[Robin Leadbeater]

Hi John,

If you use the “no ALPY calibration module” setting in ISIS it automatically finds and uses the first 7 Balmer lines in the star spectrum, plus the head of the telluric band at 6872A for calibration. Only the resulting calibration fit coefficients are transferred into the dispersion window so you can use them for subsequent calibration of further spectra (using the “predefined dispersion equation” option). The list of lines for manual entry shown there are just what happened to be left over from a previous manual run. If you delete them all for example before making the calibration you should see that only the coefficients are filled in after calibration.

HTH

Robin 

[Robin Leadbeater]

The spectrum features do look more F7 (HD222368) than G8 (HD 38751)

Robin

[Robin Leadbeater]

Hi Steve,

You could use Paolo Berardi’s spreadsheet which has RA/Dec.

I think Francois Teyssiers spreadsheet which allows you to find all the close A/B stars with low extrinction also identifies the MILES stars 

https://britastro.org/node/8152

If you do go back to the original MILES source make sure you chose the spectra uncorrected for extinction

Cheers

Robin

[Robin Leadbeater]

Hi Kate 

I am able to obtain an acceptable calibration (RMS 1.25 A) with your files using the neon file to measure the smile. I will email you a set of output files and screenshots which hopefully will allow you to reproduce it.

Cheers

Robin

[Robin Leadbeater]

Hi Kate,

What smile settings have you used?  If you do not have a calibration module, you can measure the smile using sky lines.  I tried your images assuming vertical lines with no smile (by setting the Y smile to the same as the spectrum position and the radius to a large number eg 999999) and got an RMS of 1.14 A using a pixel size of 4.68um

————————————————————————–

Wavelength fit deviation

point #1  x = 704.340  lambda = 3834.491  dlambda = 0.899

point #2  x = 725.984  lambda = 3889.299  dlambda = -0.249

point #3  x = 758.121  lambda = 3970.993  dlambda = -0.913

point #4  x = 809.533  lambda = 4102.375  dlambda = -0.625

point #5  x = 901.554  lambda = 4339.253  dlambda = 1.227

point #6  x = 1102.840  lambda = 4861.706  dlambda = -0.366

point #7  x = 1771.045  lambda = 6562.745  dlambda = 0.065

point #8  x = 1898.937  lambda = 6872.037  dlambda = -0.037

————————————————————————–

Coefficient a4 : 3.006774E-11

Coefficient a3 : -2.735230E-07

Coefficient a2 : 6.837095E-04

Coefficient a1 : 1.92972

Coefficient a0 : 2221.769

————————————————————————–

RMS : 1.116470

————————————————————————–

In his ALPY tutorial, Christian Buil suggests an RMS of 2-3 A maximum is acceptable for this calibration method

Cheers

Robin

[Robin Leadbeater]

Hi Tony,

A couple of ideas:-

It looks like there is a huge hole in the spectrum from ~4500-6600 where the response should be highest. Could there be saturation in these regions ?

Alternatively could you have set the binning zone limits very narrow or offset from the middle of the spectrum so it is missing part of the spectrum in the middle wavelengths ?

Cheers

Robin

[Robin Leadbeater]

Hi Kate,

There is still something wrong I am afraid. The RMS fit should be better than 1 Angstrom. For example I have just run a Balmer line calibration on one of my recent reference stars and get the following results (ALPY 600 2xbin 4.54um pixels, pixel size set to 8.97A   (3123/348) )

————————————————————————–

Wavelength fit deviation

point #1  x = 143.059  lambda = 3835.148  dlambda = 0.242

point #2  x = 154.045  lambda = 3888.984  dlambda = 0.066

point #3  x = 170.621  lambda = 3970.476  dlambda = -0.396

point #4  x = 197.253  lambda = 4101.992  dlambda = -0.242

point #5  x = 245.161  lambda = 4340.028  dlambda = 0.452

point #6  x = 349.413  lambda = 4861.473  dlambda = -0.133

point #7  x = 696.362  lambda = 6562.787  dlambda = 0.023

point #8  x = 762.564  lambda = 6872.013  dlambda = -0.013

————————————————————————–

Coefficient a4 : 3.601046E-10

Coefficient a3 : -1.409022E-06

Coefficient a2 : 1.183699E-03

Coefficient a1 : 4.63613

Coefficient a0 : 3146.763

————————————————————————–

RMS : 0.408720

————————————————————————–

If you email me your spectrum image fits file, I can see if i can get a better result if you like

EDIT: typo in pixel size corrected (4.54 not 5.45)

Cheers

Robin

[Robin Leadbeater]

Hi Kate, The pixels size is usually not too critical whe using  the H alpha lines as there are no other lines nearby but it can be super critical with the calibration module where it needs to distinguish between some very closely spaced lines

I find Christian Buil’s tip here

http://www.astrosurf.com/buil/isis/guide_alpy/resume_calibration.htm

works well for calculating the pixel value to use:

“A tip for calculate the scaling factor p (or virtual pixel size). Use the formula:

p = 3123 / dx

where dx is the distance between the H alpha and H beta lines measured in pixels along the horizontal axis in a raw image. Try to find the distance to the nearest pixel (the reading of the mouse pointer is sufficient). In the example, the H alpha line is at x = 882, while the H beta line is at x = 535. So dx = 882-534 = 347, and thus the pixel size to adopt is p = 3123/347 = 9.00 pixels.”

Another trick is to keep an eye on the RMS value that ISIS generates in the running commentary on the “go” page when it is running the calibration fit. If you keep on repeatedly running varying the pixel size slightly, it should be obvious when the program locks onto the right lines as the RMS will suddenly tumble to a very low value. 

Finally double check you are picking the H alpha line and not the nearby Telluric band as the reference  point. (It has been known !)

Cheers

Robin

[Robin Leadbeater]

Is this for spectroscopy?  If so you can’t use twilight or LEDs.  You need a light source with a smooth spectrum with no features. 

EDIT: you might be able to get away with LEDs in some parts of the wavelength range in higher resolution spectroscopy but I dont know of any LEDs which have a smooth enough spectrum or wide enough wavelength coverage for the ALPY for example

Cheers

Robin

[Robin Leadbeater]

Hi Nick

The defect (or cosmetic as it is called in ISIS) map is to catch the hot and warm pixels which will have unreliable values after the dark subtraction (ie the pixels will either already be saturated or will become prematurely saturated during the exposure, so the dark subtraction will not work correctly.) The same procedure is sometimes used in imaging  where it prevents dark holes appearing in the image after dark subtraction where fully saturated hot pixels are for example. The cosmetic file is generated  from the master dark and a threshold is set above which the pixels are considered defective. I have mine set to 500 currently which is well above the noise and generates around 100 pixels in a 10 min exposure with my ATK314

Cheers

Robin 

[Robin Leadbeater]

Hi Steve,

At least the ALPY is stable enough not to have to go to the extreme of taking flats with the scope still aimed at the target  like you have to do with the LHIRES if you have a camera with small scale ripples.

http://www.astrosurf.com/buil/fringes/index.html

Before I had the calibration module, I injected the light from the RELCO calibration lamp via a reversed flip mirror. Perhaps a diffused incandescent light source could also be used this way to produce a flat.

Cheers

Robin

[Robin Leadbeater]

Hi Kate,

I found my Gert Neumann panel and put it in front of the ALPY 600 spectrograph with an ATIK 428 camera. Here is the comparison between the EL panel and the internal ALPY lamp.

 (This is effectively the output from the light source x the response of the grating and CCD camera)

As claimed by the supplier, the spectrum of the EL panel  is smooth with no obvious lines (The small scale ripples in both spectra are from the camera QE and demonstrates one of  the reasons why flat correction is important).

The EL panel could potentially be used as a flat light source for spectroscopy, even though the spectrum shape is very different from the Incandescent Halogen lamp black body curve but unfortunately there is no light from it below 4000A. (The internal Halogen lamp in the ALPY also struggles in this region but in practise it still gives enough output down to ~3600A provided a large number of flat exposures are combined to reduce the noise)

A key difference between imaging and spectroscopic flats is that it is much less important that the field is evenly illuminated since you are only sampling a small slice of the field. Even some variation in illumination along the slit is not important if you are measuring at one position eg as for a star. This means even a crude flat illumination setup can be used. I bought my ALPY before the calibration module came out and used the same flat lamp setup as I used for my LHIRES – A halogen lamp waved a couple of metres in front of the telescope aperture which was covered by a diffuser (actually a pillow case!), similar to Christian Buil’s setup here

http://www.astrosurf.com/buil/isis/guide_alpy/tuto_en.htm

This worked well and I saw no difference when I eventually got the calibration module

Cheers

Robin

EDITED: to mention the grating response

[Robin Leadbeater]

Hi Kate,

Yes I know  Lothar Schanne who made those tests.  I think the Neumann panels work well for high resolution spectroscopy within a limited wavelength range, particularly H alpha with the LHIRES III spectrograph which was what Lothar was doing there. For that particular application they worked better than the usual halogen lamp, matching the defects caused by dust for example. Note that some panels from different manufacturers have some sharp emission lines which make them no good for spectroscopic flats.  

I don’t think they work as low resolution wide wavelength range flats (for the ALPY or LISA etc) though as they have a limited wavelength range with no output at the UV and IR ends compared with a halogen lamp. (Francois Cochard commented about this at the recent workshop)

I have a small one knocking about somewhere. I will see if I can dig it out and test it with the ALPY 

Cheers

Robin

[Robin Leadbeater]

OK I found a description of using the response assistant here in the LISA tutorial (about half way down)

http://www.astrosurf.com/buil/isis/guide_lisa/tuto_en.htm

it appears it reruns the reduction of whatever spectrum data is currently set up in ISIS, but without a flat correction.(This is all hidden in the background) It then combines this with the flat and the reference spectrum (which has to be of the same object which ISIS has reduced in the background) to produce the instrument response. This is effectively the same as the usual  “long hand” way of doing an instrument response but with some automation. It does seem a slightly confusing way of going about though. All very odd !

Cheers

Robin

[Robin Leadbeater]

Hi Paul,

I am at a loss to know what it is doing. It could perhaps be using the flat lamp as a reference assuming a nominal black body spectrum for the flat lamp including allowance for typical atmospheric extinction, but then why would it need the reference star library spectrum at all, and what is it comparing the library spectrum  against ?  It does seem to work though. I shall have a play, but I am not sure I would ever fully trust it !

[Robin Leadbeater]

Hi Paul,

That looks interesting

I have not used this before but cannot find a description of exactly what it does on Christian’s website (It is not mentioned in his ISIS change log for example where I normally look).  I had assumed it just automated the usual “differential spectroscopy” procedure, using the “response” and “continuum” functions under the profile tab to calculate the response using a reference star at a particular air mass which is then used with the target star spectrum at similar air mass.  Does it do something different to this?

EDIT:-  rereading your paper, am I correct in saying that the response assistant calculates the response using only the flat and the reference spectrum?  ie you do not need to measure the reference star?  

Cheers

Robin

[Robin Leadbeater]

Looking good, Tony

I can see a couple of interesting places where the two are slightly different at ~5750 and ~6300. I wonder if the differences are real? There is a telluric O2 band at 6277A but nothing as far as I know at 5750. Christian Buil has an annotated Vega spectrum here which shows the Tellurics.

http://www.astrosurf.com/buil/us/vatlas/vatlas.htm.

 If you divide your measured by the filtered library it will show up very clearly any discrepancies. It is particularly worth doing with the reference star itself which should of course match perfectly. You can use any differences to see where you might need retune your response calculation. 

Cheers

Robin

[Robin Leadbeater]

Impressive stuff but my money for the “big one” (detecting life tracers in earth-like exoplanets in the goldilocks zone) is still on transit measurements rather than direct imaging eg

https://www.spacetelescope.org/news/heic1603/

Robin

[Robin Leadbeater]

There was a piece on this on Radio 4 Front Row program on Monday. 

http://www.bbc.co.uk/programmes/b081l7zt#playt=0h10m55s

Some fuzzy reassurances made concerning environmental impact. eg “we don’t want to dazzle the fishes”

Robin

[Robin Leadbeater]

Looking forward to the video

Robin

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