[Stefan Buda]

Originally Posted by Paul Haese

My way of looking at this is that like screw drivers you need more than one type and size in your tool shed.

I also said that wide field, deep sky and planetary, all require a different theoretical approach in optimizing the setup.

You use "image scale" and I use "sapling rate", a more general term, but we mean the same thing.

Your approach of associating focal ratio with pixel size is very interesting. I never looked at it from that angle. I will have to check the math.

Originally Posted by Peter Ward

While we can ruminate on pixel size and focal length, I have found for "diffraction limited" optics...and Lord knows that is not a given
that so many other aspect conspire to smudge your images, far more than an imperfect optic.

eg:
Environmental factors (seeing, local environmental temperature, transparency )

Mechanical factors: Tracking accuracy, guiding accuracy, wind buffeting and vibration suppression, OTA rigidity, collimation retention, focuser rigidity.

Thermal control: Suppression of tube currents, optical figure retention, focus retention.

.....and apologies to Stefan for the digression..

Best Astrograph? Simple.

*Anything* made by Astro-Physics.

That is precisely the problem with choosing a good setup. There are so many factor to consider that beginners may not know what are the most important.
My reverse method is only the start of the selection process and it has been simplified to the max. For a wide field setup, it is based on two parameters only - sensor size, and desired field of view. For deep sky it is based on a single parameter - the pixel size of the sensor.
Once the best match focal length has been calculated and set into concrete, the difficult process of choosing the largest aperture can begin. It is at this stage that all other factors must be considered. I must add that a bit of departure from the calculated optimum FL is not a disaster even if I just had it set in concrete.

Perhaps someone could set out some easy to follow rules on how to proceed with the aperture maximisation.
At this stage we no longer need to consider the sensor. We only have one parameter to consider and that is the focal length. Let's ignore pocket size as a parameter.
The next logical step would be to establish the astrograph type.
We do have a focal length but we don't want to restrict it to any particular "screw driver" size. I think, the only way forward, without complicating things too much, would be to establish several FL ranges, and treat them separately.
Maybe at the end we can come up with a flow chart that starts with a sensor, covers a lot of factors and ends up with several "best" astrographs. Would there be any made by Astro-Physics among them?