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After a few happy hours playing with APT I have to say I rather like it. It’s easy to use after a little reading of the fine manual and works well even on a small laptop screen. Configuration is straightforward, though it took me a while to learn how to set the zero point so that the instrumental magnitudes are somewhere near those of the comparison stars.
I think I”m going to put some real effort into this one, with a few scripts written to convert AAVSO CCD photometry files into that required for APT, to mung the CSV output file into ensemble photometry and AAVSO and BAA-VSS format, and generally make it even easier to use so that non-Windoze users have a viable alternative.
All this will take some effort and very careful attention to detail so although beta testers would be made welcome, don’t expect a polished tool set just yet!
I’m a happy bunny. 😎
I found, downloaded and installed APT — Aperture Photometry Tool — from http://www.aperturephotometry.org/aptool/ today. It looks very promising! Because it is wrotten in Java it runs (well, walks sedately) everywhere. The same can be said of AstroImageJ for that matter.
Next plans are to evaluate it thoroughly and if it passes muster, write a Perl script to convert its output into BAA-VSS and/or AAVSO format.
Personally I’d go for the aperture if it’s within your budgetary and mechanical limitations. Most of the time the image will be seeing limited but the resolution of the larger will be better than the smaller in the brief intervals when the atmosphere in front of your telescope is steady. The extra light grasp will be invaluable if your tastes change and deep sky becomes more important, or if you branch out into fields such as VS and cometary observing.
As for focal length, note that 13*102 = 1326 and 9.5*127 = 1206.5 so the longer focal length is only 10% greater than the shorter.
Added in edit: (127/102)^2 = 1.55, so the light grasp is 55% greater, or roughly half a magnitude.
Google Maps Platform rejected your request. This IP, site or mobile application is not authorized to use this API key. Request received from IP address 80.38.183.17, with referer: https://adguard.com/referrer.html
Does anyone else get this, or is it just because I run an ad-blocker? No big deal.
a) I would recommend NOT using median stacking. Averaging and summing are essentially the same operation, just that the first divides by the number of images stacked. Median stacking is not a linear operation and (almost?) all photometry assumes a linear detector response.
b) The exposure of the stacked frames is the sum of the exposures of the individual frames.
c) I believe you should use the weighted mean of the mid-point times of the individual frames, where the weights are the exposure times of each frame. If your stack consists of equally spaced exposures of equal exposure times, the mid-point of the central frame (assuming it exists, what if you have an even number of frames?) is the same thing.
Something to be aware of if you want the highest accuracy: all the above assumes that the focus and the sky background do not vary greatly throughout the stack. If you are in doubt, perform the photometry on each frame to give a flux measurement (not the magnitude, which varies as the logarithm of the flux) and its corresponding variance. Then add up all the fluxes weighted by their corresponding variance . The stacked variance is the root-mean-square of the individual values.
All the above assumes you are doing differential photometry — that you are measuring the relative brightness of a target and a supposedly constant comparison star.
I was thinking about this problem overnight. There are several metrics which could be considered (and the considerer not to be considered to be too silly).
One is that which minimizes the person-kilometres traveled. A few people who have to travel a long distance balance a larger number who need only travel a short distance.
Another is that which minimizes the person-expenditure outlay. A few people who must take expensive routes, for example on a ferry or over a toll bridge, balance a larger number who do not need to pay for the infrastructure such as public highways over which they travel.
I have very little idea about the locations which minimize the above metrics. If forced to guess I suspect that the location would be somewhere fairly near Nottingham.
“If you stick a pin in the centre of the UK?”
https://en.wikipedia.org/wiki/Centre_points_of_the_United_Kingdom gives a point in Lancashire.
Fair point.
I was more thinking that having a price list here would be helpful to all concerned.
P.M. already sent.
“something weird associated with frame stacking”. Ah, I wonder if perhaps some frames have become rotated with respect to the others? If your stacking performs only rectilinear shifts then those near the center of rotation will be stacked perfectly but those further away will be smeared into short arcs. Removing a circularly symmetric synthetic star from the center of an arc will result in wings.
Could you try stacking, say, half the images and see if the result changes? Repeat with a differently chosen half, and again.
Not entirely sure what to make of this …
I took your FITS image and fed it through IRAF’s DAOPHOT pipeline because I knew that it could compute PSFs and subtract them from images. The image below shows an 8x enlarged view of the region of the transient after computed stars had been removed. With the exception of saturated stars and a scattering of doubles, all the bright stars were invisible, meaning that they had been successfully modeled and removed from the image. A good number of faint stars (one is very obvious in the snapshot) had not been removed because they were too faint for accurate photometry with the parameters I chose.
The double next to the transient is very clearly marked as such: the PSF of a single star has been removed from the mid-point. The transient has been removed nicely but two wings remain. I’d say that was fairly conclusive reason for your astrometric result but for the fact that the other two bright stars also show wings.
As I said, I’m not sure what to make of this but post it for your interpretation which may well be better than mine.
BTW, the image below is inverted N/S with respect to your because IRAF counts pixels from the bottom up. Causes no end of confusion …
Paul
Could it be a blend?
The residuals plot after the (presumably) Gaussian profile has been subtracted appears asymmetrical to me, with the left hump being somewhat larger than the right.
https://arxiv.org/abs/1809.00018 came to my attention a few days ago which is what made me think of this possible explanation.
Paul
Thanks! If it is an early G, which looks very likely, then the known distance (from GAIA) and reasonably well-characterized interstellar absorption in that part of the sky will let me estimate its expected apparent magnitude. Comparing that with the known magnitude (again from GAIA) lets us estimate the brightness of the secondary and, hence, its absolute magnitude, leading to a plausible guess for the latter’s spectral type.
Later …
Paul
It certainly looks like it. Typing the coordinates into https://www.aavso.org/apps/vsp/ produces a nice finder chart. It was found while investigating the X-ray transient MAXI_J1820+070 fpr which it is an AAVSO comparison star labeled “120”. The field is crowded which has led to issues with photometry too.
Thanks and good luck!
Paul
I also give my whole-hearted support for this proposal. After all, exoplanets are about as remote planets as we’re likely to study!
In a somewhat related matter, I discovered what I thought might have been a transiting exoplanet recently. In the end it transpired to be an EA variable with minima of 0.025 pm 0.005 and 0.010 pm 0.002 magnitudes. Also known as TYC 444-2670-1. Close, but no cigar, yet demonstrates that discoveries are well within the amateurs grasp.
