# How exactly did Cassini provide rock-solid attitude control to enable high resolution low light imaging? (1.2 arcsec/pixel for narrow angle camera)

The extremely cool NASA JPL video Triumph at Saturn (Part I) is really worth a watch and/or listen.

At about 45:45 it discusses the period after SOI (Saturn Orbit Insertion, July 1, 2004, 02:48 UTC) when the first images started coming in of Saturn's rings. In a JPL press conference later the same day, Carolyn Porco, Cassini Imaging Team Lead says:

I don't think you have to be a ring scientist to imagine what last night was for us. It was beyond description really. It was mind-blowing, it was every adjective you could think of.

I'm surprised at how surprised I am at the beauty and the clarity of these images. They are shocking to me.

This spacecraft allows us a very steady platform. This machine, you turn it, you point it, and it stays there. It's like a tripod in space.

At 9 to 10 AU Saturn gets about 1% of the sunlight that cis-lunar space gets, so all else being equal photographic exposures need to be 100 times longer.

Cassini's narrow angle camera has 12 micron pixels behind a 2 meter focal length f/10.5 reflector, so each pixel is only 1.2 arc seconds, and for extended objects in dim sunlight, f/10 is not a fast system.

So I would venture a guess that exposure times were perhaps seconds long rather than milliseconds.

Wikipedia's Cassini–Huygens says:

Smaller monopropellant rockets provided attitude control.

and notes that the spacecraft support the following acronyms:

• AACS: Attitude and Articulation Control Subsystem
• AFC: AACS Flight Computer
• ARWM: Articulated Reaction Wheel Mechanism

And while I'm a strong advocate for articulated permanent magnet attitude control and momentum unloading in LEO for cubesats I haven't heard of articulated reaction wheels in deep space spacecraft before. But since I'm chronically uninformed that may be my fault.

Question: How exactly did Cassini provide rock-solid attitude control to enable high resolution low light imaging? (1.2 arc seconds/pixel for narrow angle camera)

I also wonder if Cassini ever had to slew; slowly rotate in a specific direction to de-blur an image of a moon during a close flyby.

• I guess you want to read this: trs.jpl.nasa.gov/handle/2014/11604 Oct 26 '21 at 12:01
• @asdfex I don't see anything really helpful there. It's just an overview of the feedback algorithm presented as a talk for other feedback algorithm writers, and therefore over my head. I don't see anything obvious about an "articulated reaction wheel mechanism", I wonder if Cassini in fact had one? Hopefully there will be another document about the physical system as well. But thanks!
– uhoh
Oct 26 '21 at 12:17
• It had "four Reaction Wheel Assemblies (RWA, three prime and a backup RWA mounted on an articulate-able platform)", also shown in the image on the first page, with a better image here: trs.jpl.nasa.gov/bitstream/handle/2014/46224/… Oct 26 '21 at 12:54
• @asdfex holy granola, that's an amazing paper, about a really amazing system! Indeed the reaction wheels were articulated, but that may have been used only when the "four Z-facing thrusters (executed) small ∆V burns" and not during imaging, but I'll need to read the whole thing first.
– uhoh
Oct 31 '21 at 21:33