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Nick said “The FITS “standard” is irritatingly vague about things like this ..”
According to this document, in 1997 the IAU recognised this confusion and standardised DATE-OBS as the start time
https://www.cv.nrao.edu/fits/documents/standards/year2000.txt
“4.2) Henceforth, DATE-OBS shall be assumed to refer to the start of an
observation. Other interpretations must be clearly explained in the comment field.”
Cheers
Robin
The BeSS fits standard used in the spectroscopy database specifies optionally DATE-END for the end time but I think it is rarely used and DATE-OBS (start time) and EXPTIME is normally used
“Date-Time of start of exposure ● Keyword = DATE-OBS (mandatory except if DATE-END and EXPTIME are present) ● Format: char (70 char max) ● Mandatory format: yyyy-mm-ddThh :mm:ss[.ss…] ● Example: 2006-02-25T13 :34 :43.5543
Date-Time of End of Exposure ● Keyword = DATE-END (mandatory except if DATE-OBS and EXPTIME are present) ● Format: char (70 char max) ● Mandatory format: yyyy-mm-ddThh :mm:ss[.ss…] ● Example: 2006-02-25T13 :34 :43.5543
Exposure Time ● Keyword = EXPTIME (mandatory except if DATE-OBS and DATE-END are present) ● Format: float ● Units: seconds ● Valid Range: >= 0
There has been significant evolution in the spectrum, the He I N II C II lines being replaced by Fe II lines as the classification Atel suggested might happen
Cheers
Robin
The intensity of the emission component of the Balmer lines and of the He II emission which have gradually been increasing has significantly reduced tonight. Is this expected behaviour or signs of a further outburst ?
Cheers
Robin
Christian Knigge also suggested following the He II / H beta region at higher resolution looking for RV variations. Here is 4 hour time series at 10 minutes cadence from last night with the LHIRES at R~2000 . Not sure there is much to see above the noise though (SNR in a 10 min exposure is ~50)
The average
and the individual spectra (rectified)
_20210721_068_Leadbeater.png)
Thanks David. Excellent reproducibility between our spectra taken less than an hour apart though my ALPY spectrum (in pink) is lower resolution of course.
Cheers
Robin
Spectra from the past two nights. (The first predates the one in the ATel by 4 hours) I have marked the main lines but there are a few currently unidentified. Suggestions welcome
A Low resolution (R~500) spectrum taken just now (20210716.915) shows strong H Balmer P Cygni profile emission lines with a velocity of ~1400km/s and other broad emission lines including He. (Looks like it is a Nova)
https://britastro.org/specdb/data_graph.php?obs_id=10094
Continuing to take spectra.
Robin
Some more examples
http://www.astrosurf.com/buil/forum18/_transmission_TELESCOPE.png
From a thread entitled ” A la recherche des photons perdus”
https://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2277
There have indeed been various discussions on the ARAS spectroscopy forum about some modern coatings (on mirrors, lenses and camera windows) which while very efficient in the visible, drop off rapidly in the UV, for example this plot for the GSO RC telescope which effectively cuts off steeply at 4000A
https://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2398&p=13202#p13470
This has been a big issue for the UVEX spectrograph, summed up by Christian Buil in the same thread
The effect of an off axis stop is an interesting one. The star image size will remain the same (assuming seeing limited) so the maximum theoretical resolution is unchanged. The defocussing effect of field curvature should be less due to the increased depth of field with the higher focal ratio and the chromatic coma should also be less with the less steeply convergent beam. I did suggest this to someone recently though in a similar situation and they reported no obvious improvement. Best approach I would say is to see what you can get on the existing setup and then try a stopped down aperture. (What spacing do you have currently?)
If you are limited in how far away you can mount the grating then the SA200 can give you the dispersion you need for useful resolution but it does not quite match the SA100 at double the distance. The field curvature is also more severe and you can see this in your spectrum as it widens towards the red end. Getting the best result is a compromise with what is a primitive setup but think of it as good inexpensive way to “cut your teeth” before upgrading to a “proper” spectrograph if the bug bites
Sorry for any typos, I am on holiday and using a portable device. Normal service should be resumed next week.
Yes it is standard practise to align and stack shorter exposures with slitless spectra (on the zero order image) as it avoids guiding, effectively using the same techniques as planetary imagers to select the sharpest images (but no further processing eg sharpening etc) modern CMOS also means the noise penalty is lower than with CCD
Cheers
Robin
As a rough guide to what setups work I developed a simple calculator which is hosted on the Rspec website. (The Rspec software there is good for beginners using the Star analyser but you will outgrow it as you progress to more research based spectroscopy)
https://www.rspec-astro.com/calculator
There is a tutorial on the Star Analyser on the site here
https://www.britastro.org/downloads/15701
and if you are interested in its development, I talk about it here
https://britastro.org/video/11250/12234
Cheers
Robin
You ar in the right place, I developed the Star Analyser
P Cygni is an excellent test target It shows nice clear emission lines of H and He, ideal to focus on (focus on the features in the spectrum, not on the zero order star image, noting that the field curvature, particularly with long spectra means that the focus will vary along the spectrum to some extent
http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm#SA200_filter_wheel
16 inch is a big aperture for a slitless converging beam setup though unless the seeing is exceptional. The resolution is at best defined by the size of the star image relative to the length of the spectrum. The problem is as you increase the length of the spectrum the aberrations of the simple setup get worse and if you try to use a lower focal ratio to keep the star image small, the aberrations also get worse due to the steeply converging beam so these setups work best on modest apertures. give it a go though and I can help steer you to get the best out of the setup
don’t worry too much about software at this stage, concentrate on getting a clear image. It is a low resolution “broad brush” type of device and most of the interesting features the Star Analyser are immediately visible. This is an example of the sort of features you should be able to pick up.
https://britastro.org/observations/observation.php?id=20201216_234948_8cabda965bfe692f
Cheers
Robin
A nice collection of spectra building up here in the Spectroscopy Database showing the evolution
The shift in the H alpha line from predominantly blue to red while still centred on the rest wavelength is curious. I wonder what that means physically
Cheers
Robin
Prof Steve Shore comments on the evolution of Nova Del 2013 based on amateur spectra here. Not simple though !
http://www.astrosurf.com/aras/novae/Nova2013Del-1.html
Cheers
robin
Very broad P Cygni H and He by the looks. Almost overlapping with the high velocity. Almost a supernova like spectrum
Intriguingly a low resolution Star Analyser spectrum by Rob Kaufman in Australia earlier today failed to show any significant emission
After clear blue skies all afternoon the clouds have rolled in here but no doubt we will have better spectra tonight
