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According to http://convertalot.com/asteroid_impact_calculator.html the energy yield of that impact, with an assumed relative density of 3 (about that of a stony asteroid) is 75 megatons TNT which will dig a crater a kilometre in diameter and a quarter of that in depth. The calculator assumes a terrestrial impact. The lower gravity on the moon ensures that somewhat more (but not a lot more, because mass is proportional to the cube of the size of the excavated material) excavation can be performed at the same energy cost.
I do not know the depth to which it would penetrate before exploding, partly because it depends on the structural strength of both the impacting body and the lunar regolith, but note that until the asteroid hits something it travels its own diameter in 5 milliseconds, so perhaps 500m might be a reasonable guess, a distance which requires a travel time of well under a tenth of a second.
See https://en.wikipedia.org/wiki/Aperture_Photometry_Tool for an introduction and a download link.
You are quite correct in that a larger circular aperture can be used. Unfortunately that will include a larger amount of sky and increases the likelihood that the sky annulus will itself contain stars as you indicate.
Elliptical apertures really come into their own when measuring galaxies but there is nothing to stop one exploiting that feature in other circumstances.
The best way of doing photometry, in my view, is to use PSF fitting as implemented in DAOPHOT because then one is fairly sure that you are measuring only the absolute minimum of sky and that stars within the sky annulus are properly accounted for. Unfortunately the only code I have yet found is labour-intensive and very hard to script for reliable use.
Here is another version. Nowhere near as good as yours, not least because the limb was drifting out of the FOV, but I find it really quite remarkable because it was taken with an iPhone pointed at a 25mm eyepiece of a 25cm aperture 1200cm FL undriven reflector. Took two people acting in concert, a friend holding his phone to the eyepiece and taking the exposure and me driving the focus on the phone when the camera was pointing in the right direction. It’s sufficiently encouraging that perhaps I’ll get a proper phone holding gizmo.
That is excellent data, thank you! Just what I was looking for.
I recognize the difficulty of extrapolation but for present purposes any number which is good to within a factor of two is easily precise enough. For instance, 10s for mag 11 tells me that 30s should give about mag 12 because a factor of 3 in flux collection is close enough to the 2.5 times lower flux from a target one magnitude fainter. The precision value you quote is also encouraging. I chose 0.1mag as an upper limit because that corresponds to the best which can be reasonably expected without a camera.
Last night an interesting observation of the moon with an iPhone and eyepiece projection suggests that using the 25cm Dob with the DSLR at prime focus might be feasible. It might not be, but it is certainly worth trying.
BTW, have you come across Russ Laher’s Aperture Photometry Tool (APT)? It runs on any OS with Java installed and can told to use elliptical apertures — just the thing for poorly tracked images.
Hi James,
Could you report your practical limiting magnitude of your equipment please? For example, what is the faintest you can reliably measure to within 0.1 magnitudes after a 10 minute exposure?
I’m toying with getting a small equatorial mount for a Canon DSLR / telephoto lens combination and your experience will help set expectations for a roughly 50mm aperture.
There are limits to what can reasonably be extrapolated from something approaching two orders of magnitude greater light grasp!
“In my experience, size isn’t everything. it’s how you use it that counts” and “There are a hell of a lot of variables brighter than 16th magnitude.”
Have you thought about monitoring stars for exoplanet transits? Some of their stars are quite bright and within range of a 100mm scope.
Of course, if you send the same amount of money on a reflector as on a refractor you could get a markedly larger aperture … This assumes you haven’t already bought your equipment. If you have, there are still many things you can do with photometry on what you have.
In my experience, size isn’t everything. it’s how you use it that counts. With a 100mm aperture you are not going to be making observations of 20th magnitude stars any more than I am of 23rd targets with my 400mm, which estimate is based on a roughly 3 magnitude difference in light grasp. I can perhaps manage useful observations (an accuracy of 0.1 m) down to mag 20 if I am prepared to spend enough time (hours!) on the exposure. You could manage perhaps mag 17 with a similar commitment but 15 to 16 should be entirely straightforward.
There are a hell of a lot of variables brighter than 16th magnitude. Some are so bright that they will saturate your detector and so are effectively unmeasurable.
What you really need to do, IMAO, is to start observing, learn what your equipment can do and to learn how to analyse your data. Your equipment is capable of producing research quality data far beyond that of any amateur set-up as recently 50 years ago.
I would also recommend contacting the VSS, but I’m biased. I found it extremely helpful.
Go for it!
Thanks for drawing this to my attention. It is fascinating stuff which will require much study to do it justice.
A line which caught my attention comes from the extensive bibliography:
Bruno, G., 1584, ‘De l’infinito, universo e mondi’ (On the Infinite, the Universe, & the Worlds)
Very few scientific papers contain references to work performed before 1800. This one contains several.
Faced with the difficulty of getting to La Palma where my observatory is located, I purchased a 10″ Dobsonian cloud maker on eBay so that I could at least try to do something.
It was dispatched on Friday and will likely arrive just in time for the requested observations.
Sorry folks, but tonight is probably the last clear night until June, by which time it won’t get dark at night anyway.
(Added in edit) It arrived a couple of hours ago, and very effective it is too. Rain has been persisting it down since last night.
I agree, except that long years experience in the field of computer security has taught me that what the organizer may consider “local” is profoundly ambiguous. All too often it is just plain wrong. In the case I noted, the organizer was just plain wrong: she explicitly stated “GMT” but meant “BST”.
There are other considerations. For example, the term “EST” is ambiguous, By and large it means UTC-5 but is very frequently interpreted as UTC+10 in Australia.
That is why CERTs are very strongly advised to use UTC and ISO-standard date formats in their communications.
We are all, notionally at least, astronomers and invariably use UT to report events we have observed. Predictions of future events, such as are published in the BAA Handbook, are invariably given in UT.
Can those who organize events for a future date and time please explicitly specify the time(s) in UT?
I missed an exoclock meeting because the organizers quoted 16:30GMT when they meant 16:30BST. I was not the only person to do so. Had the time been advertised as 15:30UT all would have been unambiguous.
This comet is no more. It has ceased to be. It’s a stiff. Bereft of life, it rests in pieces. It is an ex-comet.
I had similar problems when using the PaleMoon browser (a Firefux derivative). Logging in again fixed it for me.
Seems strange that three separate browsers running on very different hardware should all have the same issue…
“launching astronauts to the International Space Station on Crew Dragon at 21.32 on May 27th”; “The UK has an ISS pass at 21.20,”; “and the[n] go into garden to wave them on there [sic] way.”
Am I missing something? How do I subsequently wave at the crew 12 minutes before they leave the Earth?
“Is anyone else making different observing plans in the current situation?”
Yup, I’m trying to get hold of a telescope to use until I can get back to La Palma. See the “Telescope wanted” thread.
Plans that either come to naught or half a page of scribbled lines.
Hanging on in quiet desperation is the English way.
The time is gone, the song is over, thought I’d something more to say.
