We present here the the optical efficiency (transmission vs. wavelength) for its 1st order for two of Newport (former Richardson gratings) transmission gratings:
- Ref. 54-006-630R, 200lin/mm, blazed 10.0 deg @505nm, 58x58x10mm
- Ref. 54-006-806R 300lin/mm, blazed 14.6 deg @490nm, 58x58x10mm
and a description of the laboratory set-up to take these measurements.
Efficiency of Newport 54-006-630R
and compared the results with the manufacturer (ones):
Measurements made with non-polarized light and perpendicular to the grating (not Littrow). Date: 17 Dec 2008
Efficiency of Newport 54-006-806R
Measurements made with non-polarized light and perpendicular to the grating (not Littrow). Date: 11 April 2013
- Richardson measurements were done on the master grating, at Littrow and parallel beam. Our measurements were done on a replica, 10 deg out of Littrow and in a slightly convergent beam.
- We orientated the grating according to the proposed design of an echelle spectrograph (FLECHAS). The measurement correspond as per spectrograph operation configuration. In this setup the blaze wavelength slightly shifts towards the red
- We used a convergent beam only to simplify the optical set-up. By using a laser beam and tilting it by the convergence angle, we estimated a transmission error below 3%
List of material used for the measurements
- Source of white light: halogen lamp 50 W.
- Fibre bundle
- Monochromator Jobin-Yvon HII-25. 600 lines/mm @ 500 nm. f =250. F/5. Incidence-diffracted angle = 50º. Dispersion: 6.48 nm/mm
- Collimator: doublet linos 25 mm diameter 60 mm focal length
- Transmission grating Newport 58x58x10 mm, 200 lin/mm, blazed 10°@505nm. Ref: 54-006-630R
- Detector: CCD camera ATIK 11000 4008 x 2672 pixels 9×9 um.
- OG530 bandpass Schott filter for removal of the second order spectrum
- Optical rail
- Supporters for CCD camera, grating and fiber bundle.
- Connect halogen lamp to power supply. Power cooling fan. Power halogen lamp max 12v. Do not use any density yet.
- Attach one end of the fibre bundle to lamp. The other end in a rail supported illuminates the entrance slit of the monochromator.
- Set the monochromator to a wavelength aprox. 600nm (orange).
- Wide open both entrance and output slit of the monochromator.
- Place the doublet (collimator) to the output of the monochromator
- Place the slit before the focal plane of the collimator in such a way that its image will be about 20 cm away
- Dimm the ambient light and observe with a piece of white paper as screen where the image of the slit get focused.
- Place the CCD camera in the optical rail at the focusing distance. There must be a gap space between collimator and CCD enough to insert the transmission grating. The chip is oriented parallel to table. Note that the beam is not perfectly parallel but sligthly convergent. See the discussion above.
- Power the CCD, take some exposures to center the image in the CCD.
- Insert the grating between collimator and CCD. The grooves orientation is not relevant provided that the CCD “sees” the first and zero orders simultaneously. We orientated the grating with the grooves horizontally, i.e. the dispersion was vertically.
- Take new exposures to confirm order zero and first order appears in the CCD. Check that the diffracted beam reaches the CCD without vignetting and for all wavelengths!
- Close both input/output monochromator slits to 50 μm (Δλ = 0.6 nm passband). This is the best resolution (passband)/luminosity compromise of the monochromator
- Set monochromator wavelength to 800 nm. Insert the orange OG530 filter to avoid the 400 nm wavelength contamination from second order. Start taking exposures with and without grating. Avoid saturation and signal levels in the non-linear zone (>45000 counts). Save the images.
- Do the same procedure for 700 nm
- Remove the orange filter, and take exposures at 600, 550, 500, 450, 400 and 390nm. Adjust exposure time if necessary. Use density to reduce flux to avoid exposure time smaller than one second..
- When finishing exposures, compute ratio between first order of the grating and direct monochromator light. Write the results in an excel table and plot the percentage versus wavelength.
- Set the monochromator to e.g. at 500nm. Take an exposure without the grating. Save it
- Load the image with a image processing system like Maxim DL.
- The left picture below (inverted contrast) shows a sub-window including the image of the direct monochromatic light. The following picture shows the same sub-window but out of the image to measure the background.
- The actual flux is the difference between the two sub-windows. In our case we computed 3.757e7 – 4.101e6 or 3.347e7
- Introduce now the transmission grating between the monochromator and the CCD. Take an image with the same exposure time as the direct image (next 2 pictures). In this case the CCD will image the order zero (weak spot at the right) and first order (strong spot at the left).
- Take sub-windows on the image of the first order as described above
- The absolute flux corresponding exclusively to the first order is computed as the image of the first order minus the background, in our case was 2.969e7 – 4.115e6 = 2.558e7
- The transmission at 500nm will be the coefficient between direct light and first order, i.e. 2.558e7 / 3.347e7 = 0.76 or 76% of the direct light goes directly to the first order.
- Allow some time (10min) for the halogen lamp to stabilize flux before taking exposures.
- When measuring the efficiency in the UV region, use the halogen lamp at nominal voltage of 12v.
- Avoid light pollution during exposure.
- Use simple auto-dark subtraction with exposures to reduce background noise.
- Cool down the CCD to -25 C to reduce dark current.
Carlos Guirao and Gerardo Avila
Optical efficiency of the 200 ln/mm Newport transmission grating by CAOS group is licensed under a Creative Commons Attribution-Non-Commercial-No Derivative Works 3.0 Germany License.
Based on a work at spectroscopy.wordpress.com.