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Relative Flux Calibration

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Relative Flux Calibration

I’ve not been able to get a decent match between my spectra & the Miles references of the group of stars in Taurus reported by Robin in “Testing ALPY 600”. I wavelength calibrated HD27819 using ISIS Pre-defined  Mode [Balmer Lines] giving an RMS error of approx. 0.3A and then calculated the instrument response. The corrected spectrum of HD27819 matched well with it’s Miles reference. However, when I used this to calibrate the other spectra the comparisons with their Miles references are significantly in error [see fig]. There is clearly something amiss in my set-up or  measurements or reduction method, but I haven’t been able to determine what.  I’ve repeated the process using HD27962 with the same outcome – RMS errors in this instance of approx. 0.15A, good match of corrected spectrum with it’s Miles reference, but significant errors in other spectra when it’s used for calibration. The instrument responses calculated using HD27819 & HD27962 are markedly different. Am using Celestron C6 SCT @ f/10 with ATIK 314l+ & ALPY w/o calibration module. Wondered if it might be mismatch in focal ratio with ALPY or if not might others have some suggestions w.r.t. where I might be going wrong?

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Relative flux calibration

Were the spectra taken at a similar air mass? The instrument response includes the effect of the atmosphere as well as the instrument. 

Regards Andrew

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Air Mass

Thanks for the quick response Andrew - yes, not the same, but similar air mass. The two in the graph are close together and image sequences were taken one after the other over 25mins or so. Altitude range 51-52 deg. I'm ALPY novice so I suspect something basically wrong about my set-up or what I've done to process, the errors seem to be too big compared to what's been achieved by Robin. The annoying thing is that the corrected HD27819 seemed so close to its Miles library ref.

john s

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Similar experience

John,

I find the freehand drawing of the response curve can produce markedly different results with different attempts.

Also, the placing of the star on/in the slot (and what portion is recorded) also plays it's part.

Don't get paranoid about your setup - yet.

Keep on swimming. 

Regards

Tony

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varying instrument response

Hi John,

Your response calculation technique looks sound, otherwise you would note get a good match when applied to the reference measurement. It does seem though that something must have changed between the observations.  There are a few things to check

 I ran into a similar problem when I first attempted this type of exercise soon after I got the ALPY. In this case it was due to a combination of chromatic aberration and a shift in focus. I have attached the details.

Combinations of chromatic aberration, atmospheric dispersion, focus and the position of the star on the slit can all conspire to give these sort of problems. Christian Buil has a particularly scary page on the problem somewhere. I will see if I can dig it out.  Having said that your C6 without focal reducer I would guess does not suffer from too much CA.

Other things to watch in processing is to make sure your binning zone includes all the spectrum and the background zones are free from any spectrum data. If you use the optimised binning in ISIS try switching it off as I found the similar feature in VSpec could distort the spectrum under some circumstances.

Cheers

Robin 

EDIT: here is the link I was looking for 

http://www.astrosurf.com/buil/dispersion/atmo.htm

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optimised binning

Hi Robin,

Thanks for the suggestions & references. I've only been able to look at switching off optimised binning so far, but it makes no difference to the results. On the way I checked out switching everything out [no offset, dark or flat field correction, just raw images with no calibration]. I looked at the consistency in pixel location for several absorption lines across all the images and the standard deviations seem reasonable to me.

H.alpha=0.15px; H.beta=0.13px; H.gamma=0.11px; H.delta=0.16px; CaII=0.09px 

I will look again at the data to see if the results show any obvious trends with time. However, more disturbing is that I have found what looks like a straylight problem with my set-up.  If I turn up brightness & contrast to the maximum in the calculated image means, there is a streak of light running parallel to the spectrum [in all the images, to varying degrees].  It is approx 3% of the peak of the signal and about 50 pixels away.  It wouldn't have been summed in background removal, but not a good find. Am concentrating on this for the moment.

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varying instrument response

I've experienced the same problem as John with the same sequence of Miles spectra which Robin used. The response profiles calculated for each star give an excellent match with the Miles spectrum for that star but differ from star to star. All the spectra were taken over a period of an hour at an air mass of around 1.2 with no change to the equipment including focus and no obvious change in atmospheric conditions. However, I do live about 50 miles west of Heathrow and planes are flying over at high altitude all the time. I recently asked on the ARAS Forum if anyone have noticed changes in atmospheric response due to aircraft con trails but so far there has been no response. Possibly a bit of a long shot.

I repeated the exercise a few days later and found the same variation, again different response profiles from star to star but no consistency with the first set. I now plan to experiment with varying the position of the guide star relative to the slit for the same star to see what effect this has. That will not be for a while as the forecast here is bad for several days.

I'm using a LISA with a 23 micron slit on a C11 with a focal reducer operating at f/5.5. The slit size on the sky is 3.1" which is a reasonable match to the seeing here. These is a noticeable fishtail on the spectra due to a combination of chromatic aberration in the LISA and the FR lens but I include the full fishtail that in the binning zone and do this with the same binning zone for all the stars. I use the optimal binning option in ISIS but switching that off has no effect on the derived response profiles. It is not a problem with smoothing the response profile as the effect is much larger than any variation that causes.

Cheers, David

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varying instrument response

I think this highlights the difficulties of doing low resolution spectroscopy at ground level. I did see your post on the ARAS site David but could find no references to your question via various Google searches. It seems planes are not an issue for the professionals.

I think in addition to the issues covered above if you want to get an accurate continuum you need to operate with a wide slit. That is one where the slit is significantly greater that the seeing disk so as to admit 99.99% + of the incident  star light. Thus your spectral resolution will be seeing/guiding limited. 

Obviously, this will reduce your spectral resolution compared to a narrow slit where it defines the effective resolution (all else being equal). So you trade off photometric accuracy for resolution

I think your science goals will dictate which is needed and if it is both then two set of observation may well be needed.

Regards Andrew

PS see here http://www.astrosurf.com/buil/alpy600/photometric_slit.htm

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varying instrument response

Thanks David, Andrew, for your comments & also the Buil reference [I'd not seen this - but very useful]. No condensation trails at my location, but it is loch-side surrounded by hills & conditions sometime change quickly. Once I've sorted out my stray-light problem, I will set an evening aside to track a couple of targets to see the extent to which my observations change over the period of time.

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slit width and continuum shape

Hi Andrew,

A wide "photometric" slit is certainly needed for absolute flux calibration of course but in my experience, provided precautions are taken I have found it is possible to consistently get "reasonable" (+-10% say above 4000A) broad continuum relative flux calibration with my setup using a slit width the same order as the star PSF.

(Except for the effect of changing focus on the original run and an example of the effects of scintillation on the latest run, the continuum errors my two runs using MILES stars I have documented here have been reasonably under control. You can also see some comparisons with professional spectra in what is perhaps a worse case scenario of faint targets using the modified ALPY 200 here.

http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm

Comparisons between observers on the same object posted on the ARAS forum using the reference star techniques are also usually (but with admittedly some exceptions) consistent. 

Perhaps my success though is due to the not particularly good guiding performance which effectively scans the centre of the star image across the slit.  

Speaking to a professional recently, this sort of thing is tough for them too and their approach seemed to be to specifically measure the instrument response using standard stars only occasionally on photometric nights, removing the effect of the atmosphere by measuring at different air masses and then measure just the atmospheric extinction during the observing session, typically making one standard star measurement and correcting for air mass using an atmospheric model generated for their particular local conditions. This gets round the problem of having to find a reference star close to the target but does not address the problem of selective wavelength  sampling at the slit which may be what we are seeing here, though I believe that is yet to be proved. (David, perhaps you could try running with a wider 35um or even 50um slit to see if you get any improvement?).

Buil's ideas on spectrophotometric measurements using a combination of wide and narrow slits are probably worth thinking about in this context too, though adopting the 4 spectra measurement every time would reduce throughput significantly and might still give extinction errors with our fickle weather.

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

Professionals though probably measure with the slit orientated at the parallactic angle and their CA and positioning and guiding errors are likely much lower than ours in general so perhaps selective sampling at the slit is not a big issue for them 

Cheers

Robin

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slit width and continuum shape

I don't disagree with what you say Robin. If you have a technique that delivers the accuracy you need for your objectives e.g. you SN matching work then absolutely fine. The difficulty I have is when corrected spectra or even flux calibrated spectra are presented with no estimate of their errors in the continuum level. (I don't think the BeSS standard requires it.)

If you are using your own data I am a great believer in doing the minimum processing consistent with your goals to minimise added noise and potential artifacts. As we have commented before it is different when adding data to a general use database.

Regards Andrew

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specifying precision and accuracy

Hi Andrew,

Yes I agree, this is an area which definitely needs more consideration. Even Pro-Am campaigns are rarely specified beyond a SNR figure and in most cases I suspect these sort of  systematic errors are much greater than the stochastic ones that SNR estimates, and much harder to quantify. These problems are not confined to amateur observations and databases either. For example the continuum of the ELODIE Vega spectrum originally incorporated into ISIS and which has probably used to correct hundreds of spectra in BeSS, is significantly in error.

 http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1369#p6205 

and the Indo-US library spectra sometimes suggested as reference stars have a completely synthetic continuum based on Pickles

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1582

Cheers

Robin

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AOD in ISIS anyone?

Hi John,

Apologies for the lack of a useful Buil link, but you may also want to take a look at the AOD function in ISIS if you're correcting your spectra using references taken at different altitudes, times and varying atmospheric conditions. I've used it for pro-am collaborations where high cadence spectra were required - in some cases using the same IR correction curve over several months. In recent times I've become less sure of the AOD tool (lacking the time to dig into the detail of aerosol theory among other things).

Also - for the benefit of other newcomers, make sure you scale your miles comparison spectrum to the same range as your processed spectra when comparing the two. This appears to be the case with your plots John, but I just thought I'd mention it for others. In ISIS this is the "Normalize" button on the "Profile" page. The values applied to a comparison should match the ones you used for processing your spectra (specified on the "Settings" page) otherwise you'll usually see a big offset.

Cheers from the southern colony,

Paul

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AOD

Hi Paul,

Yes it is described in part 4 of the link I posted above.

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

It seems to be what the pro I talked to was suggesting too. ie separating out the instrument response (which is presumed stable so measured infrequently) and extinction (measured on the night and adjusted for air mass using the AOD)

Cheers

Robin

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Using the AOD to do a second order correction for extinction ?

I am wondering if using the AOD technique to do a second order extinction correction coupled with our standard reference star technique as a first order correction might be useful, reducing the need to chose the reference star precisely at the same elevation as the target. ie The MILES star need not then be at the same elevation, as the effect of the difference in air mass is calculated separately using a typical AOD. 

Robin

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Using the AOD to do a second order correction for extinction

Yes, I believe so Robin. In fact that's exactly what I did for the aforementioned high cadence campaigns (gam2 Vel, zet Pup). I created a response correction, initially processing the reference star with AOD enabled. At the time I assumed it was necessary in order to compare to the (presumably corrected) published references when creating the IR profile.

I must confess though, I'm no expert. Bernard H., provided much of the guidance during my initiation into these campaigns, having spent many more hours (years !) working with both the ISIS and the pros in question. I'm still [mostly] confused about how to use this feature, particularly when it comes to determining the correction value. As such I've reverted to back the conventional method of correction using a nearby standard for each session and leave AOD switched off.

I do recall seeing some analysis (again, from Bernard) that showed only minor - almost insignificant - variations between the two methods - supporting the consistency of the 'one IR' method.

Cheers,

Paul

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Addendum

Just to tidy up one point. Although the ALPY guide unit has just the single 23 micron slit, Shelyak offer a combined 23/200 micron photometric slit. In case anyone chooses to follow this route; I think I’ve solved my problems of inconsistency of continuum measurement by purchase of the latter – using the wide slit to determine the “instrument response”. It means taking 2 sets of measurements instead of 1 each time, but it seems worthwhile to overcome the variable effects of seeing, guiding errors, geometry & parallactic angle etc.  I’ve not tested the new set-up extensively yet, but initial checks on some of Robin’s stars in Taurus get me to +/- 1.5 - 2% error w.r.t. references over range 4000A & 7000A.  Also, had a quick look at HD35155 which is in the Miles library & used as ref. HD37507 [chose A5V in Pickles] – result in attached figure, still a bit of work to do, but seems promising.

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Atmos extinction and AOD

I'd played around with AOD having gained an idea of what it might be from here...

https://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_AER_OD

Experimenting with varying target altitudes obviously give quite serious differences but using differing AOD values between 01 - 0.2 (the UK apparent variances) produced huge differences in the Aerosol component.  It seems to vary considerably from week to week (?).

I'm not sure how ISIS applies this (?) or indeed whether UK values are accounted for.  (They certainly appear different from the guides given in ISIS).

Or am I reading this wrong?

Regards

T

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Atmos extinction and AOD

Hi Tony,

In ISIS (and other amateur packages) this is usually taken account for in the single combined step for instrument and atmospheric response correction.

The approach is take a spectrum of a standard A or B type star that has a spectrum in the Miles or similar database, and that it is at a similar altitude to your target. You use your spectrum along with the one from the Miles database to calculate a combined instrumental and atmospheric response correction. Where this won't work is if you try to observe a target at a significantly different altitude, or if the sky conditions change. In that case you would need to take a fresh spectrum of an appropriate A or B type star.

When removing individual atmospheric telluric lines then it will be necessary to take a more sophisticated approach. ISIS has another tool which can be used for that but often this step is not needed for the target spectra. I also find this only approximately works rather than giving a really good telluric removal.

Cheers,

Andy

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Atmosphere

Hi Andy, all,

The response tool in ISIS divides the spectrum of our processed standard star by a Miles (or other) standard to create a response profile - which is then smoothed before applying to our target spectra during processing.

Assuming the database standard has been corrected for atmosphere, shouldn't we be doing the same (ie, invoking 'auto atmosphere' with an AOD value) when we pre-process our standard star? Otherwise, how does ISIS correctly correlate the atmosphere corrected Miles standard with our non-atmosphere corrected calibration star.

What am I missing?

Paul

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AOD

Paul,

If we choose reference with same air mass doesn't that ensure correction OK? Only if we try to use that instrument correction for target at different air mass then there will be a discrepancy. Since in practice it's probable there will always be a difference, the question would be "how big a difference?" before it's significant. [if it ever stops raining here I might try to explore that for myself].

john

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AOD

Hi John,

I'm referring to the step, in ISIS, where a Miles (or other) standard, is applied to the reference. Assuming the Miles reference is atmosphere corrected, shouldn't our standard also be corrected before it is divided by the Miles?

Paul

Edit: Or to put it another way, as I understand it, the sequence in ISIS is:

Creation of IR:

1. Process the IR spectra with AOD configured and enabled.
2. Create an IR curve of the processed spectra using a published reference and the continuum shaping tools in ISIS.

Processing target spectra:

1. Enable the AOD function (assuming you have a web connection and have correctly specified object name).
2. Specify the atmosphere corrected IR curve (from above).

ISIS will then remove the atmospheric effects before applying the IR correction.

The alternative method is to take reference spectra of a nearby star at every run. If a target and reference star are very close together then they will be equally affected by any atmospheric effects (attenuation, pollution) and as one is divided by the other those effects are self cancelling. The IR is processed as above (ie, with AOD enabled in order to correctly compare to a published reference) and then applied to the target with AOD disabled during processing.

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IR or IR + extinction

Hi Paul,

Yes, The way I understand it is if you are using the AOD function in ISIS to calculate and correct for atmospheric extinction then the resulting instrument response  just takes into account the instrument  (ie not the "IR+extinction" commonly calculated using a nearby reference star.) You are therefore correcting your spectrum in two stages, instrument response and atmospheric extinction. (I understand this is the usual technique used by professionals who know their instrument response and therefore just correct for the atmosphere for that particular observation) You can see an example of the procedure here.

http://www.astrosurf.com/buil/atmosphere/annexe.htm

EDIT: linked from

http://www.astrosurf.com/buil/atmosphere/transmission.htm

Cheers

Robin

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My brain hurts ...

Hi Robin,

The question remains, should we be using AOD as a matter of course when we process our reference star before dividing it by an atmosphere corrected standard such as Miles?

This regardless of whether we use AOD or not in our target spectrum processing (yes if reference was taken at another time / altitude, no if taken at the same time / altitude).

Paul

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In other words ...

... Always use AOD for reference spectra processing.

Use AOD on target spectra only if reference spectra taken at a different time / altitude.

Do not use AOD on target spectra if reference spectra taken at same time / altitude as target.

Edit: Apologies, it was late. What I mean't to say, and what I think is correct:

If AOD is enabled when processing reference spectra, ensure AOD is also enabled when processing target if using this "AOD corrected" reference.

Do not enable AOD when processing target spectra if reference spectra (taken adjacent to target) has not been AOD corrected.

If reference spectra has been AOD corrected (ie, it is IR only) then target spectra that it is applied to needs to be second order corrected using AOD.

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yes... ...but

Using AOD seems to me to be like trying to do absolute photometry in that a) you know the value to set the AOD at and b) it is constant across the sky and not time dependent. I imagine a value of AOD can be estimated from a single reference star I am not clear how the potential variation across the sky and with time is managed unless a series of standard stars has been observed throughout the night.

Has any one tried this for a set of reference stars? It would be good to see some data to allow a judgement to be made.

I think it is accepted that, at least in the UK, photometric quality nights are few and far between.

Regards Andrew

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more experimentation required ...

Thanks Andy,

Yes, these discussions have got me thinking again because of the pro-am stuff that I’ve been involved in where using AOD was mandated simply because of the cadence requirements of rapidly changing features and the inability to acquire sufficient reference spectra often enough as the target moved through different altitudes. As Robin has mentioned, it’s also a usual technique used by professionals. 

However, I’ve never been totally at home with its implementation in ISIS - having witnessed some weird effects that could only be corrected with bizarre AOD values. So, more often than not, I revert to the standard practice of obtaining nearby reference star spectra to correct for instrument response and extinction - particularly when taking just an hour’s worth of spectra or so. The problem it seems, at least for me, is when to use one vs another, and applying some consistency to spectral processing (which seems more important).

Thanks for the contributions to this interesting dissection. Response and extinction correction in spectroscopy (and photometry for that matter) seem to be recurring themes, so there's obviously some merit in all of this banter.

Paul