Monday, 25 September 2017

The Spring Gegenschein

We host a Star Party in East England at a Dark Sky Discovery Site close to RSPB Minsmere. The sky has reached an SQM level of 21.75 magnitudes per square arcsecond there, and it is dark enough to see the Zodiacal light after sunset in spring. There still is some light pollution, but much less than most places and it is really a very impressive site. What's more the company is great - it is a gathering of like-minded astronomers from semi-local societies. After having seen the Zodiacal light there in 2016, and again (but slightly less clearly) in 2017, I thought I'd have a go at taking lots of exposure on the patch of sky where the 'counterglow' or 'gegenschein' lies. The Zodiacal light is a line of scattered solar light, that follows the ecliptic plane (in which we're embedded) - so in spring you can see it passing by the Pleiades. There is also another scattering phenomenon caused by the dust that lies in this plane. It is called 'backscatter'. I was trying to see it with my naked eye and couldn't be sure, but there was always a hint of a patch of light in Virgo, near Jupiter. There was still too much background sky light to be sure, caused by distant light pollution scattering off atmospheric moisture droplets and thin cirrus clouds. After gathering about an hour's worth of snaps on both nights I was there, the cirrus closed in on the second night (which was clearer, darker & drier) and I thought I'd have a good chance of processing the data to show it. Sure enough after a straight gradient removal of light pollution, there was a blob at the expected position along the ecliptic. It didn't quite correspond to the position and shape I (thought I) saw it with my eyes, but it was very close and ~ 80% overlapping. It could be, the stars or Jupiter interfered with my visual perception. The standard kit lens 18-55mm did really well here at f/3.5 and 18mm, with 5 minute sub-exposures. 18 in total were useable, or 90 minutes. Here is the result. You can see the processed-out cloud at the bottom right, due to Virgo sinking. Try to pick out the constellations!

Sunday, 24 September 2017

Early Autumn Skies

In Early Autumn, you can catch a nice glimpse of the Milky Way in dark skies, rising from the SSW, where we look into the centre of the galaxy. Unfortunately it is very low down so we look through a lot of interfering atmosphere and consequently light pollution (produced by either ignorant, paranoid, or uncaring folk that install inappropriate outdoor lights). Despite this impedance, the power of long exposures, stacking and a lens that runs at f/2 (stopped to f/2.2 in an attempt to cut down abberrations) can be used to produce a good image. Cloud, twilight and mist were threatening to spoil my attempt but the moon was out of the way at last! There is so little time for astronomers to get good viewing conditions we have to seize opportunities like this. I have stacked 7 x 1 minute images taken at ISO 800 tracked with an RA motor, with my modified Canon 1000D, with a 100mm prime f/2 lens from a quiet, dark site south of Norwich. The region is North Sagittarius/Scutum and it contains M24 - the sagittarius star cloud, M17 the swan nebula, M16 the eagle nebula, and numerous other nebulae and clusters.

Another shot at the I.S.S.

Apologies for being away for a year and a half! The gaps between my blog posts have been growing wider and wider. In the meantime I've been running an astronomy society. I thought my first post back would be to see if this year's attempt at catching the ISS is any better than the Feb 2016 one. Same tech, ZWO ASI 130MC camera at f/10 on 8" SCT manually driven. It is very hard to follow the thing at the best of times, but the camera also has the problem of a rolling shutter, which makes it really unsuitable to capturing fast moving objects. This is especially true for stacking purposes. I therefore corrected the best 10 shots, by de-skewing them, and making sure they were taken within a few seconds of each other. July 27th 2017 pass over UK.