After years of doing astro imaging, I decided to use my trusty ST-X10ME CCD for a more scientific application.
Using a Star Analyser 200 I obtained a spectrum of the Quasar 3C-273 to highlight the red shift of the various Hydrogen lines of the Balmer series.
Since I am a beginner at this I had absolutely no idea of the exposure times required to obtain a decent S/N ratio.
My intention was on a total 2 hr exposure time but was cut back to one hour (6 X 10 minutes) on a BRC-250 scope due to fog.
While the spectrum is quite noisy I'm pleased with the result as the Hydrogen lines are clearly visible above the noise level.
I'd say it will require at least 4 hours exposure time to get the noise down to respectable levels.
I'll add more data when possible.
Steven,
Very good mate!
Well done.
IMHO I wouldn't bother with added exposure on this object - you've effectively (and hopefully accurately) recorded the significant features and the Red shift.
What object/ processing software did you use to calibrate your spectrum??
Steven,
OK understood....
What would have been "better" is to take a A type spectrum with exactly the same set up to calibrate the dispersion and then used the identified Balmer lines in the 3C273 spectrum to verify the red shift amount......
very nice, thanks for sharing..... only new at this myself but had a go at recording 3C 273 spectrum the other night but got clouded out before obtaining it.
Steven,
OK understood....
What would have been "better" is to take a A type spectrum with exactly the same set up to calibrate the dispersion and then used the identified Balmer lines in the 3C273 spectrum to verify the red shift amount......
I suggest checking your spectrum image, you may well find you have already captured some A stars in the same field that you can use for wavelength calibration. You can then derive the redshift direct rather than having to assume it. See for example the update in red here when I made the same measurement. http://www.threehillsobservatory.co..../spectra_3.htm
In order to make sure that there was no overlapping spectra that might introduce extraneous noise during processing of the data, the stacked image was heavily cropped to only include the 3C-373 spectrum.
I found in the heavily cropped image the emission peaks were more distinctive.
Taking on board the advice given, I recalibrated the spectrum based on a type A star.
In the attachment I have included the spectrum of a field star in the image that indicates the H-Gamma and Beta wavelengths as would be measured in 3C-273's rest frame.
The values of the red shifted wavelengths illustrated are read directly from the spectrum.
From these values the redshift z for 3C-273 are calculated as follows.
The exposure time is now 4 hours total.
Comparing to the previous 1 hour exposure there is now a noticeable reduction in noise and sharper emission peaks that allows a more accurate measurement of the redshifted hydrogen wavelengths.
The redshifted OIII wavelength is now visible above the noise level.
The redshifts are now z(H-Gamma)- 0.160, z(H-Beta)- 0.160, z(H-Alpha)-0.156.
Steven,
4 hrs is a lot of exposure!! Well done!
The SNR has improved but the resolution will remain the same.....
(Buil quotes 3C273 at Z=0.15834 so you're on the money!)
Eta Car and gamma2 vel should be on your list....
A similar exposure on a slit spectrograph of some of our SASER targets would definitely be very well regarded....
Steven,
4 hrs is a lot of exposure!! Well done!
The SNR has improved but the resolution will remain the same.....
(Buil quotes 3C273 at Z=0.15834 so you're on the money!)
Eta Car and gamma2 vel should be on your list....
A similar exposure on a slit spectrograph of some of our SASER targets would definitely be very well regarded....
Increasing the SNR has allowed the further identifications as seen in the attachment.
On a different topic is the correction for instrument response.
I have created an instrument response curve using my spectrum of Canopus and a professional spectrum of an F0V star (the closest data I have to the A9V spectrum of Canopus).
While the corrected spectrum of Canopus is OK applying the the same correction curve to a different spectral class of star produces irregular results.
Does this mean I have to create a new instrument response curve when the spectrum of a star is radically different from the response curve based on Canopus?
Steven,
The improved SNR helps but the inherent resolution of the grating set-up will not change. If you get say 20A resolution with your set-up (grating to camera distance/ pixel size etc.) then that's the wavelength accuracy you'll obtain...
Subject to atmospheric effects.... if nothing changes then your response curve should still be valid.
Drop me your fits files and I can verify.
Also, just double check with an A type star.....
Steven,
The improved SNR helps but the inherent resolution of the grating set-up will not change. If you get say 20A resolution with your set-up (grating to camera distance/ pixel size etc.) then that's the wavelength accuracy you'll obtain...
While the SNR doesn't improve the resolution it does deal with the issue of noise masquerading as a signal.
The higher the SNR, the greater the probability of a signal rather than noise.
It is more probable that the weak OIII emissions are a signal in the 4 hr exposure than in the 1 hr exposure (it might still be noise however)
Scientists take probabilities to the extreme case as was the discovery of the Higgs boson which was announced at a 5 sigma level or a 1 in 3.5 million chance that the Higgs boson was noise resembling a signal.
Quote:
Subject to atmospheric effects.... if nothing changes then your response curve should still be valid.
Drop me your fits files and I can verify.
Also, just double check with an A type star.....
I ended up creating a response curve based on the QE data of the KAF-3200ME chip used in my CCD.
This seems to be giving a more consistent result than deriving the curve from an existing spectrum combined with professional data.
The attachment shows the comparison response curves.
For the ProAm campaigns they generally ask for a SNR>400
Not sure what is happening with the IR curve. It obviously doesn't cut off cleanly at 400nm! I can get down around 370 with the Atik 314.....
Above 750nm doesn't really help.
Thanks Beren.
How are your efforts on 3C-273 going?
Unfortunately Steven I'm away from home for another two weeks before I can try again. I did manage to get one quick 60sec frame of 3C-273 and surrounds with one of my larger cameras before being clouded out. Just doing that and seeing the quasar on the screen was very satisfying. In the meantime going through Ken's books and practicing with the spectra data that I have captured and learning Rspec and BASS
PS: enjoy your input with cosmological discussion through the forums, so interesting
A common method in astroimaging is for images that are undersampled is to drizzle combine images.
In the attachment is a comparison between the 24 X 10 minute exposures that have been normally combined and drizzle combined.
The differences between the two methods are quite startling particularly in the H-gamma and H-beta peak heights, yet the H-alpha peak is relatively the same.
Initially I thought that each spectrum was scaled differently but this is not the case.
I'm not sure what is going here any explanations would be welcome.
very nice, thanks for sharing..... only new at this myself but had a go at recording 3C 273 spectrum the other night but got clouded out before obtaining it.
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