Didge Star Detection pane are way too big to be FWHM or even the Airy Disk - and if you compare circles with similar labels, the sizes of the circles vary significantly
The variation in star sizes can depend on a couple of factors. One has to do with "seeing" -- try looking at star sizes for very long exposures (120 seconds or more), since that will average out the seeing for the different stars. Another has to do with the flatness and tilt of the sensor plane -- in fact, this is how you typically measure and remove sensor tilt.
Next, remember that the Airy disk is the property of perfect optics. If the Strehl is not 1.0, you would expect the star size to be greater, even though the photon count inside the first zero of the Airy disk contains about 85% of the total energy.
The point spread function is often published in spec sheets as "spot diagrams." This is the smallest spot that you can get even if seeing is perfect. This is an example of the Takahashi FOA-60Q (has Strehl over 0.99, one of the largest Strehl ratio among consumer telescopes):
Notice that the size of each square is 100µm x 100µm. The 4 squares are the spot sizes for distance from the optical axis. The best spot size is of the order of 7.5 µm in diameter.
For a ASI2600, which has a pixel size of 3.76 µm, the best spot size from the FOA-60Q is almost exactly 2 pixels. (I bought an FOA-60Q just so I have a reference telescope for measurements, and I also use it for a solar scope even though H-alpha scopes don't really need such optical precision).
This is what the FSQ-85 spot diagrams:
The best spot diameter is about 10 µm, or about 2.7 pixels on an ASI2600.
This is the spot diagram for one of the newest Askar refractor (FRA-300):
Notice that the squares are 200 µm to a side :-).
The spot size at the optical axis is 14.7 µm. Or about 3.9 pixels on a ASI2600.
This is spot diagrams for the RedCat51:
Spot diameter of about 16µm, or about 4.2 pixels for ASI2600.
So,depending on the OTA... 2 pixels, 2.7 pixels, 3.9 pixels and 4.2 pixels. The Askar and RedCat spot sizes are quite typical of small non-Takahashi refractors (I have all four scopes, but have not used the RedCat in quite a while now.)
It is possible to get very small star sizes, but with large plate scales. The following is taken a week ago with a Sigma 135mm ART lens, (with aperture restricted to f/2.9):
Notice that the star size is only 1.75 pixels (camera is ASI6200MC, cropped to APS-C). However, the actual size is a whopping 10.3 arc seconds :-). The star sizes are quite consistent (no tilt) for the different stars, for this 300 second exposure with an Optolong L-Ultimate filter (I was at the time comparing the filter with a Radian Quad band filter).
Remember too that the star sizes need to be convolved with the "seeing," if the seeing is large compared to the point spread function ("spot diagram") and similarly, also convolved with the guiding error.
Chen
