Tabby's Star's twinkle probably the boring business of calibration Put the boffins in a cage and break out the popcorn: a new analysis suggests the “long-term decline” in the light observed from the hotly-speculated-upon “Tabby's star” tells us more about calibrating Earth instruments than alien gigastructures. The fun began last year when Kepler images showed unusual flickers in the …
At least that's my understanding of what it showed. If it showed some ups and some downs with a general trend of 20% down then I would accept this explanation, but if it has shown a consistent decline with few if any outliers going the other direction such an explanation doesn't pass the smell test.
There's no reason why calibration errors would always be in one direction, and changing at a set rate over time.
The fun with random is that they are sometimes nonrandom to our meaty pattern matcher.
In this case, the recent Twinkle Action is of interest (with obscuring comets unlikely to be the cause, so El Reg headline is a case of churnalistic errorbait), but the long-term secular "dimming" is not peculiar or distinguishable from noise when one compares against other stars in the archive.
That's the first thing to be checked: is there really a constand decline? Tricky. All the raw data should be re-ckecked, but there are a lot of factors to be considerd. The instruments that were used. How they were used. By whom they were used. The glass plates. The emulsion on the plates. The development process used. The chemicals used in it. The lab that did all this. The archiving. And so on.
A glass plate from say, 1900 would have ever so slightly different optical properties than one from 1950. The same goes for the emulsions etc. If you buy your films in bulk and use them up over a longer period the undeveloped emulsions will change. They will also pick up a bit of 'exposure' from background radiation. You can slow the process with cold storage, but a plate from the same batch processed right away would differ from one that sat in storage for a couple of years.
The early instruments would have been mechanical, maybe electromechanical. This stuff has wear and tear. For example the timer in the camera that controls the duration of the exposure, Or the timer the lab technician uses when developing the plates.
Then there is stuff like lens coatings changing their properties over time.
And probably lots more, all this is just off the top of my head.
Usually (i.e. just taking pretty pictures) all of this doesn't really matter, but If we are talking about variations right next to the noise level - well there should be a couple of papers in this.
I take your point about variation with time, but can't most of the sources of variable magnitude you mention be eliminated by choosing at least one other star of similar magnitude that appears on most of the plates and measuring that too? If all measured stars show similar variation, then the variability is due to the equipment and observation techniques.
I'd assume that this or a similar technique was SOP when comparing archived plates or film and, if not, would like to know why not.
I can't buy the idea that the apparent reduction in light, over time, from Tabby's Star is because "The problem with using a hundred years' of observations, the group argue, is that the source data from “Digital Access to a Sky Century @ Harvard includes half a million glass plates shot between 1885 and 1993, using a number of different instruments and cameras."
Going even further than MartinG, I'd expect the SOP would be to only consider plates that include sufficient additional stars to allow the entire plate to be calibrated, comparing every star (and galaxy) in the plate being calibrated with every other plate/image in which any of those stars and galaxies appear.
This wouldn't eliminate the problem with variations in the sensitivity of the emulsion across each plate but as the plates would have been prepared specifically for scientific measurement purposes I'd expect the 'noise' variability across each plate to be pretty low, and certainly way below 20%, which the team seems to think is a typical noise level (is 20% noise even science?).
Fwiw, Kepler's noise floor appears to be around 80 ppm, whilst two of the measured variations in brightness from Tabby's Star, in recent times by Kepler, were by 15% & 22%.
"It's just variations in measurement accuracy over time."
"One lot of very old measurements by a few people in a particular field may not have been interpreted accurately - so that proves that a completely different set of measurements, vastly larger, by far more people, checked and tested and re-checked in every possible way, using multiple different techniques and instruments, agreeing very well with past and current actual events, must also be inaccurate - because I don't want it to be true and I'll stick my fingers in my ears and shout "I can't hear you!" until it goes away."
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
