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Hi Peter,
The spectrum profiles you attached have a problem with the wavelength calibration which the graphs you posted do not so I went back to your spectrum image and recalibrated that in wavelength first. I then calculated the instrument response and applied it to the raw spectrum to produce a final instrument response corrected spectrum. You can see below the raw spectrum (blue), the instrument response (orange) and the final result (green).
As a check I also compared the final result (green) with the reference spectrum (pink) which I filtered to match the resolution of the measured spectrum. You can see the good agreement in the shape of the continuum which confirms the instrument response is working correctly.
I have attached the steps I took to produce the final spectrum in the attached pdf. I used Vspec as it is easier to see the individual steps but you can use any software package to produce the same result.
I hope you can follow this ok
Robin
Hi Peter,
You are definitely making progress and are almost there with this result. The wavelength calibration is good and the instrument response is approximately right but there are a couple of improvements you can still make.
If you compare your final response corrected spectrum (blue in the last graph) with the library spectrum (purple),there are some humps in your spectrum between 500-5400A which do not appear in the library spectrum. The continuum at the short wavelength end agrees well but at the long wavelength end, your spectrum drops below the library spectrum.
Can you post your uncorrected and corrected spectra fits files (blue in the top and bottom graph) as attachments (change the extension from .fit to .fits so the forum accepts them). and I will have a closer look at where the problem might be.
Cheers
Robin
Hi Nick,
Yes the Fermi coordinate uncertainty left a lot of sky to search but one of the papers in Gary’s reference describes how this was managed by one of the teams.
http://iopscience.iop.org/article/10.3847/2041-8213/aa8edf/meta
Remarkably the combined data from the three operational GW detectors constrained the location considerably (and more accurately) compared with just the Fermi data. (fig 1 in the above paper). The DLT40 supernova survey team then prioritised imaging their 20 target galaxies which fell in that region.
Robin
About 10 years ago I was interested in trying to image a Gamma Ray Burst optical counterpart and subscribed to alerts from the Gamma-ray Coordinates Network (GCN) Circulars. Having caught one I then lost interest. http://www.threehillsobservatory.co.uk/astro/astro_image_81.htm
I’ve just had a look in the GCN circular archives around the time of this GW event and there was a lot of chatter there about it eg https://gcn.gsfc.nasa.gov/gcn3/21505.gcn3 so that could be a good place to monitor for future events.
Robin
Interestingly the detection by several pro supernova survey teams of the optical transient was submitted to the Transient Name Server, (which generates alerts for possible supernovae and which I monitor it for potential candidates for spectroscopic classification.)
https://wis-tns.weizmann.ac.il/object/2017gfo
It was mag 17.3 at discovery which would have been just bright enough for a spectrum with my setup. The TNS submissions and subsequent spectrum which showed it to be something unusual, was embargoed for 2 months though so it only appeared on TNS today giving no chance to observe it. Had an amateur picked it up in the meantime though (eg as part of a supernova patrol) and posted it on TNS, they would have been credited with the discovery as IAU recognise the first to publish as the discoverer.
Robin
Here is an updated plot including the latest spectrum from last night. There has been no further change in the past 9 days so it seems likely that the ingress phase is now complete at this wavelength.
Robin
Hi Kevin,
For supernovae I use a specially modified ALPY with a lower dispersion GRISM which allows me to get down to ~mag 17.5 compared with ~mag 15.5 with the standard ALPY with my C11
http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm
Cheers
Robin
sn2017gxq was bright enough for the ALPY 600 last night despite the full moon. Here is the spectrum (black) overlaid on the best fit according to SNID
https://people.lam.fr/blondin.stephane/software/snid/
which predicts it is now just past maximum.
Robin
Thanks Paul,
Cold wet winters I am familiar with but long hot summers are just nostalgic childhood memories for me 😉
Robin
Hi Paul,
That’s interesting. I have never changed the desiccant in my cameras and I am not sure how to, though I notice that my latest camera ATIK 428 does have an externally removable plug. My ATIK 314 is 6 years old and the ATIK 16IC-S is about 10 years old now. Perhaps it is a climate thing ?
Robin
Hi Jack,
There are various manufacturing techniques, either mechanically ruling the lines to produce a master. (The gratings you can buy are then cast replicas reproduced from the master) or generating them photographically using holography. The free Richardson “diffraction grating handbook” is a good reference
http://www.gratinglab.com/Information/Handbook/Handbook.aspx
(they will send you a hardback copy or a google search thows up an (unofficial) on line pdf version for edition 6)
though it does not cover volume phase gratings for example where the “grooves” are actually variations in refractive index in the material, again produced using photography and holography
https://www.noao.edu/noao/noaonews/jun98/node4.html
Robin
Yes the Sony ICX825 in the ATIK 414 etc shows severe ripples in high resolution spectra but perversely is the best CCD for low resolution work in this respect, even better than the CCDs used in the ATIK 460/428
Robin
Note also that that the ALPY optics are specially optimised for the violet end and the sharpness deteriorates noticeably in the red/IR in any case due to chromatic aberrations. (The early ALPY’s had an IR blocking filter to stop people operating in this region until it was removed after I pointed out the effect the terrible ripples in the passband of the filter had on the instrument response)
Hi Paul,
I’ve not seen this problem with any of the atik cameras I have used but I think rotating the camera 180 deg should quickly show if that is the problem. A similar effect can produced if the CCD is not mounted precisely square (ISTR some cameras having an adjustment) or the camera adapter out of square, though that would not normally be a progressive deterioration and if this is the case it would still be possible to focus well at any particular wavelength, just not everywhere at the same time.
Cheers
Robin
That’s great David, topping the brightness charts currently. It is easily bright enough for the standard ALPY 600 now. Let’s hope the promised clear skies turn up on Thursday.
Robin
Hi Kevin,
I have just seen that the iPTF team have now classified it (type 1a) so no need to go for it, unless you want to of course.
https://wis-tns.weizmann.ac.il/object/2017haf
Interestingly the “SED machine” spectrograph they use is even lower resolution than mine. They are definitely a rival 😉
The location looks spot on to me in CdC if I overlay the DSS image so probably an epoch issue. (The TNS coordinates are J2000). It is quite close to the galaxy core though so separating it from the galaxy could be a problem.
Cheers
Robin
I think VSnet takes the photometry data but you can submit an image with or without a magnitude directly to David Bishop,(at least that what I used to do !)
dbishopx at gmail.com
Robin
Nice image David. Photometry could be useful. Do you plan to submit it to David Bishop’s website? there are no measurements or images for this SNe there yet.
http://www.rochesterastronomy.org/snimages/
Only clouds and bands of rain on this side of the country tonight unfortunately.
I suspect it is close to maximum now and since it is a type 1a I had a go at estimating how bright it should get to at maximum.
According to NED http://ned.ipac.caltech.edu there are no redshift independent distance measurements for ngc4964 but based on the redshift the distance would be ~36Mpc. (Though for a relatively nearby target like this the errors could be high)
Using 36Mpc and an absolute V mag of -19.3 at maximum for type 1a SNe gives an apparent V mag at maximum of ~13.5 (not taking into account any extinction) so your mag 14 guestimate seems reasonable.
Robin
Thanks !
I have got quite adept at swapping between the LHIRES and ALPY now. They have separate cameras (Currently the ATK 314 and a stellacam on the LHIRES and ATK428 and ATK16ic-s on the ALPY) and are all wired up so I just have to physically swap them over. The one not in use sits in a box next to the scope. I have even got duplicate 200/600 modules for the ALPY now as well which can be changed over without needing readjustment other than aligning the camera angle. I tend to stick with one setup on any particular night though.
@ Paul,
Sure, no problem. Let me know if you need any specific graphics, images etc
Cheers
Robin
https://wis-tns.weizmann.ac.il/object/2017gxq
This was discovered by Gaia on 17th at mag 17 and announced on 28th Sept but was much brighter last night (~mag 15 or perhaps brighter) last night when I took the spectrum and is still likely to be a few days from max so would be a good early evening standard ALPY candidate.
Robin
