What wavelength error is acceptable with an Alpy?

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  • #574158
    John Coffin
    Participant

    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? 

    #580122
    Robin Leadbeater
    Participant

    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

    #580123
    Robin Leadbeater
    Participant

    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

    #580124
    John Coffin
    Participant

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

    Extract spectral profile… 

    Input image : c:spectra20181028mean.fit

    Y-coordinate of binning zone : 532

    Binning zone width : 27

    Sauve the rectified image : c:spectra20181028@.fit

    Save the profile : c:spectra20181028@.dat  (uncalibrated spectra)

    Ok. 

    ——————————————

    Extract spectral profile… 

    Input image : c:spectra20181028neon5.fit

    Y-coordinate of binning zone : 532

    Binning zone width : 27

    Sauve the rectified image : c:spectra20181028@@.fit

    Save the profile : c:spectra20181028@@@@.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:spectra20181028zeta_draconis-1.fit

    Load the image : c:spectra20181028zeta_draconis-2.fit

    Load the image : c:spectra20181028zeta_draconis-3.fit

    Load the image : c:spectra20181028zeta_draconis-4.fit

    Load the image : c:spectra20181028zeta_draconis-5.fit

    Load the image : c:spectra20181028zeta_draconis-6.fit

    Substract the offset : c:spectra20181028caliboffset.fit

    Substract the dark : c:spectra20181028calibdark600_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:spectra20181028flat.fit

    Flat-field level : 38002

    Cosmetic correction : c:spectra20181028calibcosme.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:spectra20181028@s.fit (sky not removed)

    Save 0b product : c:spectra20181028_Zeta Dra.fit (2D image)

    Adopted Y coordinate : 531.51

    Tilt correction (second pass)

    Remove sky background (second pass)

    Write 2D image : c:spectra20181028@1.fit

    Write 2D image : c:spectra20181028@2.fit

    Write 2D image : c:spectra20181028@3.fit

    Write 2D image : c:spectra20181028@4.fit

    Write 2D image : c:spectra20181028@5.fit

    Write 2D image : c:spectra20181028@6.fit

    Optimal binning

    Write raw profile : c:spectra20181028@raw1.dat

    Write raw profile : c:spectra20181028@raw2.dat

    Write raw profile : c:spectra20181028@raw3.dat

    Write raw profile : c:spectra20181028@raw4.dat

    Write raw profile : c:spectra20181028@raw5.dat

    Write raw profile : c:spectra20181028@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:spectra20181028@star.dat

    Processing of calibration image : c:spectra20181028neon5.fit

    Dark coefficient (calibration) : 0.0083

    Save image : c:spectra20181028@calib.fit (2D image)

    Save spectral profile : c:spectra20181028@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:spectra20181028@pro1.dat

    Write individual DAT processed profile : c:spectra20181028@pro2.dat

    Write individual DAT processed profile : c:spectra20181028@pro3.dat

    Write individual DAT processed profile : c:spectra20181028@pro4.dat

    Write individual DAT processed profile : c:spectra20181028@pro5.dat

    Write individual DAT processed profile : c:spectra20181028@pro6.dat

    Write individual FITS processed profile : c:spectra20181028@pro1.fit

    Write individual FITS processed profile : c:spectra20181028@pro2.fit

    Write individual FITS processed profile : c:spectra20181028@pro3.fit

    Write individual FITS processed profile : c:spectra20181028@pro4.fit

    Write individual FITS processed profile : c:spectra20181028@pro5.fit

    Write individual FITS processed profile : c:spectra20181028@pro6.fit

    —————————————————————————-

    Write final profile : c:spectra20181028_zetadra_20181028_970_J_Coffin.fit

    Write final profile : c:spectra20181028_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. 

    #580125
    Robin Leadbeater
    Participant

    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 

    #580126
    John Coffin
    Participant

    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:spectra20181028zeta_draconis-1.fit

    Load the image : c:spectra20181028zeta_draconis-2.fit

    Load the image : c:spectra20181028zeta_draconis-3.fit

    Load the image : c:spectra20181028zeta_draconis-4.fit

    Load the image : c:spectra20181028zeta_draconis-5.fit

    Load the image : c:spectra20181028zeta_draconis-6.fit

    Substract the offset : c:spectra20181028caliboffset.fit

    Substract the dark : c:spectra20181028calibdark600_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:spectra20181028flat.fit

    Flat-field level : 38096

    Cosmetic correction : c:spectra20181028calibcosme.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:spectra20181028@s.fit (sky not removed)

    Save 0b product : c:spectra20181028_Zeta Dra.fit (2D image)

    Adopted Y coordinate : 532.51

    Tilt correction (second pass)

    Remove sky background (second pass)

    Write 2D image : c:spectra20181028@1.fit

    Write 2D image : c:spectra20181028@2.fit

    Write 2D image : c:spectra20181028@3.fit

    Write 2D image : c:spectra20181028@4.fit

    Write 2D image : c:spectra20181028@5.fit

    Write 2D image : c:spectra20181028@6.fit

    Optimal binning

    Write raw profile : c:spectra20181028@raw1.dat

    Write raw profile : c:spectra20181028@raw2.dat

    Write raw profile : c:spectra20181028@raw3.dat

    Write raw profile : c:spectra20181028@raw4.dat

    Write raw profile : c:spectra20181028@raw5.dat

    Write raw profile : c:spectra20181028@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:spectra20181028@star.dat

    Processing of calibration image : c:spectra20181028neon5.fit

    Dark coefficient (calibration) : 0.0083

    Save image : c:spectra20181028@calib.fit (2D image)

    Save spectral profile : c:spectra20181028@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:spectra20181028@pro1.dat

    Write individual DAT processed profile : c:spectra20181028@pro2.dat

    Write individual DAT processed profile : c:spectra20181028@pro3.dat

    Write individual DAT processed profile : c:spectra20181028@pro4.dat

    Write individual DAT processed profile : c:spectra20181028@pro5.dat

    Write individual DAT processed profile : c:spectra20181028@pro6.dat

    Write individual FITS processed profile : c:spectra20181028@pro1.fit

    Write individual FITS processed profile : c:spectra20181028@pro2.fit

    Write individual FITS processed profile : c:spectra20181028@pro3.fit

    Write individual FITS processed profile : c:spectra20181028@pro4.fit

    Write individual FITS processed profile : c:spectra20181028@pro5.fit

    Write individual FITS processed profile : c:spectra20181028@pro6.fit

    —————————————————————————-

    Write final profile : c:spectra20181028_zetadra_20181028_970_J_Coffin.fit

    Write final profile : c:spectra20181028_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. 

    #580127
    John Coffin
    Participant

    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. 

    #580131
    Robin Leadbeater
    Participant

    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

    #580132
    Robin Leadbeater
    Participant

    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

    #580135
    John Coffin
    Participant

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

    #580136
    Robin Leadbeater
    Participant

    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

    #580137
    Andrew Smith
    Participant

    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

    #580143
    Andrew Smith
    Participant

    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

    #580145
    Robin Leadbeater
    Participant

    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

    #580146
    Andrew Smith
    Participant

    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 

    #580147
    David Boyd
    Participant

    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

    #580149
    Robin Leadbeater
    Participant

    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

    #580150
    Andrew Smith
    Participant

    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!

    #580151
    Andrew Smith
    Participant

    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 

    #580152
    John Coffin
    Participant

    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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