BESOS: a prism spectrograph

Introduction

We present here our based prism low resolution spectrograph baptized BESOS or BEst Simple Optical Spectrograph (kisses in spanish). Designed in 2003, the spectrograph was proposed to overcome the  low throughput of our previous instrument LOROS (coming soon to this blog) which was an instrument based on an on-axis dispersion prism obtained from a commercial spectroscope. The total efficiency of LOROS was only 25% in the visible spectral range.  BESOS was built with only two doublets and a prism. This configuration reached almost 87 %  at 620 nm. With such efficiency and low resolution, we expected to measure the red shift of the most bright galaxies and quasars.

In this post we provide a description of the instrument, features, performances and the set of mechanical drawings.

Bending a laser beam

Using just water and sugar in order to create a mixture with a gradient refraction index  a laser beam can bent  as shown in the video below.

Youtube video: Bending a laser beam

This is the principle explaining the mirages, gradient index lenses and fibres.  Amaze your friends by telling them that you put a black hole behind the recipient in order to create a huge gravitational field which bends light beams (General theory of relativity). A friend was convinced that we put a strong magnet below !!!

When properly poured, the sweet water will create a mixture with a gradual refraction index. The bottom will have a higher refraction index than the top. When a light beam travels inside, its direction will bend continuously (principle of Fermat)

In order to success the mixture:

1. Saturate the water with sugar. You can go faster by heating the water
2. Put FIRST the fresh water in the recipient and then the sweet water at room temperature. The saturated water will sink creating the gradient.
3. Do NOT shake or mix the solution once you pour the sweet water

Bending a laser beam 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.

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

Optical efficiency of the 200 ln/mm Newport transmission grating

Purpose

We have measured the optical efficiency (transmission vs. wavelength) of the  200lin/mm, 505nm blaze 1st order, 10 deg, 58x58x10mm transmission grating of Newport (former Richardson gratings, Ref. 54-006-630R)

Assembling a fibre with SMA connectors

Purpose

In this post we want to show through pictures and drawings the process of assembling a bare fibre into a protective jacket and ended with commercial connectors for spectroscopy purposes. In particular, we have prepared a 10m, 50μm core fibre with SMA connectors and protected with a  square-locked stainless steel tube.

Linking a telescope to a spectrograph through an optical fibre (Part II)

5.3   Field acquisition (fibre viewer)

When observing with a telescope linked to a spectrograph through an optical fibre, the first problem you are confronted is the way to place the image of the star exactly in front of the fibre input end. How to do it? How to be sure that the telescope beam is properly launched into the fibre? The purpose of this section is to show and describe the most common opto-mechanical configurations to achieve this goal. We discuss the advantages and drawbacks of each of them as well.

1. Introduction

Optical fibres are widely used in professional observatories in order to detach spectrographs from telescopes. The advantages are indisputable but at expenses of a reduction of light flux available to the spectrograph. In amateur astronomy observers are confronted to the practical ways to prepare the fibre link between the telescope to the spectrograph. A careful choice of the type of fibre and a suitable opto-mechanical design are required for each particular case. In addition, their preparation and installation require special tools and skills.

The purposes of our Chapter on Optical Fibres are:

1. To enumerate the advantages and drawbacks using optical fibres to detach the spectrograph from the telescope. The astronomer has to evaluate the pros and contras to decide to use an optical fibre according to the observation purposes, instruments available (telescope and spectrograph) and budget restrictions
2. To describe the more relevant parameters of fibres like optical transmission and focal ratio degradation. These parameters should define the design of the optics to couple the fibre to both, the telescope and spectrograph
3. To recommend the most appropriate fibres for amateur spectroscopy. We revise the standard fibres used for telecommunications and advice the most appropriate fibres available on the market.
4. To provide tips and tricks to prepare and evaluate fibre links. We intent to explain the techniques to protect, polish and adapt the fibres with the cheapest components

We hope you will find useful all the information contained in these posts and enjoy doing astronomical spectroscopy!