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Hi All,

I had originally sent this message to Christian, and he suggested I post it to ARAS. So I am posting it here along with his reply.
I was unable to attach the image to which Christian refers, but it is on p. 31 of the new ISIS V 5.0 manual.

Cheers,

Keith Graham


My original message:

Hi Christian,

I have a question about the Ne spectrum and resolution. I do believe
that ISIS relies on the Ne line spectrum to determine resolution. Last
night there was a slight difference between the 3 Ne spectra I took
before the AZ Cas run and the 3 I took after the run. If I combine all
Ne spectra to use for calibration, the resolution according to ISIS is
10802. However, if I use only the last 3 combined Ne spectra, the
resolution jumps to 16114. All other factors in the ISIS processing
are the same. I am figuring that the lower resolution when using all
Ne lines is due to the wider combined neon line. So it appears to me
that the actual resolution of the final spectrum is not always the
same as the resolution shown for the Ne line spectrum. As you
mentioned previously, the spectrum used for calibration should be a
combined image of all those taken before and after the image run in
order to get the most accurate calibration – especially when telluric
lines are not useable as with AZ Cas. I would assume the ISIS uses the
midpoint of that combined line for calibration. If this is true, are
we getting a true resolution of the image when combining all Ne lines
before and after the image run? So if I am reading this correctly, it
would appear that resolution is being sacrificed in favor or obtaining
accurate calibration.

Another thought – perhaps the combined Ne line should not be wider
than in the original spectra if the images are properly aligned. I use
Maxim for the combining of images, but always get a wider combination
than in the original Ne line images. Would it be possible (and/or
beneficial) to put an algorithm into ISIS that would combine all NE
line spectra to be used as the calibration Ne image? That way ISIS
would be creating the exact image it needs for accurate calibration
and resolution.

Cheers,

Keith Graham


Christian's Reply:

Hi Keith,

Good question (ideally, to to subscribe to ARAS forum,
because the general pupose interest).

For the moment, the essential, my answer is
the Lhires III tutorial for the very new ISIS version 5. See here

http://www.astrosurf.com/buil/isis/guid ... to1_en.htm

and note location of attached figure near the botton of the web page.

Cheers,

Christian

Hi Keith,

More about the question here:

viewtopic.php?f=8&t=458

Christian

Hi Christian,

I have been playing with the new neon line procedure in ISIS V5.02 as described in the new manual. It makes perfect sense to me to use the first and last “pro” files for final calibration as opposed to the first and last neon line spectra. As you point out, this works nicely for spectra of cooler stars with many lines but I do have a couple of questions:

1. You mention in the manual the benefits of this method for the cooler stars. Is it correct to assume that this method would also work with Be stars when telluric lines are minimal or not present (as in extremely dry conditions), and would there be any reasons why this should not be used for Be stars?
2. Once the comparison of the first and last pro files is made and the lines are shifted to coincide, is this the file that can be saved and then submitted? I am assuming it is, but I do not want to miss anything.

This is a wonderful addition to ISIS. Great job!!

Cheers,

Keith Graham

Hi Christian,
Please disregard the previous reply as I think I now have the idea of what is happening in the ISIS wavelength registration process.
In reading your explanation in the manual and looking at your diagrams, I somehow got the impression that I needed to use the first and last “pro” file to determine the shift and then somehow apply that. But I finally realized that this, in itself, would not produce an accurate actual shift for each “pro” file – just the first and last.

So, let’s see if I now understand it correctly. If I have “wavelength registration” checked in the General tab, each individual “pro” file is shifted automatically during the processing of the images to calibrate with the first neon line. All of the registration is done automatically based on the first neon spectrum, so the user actually does nothing more to shift the image. Once the process is complete using the wavelength registration option, the spectrum has already been tweaked and is ready to submit – is this correct? This essentially would eliminate the final “tweaking “ process as is done when using telluric lines – is this correct?

If I now understand the above correctly, then I still would like to know about using this process with Be stars. You did mention that this is a good process for cooler stars due to their many lines. But would this also apply for Be stars under very dry conditions where there are few (if any) telluric lines? Also, when telluric lines are available, is the ISIS wavelength registration method of tweaking the shift preferable to using wavelength registration in ISIS?

Cheers,

Keith Graham

Hi Keith,

Moderators:- Should this thread be moved to somewhere more appropriate?

Hi Keith

I have not used the new ISIS tool yet but have resorted to doing this sort of thing manually where I have had significant shifts during runs and have not had sufficient neons to calibrate out the shifts. (My preference is to take frequent neons, preferably after each 20min exposure. With my remote controlled setup, taking neons only takes typically 30sec so little observing time is lost even if done for every exposure. By averaging before and after neons for each exposure, the need to re-register is avoided and the absolute calibration is even better.)

I have though had experience of using cross correlation techniques in areas other than spectroscopy. The effectiveness of cross correlation algorithms depends on the prominence of features in the signal so personally I would check how it performs on a case by case basis, at least initially (Many strong narrow features will work best and it would probably be advantageous to bandpass filter the spectrum first before applying cross correlation to avoid the algorithm attempting to lock onto gross broad shapes in the profile or pixel to pixel noise. It is not clear if this is being done here.)

The calibration is still based on the neon lines taken at the start of the series. These should be closest to correct but I personally would always do a final check (and tweak if needed) of the spectrum based on tellurics if they are available and clear enough as this will always give the most accurate absolute calibration.

Note that if there are significant shifts during a set of exposures then separately calibrating them (either using before and after exposure neons or by re-aligning them using the cross correlation algorithm) will ensure that the potential resolution loss due to bluring caused by the drift is minimised compared with say combining all the exposures and using a start and end of run neon average for calibration.

Cheers
Robin

Hi Keith,

My answer is on Spectroscope & software section. The good place.

Christian