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Hi Grant,
You are probably aware of the material below, as it has a long history, but it is posted here for the amusement and possible education of others.
Zeroth Law of Optimization: First get it right, then get it fast.
You can do anything arbitrarily quickly as long as your code is not required to get the right answer.
First Law: Intelligence trumps code.
Use a good algorithm. Optimizing a bad algorithm is akin to polishing turds.
Second Law: Know what is going on.
Unless you profile your code and take timing measurements most everywhere, you don’t have a clue what is worth optimizing.
Third Law: Don’t do it.
Only calculate stuff you actually need. You might be surprised at how much extraneous garbage is computed in libraries, etc, without your knowledge.
Fourth Law: Don’t do it now.
Covers a number of issues, including lifting computations outside loops and storing stuff in registers or cache-friendly memory.
Good work Grant!
Could you point me at the source code please, or perhaps mail me a copy? It has been a few years since I last wrote GPU code (CUDA then, ought to be OpenCL these days) and perhaps I may be able to get off my backside and do something if given a sufficiently large kick incentive.
There are tens of thousands of images stored on a server at home in the UK and it would be nice to see what can be dug out of them.
Thanks.
I made the claim As long as any satellite trails (or cosmic ray hits for that matter) do not intrude on the object or its immediate neighbourhood, our work is completely unaffected. Here is some supporting evidence which turned up quite by chance.
Below is an image from my work on U Leo and is the average stack of 32 exposures each of 30 seconds duration, two of which caught a very bright satellite in transit. The gap in the trail shows the movement during period the CCD was downloading its data between successive images. Even when diluted 31-fold the trail is still very obvious. U Leo (the central marked star just to the right of the trail) and it comparison stars are identified; none lie anywhere near the trail so photometry is unaffected. The VS is measured to be V=17.52 +/- 0.05 at 2020-01-21T00:30
Unfortunately the software released is Windoze only, which rules out a large number of astronomers. Oh well.
Looks like a free implementation should be created. Time to track down the references given in a paper on the Tycho site.
You may wish to consider running Linux on your Mac. In my experience, which is extensive on all three operating systems, Linux is by far the most developer-friendly environment. In one sense, Linux is closer to MacOS than either are to Windows. MacOS is a fancy graphical environment built on top of BSD Unix. Windows is starting to catch up now that Linux binaries can be run natively, notably the bash shell.
With the recent demise of support for Win7 the migration route to Linux for my TCS is well advanced. As far as I can tell everything I need for imaging, guiding, pointing, focussing, etc, runs well under Linux.
“Given this star is rather faint, at mag 17.3, it is a target for CCD observers. It will require long and unfiltered exposures to get a reasonable signal-to-noise ratio.”
It is rather tedious sitting here waiting for photons to arrive so I am trying to get ball-park estimates of what is likely to be achieved. As the autoguider on the 0.4m Dilworth / unfiltered CCD combination still doesn’t work I restrict each exposure to 30 seconds. The sky tonight is fairly dark but not exceptionally so. The SX 814 has a very low dark noise and so the major limit on SNR is sky glow. I have found that averaging 20 subs, for a total exposure time of 10 minutes, gives a SNR of 50 to 55 when U Leo is at altitude of 60 degrees. It will be worse at lower altitudes, of course, or if the Moon is above the horizon. A SNR of 50 corresponds to a precision of 0.02 magnitudes. The light curve in the VSS Circular suggests that the peak-to-peak amplitude is perhaps 0.1 magnitudes. A period of 3.2 hours corresponds to 192 minutes. Accordingly I can hope for at most 19 samples per period at an amplitude precision of 20%. In practice it will be less because of download times and inevitable discarded images from poor tracking.
It is going to take a lot of heavy duty signal processing to get robust results from my observations alone. If I had perhaps ten times the data I should be able to get something useful. We need lots more observers, in other words.
One unexpected side benefit is that U Leo lies in a rich field of galaxies, including a bright face-on spiral known as 2MASX J10234921+1357083 (it appears on Jeremy’s finder chart roughly halfway between U Leo and the bright star in the lower left corner; an edge-on spiral of around 17th magnitude lies to its left). Only a few hints of spiral arms have appeared on a large stack but a “pretty-picture” may yet be possible. The galaxy’s magnitude is g=15.9 and r=15.2 so it shows up on every single sub. There are many more galaxies brighter than 20th magnitude within a few arc minutes of U Leo; they are starting to appear in my data as the stacks get deeper.
Added in edit: since typing the above text the SNR has almost doubled. Presumably the sky has become notably brighter for some reason I do not understand. Perhapsthe signal processing may be easier than feared.
OK, CV it is then. The sequence for AT 2019xim must have been added after I took the data, which is not surprising. My observations were completed within 48 hours of the ATEL being released.
My SOP for all variables except the very brightest is to take 30-second subs and stack in real time. That way I can estimate the SNR and move to another object when it is high enough or very obviously too faint to be measured with sufficient precision. Stars which can vary by several magnitudes between successive observations can easily turn out to be saturated or invisible if purely dead reckoning is used to set exposure lengths.
I took 100 minutes of unfiltered 30s images in the wee hours of 2020-01-20, though the SNR is so low that to get adequate SNR they will probably have to be co-added a few at a time. Another run took place on 2020-01-04. They, as well as some precision photometry in Johnson V and Sloan r´, still need to be processed. Perhaps this data is too early for the INT run; I will try to take some more in the next few weeks.
A question arises from our earlier discussion in the forum about the use of non-standard sequences for VS observations to be submitted to the BAA-VSS database. There is no standard sequence for unfiltered data. Advice would be welcome, whether here, in the original thread, or off-line via email. Another case without any standard sequence concerns the observations of AT 2019xim reported on my members page. I have specifically withheld submission of the results for this reason.
contains two, one at 143 and the other at 153. Both are inconveniently distant at around 15-20 arcmin (a guess from the chart scale – I have not measured them).
Personally I´d use them to measure the instantaneous brightness of other stars closer to the SN and then measure the SN with respect to them. If the images of the fields are taken within a few minutes of each other, it is very unlikely indeed that all the secondary comparisons are variable on that timescale, and not very likely that even one will be. If one or more is a LPV, who cares? It’s constant for all practical purposes.
If you discover that one or more field stars varies from night to night, that in itself is worth reporting.
I am now at the telescope taking a long series of exposures and a satellite or meteor just went through a frame. Very faint. Stars down to 17th mag are easily visible but the trail only just shows up above the sky noise. Guess 15th or so? It is going to be completely invisible in the final stack.
Not sure how relevant this is to the current discussion but it shows that we don´t need Starlink to be able to pick up the present vermin of the skies.
Good point.
Presumably one could monitor the field and avoid taking data during the satellite pass. Could be an interesting exercise writing software for motion detection in the autoguider camera.
How do you deal with satellite trails now?
Depending on whether I am doing photometry or imaging (which includes astrometry for present purposes) I either discard the sub before average-stacking or rely on median-stacking to discard the satellite for me. In either case, such concerns are of interest only when the satellite passes inconveniently close to the objects of interest, otherwise it is just ignored.
There again, I take as many 30 to 60 second subs as are needed to reach a satisfactory SNR (which depends on sky brightness, for instance) and then move on to another target. A cheap and simple way of optimizing the productivity of telescope time. Vary rarely does a single exposure exceed a couple of minutes.
I think that post can be counted as a success and that my client, often known as The Great Deceiver, will be well pleased.
You appear to have been led to believe that a reduction by a factor of pi in the number of degrees in the complete sky is an improvement. In terms of the average number of satellites per square degree, however …
😉
I am going to act as Devil’s advocate because someone should do so. The very phrase “Devil’s advocate” should indicate that my personal views are not necessarily in accordance with what I espouse below and in subsequent posts.
First, 4pi steradians is equal to 41,253 square degrees. We are already down by a factor of pi ( about 3.14) from your estimate.
Secondly. not all astronomers, professional and amateurs, are wide-field imagers. A good fraction of us perform precision astrometry and/or photometry. As long as any satellite trails (or cosmic ray hits for that matter) do not intrude on the object or its immediate neighbourhood, our work is completely unaffected,
More Pollyannish sentiments may appear in due course. This will have to suffice for the time being, not least because dinner is now being served.
“go to the website and register your observatory as a future participant mentioning in the ‘Comments’ box that you are an amateur astronomer and that you are joining the BAA Exoplanet Division’s initiative to support the ExoClock Project.”
Been there, done that.
Clear skies are erratic in these parts. It was (mostly) clear last night but with ferociously high katabatic winds so no observing was done. In those conditions the seeing is typically 15-20 arcsec and the scope flaps around on a similar scale, ruining tracking, even though it is inside a dome.
Not been very good here in LP either. To be fair I am only here half the year and I have had health and equipment issues which have curtailed my observing time, but even so …
I blame that Thunberg woman for drawing attention to global warming with its consequent increased cloud-cover, which itself arises from a larger capacity for the atmosphere to hold on to evaporated sea-water.
😉
Added in edit: the phrase “and equipment”
Thanks for your in-depth explanations. I´m learning!
Although I’m a molecular spectroscopist by background I am emphatically NOT an astrophysicist. You (personally, not the generic “you”‘) can´t resolve the rotational and any hyperfine substructure of the molecular bands, which is where I cut my teeth. Not entirely sure that anyone can. Betelegeuse is bright enough for spectral resolutions of 100K-1M (my doctoral work was at a resolution of around 300,000) but do the physical conditions in the star’s atmosphere allow that kind of line resolution? That was a rhetorical question. I would be delighted to learn that rotational structure is readily observable, not least because the effective temperature of any species in question could then be nailed down to a very few Kelvin.
Roughly half my DPhil thesis concerned the rotational structure in the spectrum of CeO at ~2300K. Its spectrum is mind-bogglingly complex for such a simple diatomic molecule. There are at least eight low-lying electronic states with populations high enough to exhibit absorption spectra at 2300K. Well over 100K lines in the absorption spectrum between 300nm and 1200nm were measurable with 1980´s technology. CeO is also known to be an atmospheric constituent of a number of cool stars.
If the radius dropped I would expect the temperature to rise, not fall, as the gravitational potential energy is converted to thermal kinetic energy.
Again, curious.
If the temperature had dropped I would expect the continuum to have shifted too — Wien´s law — though as that goes only as the first power of (1/T) perhaps the effect might be too small to be easily noticeable. I certainly haven´t noticed it from your spectra but there again, I don´t have much experience in these things.
I´m now wondering whether a neutral grey filter has interposed itself between us and the star. Something akin to the clouds of dust which appear in the atmospheres of RCB variables. The typical particle size would need to be significantly larger than the wavelength of light or we would see severe reddening.
Curious indeed.
