Visual planetary & Luna observers a request

[andrew s]

While the weather prevents me making any practical steps forward in this area I have been researching the science and looking at what the observers of old achieved. 

One area I have found no data for is how frequently you would expect those" moments of exquisite seeing" to be for you to continue  observing rather than going in for a nice cup of tea or something stronger. For example would it be once a minute, once every ten minutes ? I have no idea.

The reason for asking is that calculations have been done for the probability of getting a lucky image based on telescope aperture and seeing and I would like to convert this to a visual session. However, unlike imaging the acceptable rate is I suspect psychological rather that technical. 

Also if you have any estimates on the size in arc secs of the finest detail you have seen please comment on that too along with the size of telescope used.

Thanks in advance. 

Regards Andrew 

PS If I do make any sense of all this I will post it up here.

 

[vlaiv]

On my very limited experience, over here with 8" F/6 aperture things are like this:

About 1/3 to 1/4 sessions will be acceptable for high power observation. Out of those about 1/10 will have moments of great clarity and these happen for a brief period of time about a few seconds and happen about 2-3 times in half an hour.

Other 9/10 times things will be a bit different - there will be no moments of exceptional clarity but good and average moments will alternate with frequency of about minute or so?

These are of course very rough estimates based on memory and not on actual recordings.

[mikeDnight]

Wow, this could be an interesting thread!  It's difficult to really pin exquisite seeing down but it probably happens fleetingly on most nights. The trick is being at the eyepiece when it happens. Seasonally, I find spring months to be most rewarding as regards exquisite seeing, but that could be due to the nights being milder and me being more willing to spend longer at the eyepiece. Also, with the Moon being high at a more sociable time, I find myself more absorbed by the superior definition the seeing allows. Bearing in mind that I generally use relatively small aperture scopes, which are perhaps not so seeing sensitive, I have seen some truly amazing things, plus I believe my observing site has above average steady air, ideal for lunar and planetary viewing. This could be because I'm low down in a valley with a river just a hop and a skip away, which can make things a little misty at times. In spring time I feel I can experience exquisite seeing for an hour or more, while winter seems to offer shorter periods in general, sometimes measured in seconds and occasionally 30mins  or even longer. Transparent nights are rarer for me than super steady nights, hence my leaning towards lunar & planetary. 

As regards the finest detail - my scope of 100mm resolves to its limit as far as stellar resolution goes, but seems to go beyond its theoretical limit when it comes to fine linear features. For example, on most steady nights the central rille of the Alpine Valley is detectable when the lunation is right. The vortex nature of the GRS is often easily seen, as are not only the shadow transits but also the Galilean discs as they cross Jupiter's globe. Recognizable albedo features easily seen on a sub 5 arc second Mars. And recent observations of the linear crater Werner have revealed ultra fine rille like features crossing its floor and up its terracing out into the rough upland terrain. These are not easily visible in images and very difficult visually, but after searching for their existence in reality after I'd sketched them, I found that visually, only Percy Wilkins hinted at the existence of one, but only on the crater floor. Some but not all Orbiter images of Werner hint at them as positioned in my sketches. They appear to be collapsed larva tubes on images, and only one wonderful chap from the BAA sent me some images he'd taken confirming my sketches. As far as I'm aware, no-one else has seen them or they doubt their own eyes. Or they make excuses for their visibility as they seem to be beyond the resolution of a 100mm scope. Yet they are real, I saw them before knew of their existence, and they've been confirmed by the amateur who imaged them. Amazing!

Edited July 27, 2020 by mikeDnight

[andrew s]

Thanks @mikeDnight in the "Amateur Astronomer's Handbook " Sidgwick gives examples of Pickering, Steavenson and others seeing extended detail at 1/2  to 1/15 of the classic theoretical resolution depending on what it was. So you are in good company. 

Just to help with my sums could you estimate how long you would be willing to wait for a good seeing moment before giving up as a night on a night as not worth observing ?

Thanks Andrew 

Edited July 27, 2020 by andrew s

[inFINNity Deck]

3 hours ago, andrew s said:

...calculations have been done for the probability of getting a lucky image based on telescope aperture and seeing...

 

The seeing is of course also very much depending on season, latitude and elevation. Were these factors in some way included in those calculations?

Nicolàs

Edited July 27, 2020 by inFINNity Deck
added elevation.

[andrew s]

15 hours ago, inFINNity Deck said:

The seeing is of course also very much depending on season, latitude and elevation. Were these factors in some way included in those calculations?

Nicolàs

Not explicitly as far as I know. However, the calculation was done as a function of the ratio of optic diameter to the atmospheric coherence length (Fried parameter) and the estimated variation about this average.

So whatever the observed (actual) seeing it gives an estimated of the probability of capturing a "Lucky" image as a function of aperture. 

Regards Andrew 

PS the formula for the Fried parameter derived from turbulence theory does include a zenith angle term.

Edited July 28, 2020 by andrew s

[mikeDnight]

5 hours ago, andrew s said:

Thanks @mikeDnight in the "Amateur Astronomer's Handbook " Sidgwick gives examples of Pickering, Steavenson and others seeing extended detail at 1/2  to 1/15 of the classic theoretical resolution depending on what it was. So you are in good company. 

Just to help with my sums could you estimate how long you would be willing to wait for a good seeing moment before giving up as a night on a night as not worth observing ?

Thanks Andrew 

Thanks Andrew,

I'm not being very scientific, but often I just get a feel for the kind of seeing a night will produce. But supposing the seeing is not particularly good, I might on occasion wait an hour or so to see if things improve. Some night's feel like they have some potential while others have a hopeless feel. I remember on many occasions being rewarded with some fantastic views after patiently waiting for an hour or more, but generally I've been with an observing buddy, which leads to mutual encouragement and so I'm more willing to stick it out than if I were on my own. Sometimes I can set up for observing and within 15mins get a feeling of hopelessness, but more often I can sense there could be spells of real steadiness and clarity, and when that happens, it makes the pain of freezing to death on cold winter nights worthwhile. I also find that having a purpose to my observing make it easier to stick it out and wait for those fleeting pristine moments. An example would be the Martian apparition of 2016. Even though the planet was so low that it was bouncing off the roof tops, I didn't presume the seeing would be too poor to observe. I'd set my sights on trying to see as much as I could of the northern hemisphere detail, and I was amazed by just how good the seeing often was low down after midnight. Not sure if any of this helps?