SPECTROSCOPY

A SIMPLE WAY OF INCORPORATING A SLIT APERTURE

 

Including a slit aperture in the optics of the spectrograph has a number of potential advantages:-

1) It makes it possible to record spectra of extended objects eg planets, nebulae, comets etc by isolating a narrow section of it.

2) The background brightness due to the sky is reduced, which should improve the signal to noise ratio for faint objects.

3) Interfering star images and spectra can be eliminated.

The main disadvantages are that additional optics are needed to produce in focus images of the slit and the target simultaneously at the CCD and finding and tracking the target are made more difficult.

To make the additional optics as simple as possible I decided to try an eyepiece projection arrangement. With this configuration, a slit aperture can be introduced at the prime focus position and the combined image of the slit and the target projected onto the CCD using the eyepiece.

 

The webcam, with adaptor and grating attached, was mounted to a 25mm Vixen Lanthanum eyepiece and an adjustable slit ($15 from Surplus Shed) fixed to the bottom of the eyepiece. The assembly was held together with an eyepiece projection adaptor. The distance of the webcam assembly was adjusted to bring the slit into sharp focus. This gave approximately 1:1 magnification relative to the normal prime focus image. The complete assembly was then mounted onto the telescope. The target was acquired and brought to focus with the slit set wide. The slit width was then reduced and the effect observed.

26th June 2005 VENUS IN DAYLIGHT

The first target attempted was Venus imaged in a bright daylight sky. With the slit wide open, the spectrum was swamped by the bright background.

As the slit was closed, the contrast between the spectrum of Venus and the background improved.

(Note that as slit was narrowed, the background became the spectrum of the background sky. To obtain the true spectrum of the target, this will have to be subtracted from the target spectrum)

 

 

26th June 2005 M57 IN TWILIGHT

PRIME FOCUS WITHOUT SLIT

EYEPIECE PROJECTION WITH SLIT

The effect of the slit has been to reduce the background brightness significantly. The sky colour was predominantly towards the blue end of the spectrum as can be seen in the spectrum background with the slit in place. In this area the reduction in background brightness is less pronounced.

Note the complete absence of interfering stars and their spectra with the slit in place.

(The different orientation of M57 in the two images is because the camera was rotated between the two configurations)

 

 

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