Contacter l'auteur / Contact the author

Recherche dans ce site / Search in this site

 

 

 

 

 

Don't be afraid of CCD

LRGB+Ha image of M33 recorded by Jason Ware with a Meade ACF 12" equipped with an Apogee U9 6303E CCD. Total integration time of 520 min.

Integration time (IV)

The integration time mainly concern deep sky astrophotography because in planetary photography, except Pluto, asterods and comets, all celestial bodies support snaphots.

The typical integration time to record a deep sky object on a CCD is usually between 1 and 5 minutes operating the camera in 1x1 or better in 2x2 binning (merging two pixels reduces the resolution of 50% but increases the CCD sensitivity). This way using a 4" f/6.3 scope we reach magnitude 18 in only 5 minutes of integration and magnitude 20 using a 14" at f/6.3 !

In worst cases, the integration can last 30 minutes on prime focus of 16" scopes to record faintest galaxies (or faint spectra) and 18 times more if you want to produce tricolor composites !

Why 18 times ? Because using a colored filter the incident light passing through the filter is reduced and the CCD camera requests 6 times more exposure to get the same result than an unfiltered image. Then you must take at least 3 exposures, in red, green and blue channels (some add a fourth channel, the luminance with is a grayscale image).

Add to this constraint CCD registration offsets, frame shifts that can occur with fast moving objects (Jupiter, etc). Therefore color CCD is another challenge reserved to the experienced amateur. This is however the best solution to records sky colors but at a time and financial cost. 

KAF- 0400 specifications on a C8 f/6.3 scope

S/N

ratio

B/W

exposure

Tricolor exposure through RGB filters

(2x2 binning)

Noise level

for 30 stacked

1 minute exposures

Red

(585-680)

Green

(496-585)

Blue

(380-502)

9

1 min

2.2 min

2.2 min

10 min

1.00

28

5 min

12 min

12 min

80 min

1.12

50

20 min

41 min

41 min

5.5 hrs

-

Another solution is using a "one shot" color CCD. Astrovid Starlight Xpress MX5c among other models uses a color matrix filter over the pixels composed of "secondary color" dyes in a grid of Cyan, Magenta, Yellow and Green (like the popular CMYK technique). The filters are arranged in such a way that the Luminance component of the image can be extracted with high definition.

Ideally the light reduction is only 33%. The resulting color image is fine but at the expense of lowering the resolution and longer exposures. Also, the MX5c is cooled, but not regulated. That means that you need to take a dark frame at each ambient temperature level each time you use it. Of course you can get this dark frame by image processing too.

Until early 2000, Astrovid had few competitors among them HiSis, Lynxx, Meade and SBIG. These systems didn't include automatic functions. Since that time, the technology has quickly progressed. Not only CCDs are more efficient and larger but they also include automatic functions : auto-focusing, auto exposure settings (optimization), auto-stretch (to bring out dim objects), automatic dark substraction (dark frames are stacked, averaged then substracted automatically), auto-align and stack images, etc.

A handful manufacturers also offer color cameras with a low form factor like e.g. Atik 460EX, QHY12 or very small models like SBIG ST-i Planet Cam and Autoguider, all very adapted to small installations and planetary imaging.

If color digital images of DSO were made in the 90's, at that time it was RGB or LRGB composites. It is only in early 2000's that Astrovid released the MX5c color CCD, followed some years later by the "Deep Sky Imager" (DSI) from Meade (510 x 492 pixels, 9.6x7.5m) displayed at left. To its right, M82 recorded with the Meade DSI by Terry Platt in only 3 minutes of exposure. At right of center, M20 recorded with a Meade DSI by John Hoot at prime focus of a 8" Meade SCT LX200 GPS (stacking of 20 frames of 30 seconds each). At right, Mars recorded on 22 May 2016 by John Earl with a C11 equipped with an Image Source DBK21 618 color CCD (stacking of 400 frames at 60 fps).

The overall resolution of first color CCDs was lower than a monochrom model due to having two arrays together. Although this physical limitation results were not bad at all, with time and new chips, these small color CCD have much improved and give very good resultats, specially in planetary imaging as confirms the above excellent image of Mars.

However, for deep sky imaging, if you are really into critical observing then you have to choice a monochrome CCD camera and acquire a color filters wheel with quality filters (e.g. from Optec). This combination will allow you to adjust the color channels (L,R,G,B, and Ha or CMY) integration times for the sensitivity of your chip, do astrometry and photometry works and even study any object using a single bandpass.

Common problems

Performances of CCD cameras and their defaults go two of a kind and is it in vain to expect recording good images if you do not control all factors that enter in our equation. So let's describe clearly problems that you can encounter in using a CCD camera and methods to avoid them or to reduce their effects.

Form factor

It is a technical term borrowed to physics qualifying the profile of a device (linked to the flow amount reaching a surface). Using a DSLR at the prime focus of a Schmidt-Cassegrain scope, with its length of 12-14 cm, we immediately note that its position just in front of the corrector plate creates an important obstruction, specially in scopes below 300 mm of aperture.

As we see in the below left graph, a Canon 60D DSLR fixed at the prime focus of a C8 system affects seriously the Airy disc of a star, and we lost up to 25% of the image details compared to a system without DSLR.

To read : What are Fastar and HyperStar? How do they work?, Celestron

At left, effect on the Airy disc and image resolution (MTF) of a Canon 60D DSLR mounted at the prime focus of a Celestron C8 (a document from Mike Jones). Result: it is not recommended. At center, a low form factor CCD from ATIK attached on a Starizona Hyperstar system fixed on a Celestron C8. At right, a Canon DSLR attached on the Hyperstar system on a Celestron C11 EdgeHD.

To resolve this issue, the user has to purchase a low form factor DSLR (e.g. Olympus) or a small cylindrical CCD (Atik 460EX, Nightscape 8300, etc) that he will mount on a Hyperstar system from Starizona as we see above (or on the old Celestron Fastar optical system but discontinued since 2005).

In the event (rare) of this solution should be too insecure or unstable on small and light installations, the alternative is using a small HD webcam. Otherwise there is no other choice that either using the Cassegrain focus with a focal reducer (but the weight will be similar) or better, using a larger scope in order to reduce the relative obstruction generated by the sensor placed a the prime focus.

Focusing

Focusing problems and fine focus adjustments are emphasized using electronic camera with a telescope. Since the CCD field is very small, especially in planetary imaging where we works with an eyepiece projection (or a Barlow), a special attention must be given to stabilize image shift and temperature changes. Typically a shift is equal to the square of the amplifying factor of the secondary mirror, which is 5x on a f/10 SCT. A mirror shift of only 0.001 mm creates a focus shift of 0.025 mm which is easily recorded by the CCD detector.

To read : Practical formulae

RGB image of M81 and M82 recorded by Robert Gendler with a 12" f/9 Ritchey-Chrtien scope fixed on AP1200 GEM equipped with a SBIG CCD camera.

Then the focal point positionning varies roughly as the square of the f/ratio. So using a scope at f/6.3 requires a focusing 2.3 times more accurate than at f/10. At prime focus of a f/6 scope, the depth of field (or focus) is only 0.0127 mm  or 0.005". Therefore a zero-shift electrical focuser is highly recommended (like the JMI NGS-F or Robofocus for SCT's) as a locking mechanism to tighten the all imaging train when heavy accessories are attached.

So prior taking your first CCD pictures of celestial bodies, a short calculation will be useful to check the accuracy of your CCD camera in combination with your optical system.

The software provided with most CCD cameras includes a menu dedicated to focusing that takes in charge the tedious task of finding the focal point on the CCD sensor. The camera takes a serie of shoots of a preselected area in changing slightly the focusing. These images are usually exposed about 1 s for bright objects and 10 s for the dimmer. Then the system let you select the the most sharp image. Some models let you also take immediately a dark frame (see further) before the light or raw image. Once the focus is achieved, remains to center the object, either in low resolution to increase de sensitivity or in full mode.

At left, auto-focus procedure on a star used to set an Atik 8300C color CCD connected to a TMB 203 mm f/9 apochromatic refractor. At center and right, respectively the focusing and image acquisition menus in the dedicated Canon BackyardEOS software linked to a Canon EOS 500D (aka Kiss X3) DSLR.

Optical train and backfocus

As in the time of argentic photography, near the focal plane some optical trains can be quite long and heavy up to create a problem of backfocus or to put the stability of small installations at risk. Indeed, a complete imaging train can include : an electric focuser + a focal reducer + a field rotator + an off-axis guider + an adaptive optics + a filter wheel + a CCD camera. Its total length is 20-40 cm depending whether the accessories are thin or large.

Hopefully there are solutions to shorter the length of this set as for example to place some accessories elsewhere (the external electric focuser in the focuser knob of SCT or in the Crayford, the focal reducer in the SCT visual back, etc.) and replacing the off-axis guider and the CCD with a dual-chip CCD combining the auto guider and the imager and including a filter wheel. At the end, it remains an adapter, the field rotator, the adaptive optics and a (larger) CCD camera. Depending on accessories, we can reduce the backfocus between 5 and 20 cm or up to 50% of its length.

A typical optical train (CCDs are a QSI 683wsg-8 imager

including a guide port in which is inserted a Lodestar guide CCD.

Thermal noise

What ever the camera used, the CCD or CMOS sensor being usualy not cooled, astronomy images recorded in low light conditions appear grainy, all the more at summer when the outside temperature increase the thermal noise.

In an ideal CCD camera each pixel would give a brightness level of 0 when there is no light, and a value increasing perfectly linearly with increasing light until it became saturated. In addition, the reponse of every pixel would be identical. Actual CCD cameras are far to reach this ideal objective.

The first reason is that the electron count for a pixel is a function of the number of photons that strike it plus the number of electrons due to "thermal noise" or dark current. In using electronic components, for lack of an ideal efficiency (100% of the inpout energy should be converted without loss), they dissipate some heat that generates a thermal noise that is reduced by half for every 6 or 7C decrease in temperature. This reduction in the total dark current generation rate has of course limits; most CCDs do not function well below -120C. As shows the below graph, in pratice the dark current is almost removed around -100C where the image only shows random read noise.

At left, the increase of thermal noise with temperature of a CCD. At center, a Meade DSI CCD camera equipped with a Sigma 8 mm lens covered with ice but till working by -70C at Concordia base in Antarctica. Document G.Dargaud. At right, a Spectral Instruments CCD 1300S (100 kpixels of 9 m) cooled to -100C.

This cooling can be reached in isolating thermally the device from its environment. CCDs are commonly cooled with liquid nitrogen but the treatement is expensive, Peltier junctions (thermoelectric coolers), or using mechanical pumps (cryo-coolers).

Like with photoamplifiers, a CCD sensor is thus very sensitive to infrared, ambiant temperature and temperature changes. For years many amateur sensors are cooled (the best are regulated) to around -25C or -30C with a stability better than 0.5C. Exceptionnally, some Spectral Instruments models are cooled to -100C.

But external infrared sources can potentially be seen by the sensor although invisible to your eyes. So remove all bright accessories in the neighbor of the sensor which emit infrared light or are not black anodize (or painted flat-black) which is also a good infrared absorber. Such sources are eyepieces holders, digital clocks, digital circles, dew heaters... So, to avoid any parasitic noise, it is advisable to place the CCD camera at ambient temperature and to wait for about half an hour after to have switched on to take your first pictures.

If your images show a thermal noise even using a cooled CCD camera, you can till reduce this noise and increase the signal/noise ratio thanks to a image processing in two steps, pre and post processing. procedures that we will describe next page.

M16 in Serpens. This is a composite of three RGB images recorded with three Astro-Physics scopes : a 180 mm f/7 EDF refractor, a Maksutov-Newton 235 mm f/4.3, and a Maksutov-Cassegrain 250 mm f/14.6. These scopes were equipped with CCDs SBIG ST-8 and ST-10 with a CFW-8 Color Filter Wheel, FLI MaxCam CM10-2E with a Custom Scientific Hydrogen Alpha Filter. Images were processed with CCDSoft, Mira Pro, Maxim DL/CCD, Sigma Beta, and Photoshop. Document published with the courtesy of Philip Perkins, Trent Kjell, and Roland Christen.

Readout noise

This degradation is caused by statistical errors in reading out the number of electrons per pixel (photosite). This relative sampling error decreases inversely with the square root of a pixel brightness level (DN factor). Then the final problem is that the pixels are not equal in their light sensitivity, with typical variations of 1-2 percent among the photosites in an array.

Resolution and binning mode

 Deep sky imagery with a CCD camera requires preferably larger pixel dimensions because large pixels simply collect more photons than smaller. This is what we call the binning, a mode that offers the possibility to sum signals from several adjacents columns and rows of pixels (binning 1x1, 2x2, 4x4, etc). But, drawback, in binning mode 2x2 the resolution drops of 50%, but the sensitivity is improved.

Conversely, in lunar and planetary imaging the amateur searches for the higher resolution and does not need so much light sensitivity. In this case a smaller binning mode is preferable. At last, there is a tendency toward CCD cameras using larger array sizes. A 16-bit "depth" (65536 brightness levels) is preferable to 12 bits (4096 brightness levels).

But all these aspects affect the size of the image, the download time and processing as well as the disk space need to save this file. As for the color, this is no more with 12 or 16 bits that we work, but at least 24 bits. In this case the file size is practically no more managable by amateurs standards and users of scanners know very well that problem. This is for this reason and for an image quality question too that the usage wants that amateurs work from LRGB images to get color composites instead of using color cameras.

Accuracy of the driving system

To picture deep sky objects, the last mechanical element that could potentially be a source of problems is the motorized mount. As explained previously, using such an accurate device as a CCD camera, you dramatize the stability of the instrument and it is mandatory to use a sturdy mount. Have always in mind that the mount is more important than your optics; you will get easier fine pictures with an ordinary optics fixed on an excellent mount than the contrary.

From a pure photographical aspect, an Alt-Az mounting will display the problem of field rotation while you will track an object across the sky. Field derotators made for the popular SCT's are just adding one more mechanism and a new freedom axis to drive. The Alt-Az mounting can only be use for Lunar and planetary imaging since the exposure time are short (faster than 1/10th sec or so). 

For all pictures of DSO's in high resolution the equatorial mount is mandatory (or an altazimutal set in equatorial mode). There is only one exception; scopes of 10" and larger can use an altazimutal  mount but have to use a derotater to avoid stars trails in the corner of the image. This heavy and cumbersome accessory is not recommanded for smaller scopes.

The mount must be well polar aligned and driven by an accurate system which errors are limited to a pixel. Any larger guiding irregularity will be recorded in the image. Therefore many amateurs get support from an auto-guide CCD. But the accuracy of a german mount equipped with classic gear wheels and a Periodic Error Correction system (PEC) is about 1" while a Direct drive mount (see this French page) driven by an electromagnetic field goes below the diffraction limit, reason for which many amateurs have adopted this kind of mount. The image gallery is eloquant in this regard.

AutoGuider Calculator, CCDWare

At left, an Astro-Physics 1200GTO mount suitable for 8" scopes and larger. At center, an AP 130 mm EDFS f/6 and ST-4 CCD mounted on a 600E GTO mount. At right, a Direct drive DDM60 mount made by ASA. Documents from manufacturers and Mike Cook.

Several manufacturers provide Direct drive systems to name Alcor System (France), ASA (Autria), Astelco (Germany), GTD (USA), PlaneWave (USA), SkyVision (France), etc. Note that Alcor System and SkyVision work in partnership with Optique Unterlinden (France).

When all technical (and financial) issues will be solved (advice of a qualified vendor or experimented amateurs is not useless) you will be ready to picture your favorite celestial object. The good news is that after looking at some CCD images produced by amateurs and their equivalent taken by professionals, subtle is the one that can say who is on first and who is on second!

Of course, getting closer, the bigger always wins, but do not forget that image processing can cover one's tracks... Let's compare for example M42 pictured with the HST and by Jason Ware using a RCX400 12" f/8 or Officine Stellare with a Riccardi-Honders RH of 200 mm f/3... In one generation, technology available to talented amateurs has done a giant leap !...

Now that we recorded our pictures of the celestial objects, some additional frames need to be recorded before processing our raw images on computer. This step is called the calibration or pre-processing. It is the subject of the next chapter.

Next chapter

The calibration or image pre-processing

Page 1 - 2 - 3 - 4 - 5 - 6 -


Back to:

HOME

Copyright & FAQ