1 will introduce errors into the instrument of large magnitude. L2 should be adjusted to maximize the sky intensity. Its helpful to block the disk image for this step. 3. Replace L1 and use its lateral adjustments to a
25、lign the mounting holes for D1 and D2 concentrically, as viewed through A2 and L2. This is best done after replacing D1 and viewing the sun; then make the final adjustment of L1 for concentric occulting. Check location of focus at D2 of an object between D1 and infinity (perhaps 100ft. away not crit
26、ical). To verify the alignment of L1, look into the eyepiece. If L1 is out of adjustment a double image of D1 will appear. Adjust L1 to move the D1 image under D2. 4. Replace disks D1 and D2. 10Calibration Procedure Calibration of the VSP requires the use of a second density wedge or equivalent atte
27、nuator. If the lenses, mirror, wedge and surface of disk D2 are kept clean, little shift of calibration should occur. In time, deterioration of the wedge and of the white surface of D2 may alter the calibration. The instrumental constant is obtained by placing the instruments own wedge in the main b
28、eam (S1), then matching the disc of the sun. The instrument must be clean and properly aligned prior to calibration. Otherwise, the sky readings will be grossly inaccurate. The sky conditions must be stable during the calibration procedure to ensure accurate comparison. Materials needed: 1. Wedge to
29、 be calibrated (Wf). 2. Intermediate wedge (Wi). 3. ND filter to be used in conjunction with the intermediate wedge. This increases the effective range of the intermediate wedge. 4. Rotating sector assembly. 5. Blank MS Word Cal Data Form to record data. 6. Excel spreadsheet file : VSP XXX Cal.xls f
30、or data reduction. 1. Obtain the instrumental constant by placing the instruments own wedge (Wf) in the main beam (S1) and matching the internal occulting disk with the suns image. Do so by tilting the instrument so that the sun is halfway visible adjacent to the internal occulting disk (refer to Fi
31、gure 2). 11Figure 2 Record the wedge scale reading, and then remove the wedge. Carefully repeat this step three times and record the average of the three readings on the Cal Data Form. 2. Place the Wi wedge in the main beam (S1) and match the disk and sun. To avoid having to adjust this wedge to a v
32、ery high initial density, the ND filter may have to be used also. Place this filter directly under the wedge so that it remains fixed in position directly over aperture A1. Record the intermediate wedges scale position on the Cal Data Form. 3. Use the rotating sector to cut the main beams light by .
33、 This will cause the sun and disk to become unmatched. Place the Wf wedge to be calibrated in the comparison beam (S2) and rematch the disk and sun. Record the scale reading on the Cal Data Form. 12 4. Turn off sector and adjust Wi to restore the balance of the sun and disk value. 5. Turn on the sec
34、tor and adjust Wf. Record the value. 6. Repeat step 4 and 5 alternating sector on and off and adjusting both wedges until the upper density limits of the wedges are reached or the sun and the disk become to faint to see. 7. Remove both wedges and repeat step one. If the day has been stable, the inst
35、rumental constants obtained at the beginning and end of the calibration procedure should match. If not, wait for better conditions and try again. 8. Plot the values obtained graphically with the X axis showing the wedge scale and the Y axis the log 1/n (n being the sector value 1, , , 1/8) , (log 1/
36、n = 0, .001, 002). This is done automatically if the data is entered in the above mentioned Excel spreadsheet file. 9. Find the instrumental constant along this curve and obtain its value. For log 1/n (y axis value) this value, Tw = log 1/n. To find (a), take the antilog of the Y axis value. Express
37、 in terms of millionth (a x 10-6). This will be the sky brightness value when the wedge to be calibrated is at complete transmission. This is done automatically if the data is entered in the mentioned Excel spreadsheet file. 10. Plot a sky brightness curve ( x axis = wedge scale ) (y axis = Sky Brig
38、htness x n) with the Y axis on a log scale. This is done automatically if the data is entered in the mentioned Excel spreadsheet file. 13 11. Connect the points with a straight line. This curve gives the wedge scale conversion to sky brightness. For ease of conversion, a table of wedge scale values
39、with corresponding sky brightness values may be derived. This is done automatically if the data is entered in the mentioned Excel spreadsheet file. 14Theory of Calibration In Figure 1 are shown the optical paths of the two beams. The lower beam or main beam consists of light from the sky at a small
40、angle from the sun. Its surface brightness as viewed in the instrument is B1. The upper or comparison beam consists of sunlight falling upon the wedge Wf and the diffusely reflecting occulting disk D2. The surface brightness of this reflecting occulting surface for any given setting of wedge Wf is B
41、2. If we let B = surface brightness of the sky at a small angle from the sun. B0 = average surface brightness of the sun = mean angular radius of sun (0.0047 radian) t1 = transmission of lens L1 t2 = transmission of lens L2 t3 = transmission of lens L3 Rm = reflectivity of mirror M k = concentration
42、 factor of L3, which concentrates the reflective beam on the reflecting surface of the occulting disk r2 = diffuse reflectivity of the white surface of occulting disk D2 = angle of incidence of sunlight occulting disk D2 tw = transmission of the wedge, Wf, for any given setting Tw = scale reading in
43、 cm indicating the position of the wedge Then, if we look from eye aperture, A2 at occulting disk, D2, we see B1 = B t1 t2 Rm And applying Lamberts Law we find B2 = B0 tw t3 k r2 cos( ) t2 15When the brightness of the two beams are matched, B1 = B2. Equating the two expressions we have B = t3 k r2 c
44、os( ) _ B0tw (1) t1 Rm For our purpose, can be assumed to be constant, at its mean value. The other quantities in the bracket are also constant for each instrument. Let this constant t3 k r2 cos( ) _ = (2) t1 Rm which can be determined collectively by observation with the instrument, as we shall des
45、cribe latter. Substituting (2) in (1), we have B = B 0 t w ( 3 ) Sky = k (sun ) wedge transmission Thus the brightness of the sky B, is expressed in terms of the suns average brightness B0, when tw and are known. We shall first calibrate the wedge, getting tw, and then shall go on to evaluate . In c
46、alibrating a wedge of unknown density distribution with any instrument, it is sufficient to provide two beams of light in the instrument originating from the same source, and to allow one of these to pass through the wedge, while the intensity of the other is varied in some way by known amounts. The
47、 intensities of these two beams are then balanced in each instance by shifting the position of the wedge. A smooth curve is drawn through the observed points therefore gives the desired calibration. In our instrument we have the required conditions for such a calibration. In equation (3) in the main
48、 beam instead of skylight, B, we substitute the sun itself, B0, (as substitute we must, in order that we can obtain two comparing beams from the same source) and control its intensity at will by means of rotating sectors. The sector disks are made of steel plates 1 mm in thickness and 9 cm. in diameter. They are of the symmetrical self-balancing type, one of which, N1 for reduction is shown in Figure 3. The sector is rotated by a small motor of high rpm, which is attached to the handle of the instrument. The sectors cut down the main beam in steps of two.
