26 July 2017 at 7:22 am #573799
I’m trying to measure the radial velocity of a planetary nebula acquired with an Alpy last night. The prominent emission lines (relative to the continuum) in these targets seem to offer themselves up to the task. My first target (NGC 6644) seemed very promising within +/- 10 or so km/s of the published RV of ~ 200 km/s.
However, I then repeated on NGC 6629 which has a comparatively low RV of ~ 13 km/s and I’m out by a couple of hundred km/s!
What am I missing that is obvious in all of this? The Alpy’s resolution seems to me to be within the RV measurement boundaries of several tens of km/s at the very least, even with the rough and ready means I’m employing using PlotSpectra . The wavelength stability of the Aply is astounding (over months of observations, my central wavelength and dispersion solution have not shifted at all) and aside from relatively short integrations, my profiles look perfectly adequate. I’ve double checked the published RV values for both targets in literature, and considered (included) the effect of heliocentric velocity.
Is there something significantly peculiar about planetary nebulae that I’m missing, or is it simply a fundamental limitation of the Alpy’s resolution with ‘rough and ready’ measurement techniques? I seem to recall Robin achieving excellent results with the Alpy using the cross correlation method in ISIS some years ago (I wasn’t necessarily striving for this level of accuracy – just a ball park example).
PS – apologies for the lack of image(s).26 July 2017 at 9:06 am #578369
… can be found here (for anyone interested):
Though I thought operating at about 500 Å/mm would get me within 50 km/s, clearly there are other factors at play. Fortunately I also own a Lhires (I just need clear weather).
Paul26 July 2017 at 9:46 am #578370David BoydParticipant
Have you tried measuring some RV standard stars to calibrate your capability? The Soubiran et al catalogue prepared for Gaia gives a good list – https://www.aanda.org/articles/aa/abs/2013/04/aa20927-12/aa20927-12.html.
These are 6th to 10th mag stars and there are several dozen with RVs greater than 50km/s, both +ve and -ve. They are F, G and K type stars so good for using in cross-correlation. About 20% of them have southern Decs down to -10deg. This would let you see what consistency you can expect.
David26 July 2017 at 11:58 am #578372
a 200km/s error with the ALPY does seem unexpectedly large (roughly a pixel or so). Like you I find the ALPY very stable and I would expect to be able to measure the position of a strong narrow line in an extended object like a PN to better than say 1/5 pixel. In stellar RV work one source of significant error is the position of the target on the slit. (if the star is too small it “rattles” in the slit and even if overspilling the slit, slight offsets in guide position alter the shape of the line profile and move the centroid.) I would not expect these issues with PN though as they are effectively extended objects. Since the error is variable we can also rule out an offset in the calibration lamp spectrum. PN are expanding so show different RV depending on the area examined but I understand these are only a few tens of km/s at most.
I dont know the quality of the wavelength measurement on Plotspectra. Have you tried measuring the lines in ISIS? Also since the ALPY calibration is so stable and you are comparing the PN using lines at the same wavelength, you could try just overlaying the raw uncalibrated profiles of the two PN and look for any shift directly to eliminate any potential errors in wavelength calibration.
Robin26 July 2017 at 12:15 pm #578374
I think my limited work with the ALPY suggested a few tens of km precision but high RV precision using low resolution instruments is certainly possible though for example as David demonstrated at a BAA workshop using a LISA a couple of years back with if I remember, an impressive 5km/s 1 sigma ?
Robin26 July 2017 at 12:51 pm #578375
I’ve been searching without success – I don’t suppose anyone has a link to Christian’s tutorial on using CCF in ISIS (I need a refresher).
Paul26 July 2017 at 2:16 pm #578376
There are some examples of its use with ALPY here which might be useful
Also described in the ISIS change log under v5.1.0
Robin26 July 2017 at 2:51 pm #578377
Apologies to all. I’ve just revisited NGC6629 tonight and a quick processing has me at a much better result (+/- a few km/s). I suspect a mis-targeting or blunder in file naming. I’ll post some results in due course.
Sorry for the interruption.
Paul27 July 2017 at 5:47 am #578381
Just to close the loop on this; I ended up measuring the radial velocities of 3 planetary nebulae with published values ranging from -13 km/s to 194 km/s. I used PlotSpectra’s point and click measurement tool, averaging the velocities of the prominent H and OIII emission lines.
All three ended up within 15 km/s of the published values – which is closer to what I expected vs my original post on the matter. Of note, the spectra were hastily acquired (ie, resulting in low S/N) between rain and clouds.
I’ve attached a couple of profiles. Nebulium 😉 dominates these objects, as expected, and in the case of the attached NGC 6629, the black body profile of the hot central star appears (perhaps?) to be peaking through at shorter wavelengths, although I did not correct for instrument response (I mention this only as my other two show little sign of a central star via the continuum which could be targeting, or target). Despite the low resolution of the Alpy, there are some interesting differences in the features of the acquired profiles which I’m yet to fully investigate. NII 6583Å is clearly visible in NGC 6818, with its sibling on the other side of H-a not quite developed at this resolution.
A recommended activity for Alpy owners / beginners.
Thanks, Robin, for the links to the ISIS CCF information above.
Paul27 July 2017 at 3:53 pm #578384
Impressive RV precision ! There are various interesting physical properties that can be measured from spectra like this. Francois Teyssier has a worked example on his website
Robin28 July 2017 at 12:38 pm #578385Steve CuthbertParticipant
Very impressive Paul, any chance of putting together a quick (?) `how to` for us lesser mortals 😉
Steve28 July 2017 at 2:32 pm #578386
I’m happy to oblige, though my attempts are not novel by any means, and I’m certainly no expert.
Nevertheless, I’ll write this up over the next week or so and let you know. I’m actually hoping for a break in the weather to improve on the data collected so far, though I have enough to share in the mean time. Stay tuned …
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