Measuring Transmission for Canon EF 200mm F/2.8L II

Purpose

We present our experimental results on the transmission of the 200 mm F/2.8 EF Canon objective

Transmission curve

Discussion

1. We choose this objective as a camera lens to image the echelle spectrum of our FLECHAS instrument because its excellent image quality in a very broad spectral range (400 to 700 nm). More information and reviews in these 3 links 1 2 3
2. One of our requirements is to procure optics for the spectrograph to reach the  Ca II lines (393.3 – 396.8 nm). The transmission of many objectives goes down rapidly below 400 nm. We got this Canon objective and decided to measure its transmission in the FLECHAS foreseen spectral range.
3. The transmission at 390 nm is 73 %, it is excellent for our amateur purposes. The transmission (52%) at 370 nm is still convenient for deep UV applications

Measurement procedure

List of components

• 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 with 60 mm focal length
• Objective Canon model EF 200mm, f/2.8L II USM.  Filter size 72mm. Length 138mm.
• Detector: CCD camera SBIG ST-1603ME. 1530×1020 pixels 9×9 um.
• Optical rail
• Supports for CCD camera, objective and monochromator.

Hausing for halogen lamp and fibre bundle

End of fiber bundle illuminating entrance in monochromator

Monocromator

Focusing the direct flux from monocromator into CCD

Measuring the flux through objective

Monochromator, colimator, objective and CCD camera

Complete set-up to measure optical efficiency

Complete set-up to measure the efficiency of the Canon objective

CCD direct image from monochromator slit

CCD image from monchromator slit through objective Canon

Objective mounted on the optical rail

Canon 200mm F/2.8 transmission curve

Procedure

1. Connect halogen lamp to power supply. Power cooling fan. Power halogen lamp max 12v. Do not use any density yet.
2. Attach one end of the fibre bundle to lamp. The other end attached to a support mounted in a rail illuminates the entrance slit of the collimator.
3. Set the monochromator to a wavelength aprox. 600nm (yellow).
4. Wide open both entrance and output slit of the monochromator.
5. Place the doublet (collimator) to the output of the monochromator.
6. Dimm the ambient light and observe with a piece of white paper as screen where the image of the slit get focused.
7. Adjust the collimator until the monochromator light focuses at proximately 160mm from collimator.
8. 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 Canon objective (length 138mm). The chip is oriented parallel to table. Note that the beam is not perfectly parallel but sligthly convergent. See the discussion above.
9. Switch on the CCD, take some exposures to center the image in the CCD.
10. Insert the Canon objective between collimator and CCD with rear side towards collimator.
11. Take new exposures to confirm the image is centered in the CCD.
12. 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.
13. Set  monochromator  wavelength to 375nm (counter = 200) and start taking exposures with and without objective. Avoid saturation and signal levels in the non-linear zone (>45000 counts).  Save the images.
14. Increase the wavelength (counter=+20) . Adjust exposure time if necessary. Use density to reduce flux to avoid exposure time smaller than one second..
15. When finishing exposures, compute ratio between  objective and direct monocromator light. Write the results in an excel table and plot the percentage versus wavelength.

Tips

• 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 polution during exposure.
• Use simple auto-dark substraction with exposures to reduce background noise.
• Cool down the CCD to reduce background noise.

Authors

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.

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