Testing the Spectroscope

Setting up and testing the grating/ spectrograph

This section covers the performance testing and evaluation of a spectrograph. We all like to know how our instrument should perform.

Some comments obviously are more appropriate to DIY built spectrographs, but still equally apply to evaluating the commercial instruments.

Common issues:

1. Alignment of the optical elements to the optical axis - entrance slit, mirrors (if fitted), collimator, grating, imaging lens, camera

Use of a laser collimator will quickly determine any gross mis-alignment. Look for vignetting at the top/ bottom of the slit spectrum - could indicate mis-alignment or under sized optics. The centre of the input connection (T2 fitting to scope), the centre of the slit gap, the centre of the collimating lens the centre of the grating and the centre of the imaging camera must all lie exactly on the optical axis.

2. Alignment of the slit gap to the grating grooves

If there is mis-alignment of the slit to the grating you may loose resolution and see excessive tilting of the spectral image across the imaging camera field of view

3. Smooth and repeatable rotation of the grating holder

If the front surface of the grating is not positioned exactly on the axis of rotation, or the grating is tilted relative to the axis of rotation the apparent position of the spectral image in the camera field of view will appear to move (up/down) during rotation of the grating.

4. Focus of the telescope to the entrance slit

Generally the slit gap will be less than the seeing disk presented by the telescope. Holding this disk central to the slit gap during the long exposures is critical to success - if it wanders across the gap you'll loose valuable light and the SNR will be impacted. Good effective guiding is mandatory! The height of the star spectrum as recorded by the imaging camera is a good guide. The tighter and narrower the spectral image, the better the focus on the slit. Remember that temperature variations may also impact on the focus - re-check frequently.

5. Focus of the imaging camera to the entrance slit

This is automatically achieved while checking the reference lamp and resolution. Work with a good reference line (not overexposed) and alter the imaging camera position (or the collimating lens if using a Shelyak Lhires III) until you achieve the tightest, narrowest line image you can get. Checking the FWHM as you proceed to find the lowest FWHM reading. The camera is now in focus, and the FWHM reading can be used to determine the R value of the instrument. You may find, due to the type of collimating lens being used (e.g. achromatic), that the focus varies with wavelength. Re-check when possible. Also, temperature variations may affect the stability of the instrument - again re-check when possible.

6. Alignment/ focus of guide camera

The focus of the guide camera can be done in subdued lighting on the bench. The aim is to get the slit gap in tight focus close to the centre of the FOV. Take your time.... 

Performance:

1. Resolution

2. Wavelength coverage - chromatic aberrations

3. Calibration in wavelength - reference lampsA

A good starting point is to enter your parameters into the SimSpec spreadsheet SimSpecV4 - this provides a first estimate of probable performance.

For transmission gratings, use the TransSpec spreadsheet.TransSpec V2

CAOS transmission grating test 

*****under construction*****