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Still messing about with fibre optic cables - after all these years.


Hawksmoor

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This blog entry is in two parts both of which are spectroscopy related!

With the weather over my house being 'changeable' to say the least, I have been doing a bit of sporadic spectroscopy. I find spectroscopy very interesting but like 'statistics' in mathematics I am always lacking in confidence as to my results.  Recently, I have been trying to obtain a low resolution spectral profile of Arcturus. I used Castor as a calibration star and to obtain a response curve. I know Castor was in a different part of the sky than Arcturus but at least it was captured using the same set up on the same night. The calibrated wavelength profiles looked ok to me but when I applied a response correction curve the result seemed far from convincing when compared with the Miles Arcturus reference. I have attached my charts prepared using the excellent BASS software. Any helpful comments would be appreciated.

As this forum is 'chocker' with knowledgeable folk, might I run this concept passed those that are considerably smarter than I am!

I have been trying to make a small plug in unit to attach to the telescope end of my very simple transmission grating spectrometer. The concept being; that I could transform my 'slit-less' spectrometer into one that uses a line or ring of small diameter fibre optic cables to take light from say a bright nebula or an extended object like a planet and present the light in star like points (neatly avoiding pinhole camera issues). As planned this 'simple' arrangement would not involve additional collimator and camera lenses. 

To date I have had a couple of goes at making such a 'gizmo' with no success. My failure could be for a number of reasons:

  • The concept is wrong and doomed to failure.
  • My engineering skills are 'dodgy' - to be honest this is a given but has not held me back so far.
  • My choice of fibre optic cables has been inappropriate (ie. diameter, sheathing and type). Up to now I have been using 9 micron fibre in a 1mm sheath.

Being old and obstinate, it is unlikely that I will give up soon. I should be grateful for any advice. In particular if the concept is flawed - changing my approach would ensure that I spend more 'quality' time with my wife.

Please be gentle with me as I am qualified in the arts rather than sciences and my engineering qualifications are limited to the shared ownership of a shed and tools of the more basic nature mainly acquired from Lidl's centre aisle !

George in a surprisingly 'sunny' Lowestoft - I must find my sun hat and shorts! - No the clouds are back - so pass me my balaclava helmet Anita.😶‍🌫️

Ps.24-05-2024 - Think I may have identified the problems associated with the Arcturus profile. I believe it is a miss-match of capture crop size. When you look at the bottom profile I would expect the Arcturus profile to be further to the right compared with the Castor profile. I have subsequently upscaled (resized) the image capture for Arcturus using BASS software and things look better. Note to self when capturing spectra keep the crop size consistent between target star and calibration star. (Even if I am not right about this, it is one less variable to juggle with!🤣)

George still not convinced 3.png

Wavelegth Calibrated Profiles Castor and Arcturus.png

Edited by Hawksmoor
Idiocy

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rl

Posted (edited)

A few quick thoughts from a non-expert:

  • The concept is sound. Many professional astro spectrometers are fed from an optic fibre because the spectrometer can be located statically in an airconditioned room where it is much more stable thus can detect smaller velocity changes in the doppler component. 
  • Your spectrum looks good for a single shot. If you want less noise, can you stack them in the same way as standard astrophotography?
  • How are you going to ensure the primary image of the star always hits the optic fibre accurately? You may need some sort of diagonal-with -a- hole arrangement. Can you arrange a guide camera with PHD2?
  • Optic fibres have a limit on how steep the entrance cone can be before light is lost..I think it's referred to as "etendue" and basically refers to how fast your scope is. The spot size scales with the scope's f-ratio. Slower is probably better providing the larger spot size still fits inside the fibre. But slower scopes for a fixed aperture have more focal length and are thus harder to guide...there will be a bunch of tradeoffs here. 
  • Is it worth starting on the sun just because there is so much light available you could just mount the fibre in the expanded beam behind an eyepiece. Usual safety caveats apply... at least you could get the spectrometer end functional first and in the daytime. You might get away with just pointing the fibre end at the sun if the exposures are made long enough.
  • Are you expanding out the beam at the spectrometer end with a collimation lens before the grating, and using a lens after to focus on to your camera? If so, f-ratio considerations apply again. One plus is that you get to scale the image to your pixel size. The fibre diameter becomes your slit, in effect, and the image of that slit will determine your resolution?
  • Not sure of the blaze on a simple transmission grating. How much of the light is wasted in the zero-th order spectrum? (i.e straight through). Might you get a better signal-to-noise ratio with a grating with a blaze angle optimized for a 1st order spectrum?
  • Are you using a colour camera (pretty but inefficient) or a monochrome? I have bought up a small fleet of old secondhand Atik 16ICs for this purpose; they are cheap, cooled and have about the right number of pixels without overkill, keeping the file sizes small.
  • There are a few books available on amateur spectroscopy, the titles of which all escape my ageing grey matter at this instant. Several delve into the basic principles in some detail without being overly complicated. Might be worth a read checking the concept and the maths before you commit too much time going down a dead-end. I will look them out later on if you need further direction...

Good luck..spectroscopy is fun! Especially when you get your first result off a nova and calculate the expansion velocity...

Hopefully those more qualified than myself will chime in...

RL

Edited by rl
Link to comment
7 hours ago, rl said:

A few quick thoughts from a non-expert:

  • The concept is sound. Many professional astro spectrometers are fed from an optic fibre because the spectrometer can be located statically in an airconditioned room where it is much more stable thus can detect smaller velocity changes in the doppler component. 
  • Your spectrum looks good for a single shot. If you want less noise, can you stack them in the same way as standard astrophotography?
  • How are you going to ensure the primary image of the star always hits the optic fibre accurately? You may need some sort of diagonal-with -a- hole arrangement. Can you arrange a guide camera with PHD2?
  • Optic fibres have a limit on how steep the entrance cone can be before light is lost..I think it's referred to as "etendue" and basically refers to how fast your scope is. The spot size scales with the scope's f-ratio. Slower is probably better providing the larger spot size still fits inside the fibre. But slower scopes for a fixed aperture have more focal length and are thus harder to guide...there will be a bunch of tradeoffs here. 
  • Is it worth starting on the sun just because there is so much light available you could just mount the fibre in the expanded beam behind an eyepiece. Usual safety caveats apply... at least you could get the spectrometer end functional first and in the daytime. You might get away with just pointing the fibre end at the sun if the exposures are made long enough.
  • Are you expanding out the beam at the spectrometer end with a collimation lens before the grating, and using a lens after to focus on to your camera? If so, f-ratio considerations apply again. One plus is that you get to scale the image to your pixel size. The fibre diameter becomes your slit, in effect, and the image of that slit will determine your resolution?
  • Not sure of the blaze on a simple transmission grating. How much of the light is wasted in the zero-th order spectrum? (i.e straight through). Might you get a better signal-to-noise ratio with a grating with a blaze angle optimized for a 1st order spectrum?
  • Are you using a colour camera (pretty but inefficient) or a monochrome? I have bought up a small fleet of old secondhand Atik 16ICs for this purpose; they are cheap, cooled and have about the right number of pixels without overkill, keeping the file sizes small.
  • There are a few books available on amateur spectroscopy, the titles of which all escape my ageing grey matter at this instant. Several delve into the basic principles in some detail without being overly complicated. Might be worth a read checking the concept and the maths before you commit too much time going down a dead-end. I will look them out later on if you need further direction...

Good luck..spectroscopy is fun! Especially when you get your first result off a nova and calculate the expansion velocity...

Hopefully those more qualified than myself will chime in...

RL

Thank you very much for your reply. Which has given me much to think about!

Straight off the bat, the spectral profiles posted were indeed stacked using BASS software. I did not use any software filters to re sample the spectrum hence the very wiggly-noisy profile. The telescope used was a 127mm apo refractor at F7.5. I also have a faster but smaller aperture F6 ED refractor.  It was the overall shape of the Arcturus profile once I had applied a response curve that worried me, as it only had an approximate relationship to the reference profile on the Miles data base.

My homemade transmission spectrometer has a blazed 500lines/mm grating.

I am using a mono QHY5-11 cmos camera.

I have used sunlight as the source for testing my new insert, for the reasons you gave, nice and bright and all around. No light detected so your comments regarding the fibre optic cable(light entry and exit) may be pertinent.

I am only going to use the spectrometer insert for extended objects, so keeping a star on the end of the fibre is not an issue for me but you are correct in that I am using the fibre optic in liege of a slit.

With regard to the books, I have read some and I seem to remember that in Ken Harrison’s excellent book he mentions using fibre optic cables in the way I intend but doesn’t go into detail. 
 

Thank you once again for taking the time to reply. It is much appreciated.

George

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