Forum Replies Created
-
Posts
-
Hi Tony
Can you track the rogue pixels down in the raw images? If it is in every image then it should have been in the master dark and corrected but you could add it to the cosmetic file and zap it that way.
I dont know about BASS but the ISIS cosmics removal tool works on the individual sub exposures where a rogue pixel stands out more clearly than in the combined spectrum image. It compares each pixel with its neighbours, looking at the differential between each pixel and the surrounding ones. You can set the threshold so if a pixel is poking its head above its neighbours, ISIS will zap it. (If you set it too low though, particularly if the spectrum is close to undersampled, it can zap a whole row or spectrum line so you have to use it with caution)
Robin
Hi Hugh,
Yes the acid test is to rerun the reference star using the calculated instrument response
If the flat has done its job well the instrument response should be nice and smooth (just the flat lamp spectrum and atmospheric extinction plus any wavelength selective sampling at the slit due to chromatism) I am not sure if BASS does this but ISIS even assumes a black body curve for the flat lamp so in theory with ISIS ideally all you should be left with is the atmospheric extinction. With my LHIRES at H alpha high resolution this results in a dead flat instrument response within a couple of percent every time so it is hardly worthwhile doing an instrument response correction. In practise with my ALPY there are some small ripples at a couple of percent from the sensor response which the flat has not quite dealt with and a rather nasty kink to deal with around 4000A. I am not sure where it comes from but others have also reported it. Perhaps the reflectivity of the paint used on the reflector used in the ALPY calibration module ? Note the variation in shape in this collection of my IR curves. A lot of this I believe is due to chromatic aberration in my focal reducer coupled with slight differences in focus.
Hi Nick,
Are you saying that ISIS is not finding the dark and flat files you entered in the general page when you run the data reduction?
I have developed my way of working with ISIS over several years so I probably don’t do things the standard way. I collect all the files ISIS needs to do a data reduction in one folder titled with the target name and date, copying the masters, instrument response and wavelength calibration files etc in from other folders if necessary, rather than have ISIS search for them in different folders. (See attached example from a run last night) That way, although there is some duplication of files, I can archive the folder and always rerun the reduction again using the same set of files in the future. When I run the reduction I point ISIS to that folder in the settings page and point to the individual files in that folder the general page.
Robin
Hi Nick,
Are you looking for A or B stars in general or specifically ones suitable for use as reference stars?
If the latter, then for low resolution spectra the MILES stars (included in the ISIS database) have actual spectra associated with them or alternatively Francois Teyssier has generated a list of hot main sequence stars with low interstellar extinction which can be used (with caution) with generic spectra from the Pickles library. You can find his spreadsheet which also includes the MILES stars and has tools to help with selecting stars close to the target here
http://www.astronomie-amateur.fr/Documents%20Spectro/ReferenceStarFinder.xlsm
with more information in this post on the ARAS forum
http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=821&p=3697#p3697
Cheers
Robin
A reminder that spectra to BeSS standard normally do not have telluric lines removed or heliocentric corrections applied so it is important to make these corrections before comparing them. (I remember seeing a poster by a professional which claimed to show features moving in a line profile which were in fact due to telluric lines in heliocentric corrected spectra !)
Robin
Similar variability is also seen in the He 6680A line.
Although the line is much weaker, the variability is actually greater as a percentage compared with at H alpha
Robin
Hi Kevin,
You could well be right. I was looking at the variability the other day, using some from the BAA database and some I had been sent direct.
The spectra are good quality and there is good coverage so it might be possible to characterise the variability. There is some sign of rapid variations, even from day to day though so any period might be rather short. It might be worthwhile doing a run over a full night and looking at the short term variability.
Robin
Thanks Andrew,Andy,
I am starting to wonder if this might be an amateur first. Does anyone know of any other amateur extragalactic nova spectra ?
Robin
Thanks Hugh,
I think monitoring with low resolution spectra will be useful to detect any significant changes in the strength of the emission lines and when combined with the photometry, will give an absolute measure of the flux which is difficult to do at high resolution.
(The kink in the continuum around H beta is curious. Is the instrument response is ok in that region ?)
Cheers
Robin
Now confirmed as an Fe II nova by Wiliams et al from a spectrum taken using the Liverpool Telescope.
http://www.astronomerstelegram.org/?read=12138
(My spectrum taken a day earlier is also referenced there)
Robin
I was thinking perhaps the workshop could be based around some standard targets that participants would record beforehand. By processing these and comparing the results between each other and to the published spectra, any potential issues with spectrograph setup, data acquisition or processing would then be highlighted. (Something that I know helped me during an OHP workshop when I started out trying to get good quality data.)
Robin
Hi Tony
ISIS apparently does not work with the current gnuplot . Try v4.4.6 perhaps ? eg the same problem reported here
http://www.spectro-aras.com/forum/viewtopic.php?f=10&t=2139
Robin
It seems ALPY grisms out of adjustment is a common problem. I thought I remembered commenting on this here before
https://britastro.org/node/14085
Robin
That’s great !. If you still have problems with the spectrum horizontal, you can also adjust the ALPY grism to make the lines exactly vertical. If you look at the core module,you can see the grism held in place by allen head grub screws. If you loosen them off you can rotate the grism, viewing a spectrum by eye until the spectral lines are square to the dispersion direction. (Take care not to loosen to much and hold the grism holder in place as there is a spring behind it)
For any LHIRES /LISA users which might be following this, The error is specific to the smile correction used with the ALPY. I have checked the slant correction that ISIS uses with LHIRES, LISA and this works correctly.
Robin
As a further test, the calibration also runs ok with the original X position (though with slightly lower precision and resolution and poor sky background subtraction of course) if you run without the smile correction (by putting a very large value for the radius)
I agree. I found it by looking at the position of the lines in the geometric corrected spectrum image. (This could be a work round by running the calibration a second time applying the offset seen in the image ) I am awaiting feedback from Christian
Robin
Hi John,
See answer #9 above for the source of the problem. Actually, I see now that the slant of the lamp lines is partly because the dispersion is not exactly horizontal, as seen in the star image. If next time you rotate the camera to make the star spectrum horizontal, the calibration might run ok without needing to change the X position entered or adjust the grism.
Cheers
Robin
Thanks for the files. I now see the problem. When ISIS does the geometric correction for smile, this shifts the X positions of the lines so the entered X reference position for the line that ISIS works out the other lines from is not correct and it cannot lock on. This does not give a problem if the lines are close to vertical, for example in my case the shift is only 2 pixels. If there is a significant slant though the shift can be significant (+15 pixels in your case) If you add 15 pixels to the reference position, I think you should find the calibration runs correctly. Can you try it and see what happens? (It ran for me giving an RMS of 0.3A but I have not tried it combined with the Balmer lines using the wizard)
The slant can be corrected in the ALPY by rotating the grism to make the lines perpendicular to the dispersion direction. (It is simple to do. I did this with my ALPY when I received it and I will dig out the instructions) but I am a bit surprised ISIS does not take this shift into account. I will check with Christian Buil via the ARAS forum
Cheers
Robin
You can download my lamp image here
https://drive.google.com/open?id=1I1UiZhmynSmkF19Fq3ce9foTsRF93ZgN
Robin
