Asteroid 2012 DA14 and spectrum

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Asteroid 2012 DA14 and spectrum

Postby Robin Leadbeater » Fri Feb 15, 2013 10:45 pm

Hello all,

I tracked the close encounter with this asteroid tonight with the Star Analyser in position. Attached is one 5 second exposure out of over a hundred I took. I also have a G2v star spectrum to correct for the solar spectrum so I will attempt to extract the spectrum of this object. It was moving very fast so the spectrum is not much above the background noise.

Cheers
Robin
Attachments
2012_DA14_122_50pc_annot.jpg
2012_DA14_122_50pc_annot.jpg (165.99 KiB) Viewed 8111 times
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
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Re: Asteroid 2012 DA14 and spectrum

Postby Paolo Berardi » Sun Feb 17, 2013 5:29 pm

Great observation Robin! We should expect a Ld-type asteroid reflectance, a reddish featureless spectrum, approach flat beyond 7500A.

SMASS Ld-type reflectance: dashed line in the follow graph:
http://ars.els-cdn.com/content/image/1- ... 50-gr5.jpg

I read here about 2012 DA14 class:
http://echo.jpl.nasa.gov/asteroids/2012 ... nning.html

Waiting for your spectrum! I think this is not a simple work...

Ciao
Paolo
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Re: Asteroid 2012 DA14 and spectrum

Postby Robin Leadbeater » Wed Feb 20, 2013 11:22 am

Here are the reduced spectra, taken every 10 mins from 20:50 - 22:00. The spectra are divided by a G2v star to give the reflectivity of the asteroid and normalised at 5500A.

2012_DA14_20120215_spectrum.png
2012_DA14_20120215_spectrum.png (41.49 KiB) Viewed 7797 times


The result is significantly different from that expected for a Ld spectral type (a much shallower slope) and similar to Christian's extinction corrected measurement here
http://www.astrosurf.com/buil/alpy600/d ... DA14_1.png

The 5 second exposures showing the spectra were dark and flat corrected and individually corrected for slant and background subtracted using IRIS. Visual Spec was used to bin and wavelength calibrate the spectra (using Castor as a reference) and divide the spectra by the measured spectrum of the G2v solar analogue star measured at Alt 55 deg . No correction for extinction has been made.

Cheers
Robin
Last edited by Robin Leadbeater on Wed Feb 20, 2013 12:36 pm, edited 1 time in total.
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
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Re: Asteroid 2012 DA14 and spectrum

Postby Robin Leadbeater » Wed Feb 20, 2013 12:13 pm

Sorry there was an error (now corrected) in the Alt of the asteroid at the times of observation shown on the graphic (The the Alt figure was actually the Dec) The altitude was 33 deg at the start of the run increasing to 52 deg at the end

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 905
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Re: Asteroid 2012 DA14 and spectrum

Postby Andrew Smith » Wed Feb 20, 2013 1:12 pm

Impressive work as always Robin, do you think the variations in the individual spectra are traditional noise sources or due to the atmosphere or possibly the asteroid itself?

I have been looking at my SA work in an attempt to get solid reproducible results and have found considerable exposure to exposure variation (30 – 60 sec exposure range) beyond what you would expect from photon and CCD noise sources and can only think it due to the atmosphere. I suspect both the seeing which is normally poor for me 2.5 – 4.5 arc sec and thin cloud/vapour trails.

Your thought would be appreciated.

Regards Andrew
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Re: Asteroid 2012 DA14 and spectrum

Postby Robin Leadbeater » Wed Feb 20, 2013 4:13 pm

Hi Andrew,

I have not worked the numbers but in this case (with the possible exception of the drop off a the red end in the second spectrum which I need to take a closer look at) I suspect the main culprit is the large amount of background included in the observation, made all the more problematic if the background is changing with the asteroid location and having to select different areas for each exposure to avoid other stars , making accurate background subtraction difficult. This is ultimately generally the limiting factor with slitless spectroscopy. In this case, because the asteroid spectrum is trailed, the background included with the spectrum is much higher than would be the case for a stationary target. (I estimate around 30x in this case) I just did a check and typically at maximum intensity at ~5500A the background counts were roughly the same as in the background subtracted spectrum, which would decrease the SNR by sqrt 2 relative to a zero background even in the absence of systematic effects. At the extreme ends of the wavelength range the ratio is much worse of course as the instrument sensitivity drops but the background stays the same and the result can be seen in the way the variability builds up there. (Any small systematic errors in the background subtraction also have a much larger effect in these regions too)

From the work done using the LISA, I suspect in typical skies, slit spectrographs like the LISA can go as deep and possibly deeper than the Star Analyser because of the lower background, despite the much higher resolution

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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Re: Asteroid 2012 DA14 and spectrum

Postby Paolo Berardi » Wed Feb 20, 2013 11:03 pm

Robin, you got the best from 2d spectrum images! Effectively the reflectance profile has a slightly lower slope than L/Ld type one. I see Christian similar result (slope even less pronounced), so this is probably close to the real reflectance spectrum. I also understand that a small residual offset in background can produce a variation of the NEO reflectance curve. Certainly everything is much easier with the slit (except for asteroid tracking!)...

Ciao
Paolo
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