About six months ago I got interested in photographing the Moon, and I’ve been posting some of the results on Facebook. Mostly I’ve used a simple telescope webcam, which can produce high-resolution images of relatively localized areas such as the one above (showing the lunar north pole at the top and the Alpine Valley at the bottom, with the craters Aristoteles and Eudoxus to the east of it). Then a few days ago I posted a wider field of view using a DSLR on the same telescope, and Rhodri Evans asked how that compared with using the DSLR with its own lens.
That prompted me to try a few tests last night (with the Moon at first quarter). For anyone who may have just stumbled across this blog, I should stress that I know almost nothing about astrophotography, so please don’t take this as “best practice” advice. On the other hand, it does give an idea what a beginner can achieve with bottom-of-the-market equipment (I’ll give details of the hardware and software at the end of the post).
To start with, here is a picture of the Moon taken through the 75-300 mm lens that came with the camera. This uses the highest zoom (300 mm) at f/16, 1/250 second exposure, ISO-800 (settings visually optimized using Live View). It’s the sort of picture anyone with a DSLR could take.
Now here’s a single frame taken with the same camera attached to the telescope (i.e. effectively using the telescope as a long telephoto lens). My telescope has a focal length of 600 mm, but I used a 2x Barlow lens as well, which doubled the focal length to 1200 mm (i.e. four times the camera’s own lens). The telescope aperture is 120 mm, which I guess makes it f/10, and again I used a 1/250 exposure and ISO-800.
I also took a short movie (20 seconds, 500 frames) using the same settings, and stacked the result into a single image using Autostakkert. The result is a definite improvement (I tried the same trick with the camera-only shot, but in that case it made things worse).
Here’s a side-by-side comparison of those three photos. The telescope is an improvement on the camera alone, but stacking is what really works the magic.
As I said at the start, I normally use a small astronomical webcam to capture images, because it’s so easy to use. Optically this is identical to attaching the DSLR to the telescope (aperture 120 mm, focal length 1200mm with the 2x Barlow lens), but the field of view is much smaller because the sensor array is tiny (4.8 mm rather than 22.3 mm). For the same reason the image drifts across the screen more quickly (I don’t have a tracking mount), so I only have time to grab a 10-second video, or 300 frames at 30 fps. Nevertheless the results – such as the example at the top of this post – are impressive after stacking in Autostakkert.
Here is a side-by-side comparison of that picture with the same area seen in the single-frame DSLR+telescope and DSLR-alone images:
For reference, my telescope is a Skywatcher Startravel-120 refractor on an equatorial mount, which cost me £389. There are plenty of other good telescopes available around this price. The webcam is a ZWO ASI120MC, which at around £170 is pretty much the cheapest astronomical camera on the market. Similarly my DSLR is from the bottom of the market – a Canon EOS 100D, which I got complete with two zoom lenses for just £379.
The ZWO camera came with two pieces of software – SharpCap for capturing videos and Autostakkert for stacking – but they’re both freely available online (here and here respectively). In the examples shown here, I used GIMP (also free) to crop, rotate and contrast-enhance the images.