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I now have software which measures everything it can find in an image and records all those data which have a formal error better than 0.15 magnitudes (which corresponds to a SNR ~7) in a SQL database, along with other meta-data such as the sequence used and relevant excerpts from Gaia-DR2. I will be able to report cases as discussed above to the BAA photometry database if given permission to do so.
I can do rather more too. An article may be written for a forthcoming VSSC. The software will be made freely available on my web site in due course. Whether access to the database will also become forthcoming depends on a number of matters which include security, bandwidth and storage capacity.
APT has a variety of sky background models. You might find one that works well in this situation.
APT is the workhorse of my photometry though I’ve never used it where the “sky” varies that much on such a small scale. It appears to work fine where the background gradient is relatively small, such as variables in M31 and M33.
Otherwise I would suggest PSF-fitting photometry such as IRAF/DAOPHOT. Again I’ve never used it in such extreme circumstances but the documentation has reassuring words about what to do where a star is positioned within nebulosity. Be warned: this package has a steep learning curve and is generally labour intensive.
I could well have an old PC with installed // port in my collection of antiques. If so, and it meets other hardware requirements, you can have it for the price of shipping — free if you are willing and able to collect.
Contact me by email if you’re interested.
(Added in edit: A very quick rummage in the loft unearthed a Pentium-II system. Unfortunately it doesn’t have a PSU but I may be able to find one. Alternatively, perhaps just the motherboard may be sufficient and you could use the rest of your old kit to complete the system.
BTW, how did your old system die? It may well be reparable with components from my parts collection.)
I can’t help but think of the following …
Arthur: I am your king!
Dennis’ Mother: Well I didn’t vote for you.
Arthur: You don’t vote for kings!
Dennis’ Mother: How’d you become king, then?
Arthur: The Lady of the Lake, [Angel chorus begins singing in background] her arm clad in the purest shimmering samite, held aloft Excalibur from the bosom of the water signifying by Divine Providence that I, Arthur, was to carry Excalibur. [Angel chorus ends] That is why I am your king!
Dennis: Listen. Strange women lying in ponds distributing swords is no basis for a system of government. Supreme executive power derives from a mandate from the masses, not from some farcical aquatic ceremony.
Arthur: Be quiet!
Dennis: You can’t expect to wield supreme power just ’cause some watery tart threw a sword at you!
Arthur: Shut up!
Dennis: I mean, if I went ’round saying I was an emperor just because some moistened bint had lobbed a scimitar at me, they’d put me away!
Arthur: Shut up! Will you shut up?! [Grabs Dennis and shakes him]
Dennis: Ah, now we see the violence inherent in the system!
Arthur: Shut up!
Dennis: Oh! Come and see the violence inherent in the system! Help! Help! I’m being repressed!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.
