Author Archives: Roger Samworth

Lunar data mining and “Alexander’s beaded rim”

October’s “Sky at Night” magazine has quite a few lunar images. The regular “Moonwatch” article is about the crater “Alexander” and its environs. Alexander is a difficult to identify eroded crater. In particular it talks about “Alexander’s beaded rim” about which it says “The peaks that define Alexander’s western rim appear like tiny stars arranged in a beautiful arc”. I have to confess that from the images in the magazine, this clair-obscur effect totally eluded me.

So, as I now have quite an extensive set of window-sill based images (of varying quality!) of the Moon in a variety of phases a bit of data mining was indicated. I found the following image which I cropped and labelled the features to be approximately the same as that in the magazine;

Then, after a bit of contrast stretching in GIMP, we get this:

NOW I can see an effect, although whether it is the one they were talking about, I don’t know! It is pretty though.

While looking at this image set, nearby there was also a pretty good manifestation of the Lunar “V”.

Another attempt at Mons Rumker 11/09/2019

I have been trying for a long time to get a reasonable image of Mons Rumker to add to my lunar dome images. See

Rumker, being so close to the limb, is very tricky to get the terminator lighting and libration state right. Here is another go tonight. Slightly better than the last effort above, I fancy, but still not wonderful.



Another DSO challenge – the Hickson catalogue

The appearance of Pegasus reminds me that there are a couple of well-known compact galaxy clusters in that constellation. One of them, Stephan’s Quintet is included in a catalogue of galaxy clusters – the Hickson catalogue

It is a pretty challenging set of targets for all you astrophotographers out there! There is a sub-set of the easier ones (!) at

I have only managed to image 3 out of this list, so far, and here are some PD images from my archive to wet your appetite.

Hickson 44, comprising NGC3187, NGC3193, NGC3190 and NGC3185

Hickson 68, comprising NGC5350, NGC5354, NGC5353, NGC5355 and NGC5358

Hickson 92 (including Stephans Quintet), comprising NGC7320C, NGC7320, NGC7319, NGC7318B, NGC7318A and NGC7317

The other well-known cluster in Pegasus is the Deer-Lick group, comprising NGC7336, NGC7335, NGC7331, NGC7340 and NGC7337

Happy imaging!



GIMP, Reprocessing and C12 / NGC6946

Many thanks to Andy for posting his instructions for image processing – obviously caught the bug!

Its nice that someone else is also trying things out. The simple method there for reducing star trails had never occurred to me for example!

There are many different ways of achieving an objective once you have a general purpose toolbox, and often a combination of methods is the best option, The GIMP “pyastro” plugin is pretty good for rounding stars for instance, but doesn’t always work, especially when the stars are in front of an extended DSO.

That brings me to C12 / NGC6946. Rob has just posted a really good image of this. I posted one way back in 2015 (that is very poor compared to Rob’s) at

It suffers from all sorts of problems such as star trails and halos, so can we do anything about it?

I’ll not bore everyone by recounting the exact sequence of processing steps, as it will be different for every image.

Suffice it to say it was a combination of despeckle / star trail reduction / and dehaloing as I described here:

The dehalo method for deep sky objects using GIMP.

Anyway, here is the original as posted, and the reprocessed version.

Still not a patch on Rob’s, of course!

Clustering 08/09/2019

I wasn’t ambitious enough to go hunting local group galaxies last night, but it occurred to me that I hadn’t got a half decent image of the Perseus double cluster, for all the same reasons I hadn’t got an image of M31. So, equipping the PD with the 5-100mm zoom lens again, I tried to rectify the situation. Here is NGC669 / NGC884 / or C14, as you choose.


By this time, the Pleiades (M45) had come into view. Although that means winter is coming, it is always nice to welcome back and take a snapshot of old friends.


Here is another version with the nebulosity suppressed a bit:

Local project?

How about this for a project?

Observe/sketch/image as many of the Local Group of galaxies as possible.

Meet the Shy Side of the Local Group

They are well placed at the moment. There is the obvious M31, M32, M110 and M33. There is also the rather less obvious NGC147 and NGC185 (C17 and C18). There are also others.

Here are images of M32 and M110 I have extracted (not too successfully!) from existing images of M31,

and here is a reprocessed image I obtained last year of NGC185. So far I have never managed NGC147.

Anyone up for a challenge?