Does that mean that when I see a spike from a bright star extending over a degree away from the star that the entire length of that spike is within the first maximum away from the star?
Does that imply a 2-degree visual spike? It is easy to figure out, since the entire central maximum is approximately D/W times the Airy disc diameter, where D is the aperture diameter and W the vane width. It is never seen in its entirety, though, since the portion close to the minimum is of extremely low intensity. Of course, a sufficiently bright point-like object would ultimately show second and/or third maxima of the vane pattern, but such objects are not a part of the usual sky inventory.
That means that as the spider vanes get thinner, the length of the spike gets longer, though the minimum after the first maximum near the star should be invisible.
On a 317.5mm primary with 0.5mm blades, that would make the spike length 635X the width of the Airy disc.
That would be substantially less than the visible length of the spike I see.
The Airy disc on the 12.5" f/5 scope is .00665mm (2.43932 x lambda in mm x focal ratio) , so the length of the spike should be 4.23mm.
At the image scale of the 12.5" f/5 scope, .0361 degrees per millimeter, a 2 degree spike (and it's longer than that, but I'm not sure by how much) would cover 55.4mm, or over 13X the width of the first maximum.
Ergo, I am seeing well out into further maxima well beyond the width of the first maximum if your formula D/W x Airy Disc holds up.
The spike length (as you would expect) shortens as the magnitude of the star goes down. By the time I get to 3rd magnitude, the spikes fit entirely inside a field 22mm wide.
And, by 5th magnitude, inside a 10mm field.
So tell me, is there something wrong with my math, or does that mean that many more maxima are visible in the star spikes than just the first one near the star?