Wavelength Calibration Relco

[Francois Teyssier]

Hi,

I propose an exchange about the calibration of ALPY600, LISA, and other low resolution spectroscopes with the Relco lamp.

1. Spectroscope
2. List of lines you are using
3. Degree of the polynome

[HughAllen]

Hi Francois,
Thanks for the work to recover this site, it's great to be back. Here are my Relco calibration details:

Spectroscope: Alpy 600
Calibration lines:
7383.98
7272.94
7147.04
6965.43
6506.53
6266.49
5852.49
5400.56
4764.87
4545.05
4158.59
3946.10
Degree 4 polynomial

Cheers
Hugh

[David Boyd]

Hi Francois,

Yes, good to be back.

I use a LISA on a C11. I now use the following 22 lines from the LISA Ar/Ne lamp in a .lst file with a 4th order polynomial. This typically gives an rms in ISIS of around 0.08 and a resolving power of about 1100.

4
1.8
7383.980
7272.936
7147.042
7067.218
6965.431
6871.289
6598.953
6506.528
6402.248
6266.495
6143.063
6096.163
5944.834
5852.488
5764.419
5495.874
4965.079
4879.864
4609.567
4510.733
4259.362
4158.590

I have analysed calibration spectra from several LISAs over the years for different observers and found that every one needed a different set of lines to give optimum performance. So, in my experience, experimentation is needed to find the lines which give the best result for your device.

Regards,
David

[John Coffin]

Hi, I am using an Alpy 600 and have never been able to get an accurate calibration during the six years I've been using it. I normally use ISIS but have switched to Demetra to see if that made any difference. I have played around with different degrees of polynomial and various calibration lines. In fact I used Hugh's list and that failed to calibrate at all. With a 4th polynomial the RMS was 0.22

What I have found is that whatever I do there is still a blueshift of roughly 2 Angstroms. In a very old thread with Robin Leadbeater, https://britastro.org/forums/topic/what ... h-an-alpy we wondered if the optical path of the calibration lamp and the light from the telescope were different and that caused the issue.

To quote Robin '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?'

I am resigned to accepting this degree of accuracy with my Alpy. At one point I made an automatic wavelength adjustment to my spectra, but was told by the BAA database manager that I should submit them as they are without any changes

[John Coffin]

I've had another thought. I have been using my Alpy with a Celestron 8 Edge HD and focal reducer x 0.7. I wonder if the optics of this set up has some effect on the spectrum and somehow distorts the wavelengths towards the blue. I can't think how this could be, but I could try the Alpy on a different scope. I have an 80mm refractor.

[Robin Leadbeater]

I will be interested to see what you find. I still have the same issue. I use ISIS which gives a good fit using auto calibration using the calibration lamp but gives an increasing offset towards the blue on stars. I noticed among spectra collected during a STAROS campaign that some ALPY users seem to have the same problem but others did not. I suggested to the STAROS team it might be an interesting thing to follow up but did not get a reply

Cheers
Robin

[HughAllen]

Hi, I am using an Alpy 600 and have never been able to get an accurate calibration during the six years I've been using it. I normally use ISIS but have switched to Demetra to see if that made any difference. I have played around with different degrees of polynomial and various calibration lines. In fact I used Hugh's list and that failed to calibrate at all. With a 4th polynomial the RMS was 0.22

Hi John,
An RMS of 0.22 is pretty typical I find, and I think an RMS of this value is typical for other users as well and shouldn't significantly affect the accuracy of your calibration. So I wonder what is behind your comment that your calibration using the same lamp lines 'failed' - could you give an example? As a typical example from me, here's the calibrated, response corrected reference star spectrum (zoomed into the blue end) that I used for a TCrB observation made last year. It does show the same slight blue shift error towards shorter wavelengths on which both you and Robin have commented. But the error is very consistent and so doesn't affect the capability to make accurate comparisons of different star spectra
Cheers
Hugh

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