Varying response profiles

[David Boyd]

I may have found the reason why the response profiles I measured for Robin’s Miles stars varied from star to star.

I replaced the 23 micron slit in the LISA with a 50 micron slit and repeated the exercise. This time the response profiles of all of the stars were almost identical with only minor differences arising from the subjective smoothing process. Comparing raw uncorrected spectra of each star with the narrow and wide slits, it is clear that I was progressively losing the red end of the spectrum with the narrow slit compared to the wide slit. The effect was worse with stars of later spectral type.

My measurements of Miles stars were all made in January and February when the seeing was consistently bad, 4” or worse as measured from photometry on the same nights. With my setup, >4” seeing gives a star size at the slit >31 microns. With a 23 micron slit this means the star image is considerably larger than the slit which means I am losing light.

The wavelength-dependence of diffraction indicates that for my setup the seeing disk is about 5 microns wider for light at 7400A compared to 3900A. So as well as losing light, progressively more light is likely to be lost towards the red end of the spectrum, which is what I see. With the wide slit, the full seeing disk passes through the slit so no light is lost and the spectrum is unaffected. The response profiles for all the Miles stars should be the same, as indeed they are.

On nights with seeing better than 3”, the full star image passes through a 23 micron slit and the effect will be much less obvious which agrees with my experience.

I don’t recall seeing this mentioned as a possible source of error compared to the problem Christian Buil has mentioned with atmospheric dispersion.

David

[Robin Leadbeater]

Hi David,

I am not sure I follow this. I can understand how the wavelength dependency of the Airy disc size due to diffraction could potentially cause this effect in a diffraction limited situation but in your setup I would expect this to small compared with the slit width and the seeing.  Also the atmospheric effects which affect seeing and atmospheric dispersion are due to refraction, so wouldn’t they reduce with increasing wavelength?  Comparing the star FWHM from photometric B and I band measurements could be interesting. Do you know if that this affect on seeing is seen there ?

Robin

[Dr Andrew Smith]

I suspect it is an effect of any misalignment of the slit with the parallactic angle having less effect with the wider slit.

This may be relevant http://www.eso.org/sci/facilities/paranal/instruments/vimos/inst/atmos/atmos_paper.pdf

Regards Andrew

[Robin Leadbeater]

Hi Andrew,

Yes the effect of atmospheric dispersion (particularly if combined with CA) is well known in amateur spectra too eg  

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

but that does not seem to explain this case where the results were ok when the seeing was better, unless the target was also higher in the sky or the location was such that the parallactic angle fortuitously aligned better with the slit

Cheers

Robin

[David Boyd]

Thanks for your comments. There may well be another reason for the effect I’ve seen but I’m struggling to identify what it is.

I think it would be difficult to detect such an effect photometrically because of focus variation between filters.

My first thought was that it was the problem Christian identified with parallactic angle as I cannot set my LISA to that orientation because the cameras which project from both ends of the LISA foul the pier when near the meridian. However I would have expected the effect of that to be worse at the blue end.

Thanks for the reference Andrew, I’ll digest that tomorrow and keep thinking.

David

[Dr Andrew Smith]

Hi Robin, you are right there are probably too many, unknown or uncontrolled, variables to be sure. 

Regards Andrew

[David Boyd]

Do either of you know of a reference which quantifies the size of atmospheric chromatic dispersion as a function of zenith angle at sea level? A google search has not thrown up anything useful so far. All the references I can find address the issue for large telescopes on mountaintops. It is difficult to tell from Christian’s webpage what the actual size of the effect is.

David

[Dr Andrew Smith]

David is this what you need http://www.astrosurf.com/prostjp/Dispersion_en.html

This may also help http://adsabs.harvard.edu/full/1996PASP..108.1051S and references it contains.

Regards Andrew

[David Boyd]

Thanks Andrew. These look very useful. Neither came up on a google search. Wrong search words I guess.

David

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