Hi David,

I would not bother attempting flat correction of slitless spectra. Normal flat correction techniques do not work because each point is a mix of spectral and spatial information. (The technique used on professional setups is complex and involves generating effectively a "flat cube"  for each wavelength at each point in the field.) My advice would be to take a conventional flat beforehand to check for any obvious dust donuts etc and place your target and reference star zero orders as near as possible on the same pixel avoiding any dust spots so any gross flat effects eg vignetting etc cancel.

Cheers

Robin

Thanks Robin, I'll do as you advise. Here's a not so egregiously black-clipped image. I'll have a look at the pixel brightness values to see if I can get anything reliable from the green pane.

https://britastro.org/node/23563. I get 12.6 unfiltered last night using Gaia DR2 G mags. 

Here is a spectrum of Nova Cas 2020 recorded with a LISA spectrograph on a C11 SCT and calibrated in absolute flux using a concurrently measured magnitude of V = 13.395. Using PlotSpectra, the integrated absolute flux of the H-alpha and H-beta lines are 3.53E-12 and 2.52E-13 erg/cm2/sec respectively. These results are consistent with the values 3.54E-12 and 2.54E-13 reported in ATel 13905. The apparent FWHM of the H-alpha line is 508 km/s and there is evidence of weak P Cyg absorption on the blue side of H-alpha extending to -800 km/s relative to the peak of emission. There are also emission lines of Fe II 5018, Fe II 5169, [O I] 5577 and an unidentified emission line at ~6160 A.

David

I saw the nova last night and estimated the mag. to 12.7. 

I find the following from this morning using T5 (250 mm) in New Mexico at JD 2459062.902. I used AstroimageJ

With V filter, nova was 12.9 using 11.2 ,14.3, and 12.6, reference stars in AAVSO chart

With R filter, nova was 12.4 using 12.6 in AAVSO chart

With B filter, nova was 13.3 using 12.6 and 13.7 in AAVSO chart

Seems quite bright through the red filter. Stewart

I make it 11.8 mag through V filter this morning, Aug 5th at 01 :43. (T18, Nerpio, Spain.)

I make it 11.1 mag through the R filter.

For both measurements the reference star was 11.2 on the AAVSO chart. I'll try to get these uploaded via Muniwin once I get the hang of it.

I took images last night through a V filter and measured a calibrated stack in Maxim (AAVSO X25582AO) 

N Cas 2020 measured 11.548 CCD-V

REF 112 set 11.186

CHK 126 measured 12.599

This is a spectrum from the early hours of 12th August. It looks very different from those of the 30th and 31st July in this thread. The H alpha line is much reduced and there appears to be a P Cygni profile, presumably due to expansion of the nova. I haven't tried to identify the other broad absorption features. I would be interested in other people's comments on what this spectrum tells us about the evolution of the nova. I see from the database that others have also noticed the changing profile.

Observations from last night and this morning suggest Nova Cas is rebrightening after a very short four day decline.

At this stage the light curve resembles several other Nova I have observed over the years - V1493 Aql (N Aql 1999#1), V2274 Cyg (N Cyg 2001) and the Fe II Nova V2362 Cyg (N Cyg 2006), although the time scales of fading and rebrightening are different.

The light curve below is from visual observations and unfiltered CCD from SLOOH.  Thank goodness for remote scopes - the night sky here in Brum has been very cloudy of late, although I did have a forty minute gap last night (17th).

Gary

The H alpha line has become very prominent again and the P Cygni profile has disappeared. There appear to be emission lines for Fe II, He I and [O I]. The spectrum was recorded around midnight on 18th/19th August with an Alpy 600 and the resolution is ~ 550.

Interesting passing comment in the latest ATel 13967 on the nova:

Quote:    "...dust formation will almost certainly take place sometime in the next several weeks."

So I guess a dramatic fade can be expected??? Well worth watching at every opportunity.

Mike.

The Nova is now fading from it's second maximum, very slightly fainter than the first.   Most unusual.  Reminds me of V5558 Sgr (Nova Sgr 2007).

Gary

H alpha is still prominent, possibly even brighter. It needs someone cleverer than me to do a flux calibrated spectrum. I've attached a plot of three spectra taken over the last three weeks to show the changes. This object continues to fascinate me. Any interpretation of what we are seeing would be welcome.

If ever we get a clear sky in Leicester, I’ll take a look. My 12” Newtonian should pick it up.

John,

I calibrated your 3 spectra in the BAA database in absolute flux using V magnitudes interpolated at the times of your spectra from V magnitudes in the AAVSO photometric database. I have labelled the main emission lines which include the Balmer series of hydrogen recombination lines and collisionally excited lines of singly ionised iron Fe II. The latter have strengthened during the recent rebrightenings of the nova. There are also forbidden lines of oxygen [O I] formed in the lower density expanding nebula.

David

Thanks very much for that. Maybe next year you will be able to teach us how to do this, perhaps in another spectroscopy workshop.

Last nights spectrum shows P Cygni profiles in He I, Fe II, Na I and [O I] which have deepened or appeared since 5th September.

1920p

 I have the following for V1391 Cas  from  23 Sept 2020:

V filter  12.95,

B filter 14.4 ,

I filter    10.19

Values are in the database. Stewart

It looks as if there are P Cygni profiles in sodium. Is the sodium interstellar, and if so, what causes the P Cygni profile? Is the emission component due to photoexcitation by UV from the white dwarf or some other mechanism? Thanks. 

Recent Astronomers telegrams report that dust formation is happening in this nova explaining the recent deep fade.  See ATel14272 and ATel14267 for details.  Also recent images illustrating the fade are on my members page.

This classic DQHer dust dip started slowly in late November and increased in mid December.   Here is my plot so far...

Gary

Imaged the nova last night and it is barely detectable now.  Presumably if it does behave like DQ Her it should brighten again in the next month or so?  Well worth keeping an eye on...!

Mike

I've seen a number of 'dust dips' over the years, and 6 weeks in low state seems to be a fair average time before a rise sets in again in the very obvious ones.  There are exceptions -  for example, V2362 Cyg (Nova 2006) faded 4 mags from 8-12 before a 16 week plateau set in, then rebrightened to mag 10 before a very rapid decline followed by a gradual fade to quiescence.  This was a dusty Nova, but the hot dust was probably confined to the inner region of the system.   Going on the quite unusual behaviour so far seen in V1391 Cas, it will be interesting to see what happens next over the coming months.

Gary

The nova is just visible on an image taken tonight at around mag 20. I've not seen it on stacks with a similar limiting magnitude to tonight's image so it may be on the rise again.