ROAST

A few nights ago I took some more images of Markarian’s Chain, and while it wasn’t a great session (clouds), I did learn something about when to observe.

I took 210 photos with 30-second exposures, and the Mini rejected 14 of them — all due to satellites, mostly Starlinks I think.

Here’s a quick manual stack of the rejected frames:

They haven’t been demosaiced or aligned (hence the green tint); I just wanted to see the satellite tracks, all in a pretty small area of the sky.

What I learned was this: sessions should start as late as possible. I started imaging at 9:10pm, and finished at 23:12. All the failures happened before 22:22pm, by which time the sun had moved far enough below the horizon that the satellites weren’t illuminated any more — all those Starlinks are in low orbit.

Unfortunately, that means that in the summer (shorter nights) we’ll get many more trails. :-(

Last night was meant to be clear, but it started off with some cloud which then cleared until about midnight, when it hazed over again. So counts were limited. But here’s a Megastack of M51, the Whirlpool galaxy:

250 images taken over two nights, all 60 second exposures; 200 with gain 60, 50 with gain 100 (which was a mistake — high gain values are noisier). Compare with my first effort.

And this is Markarian’s Chain, a linked series of galaxies in the Virgo area, part of the major cluster which includes our own galaxy:

100 60-second exposures, gain 80 (again, too high). As always, click for bigly. How many galaxies can you see?

This is M106, aka NGC4258 — a galaxy with no nickname in Canes Venatici (”Hunting Dogs”). 140 one-minute exposures, gain 60; I was going for 200 but the clouds rolled in.

I also caught a satellite, at top left. Actually, examining it closely it’s probably a plane — it’s a series of dots, and satellites don’t usually flash like this.

And on the lower right is NGC4217, a spiral galaxy viewed edge-on. Click to view at full size, and you can see more galaxies.

[Edit 2026-04-18: Fixed incorrect byte ordering of colour channels.]
It’s been 25 years — blimey! — since I did any serious ARM coding. The last was fixing some bugs in my Lisp interpreter in 2001, after which my Iyonix died and I moved over to Linux.

Back with RISC OS now, and in the intervening years the ARM processor has had some substantial changes. Originally it was the Acorn RISC Machine, of course, and it first appeared in the Acorn Archimedes computer in 1987. It was a genuine breakthrough at the time, a custom-designed (by Sophie Wilson et al) 32-bit processor running at 8MHz. I’m now using a 4té², a repackaged Raspberry Pi 4b containing an ARM Cortex-A72, running at 1.8GHz — over 200 times faster.

And while the original ARM chips were indeed Reduced Instruction Set Computers, with only about 25 instructions¹, these days it’s something of a misnomer. So what’s been added? SIMD and NEON, mostly. This article is a simple introduction to using some SIMD instructions; we’ll cover NEON² later.
[Read more…]

It’s been a long wait, but we finally had some clear skies last night. And here’s a “Megastack” of M101, the Pinwheel galaxy. The Dwarf Mini can combine images taken over different observing sessions, at different times and dates. Here it’s a stack of 295 images taken over three nights — the Megastack processing takes a while, 50 minutes in this case:

Some post-processing in PhotoDesk: gamma, equalisation and cropping. Compare with the image taken on March 18.

I also managed a not entirely successful shot of Bode’s galaxy, this time getting the Cigar galaxy, M82, in the shot:

Better luck next time; clouds started rolling in for this.

Finally, the Moon:

Again not very good, due to haze.

The globular cluster M13 has a number of variable stars, which change in brightness by various amounts over time. Some, like Cepheid variables, are very predictable and can be used to estimate the distance of objects.

I surmised it might be possible to catch the variations with the Dwarf Mini, so after taking the 27 March image, I took another the next night in a brief cloudless spell and tried comparing them. Spoiler: this hasn’t worked very well!

The image on the left is an animation of the two images. Both were taken with the same settings: 50 30-second exposures, 60 gain, Astro filter.

First of all the images had to be aligned. Because they were taken at different times the second had to be rotated by a small amount (about 2.1°), which was done in ArtWorks — it’s a bit easier than in PhotoDesk. The images were then imported into PhotoDesk and aligned using a layer for each image.

There are some obvious problems here. First, the images should have been taken at similar times. The second was taken earlier in the evening, so M13 was lower in the sky resulting in greater atmospheric attenuation, which alters the colours slightly — that’s why they’ve been converted to greyscale — and dims the image a bit. This would also reduce or eliminate any need for rotation.

Second, the weather for the second image was worse, with occasional cloud and some high haze.

Third, it turns out that most of the variables have fairly long cycles, between five days and three months, so the two photos should be separated by more than a day. And the amount of variation isn’t that much, so the two images do need to be as identical as possible.

So although it looks like some stars are varying, mostly fainter ones, this is an artefact. And they’re not quite aligned perfectly either.

This Sky and Telescope article has a useful guide to M13 variable stars. About halfway down the page there’s an annotated image with the variable stars labelled and some asterisms — star patterns — outlined as guide aids. My image is on the right, and I’ve matched the asterisms, shown in green — an encouraging start — and marked a few of the variable stars, in red. It’s been zoomed in and cropped.

So my first attempt was, basically, rubbish. But I’ve learned a lot here…

Another Dwarf Mini session last night. I plugged in a power bank for an extended session, but I stupidly forgot to turn it on 😢 so halted things when the Mini’s battery was down to 25%, after about 3 hours. Doh.

Anyway, this is M13, the globular cluster in Hercules:

As always, click for a bigger version. M13 is a small satellite galaxy of our own, and contains perhaps half a milliion stars. Stars are packed about 100 times closer together than in our neighbourhood, so the night-time sky of a planet in the cluster must be spectacular.

Here I took 50 30-second shots with 5 failures, gain 60, Astro filter. Slight sharpening and saturation in PhotoDesk.

There’s a feature called the propeller in this cluster, which the Mini has managed to resolve — see right. It’s just a chance arrangement of dust lanes in the cluster and our viewpoint.

And below is another attempt at Bode’s Galaxy, M81:

This time I was more conservative with the settings, which has significantly improved the result. 200 images with no failures, 30 seconds each, gain 60 and Astro filter. No processing!

Today started off with thick fog, which gradually cleared to hazy sunshine and milky skies. Not the best conditions for astrophotography, but I decided try shooting the Pleiades, or Seven Sisters, a prominent star cluster in Taurus.

It’s not well positioned from our location at this time of year, being lowish in the western sky. And it’s only visible from our front garden, with a bright street light just outside. Nevertheless I had a go, and set up a Heath Robinson affair of a towel — life’s essential — draped over another tripod to shade the Dwarf Mini from the glare of the street lamp.

Here’s how it went:

Not bad at all, considering the conditions! I set up the Mini to take 100 shots, with 60-second exposure and gain 60. But as you can see from the screenshot below, by 75 shots the hedge was getting in the way so I stopped the session. Unsurprisingly there were 12 failed shots, so this is a stack of 63. It’s been processed in PhotoDesk with equalisation and quite a strong gamma adjustment to bring out some of the faint blue nebulosity. This target really needs dark skies and long exposures, so we’ll have to wait until later in the year for a better result.

I also took a quick shot of the 3-day-old Moon:

A very clear night, and an opportunity to attempt some better shots of the galaxies I looked at last time.

I set up an equatorial mount, not “Perfect” but within 1°. I upped the exposure time to 60 seconds, and used a gain of 100. Click on the pictures for bigger versions. All have minimal PhotoDesk processing after passing through Stellar Studio — just small gamma and equalisation adjustments, and cropping.

So here’s M81, Bode’s galaxy, again. 50 exposures, two failures:

The thing at the top is the Cigar galaxy, which I didn’t realise was there.

This is M101, the Pinwheel galaxy. 50 exposures, no failures:

And a new one, the small but pretty M51, the Whirlpool galaxy. 50 exposures, no failures again:

This has a companion galaxy, NGC5195, which is pulling out one of the spiral arms. Look carefully and you can see several smaller, more distant galaxies at lower right.

I also did the Hamburger galaxy, but I was jinxed again, this time by light pollution from our bathroom window.

[Update 20 Mar 2026]
Managed to get the Hamburger last night, although there was a lot of high cloud/haze. 100 shots, 30sec, gain 60:

Gradgrind has been updated to 0.93beta. Changes:

• GIMP palettes changes:
  • Doesn’t now ignore duplicates
  • Added paged display for large palettes (uses left/right arrows)
  • Description for these shows currently displayed colour numbers
  • Includes (commented) description when saving as gpl file
  • Added some sample GIMP palettes
• Sprite save icon removed. Can now drag the gradient/swatches displays to save them as sprites. Filename has “S” or “G” appended.
• Tidied up swatches display, and added option for outlines (toggle with ADJUST)

Download here.