Contacter l'auteur / Contact the author

Recherche dans ce site / Search in this site

 

 

 

 

 

Don't be afraid of CCD

Astrel Instruments AST-DU-4.3-A color CCD with a 4.3" touch screen and Wi-Fi (100 €).

Guiding systems (III)

When speaking of astrophotography, there are two kinds of applications : planetary imaging and deep sky imaging. If the first can easily manage snapshots on relatively light installations, the second requires long integration (exposure) times and thus a sturdy and accurate mount. Explanations.

The size of a CCD pixel being very small, some 10 microns, and the complete chip up to 10 times smaller than the 35 mm format, this simple relation explains that a CCD camera are easily capable of showing very small guiding errors with fiendish accuracy. Therefore a sturdy equatorial mount and an accurate focusing system are required to get good pictures, specially for deep sky imaging. We will come back on the mount later.

In these conditions, the amateur must be assisted with an accurate guiding system. Guiders are available in various forms and configurations, from the simple piggyback guide scope to the autoguider equipped with a dedicated CCD.

The main disadvantage of small CCDs like webcam-based guiders is their low sensitivity and their capability at guiding on anything but bright stars. Choosing the right combination Scope-CCD can thus make the difference between succeed and fail in picturing faint or deep sky objects.

Many accessories can ensure this task that we can gather in the three following categories :

1. The guide scope in piggypack on the main scope

2. The off-axis guider with or without computer

3. The combined (auto)guider and imager CCD.

Let's review each of these solutions.

1. The guide scope

The first solution is the guide scope (in the general sense as it can be a reflector) in piggypack on the main instrument, and equipped with a CCD camera connected to a computer, itself linked to the scope mount as we see below.

The advantages of this set up are as numerous as its disadvantages. Indeed, among its advantages, the piggyback (attachment in parallel) solution requires only one mount and is more flexible than the other solutions in that sense that the user can point the guide scope independently from the main scope to any target, even off the main scope axis. In addition, the two scopes being physically separated, the light beam of the guide scope does not affect the one of the main scope. So the user can use the guide scope without filter or flip-mirror and can thus acquire a guide star up to 30 % dimmer than through a filter, a serious advantage knowing the difficulty of this task through a filter that usually restricts its use to the brightest stars.

Typical piggyback setups. At left, a Sky-Watcher Esprit 100 f/5.5 apochromatic refractor with field flattener supporting a carbon fiber Stellarvue SVR105T apochromat, both connected to Canon T3i DSLRs and fixed on a sturdy Takahashi EM200 Temma 2M mount. In-between, there is the small Orion 50 mm refractor "Magnificient Mini Autoguider". At center, a 66 mm refractor equipped with the "Orion StarShoot autoguider". At right, an Orion 120 mm f/5 refractor used as guide scope on a C14 HedgeHD. The CCD (not showed) is an Atik 1000M. Documents Jerry Gardner, Alan Dyer and Serge Petiot.

But there are also drawbacks in using a piggyback guide scope. Such a set up works very well when instruments are fixed for good or the installation rigid. In the other cases, forget it. Why ? Because its first disadvantage is the total charge of both scopes and their accessories that can be at the limit of the mount specifications without to mention the possible torque issue on small mounts. A small mount like Vixen Super Polaris EQ-4 supports a load of 4 kg maximum and it is thus practically not possible to piggyback it a classic guide scope (the OTA weight of the Sky-Watcher 70 mm f/7.1 refractor is 4 kg). In all cases, the EQ-4 mount is not designed to be computer-assisted (it can just receive a motor drive and a hand controller for manual corrections).

In this matter, there is no compromise. Better to keep a safety margin in oversizing the mount instead of using an undersized system. The margin of safety is very important on small mounts and should be at least 1 kg , making the use of a guide scope practically impossible.

To avoid this kind of precarious handy work, some manufacturers provide small guide scopes weighting between 300-500 g among which the SBIG kit including a 100 mm f/2.8 lens designed for the ST-i autoguider, the small Borg FL refractor at two fluorite elements fluorite from 35 to 90 mm in diamètre f/4.5 to f/7 weighting between 175 g and 2 kg depending on models (or the apochromatic BORG Oasis Studio models from 50 to 125 mm in diameter often used with DSLRs) or even the small short Orion refractor of 50 mm f/3.2 (Guide Scope Mini 50) and 60 mm f/4 (Multi-Use Guide Scope) accepting for example the Orion StarShoot series of CCD autoguiders combining a guider and an imager at rates up to 200 fps.

At left, autoguide CCDs SBIG ST-i (red cylinders) equipped with their kit (100 mm f/2.8 telelens weighting 368 g, rings and adapter, 349$) and a SBIG SG-4 CCD (black box) used as guide scope on Takahashi FSQ-106ED f/5 apochromats on which is attached a color CCD QHY10. At center, an Astro-Physics StarFire apochromat of 160 mm f/7.5 EDF equipped with a Borg 60ED guide scope and a CCD, all being fixed on an AP 900GTO mount. At right, an Orion 50 mm scope (190$) connected to an autoguider from StarShoot series (total 440$).

As for small catadioptric GoTo telescopes, Celestron NexStar for example as NexStar 5SE of 125 mm in diameter can use the NexGuide autoguide CCD camera that also takes into account the effect of backlash (the play on the declination axis) during autoguiding. Ideally, the best to do is to attach in piggyback one of the small guide scopes described above weighing less than 500 g (~ 55 mm in diameter) equipped with a small autoguide CCD camera. However, the mount being not very robust, its performances in deep-sky imaging are limited.

By contrast, when we want to take advantage of a large guide scope (from 100 mm in diameter showing e.g. a weight of 3 kg for the Takahashi FC-100 DF f/7.4 APO) either for guiding or imaging, it is recommended to use it only in conjunction with sturdy german mounts able to support heavy loads, and including of course an electronic module for autoguiding at least compatible with SBIG ST-4. Among these models name Sky-Watcher EQ-5 Pro with the ST-4 USB kit (load capacity of 10 kg counterweights excluded), Takahashi EM200 Temma 2M (18 kg), Losmandy G11 (27 kg), Sky-Watcher EQ8 GOTO (50 kg), Astro-Physics 1200GTO (63 kg) and other Software Bisque Paramount ME II (109 kg). Depending on models, these mounts must be able to support for example two instruments of 150 mm in diameter (2 OTA of 7 kg each) or of 180 mm in diameter (2 OTA of 15 kg each) and their accessories (up to several kilos). We can also use the sturdiest altazimutal forks (not those using a single fork arm) supporting catadioptrics.

The use of a guide scope can also have an impact on the tracking system, what we call the differential flexure. This potential problem is more apparent at high magnification and it is not directly linked to the weight of the guide scope. During the guiding, the guide scope can slightly move in its mounting rings. Very small moves (we speak of a scale from some hundreds of microns to some millimeters) will be interpreted by the guide CCD as a shift from the nominal position, and thus the system will try to compensate for a guiding error that in fact does not exist.

Two high-end installations used under the dark skies of south Arizona. At left, a C14 equipped with a SBIG-1301E CCD with a piggyback Borg 76 f/6.6 ED refractor. At right, an Astro-Physics 180 mm f/9 EDT refractor fixed on a Software Bisque Paramount GT-1100ME mount and connected to a SBIG ST-10XME water-cooled CCD with for guide scope a Takahashi FSQ-106N f/5 quadruplet fluorite refractor equipped with a SBIG ST-237A CCD. Documents Huachuca Astronomy Club and Frank Barnes.

At last, consequence of this setup, the amateur needs to purchase a second scope in that purpose. If a small 100 mm telelens is enough for wide-field pictures (e.g. in conjunction with a SBIG ST-i or Lodestar autoguider, see below), to track with accuracy small deep sky objects with a dim star requires a larger guide scope, usually from 80 to 120 mm in diameter and from 480 to 1000 mm of focal length.

Note that in all cases, this solution requires an external control, usually via a portable computer linked by cables to the CCD and to the mount.

Although performing, this solution can be expensive. It also becomes outmoded or obsolete because of the invention of the off-axis guider and till more with CCD cameras combining the guider and the imager.

2. The off-axis guider with or without computer

As all amateurs cannot afford several scopes, as in the old good time, another solution is to not use a guiding-scope but to attach the guiding CCD on an off-axis guider, aka OAG, as we see below that will drive alone the motors of the mount while the amateur will record his pictures with the main camera. In this case, the amateur also needs of an external computer to control the system.

More recently, some manufacturers have imagined a solution that eliminates the need of computer : it is the autoguider CCD (e.g. SBIG SG-4, Celestron NexGuide, Orion StarShoot, Starlight Xpress, Lodestar, etc.) that performs the guiding functions standalone, without external computer. Of course, the system is fed and connected to the telescope motorized mount.

But how to assembly all these parts together ? The simplest solution is to use an off-axis guider as we see below at center. The alternative is using a second small scope in piggyback to the main one as we see below at right with constraints described above.

Pedro Ré's SBIG ST-7 CCD

with a True Technology flip-mirror

(2650$+175$).

Celestron C8 CPC equipped with an Orion

StarShoot Autoguider and DSLR

(280 € + off-axis adapter)

Celestron NexGuide autoguider

on a refractor in piggyback

(399 € or 299.95$)

The most elegant and practical solution is to insert a small mirror or a flip-mirror on-axis between the OTA and the CCD camera. If in the past this accessory was cumbersome and is till sometimes, today there are narrow models. Among these OAG name the SBIG OAG-8300, Innovations Foresight ONAG-XT guider, Lumicon Giant Easy-Guider, Celestron OAG, Meade OAG, Orion thin off-axis guider as well as older models like the True Technology OAG among others.

These units have 2 outputs and allow to use either a powerful illuminated reticle eyepiece at 90° or a CCD guider camera at the same time as the main camera or the DSLR in order to ensure tracking corrections. Below is an example of on-axis full-frame guiding system of last generation adapted to Schmidt-Cassegrain telescopes. At right, the minimalist "thin and slim" version from Orion.

To watch : On-Axis Guiding for Astrophotography

To read : The ONAG from Innovations Foresight (PDF), Dennis di Cicco, S&T, 2015

At left, the optical divider ONAG-XT from Innovations Foresight (989$). At center, its assembling at the heart of a guiding system connected to two Atik CCD cameras (a guider and a cooled imager). At right, the Orion thin off-axis guider (TOAG, 149.99$) very useful to reduce the overall length of the optical train.

With time, manufacturers have developped new more compact solutions combining in the same box the guide CCD and the imager CCD camera, eliminating the need for the off-axis guider.

3. The combined (auto)guider and imager CCD

Among the first CCD cameras combining a guider and an imager, the Astrovid Star 2000 system (e.g. MX5 or MX9 camera) used a special device splitting each pixel in two independently readable halves, one integrating long exposures while the other selects a star in the field up to magnitude 11 for guiding the telescope.

Thanks to a further miniaturization, these last years some manufacturers have developped a till more ingenious solution combining this time a dual-chip including the autoguider and the imager. As we seen below, modern autoguiders/imager are available in various forms and price range (e.g. Orion StarShoot G3, Opticstar PX-125C, SBIG ST-7EA, STL and STT series, etc.). 

Other manufacturers have designed a CCD camera including a small diagonal mirror that redirects a part of the light beam to an external guider (e.g. SBIG STF-8300 with FW5 and OAG, Quantum Scientific Imaging QSI 540wsg or QSI 683wsg-8, etc.).

Remains one issue, the guide star acquisition through possible filters that we evoked earlier which is the main weakness of most (auto)guiding systems. To resolve this issue, some manufacturers like SBIG have placed the guide CCD sensor within the filter wheel instead of behind the filters. With such a device at your disposal, you have no more excuses to select a faint guide star.

To read : SBIG's New STT-8300 Camera (PDF), Dennis di Cicco, S&T, 2013

QHY IC8300 CCD (dual chip and WiFi connexion)

Four CCD cameras combining an autoguider and an imager available in 2016. From left to right, the CCD Orion StarShoot G3 (448 € or 399.99$), Opticstar PX-125C (129£ or 237$), SBIG STT-8300 (4550 € or 3800$), and QHY IC8300 (2200$), the first of an innovating serie equipped with a Wi-Fi connexion and thus accessible remotely over Internet.

Drawback of this integrated solution, if an entry-level model is relatively light (~350 g) and cheap (< 500 € or 400$), with a filter wheel and a regulated cooling system a complete high-definition dual-chip CCD is of course more cumbersome than a low form factor CCD, heavier (~2.4 kg for SBIG STT-8300 with its filter wheel) and rather expensive (~4550 € or 3800$ for SBIG STT-8300 in 2016 but there are also models exceeding 32000$). These dual-chip models represent today the most performing solution offered to amateurs for deep-sky imaging.

Of course, none of these solutions eliminate the computer that is always mandatory for the inital setup (selection and lock on the target, focusing, etc.) and to download each image or video recorded by the CCD camera (main or combined).

Looking ahead, some manufacturers already offer Wi-Fi CCD cameras (the color model AST-DU-4.3-A from Astrel Instruments with a 4.3" touch screen at 100 € in 2016, QHY IC8300 at 2200$ in 2015, etc.). We can also imagine that they will be equipped with a flash storage memory (SD card) which will prevent losing time after each shot to download the image.

Next chapter

Integration time

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


Back to:

HOME

Copyright & FAQ