
Autoguiding
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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
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