Relative Flux Calibration

[john simpson]

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

[Paul Luckas]

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.

[Robin Leadbeater]

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

[Paul Luckas]

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

[Dr Andrew Smith]

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

[Paul Luckas]

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

[Paul Luckas]

… 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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