Airy Disk size vs. Spot Diagram optical performance: What's the catch? [Deep Sky] Acquisition techniques · John Stone · ... · 6 · 155 · 2

I'm  a little confused and am hoping an optics expert can clear this up.

I recently became aware that the airy disc size on my TOA-130 telescope in green light is ~10.5 µm.  (In fact, the airy disc size is only governed by f-ratio so all F7.7 telescopes will have this airy disc size regardless of their aperture).

When I calculate how much of the sky that airy disc covers it comes out to 2.18" with the TOA + 645 flattener.



When I go checkout the spot diagrams of the Takahashi 645 flattener I find 1 µm spots RMS on-axis and 2 µm RMS at full-frame.

https://www.cloudynights.com/topic/822975-toa-130-toa-150-and-645-flattener-results/

What I don't understand is why are this super small spots prized when the airy disc is 5x - 10x larger?

Also, the TOA is known for being a super sharp telescope but how can that be when it's airy disc is so much bigger than what's you get with say a RASA 11 with <4.4 µm RMS spots with and airy of 2.96 µm.

https://www.celestron.de/media/mageworx/downloads/attachment/file/1411/RASA_BOOKLET_11in_36cm_2018_LR.pdf

While the RASA the optics are not diffraction limited (what you see is governed by the RMS performance instead of the airy disc) but even then it has 50% better resolution than the TOA-130...  [1.46" * 1.49 = 2.18"]



I must be missing something because I don't see a lot of people reaching for a RASA during galaxy season even though these numbers seem to suggest it has significantly better resolution.

What am I missing?

RASA has a better resolution (in microns) because it's F ratio is small (airy disk radius = 1.22* lambda* F). But because the focal length is small too so the physical size of the galaxy image is also quite small. What you want is high "size of the object to the spatial resolution ratio". It is more intuitive (to me at least) to understand these in terms of angular resolution rather than spatial resolution.

John Hayes:
John,
Putting aside the issue of atmospheric seeing, you are 100% correct that the Airy diameter in the focal plane depends only on the focal ratio.  The rms spot size numbers that you see come from the optical design data that relies only on geometric ray tracing.  When you design a lens, the goal is to get the rms for the geometric spot size to the same size or smaller than the Airy size.  Those numbers are just for design purposes and have nothing to do with the actual size of the diffraction limited image in the focal plane.  Manufacturers love to put those rms numbers in their ads but they are meaningless in terms of how the telescope actually produces an image in the real world where diffraction exists.

John

Thanks for clearing this up, but I'm still hoping for your input on the actual optical performance in the focal plane.   

To me, the numbers seem to suggest that I sell the TOA and replace it with a RASA because if it performs to design spec (a big if) I'd get better resolution from the RASA and also enjoy the benefits of much shorter integration times (while suffering the pain of tilt/focus of a F2.2 optic).

Assuming we put aside manufacturing variances (which is a big assumption, I know) why do you believe more people choose the TOA-130 for fine resolution, like galaxy imaging, than say something like the RASA11?  

This thinking obviously must not play out in the real world because a lot of people smarter than me consistently praise the quality of the images produced by the TOA series telescopes. 

I'm trying to understand why that might be, and what insight(s) I'm missing.

Thanks again for everyone's attention and opinion.

John Stone:
To me, the numbers seem to suggest that I sell the TOA and replace it with a RASA because if it performs to design spec (a big if) I'd get better resolution from the RASA and also enjoy the benefits of much shorter integration times (while suffering the pain of tilt/focus of a F2.2 optic).

Assuming we put aside manufacturing variances (which is a big assumption, I know) why do you believe more people choose the TOA-130 for fine resolution, like galaxy imaging, than say something like the RASA11?  

This thinking obviously must not play out in the real world because a lot of people smarter than me consistently praise the quality of the images produced by the TOA series telescopes. 

I'm trying to understand why that might be, and what insight(s) I'm missing.

Angular resolution under perfect conditions depends only on the aperture size; however, when you mix in seeing, that often becomes a limiting factor--particularly when the aperture gets bigger than maybe 4".   One thing that I want to emphasize is that if the design shows a rms spot size of 1 micron for a F/7 system, it will absolutely perform to the "design spec".  Just because RASA has a diffraction limited spot similar in size to the rms spot size does not mean that it performs any better than your TOA.  They will both be diffraction limited and that means that the aperture size will be the factor determining angular resolution.  Of course as the aperture gets bigger, the role of atmospheric seeing becomes more dominant.  Under 3" seeing a 1-m telescope won't deliver an image any sharper than your refractor.  I love the RASA scopes but keep in mind that they also their own limitations.  For one thing, the camera has to be in the pupil, which limits using things like filter wheels.  That makes them hard to automate with mono-cameras.  They work with mono cameras but they are best used with OSC cameras.  The really strong points of RASA are 1) the field size and 2) the optical speed.  Along with Hyperstar, they are a modern day replacement for the Schmidt camera.

John