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.
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):
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: