Creating the Instrument Curve (long post)

[johnb]

Ok so I have been playing with the SA100 for a while now and have sorted Spectrum Capture and Calibration using RSpec but the bit that I still struggle with it’s the IR Curve, I believe once I have a decent on then I will be able to apply that to all the spectrums I capture

So sorry for the length of the post but it will help me and maybe others – please note that I am using RSpec

So this is the Calibrated Vega Spectrum

This is the Library Ref for A0V cropped for the range 4000 – 8500

Now we have the cropped smooth curve

And now we have my Calibrated Vega and the Smoothed IR prior to the division (to produce the IR )

Now post the division

Now the IR Curve with a final smoothed and saved as the final IR

And finally my original Vega with the IR Applied

Feedback really appreciated

Regards

John B

[Gasman]

Doesn`t look too bad to me John. Must admit I don`t always correct for IR (maybe I should!) . I suppose its useful when comparing spectra with other observers whereas I`ll check curves from one night to the next using the same setup. Maybe Ken or Robin could offer you better advice?

Best

Steve

[johnb]

Thanks for that, I must admit im not 100% sure why I would IR Correct but it does seem to be part of the process ( and I think I only need to do it the once, unless I change my setup) so I wanted to start out the right way, do you Normalised for Enhance Spectra ?

Regards

John B

[jnp]

Hi John,

Most cameras are most sensitive in yellow/green so red and blue ends are shown dimmer. It's not just the camera, the grating, lenses etc all have some IR component).  The purpose of IR is to display the spectrum with the real intensities. This will typically boost the red and blue ends. An audio analogy is to think of the 'loudness' control that boosts treble and bass .

Your process for generating the IR looks OK. A variation of the process is to divide the target by the reference spectrum and then smooth the result.

There have been various debates about being able to reuse a single instrument response. In theory, yes you can, especially if you avoid the wavelengths where the instrument response would be poor (e.g. ultraviolet < 400nm). In practice, it might not always work well because other factors come into play, especially the elevation of the star. The lower down, the more 'red' the spectrum will be. Other issues may be caused because the star is not exactly the spectral type. The standard reference spectra are from the Pickles library. Apparently, MILES library reference spectra can be used when greater accuracy is required.
 
However, IR should work reasonably OK if the elevation of your target and reference stars are similar.

regards
John

[johnb]

Thanks for that, its reassuring to know that my method and result is reasonable, this is a learning curve so I think for the time being I will stick with a single IR and get a collection of captures, the only thing I may do is create a IR Curve that starts at 3800 rather than 4000 otherwise on the A type stars i will lose some of the Hb lines

Fascinating Hobby

Regards

John Berman

[robin_astro]

Hi Guys,

Provided you measure carefully, you can indeed use a single instrument response for any object and it even works approximately over a wide range of air masses.  Some people claimed otherwise so I did a series of tests last year.  See the examples attached.  There are many areas where care is needed though for this to work well and even small errors have a big effect at the extreme ends of the response.   It is true though that for many purposes there is no need to do any processing of the raw spectrum beyond wavelength calibration as most of the interesting features are immediately apparent. 

Cheers

Robin

instrument_response.pdf

[johnb]

Robin

Thanks for that, very useful. I'm aware I have been obsessing over this area for some time, I think for the time being I will leave IR behind and concentration on getting decent spectrum's and calibrating etc, as you say with the IR there is so much to see.

regards

John b