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A quick searcn turns up one for which I only have the abstract as the full text is behind a paywall.
Optimal Addition of Images for Detection and Photometry, Authors: Fischer, P ; Kochanski, G P, In: Astron. J. 107 (1994) 802-810
Abstract:
In this paper we describe weighting techniques used for the optimal coaddition of CCD frames with differing characteristics. Optimal means maximum signal-to-noise (s/n) for stellar objects. We derive formulae for four applications: 1) object detection via matched filter, 2) object detection identical to DAOFIND, 3) aperture photometry, and 4) ALLSTAR profile-fitting photometry. We have included examples involving 21 frames for which either the sky brightness or image resolution varied by a factor of three. The gains in s/n were modest for most of the examples, except for DAOFIND detection with varying image resolution which exhibited a substantial s/n increase. Even though the only consideration was maximizing s/n, the image resolution was seen to improve for most of the variable resolution examples. Also discussed are empirical fits for the weighting and the availability of the program, WEIGHT, used to generate the weighting for the individual frames. Finally, we include appendices describing the effects of clipping algorithms and a scheme for star/galaxy and cosmic ray/star discrimination.
Two much more recent papers I have read in full are https://arxiv.org/pdf/1512.06872.pdf and its sequel https://arxiv.org/abs/1512.06879 which specifically treat optimal co-addition.
Sigma clipping destroys photometric accuracy. If all I want is a pretty picture I tend to use median stacking but for scientifically useful images that’s only an option for astrometry.
I use 30s subs, so bad images can be discarded with out losing too much data, but need to use arithmetic mean co-addition for the great majority of my work.
They will be illuminated for up to 2 hours after sunset and before dawn. That’s all night at these latitudes at this time of year. Over on Twitter there is a great deal of discussion which includes amateur and professional astronomers.
https://twitter.com/doug_ellison/status/1132443682018226176 is a good place to start.
One person points out that when in a few years time the whole 12,000 constellation is in orbit, SST will be on stream and observing much of the sky every night. Even if spread out as far as possible there will be one Starlink satellite every 3.4 square degrees. The field of view of LSST is 9.3 square degrees. (Apologies for having 1.5 in the first version of this post).
It’s going to be a nightmare.
Yes, you should take frequent flats if you wish to do photometry at the highest precision possible. The out of focus dust ring-images change in size and intensity as the focus changes.
Most everyone, myself included, ignore this effect and use a set of flats for lengthy periods.
A quick kick of the tyres left me impressed but there is something I spotted. Is the longitude positive going east or west? For instance, La Palma is roughly 17W. Is that longitude 17 or 343?
Tracie, I understand your concerns but my teenage observations were made from light-polluted suburban Derbyshire, not a dark site. Today, despite my aging eyes, I still have no difficulty at all observing it from home 10km due south of brilliantly lit Cambridge (the VS is circumpolar …) and in a garden lit by neighbours.
I do not know where Tracey’s members live in North Wales but I remember the skies being markedly darker (and cloudier) there than they were in Derbyshire and my guess is that they are likely darker than where I now live.
Much the same with me, but the other way round. TheSkyX is installed but, strangely perhaps, prefer The Sky 6. MaximDL 6 is used for camera control, including guiding. My experience is that it guides very well as long as there is a bright enough star within the field of the SBIG-8 / AO-7 AG combination.
For VS work I tend to take enough 30s subs to achieve an acceptable SNR and then move on to the next target. MaxIm DL’s stacking function is invaluable for this. Given that some variables can change by several magnitudes from night to night, my approach can save significant telescope time. To avoid confusion: Maxim DL is used only as a first-look eyeballing procedure, not for final analysis where a local installation of astrometry.net and SWarp is used for stacking.
Naked eye observations of delta Cephei introduces people to variable star observing. This particular star has the benefit of being bright (visible in light polluted areas), good comparison stars nearby, circumpolar (it doesn’t matter what time of year it is and so suitable for long-term observation) and varies markedly over a short time scale (conducive to making it interesting to beginners).
Pooling the results of several of your members will help bring out the variations more clearly and phase-folding observations made over a period of weeks or months should show the nice sawtooth light curve of a classical Cepheid.
Somewhere I still have the observations I made as a young teenager. It’s how I became interested in observing variables.
Linux can certainly run a telescope control system natively but you’d have to use alternatives to the packages with which you are familiar.
WINE is getting pretty good and may be able to run MaxIm DL et al. but when I tried to run Astrometrica under WINE on a Ubuntu 18.10 system it failed because of the lack of some critical component in WINE. You may have better luck.
An alternative to WINE is to run Windows 7 (or whatever you have) in a virtual machine. Again, it may not be completely plain sailing but it may work just fine.
I can go into much more detail but perhaps that’s best done off-line.
This is a question to which I genuinely don’t know the answer. Is some component of sky glow caused by long-lived excitation and relaxation (aka phosphorescence) of molecular species in the upper atmosphere? Some so-called forbidden transitions have a lifetime of the excited state measured in minutes or hours. A commonly encountered example is found in alarm clocks where the hands and figures can remain glowing for hours after the lights have been switched off.
This mechanism may be a factor in the sky become darker long after it has been directly illuminated as the phosphorescence fades.
I support the Dobsonian suggestion though, as noted, it’s not suitable for long-exposure imaging. Lucky imaging of planerts should be fine as each exposure is so short that trailing is likely to be completely overwhelmed by the seeing and / or diffraction. (Maximum drift is 15 arcsec per sec for an equatorial target, or 0.15 arscec at a frame rate of 100 fps where you would need almost a one metre aperture for diffraction to be an issue.)
Indeed, back in the day I kept an 18″ Dob. in my kitchen. Made of plywood, it was still a reasonable task to lug its two sections out onto the back yard for an observing session. Current designs are lighter still. There is absolutely no way that a traditionally mounted 18-incher could be regarded as portable.
I suggest that you send this to Roger for the “literature watch” section of the news letter. Although I started that idea, others’ contributions are always welcome.
Declaration of ulterior motive: anything which makes my life easier …
Should be easy enough to do. Point your scope at the celestial pole which is above your horizon. Take an hour-long exposure every month or two. Use your favourite astrometry software to find the centre of the star image arcs. Plot the results in a coordinate system where the start (or centre or end) of each arc is fixed. Precession is ~20 arcsec / year, easily measurable by anyone capable of arcsec precision. Note that this approach neglects the proper motions of the reference stars, which is likely to be accpetable onver the timescale of the project.
I’d try it but my fork-mounted scope can’t be pointed much above +75 Dec. That said, it might be a fun project for another scope which is (a) on an GEM and (b) not used for anything right now.
Thanks for giving me the idea!
A few months ago I discovered an EA variable in images taken by Kevin Hills where the primary minimum is around 25 mmag and the secondary is about 10 mmag. Not an exoplanet discovery in this case but well within the range of variability resulting from exoplanet transits, which tend to lie between 5 and 30 mmag.
I’m with Roger on this one.
Added in edit: I should analyse the rest of Kevin’s data to see what else turns up.
Thanks Nick, they are beautiful.
When I get chance (not for a couple of weeks or more) I’ll write a circular-arc Hough transform to find the NCP in each image. Assuming it works well I’ll make the source code available here.
Variables. I spend a few minutes on one target, then perhaps 20 on another, then off to another. Repeat for a few hours.
Setting the dome and letting them drift is the essence of my request. There are a few dozen eruptives in the BAA-VSS program. In what order should I observe them on a given night so that they drift past the open slit which has been set to a particular azimuth? Ordering by increasing RA is useful if the azimuth is 180 degrees and the declination is south of +28 degrees (my site is on La Palma at 28N) but many variables are at much higher declinations. In practice my chart collection is ordered primarily by RA (in 30 minute zones) then by Dec, south to north. I just flip pages in the binder and observe in order. It’s not a bad solution, by and large, but neither is it a good one if the objects are at high Dec or far from the meridian.
Of course, another approach would be to persuade the BAA-VSS to adopt some variables which barely rise from 52N. Somehow, I think I may have difficulties with that one. 😉
