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What wavelength error is acceptable with an Alpy?

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John Coffin's picture
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Joined: 24/10/2017 - 13:58
What wavelength error is acceptable with an Alpy?

I have been working hard to calibrate my spectra collected by my Alpy. I have spent a lot of time trying to get the spectrum horizontal and have also adjusted the grism. 

I have found the absorption lines of my spectrum of zeta draconis matched those in the Miles database from H beta to H epsilon, but H alpha was blue shifted by about 3 Angstroms. 

It was a moonlit night with intermittent cloud and zeta draconis was at a lowish altitude of 36 deg, so could that explain why my instrumental response curve had a hump around 3940?

I am using ISIS with an Alpy calibration lamp. The lamp images were not saturated and I used the mean of 13 images to make a "master" calibration image. 

I used the "Alpy with calibration module" option in "settings" and the predefined dispersion option on the general tab. I used the calibration assistant in the calibration tab, and ticked the UV calibration box. 

My spectrum was tilted by 0.28 degrees. The calibration lines are not vertical despite adjusting the grism. 

Here is the neon image. 

The tilt increases towards the right.

Do I need to adjust the grism again, a very difficult process to get right, or is this kind of error at the red end of the spectrum normal for an Alpy? 

Robin Leadbeater's picture
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Wavelength calibration

Hi John,

I am not sure the wavelength calibration of the MILES stars is all that accurate. I remember seeing some apparent discrepancies when I was trying to do some radial velocity work.  Do your individual errors for the lamp lines look good particularly those bracketing the H alpha line? (from the report that ISIS generates in the "Go" window. )   

I would not worry about adjusting the Grism further if ISIS is finding the lines to do the wavelength calibration OK (The RMS error should be low, <0.5A and there should be no obvious larger errors in any individual line as reported by ISIS)

The kink in the instrument response of the ALPY around 4000A is real and often seen, though the cause is not clear. See my recent post here for example.

https://britastro.org/comment/5542#comment-5542

Cheers

Robin

Robin Leadbeater's picture
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H alpha error

OK I have checked the MILES zet Dra spectrum and the Balmer lines do appear to be at the expected wavelengths ( eg 4861, 6563A) There will be small adjustments to make for the radial velocity and heliocentric correction but these will not add up to as much as 3A so there looks to be something going on here.  Can you post a copy of what ISIS reports in the "Go" window please?

Thanks

Robin

John Coffin's picture
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calibration problems

Here is the text of the ISIS report after pressing the Go button on the calibration assistant.

Extract spectral profile... 

Input image : c:\spectra\20181028\mean.fit

Y-coordinate of binning zone : 532

Binning zone width : 27

Sauve the rectified image : c:\spectra\20181028\@.fit

Save the profile : c:\spectra\20181028\@.dat  (uncalibrated spectra)

Ok. 

------------------------------------------

Extract spectral profile... 

Input image : c:\spectra\20181028\neon5.fit

Y-coordinate of binning zone : 532

Binning zone width : 27

Sauve the rectified image : c:\spectra\20181028\@@.fit

Save the profile : c:\spectra\20181028\@@@@.dat  (uncalibrated spectra)

Ok. 

--------------------------------------------------------------------------

Lines search zone  

|  -5  |  373  |  3  |

|  -5  |  384  |  3  |

|  -5  |  401  |  3  |

|  -4  |  428  |  4  |

|  -5  |  478  |  3  |

|  -3  |  584  |  5  |

|  -5  |  790  |  7  |

|  -5  |  809  |  7  |

|  -6  |  878  |  6  |

|  -5  |  929  |  7  |

|  -6  |  1070  |  6  |

|  -8  |  1124  |  6  |

|  -10  |  1202  |  4  |

Gaussian fit on : +/-5 pixels  

--------------------------------------------------------------------------

Reciprocical inverse dispersion equation  

Coefficient a4 : 2.039047E-10

Coefficient a3 : -1.032328E-06

Coefficient a2 : 1.289598E-03

Coefficient a1 : 4.25183

Coefficient a0 : 2113.351

--------------------------------------------------------------------------

Fitting deviation (wavelength)

point #1  x = 373.309  lambda = 3835.394  dlambda = -0.004

point #2  x = 384.441  lambda = 3889.156  dlambda = -0.106

point #3  x = 401.092  lambda = 3969.698  dlambda = 0.382

point #4  x = 428.333  lambda = 4101.743  dlambda = 0.007

point #5  x = 477.589  lambda = 4341.144  dlambda = -0.664

point #6  x = 584.329  lambda = 4860.809  dlambda = 0.531

point #7  x = 790.079  lambda = 5852.713  dlambda = -0.223

point #8  x = 809.454  lambda = 5944.744  dlambda = 0.086

point #9  x = 877.733  lambda = 6266.474  dlambda = 0.016

point #10  x = 929.332  lambda = 6506.631  dlambda = -0.101

point #11  x = 1070.153  lambda = 7147.050  dlambda = -0.010

point #12  x = 1123.593  lambda = 7383.774  dlambda = 0.176

point #13  x = 1201.871  lambda = 7723.850  dlambda = -0.090

--------------------------------------------------------------------------

RMS : 0.351305 (in angstroms)

--------------------------------------------------------------------------

Ok.

Here is the report from the Go page.

Load the image : c:\spectra\20181028\zeta_draconis-1.fit

Load the image : c:\spectra\20181028\zeta_draconis-2.fit

Load the image : c:\spectra\20181028\zeta_draconis-3.fit

Load the image : c:\spectra\20181028\zeta_draconis-4.fit

Load the image : c:\spectra\20181028\zeta_draconis-5.fit

Load the image : c:\spectra\20181028\zeta_draconis-6.fit

Substract the offset : c:\spectra\20181028\calib\offset.fit

Substract the dark : c:\spectra\20181028\calib\dark600_b2_minus10-.fit

Dark coefficient (1) : 0.0333

Dark coefficient (2) : 0.0333

Dark coefficient (3) : 0.0333

Dark coefficient (4) : 0.0333

Dark coefficient (5) : 0.0333

Dark coefficient (6) : 0.0333

Divide by the flat-field : c:\spectra\20181028\flat.fit

Flat-field level : 38002

Cosmetic correction : c:\spectra\20181028\calib\cosme.lst

Smile correction - Y0 = 658  Radius = 11056

Transverse registration at intermediate Y coordinate = 532.00

Spectrum Y coordinate Y 1 = 533.31

Spectrum Y coordinate Y 2 = 533.47

Spectrum Y coordinate Y 3 = 533.53

Spectrum Y coordinate Y 4 = 533.73

Spectrum Y coordinate Y 5 = 533.76

Spectrum Y coordinate Y 6 = 533.58

Remove sky background

Save image : c:\spectra\20181028\@s.fit (sky not removed)

Save 0b product : c:\spectra\20181028\_Zeta Dra.fit (2D image)

Adopted Y coordinate : 531.51

Tilt correction (second pass)

Remove sky background (second pass)

Write 2D image : c:\spectra\20181028\@1.fit

Write 2D image : c:\spectra\20181028\@2.fit

Write 2D image : c:\spectra\20181028\@3.fit

Write 2D image : c:\spectra\20181028\@4.fit

Write 2D image : c:\spectra\20181028\@5.fit

Write 2D image : c:\spectra\20181028\@6.fit

Optimal binning

Write raw profile : c:\spectra\20181028\@raw1.dat

Write raw profile : c:\spectra\20181028\@raw2.dat

Write raw profile : c:\spectra\20181028\@raw3.dat

Write raw profile : c:\spectra\20181028\@raw4.dat

Write raw profile : c:\spectra\20181028\@raw5.dat

Write raw profile : c:\spectra\20181028\@raw6.dat

Intensity of individual profiles (ADU)...

Mean (median) for profile #1 : 277922 (48733)

Mean (median) for profile #2 : 267284 (46730)

Mean (median) for profile #3 : 274253 (46119)

Mean (median) for profile #4 : 283042 (47260)

Mean (median) for profile #5 : 261331 (48022)

Mean (median) for profile #6 : 247607 (44406)

Standard summation of individual profiles

Save uncalibrated spectral profile : c:\spectra\20181028\@star.dat

Processing of calibration image : c:\spectra\20181028\neon5.fit

Dark coefficient (calibration) : 0.0083

Save image : c:\spectra\20181028\@calib.fit (2D image)

Save spectral profile : c:\spectra\20181028\@calib.dat

Spectral calibration

Computed A0 parameter : 2113.160 (actual polynom A0 = 2113.351)

Predefined dispersion polynom :

Coefficient a4 : 2.039047E-10

Coefficient a3 : -1.032328E-06

Coefficient a2 : 1.289598E-03

Coefficient a1 : 4.25183

Coefficient a0 : 2113.351

----------------------------------------------------------------------------

Normalization spectral range : [6650  -  6750]

Normalization value (final profile) : 42238320.9

Normalization value (individulal profile #1) : 7339427.7

Normalization value (individulal profile #2) : 7005366.1

Normalization value (individulal profile #3) : 7012000.4

Normalization value (individulal profile #4) : 7058567.1

Normalization value (individulal profile #5) : 7157459.9

Normalization value (individulal profile #6) : 6665499.8

----------------------------------------------------------------------------

Spectrum #1   ( b ) - ( v ) intensity = ( 0.839 ) - ( 0.996 )

Spectrum #2   ( b ) - ( v ) intensity = ( 0.862 ) - ( 0.994 )

Spectrum #3   ( b ) - ( v ) intensity = ( 0.913 ) - ( 0.997 )

Spectrum #4   ( b ) - ( v ) intensity = ( 0.951 ) - ( 0.998 )

Spectrum #5   ( b ) - ( v ) intensity = ( 0.786 ) - ( 0.996 )

Spectrum #6   ( b ) - ( v ) intensity = ( 0.798 ) - ( 0.997 )

( b ) - ( v ) mean intensity = ( 0.858 ) - ( 0.996 )

( b ) - ( v ) median intensity = ( 0.862 ) - ( 0.997 )

----------------------------------------------------------------------------

Write individual DAT processed profile : c:\spectra\20181028\@pro1.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro2.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro3.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro4.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro5.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro6.dat

Write individual FITS processed profile : c:\spectra\20181028\@pro1.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro2.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro3.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro4.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro5.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro6.fit

----------------------------------------------------------------------------

Write final profile : c:\spectra\20181028\_zetadra_20181028_970_J_Coffin.fit

Write final profile : c:\spectra\20181028\_zetadra_20181028_970_J_Coffin.dat

Intermediate files removed

----------------------------------------------------------------------------

Acquisition starting date : 28/10/2018 23:16:27

Duration : 42.0 secondes

Mid-exposure date : 28.970/10/2018

Mid-exposure Julian day : 2458420.4700

Resolution power : 521.5

Ok. 

Robin Leadbeater's picture
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try with just lamp lines

OK John,

ISIS seems to have found the lines ok and the fit looks good. I am wondering if there has been some sort of shift  between the lamp spectrum and the star spectrum.  With the mixed Balmer and lamp line calibration you are using, the lines used for calibration up to 4861 (H Beta) are the Balmer lines in the star and the lines higher than that are from the calibration lamp. Normally the ALPY is very stable but if everything is not quite tightened up, there could have been a shift between the star and lamp exposures. Did you take the lamp spectrum under the same conditions as the star ? 

Can you also run the calibration using just the lamp lines ? If there was a shift, we should then see all the Balmer lines shifted, not just H alpha

Cheers

Robin 

John Coffin's picture
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Joined: 24/10/2017 - 13:58
lamp lines only

Dear Robin, all the Balmer lines were blue shifted by about 2 A, though H zeta and H epsilon were shifted by 3 and 2.5 A respectively. 

I'll check all the connectors, though I thought they were tight really. 

Here's the report

Load the image : c:\spectra\20181028\zeta_draconis-1.fit

Load the image : c:\spectra\20181028\zeta_draconis-2.fit

Load the image : c:\spectra\20181028\zeta_draconis-3.fit

Load the image : c:\spectra\20181028\zeta_draconis-4.fit

Load the image : c:\spectra\20181028\zeta_draconis-5.fit

Load the image : c:\spectra\20181028\zeta_draconis-6.fit

Substract the offset : c:\spectra\20181028\calib\offset.fit

Substract the dark : c:\spectra\20181028\calib\dark600_b2_minus10-.fit

Dark coefficient (1) : 0.0333

Dark coefficient (2) : 0.0333

Dark coefficient (3) : 0.0333

Dark coefficient (4) : 0.0333

Dark coefficient (5) : 0.0333

Dark coefficient (6) : 0.0333

Divide by the flat-field : c:\spectra\20181028\flat.fit

Flat-field level : 38096

Cosmetic correction : c:\spectra\20181028\calib\cosme.lst

Smile correction - Y0 = 658  Radius = 11056

Transverse registration at intermediate Y coordinate = 533.00

Spectrum Y coordinate Y 1 = 533.31

Spectrum Y coordinate Y 2 = 533.47

Spectrum Y coordinate Y 3 = 533.53

Spectrum Y coordinate Y 4 = 533.73

Spectrum Y coordinate Y 5 = 533.76

Spectrum Y coordinate Y 6 = 533.58

Remove sky background

Save image : c:\spectra\20181028\@s.fit (sky not removed)

Save 0b product : c:\spectra\20181028\_Zeta Dra.fit (2D image)

Adopted Y coordinate : 532.51

Tilt correction (second pass)

Remove sky background (second pass)

Write 2D image : c:\spectra\20181028\@1.fit

Write 2D image : c:\spectra\20181028\@2.fit

Write 2D image : c:\spectra\20181028\@3.fit

Write 2D image : c:\spectra\20181028\@4.fit

Write 2D image : c:\spectra\20181028\@5.fit

Write 2D image : c:\spectra\20181028\@6.fit

Optimal binning

Write raw profile : c:\spectra\20181028\@raw1.dat

Write raw profile : c:\spectra\20181028\@raw2.dat

Write raw profile : c:\spectra\20181028\@raw3.dat

Write raw profile : c:\spectra\20181028\@raw4.dat

Write raw profile : c:\spectra\20181028\@raw5.dat

Write raw profile : c:\spectra\20181028\@raw6.dat

Intensity of individual profiles (ADU)...

Mean (median) for profile #1 : 278613 (48845)

Mean (median) for profile #2 : 267945 (46851)

Mean (median) for profile #3 : 274931 (46238)

Mean (median) for profile #4 : 283742 (47380)

Mean (median) for profile #5 : 261979 (48189)

Mean (median) for profile #6 : 248222 (44512)

Standard summation of individual profiles

Save uncalibrated spectral profile : c:\spectra\20181028\@star.dat

Processing of calibration image : c:\spectra\20181028\neon5.fit

Dark coefficient (calibration) : 0.0083

Save image : c:\spectra\20181028\@calib.fit (2D image)

Save spectral profile : c:\spectra\20181028\@calib.dat

Spectral calibration

--------------------------------------------------------------------------

Primary dispersion : 

Coefficient a4 : -1.941613E-13

Coefficient a3 : 5.199794E-09

Coefficient a2 : -4.783460E-05

Coefficient a1 : 0.39175

Coefficient a0 : -710.014

--------------------------------------------------------------------------

RMS : 0.091500 (en pixels)

--------------------------------------------------------------------------

Primary wavelength fit deviation

point #1  Lambda = 3770.630  px = 326.562  dx = -0.067

point #2  Lambda = 3797.300  px = 332.190  dx = 0.154

point #3  Lambda = 3835.390  px = 340.215  dx = -0.078

point #4  Lambda = 3889.050  px = 351.495  dx = -0.013

point #5  Lambda = 3970.080  px = 368.476  dx = 0.043

point #6  Lambda = 4101.750  px = 395.954  dx = -0.049

point #7  Lambda = 4340.480  px = 445.487  dx = -0.017

point #8  Lambda = 4861.340  px = 552.926  dx = 0.039

point #9  Lambda = 6562.850  px = 910.356  dx = -0.070

point #10  Lambda = 6869.000  px = 976.968  dx = 0.067

point #11  Lambda = 7605.000  px = 1140.339  dx = -0.009

--------------------------------------------------------------------------

Inverse dispersion equation  

Coefficient a4 : 2.999117E-10

Coefficient a3 : -1.336891E-06

Coefficient a2 : 1.632851E-03

Coefficient a1 : 4.09469

Coefficient a0 : 2135.790

--------------------------------------------------------------------------

RMS : 0.311249 (in angstroms)

--------------------------------------------------------------------------

Wavelength fit deviation

point #1  x = 397.713  lambda = 3945.982  dlambda = 0.118

point #2  x = 441.652  lambda = 4158.959  dlambda = -0.369

point #3  x = 513.877  lambda = 4510.640  dlambda = 0.090

point #4  x = 520.824  lambda = 4544.520  dlambda = 0.530

point #5  x = 544.099  lambda = 4658.045  dlambda = -0.145

point #6  x = 565.974  lambda = 4764.726  dlambda = 0.144

point #7  x = 607.204  lambda = 4965.603  dlambda = -0.523

point #8  x = 696.858  lambda = 5400.458  dlambda = 0.102

point #9  x = 791.067  lambda = 5852.415  dlambda = 0.075

point #10  x = 878.731  lambda = 6266.457  dlambda = 0.033

point #11  x = 930.316  lambda = 6506.578  dlambda = -0.048

point #12  x = 1071.150  lambda = 7147.069  dlambda = -0.029

point #13  x = 1124.583  lambda = 7383.958  dlambda = 0.022

--------------------------------------------------------------------------

Coefficient a4 : 2.999117E-10

Coefficient a3 : -1.336891E-06

Coefficient a2 : 1.632851E-03

Coefficient a1 : 4.09469

Coefficient a0 : 2135.790

--------------------------------------------------------------------------

RMS : 0.311249

--------------------------------------------------------------------------

----------------------------------------------------------------------------

Normalization spectral range : [6650  -  6750]

Normalization value (final profile) : 42355639.7

Normalization value (individulal profile #1) : 7359979.4

Normalization value (individulal profile #2) : 7024372.2

Normalization value (individulal profile #3) : 7031455.9

Normalization value (individulal profile #4) : 7078341.5

Normalization value (individulal profile #5) : 7177964.8

Normalization value (individulal profile #6) : 6683525.9

----------------------------------------------------------------------------

Spectrum #1   ( b ) - ( v ) intensity = ( 0.836 ) - ( 0.996 )

Spectrum #2   ( b ) - ( v ) intensity = ( 0.860 ) - ( 0.994 )

Spectrum #3   ( b ) - ( v ) intensity = ( 0.910 ) - ( 0.997 )

Spectrum #4   ( b ) - ( v ) intensity = ( 0.948 ) - ( 0.998 )

Spectrum #5   ( b ) - ( v ) intensity = ( 0.784 ) - ( 0.996 )

Spectrum #6   ( b ) - ( v ) intensity = ( 0.796 ) - ( 0.997 )

( b ) - ( v ) mean intensity = ( 0.856 ) - ( 0.996 )

( b ) - ( v ) median intensity = ( 0.860 ) - ( 0.997 )

----------------------------------------------------------------------------

Write individual DAT processed profile : c:\spectra\20181028\@pro1.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro2.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro3.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro4.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro5.dat

Write individual DAT processed profile : c:\spectra\20181028\@pro6.dat

Write individual FITS processed profile : c:\spectra\20181028\@pro1.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro2.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro3.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro4.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro5.fit

Write individual FITS processed profile : c:\spectra\20181028\@pro6.fit

----------------------------------------------------------------------------

Write final profile : c:\spectra\20181028\_zetadra_20181028_970_J_Coffin.fit

Write final profile : c:\spectra\20181028\_zetadra_20181028_970_J_Coffin.dat

Intermediate files removed

----------------------------------------------------------------------------

Acquisition starting date : 28/10/2018 23:16:27

Duration : 42.0 secondes

Mid-exposure date : 28.970/10/2018

Mid-exposure Julian day : 2458420.4700

Resolution power : 507.6

Ok. 

John Coffin's picture
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Joined: 24/10/2017 - 13:58
lamp lines more info

I took these lamp lines indoors. The one I took last night was saturated towards the red end of the spectrum and gave me a similar result and were taken at the end of the evening after the calibration star. The ambient temperature was 2 C last night.   Next time I will take some in the middle of the target exposures, and again with the reference star and see if that makes any difference. 

Robin Leadbeater's picture
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wavelength shift

OK,   I think the constant shift with just the lamp confirms that there was some shift in the spectrograph (It is very small. I think 3A equates to about 5um or about a pixel).  The changing temperature might have been a factor. With the ALPY I  usually take a lamp spectrum each time I change target  during an observing run even though I normally dont see any shift during the night. (With the LHIRES I  see a lot of movement so I take a lamp spectrum at start and end on every target and every half hour or so during long exposures but it is much less stable thermally and mechanically than the ALPY)

Cheers

Robin

Robin Leadbeater's picture
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lamp illumination

One other thing that could be worth checking is that the servo controlled reflector is set so the calibration lamp is shining directly down on the slit. If the slit is illuminated at a significant angle, it can cause small shifts relative to the star spectrum. (I've not heard of  this with the ALPY but I suspected this problem with the original manual LHIRES system)

Cheers

Robin

John Coffin's picture
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Lamp illumination

The reflector isn’t perfectly straight and the bulb emits light from one side of itself.

Robin Leadbeater's picture
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Joined: 05/03/2014 - 00:50
lamp calibration errors

Ok the reflector position looks ok. ( I was concerned that the reflector might not be rotating to the correct position. It can be adjusted by operating  the switches in a particular sequence) Not sure what any offset in the lamp might do (The asymmetry in the lamp internally is normal) and I cant see mine at the moment as it is mounted on the scope but is probably best not to change anything until you have checked the calibration with  lamp spectra taken at the same time as the star spectrum.

I have been having a look at some of my past calibrations and I am also seeing  wavelength errors between lamp and star (I  use just the lamp without the star Balmer lines) In my case H alpha is spot on but there is an increasing error towards the blue  to around 2-3A by 3900A where the lamp lines end, so errors of this order might be typical. What are other other ALPY users seeing?

We might be seeing a fundamental limitation in the calibration accuracy using the internal lamp.  At this level (~1/4 the slit width) effects such as the position of the star on the slit and subtle differences in line shape due to optical aberrations come into play.

Cheers

Robin

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Statistical errors ?

It did occurs to me that with a non-linear dispersion you might get errors due to more lines being fitted at the red end compared to the blue. This would then give a better fit in the red while a poorer one in the blue even if the overall fit looked good.

Regards Andrew

Robin Leadbeater's picture
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fit errors

A good point.  Buil seems to have done a reasonable job of selecting evenly spaced lines for the fit just using the lamp though and the fit residuals are much smaller than the errors seen. The use of a 4th order fit though does make me slightly nervous that we might be "fitting elephants" .

lamp lines used  (EDIT 2018-11-02    Note these are the fit values, not the published wavelengths )

3946.331
4158.342
4510.673
4544.769
4657.878
4765.023
4965.325
5400.742
5852.471
6266.287
6506.282
7147.661
7383.626
andrew.j.smith1905's picture
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Extrapolation into the blue

I had a play with Excel and fitted the data to different orders all were good fits but you can see the difference in the extrapolation into the blue. Order 1 fitted at R=0.9998 ,orders 2,3,4 fitted to R=1 the graphs are order 1,3,4. The blue line is the fitted data and red the extrapolation using the ISIS fit.

Regards Andrew

Attachments: 
Robin Leadbeater's picture
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Residuals

Hi Andrew,

The extrapolation is into the red not the blue ?

Do you have the residuals for the blue points for the different fits? Particularly how much improvement in the fit is there going from 3 to  4 ?

Robin

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Not to hand

Hi Robin, no I just used the trend function in Excel which just reports the R value. I will see if I can just fit it in Excel proper rather than on the graph.

I nitially I was refering to wavelength then I was referring to the colour on the plots not wavelength. I should have changed the colour to avoid the confusion. Just used the defaults without thinking.

Regards Andrew 

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Calibration lamp wavelengths

I am curious about the wavelengths which both John (implicitly in the ISIS listings) and Robin give for their calibration lamps. These are different from the standard Ar and Ne wavelengths given by NIST, for example here https://physics.nist.gov/PhysRefData/ASD/lines_form.html

If you look at the discrepancies which ISIS lists in John's calibration fit, these appear to be almost exactly what is needed to bring the lines into agreement with the NIST values.

I have the Shelyak Ar-Ne lamp in my LISA and use a .lst file in ISIS which contains the NIST wavelength values. This routinely gives me a 4th order polynomial fit with rms residual less than 0.1.

David

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line wavelengths

Hi David,

Ah sorry, I see the wavelengths I posted (and listed by ISIS in the reports) are the values calculated from the fit, not the true values. 

There does not seem to be a problem with the quality of the fit to the lines, at least within the range of lines used. (RMS ~0.2-0.3A seems typical for the ALPY.)  The issue is apparent systematic differences in wavelength up to 3A between the lamp  and the star)

Cheers

Robin

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Residuals

Hi Robin the 3rd order fit has 0.57 and the fourth 0.34. The fourth is as the ISIS fit. Though looking at this again I don't think this is an issue.

DAVID makes an interesting point about the line wavelengths though.

Regards Andrew 

I will need to do this again as I used Robin's figures!

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Under sampling

The error is about 1/2 a pixel. I wonder if it is due to undersampling in which case it will be a practical limit unless you use smaller pixels. 

Somewhere I have a paper on this and I will try to find it.

Regards Andrew 

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3 A error

Simple answer  is I don't know.

Can you process a calibration frame as if it were a star? Rename it so ISIS is not confused. Are the lines in the right place? 

If they are then I can only think it is the fact that the calibration light is not illuminating the spectrograph in the same way as the star.

If the are not then it may be other issues like undersampling.

Regards Andrew

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My repeat spectra are still inaccurate round H alpha

Last night's specrum of Pi Andromeda (HD 00369) was still about 3 A out at H alpha but very accurate from H beta to epsilon. 

If this is an error due to undersampling why would it only affect H alpha?

If undersampling is the issue, can I calibrate the dispersion of a spectrum collected binned x 2 using a reference star that was not binned? 

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calibrating the lamp image with the lamp image

Hi Andrew, I've processed the calibration image as is if were a star, of course I had to use the same image to calibrate it. The result was odd, but the neon 6965 line was incorrectly calibrated.

The lines are broad and fuzzy in this area, so perhaps I need to focus it better there?

I understand that if I do this I will lose definition at the blue end of the spectrum. Thank you for your thoughts on this issue. I am very grateful for everyone's input into this. 

I've attached the files of these images.

Regards, John

Attachments: 
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last night's run

Hi John

"Last night's specrum of Pi Andromeda (HD 00369) was still about 3 A out at H alpha but very accurate from H beta to epsilon. "

I presume this was using the mix of lamp lines and Balmer lines for calibration?.     If so, the Balmer lines from H Beta down will always be in exactly right the right place because ISIS used them for calibration. 

Similarly if you just used the lamp lines for calibration and produce a spectrum of the lamp, I suspect you will find that the lamp lines are in exactly the right place (within the accuracy of the fit ~+-0.5A)  

The problem arises when we use lamp lines to calibrate star spectra.  There is a shift somewhere between the star spectrum and lamp spectrum.  Based on the latest result this does not seem to be a stability problem.

There are differences between the optical paths for lamp and star which could produce small systematic shifts. We may have found the limit of absolute accuracy of using the internal lamp for calibration. (note this is a systematic error, not a lack of precision/repeatability  so not a problem when studying changes in wavelength eg in radial velocity measurements of binary stars.) 

To take this further I would say we would first need to compare results from other ALPYs to determine if it is a limitation of the instrument or if this particular example is worse for some reason. Since I am seeing signs of similar size errors in star lines with my ALPY when using just the internal lamp, I suspect it could be a limit in absolute accuracy. In which case there is no need to be overly concerned but longer term there may be some modifications that could be made to the ALPY or the calibration technique to improve the calibration accuracy further.  

Using a mix of lamp and star for calibration confuses the issue. I would say The definitive test would be for ALPY owners to take a spectrum of a star with known reliable RV, Calibrate just using the internal lamp and look for errors in the star lines.

This could be a good project for the workshop

Cheers

Robin

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spectrum of lamp

Hi John,

 "I've processed the calibration image as is if were a star, of course I had to use the same image to calibrate it. The result was odd, but the neon 6965 line was incorrectly calibrated."

The spectrum plot looks odd. Did you tick "sky not removed"  If you don't do this it subtracts the lines from them selves, leaving almost nothing behind)

Robin

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Lamp spectrum

No, I didn't tick that box. Here is the new improved lamp spectrum which looks accurate. I noticed that the intensity of the lines that ISIS uses for calibration are much weaker than those above 7000 A. Perhaps I should not worry about saturating those brighter lines and try using a longer exposure in order to make the ones used by ISIS brighter. 

What do you think is the way forward now? I was hoping to submit spectra to our database. Should I put a comment in the FITs header about the calibration method and its (?) inherent problems, ie mixed method with accuracy upto H beta, 3A blueshift near H alpha.

What would be the best way to calibrate now? Would it depend on the nature of the target? 

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way forward

Hi John,

I wouldn't hold back from submitting spectra. (You could have a look in the databases for other ALPY spectra to see how their calibration looks.) I think a wavelength calibration accuracy of ~1/4 the resolution is still good (At the end of the day my ALPY spectra using just the lamp lines are no more accurate. The errors are just at the other end of the wavelength range). There is still something niggling to do with the difference in how the ALPY sees the lamp compared with the star to nail down here but I suspect it will turn out to be something common to ALPYs in general.

Dont refocus to sharpen up the lines in the far red to the detriment of the blue end. The ALPY performance is biased towards the blue end where it actually outperforms the LISA

I think ISIS is finding the lines OK so exposing more probably will not make much difference but exposing for the brightest line that ISIS uses is a good idea and if you want to try going further you can sum several exposures to increase the signal/noise in the faint lines in the same way as summing flats works  (I used to do this but the results using just one well exposed lamp exposure seemed to work just as well.)

Cheers

Robin 

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Thanks

Thanks for all your advice. Much appreciated!

John

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Mixed method

I would avoid using the star plus lines method as it is mixing two different dispersion if the light path is different between the two.

I think it would be best to use the lines to get the dispersion then adjust any offset with an A star. The ALPY with Ne Ar has enough lines without the star lines for a good fit.

Regards Andrew 

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mixed method

Yes I only use the lamp lines. The fit does deteriorate below 3900A though. (The lowest line ISIS uses) which is why Buil added the mixed star and lamp option

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

see comparison of methods 2 and 3

Cheers

Robin

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Wavelength calibration

My understanding is that the mixed method (lamp + Balmer lines) was used in ISIS to compensate for the original Ne-only lamp bulbs only having usable lines at the red end. When Shelyak introduced the replacement Ar-Ne bulbs, I was told that method was redundant and I should just use the lamp lines. In my LISA the Ar-Ne bulb produces sufficient lines across the whole spectrum to use for calibration without needing a stellar spectrum. I always had problems getting a good result with the mixed method and found using just the Ar-Ne lines worked much better.

I probably missed it somewhere in the earlier discussion whether John's lamp has the Ar-Ne bulb. Some of these bulbs can deteriorate over time so it may be worth asking Francois Cochard at Shelyak if the emission line spectrum you get from the Alpy calibration module is what he expects to see. I've found Francois very willing to help resolve problems like that.

One small point on an earlier posting by Robin. Robin said "I see the wavelengths I posted (and listed by ISIS in the reports) are the values calculated from the fit, not the true values". In my experience, the lambda values which ISIS lists in the report are the true wavelength values you give it for the lines. The dlambda values are the corrections it has applied to these values to get the best fit.

David

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mixed calibration

Hi David,

The ALPY calibration module has always had the NeAr lamp. The link I posted was specifically for calibration the ALPY. The first line used in the  ALPY calibration is 3946.1 and the calibration deteriorates below there. eg

 http://www.astrosurf.com/buil/isis/guide_alpy/calibration/fig_fr22.png

What  is the first line you use with the LISA? (note some lines which might be resolved at the LISA resolution could be blends with the ALPY so not useable)

When calibrating the ALPY with the NeAr lamp, ISIS definitely reports the fit values not the published values. You can see examples on the page linked. For example

http://www.astrosurf.com/buil/isis/guide_alpy/calibration/fig_fr19.png

and for a bad calibration

http://www.astrosurf.com/buil/isis/guide_alpy/calibration/fig_fr23.png

Cheers

Robin

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Calibration

Hi Robin,

The first really usable line in my Ar-Ne lamp spectrum is at 4158.590A. Below that the lines are either blended or too faint unless I overexpose the red end. Including the 3946.097A line gives a considerably worse fit at the blue end. So the accuracy of my wavelength calibration drops off below 4000A and I normally don't report my spectra below 3900A.

The ISIS wavelength fitting report issue is curious. Here is a report I got recently.

The lambda values listed are all exactly the values I provide in my .lst file. Maybe it depends on the spectral calibration mode you use?

David

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Ofsetting the lamp calibrated spectrum

I like Andrew's idea of calibrating using the lamp and then ofsetting the spectrum to match the star lines. This is similar to what is done with the LHIRES where telluric lines at H alpha are used to correct for any offset.  You would have to consider the RV of the star though and for highest accuracy perhaps the Heliocentric correction.

Cheers

Robin

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3946.097 line not reliable for calibration ?

Hi David

"Including the 3946.097A line gives a considerably worse fit at the blue end." 

I have had a look at the high resolution echelle spectrum of the lamp that Shelyak use in Richard Walker's atlas. (plate 98 order 57)  The 3946.097 Ar line that ISIS uses for a lamp line fit with the ALPY is in fact a close triplet 3946.097, 3947.505, 3948.979  so definitely incorrect.  I don't think this is connected with what John is seeing but I suspect this may be the cause of my increasing error towards the blue using just the lamp calibration.

I took a spectrum of MILES star HD206165 last night to experiment with. It is a B2i star so lots of narrow H and He lines which hopefully should show up any errors

Cheers

Robin

 

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Accuracy of Alpy lamp calibration

Hi John,

The thread that you started has prompted me to re-visit the accuracy of my own Alpy calibration. I use BASS so calibration is a manual process. I use 13 lines from the Alpy calibration lamp spectrum and a 4th order polynomial. From my observing session a couple of nights ago the calibration lamp fit had an RMS of about 0.53A. I had selected SAO 25665 as reference star for X Per and b Per observations. SAO 25665 has a very low radial velocity according to SIMBAD, giving the spectra and results that I attach below. The calibration accuracy of the Balmer lines in the SAO 25665 spectrum is pretty good even into the near UV. And like you have found, the accuracy is worst, although still reasonable, for Hα. I wonder what could be the explanation for the dveiation in the Hα accuracy? By the way, I marked on the spectrum the position of the 13 calibration lamp lines that I use - I stack 3 x 40sec calibration lamp exposures.

Cheers

Hugh

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My errors

Here are my errors using the ISIS automatic calibration on lamp lines only (Heliocentric and RV corrected). It confirms what I have generally been finding with low errors at the red end increasing at the blue end.  As expected the lines in the lamp spectrum are in exactly the right place  (to better than +-0.5A).  I also had a look at Hugh's lines in my calibrated lamp spectrum and where different lines are used they are still exactly in the right place so it does not seem to be dependent on the lines used. (Note I could not use the 6562.85 lamp  line (H alpha) as the Hydrogen has all leaked from my lamp which is a few years old now)

Cheers

Robin

lamp lines HD206165 ALPY calib
wavelength error
7383.98 7065.2 -0.1
7147.04 6678.2 0.2
6506.53 6562.8 -0.3
6266.49 5875.7 0.2
5852.49 4921.9 -0.5
5400.56 4861.3 -0.7
4965.08 4387.9 -0.6
4764.87 4340.5 -0.5
4657.9 4101.7 -1.0
4545.05 4026.2 -2.3
4510.73 3970.1 -3.0
4158.59 3888.6 -2.8
3946.1 3835.4 -3.6
3797.9 -4.0
3770.6 -4.0
3750.1 -4.2
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Dropping the 3946.1 line

Hi Robin, what happens if you drop the 3946.1 line from the calibration?

Regards Andrew 

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3946.1 line

Hi Andrew,

Yes I was planning to do a manual run without that line (and will still do, and I will also try Hugh's line set, which uses that line but also other different lines) though after looking at Hugh's errors which are good past that wavelength and mine which deteriorate before then,  I now suspect that may  not be the problem.  I also had a closer look at this line. As I said,it is a triplet and therefore suspect but in Richard Walker's atlas the 3946.1 line of the triplet is significantly stronger so I doubt the error in that line is more  than 1A at most.

Cheers 

Robin

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comparison of calibrations with different lines

OK, I have now manually calibrated using:- 

The standard set of lines used by ISIS for the automatic fit (lamp lines only)

The same set with the 3946 line removed

The set of lines used for Hugh's calibration in post #36, less the H alpha line which I do not see in my spectrum

The calibrated star spectra were  identical every case with errors the same as  those in my post #37, within the fit uncertainty of +-0.5A, even down to 3750A  

Lamp spectra calibrated using these fits showed no significant wavelength errors so  the problem, in my case at least, appears to be caused by some systematic difference in the way the spectrograph sees the internal calibration lamp compared with the star. The error is  wavelength dependent rather than a simple offset so is going to be difficult to correct for.

I think my next step will be to check what my "filly dot" lamp, which is mounted over the telescope aperture,  gives.

Cheers

Robin

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Further investigations on 3/11/18

Method

Spectra of Zeta Cas and Z And were collected, the latter was exposed for 240s to avoid saturating the H alpha emission line and then a further set of exposures for 600s were taken in order to show more detail in the continuum and other emission lines.  Towards the end of the series taken for 600s the telescope reached the meridian and the lamp image for that set was not taken until after the meridian flip. On the other occasions the lamp image was taken immediately after the end of a capture sequence.

Results

The spectra were processed in ISIS in three different ways.

Balmer lines only

This couldn’t calibrate Zeta Cas so was not suitable for Z And either

Mixed Balmer and lamp lines calibration

Each time the lamp lines for the particular target were used.

Zeta Cas               H alpha blue shift 1 A, accurate from H beta onwards towards the UV end.

Z And (600s)       H alpha red shift 2 A, gradually increasing so at H delta the red shift was 3.4 A.

Z And (240s)       H alpha blue shift 6 A, reducing so at H delta the blue shift was 4.0 A.

In all targets using the mixed method the wavelength errors become more shifted towards the red at shorter wavelengths.

Lamp lines only calibration

Zeta Cas               H alpha blue shift 1 A, increasing so at H delta the blue shift was 4.5 A.

Z And (600s)       H alpha red shift 0.7, accurate calibration from H beta to H delta. H epsilon was blue                        shifted by 1.5 A.

Z And (240s)       H alpha blue shift 6.0 A, increasing so that at H delta the blue shift was 7.7 A.

So in all the targets calibrated by the lamp lines alone, the wavelength error shifts towards the blue at shorter wavelengths. 

I will be very interested to see the result of your Filly Dot experiment. I may have to throw away my calibration module. (I'll keep it for the flat fields actually).

Cheers, John

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Filly lamp tests

Hi John,

Your  wavelength error trend using just the lamp lines seems similar to mine qualitatively eg post #37.  Your variable offset as shown at H alpha is different to what I see though. My calibration is consistently good at H alpha.

I now have some data for the Filly lamp (I used the "cross" lamp which is just Argon)  There does appear to be  something subtly different. I will report back when I have figured out what is going on.

Cheers

Robin

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ALPY v Filly lamp

Hi John,

Don't throw out your calibration module just yet !

There is a just detectable systematic difference between my ALPY internal lamp and the Argon "Filly Cross" lamp mounted at the edge of the telescope aperture in line with the slit but it is small (Calibrating the Filly lamp with the ALPY lamp gives +1A error around 7000A, 0 error around 5000A and -0.7 around 4100A).  

Cheers

Robin

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What a puzzle.

The only thing I can think of is the the dispersion direction of the prism and grating are not aligned in such a way that the path differences with a fully illuminated entrance pupil and a partially illuminated one results in more dispersion from the prism in the fully illuminated case.

Clutching at straws!

Regards Andrew

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distortion of line profile ?

The errors are within the FWHM of the slit image so I wonder if it could be something distorting the line profile eg :-

Coma from the ALPY camera lens distorting the line shapes more at the edges, which will depend on the f ratio of the incident light and appear as a variation in dispersion.

On stars, some sort of  chromatic aberration in the telescope optics producing a wavelength dependent asymmetry in the psf of the star which alters the distribution of light across the slit and hence the line profile.

Cheers

Robin

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line profiles look ok

Having said that, comparing the line profiles of both the narrow lined star I used and the internal lamp, they have the same FWHM within 5% and there is no sign of asymmetry in line profiles across the spectrum in either case (dominated by the slit width) so it looks like this is not the cause.

Robin

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Filly lamp

Hi Robin could you try the filly lamp at 90 deg to the correct position to see if the off axis lines mirror the behaviour of the star.

Regards Andrew 

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Any ideas why my H alpha calibration should vary so much?

Dear fellow Alpy users, please see my post #41

Everything In my Alpy is screwed down tight as far as I can tell. Yet I am getting these large disparities in the calibration of H alpha.

It suggests something does change when the instrument moves from one star to another, i thought of checking the Relco bulb isn’t loose, presumably the grism  is unlikely to move as it’s held in place by a spring.

The best calibration using the lamp lines occurred when there was a meridian flip between the target and the acquisition of the lamp lines.

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something moving ?

Hi John,

If something is moving in the spectrograph, you should see this in the lamp spectrum ie calibrating the lamp spectrum using an earlier lamp spectrum should show an error.  The spring in the core module is there to hold the collimator lens in position, To hold the grism in position, the allen head grub screws should have been retightened after adjusting to make the lines perpendicular to the dispersion direction.

Cheers

Robin  

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Moving innards

I’ll do as you say. Thanks.

John

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mechanical instability ! (Astronomer error!)

I took your advice and compared the lamp images from the evening where I had calibration problems. There was a shift of minus 3 A between the first and second lamp image. The meridian flip was between the first and second images. Thereafter the lamp image shifted plus 0.4 A and stayed there.

I thought I had checked everything was tightened, but today I’ve discovered the two Allen head bolts holding the slit in place had worked loose. No idea how! 

 I don't think this explains everything though. The star spectra have a different amount of shift at different wavelengths, while the lamp spectra have the same shift all the way along. I can only think that the different light paths of the lamp and the stars are at the root of the problem as you have been saying.  

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