Introduction The scope of this document is to guide an experienced technician in optimizing the signal entering the photomultiplier tube of the Brewer Spectrophotometer by ensuring the optical path is correct and the optical components are positioned correctly. Some Brewer Spectrophotometer have been found to have ‘Non Focused’ spectrometers from production. This means that the light does not travel along the optical plane horizontally but rather travels at different angles through the spectrometer before reaching the exit slit at the bottom. In addition, it has been found that the correction lenses found where the light enters the spectrometer after the fore-optics and where the light exits the spectrometer at the exit slit have been installed at the incorrect orientation. These irregularities can lead to poor distribution of light when reaching the exit slit and lead to the assumption of poor sensitivity of the photo multiplier tube when it is a signal positioning issue. Method Remove the Spectrometer Make sure you have a clear area to place the Spectrometer with blocks to stabilize when the spectrometer is placed down. Remove the spectrometer carefully. There are four screws that hold the spectrometer in place of various sizes. Make sure that you have the correct hex bits handy with one or two extensions.
With one hand hold the spectrometer to ensure it is secure as you remove the screws. There should be two screws on the right-hand side, one on the left, and one in the bottom centre. Check the Correction Lens Remove the two machine screws that secure the collar to the spectrometer. Put on gloves and press on the edges of the correction lens to pop one side of the lens out and remove carefully. Make sure to note how the correction lens was installed in the instrument.
The above diagram illustrates the correct orientation of the correction lens. To find the correct orientation of the correction lens place a clean Kimwipe on a flat, clean surface. Carefully place the correction lens flat on the Kimwipe and gently press around the edges of the lens. If the lens rocks the cylindrical convex side of the lens is face down and wherever you can press and rock the lens is the top and bottom. The correction lens needs to be inserted with the axis of the cylindrical surface being within 1 degree from horizontal. To make sure that you have the lens in the optimal position pick the lens up from its edges and point it at a thin vertical structure i.e. window frame. Notice how the object you’re looking at appears though the lens. You want the object to appear thin and in line with the object you are viewing. You have now found the orientation of the lens with the rocking test and the viewing test, mark the side of the lens showing the optimal orientation and reinstall it into the spectrometer. Repeat for the second correction lens if working with a double spectrometer instrument. Grating Mirror Reflection and Refraction With the spectrometer secure you now start to remove the top grating mirror. Remove the bottom two screws using a 3/32 Allan key. With the grating mirror hanging from the top two screws, grab the back adjustment screw on the mirror mount and remove the top two screws. Carefully remove the grating mirror from the mount.
Attach the removed grating mirror to a loose grating arm. Install the grating arm into the rotating and tilting optical mount on the optics bench. Take a bubble level and place it on top of the grating mirror and level the mirror using the controls on the mount.
With the mirror leveled, turn on the 405nm laser on and aim it towards the grating mirror.
Ensure that the laser is level by using two grating blocks to shoot the laser through. If the laser can pass through both blocks with a strong signal on the other side, you have ensured that the beam is horizontal.
Hit the grating with the laser and find the reflection and the refracted beam. Place a grating block at both beams at the same height as was used to test the horizontal beam. Note how the beam is hitting the two grating blocks and adjust the mirror to have both beams go through the hole for the reflected and refracted beams.
To raise one beam while lowering the other the grating must be rotated and that is done by raising and lowering the other two blocks on the grating.
Not how the grating was adjusted. Re install the grating into the spectrometer. Flip the spectrometer over and repeat the process for the other grating mirror. Maximizing Counts and Finding HG Line Determine how square the spherical mirrors are to the bulkhead. Take a caliper and measure from the back of the mirror to the bulkhead to make sure that they are equal distance. Use a block to match the height of the mount of the high end of the mirror. Note the two measurements and how close they are together. You may need to make adjustment to square the mirror. Close the spectrometer and return to the instrument computer. Run a ‘FRHP’ to maximize counts off the standard lamp. Enter ‘MM’. Once in MM, entre ‘SL’ to have the instrument in the correct orientation. Once the instrument is oriented correctly run ‘R,0,7,4;O’ to see how many counts the instrument is getting off of the standard lamp. Loosen the mirror set screw and then adjust the thumb screw in one direction by 10 teeth. Note how you adjusted the grating mirror. If the beam was shooting low off the grating the spherical mirror will be compensating the point the beam back up. After a small adjustment is made, close the spectrometer and rerun the R command by typing ‘R;O’. If counts go up continue adjusting in the direction you did before. If counts go down adjust the thumb screw in the opposite direction. Once the top spectrometer has optimized counts repeat the process for the bottom spectrometer. Exit MM and run a ‘HP’ to optimize the micrometer alignment. Re enter MM. Enter ‘SL’ and turn on the mercury lamp by entering ‘B,2’. Reenter ‘R,0,7,4;O’ to see how many counts the instrument can get off the mercury lamp. You may need to adjust the micrometer offsets to re-locate the mercury line. Once the counts are optimized exit MM and then run ‘HPHGHGFR’ to find the new offsets. Once found set the new offsets in CF. Now that the new offsets are found enter ‘CS’ to run multiple commands and run the following string. FR HP HG UVFOCUSD HP HG UVFOCUSR HP HG
This string takes approximately 3 hours to complete. Once the string is complete go into the UVFOCUS Files and see how far off each half of the spectrometer is.
FOCSAVGR file indicating how far off the focusing is and which direction to move the mirror. First the top of the spectrometer must be adjusted. The focus here affects the wavelength focus and will change the focus of the bottom as you adjust the top. There are two adjustment screws on both spherical mirrors secured with a nut. Use a 3/32 head on a small ratchet and make sure to be careful not to touch the spherical mirrors and wear gloves while doing the procedure. The spectrometers are very sensitive. Per 60 Degree rotation on a hex ratchet we have found that it moves 3 points in the FOCUSAVG file. This is a guideline and can change depending on how far you are from the optimal focus. Repeat focusing until the top and bottom spectrometers are optimized.