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Another 28 Tau

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  • alunhalsey
    Hi Managed to get out under the stars last night although there was a thin mist/fog developing in the early hours at the time of capture.. Using my C9.25 and
    Message 1 of 18 , Oct 26, 2017

      Hi


        Managed to get out under the stars last night although there was a thin mist/fog developing in the early hours at the time of capture..

       Using my C9.25 and Lhires III I had another go at 28 Tau,only this time round I collected dark,flat and bias frames as well. Slit used with the 2400 grating was the 23um and I set the 314L to take 240sec exposures again in the Ha region..

       Have posted profile to my folder in the photos section,stack of 6 frames with 6 darks,8 bias and 8 flats,using BASS throughout to stack, pre-process and calibrate using the on board neon lamp. No instrument response correction.


      https://uk.groups.yahoo.com/neo/groups/astrobodger/photos/albums/2100753696/lightbox/1466377905?orderBy=ordinal&sortOrder=asc&photoFilter=ALL#zax/1466377905


        Thank you for looking


           Regards


              Alun


    • John Paraskeva
      Hi Alun, Thanks for sharing. Good to see you are using darks, bias and flats now. Must have a go at this one. Perhaps conditions were not good, but I notice
      Message 2 of 18 , Oct 26, 2017
        Hi Alun,

        Thanks for sharing. Good to see you are using darks, bias and flats now. Must have a go at this one.

        Perhaps conditions were not good, but I notice your 26 Oct with 23 micron slit is not as well resolved and is noisier as your 16th Oct posting using 35 micron slit. Could be a processing issue, so worth a experimenting by excluding some files?

        regards
        John



        From: "ALUN@... [astrobodger]" <astrobodger@...>
        To: astrobodger@...
        Sent: Thursday, 26 October 2017, 16:40
        Subject: [astrobodger] Another 28 Tau

         
        Hi

          Managed to get out under the stars last night although there was a thin mist/fog developing in the early hours at the time of capture..
         Using my C9.25 and Lhires III I had another go at 28 Tau,only this time round I collected dark,flat and bias frames as well. Slit used with the 2400 grating was the 23um and I set the 314L to take 240sec exposures again in the Ha region..
         Have posted profile to my folder in the photos section,stack of 6 frames with 6 darks,8 bias and 8 flats,using BASS throughout to stack, pre-process and calibrate using the on board neon lamp. No instrument response correction.


          Thank you for looking

             Regards

                Alun



      • alunhalsey
        Hi John, Thank you,and yes I finally got into the habit of taking darks etc and using them,did notice though that whilst loading the calibration frames into
        Message 3 of 18 , Oct 26, 2017
          Hi John,

              Thank you,and yes I finally got into the habit of taking darks etc and using them,did notice though that whilst loading the calibration frames into the stacking/preprocessing routine in BASS I didn't take flat darks,so have added them to the list for next session:)
             Regarding the noise,it was getting foggy as the morning drew on so possibly contaminated a few frames,will check through and have another go. 

          Oh,one other point,I deleted my original message,due to my posting the wrong image at first,so may cause confusion to those that receive group messages via email.Sorry.

              Regards

                 Alun
        • andyjwilson_uk
          Hi Alun, A nice spectrum and good to hear your are getting to grips with the image calibrations. With regard to your 26th vs 16th October spectra, I m tempted
          Message 4 of 18 , Oct 26, 2017
            Hi Alun,

            A nice spectrum and good to hear your are getting to grips with the image calibrations.

            With regard to your 26th vs 16th October spectra, I'm tempted to think that the 26th looks like better signal but perhaps not quite such quite a high resolution. Though such things are very difficult to judge from target spectra alone. A few things to think about.

            - Fog and mist should not affect the resolution, since the star can be blurred across the slit, but the spectrograph would still give the same resolution.

            - Whether the star is exactly on the slit has a big impact on how good a signal you get.

            - The telluric lines look stronger in the 16th than 26 October. This could simply be weather conditions, or also if the spectra were taken with the star at different altitudes.

            - The LHIRES III does suffer from small wavelength shifts due to temperature differences (as do most spectrographs). I tend to focus mine for a given season and then only adjust the doublet focus when needed. Otherwise I need to retake flats each time I refocus, which would be annoying during the night.

            - For long exposures there can be a slight wavelength shift. As the telescope tracks so gravity will act on the spectrograph at different angles and cause slight flexure. I tend not to worry too much about this, even when I do 2 hour exposures, but it is worth knowing about as it is a potential cause of lower resolution for long exposures.

            Best wishes,
            Andy
          • alunhalsey
            Hi Andy, Thank you:) I took the spectra for the 16th whilst the star was low in the east whist last night 28 Tau was near on overhead. Could be that I may
            Message 5 of 18 , Oct 26, 2017
              Hi Andy,

                   Thank you:)    I took the spectra for the 16th whilst the star was low in the east whist last night 28 Tau was near on overhead. Could be that I may have placed the star slightly off slit,or maybe the autoguiding wasn't as steady,PHD2 can be a bit temperamental at times;Having just checked each profile separately in BASS and found they all have the same amount of noise. Regards the flexure,I am tempted to rig up some sort of support arm attached between scope and Lhires (got idea from Ken Harrison,thank you Ken!) although if I do follow this up I may have to lose the side by side configuration on the EQ6!

                Having read and been told about the focus changes when the temperature cools I have been playing with the idea of attaching dew beating heater bands/pads to the Lhires so as to help minimise the changes due to temperature drops. Whether or not the idea would work by applying a small amount of heat near the area of the collimating lens,or more importantly applied to the casing to warm the unit as a whole,I don't know but could be worth trying out. Thoughts??

                 Regards

                    Alun

                 
            • andyjwilson_uk
              Hi Alun, To be honest I would just concentrate on getting spectra for the time being. Most of those who submit spectra to databases like the BAA, BeSS and
              Message 6 of 18 , Oct 26, 2017
                Hi Alun,

                To be honest I would just concentrate on getting spectra for the time being. Most of those who submit spectra to databases like the BAA, BeSS and ARAS, don't worry about the flexure or temperature changes during an observing session. They just keep an eye on their results to make sure they maintain a consistent quality.

                With flexure, it is not the weight of the spectrograph and camera on the telescope, but internal flexure within the LHIRES III. This only needs to be very small to be noticeable as a tiny wavelength shift. So a support arm could help by reducing the stress on the spectrograph due to the camera, but you could also go to the effort and find it makes very little difference. It is only ever a problem if you stay on the same target for many hours, and this can be mitigated by taking neon calibration spectra at intervals during your observing run. So I would not concern yourself with this for now.

                Temperature variations can be a nuisance, but I'd get spectra first and then see if you start to notice a problem. The easiest way to spot them is by looking at your calibration spectra at different points during your observing session. If they start out in focus, but then get fuzzy as the temperature drops then that is almost certainly due to contraction of the optical paths due to the drop in temperature. Even so, I find that the spectra are still usable, and it is a rare night when temperature variation is a real problem.

                When I started out, I focused the doublet in my LHIRES III at the start of each observing session. However, after a few months I realised it was stable from night to night, and only needed adjusting with the bigger temperature swings between the different seasons.

                With your two observing sessions, what would be interesting is to compare the neon calibration spectra. You can get the resolution by measuring a line on the processed neon spectra. Even better is to also have neon calibrations at the start and end of each run on individual targets. Then you can tell if there has been any shift in the wavelength calibration as well as a shift in focus. Even then, it is worth noting that small shifts are common, and as long as it doesn't start giving you multiple lines due to a big shift then things should be fine.

                Clear skies,
                Andy


                ---In astrobodger@..., <ALUN@...> wrote :

                Hi Andy,

                     Thank you:)    I took the spectra for the 16th whilst the star was low in the east whist last night 28 Tau was near on overhead. Could be that I may have placed the star slightly off slit,or maybe the autoguiding wasn't as steady,PHD2 can be a bit temperamental at times;Having just checked each profile separately in BASS and found they all have the same amount of noise. Regards the flexure,I am tempted to rig up some sort of support arm attached between scope and Lhires (got idea from Ken Harrison,thank you Ken!) although if I do follow this up I may have to lose the side by side configuration on the EQ6!

                  Having read and been told about the focus changes when the temperature cools I have been playing with the idea of attaching dew beating heater bands/pads to the Lhires so as to help minimise the changes due to temperature drops. Whether or not the idea would work by applying a small amount of heat near the area of the collimating lens,or more importantly applied to the casing to warm the unit as a whole,I don't know but could be worth trying out. Thoughts??

                   Regards

                      Alun

                   
              • alunhalsey
                Hi Andy, Once again,very good sound advice,thank you. :) Will check the neon line spectra just to see what differences there are between the two sets from
                Message 7 of 18 , Oct 26, 2017
                  Hi Andy,

                       Once again,very good sound advice,thank you. :)   Will check the neon line spectra just to see what differences there are between the two sets from the 16th and 26th. Hoping this persistent cloud clears out
                   of it soon and can get out there and use the kit instead of looking at it thinking of things to do to it! 

                     Again I thank you for your advice :)

                          Regards

                           Alun
                • alunhalsey
                  After following everyone s invaluable advice,thank you Andy,Ken and John,I have managed to iron out a few kinks in the Lhires and more importantly in my
                  Message 8 of 18 , Oct 31, 2017
                    After following everyone's invaluable advice,thank you Andy,Ken and John,I have managed to iron out a few kinks in the Lhires and more importantly in my technique. Found that my neon bulb was sat slightly lower in the
                    slit,after adjusting it I am now getting tighter lines. Have also realised my exposure times are too short,hence the noisy continuum and have also had to lose my JMI motor focus unit on the C9.25 so as to allow the slit to be positioned parallel with the RA axis. Had another go at 28 Tau using 35um slit and 600 second exposures on the night of the 29th,3 images preprocessed and stacked, calibrated in BASS. results of which I have posted to my photo album...



                       Regards

                          Alun
                     
                  • John Paraskeva
                    Hi Alun, Your 28 Tau looks good, especially considering I needed to resort to 2x2 binning. I haven t got around to adding an internal calibration lamp yet.
                    Message 9 of 18 , Oct 31, 2017
                      Hi Alun,

                      Your 28 Tau looks good, especially considering I needed to resort to 2x2 binning.

                      I haven't got around to adding an internal calibration lamp yet. It's on the list.

                      Be interesting to compare lines from waving a neon lamp over the end of the scope, with the internal lamp.
                       
                      best regards

                      John



                      From: "ALUN@... [astrobodger]" <astrobodger@...>
                      To: astrobodger@...
                      Sent: Tuesday, 31 October 2017, 15:55
                      Subject: Re:: Re: [astrobodger] Another 28 Tau

                       
                      After following everyone's invaluable advice,thank you Andy,Ken and John,I have managed to iron out a few kinks in the Lhires and more importantly in my technique. Found that my neon bulb was sat slightly lower in the
                      slit,after adjusting it I am now getting tighter lines. Have also realised my exposure times are too short,hence the noisy continuum and have also had to lose my JMI motor focus unit on the C9.25 so as to allow the slit to be positioned parallel with the RA axis. Had another go at 28 Tau using 35um slit and 600 second exposures on the night of the 29th,3 images preprocessed and stacked, calibrated in BASS. results of which I have posted to my photo album...



                         Regards

                            Alun
                       


                    • Ken Harrison
                      John. I currently use a Filly Ar lamp for reference in regions around H Beta...... This is positioned in front of the telescope aperture in line with the slit
                      Message 10 of 18 , Oct 31, 2017
                        John.
                        I currently use a Filly Ar lamp for reference in regions around H Beta......
                        This is positioned in front of the telescope aperture in line with the slit height.
                        Seems to work well for me.
                        There was a document comparing front lamp positions v's possible errors; I'll try to find it.
                        Ken


                        On 1 November 2017 at 09:51, John Paraskeva johnparaskeva@... [astrobodger] <astrobodger@...> wrote:
                         

                        Hi Alun,

                        Your 28 Tau looks good, especially considering I needed to resort to 2x2 binning.

                        I haven't got around to adding an internal calibration lamp yet. It's on the list.

                        Be interesting to compare lines from waving a neon lamp over the end of the scope, with the internal lamp.
                         
                        best regards

                        John



                        From: "ALUN@.... uk [astrobodger]" <astrobodger@... >
                        To: astrobodger@...
                        Sent: Tuesday, 31 October 2017, 15:55
                        Subject: Re:: Re: [astrobodger] Another 28 Tau

                         
                        After following everyone's invaluable advice,thank you Andy,Ken and John,I have managed to iron out a few kinks in the Lhires and more importantly in my technique. Found that my neon bulb was sat slightly lower in the
                        slit,after adjusting it I am now getting tighter lines. Have also realised my exposure times are too short,hence the noisy continuum and have also had to lose my JMI motor focus unit on the C9.25 so as to allow the slit to be positioned parallel with the RA axis. Had another go at 28 Tau using 35um slit and 600 second exposures on the night of the 29th,3 images preprocessed and stacked, calibrated in BASS. results of which I have posted to my photo album...



                           Regards

                              Alun
                         





                        --
                        "Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....

                      • Ken Harrison
                        Thanks to Robin, I refound the article I was thinking of... http://astrospectroscopy.eu/Einsteiger/Kalibrierung/Kalibrierung_e.htm Conclusions at bottom of the
                        Message 11 of 18 , Nov 1, 2017
                          Thanks to Robin, I refound the article I was thinking of...

                          Conclusions at bottom of the page.


                          On 1 November 2017 at 10:20, Ken Harrison <kenm.harrison@...> wrote:
                          John.
                          I currently use a Filly Ar lamp for reference in regions around H Beta......
                          This is positioned in front of the telescope aperture in line with the slit height.
                          Seems to work well for me.
                          There was a document comparing front lamp positions v's possible errors; I'll try to find it.
                          Ken


                          On 1 November 2017 at 09:51, John Paraskeva johnparaskeva@... [astrobodger] <astrobodger@... > wrote:
                           

                          Hi Alun,

                          Your 28 Tau looks good, especially considering I needed to resort to 2x2 binning.

                          I haven't got around to adding an internal calibration lamp yet. It's on the list.

                          Be interesting to compare lines from waving a neon lamp over the end of the scope, with the internal lamp.
                           
                          best regards

                          John



                          From: "ALUN@....u k [astrobodger]" <astrobodger@... >
                          To: astrobodger@...
                          Sent: Tuesday, 31 October 2017, 15:55
                          Subject: Re:: Re: [astrobodger] Another 28 Tau

                           
                          After following everyone's invaluable advice,thank you Andy,Ken and John,I have managed to iron out a few kinks in the Lhires and more importantly in my technique. Found that my neon bulb was sat slightly lower in the
                          slit,after adjusting it I am now getting tighter lines. Have also realised my exposure times are too short,hence the noisy continuum and have also had to lose my JMI motor focus unit on the C9.25 so as to allow the slit to be positioned parallel with the RA axis. Had another go at 28 Tau using 35um slit and 600 second exposures on the night of the 29th,3 images preprocessed and stacked, calibrated in BASS. results of which I have posted to my photo album...



                             Regards

                                Alun
                           





                          --
                          "Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....




                          --
                          "Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....

                        • andyjwilson_uk
                          Looking good Alun. That spectrum has a nice strong signal. Cheers, Andy ... After following everyone s invaluable advice,thank you Andy,Ken and John,I have
                          Message 12 of 18 , Nov 2, 2017
                            Looking good Alun. That spectrum has a nice strong signal.
                            Cheers,
                            Andy


                            ---In astrobodger@..., <ALUN@...> wrote :

                            After following everyone's invaluable advice,thank you Andy,Ken and John,I have managed to iron out a few kinks in the Lhires and more importantly in my technique. Found that my neon bulb was sat slightly lower in the
                            slit,after adjusting it I am now getting tighter lines. Have also realised my exposure times are too short,hence the noisy continuum and have also had to lose my JMI motor focus unit on the C9.25 so as to allow the slit to be positioned parallel with the RA axis. Had another go at 28 Tau using 35um slit and 600 second exposures on the night of the 29th,3 images preprocessed and stacked, calibrated in BASS. results of which I have posted to my photo album...



                               Regards

                                  Alun
                             
                          • John Paraskeva
                            Thanks Ken (and Robin) for this link.  I was aware of calibration issues, if the internal lamp position is not perfect. The Waving the lamp method is not
                            Message 13 of 18 , Nov 2, 2017
                              Thanks Ken (and Robin) for this link. 

                              I was aware of calibration issues, if the internal lamp position is not perfect. The Waving the lamp method is not without issues either, but seems more reliable.

                              regards
                              John



                              From: "Ken Harrison kenm.harrison@... [astrobodger]" <astrobodger@...>
                              To: astrobodger@...
                              Sent: Wednesday, 1 November 2017, 20:57
                              Subject: Re: : Re: [astrobodger] Another 28 Tau

                               
                              Thanks to Robin, I refound the article I was thinking of...

                              Conclusions at bottom of the page.


                              On 1 November 2017 at 10:20, Ken Harrison <kenm.harrison@...> wrote:
                              John.
                              I currently use a Filly Ar lamp for reference in regions around H Beta......
                              This is positioned in front of the telescope aperture in line with the slit height.
                              Seems to work well for me.
                              There was a document comparing front lamp positions v's possible errors; I'll try to find it.
                              Ken


                              On 1 November 2017 at 09:51, John Paraskeva johnparaskeva@... [astrobodger] <astrobodger@... > wrote:
                               
                              Hi Alun,

                              Your 28 Tau looks good, especially considering I needed to resort to 2x2 binning.

                              I haven't got around to adding an internal calibration lamp yet. It's on the list.

                              Be interesting to compare lines from waving a neon lamp over the end of the scope, with the internal lamp.
                               
                              best regards

                              John



                              From: "ALUN@....u k [astrobodger]" <astrobodger@... >
                              To: astrobodger@...
                              Sent: Tuesday, 31 October 2017, 15:55
                              Subject: Re:: Re: [astrobodger] Another 28 Tau

                               
                              After following everyone's invaluable advice,thank you Andy,Ken and John,I have managed to iron out a few kinks in the Lhires and more importantly in my technique. Found that my neon bulb was sat slightly lower in the
                              slit,after adjusting it I am now getting tighter lines. Have also realised my exposure times are too short,hence the noisy continuum and have also had to lose my JMI motor focus unit on the C9.25 so as to allow the slit to be positioned parallel with the RA axis. Had another go at 28 Tau using 35um slit and 600 second exposures on the night of the 29th,3 images preprocessed and stacked, calibrated in BASS. results of which I have posted to my photo album...



                                 Regards

                                    Alun
                               





                              --
                              "Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....




                              --
                              "Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....



                            • alunhalsey
                              Thank you John,Andy :) Did a neon line test last night John using the internal and external lamps. The internal lamp only needed 0.3sec exposure,whilst the
                              Message 14 of 18 , Nov 2, 2017
                                Thank you John,Andy :)

                                   Did a neon line test last night John using the internal and external lamps. The internal lamp only needed 0.3sec exposure,whilst the external needed at least 15secs to get a good signal. Just held the lamp in one position in front of corrector plate,but had to be placed at central point between secondary holder and outer 
                                edge of corrector plate so as to ensure lines filled frame, Have uploaded result to my folder,alignment is a match so either lamp will work,must add though that the external neon lines were slightly tighter than those 
                                of the internal.


                                Internal lamp.....


                                  External lamp.....


                                    Regards

                                       Alun
                                 
                              • andyjwilson_uk
                                Those are good looking neon lines Alun. Interesting that the external ones are sharper. It is worth noting that for critical wavelength calibration near to
                                Message 15 of 18 , Nov 2, 2017
                                  Those are good looking neon lines Alun. Interesting that the external ones are sharper.

                                  It is worth noting that for critical wavelength calibration near to H-alpha you can use the atmospheric telluric lines. Using the easiest method for wavelength calibration is often sufficient, as long as it gives good results.

                                  For example when submitting spectra to the BeSS database, the telluric lines are left in the spectra. This allows researches to redo the wavelength calibration of the spectra should they wish to do so. Noting of course that spectra submitted to any database should have a good wavelength calibration.

                                  Cheers,
                                  Andy
                                • alunhalsey
                                  Thank you for the tips Andy,I prefer to leave the Tellurics in place. But I have noticed that when calibrating the neon lines (external or internal source),if
                                  Message 16 of 18 , Nov 3, 2017

                                  Thank you for the tips Andy,I prefer to leave the Tellurics in place. But I have noticed that when calibrating the neon lines (external or internal source),if I use Linear then the wavelength do not match to the peaks (more so in the 6532A line) yet if I were to select Quadratic then everything matches up and gives good wavelength calibration on the star profile. Does it matter that I am not using Linear with just the 3 neon lines?

                                       Regards

                                          Alun
                                • andyjwilson_uk
                                  Hi Alun, That is a good point. The short answer is for the LHIRES III around H-alpha a quadratic fit to the 3 neon lines is best. I m not sure if you read my
                                  Message 17 of 18 , Nov 4, 2017
                                    Hi Alun,

                                    That is a good point. The short answer is for the LHIRES III around H-alpha a quadratic fit to the 3 neon lines is best. I'm not sure if you read my BASS tutorial on the BAA website, but that would indicate that by default a linear fit is best, but it turns out not to be so in this scenario. If I ever get the time to update the tutorial I'll add a comment about this.

                                    Longer answer as follows :-)

                                    Usually when fitting a wavelength calibration it is better to use a polynomial with less degrees of freedom than the number of calibration lines. So for 3 lines this normally means a linear fit with 2 degrees of freedom is best. However, for the LHIRES III the wavelength calibration is know to be non-linear. So near H-alpha it is better to use a quadratic for wavelength calibration using the 3 neon lines. This does mean you get no error measurement of the fitted points in BASS. So to mitigate this you review the spectrum to make sure the wavelength calibration looks sensible (I am sure what most people do anyway).

                                    If you ever use the telluric lines to refine your wavelength calibration then you can easily fine 5+ lines near H-alpha to play with, which makes fitting a higher order polynomial more straight forward. However, don't worry about using tellurics for the time being, unless you have time on your hands and are interested. I find this is only needed when very high accuracy is required and you are comparing multiple runs.

                                    Cheers,
                                    Andy



                                    ---In astrobodger@..., <ALUN@...> wrote :


                                    Thank you for the tips Andy,I prefer to leave the Tellurics in place. But I have noticed that when calibrating the neon lines (external or internal source),if I use Linear then the wavelength do not match to the peaks (more so in the 6532A line) yet if I were to select Quadratic then everything matches up and gives good wavelength calibration on the star profile. Does it matter that I am not using Linear with just the 3 neon lines?

                                         Regards

                                            Alun
                                  • John Paraskeva
                                    I haven t faced this issue in anger, since my 1200 l/mm grating gives me 7 Neon lines. I am due to test with a 1800 l/mm grating soon. Looking at the Ne
                                    Message 18 of 18 , Nov 4, 2017

                                      I haven't faced this issue in anger, since my 1200 l/mm grating gives me 7 Neon lines. I am due to test with a 1800 l/mm grating soon. Looking at the Ne spectrum I will probably only get 4 lines to use, so a quadratic would be fine.  

                                      Using a polynom with fewer degrees of freedom than the number points is generally a good thing, not just for spectra.  If you wanted to extrapolate outside the range of the calibration points, then a linear fit would probably be more stable. However, as we are mostly concerned with the H Alpha region within the calibration points then the quadratic my be better. Depending of the profile, adding telulric lines from "-atmos_Ha.dat" file to the chart may help validate the calibration.

                                      cheers
                                      John 



                                      From: "barnards.star12@... [astrobodger]" <astrobodger@...>
                                      To: astrobodger@...
                                      Sent: Saturday, 4 November 2017, 8:58
                                      Subject: Re:: Re: : Re: [astrobodger] Another 28 Tau

                                       
                                      Hi Alun,

                                      That is a good point. The short answer is for the LHIRES III around H-alpha a quadratic fit to the 3 neon lines is best. I'm not sure if you read my BASS tutorial on the BAA website, but that would indicate that by default a linear fit is best, but it turns out not to be so in this scenario. If I ever get the time to update the tutorial I'll add a comment about this.

                                      Longer answer as follows :-)

                                      Usually when fitting a wavelength calibration it is better to use a polynomial with less degrees of freedom than the number of calibration lines. So for 3 lines this normally means a linear fit with 2 degrees of freedom is best. However, for the LHIRES III the wavelength calibration is know to be non-linear. So near H-alpha it is better to use a quadratic for wavelength calibration using the 3 neon lines. This does mean you get no error measurement of the fitted points in BASS. So to mitigate this you review the spectrum to make sure the wavelength calibration looks sensible (I am sure what most people do anyway).

                                      If you ever use the telluric lines to refine your wavelength calibration then you can easily fine 5+ lines near H-alpha to play with, which makes fitting a higher order polynomial more straight forward. However, don't worry about using tellurics for the time being, unless you have time on your hands and are interested. I find this is only needed when very high accuracy is required and you are comparing multiple runs.

                                      Cheers,
                                      Andy



                                      ---In astrobodger@..., <ALUN@...> wrote :


                                      Thank you for the tips Andy,I prefer to leave the Tellurics in place. But I have noticed that when calibrating the neon lines (external or internal source),if I use Linear then the wavelength do not match to the peaks (more so in the 6532A line) yet if I were to select Quadratic then everything matches up and gives good wavelength calibration on the star profile. Does it matter that I am not using Linear with just the 3 neon lines?

                                           Regards

                                              Alun


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