Autoguiding tips

Written by gemini on Dec. 5, 2012 .

Autoguiding – Advanced Techniques

András Dán, MSc, Gemini, Nov. 2012.

Introduction

It is time we take a systematic approach at this very important topic of astrophotography to get the most out of our expensive gear – mount, guide scope, guide camera, main camera.

The aim of this document is to help understanding the process of autoguiding and to get the best result possible with your gear. It is intended for both beginners and experts.

Autoguiding is an attempt to track the sky with smaller error than the pixel scale (smallest visible error) of our imaging (main) camera. While trying to correct mount errors we want to disregard seeing blur (except with active optics using high frequency).

Let's start by creating a list of the players of the game.

Guide star – its position on the guide chip is measured by the guide software at regular intervals, normally with a few seconds (1-5) of integration. The measured so called “guide error” is the result of mount tracking errors (refraction and mechanical errors) and seeing blur.

Mount – it tracks the sky as precisely as its polar alignment, software and mechanics allow.

Guide camera – it takes images of the guide star and sends them to the guide software for analysis.

Guide scope (separate) – images the guide star for the guide chip. In case of off axis guiding or dual chip camera we have no guide scope.

Guide software – this is the brain of the system, interprets the guide chip data and sends guide commands to the mount.

Main telescope – don't blame everything on the guider, the main telescope is half of the story

The process of guiding

It all starts with taking an integration of the guide star. Let this be 2 sec.

When finished, the image is downloaded and analyzed by the software, the centroid position is calculated (to 1/100^th of a pixel normally). Let this be 0.2 sec.

Comments

Edoardo on 12/07/2012 11:06 a.m.

Una guida molto utile e ben fatta, sarebbe bello avere la traduzione completa in italiano.
Io l'ho letta aiutandomi con un traduttore automatico, ma alcuni passi non sono ben comprensibili per cattiva traduzione.
Andrea on 12/07/2012 3:16 p.m.

Grazie Edoardo, sono contento che ti piaccia! Magari durante le ferie lo traduco ma serverá una mano per togliere i errori grammatici :-)
Ignacio on 03/21/2013 5:40 p.m.

Excellent document! Thanks for posting it.

One thing that called my attention, is that you seem to show better performance going thru the telescope/USB/PC interface in closing the loop, rather than the more direct path thru the ST-4 port.

I always use the telescope/USB interface, so I cannot make a comparison. However, I have always read that the ST-4 path is more direct and subject to less delay, a key aspect of performance in closed-loop control systems. Maybe this is not an issue anymore, as laptops and USB connections got very fast, but was wondering what is the general experience.

Regards,
Ignacio
Andrea on 03/21/2013 7:24 p.m.

Grazias Ignacio,
your remark is very interesting! I have had this result with a specific brand of CCD and it might be different with others. However, opto couplers used in the "relays" are not very fast and you have two of them (CCD and controller). Since the correction command is calculated in the PC, it is more "staight" if it goes directly to the mount controller and not to the CCD, then to the controller.
Regards,
Andrea