Design Specification¶

Author: Michael Fulbright

Status: Initial draft

Date: 2020-09-04

Introduction¶

Purpose¶

The main purpose of this document is to outline the requirements of the Alpaca Digital Setting Circles Driver (hereafter “DSC driver”). This specification will cover the user experience as well as address some details of the technical implementation.

The digital setting circles (DSC) is a device which interfaces which each movable axis of a telescope and tracking change sin position. The most common use is on a dobsonian telescope which has altitude (ALT) and azimuth (AZ) axes. The ALT axis moves from the horizon to straight overhead (the zenith), while the AZ axis corresponds to the distance from North around the horizon. These together allow the specification of any point in the sky.

By reading the changes in the ALT and AZ position of the telescope a program can track the telescope and determine where it is pointing in the night sky. A planetarium program can be used which also has a database of sky objects and so the telescope position can be plotted against the known objects. This allows user (“observer”) to find objects by moving the telescope until it is at the desired object.

Scope¶

The DSC Driver will allow users to:

connect to a DSC using an application which supports Alpaca
edit the configuration of the DSC (serial port, encoder resolution, etc)
edit the geographical location where the observer is location
synchronize the telescope location via a planetarium program
once synchronized the user can get locate objects using the planetarium program

Technical Overview¶

Supported Platforms¶

The DSC driver will support Windows and Linux targets initially. In theory the driver should work on any platform which supports Python 3.

Communication Interfaces¶

The DSC driver will listen on a TCP port for REST API requests which are how an Alpaca driver communicates with clients.

The DSC driver will also communicate with the DSC device. This is normally a serial interface such as RS232 or a USB<->RS232 adapter. Wireless operation using a serial stream via Bluetooth or WiFi is also possible.

Assumptions and Dependencies¶

The DSC driver uses Python 3.7 and depends on <insert python web framework> web framework to implement the REST API server needed for Alpaca. The pyserial module is used for communicating with serial devices.

Currently the driver assumes an ALT/AZ arrangement for the telescope. It would be possible to support a RA/DEC arrangement (like a German Equatorial Mount (GEM)), but this is beyond the scope of the current implementation.

Requirements¶

Functions¶

Alpaca Telescope Driver¶

The Alpaca driver listens on a TCP socket for REST API requests from clients. There are several classes of devices supported by Alpaca such as cameras, telescope, filter wheels, etc. The DSC Driver is a telescope device. The entire API for a telescope device is not implemented however, as this is not required for a DSC device.

The entire Alpaca API for a telescope device is NOT implemented, but only those sufficient to get a planetarium program working. The program Cartes du Ciel was used for testing - other programs may require additional API components to be implemented.

The following Alpaca API interfaces are implemented:

Name	GET	SET	Notes
alignmentmode	YES	NO	Returns ALPACA_ALIGNMENT_ALTAZ
altitude	YES	NO	Need to implement SET
aperturearea	YES	NO	Need to implement SET
aperturediameter	YES	NO	Need to implement SET
athome	YES	NO	Returns FALSE
atpark	YES	NO	Retursn FALSE
axisrates	YES	NO	Returns empty list
azimuth	YES	NO
canfindhome	YES	NO	Returns FALSE
canmoveaxis	YES	NO	Returns FALSE
canpark	YES	NO	Returns FALSE
canpulseguide	YES	NO	Returns FALSE
cansetdeclinationrate	YES	NO	Returns FALSE
cansetguiderates	YES	NO	Returns FALSE
cansetpark	YES	NO	Returns FALSE
cansetpierside	YES	NO	Returns FALSE
cansetrightascensionrate	YES	NO	Returns FALSE
cansettracking	YES	NO	Returns FALSE
canslew	YES	NO	Returns FALSE
canslewaltaz	YES	NO	Returns FALSE
canslewaltazasync	YES	NO	Returns FALSE
canslewasync	YES	NO	Returns FALSE
cansync	YES	NO	Returns TRUE
cansyncaltaz	YES	NO	Returns FALSE
connected	YES	NO
declination	YES	NO
description	YES	NO
destinationsideofpier
doesrefraction	YES	NO	Returns FALSE
driverinfo	YES	NO
equatorialsystem	YES	NO	Returns FALSE
focallength	YES	YES
interfaceversion	YES	NO
ispulseguiding	YES	NO	Returns FALSE
rightascension	YES	NO
sideofpier	YES	NO	Returns 0
sideraltime	YES	NO
siteelavation	YES	NO	Need to implement SET
sitelatitudev	YES	NO	Need to implement SET
sitelongitude	YES	NO	Need to implement SET
slewing	YES	NO	Returns FALSE
supportedactions	YES	NO
synctocoordinates	NO	YES
targetdeclination	YES	NO
targetrightascension	YES	NO
tracking	YES	NO
trackingrate	YES	NO
trackingrates	YES	NO
utcdate	YES	NO

Interface to DSC Encoders¶

The DSC driver also maintains communication with the encoders of the DSC device. This gives the position of the ALT and AZ axes of the telescope. The DSC is polled at regular intervals for its current position and the driver then recomputes the sky position that the telescope is currently pointed. This computation depends upon the user first performing a synchronize (or sync) operation which involves pointing the telescope at a known star or sky objects and telling the planetarium program to synchronize the mount. The DSC driver uses the raw ALT/AZ encoder positions and the RA/DEC coordinates of the target chosen in the sky for syncing and computes a mapping from raw encoder position to sky position.

Encoder Synchronization¶

The DSC encoders report back the change in the position of each axis since the DSC was powered on. The changes are relative to the original position. In order to map these values to the position of the telescope in the sky the user must synchronize the DSC encoders. The process is as follows:

User points the telescope to a star or sky object in the planetarium catalog.
Once the object is centered in the field of view (FOV) of the telescope the planetarium program is told to “sync” the position of the telescope.
The DSC driver receives the sync request and records the raw DSC encoder values.
Using the encoder resolution for each axis the raw encoder values are converted to degrees.
Using the geographic location of the observer site and the current time the current position of the object in the sky (ALT/AZ) is computed.
A linear mapping between the raw encoder values and the actual ALT/AZ position is computed and applied to future value read from the DSC device.
It is assumed the dobsonian base is level for this simple synchronization to work.

This simple mapping works well within the vicinity of the object chosen for synchronization but will become more inaccurate as the observing position is farther and farther from the synchronization position. The easy remedy is to chose a new synchronization point when moving to a new area of the sky.

Observing Profile¶

A profile is stored for each observing configuration which contains the following information:

location
- location name
- latitude (decimal degrees)
- longitude (decimal degrees)
- altitude (meters)
DSC configuration
- serial port for DSC
- communication speed
- ALT/AZ encoder resolution
- Whether ALT and/or AZ direction is reversed
equipment information
- aperture of telescope
- focal length of telescope

This profile is stored as a YACC file under a system configuration directory which depends on the system platform. For Linux is it stored in the “.config/AlpacaDSCDriver” directory in the user’s home directory. In Windows it is stored in the directory “%APPDATA”/AlpacaDSCDriver”.

Web dashboard¶

The DSC driver also has a built in web server which is used to monitor the current status of the DSC driver as well as config the driver. The status page displays the following information:

current raw encoder counts
current sky position as ALT/AZ (if synchronized)
current sky position as RA/DEC (if synchronized)
whether the mount is tracking (for dobsonians on an equatorial platform - not currently implemented)
current observational profile

A button exists that will lead to an alternate web page allows configuring the observing profile mentioned in the previous section “Observing Profile”. The user can also create a new profile, load an existing profile, or save the current profile under a new name.