Don't be afraid of CCD

Don't be afraid of CCD

Professional CCDs (VI)

After have described CCD cameras used by amateurs, let's see what are the latest technologies used by professionals.

8k and 12k CCDs

In 1995, at a time where amateurs till used 512 pixels CCDs, at Hawaii CFHT Observatory, David Jewitt (today at UCLA) and his colleagues already used CCDs offering a resolution 256 times higher ! One of the first model was a 8192x8192 pixels device (UH8K) usually attached at the prime focus of the 2.2 m telescope. It was used to discover the first transneptunian asteroid.

In 2001, astronomers ordered a 12 kpixels CCD of 15 microns (CFH12K). Its field reaches 42x28 arc-min at f/4 what means that is covers the entire Moon !

Attached to the CFHT, in this configuration its resolution was 0.206 arc-sec per pixel. In other words, even without doing high resolution, if astronomers enlarge the image they can detect features as small as 1 km wide on the Moon surface ! This is fine you will tell me, but where is the benefit ? This CCD camera is in fact the most powerful we have ever made in term of resolution and exceeds by a 10-factor the most sophisticated amateur CCD camera of that time !


8192 x 8192 pixels CCD	12 k-pixels CCD of 15 microns

Take an example in comparing the CFH12K to the Lynxx CCD made by Spectra Source Instruments for amateurs at the same time (~2000). This latter uses a TC255 chip measuring 336x243 pixels of 10 microns each. Its matrix has the same size as the one of SBIG ST-4 described on the first page. It is equipped with an electro-mechanical shutter and has a 12-bit A/D converter allowing 4096 levels of gray, a substantial progress compared to ST-4. Although its size is modest its offers an enviable resolution of 0.25 arc-sec per pixel on a 12" f/39 scope, near the one of the 12k-CCD. Only drawback, at this focal ratio the field does not exceed 87x63 arc-seconds, while at f/4 its resolution drop to... 1.8 arc-second per pixel, 9 times lower that the one of the 12k-CCD ! If the Lynxx creates images of 160 KB in size containing 4096 brightness levels, the 12k-CCD requires 200 MB of disk space to store each image ! Hard to beat professionals...

MegaCam, 340 megapixels

In 2003, CEA engineers built the MegaCam : 40 CCDs of 2048 x 4612 pixels each for a total of 340 megapixels ! This CCD covers a field 1 x 1° wide (the field of 4 full Moon) with a spatial resolution of 0.187"/pixel in order to sample correctly the signal when seeing is at best (CFHT where the MegaCam is installed reaches an average resolution of 0.7"). Drawback, the weight of the system that contains the MegaCam is 375 kg and measures 1.7 m of height for 1.2 m in diameter !


At left, close-up on the MegaCam during its design by CEA at Saclay in France. At right, close-up on the first deep sky image recorded with the MegaCam and published on IAF Terapix website in September 2004. This is a RGB composite of 250 individual frames. More than 600,000 objects are visible up to magnitude 26.

OmegaCAM, 268 megapixels

Since 2011, the summit of Mt Paranal where VLT telescopes are installed is complete with the VST (VLT Survey Telescope) of 2.65 m of diametre.

It is equipped with the OmegaCAM CCD of 16k x 16k pixels (26 cm on each size !) recording images of 268 MB. As VLT or the future LSST (see below), it generates 30 TB of raw images each year that are transmitted to ESO for processing.

The Zwicky Transient Facility, 606 megapixels

The Zwicky Transient Facility (ZTF) is a robotic CCD camera made of 16 CCDs of 6144x6160 pixels each offering a resolution of 606 megapixels. It is installed since 2017 on the Mont Palomar's Samuel Oschin Schmidt telescope of 1.22 m (48"). Offering a field of 47° squares, the ZTF is capable of scanning more than 3750° squares per hour up to magnitude 20.5. The camera is used to detect transient light phenomena such as novae, supernovae or TDE, some of which being associated with X-ray, γ or neutrino emissions for which other observatories will take over (for example satellites for emissions X and γ and the IceCube for neutrinos).

Hyper Suprime-Cam, 870 megapixels

In 2012, the 8.2 m Subaru telescope has been equipped with the Hyper Suprime-Cam (HSC). Built by National Astronomical Observatory of Japan in collaboration with international academic and industrial partners, the systems includes CCD sensors, lenses, filters, and a shutter. The camera is at the scale of the telescope : the HSC camera is about 3 m in length, the first lens is 82 cm in diameter for a total weight of about 3 tons !

The HSC camera includes a highly sensitive CCD offering a resolution of 870 megapixels and covers a field of 1.77° in diameter or seven times the apparent surface of the full Moon. This CCD matrix is installed inside a vacuum cryogenic dewar and is operated at -100°C where the dark current becomes negligible. Mechanical alignment of optics is ensured by six customized actuators.

Gigacam, 1.4 gigapixel

En 2007, IfA astronomers in collaboration with MIT Lincoln Laboratory engineers pushed the feat still further with the Gigacam GPC1 : 4096 CCDs offering a total resolution of 1.4 gigapixel. The image is 40 cm in diameter, the diameter of CCD opening measuring 56 cm !


At left, one of the 64 OTAs constituing a row or a column of CCDs equipping the GPC1 Gigacam. Each OTA includes 8x8 CCDs that can be controlled and read individually. Each OTA shows a resolution of 360000 pixels. At right, GPC1 installed in its mount of 56 cm of diameter. In total, GPC1 shows a resolution of 1.4 gigapixels. Documents IfA.

The system is constituted of an array of 64 x 64 CCDs offering each a resolution of 600x600 pixels. CCDs are mounted in 8x8 array on a silicon chip called "orthogonal transfer array" (OTA) of about 5 cm². Each CCD of each OTA can be controlled and read individually. In total, 66 OTAs are placed at the telescope focal plane (the 4 corners are omitted because they are too far of the optical axis). The system can record objects to magnitude +24 in 60 seconds of integration, or 10 million times dimmer than the weakest objects visible naked eye !

The 1.80 m Pan-STARRS 1 telescope equipped with the 1.5 Gpixels gigacam installed on top of Mt Haleakala in Hawaii. Document PS1SC.

The GPC1 camera was mounted in August 2007 on the first of four 1.80 m PS1 (Pan-STARRS-1) telescopes currently installed on top of Haleakala on Maui island, Hawaii. The system has a 3° field of view with a spatial resolution of about 0.3"/pixel. PS1 can survey 6000 deg² per night. The whole available sky as seen from Hawaii will be observed 3 times during the dark time in each lunar cycle.

Thanks in particular to its pipeline processor architecture ensuring the chain-execution of instructions, the computing infrastructure associated to this system can process the 6000 images recorded each night in 15 hours of processing. Data are thus practically immediately available to astronomers associated to this program.

Thanks to these telescopes at the cutting edge that rejoign those at Kitt Peak already operational, in 2010 astronomers had catalogued 90% of the largest NEO including PHA's potentially hazardous that might precipitate a global disaster.

Performance side, the night of January 29, 2016, thanks to PS1 astronomers discovered 19 new NEA. PS1 has also to its credit the discovery of several supernovae and new Trojan satellites around Neptune.

Today, four identical telescopes are equipped with the Gigacam and have for task to monitor trajectories of potential hazardous asteroids. They mainly work in the visible spectrum between 500 and 800 nm as well as in standard photometric G, R and I bands, including Z band, in which works SDSS, and in infrared in Y band at 1 micron, where by nature CCDs are always very sensitive.

LSST 3.2 gigapixels CCD

The future 8.40 m LSST synoptic telescope, alias Vera Rubin Observatory that will be partly operational in Chile from 2020 will be equipped with a 3.2 gigapixels CCD camera. The size of the CCD assembly is 1.65 m in diametre and its weight exceeds 3 tons. This system cooled down to -120°C will cover a field over 3° wide which surface is equivalent to 40 times the apparent size of the Moon.

As we see below, at the focal plane the diameter of the CCD area is about 64 cm. Thanks to this system, this telescope that will work between 400-1060 nm will be able to record images of 3 GB each 17 seconds, generating 30 TB of data each night.


Renderings of the LSST 3.2 gigapixels CCD camera of 1.65 m in diameter, and a mockup of the field covered at the focal plane by the gigacam presented by Suzanne Jacoby from LSST. The CCD area is approx. 64 cm in diameter and covers a field over 3° wide. Documents SLAC/LSST.

The LSST mission will be to regularly take pictures of the sky to detect any change from one period to another in order to better understand the nature of the dark energy and the dynamics of the Universe.

But where will they stop ?...

Know-how

One does not act as "CCD imagery guru" from one day to another. If you can master a CCD camera in a few hours, to get high resolution images using a CCD camera or achieving a quality stacking and image processing require experience that one can only acquire by a regular practice.

That said, being given the relative ease of use of preprocessing and postprocessing software, the apparent difficulty of the first contact with a CCD and image processing tools should not put off any amateur interested in the subject.

For more information

Reviews

Astronomy Photographer of the Year: Equipment Analysis 2019-2021, Skies & Scopes, 2021

Astrophotography Cameras 2021: which are the best?, BBC/Sky at Night

Best Cameras for Astrophotography within your budget in 2021, Antoine & Dalia Grelin

The best camera for astrophotography in 2021: tools and lenses to shoot night skies, Jamie Carter and James Artaius

Astrophotography Cameras, AstroBackyard, 2019

CCD camera vendors

What Is a CCD?, Spectral Instruments Inc.

CCD's manufacturers (1001 Links, letter M)

2010 CCD camera buyers guide (by subscription)

Teleskop Service (most brands)