Relco SC480 starter lines position depends on bulb orientation

[drjolo]

In my LowSpec spectroscope I placed Relco SC480 starter bulb to be able to do absolute spectrum calibration. Last night I made some first tests and tries to measure Doppler shift one of the fast stars http://simbad.u-strasbg.fr/simbad/sim-id?Ident=hd237090&submit=submit+id  - 120km/s. Hovewer the results I achieved were far from true (about +30km/s when velocity correction was applied). I eliminated few possible error sources and noticed, that position of emission lines recorded from Relco starter depends on the bulb orientation (rotation along vertical axis). Starter is placed about 25mm in front of the slit, and is oriented parallel to the slit direction:

I recorded three test starter spectra after rotating it 90 degrees in the mouting hole. The lines are significantly shifted depending on the starter orientation:

This is screenshot from BASS Project software. All images were tilt and slant corrected in the same way and active region is the same for each image. Lines are shifted by several pixels (dispersion is 0.61A/px). I do not think I will be able to do proper calibration in such configuration. Am I doing something wrong? There must be a way to do proper calibration with starter bulb in front of the slit - many commercial devices work that way.

[Thalestris24]

50 minutes ago, drjolo said:

In my LowSpec spectroscope I placed Relco SC480 starter bulb to be able to do absolute spectrum calibration. Last night I made some first tests and tries to measure Doppler shift one of the fast stars http://simbad.u-strasbg.fr/simbad/sim-id?Ident=hd237090&submit=submit+id  - 120km/s. Hovewer the results I achieved were far from true (about +30km/s when velocity correction was applied). I eliminated few possible error sources and noticed, that position of emission lines recorded from Relco starter depends on the bulb orientation (rotation along vertical axis). Starter is placed about 25mm in front of the slit, and is oriented parallel to the slit direction:

I recorded three test starter spectra after rotating it 90 degrees in the mouting hole. The lines are significantly shifted depending on the starter orientation:

This is screenshot from BASS Project software. All images were tilt and slant corrected in the same way and active region is the same for each image. Lines are shifted by several pixels (dispersion is 0.61A/px). I do not think I will be able to do proper calibration in such configuration. Am I doing something wrong? There must be a way to do proper calibration with starter bulb in front of the slit - many commercial devices work that way.

I've noticed that too and realised I need to make a holder for it that fixes its position (something I still have to do). It's probably an effect linked to the variable spread of the source close to the slit. I've actually been using a Sylvania 70W starter bulb which is bigger than the Relco. The Alpy calibration unit has the bulb fixed in place and I think uses a DC power supply which will only cause one side of the bulb to glow. I had thought that wasn't such a good idea but now I'm not so sure....

Louise

ps it's a good idea to use a reflector - I just have some Aluminium foil wrapped around mine. There may be a better way...

Edited February 8, 2020 by Thalestris24

[robin_astro]

Small wavelength shifts can sometimes be seen in spectrograph lamps depending on the illumination of the slit (In commercial instruments the light from the lamp is diffused either scattered off a separate surface in the case of the ALPY or by a translucent cover over the lamp in the case of the LHIRES to minimise this effect. ) These shifts are normally very small though (just a mainly an asymmetry in the shape of the line, rather than several pixels as here. Is the effect repeatable ?  ie are you sure it is the lamp orientation which is causing the shift and not some other movement in the instrument as you adjust the lamp. (The LHIRES was notorious for this if all the screws were not tight, the slightest touch could move the lines)

Note though that you cannot rely on the lamp for very high absolute wavelength calibration in any case as there are always some systematic differences between the lamp and the sky due to the different light paths. For high absolute accuracy RV measurements (rather than measuring relative changes eg in binary system) you calibrate out any systematic offsets using measurements of RV standard stars

Cheers

Robin

Edited February 8, 2020 by robin_astro

[Thalestris24]

2 minutes ago, robin_astro said:

Small wavelength shifts can sometimes be seen in spectrograph lamps depending on the illumination of the slit (In commercial instruments the light from the lamp is diffused either scattered off a separate surface in the case of the ALPY or by a translucent cover over the lamp in the case of the LHIRES to minimise this effect. ) These shifts are normally very small though (just a mainly an asymmetry in the shape of the line, rather than several pixels as here. Is the effect repeatable ?  ie are you sure it is the lamp orientation which is causing the shift and not some other movement in the instrument as you adjust the lamp. (The LHIRES was notorious for this if all the screws were not tight, the slightest touch could move the lines)

Note though that you cannot rely on the lamp for very high absolute wavelength calibration in any case as there are always some systematic differences between the lamp and the sky due to the different light paths. For high absolute accuracy RV measurements (rather than measuring relative changes eg in binary system) you calibrate out any systematic offsets using measurements of RV standard stars

Cheers

Robin

I think the shift can be quite noticeable (without a diffuser). Clearly, if both electrodes in the bulb are glowing then it could appear as two sources with light hitting the slit at different angles. I'll try and sort out some sort of diffuser for mine

Louise

[robin_astro]

5 minutes ago, robin_astro said:

Small wavelength shifts can sometimes be seen in spectrograph lamps depending on the illumination of the slit

See here on Christan Buil's page an extreme example where the lamp is mounted in front of the telescope aperture and the line shifts depending on the position of the lamp  (about halfway down the page)

http://www.astrosurf.com/buil/isis/He_calibration/method.htm

[drjolo]

Thanks for quick response - I didn't realize that I would need RV standard stars for high accuracy RV measurements. There is whole lot of things to learn. I will check again if that shifts in my setup are caused only by bulb rotation and if they are repeatable. I will fix that bulb in the holder so it will not rotate, add some small diffuser and then maybe the line shift will be more stable, so it can be measured with RV standard stars. 

 

[Bajastro]

Robin, which stars are the best for RV calibrations?
Last time, when I observed some H-alpha profiles in Be stars I used Capella spectra for wavelenght calibration. It isn't very good example, because this star is spectroscopic binary. No spectral lamp used.

[robin_astro]

2 hours ago, Bajastro said:

Robin, which stars are the best for RV calibrations?
Last time, when I observed some H-alpha profiles in Be stars I used Capella spectra for wavelenght calibration. It isn't very good example, because this star is spectroscopic binary. No spectral lamp used.

Wavelength calibration based on lamps taken before and after the star spectra are usually good enough for amateur work but if you need higher absolute accuracy andthe spectrograph is stable enough to make it worthwhile, there are a number of catalogues of RV standard stars, for example this list of bright northern hemisphere targets on the ESO website

https://www.eso.org/sci/facilities/lasilla/instruments/harps/tools/observing_tools.html

but any non binary main sequence star and the RV figure given in SIMBAD should be good enough for most purposes. 

 For Be stars at high resolution around H alpha I would recommend rather than calibrating using star spectra, using a simple neon lamp to get the dispersion equation and then use the atmospheric water lines to trim out any offsets which can be done to high accuracy. Visual Spec, ISIS for example have nice tools to do this and the BeSS validation team use these lines to check the calibration when validating spectra for the database.

Cheers

Robin