Amateur Astronomy


#1281

Incredible image. The milky way is an awesome sight in the southern hemisphere. I remember waking up in the outback and being bowled over with the clarity and brightness of the night sky.


#1282

Given the crappy weather we’ve been having, there’s an almost miraculous clearance going on right now from the north west. Only a small bit of light cloud in Dublin at 10pm Sunday night, and the infrared sat picture looks like there’s a clear window for the next six to eight hours or so. Moonset is at 2.35am, so good opportunity to catch the Perseids near maximum after that. I’m going to hit the hay early and see if it’s worth travelling somewhere dark later.

EDIT: 2am, still in the middle of that miraculous clear window, and sky looks great. I’m off to watch a nice moonset and bag some meteors.

EDIT: 5am. The weather held up well so I watched for an hour from Brittas Bay. Decent binocular views of the Pleiades and Andromeda Galaxy. The meteors weren’t spectacular – nothing for the first 10 minutes, then 3 in 3 minutes, another fifteen minute gap followed by 3 in 1 minute, and a handful more over the next half hour. I need better than 30 per hour to keep my interest up, and by that measure it was disappointing. It was also unseasonably chilly due to the northwesterly, barely in double digits. I won’t complain, though. It’s clouded over again now (coming up to 6am) so it really was a piece of rare good luck to get the overnight clearance.

Of course, it’s a day before the maximum, and the Perseids are fairly sharply peaked. The blurbs that tell you that you can watch a week before or after max are leading you down the garden path. But unfortunately the Moon doesn’t set until 3.30am tomorrow and with the shower radiant in the northeast you’re already losing meteors into the sunlight soon after four.

There’s always next year. The 2020 Perseids will be moonless.


#1283

Will post proper reply later as on work break, it’s a particularly dark place, the location was an old abandoned place called inneston on the yorke peninsula in south Australia. Other star shots I took in outback Queensland either had more light pollution from multiple mines running 24/7 or had more dust or water vapor I don’t know but this location is nearer a major city but is darker in my opinion.

I’m not using a lens people recommend for astrophotography. Someday I’ll get one. I’m using a pentax kp camera with live view, manual focusing. Lens is a bright f1.4 55mm da* pentax portrait type lens. I use a pentax gps module which allows the sensor anti shake to track the stars with the rotation of the earth allowing me longer exposures without star trailing. It’s an app c sensor I think the ISO was 3200 . I’ll check the files later. I just take a few photos then rotate the tripod and overlap to get a panorama. Using Microsoft ice to stitch images. No image stacking or any dark images like proper astrophotographers do. Someday I’ll do it properly. Results are quite good without. I did turn the lens down to f3.2 or f,3.5 as it has bad chromatic abberation at f1.4 unfortunately. Anyway you can turn night into day and end up overexposing the milky way at f1.4 and iso 3200!!


#1284

On the second night there I took shots with stars reflecting in a lake. Moon was still up so you lose the colours of the milky way and it’s absolutely nothing on that other photo, it was also cloudy but I’ll share it later anyway. Oh and I had to use a crappy wide angle zoom lens to get the lake. I since ordered a bit better brighter zoom lens but dunno when I’ll get out there again


#1285

Those are stunning results with a consumer DSLR. Mind you it’s not a cheap camera – about a grand’s worth over here, and the full frame version would cost more than half as much again as the APS-C. Then it’s a couple of hundred extra each for a basic lens and the GPS unit.

That idea of combining the GPS with the anti-shake for astrotracking is insanely clever. I see they call it an astrotracer in the promotional blurbs. I’m not sure you could do much better with the several-hundred-quid Rokinon fast lenses that the astrophotography buffs rave about (sold as Samyang in Europe, Rokinon in the US).

Good job!


#1286

PS my brother got some shots with a camera phone that, for a phone,are brilliant. He is on the pin he might post one too


#1287

Also is Andromeda something you think I could get from here? Or is it northern hemisphere only?
I sometimes take a short but high amplification photo of the milky way then use it to get orientation and spot interesting things then I’ll put on a zoom lens. Thats how I got orion and the other nebula photo I put up here. At 300mm zoom you have very little time to get a shot lined up and focused before it rotates out of your field of view. The problem is as you zoom in you have to make sure you are still on target so you manually refocus.if you take too long it’s already passed.

The astrotracer is basically the only reason I got that specific camera


#1288

Unfortunately the Andromeda Galaxy is at the northern end of the Andromeda constellation, around 44 N declination. From Inneston / Adelaide, it will never get more than 13 degrees above the horizon (about the same as the midwinter Sun in Ireland, for comparison). Maybe with your clear skies that’s high enough to get a shot. Obviously from northern Australia, being more than 20 degrees higher, it’s a much easier spot. It’s a southern spring / summer object – October / November would be the best months, so you have to contend with shorter nights too. Here it is at midnight Adelaide local time on November 1st. The galaxy is the fuzzy blob. Culmination gets earlier as the year gets later. It eventually runs into daylight around the start of December.

I’d normally locate Andromeda using Cassiopeia (a great “pointer” constellation) to locate the great square of Pegasus. Cassiopeia is unfortunately below your horizon, however that square in Pegasus is a notably empty part of the sky and easy enough to spot. Then, counting the corner of Pegasus as 1, count to 3 along Andromeda (to Mirach). You’ll normally see two stars in a line above (below for you) Mirach with the naked eye, depending on quality of the sky and your vision. On a clear night you’ll see the galaxy with the naked eye too, although you might have to avert your gaze to catch it with your more sensitive peripheral vision.


#1289

Took these with the Huawei P20 Pro out at Inneston alongside my bro! He asked me to upload some here, unfortunately most photos are on the 10 million setting instead of the 40 mil :see_no_evil: ->

Also had no tripod for the phone but still happy with what I got!

Blindjustice stargazing in the shadows :sweat_smile:- >


#1290

Impressive for a phone alright. When you zoom in though, you can see the huge difference in noise level between that and DSLR. It’s a bit like a Van Gogh painting. :icon_biggrin:


#1291


#1292

Wow. Don’t think I’ve ever seen stars reflected on water before. I know I’ve never seen the Magellanic Clouds reflected on water.


#1293

Hmm I’ve a correction to make. The shots were 20 seconds long. Iso 12800 and f4.0. so I reckon I can get cleaner shots in future using a lower iso with longer exposures. We were facing roughly south east at approx 11pm Saturday 3rd August for your reference and having the galactic center almost centre of the photo was pure fluke. It’s how the image composite editor cropped the photos as a group.

It’s also probably Time I started doing some image stacking. I think some details are even overexposed. …this kind of thing leads down a real rabbit hole , time consuming but a fun hobby.

Glad you got some meteors!


#1294

How many Earth-like planets are around sun-like stars?

Interesting study. Even if it is 1 in 1000 that is a lot of potential earths


#1295

TESS update:


#1296

WOOHOO!!! Youtube just threw me a random recommendation and it was one of my favourite physicists! I didn’t even know she had a channel. Hossenfelder is one of those scary bright thinkers who’s not afraid to think outside the box (e.g. her work on superdeterminism and her rejection of the Everettian MWI). She’s also capable of explaining an idea simply, as here with Modified Newtonian Dynamics. Can’t wait to see what else is on the channel, and delighted to see she already has 16k subs.


#1297

Elsewhere on the site, a post about Dun Laoghaire’s problems contained a link to an IT article with this picture:

I thought there was something very neat about it, so I played around with it to try to highlight the water ripples:


What we’re looking it is an excellent example of a single slit interference pattern. The mouth of the harbour is the single slit, through which plane water waves (i.e. with roughly straight wave fronts) are passing from the Irish Sea. The behaviour of any wave passing through a slit is to spread out in a circular pattern on the other side. This is the phenomenon of diffraction.

But crucially, if the slit is significantly wider the wavelength of the wave, each side of the slit acts as a separate barrier. The waves propagate in their circular motion from each side, and the two diffracted waves interact with each other to make an interference pattern. You can see this in the complex overlapping waves just inside the mouth of the harbour in the photo. Here’s a more controlled version of it in a wave tank:

If we measure the ripples along the top edge of this image we find there are alternating regions of high waves and flat water. It’s a one-dimensional interference pattern where the slit and the resulting pattern are lines, roughly speaking. We can also have a two-dimensional version where the slit is a circular hole and the pattern is measured in a plane. It’s hard to do this with water, but we can do it with light:

image

The circular pattern is called an Airy disc after the astronomer royal, George Biddell Airy, who was the first to mathematically describe it in 1833. However, it was first studied in detail by Italian Jesuit Francesco Grimaldi around the mid-1600s. He coined the word diffraction from Latin diffringere, “to break up”, due to the pattern’s similarity to flowing water breaking up around a stick. French Jesuit Honoré Fabri described Grimaldi’s work in Dialogi physici, and this is where Isaac Newton read about it before performing the experiments described in his Opticks of 1704.

The Airy disc is the bane of astronomers. The famous astronomer John Herschel had observed in 1828 that under high magnification even the most perfect telescopic image of a star appears like a planetary disc, surrounded by concentric light and dark rings. Even without high magnification a star’s image will appear spread out across a disc whose size depends on its brightness:

image

In the above image, diffraction is caused not only by the telescope’s circular aperture, but the supporting cross-struts in the telescope tube which give rise to the cruciform spikes. One irony of history is that although Grimaldi accurately described diffraction from experiments done with pinhole images of the Sun in a darkened room, he also devised a method for measuring the diameters of stars using a telescope. Of course, he was doing no more than measuring their brightnesses, owing to the same diffraction effect.

This became a much greater conundrum as astronomers started to look for stellar parallaxes. As early as 1672, astronomers Cassini and Flamsteed had obtained a parallax distance to Mars, by two different methods. This allowed the dimensions of the whole solar system to be determined, and for the first time we started to have a sense of the scale of our surroundings. But even using the 300 million kilometre diameter of Earth’s orbit around the sun as a baseline, no parallax shift of any star had been successfully measured by the time of Airy’s calculation.

This was a huge stumbling block for the heliocentric theory for a long time after Copernicus and Galileo. Contrary to popular misconception, the objections weren’t all based on dogmatic readings of the Bible. Basic geometry showed that if the stars were distant suns, the lack of parallax implied a distance scale that was simply implausible by any conception of the time. In the old Ptolemaic model, the stars were fixed to the next celestial sphere out from the planet Saturn. For a long time, even in the scientific era, it was simply beyond human grasp that the nearest star would turn out to be 25,000 times the distance to faraway Saturn.

The problem was even greater if the disc-like appearances of stars in the telescope were real. If they were at the incredibly great distances evidenced by the lack of parallax shift, then they must also be incredibly huge. In fact, by one early (and very mistaken) estimation they were bigger than the entire universe was supposed to be! Fortunately, Airy’s calculation showed how point-like stars could produce extended images in the telescope. And just five years later Friedrich Bessel made the first parallax measurement of a star, 61 Cygni at a distance of 11 light years. To give an idea of the practical difficulties, the measurement involves a triangle whose baseline – the diameter of the earth’s orbit around the Sun – is more than a quarter of a million times smaller than its height, the distance to 61 Cygni. Bessel used a heliometer, a device which splits an image into two parts and allows them to be converged using a micrometer screw. This allows very precise measurements of the angle between a target star and a reference star (which is much further away and not subject to parallax).

One of the wonderful things about astronomy is the way that advances in both technology and mathematical theory have allowed us to make sense of observations that would otherwise just seem crazy. If there are lunatics (and there are) who still believe the stars are sprinkled on the inside of a snow globe a short distance above our heads, it is a failure of education to convey the hard won and astonishing insights we’ve been able to glean from experiments like these. Today we are on the third generation of incredible spacecraft which have extended the reach of parallax measurements far beyond Bessel’s dozen or so light years. Hipparcos in 1989 had a reach of a few hundred light years, and the Hubble telescope extended that to 10,000 light years in the 1990s. Thirty years later Gaia is making parallax measurements with a technique which is not much different in principle to Bessel’s heliometer – just a million miles out in space on a billion dollar spacecraft, using materials Bessel couldn’t have conceived of :slight_smile: . By the end of the mission the measurements will extend to more than 30,000 light years, which is further than the centre of our galaxy.


#1298

We haven’t had a single item about the Juno spacecraft yet, and it’s already three years into it’s five year mission around Jupiter. I’ve been looking at some of the pictures from Junocam and they are stunning. Juno’s scientific mission involves measuring the planet’s gravitational and magnetic fields, its chemical composition, and atmospheric dynamics. Not one of these objectives needs a visible light camera! So Junocam was added specifically as part of a public outreach endeavour (and, no doubt, to generate the sort of PR you need to justify a billion dollar budget). The public would get to point the camera and process the images, and it turns out that citizen scientists and artists have taken up the challenge very enthusiastically.

Juno is on a highly elliptical 53-day polar orbit that takes it to within four thousand kilometres of Jupiter’s cloud tops, and as far away as eight million km. Junocam starts clicking when it swoops in toward perijove, as the closest point is called. (Obviously in space no one can hear you click :blush: ). Perijove (which NASA abbreviates to PJ) also exposes the camera to a blast of Jupiter’s intense radiation, and so it was only expected to last the first eight encounters. As of PJ22 last week it is still going strong. Indeed, it has been a victim of its own success. NASA stopped the public voting on selecting camera targets after PJ8, as the images have turned out to be so scientifically valuable. The public still gets to download and process the images.

It would be impossible to do justice to even a selection of Junocam images here. You can Google them, and find them on various Flickr accounts. Seán Doran, an Irish guy living in London, has done some of the most viral images (on Flickr here). Here’s his animation of the flyby on PJ6 (music by Ligeti, which I think I recognise as part of the soundtrack from Kubrick’s 2001):

Doran uses images pre-processed by Gerald Eichstädt, a German mathematician and software guru. Using the NASA data directly is no mean feat. Like most astronomical cameras, Junocam’s sensor only does greyscale for maximum resolution. The colour information comes from different colour filters on separate frames. But the Juno spacecraft is spin-stabilised so successive single-colour frames are taken from different rotation angles. Complicated map projections and colour calibration are needed to produce even a single combined colour still.

I thought I would take just a single Junocam image for closer examination. This was uploaded last week by Kevin Gill, a NASA software engineer who processes Junocam images for his own curiosity and enjoyment.

This is the shadow of Jupiter’s moon Io moving across the planet’s surface during PJ22. I was struck by how sharp and how big the shadow is. In fact it’s too big – I’ve seen other Io eclipse photographs and they look nothing like this. But we’ll get to that. First about the sharpness … eclipses are all about geometry. Here’s how a solar eclipse from Earth is usually explained:

The deep shadow, or umbra, is the region where the Sun is totally blocked, while the penumbra is the region where a partial eclipse of the Sun would be seen. The penumbra is not uniform as the fraction of the Sun that is eclipsed decreases toward the edges. This gives a gradation of shadow across the whole region.

The above picture is not to scale as the Sun is 400 times bigger than the Moon and 400 times further away from the Earth, but it gives a good idea. Imagine you were to start shrinking the sun. If you follow the geometry you see that the umbral region would start getting bigger, while the penumbra would get smaller. Consequently, the shadow would get sharper as the umbra and penumbra converged.

On Jupiter, the Sun appears only one fifth of the size that it does from Earth. It therefore casts very crisp shadows, and this is what we are seeing in the Junocam image. For comparison, look at this image of a solar eclipse – the last total eclipse in the British Isles in August 1999 – taken from the Mir spacecraft. It’s a fuzzy mess, even though most of what we are seeing is umbra as the penumbral shadow is mostly too subtle for the camera. The maximum umbra for a solar eclipse on Earth is 257 km wide, while the penumbra is many thousands of km wide.

So what about the size of the Io shadow on Jupiter? Here’s a more “normal” image from the Hubble space telescope showing a transit of Io and its shadow:

Relative to Jupiter the shadow is tiny compared to what we see in the Junocam image. It’s reminiscent of the complaints of a certain lunatic fringe who claim that all NASA photographs are faked. As evidence, they show pictures of the globe in which the continents are different sizes. Their misconception is that all images that show a planetary disc must be showing an entire hemisphere. While this is approximately true for images taken from very far away, it is certainly not the case for close-ups. Here’s my schematic for an observer who is a height h above a planet:

The radius of the visible disc is r, which is clearly smaller than the planetary radius R. By including the distance to the observer’s horizon d, we can give it a proper geometrical treatment. Consider the two triangles with sides r, d, and h, and sides R, d and R+h. They are both right angled and share the angle made by d and h, so they are similar. Therefore:

By Pythagoras theorem we also have:

Now, is the amount by which objects are going to appear bigger in images taken from different heights. I’m going to outrageously try and glean this directly from the Junocam picture of the Io eclipse. Let’s call the result of that a. Combining the above equations we get:

… and this can be rearranged to:

I said I was going to be outrageous, and here I’ve literally measured the ratio of the visible disc diameter to Io’s shadow in a crappy drawing program. Io is bent into an ellipse around the curve of the planet, but the major axis gives the undistorted dimension:

I get a ratio of 6.6. I then use the actual geometry of the Sun-Io-Jupiter positions and sizes to calculate the ratio if an entire hemisphere was visible (as in the Hubble telescope image), and I get 46. This gives me:

And finally:

As it happens, a NASA credit gives us Juno’s height for this image at approximately 8,000 km. Given the rough and ready nature of my measurement I think that’s a fine agreement. The lunatic fringe can read it and weep. :smile:


#1299

Titbit of information I recently learned.
When a satellite uses a planet for slingshot purposes, it’s in effect stealing the planet’s gravity.
So, if only by a miniscule amount, the planet actually slows down.


#1300

It’s probably more accurate to say that it’s stealing the planet’s orbital energy. In the reference frame of the Sun, the planet slows down and the spacecraft speeds up. It can work both ways too. A spacecraft can use a planet to slow down (by flying ahead of it in its orbit), and it speeds up the planet in the process. The BepiColumbo mission to Mercury will be doing this, as the spacecraft needs to lose energy to get from Earth’s orbit to Mercury’s. That said, the spacecraft got most of its energy from Earth in the first place, so actually the net effect is that we are transferring some of the Earth’s orbital energy to Venus and Mercury.

The thing that used to puzzle me most about slingshots was that they often do a fuel burn at the same time. It’s more efficient to fire the engines when close to the slingshotting planet than any other time. This seemed to me to violate conservation of energy, but it’s explained in terms of the exhaust gas being left with a lower gravitational potential energy. If you burned it higher up, the exhaust would have the potential to speed up as it fell toward the planet – none of which would help the spacecraft. It’s called the Oberth effect.