LHires III shifts with long exposures - how to handle?
Posted: Wed Dec 30, 2020 12:46 pm
Dear LHires III/L200 owners,
I am opening this topic to discuss how to process calibration shifts of long (multiple hours, e.g. 12 x 10 minutes) of exposures.
As I have experienced so far, many of us approaching it several different ways, with no mainstream resolution I'm aware of (whilst we produce such spectra in masses).
Via private discussions, I've met these approaches:
1) neglect - because:
a) it's a hobby / just doing short exposures (such that I understand)
b) let the database owner / professional notice the skew, when checking gathered spectra seriewise (drop/fix)
c) checks it later if ever happened (e.g. tellurics around H-alpha), drop/fix when error above threshold
d) does think it usually never happens with the owned LHires III (I have met 2 people in private mails of this thinking, in the last 2 years).
Antiproof: see the references below.
2) those who handle it in a way
a) ISIS -> Wavelength registration (and do calibration only for 1 exposure)
b) calibration before first, after last exposure - then use their average, accept the skew as a resolution degradation
c) calibration between each exposures (automatic) - a few of us able to do (including me):
i) use sum/average of these calibrations
ii) process individual exposures as spectra in 2D, average them in 1D as a final step (huge manual effort with ISIS)
iii) https://www.astrophoto.at/spectrocalc.html
3) "lateral" (http://www.astrosurf.com/buil/isis/tuto ... ion_us.htm) - special use only, we may possibly never used
For long exposures (= topic of this thread) like 1+ hours of exposures, obviously 2)+ is the way to go.
So far, my favourites have been 2.a) and 2.c.ii), of which I have these problems:
2.a) is useable only when spectrum is high-SNR, and/or have enough spikes to fit on (error is hard to calculate, ISIS does a complex approximation here).
Earlier discussion: http://www.spectro-aras.com/forum/viewt ... ?f=8&t=458
Since error is unknown (can be any - saw strange behaviours), I'd never use it for a precise RV measurement (e.g. CCF) - as the best precision is only the 1st spectra's CCF precision + fitting errors of further spectra. However, for measuring EWs (and RVs in a viusal way) it's still acceptable.
2.c.II): sum or avergae of spectra in 1D manually, causes in ISIS the fits headers to be wrong, need to be edited manually (the DATE-OBS field is what I often mistype, as cannot be copy/pasted in ISIS - unlike all the other fields). I even miss the possibility of a sigma clipping of such an average (correcting spikes, e.g. comsic ray filter often fails when its hits the spectrum - so that excluding that single spectrum at that wavelength region's pixel would be desired, something IRAF can do).
Note: in case of my Nova Cas 2020 serie (made a few times 8 hours), even the Y of spectra have moved (nearby star for guiding + my polar alignment isn't the best), whilst ISIS not finding the Y automatically so need to adjust it manually. One more reason to do it per-exposure.
This sum (or avg) in 1D is not how the way ISIS works by default (same can be true for BASS), so this earlier discussion on aavso is broken (of course, till method 2.b you don't meet such a problem):
https://www.aavso.org/batch-spectra-upload
I suggest continuing discussion here (it was offtopic on aavso). Misunderstanding must have been, as I told I am averaging the results in 1D one by one (using ISIS's sum feature of many spectra, not the images!).
Although 2.b) is better than 1), any of the 2.c) is even better than 2.b), and I consider the 2.c.ii/iii) method even better (something "unfortunately" I can easily produce).
This year I've been introducing too much manual mistakes, and for this reason I'm thinking about switching processing software.
I appreciate mentioning other alternatives, too (e.g. Audela step by step in English, for exactly this use-case with per-exposure calibrations).
For 2.c.iii) (SpectroCalc), I'm curious if anyone is using it, and uploads for which database.
Or, it would be nice only to resolve the fits headers problem in the ISIS sum feature (should not be hard).
References:
http://www.spectro-aras.com/forum/viewt ... f=25&t=459
http://www.spectro-aras.com/forum/viewt ... ift#p14222
http://www.spectro-aras.com/forum/viewt ... ion#p10924
...
Thank you for reading it, and I wish a happy new year for all of you!
Peter
I am opening this topic to discuss how to process calibration shifts of long (multiple hours, e.g. 12 x 10 minutes) of exposures.
As I have experienced so far, many of us approaching it several different ways, with no mainstream resolution I'm aware of (whilst we produce such spectra in masses).
Via private discussions, I've met these approaches:
1) neglect - because:
a) it's a hobby / just doing short exposures (such that I understand)
b) let the database owner / professional notice the skew, when checking gathered spectra seriewise (drop/fix)
c) checks it later if ever happened (e.g. tellurics around H-alpha), drop/fix when error above threshold
d) does think it usually never happens with the owned LHires III (I have met 2 people in private mails of this thinking, in the last 2 years).
Antiproof: see the references below.
2) those who handle it in a way
a) ISIS -> Wavelength registration (and do calibration only for 1 exposure)
b) calibration before first, after last exposure - then use their average, accept the skew as a resolution degradation
c) calibration between each exposures (automatic) - a few of us able to do (including me):
i) use sum/average of these calibrations
ii) process individual exposures as spectra in 2D, average them in 1D as a final step (huge manual effort with ISIS)
iii) https://www.astrophoto.at/spectrocalc.html
3) "lateral" (http://www.astrosurf.com/buil/isis/tuto ... ion_us.htm) - special use only, we may possibly never used
For long exposures (= topic of this thread) like 1+ hours of exposures, obviously 2)+ is the way to go.
So far, my favourites have been 2.a) and 2.c.ii), of which I have these problems:
2.a) is useable only when spectrum is high-SNR, and/or have enough spikes to fit on (error is hard to calculate, ISIS does a complex approximation here).
Earlier discussion: http://www.spectro-aras.com/forum/viewt ... ?f=8&t=458
Since error is unknown (can be any - saw strange behaviours), I'd never use it for a precise RV measurement (e.g. CCF) - as the best precision is only the 1st spectra's CCF precision + fitting errors of further spectra. However, for measuring EWs (and RVs in a viusal way) it's still acceptable.
2.c.II): sum or avergae of spectra in 1D manually, causes in ISIS the fits headers to be wrong, need to be edited manually (the DATE-OBS field is what I often mistype, as cannot be copy/pasted in ISIS - unlike all the other fields). I even miss the possibility of a sigma clipping of such an average (correcting spikes, e.g. comsic ray filter often fails when its hits the spectrum - so that excluding that single spectrum at that wavelength region's pixel would be desired, something IRAF can do).
Note: in case of my Nova Cas 2020 serie (made a few times 8 hours), even the Y of spectra have moved (nearby star for guiding + my polar alignment isn't the best), whilst ISIS not finding the Y automatically so need to adjust it manually. One more reason to do it per-exposure.
This sum (or avg) in 1D is not how the way ISIS works by default (same can be true for BASS), so this earlier discussion on aavso is broken (of course, till method 2.b you don't meet such a problem):
https://www.aavso.org/batch-spectra-upload
I suggest continuing discussion here (it was offtopic on aavso). Misunderstanding must have been, as I told I am averaging the results in 1D one by one (using ISIS's sum feature of many spectra, not the images!).
Although 2.b) is better than 1), any of the 2.c) is even better than 2.b), and I consider the 2.c.ii/iii) method even better (something "unfortunately" I can easily produce).
This year I've been introducing too much manual mistakes, and for this reason I'm thinking about switching processing software.
I appreciate mentioning other alternatives, too (e.g. Audela step by step in English, for exactly this use-case with per-exposure calibrations).
For 2.c.iii) (SpectroCalc), I'm curious if anyone is using it, and uploads for which database.
Or, it would be nice only to resolve the fits headers problem in the ISIS sum feature (should not be hard).
References:
http://www.spectro-aras.com/forum/viewt ... f=25&t=459
http://www.spectro-aras.com/forum/viewt ... ift#p14222
http://www.spectro-aras.com/forum/viewt ... ion#p10924
...
Thank you for reading it, and I wish a happy new year for all of you!
Peter