I don’t often acquire images of deep sky objects (galaxies, star clusters, nebulae), but whenever I do, it is because I either get under dark skies or I want to test something new.
The main reason I don’t do this often is because, not having a fixed observatory, I must drive to my observing place in the middle of the night and I can’t stay at that location for more than a few hours (usually for a maximum of 3 hrs). Plus, my observing spot is rather heavily light polluted (at least for deep sky imaging). Since most sessions occur between working days I must be ready for work in the morning. All of these make deep sky imaging quite a challenge for me, and a good image of a galaxy for example needs a few hours of data and good sky conditions. Plus, my EQ-6 mount is far from good when it comes to long exposures at long focal lengths. I did complete my tracking routine using the PHD software but the mount itself can’t cope with large instruments like the 355mm F/5 homemade Newtonian, which weights around 18 kilograms. So any serious long exposure images using this scope were out of the question. Until a few days ago…
…when I’ve decided to test my newest camera, the ZWO ASI 174MM.
There is a deep sky imaging method that is used by amateurs having similar difficulties like mine: instead of acquiring only a few long exposures (say from 5 to 20 minutes per frame), one images an object with many, many short exposures (a few seconds) and then composes them into a single final image. This method ensures a very good noise level correction due to the large number of frames (usually in the hundreds) and also a very sharp image due to the low exposure time which can “freeze” a bit the seeing (a long exposure time usually is affected more by atmospheric seeing compared to a short exposure). Plus, the requirements for expensive cooled cameras don’t play a significant role anymore: you can do this type of imaging with much cheaper cameras, with no active cooling at all.
This was the basis of my latest imaging session which was dedicated to a single object: the Messier 66 galaxy from the “Leo triplet” galaxy group, in the constellation of Leo.
The galaxy is about 36 million light years away, which means that it is quite far, but it does show a lot of fine details in the spiral arms, many of which I was surprised to detect in my image.
The image, enlarged and rotated a bit from original, showing details in the arms:
And the original size image, showing almost the full field of the camera (I did use some crop due to the poor tracking):
A lot of small faint galaxies are also present in the image, some of which have a magnitude of around +20!
The image was acquired using the 355mm F/5 homemade Newtonian with the ASI 174MM camera placed at the focal plane. No filter was used. No field-flattener was used (and this is visible in the image as the stars towards the corners have elongated shapes). The sky conditions were far from good in terms of transparency, with a visual magnitude of +5 in Leo, lots of humidity, minus 7 degrees Celsius, and of course some light pollution from Bucharest (the location is just 10 km to the South). On the other hand, the seeing was good (6/10). Dew formed on my secondary mirror after only 30 minutes from arriving at the observing location, and had to be removed about 10 times during the session. This dew problem had another effect: focusing (which shifts during the session) was done poorly a few times, making the stars appear less point-like.
The final image is a stack of 600 frames: 300×5 seconds, and 300×8 seconds. So, the total exposure time was 65 minutes. The gain setting was at the maximum (400 in this case), and this is were the camera’s performance was visible. I had no dark or flat frames acquired, and thus I had to create a synthetic flat from the actual data. A lot of post processing in AstraImage and Photoshop had to be performed, but at the end I feel that the procedure was somehow easier compared to DSLR image processing.
Due to the shape of my secondary mirror holder (visible in this image), I had to improvise a second diffraction pattern by using a thick wire mounted at 90 degrees on the mirror’s holder. This is why my stars have a “nice” cross shaped diffraction pattern, and not a single spike pattern.
Despite all, I believe the final result is actually quite good, showing a lot of detail in the galaxy. This is why there is a “HR” in the title of the post: HR stands for High Resolution, but it is usually used for planetary/lunar images. For this image I’ve used it to signal the good details visible in the final image, but this is still far from what an 355mm scope can do in terms of resolution if the conditions are right. Perhaps next time I shall use also a Barlow lens to get a higher sampling factor, and thus better details. I should add some colors data also…