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My image from just now (Nov 26.76) is on my members page here. It is just saturating in a 10s exposure but the unfiltered magnitude looks to be around 9.3.
I don’t have any experience with Canon mirrorless cameras but I have a Sony A7 and it works very well. I have got used to the fact that I have to turn the camera on to see things through the viewfinder and you get a lot of image sensor are in a very lightweight body. The very short back focus is also nice since, with a suitable adaptor, you can use some very good, old prime lenses that you can’t use on EF mount cameras. It is worth noting that you will need an adaptor to use your existing EF lenses on this camera since the EF-M lens mount on the mirrorless cameras is designed for much shorter back focus lenses. I hope that the Canon mirrorless cameras use the same raw format as their DSLRs do. It is a well designed format that is really raw. The Sony raw format is absolute rubbish.
Paul is right that there will be lots of opinions. Here is mine based on many years of imaging work.
My calibration steps involve having a library of dark frames of different exposures so that I never have to scale darks. Since I don’t scale my darks I don’t need bias frames. I generate the darks when it is cloudy and generally average 30 – 100 raw dark frames at each exposure to get my library dark (I do 5, 10, 15, 30, 60, 120, 300s darks). My camera has good temperature regulation and so I can use dark frames from 6 months ago and they are fine. If you have a CMOS camera you probably will find that scaling darks is not always successful so, again, it is a good reason to keep a library of darks at different exposures rather than trying to scale them.
I have always used sky flats, again taking 30 – 100 raw flats, subtracting the flat dark and then normalizing and averaging them to get the master flat. Since the right conditions for making twilight flats don’t come along very often I tend to re-use flats for a month or so. I’m lucky, I have a permanent observatory and the camera never comes off the telescope but new dust spots appear with monotonous regularity. You can see an example of that in the lower right of this image.
When you do the averaging is best if you can use floating point format output FITS files rather than 16-bit integers. This applies both for the calibration frames (flats, darks) and for your final stacked images. Certainly don’t using summing with integer files since you will end up saturating brighter stars as their summed pixels hit 2^16.
I’d just like to emphasise Jeremy’s comments. Ultimately it would be good to aim to get some filters but proper photometric filters are very expensive and a lot of work can be done without them or even using the much cheaper filters designed for colour imaging.
If you are interested in trying something slightly different you might want to look at our project to monitor the comet 29P/Schwassmann-Wachmann. Have a look here for details. This comet is well placed at the moment and normally sits around mag 16-17 but it can flare up and catching the eruptions early is very important. Richard Miles runs this project on behalf of the comet section. It might be a little faint for your kit but try some images of the field and see what you get.
It is a real shame that we have got to this point but I think it reflects the fact that many people now expect to be able to browse the web and get information for free. Most sources of quality journalism are suffering the same fate. Some publishers have embraced the low-cost route but many of their current books look like printed versions of someone’s web page.
The Internet has been, generally, a great force for good but quality publishing has taken a big hit. I’m not so sure that the democratisation of news and comment has been a particularly positive thing either.
Here is one of the superhumps from tonight. Unfortunately the sky clouded over at the end of this run.
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.
