Astrometry.net
My digital finder journey started out using astrometry.net as the core plate-solver. I could see it was being used by many others, both amateur and commercial, and it was easy to access from my own python code. It has proved very reliable with very few failures to solve and it even works when stars are barely visible to the eye. It is quite fast if you specify the pixel scale of the image accurately, and then blind solving is about the same speed as when an RA and Dec seed is given.
Tetra3
I’ve been monitoring others’ progress using Tetra3. I was told it can give very fast results, but needs extra work in preparing the database. Also it wasn’t as reliable as astrometry.net so I had no reason to change.
But then I started developing 'eFinder Lite’. Running on a Pi Zero 2 it really struggled with astrometry.net. 10 seconds to solve wasn’t unusual (compared with <1 second on a Pi5).
So I loaded up Tetra3 on to my test rig and started feeding it test images. For about a week I barely got a solve but then it all came together. Indeed the database needs to be built customised to your camera and lens. Then for a solve Tetra3 needs again accurate information about your image.
On a Pi5 I could get down to 200ms solve time, and on a PiZero2 about 2-3 seconds. Importanly, reliability was proving to be quite good.
Cedar-detect & Cedar-solve
I noticed that PiFinder uses cedar-detect & solve extension of Tetra3. Cedar is a suite of applications that can deliver a complete camera control, star extraction and plate-solve system. It is designed for speed and continuous solving at 10Hz is even possible, (or faster!). Additionally the author has optimised the database used by the solver too.
Installation of cedar-detect & solver isnt completely straightforward, but helpful instructions for migration from Tetra3 are given. The same requirements for careful generation of a bespoke database and args when calling cedar-detect are necessary.
Results are impressive. With the right images, total detect and solve time of around 12ms on a Pi5 are possible and 200ms on a Pi Zero2. I say 'right images' as the star detect process whilst extremely fast, is not as robust as Tetra3 or astrometry.net. Care must be taken to prevent stars saturating, but a simple exposure control can take care of that. For my eFinder focus utility, I need to revert to Tetra3 as cedar-detect cannot register out-of-focus stars, but utility is only used once per session at most and so Tetra3 is fine. Even with UK summer night skies, I’m getting reliable results with 0.1 second exposures on my Pi HQ camera.
Conclusions
For me it is ‘horses for courses’. Astrometry.net remains the most reliable solver, easy to integrate into my code and the on-line server is a useful asset. For my systems that use a Pi5 (ScopeDog and standard eFinder) I am in no hurry to change the plate-solver. No need. The second it takes to solve an image is not excesssive when part of the overall observing process.
Where speed is important or processing power is limited, then Tetra3 wins hands down. For most of these applications, using Cedar-Detect & Solver is an obvious choice. As shown by PiFinder, this opens up the prospect of replacing encoders with a digital finder, running faster than 1Hz. But take note of the stricter image quality requirements (although I suspect the author is working on this).
My eFinder Lite prototype is now reliably solving images in 0.2 seconds. Using the Pi Zero 2, the whole eFinder has just one cable. A USB to the Nexus DSC USB port. This provides power and data each way.
edited 31st July