The giant sunspot AR2192

Through a rare break in the clouds, I have been able to snap a photo of a true giant. Here is Helios in all its glory.


Our Sun, with tree shadows superposed

There were scattered clouds, but the shadows on the disk are due to trees in the foreground (out of focus). Other blotches on the disk are more distant, i.e. they are on the solar surface, the photosphere:

giant spot

Huge sunspot complex

The extent of this sunspot group is much bigger than Earth. We can clearly see the darkest core (umbra) and the lighter boundary (penumbra) of the sunspot. Magnetic field lines penetrate the photosphere at the locations of the sunspots, aligning and cooling the plasma which then appears darker against the hotter surface. Sharper views are possible with a bit more effort where we can see the granules produced by convection, and the way finer details in the penumbra align following the magnetic field lines, like iron filings along the field lines of a magnet. The sunspot evolves as the Sun’s rotation carries it across the disk. NASA’s Solar Dynamics Observatory (SDO) have pretty awesome views of it, and in multiple wavelengths. We live in a time when such content is only a few clicks away.

A magnetogram of the complex, courtesy of SDO:


SDO magnetogram of the giant sunspot complex.

Other, smaller spots can be noticed:

other spots

Smaller sunspots

An interesting project would be to follow the evolution of the larger spot before it disappears from view. Of course, SDO will have such a movie archived in due time. The wonders of the global web…

Only a simple mylar filter was placed in front of the objective lens of the small refractor I used, and a DSLR camera was placed in the prime focus for this shot (1/800 s at ISO 100).

Astrophotography on the (relative) cheap

Having a rare clear night helps when in the Netherlands. Doubly so if circumstances are such that one can have in one’s sight a couple of “standard” astro-photo targets such as M31, our nearest (proper) galactic neighbor, and a really good open star cluster, in this case the Double-Double cluster.

I use a Meade ETX-80 scope, paired with a Canon 1100-D DSLR. With such a setup, long exposures are a no-no, and the results of my run will serve to demonstrate this fact.

Take a look at the final image of M31; it is an equivalent of a 2 minute guided exposure, obtained by stacking a number of individual 2 second exposures. This is the limit of the tracking accuracy of my mount, which makes it almost redundant.

M31 - 2 min. stack

M31 – 2 min. stack

I have imaged using the raw (CR2) format, readout noise subtraction done in camera. Stacking was done using the free Deep Sky stacker (dark and bias frames applied, no flats). The CR2 images were first converted to the DNG format (the DNG converter tool – standard feature of the Astro Distro Linux did that). The stack was saved as a FITS file (a cube actually, one plane per color) and then re-composited into a color image using DS9 (log scaling with an exponent of 3000). The color scaling was done by eye, I don’t claim that I have achieved the correct color multipliers. Finally, the end result was exported as a JPG file, scaled and saved as a PNG for publishing.

Pushing the setup to the limit of the noise has its consequences, as is plainly visible. Visible are also the remaining hot pixels which were formed into snakelike artifacts by the stacking. The two satellites of M31 are also apparent, as is a hint of the disk and some of the dust in it. M31 has a very bright bulge, and we almost always see that (with amateur scopes anyway) thinking that it is the whole thing. It is not.

Now, here is what I got with in-perfect tracking, exposing for 2 minutes, no stacking, same setup. The image was converted from CR2 to DNG format and processed in the Gimp (its DNG plugin does wonders by the way):

M31 trails

M31 trails

This was much more straight forward. Reason? The signal to noise was much better (understandably). So, stacking can only do so much. Especially for faint targets. In this case, I bet that a stack of say 20 2 minute exposures even with trailing stars would  benefit the Andromeda galaxy. It is already a blur, so who cares, right? 😉 I might do such a project…

Same, but for the Double-Double cluster in Perseus:

Double Cluster

Double Cluster


Double Cluster trails

Double Cluster trails