Size of features depending on scope aperture?

[vlaiv]

I was just out and I had a first light with my Mak102 - quick lunar session. Seeing was rather good but quickly started to deteriorate as temperature started to drop. It was also first light for few new high power eyepieces.

Scope is really sharp and although I'm not lunar observer - I decided to try to push the little scope (and my eyes) and look for distinct feature that is both small and that I could remember to later check for size / angular size to see how sharp my scope really is (at the eyepiece I thought it was rather sharp and surprised me).

Here is what I saw:

It was very distinct view - almost like in second image - there is no mistaking it for something else.

What surprised me is that these two features are : about 1.9km in diameter and separated by 4.6km.

At distance of 384000km that gives:

1.02" and 2.47". According to Dawes, resolution limit of 4" scope is 1.137" while Rayleigh gives 1.26" as resolution limit. Now image did not look like it was about to break or anything like that - it was not blurry, two peaks were distinct features with quite a bit of space between them (like 2-3 times their size). In fact second image really depicts what I saw.

What is your experience like in terms of seeing features of certain size - is it in line with "resolution limit" of telescope or do you feel that there is more "oomph" to scope then numbers suggest?

 

[John]

Seeing conditions are the master here but when they are good I've been able to see sections of the central rille that runs down the lunar alpine valley with my 12 inch dob at around 300x. The rille is 1000 metres across at it's widest which would equate to a resolution of around .6 of an arc second.

That is the finest detail that I've been aware of resolving on the moon so far. On really good nights I've also managed to see 11 craterlets in Plato the smallest of which are just under 1km I believe.

The illumination situation of the features being observed plays a big part in their visibility - shadow really does help in picking up a piece of terrain.

[Sunshine]

All i can add to this technical conversation is, Mak's kick butt no? when seeing is good of course.

[Charles Kirk]

Dawes Limit assumes point sources, so it is more directly applicable to separating double stars than fine lunar and planetary detail.

[vlaiv]

36 minutes ago, Sunshine said:

All i can add to this technical conversation is, Mak's kick butt no? when seeing is good of course.

Seems so , this is my first time with a mak and I'm rather happy of what it delivered. In fact, it went beyond my expectations.

Must make comparison one night - 4" achro (F/10) vs 4" mak to see the differences. Hopefully, seeing will be as good as tonight (before it started getting worse, of course).

18 minutes ago, Charles Kirk said:

Dawes Limit assumes point sources, so it is more directly applicable to separating double stars than fine lunar and planetary detail.

Indeed. I think that simulation is in order - I can download image and blur it by 4" 30% CO aperture and make apparent size similar to that of x240 (taking into account average pixel size and average viewing distance). That way we can see what theory says it should look like and if it can be easily resolved.

[Alien 13]

41 minutes ago, Sunshine said:

All i can add to this technical conversation is, Mak's kick butt no? when seeing is good of course.

Totally agree, what I have noticed about Maks is their solid build quality and pin sharp optics. I smile everytime I take mine out of its bag.

Alan

[vlaiv]

Ok, resolution matches what I saw

This is what it should look like (disregard bright edges - that is convolution artifact) at x240 power with 4" scope if viewed on 23" 1920x1080 computer monitor at 91cm away.

(this is based on 0.265 pixel size).

[Charles Kirk]

That's interesting, 19th Century theory still works!

38 minutes ago, vlaiv said:

Must make comparison one night - 4" achro (F/10) vs 4" mak to see the differences. Hopefully, seeing will be as good as tonight (before it started getting worse, of course).

That would be a very interesting comparison.

 

[vlaiv]

2 minutes ago, Charles Kirk said:

That's interesting, 19th Century theory still works!

Well, I'm in favor of theory working, but view was so nice and sharp that I started questioning it

[johninderby]

I used to be impressed with the Skymax 180s ability to take high magnification but it is so comprehensively out performed by the 8” classical cassgrain. The cassegrain delivers great performance at 550 mag on the moon with a Morpheus eyepiece  providing seeing is reasonable. Trying to push that mag with the mak was a waste of time as it was empty mag that didn’t show more detail.

I find at 550 mag the surface of the moon looks grainy. Yes it’s a bit more aperture but that doesn’t explain the difference. Looking forward to using it with the AZ100. Hopefully the forecast for Friday night is correct. 🙏🏼

Edited January 2, 2020 by johninderby

[Sunshine]

10 minutes ago, vlaiv said:

Well, I'm in favor of theory working, but view was so nice and sharp that I started questioning it

I'm not following, sorry, does this mean you were seeing something that led you to believe the scope was performing better than it actually was?

[RobertI]

Interesting experiment Vlaiv. Coincidentally I have been reading about this very subject in my lovely little book 'Frank's book of the telescope' written by the telescope maker Charles Frank in 1959. I frequently delve into this book, which is as relevant today as it was back then. In the chapter discussing amatuer intruments, he notes:

"...some of these lunar features, frequently observed by users of such telescopes, are known to be of a size which theory would suggest to be incapable of resolution. When this happens it would be unwise for the observer to be believe that what he sees is the full and final form of the feature; sometimes this evident resolution is spurious , being the result of overlapping in the very complicated diffraction pattern which viewing areas of high contrast by optical means must involve..... All the same, it remains a fact that excessivly fine detail, especially when of a linear type, can be seen. Another way in which subjects below the resolving power of the telescope can be detected on the surface of the moon is by detection of the shadows....then it is that very ridges of no great height  can be seen thrown into exaggerated relief......  "

May not help 'resolve' your question (excuse the pun) but is interesting to note the same debates were happening 50 years ago!

Edited January 2, 2020 by RobertI

[vlaiv]

37 minutes ago, Sunshine said:

I'm not following, sorry, does this mean you were seeing something that led you to believe the scope was performing better than it actually was?

Yes, that was the impression that I got behind the eyepiece.

I'm not overly experienced Moon observer (one of reasons for getting this scope was to spend more time doing lunar observing), and that might have been the reason behind it. When I looked the airy disk - it looked about right for aperture at that magnification - it was all there - little "ball" in the center and first and second diffraction rings. First being broken in 2-3 segments almost all the time and second just glimpsed.

Somehow sharpness of lunar view was greater than I would expect for that aperture at such magnification (judging by airy pattern seen). I've heard that the Moon can take crazy mags and still give good image, but never tried myself.

I can only explain it by very large contrast range on the moon as a target and blur that comes from airy pattern is not enough to make image soft - that is what I gathered from producing above image that should resemble view that I saw. In principle that is what I saw in terms of detail, but position of the sun - how shadows and highlights combine and brightness of the target made it look much more alive and sharper than the image depicts.

[Stu]

3 hours ago, John said:

Seeing conditions are the master here but when they are good I've been able to see sections of the central rille that runs down the lunar alpine valley with my 12 inch dob at around 300x. The rille is 1000 metres across at it's widest which would equate to a resolution of around .6 of an arc second.

That is the finest detail that I've been aware of resolving on the moon so far. On really good nights I've also managed to see 11 craterlets in Plato the smallest of which are just under 1km I believe.

The illumination situation of the features being observed plays a big part in their visibility - shadow really does help in picking up a piece of terrain.

What's interesting is that I've seen the Alpine Rille in my 4" Tak a couple of times, but I think I've only managed 4 Plato craterlets with it. I guess the fact that the Rille is a long feature makes it easier, somewhat like the Cassini Division? Illumination has to be optimum otherwise it's not possible.

[John]

6 minutes ago, Stu said:

What's interesting is that I've seen the Alpine Rille in my 4" Tak a couple of times, but I think I've only managed 4 Plato craterlets with it. I guess the fact that the Rille is a long feature makes it easier, somewhat like the Cassini Division? Illumination has to be optimum otherwise it's not possible.

That is interesting. I've yet to see the Alpine Rille with any scope other than my 12 inch dob but that might just be a case of things not quite falling into place when I've been trying with one of the refractors.

I also think these targets that are pushing the envelope in performance terms place high demands on the last links in the observing chain, ie: the eye and the brain. We don't talk about those observing tools that much but they are crucial of course.

 

[mikeDnight]

I've always felt there's more oomph in a scope than the numbers suggest. When looking at linear features who's width is below, and sometimes well below the resolution limit of a scope for example, is often resolvable due to its length. Many lunar features and ultra fine divisions within Saturn's rings are good targets when pushing a scope over the presumed limit. Jupiter and Mars also reveal very tiny but fascinating details when the seeing allows. Infact I spend a lot of my time really studying things so as to try and tease out the most intricate, or most subtle detail which could be considered as being at the very limit of my scope. It's amazing fun, and as a consequence I've been rewarded with very many unbelievably breathtaking views. My friends who observe with me have the same mind set and use their scopes to their limit, and on occasion, beyond. 

Interestingly, we all tend to use scopes of relatively small aperture as our preference, currently 3" to 4.7". Ours are all refractors, but as your own observations are proving, Maksutov's can play this game too. You obviously have a very good and capable scope in your 4" Maksutov, and perhaps you are a lunar observer but you just don't know it yet. It's addictive, so tread carefully!  

[Sunshine]

 I should really consider a bino viewer for my Mak, i have a feeling it would be a whole other level of lunar awesomness, unfortunately, it will probably create a nice hole in my wallet.

[mikeDnight]

4 hours ago, John said:

Seeing conditions are the master here but when they are good I've been able to see sections of the central rille that runs down the lunar alpine valley with my 12 inch dob at around 300x. The rille is 1000 metres across at it's widest which would equate to a resolution of around .6 of an arc second.

That is the finest detail that I've been aware of resolving on the moon so far. On really good nights I've also managed to see 11 craterlets in Plato the smallest of which are just under 1km I believe.

The illumination situation of the features being observed plays a big part in their visibility - shadow really does help in picking up a piece of terrain.

The Alpine Valley had always been on my Wish To See list, until one Sunday morning back in 2003/4 (can't remember which), at around 3am, I saw it for the first time in my Tak FS128. The moon was high but I was in an uncomfortable position as the tripod was low, so not a great experience.  I have been lucky enough to see it several times since both in an SW 120ED but oddly with more success through my Tak FC100DC. On one occasion in the spring of 2018 I was observing the central rille using my FC100DC with binoviewer, barlow and a pair of 15mm Vixen LV eyepieces (the best view I've ever had of it to date), when I had a call from Paulastro, who was also seeing it. Paul was using a 127mm Maksutov at the time and was buzzing with the same excitement that I was. I think liberation as well as seeing plays an important roll in determining whether the rille will be visible or not. For the most part it looks like a fine broken bright line to me, perhaps because the Sun is striking the northern wall of the rille, but on the occasion Paul phoned, the rill was the most complete I've ever seen it. 

I've heard it said that smaller apertures are less affected by atmospheric turbulence than larger scopes, and so perhapse that works to the smaller scopes advantage. Whatever the reason, I'd say to anyone who hasn't yet seen it, keep looking. With the high spring moon soon to be upon us and the milder nights, sure success is bound to eventually reward those who persevere. 

[Stu]

3 minutes ago, mikeDnight said:

For the most part it looks like a fine broken bright line to me, perhaps because the Sun is striking the northern wall of the rille

That matches my experience Mike. I don't know why, but I was looking for a dark line so took me a while to work out that I was seeing it! Not sure if it is possible to see as a dark line when in shadow?

[vlaiv]

2 minutes ago, Stu said:

That matches my experience Mike. I don't know why, but I was looking for a dark line so took me a while to work out that I was seeing it! Not sure if it is possible to see as a dark line when in shadow?

I doubt it. It will never be only shadow. One side and bottom will be in shadow, but opposite side will in sun light (same as craters - one side is always in sunlight). I think that sunlit side is going to be much brighter than shadow is darker than surroundings (sun is really strong source of light) and hence it is more likely to be seen as bright line vs gray surrounding than it is as dark.

[mikeDnight]

I think the dark shadow may well be visible Stu, but how dark is dark? Im reasonably confident ive seen the shadow but that its very subtle, partly because its not that much darker than the valley floor, and partly because its overpowered by the brightness of the north wall. That's my theory for what its worth! Trying to see the rille shadow will make an interesting little project this spring. 

Edited January 3, 2020 by mikeDnight

[John]

I think the angle of illumination of these very fine lunar features is important to detecting them. Its possible that with some, that the "window of opportunity" is very short - even an hour either side of the optimum time might make all the difference.

 

[Voyager 3]

Any other recent sightings of alpine rilles ? Interesting topic !