Astrophotography with CCD cameras
I am presently working with two black&white surveillance cameras that can add multiple frames internally. That results in exposure times from 1/10000 seconds up to 2.5 seconds in case of the Mintron MTV 12V1C-EX (left) and from 1/25 seconds up to 10 seconds for the Watec WAT 120-N (right). Both cameras are equipped with high-sensitive low-noise SONY CCD-sensors but not cooled. Even in summer nights at moderate temperatures the number of hot pixels is very low. Especially the Watec camera can be compared to native CCD-cameras for astrophotography. It just lacks cooling and exposures longer than 10 seconds. On the other hand, this is even an advantage because with maximum 10 seconds exposure the polar alignment and guiding accuracy are far less critical. The tracking accuracy of my mount is sufficient and a rough polar alignment is enough to get decent images made by stacking of several hundered frames of 10 seconds each. Also, as mentioned elsewhere on my website the seeing, wind gusts, satellites, aircrafts, clouds and many other nuisances are less critical with such comparatively short exposures. I always have the possibility to define a personal critical quality level for each series of frames and an I am free to discard all frames that are below that level. Usually the rate of discarded frames is low but can be as high as 50% if the conditions are poor.
Why am I using for some years now a CCD camera and not a camera for chemical film? Astrophotography with SLR cameras and chemical film is simple, cheap and ... often frustrating. And the latter for various reasons:
1. It takes a lot of time until all frames are exposed.
2. It takes some time until the film is developed.
3. The images are under or over exposed.
4. The target is not as nicely centered as planned.
5. The target is so small that details are barely perceptible.
6. It needs hours of guiding in the cold in an awkward position.
7. The film manufacturer "improved" the emulsion again but the response of the emulsion with time and the H-alpha sensitivity were reduced.
8. The film is on the one hand very sensitive but on the other hand very grainy.
Dedicated CCD cameras for astrophotography have always been and are still quite expensive, need a lot of power and maybe water circulation for cooling and need some time for read-out of the data (even with USB 2.0 nowadays). This reduces its mobility, however, a mobile use of CCD cameras is possible but needs some preparation. Then I stumbled about OES's CMOSfB camera, which was (compared to real CCD cameras) a bargain, thermoelectrically cooled and according to the data sheets equiopped with a sensitive and low noise CMOS chip.
All I achieved were a few images of the moon. The reasons were that the camera needed a current of 1.5 Ampere, that the notebook needed another 3 Ampere and the mount 0.5 Ampere, alltogether 5 Ampere. And the mount had to be exactly polar aligned. So I started with power supply from AC transformers but did not succeed in aligning the mount well enough because my house was blocking the sight of Polaris. Without guiding, stars were always short lines and the download times were quite long. Test images of M42 showed that the camera had some capability but a weight of more than 1 kilogram was quite large. Operating the camera was not done quick and simple.
Then the Mintron MTV-12V1C was introduced: A CCTV surveillance camera of industrial quality that could expose up to 2.5 seconds per frame. Images of some users showed that this simple to operate camera had the capability to image many deep-sky targets with ease and without cooling. That was very attractive for me. However, the output was an analog video signal that required a frame grabber for digitizing the images in a PC. At that time (USB 1.1 was still standard!) I did not find a suitable frame grabber, that could deliver the full resolution, for my notebook and decided to use the camera with my PC at home and to take images from my backyard in a small town.
I was quite impressed by the results I could obtain with the Mintron camera. There were some points that could have been improved. The background was quite uneven and quite illuminated at the left edge due to the infrared radiation of the amplifier. The integration time of 2.5 seconds could be longer and a USB port for data transmission would have been a really nice feature. However, I could carry along with the weak points of this camera and the max. 2.5 seconds exposure per frame were alright to obtain round stars even without good polar alignment.
A year later in early 2004 I upgraded to the Watec WAT-120N that has a similar CCD chip as the Mintron but can expose until 10 seconds per frame, has a far more even and dark background and can be controlled via an external control box. With this camera I was able to image galaxies, even next to the full moon and from near the city center of our town. Quite impressive! The polar alignment of the mount was easier than I thought after I found that the tiles on my backyard floors are exactly east-west aligned. All I have to do now is to align the tripod legs with the tiles and to level the mount. The house is still blocking the sight of Polaris from my backyard but now I know how to setup and align the mount very quickly. The stars are since April 2004 much rounder.
I use the Watec since more than 3 years now and imaged everything that did fit on the CCD chip: Globular clusters, planetary nebulae, galaxies, reflection and some emission nebulae and a few open clusters. With my C9.25 at f/6 and stacking about 180 images of 10 seconds exposure each I can reach stars of 20th magnitude. The anti-blooming gate and the non-linearity near saturation make it easy to image details even near the core of bright galaxies and faint details in the outskirts at the same time. The 12 by 16 arcminutes field of view is sometimes a limitation that requires assembling mosaics from several images.
The new G11 Gemini mount makes setup, polar alignment and targeting the deep-sky wonders much more easy. Its pointing accuracy and tracking capability are impressive. I still do not guide and see potential for even longer exposure times than 10 seconds and maybe a larger CCD chip and - maybe - even a little bit of cooling ...
Watec WAT-120N CCTV camera
Mintron 12V1C-EX CCTV camera
Sensitivity comparison of Watec WAT-120N and Mintron 12V1C-EX
CCTV-Camera attached to the telescope
Comparison of different frame grabbers
OES CMOSfB camera
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