Copernicus

It doesn't take many lunar observing sessions before ending up with a picture of Copernicus. Here's a picture I took during an observing session in March. It was a near full moon so the surface is looking rather flat.


I got the webcam orientation a bit wonky so north is roughly pointing towards 4 o-clock. Copernicus is a relatively new crater being around 800 million years old. Typical of many Copernican period craters it has a prominent ray system and the crater hasn't been flooded with lava - some of the features inside the crater are obvious even with this little shadow.

The relatively bright crater in the bottom-right corner is Pytheas, a crater of similar age to Copernicus and located in the southern part of Mare Imbrium. To the right of Copernicus in this picture is the mountain range Montes Carpatus which is 2-3 billion years older than the two craters just mentioned.

The two medium-sized craters situated at 11 o'clock are Reinhold and Reinhold B, both of which are much older craters than Copernicus.

It's a shame I didn't pick a slightly shifted field of view - a little more towards the top-right would have given a better view of Montes Carpatus, but just out of shot in the bottom-left is Eratosthenes which would have made a great target alongside Copernicus. Maybe next time.

Mersenius and Gassendi

Mersenius and Gassendi are two large craters in the southwestern part of the Moon and are the next step in my quest to image the entire lunar surface.


Of the two large craters in the middle of the picture, Mersenius is on the left and Gassendi on the right. The large, smooth, dark region at the bottom of the picture is Mare Humorum and North is roughly towards one o'clock.

Mersenius is 84km in diameter, 2.3km deep and was formed nearly 4 billion years ago. The interior of the crater has been flooded by basaltic lava which has solidified into a central dome shape and covered many other features. The rim of the crater is heavily worn particularly at the northern edge. The small crater Mersenius N can be seen lying across the southwestern rim.

Gassendi is a larger and apparently shallower crater. Similarly to Mersenius, Gassendi has been filled with lava but some of the multiple central peaks are still visible. The crater situated on the northern rim is Gassendi A and the appearance of the two craters has been likened to a diamond ring.

Spectroscopy

At the beginning of this year I added a new toy to my stargazing kit - the Star Analyser 100 from Paton Hawksley. I'd always had a hankering for getting into stellar spectroscopy and this looked like the perfect starting point - spectrometers are VERY expensive, the Star Analyser could get me started for under £100.

Spectroscopy is essentially analysing the light from an object and seeing how the intensity of the light varies as a function of wavelength (i.e. looking at the spectrum of the object). This can reveal all manner of things such as the temperature of the object, what it is made of and how fast it is moving. This is detailed analysis that needs finely tuned equipment and the Star Analyser tries to fill a gap in the low end of the market - it won't show the detail but it's fun, easy to use and informative.

The Star Analyser is just like a standard filter - it screws into any eyepiece and off you go. When combined with my webcam it means I can take pictures of stellar spectra for further analysis. Here are some spectra I took of Arcturus:



As you can see, the results are very consistent and show some detail - the red appearance of Arcturus is obvious and the dark line on the middle-right is one of the Fraunhofer absorption lines due to oxygen in the Earth's atmosphere.

To show the difference between a relatively cool K-type star like Arcturus and something a bit hotter, here's a spectrum taken of Sirius an A-type star:



The spectrum is much more green/blue and other features are visible such as the H-Beta line in the light-blue section.

Another interesting use of the Star Analyser is to compare stellar magnitude. I find it much easier to compare the brightness of two spectra rather than looking directly at the stars. Providing I keep the webcam settings the same and compare stars of the same spectral type then I get a decent comparison of magnitude. I tried this with the main stars in the Plough (which are mostly A-type stars) and got a magnitude comparison and therefore distance approximation. I haven't had much chance to play around with this yet and feel it needs a new post anyway! I'll also follow this post with a description of how to get a spectrum from using the Star Analyser.

Saturn

It's been a while since my last post and the almost permanent twilight of this time of year means I've produced no new material for about 6 weeks. Still, this gives me the chance to catch up on some webcam pictures from earlier this year.

For stargazers like myself who aren't that interested in starting an observing session at two in the morning, Saturn came back into range in around early March. For pre-midnight observing it was still lurking in the haze near the horizon and, for my location, in the direction of Birmingham city centre but Saturn is Saturn and it was my first chance to see it through the new 'scope.

It's just not possible to tire of seeing those rings and I was at the eyepiece for most of session - it's not the same looking at a laptop screen! I recorded a couple of decent videos, the best of which produced this:



The rings are still quite narrow but they should open out nicely throughout the year. And I'll have to try and make the most of it - after more than a decade in the northern sky Saturn is now most definitely heading south which will soon mean about 13 years of less favourable viewing.

Kepler

Another of my early lunar targets is Kepler and its near neighbours Marius and Reiner. Kepler is an impact crater located just above the equator on the west-side of the Moon (west of Copernicus).

Kepler has a pronounced ray system similar to that seen around other prominent craters such as Copernicus and Tycho. Ray systems are formed by radial streaks of ejecta thrown up during the formation of an impact crater. Larger chunks of ejecta can also form smaller secondary craters around the main impact site. Ray systems usually have a higher albedo (reflectivity) than the surrounding material so appear brighter.

Here is a picture of Kepler created from a 4 minute webcam video:



Kepler is the crater with the ray system in the top-right, Marius (top) and Reiner (bottom) are towards the left. Marius is another crater that has been flooded by basaltic lava leaving the interior flat and smooth with no central rise.

Aristarchus

After getting plenty of practice on Mars, webcam astrophotography of the Moon is pretty easy - finding the target couldn't be easier and there are plenty of features to focus on.

My recent observing sessions have coincided with a near-full moon so my first lunar targets have been towards the edge of the disk. In this post I'll be looking at Aristarchus which is one of the best known lunar regions.

The Aristarchus crater is located in the north-west of the Moon at the south-east edge of the Aristarchus plateau. It is one of the brightest lunar features with an albedo of nearly double that of most other features. Next to Aristarchus is the slightly smaller crater Herodotus which is darker due to the crater floor being flooded with lava. Evidence of earlier volcanic activity is also seen in the prominent rille Vallis Schröteri which winds its way northwards from Herodotus.

This region is a satisfying target for astrophotography and here are a couple of images from my observing sessions on Feb 26th and March 27th 2010.




The brightness and depth of the features shows up well in both pictures, particularly the bottom picture which has a more prominent terminator and a slightly sharper angle of observation.

From a technical point of view, each picture is based on stacking the best 400 or so frames from a 4 minute video. I did very little processing the stacked image - simply increasing the contrast, decreasing the brightness and making a few small adjustments on the layers to strengthen some of the finer detail.

Registax Basics

As I mentioned earlier I use Registax to process my webcam videos. There are quite a lot of settings and controls which at first glance can be intimidating, but it's easy to use with a little practice.

The first major step is alignment. For my reference frame I try to pick a relatively high quality frame from the first 50 or so. Then comes the selection of alignment method. So far I've dealt with 2 types of target - planetary and lunar. For planetary videos I use centre of gravity alignment and drop the luminosity threshold to around 30%. This set-up means the alignment process will work for a target with few clear features that jumps around between frames, which is the case for targets such as Mars. For lunar videos I use a single, large alignment box (usually 256) centred on the most obvious feature in the reference frame. I have tried using multi-align but find that it rarely works. Since my videos are quite short (around 4 minutes) there should be little image rotation and I doubt multi-align would significantly improve my final image.

After alignment has completed the images need to be stacked. As a rule of thumb I aim to stack around 400 images which can be varied by changing the quality settings. For a good quality video the lowest quality should be around 95% of the reference frame.

The next few steps are straightforward until the final stage of adjusting wavelets and picture settings. This is something of a process of trial and error but there are some settings that work more often than others. The main controls are the 6 picture layers, the contrast and the brightness. Starting with the easy ones, I find that turning the contrast up (around 130) and brightness down (around -25) usually helps. The layer sliders enhance the detail in different parts of the image but if over-used can create an unnatural looking image. I find that enhancing layers 4 and 5 often has the best effect, usually picking values in the 10-30 range. Since it can be hard to decide on a 'best' image, I always create a set of images using a range of settings and then compare these side by side to pick my favourite.

All in all Registax is very easy to use and can turn a video into a crisp picture in about 15 minutes.