There was recently a raging debate about this concept in the ATM forum based on a conversation in this forum. Apparently, there is no relationship between long focal ratio and steadiness of seeing/depth of field.
I am unaware of that "raging debate". From my standpoint though, as I did my own specific testing over a stretch of time, that "debate" would be moot for me as I have my personal experience which of course will take precedence. However, I would agree with what you said above...as this is what I saw. Steadiness of the image was not impacted. What was different however was the degree of damage to the view. If the local seeing cause the image to jitter and de-focus 5 times in 5 seconds, as example, in the short focal ratio scope, then it also jittered and de-focused the same rate in the long focal ratio scope. Where the difference was was in the impact to the image. So the jittering/defocus of the image was much less severe in the longer focal ratio scope. It was quite easy to see when over a stretch of several minutes the seeing was tumultuous how the impact would always be much milder in the long focal ratio instrument.
Mind though that these "advantages" in my testing did not really become evident until one got close to f/15 and they did not become blatantly obvious until I reach f/20. So there are specific bounds/conditions for this to become apparent. Most folks talk about f/10 and such as "long" and having these mystical advantages. Well, the advantages are not mystical and they do not happen until one gets significantly longer in focal ratio. But they are there and was easy to see. Others can debate all they want. I decided to go a step further and see for myself empirically, which trumps any debate. And like I do all the time when I run my personal experiments, I controlled the situation as much as possible and was careful to mitigate as many confounding variables as possible in the process, and never base my conclusions on a single experience but on multiple runs of the experiment to ensure repeatability. I am satisfied that there is no debate that the behavior is indeed there. As for the exact cause, I can only posit that may be due to depth-of-focus since I could rack the focuser so much further with the long focal ratio f/20 scope and have the image stay sharp - was quite astounding. However, I am a visual observer and care little really about driving optical reasons. More important for me is knowing what behavior will happen when I am using an instrument. The wherefore and whys are less important for me so I don't get hung up on it if people want to argue the root cause as that is also irrelevant for me. All I really care about in my observing life is that I know myself, the sky, and my equipment well enough that I can get repeatable results all the time when I choose certain tools, approaches, targets, and conditions. And of course adding to that cadre based on experience and not debate as time progresses. Others can believe what they choose based on whatever theory/science they want. I will believe behaviors that I see and can reliably replicate. The behavior is there and very real for suitably long focal ratios when a human eye is at the eyepiece (i.e., for the entire "system" as component attributes are again subject to relentless debate by the many whose cup is always full). End of story.
And again...excellent report and observation Piergiovanni
Edited by BillP, 05 April 2019 - 01:24 PM.