Fiber modal noise mitigation for High Res

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Fiber modal noise mitigation for High Res

Postby Andrew Smith » Fri Feb 09, 2018 1:39 pm

The original fiber agitator I had with the MRes spectrograph did not prove effective with the my new High Res spectrograph se my previous post. In view of this and taking advice from this paper https://arxiv.org/abs/1802.01642 I have looked at both linear and circular agitation (see results below) and concluded that I will build an agitator that provides both types of agitation.

Some key parts are on order but a key additioanl constraint is to have it enclosed so as not to attract an attack by my Burmese cat!

Modal Noise Mitigation.png
Modal Noise of Flats
Modal Noise Mitigation.png (253.77 KiB) Viewed 613 times


The data is the same line through the average of 4 flats filtered with an average 3x3 Kernel filter as the camera ASI1600MM Cool greatly over samples the spectra.

I also still need to work to get the best from this camera.

Comments welcome.

Regards Andrew
Andrew Smith
 
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Re: Fiber modal noise mitigation for High Res

Postby Christian Buil » Sat Feb 10, 2018 3:15 am

Hi Andrew,

First, congratulation for your fine High res spectrograph

But, the modal problem is severe in your actual setup. It is a real problem of course.

The spectrum of sunlight in your first message does not seem so deteriorated (Halpha line). You confirm ?

You use a tungsten lamp here I suppose ?

Injection into the fiber can be a critical point. If possible, enter with a well-open optical beam (f / 4 - f / 6). One question: do you have a diffuser between the source and the fiber input ? If no, try this.

Christian
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Re: Fiber modal noise mitigation for High Res

Postby Andrew Smith » Sat Feb 10, 2018 2:13 pm

Thanks for your comments Christian.
Firstly it was a tungsten lamp with diffuser but as you will see this doesn't seem to be the issue. It was H alpha and I agree it did not seem to have the issue.

This lead me to do some more tests. Firstly with the set up with an 5m 200 micron fibre to the guide-head then 5m 50 micron to the spectroscope (as before).

Then I used a 2m 50 micron fibre direct from the tungsten lamp to the spectrograph. To my amazement the noise was gone!

I then reverted back to the first configuration but agitated both fibers with a much improved pattern.

I did do some other checks swapping fibres but it it seems it is the combination fibres and guide-head that is the issue.

Modal Noise 2.png
Modal noise take 2
Modal Noise 2.png (250.28 KiB) Viewed 560 times


Note I did a 3x3 Kernel average filter to reduce the noise to better show the modal variations.

Any further comments most welcome.

Regards Andrew

PS Just ordered some 150 micron square fibre to try instead of the 200 micron circular fibre. Will report the results in due course.
Andrew Smith
 
Posts: 289
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Re: Fiber modal noise mitigation for High Res

Postby Christian Buil » Sun Feb 11, 2018 2:55 am

Some old unpublished results about modal noise (VHIRES-MO spectra).

On the left a spectrum of Vega taken with a 24-inch Newton telescope f/3.5 without shaking the fiber. On the right, a spectrum of Vega taken with a Celestron 11 f/6.3 (focal reducer) telescope without shaking the fiber :

Image

Note the importance of fiber input parameter (telescope focal ratio,...)

Below, a spectrum of Vega taken by using a C11 f/6.3 and shaking the fiber with a 0.1 Hz frequency over a wide amplitude (nearly +/- 10 cm) :

Image

(50 micron diameter / 10 meters fiber in all situations)

I also note different behavior by using several fibers of the same reference. Try to exchanging input and output port of the fiber.

This subject is always delicate (sometimes also underestimated by eShel spectrograph users for example -> evaluation of the setup is a recommended task).

Christian
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Re: Fiber modal noise mitigation for High Res

Postby Andrew Smith » Tue May 15, 2018 1:31 pm

After much research I concluded that the problem with the Flats giving a high modal noise was due to the Shelyak guide-head unit. Following discussions with Francois I sent the unit back and he realigned the feed from calibration unit onto the entrance hole.

Testing on its return shows I now have 5 time more illumination than before and the modal noise is significantly reduced so that "shaking" the fibre removes it.

Thanks to Francois for his efforts which were free of charge.

Regards Andrew
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Posts: 289
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