Back in January 2008, I was fortunate enough to get my hands on a DMK41AF02 from Astronomy Cameras. It’s a monochrome webcam with a 1/2″ CCD that can capture at 15fps in 1280×960 pixel resolution – just perfect for lunar astrophotography where a large field of view really matters.
The images below were captured on the 27th January 2008 in average seeing, but weren’t processed until now due to limitations in Registax 4. The large frame size produced by the DMK41AF02 meant that a 2 minute AVI was over 2 gigabytes in size (using the Y800 codec), and Registax 4 just couldn’t load those large files.
Luckily though, in recent months I’ve been on the Registax 5 beta-testing program (it’s due for release any week now!) and so I’ve been able to use Registax 5 to process these AVI’s, as it handles large file sizes without a problem.
It was worth the wait – the large field of view combined with a mosaic of several frames has allowed me to capture some larger than normal patches of the moon in high-resolution, when compared with the DMK21AU04 which has 640×480 pixel resolution.
Please continue reading to see more images from the session and for more information and a review of the DMK41AF02 in practise.
While the large resolution (and field of view) is immensely valuable for capturing lunar images, the trade-off is that the camera can only capture at a maximum 15 frames per second (fps). You can still ‘freeze the seeing’ with a fast shutter speed, especially on bright targets like the Moon, but you just can’t capture as many frames in a short period of time as you can with say the lower resolution DMK21AU04 which can capture at 60 fps.
The DMK41AF02 uses the 1/2″ Sony ICX205AL CCD, which has a pixel size of 4.65 microns. This helps to give a large image scale across the large resolution, however the more shallow well depth combined with the 8-bit ADC means the camera isn’t as light-sensitive as those cameras with a larger pixel size. This will reduce its effectiveness for fainter objects like imaging the planets, but such a large field of view isn’t necessary for planetary imaging.
Ideally I would’ve liked to have used the USB version (DMK41AU02) but none were in stock at the time so I went with the firewire version. My laptop doesn’t have a powered 6-pin firewire port, so I needed an external powered firewire hub to power the camera, and a PCMCIA card to download the data.
The large frame size puts quite a demand on your disk speed so you’ll definitely want a 7200 rpm drive. I was able to capture to an external harddrive using my reasonably old Dell laptop and only dropped a small percentage of frames out of each 2 minute run.
The two images below show an excellent comparison of how impressive the larger field of view of the DMK41AU02 is when compared with the 640×480 resolution of the DMK21AU04. Both images are of the crater Tycho, both using the 12″ Newt and 5x powermate (same focal length), imaged under a similar lunar phase showing similar lighting and contrast. Please click the images to see the full size and expand them if your browser resizes it.
The image of Tycho with the DMK41AF02 was captured at 15fps, taking 2 minutes to accumulate 1800 frames. With the DMK21AU04 running at 60fps, an equivalent 1800 frames was captured in only 30 seconds. For lunar images there isn’t really any capture time limitations as there is with planets like Jupiter, so 30 seconds versus 2 minutes isn’t necessarily a good or a bad thing, as the seeing can change quite dramatically in the course of a few minutes. I don’t have any details of the shutter speeds or gain used for both captures.
So what is the DMK41AF02 like as a planetary imaging camera?
It just so happens that the seeing improved after capturing the lunar images above, so I turned my scope towards Saturn and captured data in good seeing with both the DMK21AU04 and the DMK41AF02 – again with the 12″ Newt and 5x powermate, with Astronomik RGB filters mounted in an Atik manual filter wheel.
The first image below was taken with the DMK41AF02. It has been cropped but the image scale is as captured, at approximately 10.5 metres focal length.
It was captured at 15fps, shutter speed 1/15s with gain at max for 2 minutes in each channel (1800 frames per channel). 500 frames from each channel were stacked using Registax 5.
The image below was captured with the DMK21AU04. It has been slightly cropped but the image scale is as captured.
While looking at the raw data, and while processing, a couple of differences became clear:
- As mentioned above the smaller pixel size in the DMK41AF02 gives a larger image scale than the DMK21AF04 with the same telescope and powermate.
- Also as mentioned above, the smaller pixel size means the camera is not as sensitive as the DMK21AU04 with the larger 5.6 micron cells. This is clear when you look at the faint crepe ring on Saturn, which is difficult to capture well, but is more evident in the DMK21AU04 image.
- The smaller pixel size in the DMK41AF02 produces more dark current and readout noise than the larger pixels, and this was evident in the raw data which was much grainier in the DMK41F04 when compared with the DMK21AU04 (see comparison below).
- Saturn was captured at 15fps in both cameras in this example, but for brighter objects like Venus and Jupiter, a faster framerate is desired – especially for Jupiter where you are limited in your capture time due to the fast rotation of the planet. You want to capture as many frames as possible in the shortest time.
- For planetary imaging you want to ‘freeze the seeing’ and use the fastest shutter speed you can. For Jupiter, ideally this is 1/30s if capturing at 30fps. Using the DMK41AF02, you will be limited to 1/15s capturing at 15fps. This is still passable, but will require above average seeing to get a similar quality image.
So out of all that, I’d say that the DMK41AF02 definitely has the capabilities to be a planetary imaging camera, and you can see that from my image above, but it’s not the best camera for the job if you want to get the best results on the planets. The DMK21AU04 still represents the best planetary imaging camera available at a relatively low cost.
As a lunar imaging camera, the DMK41AF02 really excels. The huge field of view makes it much easier to capture whole features on the moon when imaging at long focal lengths – something that usually required a 2-4 panel mosaic with the smaller resolution in the DMK21AU04.
Combine that large field of view with a mosaic, and you can capture large areas of the moon in a single session to produce wonderful broad vistas of our nearest celestial neighbour.
The excellent IC Capture software comes standard with the DMK range, and can be used with any of the DMK cameras. I was even able to have both cameras plugged in and quickly switch between one device and the other.
In summary, the DMK41AF02 is a top performing web cam ideally suited for lunar astrophotography.
For further reading, please see my other articles and reviews:
- RGB Planetary Imaging with a Monochrome Camera
- Planetary Imaging and Image Processing
- Astrovid Voyager X – Planetary Imaging Camera
Thanks for reading. If you have any questions, please feel free to comment below or contact me directly.