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I get 14.87 tonight (2020-11-10.8) which is the brightest I’ve seen it. All of my recent obs are in the VSS database.
This was my first opportunity to image the field. The sky was transparent but the Moon was bright and the wind was strong. Quite a lot of my subframes were badly trailed but this is what I managed to recover. The nova would be in the blue circles, comp stars are in the red circles. This is an unfiltered image referenced to Gaia DR2 G.
Alex,
Did it give that number to 6 significant figures too?
I would take that 15 arcsec with a rather large pinch of salt and it would certainly get you into a religious argument with certain people. I remember an endless discussion with a certain Mr. CJRL on this subject many years ago.
At that level of precision people will also argue about whether you should point the polar axis at the refracted pole or the true pole (they are around 50″ apart at 52N) or somewhere between the two. Even tiny amounts of differential thermal expansion can move the polar axis by many arcsec. I suppose at least Leeds doesn’t have many earthquakes to worry about.
This is in one of my lesser patrolled fields around M31 (my field 15). I try to get a deep image of the core of M31 every available night but I’ll keep an eye on this field too. Nothing there at the moment.
It is difficult to get a reliable magnitude but 2020 vak was around 16.6 (unfiltered vs Gaia DR2 G) on my image of Oct 26.8.
You’re right. I tend to be very lax and use the term nova for anything that varies in M31. Transient would be a much better term to use. That spectrum doesn’t look very nova like. It is very close to the core though so how do you stop the spectrum being polluted by the background?
Here it is on my patrol image from early evening on Oct 26. M31 novae are very frequent and discovering them is very competitive. This image also shows AT 2020vak much closer to the core.
Don’t get me started on NASA’s “discovery of xxx” press conferences and their wonderful mixture of units. From the BBC article: ““The amount of water is roughly equivalent to a 12-ounce bottle of water in a cubic metre of lunar soil.” I suppose it is not as bad as saying a bath full of water in something the size of a house. Indeed, over half a pint of water in a cubic metre of soil does sound an awful lot though. My garden possibly had less than that at some point in the last summer! I suppose I should really look at the Nature papers rather than rely on second hand reports on the BBC but life is too short…
Astrometrica reports two different SNRs for the same measurement. In astrometry reports it uses something that Herbert Raab calls “peak SNR”. This is the SNR in the single brightest pixel. In the photometry report it uses what we would think of as the normal SNR definition (i.e. the SNR in the photometric aperture). Both I think use a process similar to what we have been discussing on this thread but I only really care about the photometric version since that one (or actually the log of it) is included in ADES astrometry that gets sent to the MPC. I’m trying to find out how Astrometrica estimates the gain, G (e-/ADU) from the image since the photometric SNR it reports does not scale correctly.
All the discussion about estimating the sky standard deviation is very interesting but for bright sources the photon noise dominates and getting G wrong really messes things up.
The one they use in VPHOT, described on page 4 here. Essentially you calculate the mean and SD then reject pixels more than 3 sigma from the mean and repeat until no more pixels are rejected. In the past I’ve used the median to get the sky level then used pixels below the sky level to determine the SD.
It looks like gain is normally defined upside down (i.e. e-/ADU rather than ADU/e-) which is odd but would explain the reciprocal.
Mean has better properties than median so I use an iterative approach which calculates the mean and rejects outliers.
Thanks Dominic. Well beyond the call of duty to be doing stats at 2am!
That all looks reasonable. M/(M-1) and (M+1)/M aren’t quite the same but they are very similar and in any case tend to unity for the real situation of largish M. For the photon noise SG makes sense when G is defined as ADU/photon.
One further practical question about estimating G from the image. I am using the magnitude zero point derived by fitting an ensemble of stars to a catalogue. This links the magnitude (and hence photon flux) to the ADU count. Is there an alternative/better way of doing this?
I’ll try this later and will see if I can start getting some consistent SNR estimates.
If you really want to put delta-T in a program then there are polynomial approximations such as the ones here. As has been said though you are unlikely to require this for lunar phases. It is important for high precision ephemerides of relatively close objects (including satellites) since it does affect the parallax correction and, as Dominic says, it is crucial for calculating total eclipse paths. It is probably worth including in telescope pointing algorithms too. Should anyone ever decide to remove leap seconds from UTC it will become important for the general public in a few thousand years!
The attached are updated elements of 2020 SW including my astrometry from last night. These should help if you are using any program which uses the MPC elements. This is actually a text file but for some reason you can’t upload those.
I managed to get some astrometry of this very small object this evening and it was a bit off track. An updated ephemeris is attached. Tonight it is around 19.7, tomorrow night (Tuesday) it will be 18.6 moving at 5″/min and Wednesday night it will be up to 16th mag but accelerating through 30″/min. It then moves rapidly south and we lose it as it comes within the geostationary satellite belt for a short period around Sept 24.45.
Note the ephemeris is for my location and the position will be different for you. This will become more significant the further from Essex you are and the closer the object gets. Let me know if you want an ephemeris for your location.
Here’s an image tonight in pretty good seeing (FWHM = 2.4 arcsec). It is in a very crowded field and, as Gary says, now it has faded it is difficult to separate from the star just to the west.
Here is a link to an animation showing how the star and nebula have changed over the last two years. I don’t think animated GIFs work in this forum.
