Optical fibre connection.....

[chiltonstar]

I have been playing with a small Beck spectroscope I have, which I want to link to one of my scopes with an optical fibre; anyone any idea of a suitable product? 

The resolution seems quite good, eg below of the Na D lines from a discharge lamp with a modified Tesco webcam as detector, giving a half bandpass resolution of about 0.3 nm. The Na doublet is the pair of lines at the centre, at 589.0 nm and 589.6 nm.

There is the additional problem of course of the non-linear scale (prism)

Chris

.......

[Gasman]

Hi Chris

I have only used the supplied SMA type of optical cable which came with my spectrometer which I gather is now an older means of connection. I have been advised that the type of optical cable used for audio should be fine but I haven`t tried it! You just have the problem of connecting it !!

Steve

[Merlin66]

This is discussed in 'Astronomical Spectroscopy for Amateurs' p210

I used an OM2 ST-ST Duplex, P/cable 50/125

The real issue comes down to getting (and holding!) a target star on the end of a 50 micron fibre cable...and also the f ratio degradation -i.e. goes in a f8 comes out at f5!

Then aligning the fibre with a slit gap - depending on the spectroscope, this can improve resolution but at a cost - loss of through put.

[chiltonstar]

This is discussed in 'Astronomical Spectroscopy for Amateurs' p210

I used an OM2 ST-ST Duplex, P/cable 50/125

The real issue comes down to getting (and holding!) a target star on the end of a 50 micron fibre cable...and also the f ratio degradation -i.e. goes in a f8 comes out at f5!

Then aligning the fibre with a slit gap - depending on the spectroscope, this can improve resolution but at a cost - loss of through put.

Ken: I have of course read the section in the excellent book mentioned above (recommended to all spectroscopists) but was wondering about using a fibre bundle approach, eg:-

http://www.fibrephotonics.com/page/173/Profile-Cross-Converter-Spectroscopy-Fibre-Bundle.htm

I've used this kind of entrance optics in the past with a high-res spectrometer....

Chris

[robin_astro]

Ken: I have of course read the section in the excellent book mentioned above (recommended to all spectroscopists) but was wondering about using a fibre bundle approach, eg:-

http://www.fibrephotonics.com/page/173/Profile-Cross-Converter-Spectroscopy-Fibre-Bundle.htm

I've used this kind of entrance optics in the past with a high-res spectrometer....

Chris

Hi Chris,

The problem is the star image size (typically ~25um) is smaller than the diameter of a single fibre and there are lots of gaps in a bundle of circular fibres for the light to be lost. From  the various attempts I have followed, efficient  DIY telescope fibre feed/guiding units are tough things to design and build. (By comparison the spectrometer is the easy bit!)  which is reflected in the cost of commercial units eg the Shelyak fibre feed for their eShel spectrograph is 2500 euro 

If you are interested finding out more the German VdS spectroscopy group have some some expertise in this area.

http://spektroskopieforum.vdsastro.de/

Also the CAOS group 

http://spectroscopy.wordpress.com/

Cheers

Robin

[robin_astro]

This presentation in particular, given by the CAOS group at a VdS spectroscopy section conference in 2012 has some useful information about the theoretical and practical considerations.

http://spectroscopy.files.wordpress.com/2009/05/2012_0505_aspekte_bad_boll_fibres_is.pdf

(Note that although the CAOS groups are "amateurs" in this area, they work for ESO so have access to some rather specialist resources and skills) 

Cheers

Robin

[chiltonstar]

Many thanks for these - something to read during a cloudy evening (ie nearly every night)....

I'd planned to start with some reasonably bright sources, defocusing the image to a 3 or 4mm circle to make alignment easier and to roughly match the bundle diameter, before I tried anything too difficult (my aligment skills, and patience, are a bit to limited to envisage aligning a 25 micron spot on a single 50 micron fibre, and keeping it there....).

Chris

[robin_astro]

 (my aligment skills, and patience, are a bit to limited to envisage aligning a 25 micron spot on a single 50 micron fibre, and keeping it there....).

Hi Chris,

Acquisition, focussing and guiding on a fibre (or slit)  is straightforward once you have the hardware side sorted. The fibre sits in a "hole" in a mirror which you view with a camera so in essence it is no more difficult than guiding a star in say an off axis guider image eg this is what Saturn looks like in the Shelyak unit

http://www.astrosurf.com/buil/eshel3/hole3.jpg

Cheers

Robin