Autoguiding

AUTOGUIDING

In olden times astrophotographers shot using 35mm film, frequently taking exposures of an hour or more. To avoid trailing of the image, they would use a separate guidescope on the same telescope mount, and watch a guide star in a cross hair eyepiece. They would keep the object centered by pressing buttons on a handbox connected to the mount’s drive system to correct for speed variations and/or drive misalignment. We’ve all seen some of these early photos and the results were worth the effort - but it must have been very challenging physically. There are folks doing great work with this technique today as well, but more modern guiding systems are increasingly popular.

Shorter exposures are generally required with electronic imaging due to the elimination of reciprocity failure, a characteristic of film causing the effectiveness of a given exposure to drop as the exposure time grows longer. Electronic imaging also lends itself to the stacking of short exposures to simulate a longer one. Still, even the best mounts (in the $10,000 class) can’t deliver the tracking required for exposures of more than a few minutes at moderate image scales. Guiding is still an important tool. The most frequently used guiding setups these days are electronic.

The first electronic autoguiding system with which I’m familiar was an optional accessory for the Accutrack drive corrector. Drive correctors were used to vary the frequency of the AC power supplied to the telescope’s drive motor, to control the tracking speed. My old C14 (80‘s vintage) came with an Accutrack unit and also included an optical sensor which could be inserted into an eyepiece holder. It kept track of the light level detected, and automatically sped or slowed the drive to correct for drift - at least that’s what it said it did. I never tried it. I’m sure it wasn’t very sensitive and it must have been challenging to find a sufficiently bright guide star near the object to be imaged. It could, of course, only be used with a mount that had an AC motor for the drive.
More modern mounts using steppers or servo motors require something more sophisticated.

The first popular dedicated autoguider was the SBIG Star Tracker 4 (ST-4). This was a miniature dedicated CCD imaging camera primarily intended for use as a guider. It was cooled, as premium CCD imagers are to this day. This permitted high sensitivity and relatively long exposures while keeping noise to a minimum. Another popular early dedicated autoguider was the Meade 201. This unit was much simpler than the ST-4, having no external computer module. It was self-contained in a small housing. It was uncooled, though, making it effectively less sensitive than the ST-4. More elaborate cameras with imaging capabilities from both Meade (208, 216) and SBIG often included the ability for standalone autoguiding when not being used for imaging. Later SBIG cameras incorporate a second imaging chip with support electronics for autoguiding while the main chip is used for imaging. When using any of the above systems, or any other SBIG-compatible guiders, the dedicated controller observes a guidestar and emits control pulses based on the direction in which it detects the guide star to be moving - plus or minus in right ascension, plus or minus in declination. These four signals were originally fed to a telescope mount via the same connections previously used for a four-button handbox for manual guiding. Manufacturers soon began to include an accessory RJ-style jack intended specifically for use with this type of autoguider, usually labelled “CCD” or “autoguide”. The amazing thing is that they very nearly standardized the connections! Vixen uses the usual connector but a strange pinout; most other manufacturers are in agreement. Any mount advertised as incorporating an autoguide port is referring to this system and should be compatible with any of the above-mentioned autoguiders.

These days the 201 and ST-4 are no longer available. The only available dedicated autoguiding camera with which I’m familiar is the STV, which sells for about $2000. Folks looking to gain autoguiding capabilities for less expense (or with mounts lacking a dedicated autoguide port) are exploring other options. Webcams or webcam-like cameras (LPI, DSI, NexImage, TouCam, etc.) are being used by many to guide popular computerized mounts via a laptop or PC. Nearly any computerized, driven mount capable of RS-232 serial communications with a PC will support this function.

These cameras aren’t capable of connecting directly to a telescope mount; they require a laptop or PC plus software that can analyze an image, detect relative motion of a guidestar, and issue guiding commands to the mount via a serial port. One popular (and free!) program is GuideDog. Other available software includes K3CCDTools, Astrovideo, AstroSnap, and Autostar Suite (AS works with Meade products only). This software can control all the standard webcams and can issue serial command strings to all of the popular computerized mounts. The mount’s controller decodes the ASCII string, parses the motion command, and adjusts the telescope’s position accordingly. This mode doesn’t use or require the dual-axis autoguide port mentioned earlier.

Disadvantages include the necessity for a computerized mount as well as possible issues with command decoding delays. For a more rigorous solution, GuideDog and most other guiding programs can issue pulses sinilar to those emitted by the earlier dedicated guiders via the computer’s parallel port. Interfaces are available from Shoestring Astronomy to isolate these pulses and feed them into a conventional autoguide port. This eliminates command decode timing issues and will work with older, non-computerized mounts if they have an AG port. Even though my mounts are computerized, I usually use this setup to avoid communications issues - and to permit switching of mounts without having to reconfigure my cables or software.

Off-axis guider pickoff prism

The guide camera can be connected to either a second telescope on the same mount as the imaging telescope, or it can be attached to an off-axis guider. This device is placed in the light path to the imaging camera and contains a small pickoff prism which directs a small portion of the light to a guide port mounted on its side.

There are tradeoffs to either technique. A separate guidescope is more expensive and difficult to mount. Any motion of the guidescope relative to the imaging scope will cause guide errors. If the guide scope is a catadioptric and there is any mirror shift, the guiding camera won’t see it and it won’t be compensated. The off axis guider avoids these issues at the cost of tricker setup and loss of sensitivity due to the small pickoff prism.

Jrcrilly
2/12/2006