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Hi John
“Last night’s specrum of Pi Andromeda (HD 00369) was still about 3 A out at H alpha but very accurate from H beta to epsilon. “
I presume this was using the mix of lamp lines and Balmer lines for calibration?. If so, the Balmer lines from H Beta down will always be in exactly right the right place because ISIS used them for calibration.
Similarly if you just used the lamp lines for calibration and produce a spectrum of the lamp, I suspect you will find that the lamp lines are in exactly the right place (within the accuracy of the fit ~+-0.5A)
The problem arises when we use lamp lines to calibrate star spectra. There is a shift somewhere between the star spectrum and lamp spectrum. Based on the latest result this does not seem to be a stability problem.
There are differences between the optical paths for lamp and star which could produce small systematic shifts. We may have found the limit of absolute accuracy of using the internal lamp for calibration. (note this is a systematic error, not a lack of precision/repeatability so not a problem when studying changes in wavelength eg in radial velocity measurements of binary stars.)
To take this further I would say we would first need to compare results from other ALPYs to determine if it is a limitation of the instrument or if this particular example is worse for some reason. Since I am seeing signs of similar size errors in star lines with my ALPY when using just the internal lamp, I suspect it could be a limit in absolute accuracy. In which case there is no need to be overly concerned but longer term there may be some modifications that could be made to the ALPY or the calibration technique to improve the calibration accuracy further.
Using a mix of lamp and star for calibration confuses the issue. I would say The definitive test would be for ALPY owners to take a spectrum of a star with known reliable RV, Calibrate just using the internal lamp and look for errors in the star lines.
This could be a good project for the workshop
Cheers
Robin
Hi John,
“I’ve processed the calibration image as is if were a star, of course I had to use the same image to calibrate it. The result was odd, but the neon 6965 line was incorrectly calibrated.”
The spectrum plot looks odd. Did you tick “sky not removed” If you don’t do this it subtracts the lines from them selves, leaving almost nothing behind)
Robin
Hi David,
Ah sorry, I see the wavelengths I posted (and listed by ISIS in the reports) are the values calculated from the fit, not the true values.
There does not seem to be a problem with the quality of the fit to the lines, at least within the range of lines used. (RMS ~0.2-0.3A seems typical for the ALPY.) The issue is apparent systematic differences in wavelength up to 3A between the lamp and the star)
Cheers
Robin
Hi Andrew,
The extrapolation is into the red not the blue ?
Do you have the residuals for the blue points for the different fits? Particularly how much improvement in the fit is there going from 3 to 4 ?
Robin
Ok the reflector position looks ok. ( I was concerned that the reflector might not be rotating to the correct position. It can be adjusted by operating the switches in a particular sequence) Not sure what any offset in the lamp might do (The asymmetry in the lamp internally is normal) and I cant see mine at the moment as it is mounted on the scope but is probably best not to change anything until you have checked the calibration with lamp spectra taken at the same time as the star spectrum.
I have been having a look at some of my past calibrations and I am also seeing wavelength errors between lamp and star (I use just the lamp without the star Balmer lines) In my case H alpha is spot on but there is an increasing error towards the blue to around 2-3A by 3900A where the lamp lines end, so errors of this order might be typical. What are other other ALPY users seeing?
We might be seeing a fundamental limitation in the calibration accuracy using the internal lamp. At this level (~1/4 the slit width) effects such as the position of the star on the slit and subtle differences in line shape due to optical aberrations come into play.
Cheers
Robin
One other thing that could be worth checking is that the servo controlled reflector is set so the calibration lamp is shining directly down on the slit. If the slit is illuminated at a significant angle, it can cause small shifts relative to the star spectrum. (I’ve not heard of this with the ALPY but I suspected this problem with the original manual LHIRES system)
Cheers
Robin
OK, I think the constant shift with just the lamp confirms that there was some shift in the spectrograph (It is very small. I think 3A equates to about 5um or about a pixel). The changing temperature might have been a factor. With the ALPY I usually take a lamp spectrum each time I change target during an observing run even though I normally dont see any shift during the night. (With the LHIRES I see a lot of movement so I take a lamp spectrum at start and end on every target and every half hour or so during long exposures but it is much less stable thermally and mechanically than the ALPY)
Cheers
Robin
OK John,
ISIS seems to have found the lines ok and the fit looks good. I am wondering if there has been some sort of shift between the lamp spectrum and the star spectrum. With the mixed Balmer and lamp line calibration you are using, the lines used for calibration up to 4861 (H Beta) are the Balmer lines in the star and the lines higher than that are from the calibration lamp. Normally the ALPY is very stable but if everything is not quite tightened up, there could have been a shift between the star and lamp exposures. Did you take the lamp spectrum under the same conditions as the star ?
Can you also run the calibration using just the lamp lines ? If there was a shift, we should then see all the Balmer lines shifted, not just H alpha
Cheers
Robin
OK I have checked the MILES zet Dra spectrum and the Balmer lines do appear to be at the expected wavelengths ( eg 4861, 6563A) There will be small adjustments to make for the radial velocity and heliocentric correction but these will not add up to as much as 3A so there looks to be something going on here. Can you post a copy of what ISIS reports in the “Go” window please?
Thanks
Robin
Hi John,
I am not sure the wavelength calibration of the MILES stars is all that accurate. I remember seeing some apparent discrepancies when I was trying to do some radial velocity work. Do your individual errors for the lamp lines look good particularly those bracketing the H alpha line? (from the report that ISIS generates in the “Go” window. )
I would not worry about adjusting the Grism further if ISIS is finding the lines to do the wavelength calibration OK (The RMS error should be low, <0.5A and there should be no obvious larger errors in any individual line as reported by ISIS)
The kink in the instrument response of the ALPY around 4000A is real and often seen, though the cause is not clear. See my recent post here for example.
https://britastro.org/comment/5542#comment-5542
Cheers
Robin
In this case though what we need is the geometric median which minimises the distance travelled. My first student vacation job was with the GPO, writing a computer program to decide where the international exchange should be sited to minimise cabling costs (distance from exchanges, weighted by the traffic). I think it came out near Reading (This was in the 60’s and the computer CPU time was charged by the second)
Kevin does have a valid claim to the centre of population though.
I filter the spectra to match the resolution and shift them slightly if needed to line them up before division. With MILES spectra the match is generally very good then and little editing is needed (I edit out any artifacts rather than choosing points on the curve as that way I can smooth less and include more subtle effects in the instrument response. eg as here)
https://britastro.org/comment/5542#comment-5542
Cheers
Robin
Definitely the normal (ie the original) spectrum from MILES for response correction. You want the original spectrum as it was measured, not what it would look like without interstellar exinction. (When you see MILES referenced in literature it is usually the dereddened spectra as it is normally used for modelling of stellar populations in galaxies but fortunately for us the raw uncorrected spectra were also made available)
Cheers
Robin
Hi David,
This is a good technique I sometimes also use if I suspect there has been a shift in focus during a long run. My setup suffers from chromatic aberration which can affect the response depending on focus due to selective sampling of different wavelengths by the slit as here for example.
https://britastro.org/comment/5542#comment-5542
Robin
Hi John,
The error in calibration for a given altitude difference depends on altitude. The best match would be to the mid point altitude but in practise except at low altitudes the error is small for 5-10 deg difference in altitude. For more details see my recent post here
https://britastro.org/comment/5536#comment-5536
The ARAS forum link there gives links to Francois Teyssier’s spreadsheet for choosing reference stars which includes MILES and other potential stars and takes the error into account. There is also a chart linked from the ARAS post showing error relative to altitude.
Cheers
Robin
From Francois Teyssier’s “Low Resolution Spectroscopy Observers Guide” list of principal lines in symbiotics p 41
http://www.astronomie-amateur.fr/Documents%20Spectro/SpectroscopieBasseResolution_En.pdf
Cheers
Robin
Hi Tony
Can you track the rogue pixels down in the raw images? If it is in every image then it should have been in the master dark and corrected but you could add it to the cosmetic file and zap it that way.
I dont know about BASS but the ISIS cosmics removal tool works on the individual sub exposures where a rogue pixel stands out more clearly than in the combined spectrum image. It compares each pixel with its neighbours, looking at the differential between each pixel and the surrounding ones. You can set the threshold so if a pixel is poking its head above its neighbours, ISIS will zap it. (If you set it too low though, particularly if the spectrum is close to undersampled, it can zap a whole row or spectrum line so you have to use it with caution)
Robin
Hi Hugh,
Yes the acid test is to rerun the reference star using the calculated instrument response
If the flat has done its job well the instrument response should be nice and smooth (just the flat lamp spectrum and atmospheric extinction plus any wavelength selective sampling at the slit due to chromatism) I am not sure if BASS does this but ISIS even assumes a black body curve for the flat lamp so in theory with ISIS ideally all you should be left with is the atmospheric extinction. With my LHIRES at H alpha high resolution this results in a dead flat instrument response within a couple of percent every time so it is hardly worthwhile doing an instrument response correction. In practise with my ALPY there are some small ripples at a couple of percent from the sensor response which the flat has not quite dealt with and a rather nasty kink to deal with around 4000A. I am not sure where it comes from but others have also reported it. Perhaps the reflectivity of the paint used on the reflector used in the ALPY calibration module ? Note the variation in shape in this collection of my IR curves. A lot of this I believe is due to chromatic aberration in my focal reducer coupled with slight differences in focus.
Hi Nick,
Are you saying that ISIS is not finding the dark and flat files you entered in the general page when you run the data reduction?
I have developed my way of working with ISIS over several years so I probably don’t do things the standard way. I collect all the files ISIS needs to do a data reduction in one folder titled with the target name and date, copying the masters, instrument response and wavelength calibration files etc in from other folders if necessary, rather than have ISIS search for them in different folders. (See attached example from a run last night) That way, although there is some duplication of files, I can archive the folder and always rerun the reduction again using the same set of files in the future. When I run the reduction I point ISIS to that folder in the settings page and point to the individual files in that folder the general page.
Robin
