27 January 2017 at 10:17 pm #573678
Managed to get a few observation on 23rd…
The first (shown in attachment) is an apparently innocuous class A, but it raised a question about Alpy calibration. Thus, to try and get the best calibration I used 13 lines in the Alpy lamp and a 3rd order polynomial (acquisition and reduction in Demetra). The Balmer lines were then all nearly in their nominal positions, but blue-shifted by around 1.5A on average. I then noted that this star is in fact a spectroscopic triple…
So, would you expect any significant Doppler shift with this object and, if so, would it be detectable with an Alpy? I reckon the shift corresponds to a radial velocity of 100km/s (although this is my first Doppler calculation :). I know the Alpy is resolution limited (so can’t separate features more than ~10A?) but I think measurement of the centre of mass of the features (as I did) should allow a kind of ‘hyperacuity’ (compare the use of centroid star images in guiding )?
Attachments:27 January 2017 at 10:59 pm #577846
If it is a radial velocity effect, the wavelength error should be proportional to the wavelength ie the error should be greater at H alpha than at H gamma say. If as you say the error is roughly constant in wavelength and given that Demetra is in beta testing, I think I would double checking against a reduction done using ISIS for example in case Demetra is incorrectly measuring the relative positions of lamp and star spectrum for some reason.
Small absolute calibration offsets between internal lamp and the sky are not uncommon though, due to slight differences in the geometry of the light paths and we are talking a fraction of a pixel here so even if the error persists, it may not be anything connected with the star. (Are you seeing it consistently on any other stars?) Measurements of the solar spectrum (eg the daylight sky) can be used to quantify these sorts of instrumental offsets.
Another thing to check is instrument stability. I measured my ALPY to be very stable but changes in temperature and orientation can produce small shifts. Did you measure the lamp spectrum at the same time as the star with the telescope still aimed in that direction? Are lamp spectra reproduceable before and after the observation
Robin27 January 2017 at 11:18 pm #577848
Many thanks for all those suggestions…
Yes, I should give it a try in ISIS too..
I haven’t investigated thoroughly yet to see if its a consistent thing. I can see temperature changes could be critical, and by ‘orientation’ I presume you refer to flexure effects…
Next time, I’ll gather a few more calibrations throughout the night.
Kevin28 January 2017 at 11:10 am #577849
Re-reading my post this morning it is a bit rambling so I thought I would summarise my thoughts.
The repeatability of the ALPY is good enough to measure a to wavelength precision of 1.5A given good technique (eg to detect relative changes in radial velocity in a given target) but an absolute accuracy of 1.5A is probably at the limit or perhaps beyond it without an external reference.
Even 1.5A precision can be challenging as many factors come into play at the sub pixel precision level. For example you mentioned the potential skewing of line profiles due to two or more stellar components. A similar effect can occur instrumentally if for example your star is slightly offset in the slit so the measurement is made on the downward slope of the star point spread function. The resulting shift in the centroid of the line can be significant at the sub pixel level. If high wavelength precision is specifically needed with a slit spectrograph then making sure the star image is significantly larger than the slit and even deliberately dithering the star across the slit can help. (fibre fed spectrographs have the advantage here as they scramble any gradients across the fibre aperture)
BTW did you take account of the intrinsic RV of the star (19.8km/s from SIMBAD) and the heliocentric correction which might account for some of the difference ?
Robin28 January 2017 at 11:26 am #577851Andy WilsonKeymaster
I check, or average out, mechanical shifts in my spectrograph by taking neon calibration frames at the start and end of each stellar spectra. However, I’m using an LHIRES III which is more prone to flexure than the Alpy, plus I frequently stay on the same star for an hour or more. So you may not need to do this for your setup.
When checking a radial velocity, then ISIS has a useful tool in the “Misc” tab for calculating the heliocentric velocity correction, so the rotation of the Earth and its orbit around the Sun (bigger contribution) can be calculated and removed. Then I find SIMBAD is an excellent resource for checking the expected radial velocity of the star.
Andy28 January 2017 at 11:33 am #577853
Here are some brief tests I made on the precision of RV measurements with my ALPY
Note though these measured the precision/repeatability not absolute accuracy of wavelength calibration
Robin28 January 2017 at 11:44 am #577854
Just noticed on your posted spectrum plot that the wavelength scale is way off for some reason
Robin28 January 2017 at 12:00 pm #577855
Hi Robin and Andy
Thanks for comments…
Its nice to know I am up against the absolute limits here (although I understand you could, in principle, see differences in same target, over time, up to around 1.5A)
Robin – the comment on the relation between PSF, slit, and spectrum, is very interesting (although I don’t understand the physics at work here :).. My guiding is not as good as I would like (around 0.8s rms) so I think I may be getting some dither by accident than design :). I calculated heliocentric correction in BASS and it gives a blue shift of 16.8 km/s. This would seem to nearly cancel the absolute radial velocity… so it looks like my offset is definitely an artefact..
I may take a few extra calibrations over the next session just to see how stable it is..
Kevin28 January 2017 at 12:23 pm #577857
My guiding is also bad enough to get round this problem but there is a good example of this by Tim Lester who has a superbly stable setup running at a resolution R~9000 where in this case his good guiding and sharp star image worked against him.
Robin28 January 2017 at 12:28 pm #577858
I have uploaded correct version – no idea where the other one came from – I am doing postprocessing in BASS and I find it loses the plot (literally) occasionally .. (or maybe my finger error more likely)
Kevin29 January 2017 at 5:10 pm #577867
I reworked this one in ISIS and used their autocalibration for the Alpy.. the errors on the Balmer lines were an order of magnitude smaller (~0.23A for first 4 Balmer lines starting at Ha – it gets worse at blue end..)
Incidentally, I managed this only after reading the the thread in
The nominal value of the Atik460 pixel size (4.54) is too big for me.. it works best with 4.45 (I wonder if there is a typo in their literature – swapping 4 and 5!?)
Kevin31 January 2017 at 1:07 pm #577884Tony RoddaParticipant
It’s not a typo.
My Atik460 needs 4.43um in ISIS. It was explained at the workshop as fine tuning needed to exactly match the FL but my memory might be failing.
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