The Moon with a 200mm Newtonian and two cameras – November 1, 2014
I’ve just sold my C11 SCT…My long-time friend…the C11.
And until my new larger scope will arrive (which will be subjected to large modifications to be handled by the NEQ6 mount) I’m using for my lunar images another, even longer-time friend, an 8″ F/5 Newtonian.
Plus, I had the chance of buying a second-hand DMK21AU04.AS camera (mostly for my wife which I hope will eventually get the lunar-imaging virus).
I’ve wanted (for some time) to compare this camera with the ASI120MM, both on the same scope, since one is almost double the price compared to the other, while the actual resolution is four times smaller. Also, the imaging sensors are quite different: one is a CCD while the other is a CMOS.
Well, together with my wife and her brother, we went to my usual observing place and mounted the equipment before waiting for one hour for the mirrors of the 8″ scope to cool down.
The equipment consisted of a (highly) modified 8″ F/5 Newtonian, a 2.25x Baader Barlow lens and the two cameras both filtered with the same Baader Red filter. The images bellow are stacks of 800 frames out of 2000, in variable seeing conditions, from 4/10 to 6/10.
Before starting the actual comparison between the ASI and DMK cameras, I had some rather good seeing moments (despite the Moon being only 32 degrees above the horizon) and started a brief imaging session of some of the more “good-looking” lunar features.
The first one: Rupes Recta and the trio of craters Alphonsus, Arzachel and Ptolemaeus. The image is the resulting work of two-people, myself doing the settings and processing while my brother-in-law correcting for the mount’s periodic error.
And because I’ve imaged the same region a day before, I’ve also placed part of the above mosaic in a two-day comparison image with the one acquired on October 31th (the one on the left):
Another nicely looking region (also at the eyepiece in good seeing moments before the imaging session) was the South Pole, with Clavius dominating the view. To bad that for the image below I had some rather poor seeing conditions…
A few minutes later the seeing improved, and I could take some images with the ASI camera, while my wife took some with the DMK camera afterwards, allowing the following comparison between the final images. I’ve processed them in the same manner, except the final resizing (done to better compare the actual resolution on the surface of the Moon):
Another comparison between the cameras, this time with the same resize factor (120%), showing crater Plato in two images acquired in a bit different seeing conditions: my image with the ASI was taken in 5-6/10 seeing, while my wife’s image was in some poorer seeing (3-4/10). Still, the images show the advantage of the smaller ASI pixel size, with the small four craters near the center of Plato being far better visible not only due to seeing but mostly due to the higher image scale.
And another fascinating area imaged in some good conditions with the ASI, with Rima Hadley visible in the lower part of the image:
The seeing was again good for the following image, which shows Copernicus in shadows, imaged again with the ASI.
My final conclusion of this brief test with the two cameras is that both perform more or less the same regarding the noise level and sensitivity at the same resolution (with the ASI working in ROI mode at 640×480 pixels to be comparable with the DMK), but the higher frame rate of the ASI is definitely an advantage. Of course, the settings for both cameras were kept equal in all comparison images, and the optical chain was the same, with the Baader 2.25x Barlow lens placed almost at the same distance form the sensors (an error of about 2 mm is still present, with the ASI being slightly closer to the Barlow lens).
The advantages of the ASI are the lower price, the higher resolution, the higher frame rate at 640×480, and the smaller pixel size, which works as an invisible Barlow lens allowing at the same settings (compared to the DMK) a higher image scale without extra noise due to lower light (from higher amplification factors).
But there are also some other issues which I’m still going to investigate further, like the data transfer from the CMOS sensor of the ASI compared to the CCD of the DMK. For fast moving targets (like the ISS onto the Sun’s surface in a transit event) the ASI fails to produce a “true” image, while the DMK doesn’t. I will instigate this in a future ISS pass.
But, the results and conclusions place the ASI above the DMK (from my point of view), at least for lunar imaging and especially lunar mosaics. The DMK images are very much comparable with the ASI’s ones, but the lower image scale means that the smallest details will never be present in the final data.
I am very glad I did buy the DMK, but only because it was at a second-hand price. After using the ASI for some time now, and considering it’s low price, I wouldn’t buy any other new cameras on the market today.
(November 2, 2014)