Meteor Spectroscopy

[Andrew Smith]

Hi I think this is probably a question for Christian but don't feel put off if you can contribute.

I am setting up a system to try to capture spectra of Meteors (see the post by Martin in the results section or the Nemetode Site http://www.nemetode.org/index.html although I will be using a standard astro CCD at first rather than a video camera fitted with an objective grating.

As the meteor travels its own path relative to the grating the direction of dispersion and the track of the meteor acting as a long slit can give large values of slant and tilt. My question is is how is this best to "normalise" the spectra to be level and upright? Is it best to use slant and tilt in ISIS or as some do a rotation in IRIS or something else?

Thanks for any guidance.

Regards Andrew

[Robin Leadbeater]

Hi Andrew,

In my simple tests some years ago, I just used ISIS to correct for slant which I believe just works by shuffling the rows horizontally (If the grating is orthogonal to the camera there should be no tilt correction needed) It did appear to blur the spectrum somewhat though.

Have you seen Martin's paper here
http://www.meteorastronomie.ch/images/M ... 4_2015.pdf
from this page
http://www.meteorastronomie.ch/ergebnisse_Spektren.html
It describes in detail his procedure to remove various distortions to produce an orthogonal projection of the spectrum

What setup are you using? (bigger is better where the camera is concerned, giving higher probability of capture at higher resolution.) From what I understand the current amateur systems with small CCD are considered too low resolution to be interesting to pros and the limited dynamic range of the commonly used 8 bit video cameras is a problem so I think proper 16 bit CCD astro cameras are the way to go for meteor spectroscopy. I had dreams of building a dedicated system using an SBig 8300 and a nice (expensive) wide angle APO lens with a 300/600l/mm grating on it but I have not won the lottery yet

Good Luck !
Robin

[Andrew Smith]

Thanks for the input Robin, I have looked at the papers from Matrin but am still getting my head round them. I attended a Chester Astro Soc meeting where William Stewart gave a talk on modern Meteor detection. This included doing spectroscopy. I have exchanged emails with Bill Ward who has beeing doing this for some time using Video Cameras and fast CCTV lenes and 600 l.mm gratings.

Like you I thought that larger 16 bit CCDs with longer lenses might be the way to go. So I am going to use an old CCD and a new (expensive) 20mm F1.4 lens with a 50x50mm 600 l/mm grating to try to see what I can do. I still need to make an enclosure though. Initially I will try on planes flying in and out of Manchester airport as they are more reliable than Meteors. While the old CCD (1.2 inch video size) is not as big as my 8300 the 8300 is in use on the echelle (or not given the weather!).

To stop me haveing to wade through large numbers of blank images I intend to use an old Video camera and a 4mm CCTV lens to monitor the same area of sky as the spectroscope using UFO software which triggers on the motion from the meteor so I only need to look at selective images.

I tend to agree with your point on the spectrum correction but the norm in meteor spectroscopy seems not to bother aligning the dispersion with the CCD as they rotate the grating to best suit the trajectory of the meteors from various showers and or avoid the spectra for local lights. A whole new world to explore.

Regards Andrew

[Martin Dubs]

Hi Andrew and Robin,

I appreciate that you are interested in my method of linearization of meteor spectra. I agree that it is not easy, maybe I have to explain it in more detail. I also try to determine the transformation parameters in an easier way, but so far there is still a lot of work ahead.
The idea to use a high resolution CCD together with a video camera for detection of the meteors is an excellent idea. I myself use a short focal length video camera for the survey within the swiss meteor network and a longer fl video camera for spectroscopy. I am still not quite satisfied with the resolution.
At the moment I analyze a video taken by Koji Maeda (thank you for giving permission to use this beautiful meteor spectrum for illustration of my method) with a Sony alpha 7 with a 24 mm f1.4 lens which gives excellent spectra. Actually I analyze the video frames, but the method also works with the still image as shown below:

original image recorded by Koji Maeda, reduced by factor 4

m20151127_222709_jpmz1_hepsmall4.jpg (21.81 KiB) Viewed 7837 times

After linearization and rotation (only approximate, still working on it):

linearized with distorsion correction

m20151127_222709_jpmz1_hep-dnrs4.jpg (20.63 KiB) Viewed 7837 times

This can be analyzed with IRIS (ISIS works also, but wavelength calibration is not automated yet).
I find it easier to calibrate when the dispersion runs parallel to the x-axis, a small rotation can be corrected. This was done here.
The direction of the meteor can then be corrected with the slant command (not done in this image, by coincidence the angle was very small, approx. 1°). The advantage of my method is that standard spectroscopy software can be used afterwards to extract a number of spectra with the same calibration. In the original images the dispersion is different for each position of the meteor and the spectra have a different smile, so they cannot be registered and stacked in case of the video frames.
In order to get resolution of the spectra the flight direction should be perpendicular to the dispersion. This can be achieved to some extent by rotating camera plus grating (of course this works only when you know where the meteor is coming from).

I hope this helps a bit, questions are welcome.

Regards, Martin

[Andrew Smith]

Thanks Martin, what a magnificent spectra! I think I grasp your method now but still need to do the characterisation of my system - I only just got the lens.

One point I have not understood is why for meteor spectroscopy the grating is placed normal to the optic axis rather than with the blaze direction along the optic axis and then offsetting the whole unit to point in the required direction. Is this just a practical expedient or for some more subtle reason I have not yet grasped?

Thanks again for you work in this area.

Regards Andrew

[Martin Dubs]

Hi Andrew,

both the lens and the grating equation have some rotational symmetry. For a simple solution to the transformation corresponding to linear spectra the axes need to be collinear. In that case the combined effect of lens distorsion and orthographic projection of the spectra result in an equation which has rotation symmetry, which can be determined easier than a complicated function involving several angles (see the simple form of equation 11 in the WGN paper for the collinear case).
If you want the blazed spectrum in the center of your image, all you have to do is align the camera axis at the blaze angle offset from the meteor direction (assuming you know where the meteor appears )

If you know the axis direction exactly a single spectrum with sufficient lines across the field of view (including higher order lines) can be used to determine the transformation equation. In practice several spectra are recorded, from which both the transformation coefficients and the position of the axis can be obtained. The details are described in the WGN paper.

Regards, Martin

[Andrew Smith]

Thanks Martin a good explaination. Regards Andrew

[Robin Leadbeater]

Hi Martin,

In practice several spectra are recorded, from which both the transformation coefficients and the position of the axis can be obtained.

Flat field correction on this type of setup must be difficult. When recording spectra at different positions, have you looked for any potential field illumination effects on the spectrum shape depending on where the spectrum falls?

Cheers
Robin

[Martin Dubs]

Hi Robin,

your question is right on the mark. This is what I am working on at the moment. As you say it is quite difficult. There are several effects which influence intensity calibration:
- grating efficiency
- detector efficiency
- atmospheric extinction
- flatfield correction.
With a video camera (8 bit) you have in addition a small dynamic range, so S/N is usually quite bad. In addition, the meteors do not all have the correct brightness for optimum S/N without saturating the CCD.
With a wide field lens the light falls off quite a bit towards the corner, so a flat field is necessary. I think it is best to apply it from an image without grating in order to avoid higher orders entering from large angles. This should compensate for vignetting and distorsion of the lens which spreads the signal near the corners. In principle the transformation to orthographic projection should also be taken into account but I have not done this at the moment. The grating efficiency is also sensitive to the incidence angle but to a smaller degree. I checked with some calculations. I made some attempts at IR correction, but it is still preliminary:

red: uncorrected spectrum
blue: IR applied

M20151124_043551_IR.png (8.51 KiB) Viewed 7741 times

I used Venus for reference, no atmospheric extinction correction applied (H Venus: 28°, Meteor 31.5°at peak brightness). Unfortunately there is no easy way to check the results.

Regards, Martin

[Martin Dubs]

Hello,

in the meantime I have completed the analysis of Koji Maedas meteor spectrum of M20151127. It took me a while to figure out how to process and analyze a time series of meteor spectra efficiently. The results you can find here:
http://www.meteorastronomie.ch/ergebnis ... um20151127
Interesting is the time resolved spectrum, registered to the zero order peak, from which you can see that the spectrum indeed has constant dispersion:
http://www.meteorastronomie.ch/images/M ... r_win4.mp4
with the meteor train well visible.

Enjoy the holidays,

Martin