Dawes or Rayleigh limit - real life stories

[markclaire50]

Hi. Does anyone have a story of pushing their scope to split a double (or any equivalent e.g. resolving moons of saturn, uranus or neptune to their discs: I've not included jupiter moons as I'm sure these are too easy, with perhaps Europa as greatest challenge) with respect to whether your scope achieved this at the Dawes or Rayleigh limit. I don't pretend to understand these limits fully, except that I believe Rayleigh is more applicable to extended objects like moons, whereas Dawes may be applicable to splitting point sources like stars. But, in reality what do people actually find? How far have you pushed your scope to reach these theoretical limits and were you successful? 

My initial input as an embryonic observer--I've split beta Mons with 2.6" separation with my 127mak. There was clear black between, so I'm very optimistic it can go much lower. I'll be looking for something around 1.1" which is its rayleigh limit, as my first attempt. 

I look forward to hearing people's stories. 

Thanks

Mark 

[markclaire50]

Any stories on this? 

[John]

Try 32 and 52 Orionis with your 127 mak. I find them very good tests with my 4 inch refractors. 32 has a separation of around 1.4 arc seconds and 52 around 1.1 or 1.2 arc seconds. They both need high magnifications and steady seeing conditions. 32 is near Bellatrix and 52 near Betelgeuse.

 

 

[Ags]

Interested to see how you get on...

[markclaire50]

1 hour ago, John said:

Try 32 and 52 Orionis with your 127 mak. I find them very good tests with my 4 inch refractors. 32 has a separation of around 1.4 arc seconds and 52 around 1.1 or 1.2 arc seconds. They both need high magnifications and steady seeing conditions. 32 is near Bellatrix and 52 near Betelgeuse.

 

 

Thanks John! ?

[markclaire50]

1 hour ago, Ags said:

Interested to see how you get on...

I will let you know. 

[rl]

I managed to follow zeta bootis (two equal brightness stars) down to below 1" with a 5" refractor during the 1980s- 1990s. Over a couple of years it coalesced from touching but distinct Airy discs, to an ellipse, to a circle. There were only a handful of nights in a season steady enough to be sure. This and Porrima are the only cases where I have seen stellar orbital motion myself.

RL

[markclaire50]

1 hour ago, rl said:

I managed to follow zeta bootis (two equal brightness stars) down to below 1" with a 5" refractor during the 1980s- 1990s. Over a couple of years it coalesced from touching but distinct Airy discs, to an ellipse, to a circle. There were only a handful of nights in a season steady enough to be sure. This and Porrima are the only cases where I have seen stellar orbital motion myself.

RL

Hi. Mmm. So, would you say that was running near Dawes or Rayleigh limit? 

[John]

It's quite important to understand the difference between the Dawes and Rayleigh limits, their intentions and implications etc. This article is quite lengthy but very relevent I think:

Understanding Resolution(1).pdf

[markclaire50]

7 hours ago, John said:

It's quite important to understand the difference between the Dawes and Rayleigh limits, their intentions and implications etc. This article is quite lengthy but very relevent I think:

Understanding Resolution(1).pdf 296.69 kB · 6 downloads

Thanks John. I'm looking forward to reading it. 

[rl]

Excellent article from John.

I watched my 5" scope go through both the Rayleigh and Dawes limits. It was (still is) a f/10 doublet, with a 4.8mm Nagler MK1 eyepiece. 

RL

[markclaire50]

6 hours ago, rl said:

Excellent article from John.

I watched my 5" scope go through both the Rayleigh and Dawes limits. It was (still is) a f/10 doublet, with a 4.8mm Nagler MK1 eyepiece. 

RL

Hi RL

when you say you saw it go through the limits, do you mean reached the limits? I'm not being pedantic. Genuine interest, as I thought the limits meant you can't go through them. ?

[rl]

Well, I started looking at Zeta bootis around 1985. I used to look at the predicted separation in the pink BAA annual handbook and see how well I could separate it. If recall correctly the separation dropped from about 1.2 arcsec to 0.8 arcsec in this timeframe (1985-1995) and I was able to track the separation from cleanly split with a gap to figure-of-eight, to elliptical, to a single star as seen today. So I must have gone through both Rayleigh and Dawes limits at some point knowing the separation from the handbook. The Rayleigh limit for my scope is 1.06" and the Dawes 0.9"

I've long since thrown away the BAA handbooks but someone might still have a set and can look up the exact separation and PA for each year but I remember being quite satisfied with the scope's performance at the time; 0.9 arcsec was detectable as a contact double with a waist where you could have got a rough position angle. 

If you're playing this game seriously it's important to have up-to-date information on the orbital details. Many older sources can be inaccurate. 

Theoretically the Rayleigh criterion is much better defined than the Dawes in that the centre of one Airy disc is placed over the middle of the first dark band of the second star. This still leads to some overlap between the discs but the waisting in the middle is clear and constant in shape.  Dawes' limit  is a lot more subjective and really amounts to the point where you can just be certain there is a waist in the middle of the combined discs. The wavelength of light is also critical; here the rods in your eye impose a natural filter at about 500-550 nanometres.

It's important to appreciate that from the practical observational point of view neither Rayleigh or Dawes is a hard limit cast in stone where one side of the line the double is clear, the other it's invisible and all you see s a single star. There is a gradual coalescence of the Airy discs as the separation goes down and the exact point where an observer draws the line between split and unsplit is somewhat subjective; it's hard to say if the first Airy disc is exactly on the first dark band...even for Takahashi owners! A lot of experience and preferably averaging over multiple observations is required; you are working at the limits of the scope at maximum useful magnification where atmospherics wreak the maximum havoc. 

If you google zeta bootis there is quite a lot on it; many others have tried exactly what I did! It was a useful star because it's bright enough to be easily found and the two components have almost exactly the same brightness. It's too close now. 

RL

[markclaire50]

44 minutes ago, rl said:

Well, I started looking at Zeta bootis around 1985. I used to look at the predicted separation in the pink BAA annual handbook and see how well I could separate it. If recall correctly the separation dropped from about 1.2 arcsec to 0.8 arcsec in this timeframe (1985-1995) and I was able to track the separation from cleanly split with a gap to figure-of-eight, to elliptical, to a single star as seen today. So I must have gone through both Rayleigh and Dawes limits at some point knowing the separation from the handbook. The Rayleigh limit for my scope is 1.06" and the Dawes 0.9"

I've long since thrown away the BAA handbooks but someone might still have a set and can look up the exact separation and PA for each year but I remember being quite satisfied with the scope's performance at the time; 0.9 arcsec was detectable as a contact double with a waist where you could have got a rough position angle. 

If you're playing this game seriously it's important to have up-to-date information on the orbital details. Many older sources can be inaccurate. 

Theoretically the Rayleigh criterion is much better defined than the Dawes in that the centre of one Airy disc is placed over the middle of the first dark band of the second star. This still leads to some overlap between the discs but the waisting in the middle is clear and constant in shape.  Dawes' limit  is a lot more subjective and really amounts to the point where you can just be certain there is a waist in the middle of the combined discs. The wavelength of light is also critical; here the rods in your eye impose a natural filter at about 500-550 nanometres.

It's important to appreciate that from the practical observational point of view neither Rayleigh or Dawes is a hard limit cast in stone where one side of the line the double is clear, the other it's invisible and all you see s a single star. There is a gradual coalescence of the Airy discs as the separation goes down and the exact point where an observer draws the line between split and unsplit is somewhat subjective; it's hard to say if the first Airy disc is exactly on the first dark band...even for Takahashi owners! A lot of experience and preferably averaging over multiple observations is required; you are working at the limits of the scope at maximum useful magnification where atmospherics wreak the maximum havoc. 

If you google zeta bootis there is quite a lot on it; many others have tried exactly what I did! It was a useful star because it's bright enough to be easily found and the two components have almost exactly the same brightness. It's too close now. 

RL

Thank you RL for a very comprehensive reply! I hear that Rayleigh limit is more relevant than Dawes for deciding if an extended object can be resolved e.g. a disc. Say moons of Jupiter. Did you ever resolve Europa into a disc as I think it would be at the Rayleigh limit for the 5" even at opposition?

Thanks 

Mark 

[rl]

I'm not sure the Rayleigh is more relevant; it is certainly better defined but the choice of definition was failry arbitrary in putting the centre of one disc over the first dark ring. It has stood the test of time very well but but Rayleigh could just as easily have chosen the first light ring which would have included a definate gap. The Dawes is totally empirical and more subjective but applies to what people actually perceive at the eyepiece which is maybe what matters. 

The moons of Jupiter are nonstellar even in a humble 6" newtonian; compare them in the same scope with any 6th magnitude star and you will see a difference given enough magnification. All four are about 2 arcsec diameter from memory and the disc you see is noticeably bigger than an Airy disc for a stellar point source. People with scopes 12" and upwards have managed to image very rough details on Jupiter's moons that with a lot of imagination bear a dubious resemblance to blurred voyager photos .... but with small/ medium scopes all you see is an obviously enlarged disc with no real detail. 

Slightly off subject, one of the things that strikes me on this subject is the way that technology has not really improved on the basic physics (why should it, you may ask...); the same rules that worked 150 years ago have stood the test of time and are still used to judge modern scopes.  Both Airy and Dawes used handmade long focus achromat scopes with simple crown/flint combinations, with no coatings. The results we get nowadays with ED glass and superb coatings are really just the same in terms of double star resolution. There are obviously other benefits of progress in terms of colour correction and contrast and scope length but from my own experience a well-made simple long focus achromat can deliver all the detail in a double star that can be seen with the best of modern ED triplets. Apart from the 5" I have a 4" cheapo skywatcher crown/flint achro amd a 6" uk made crown/flint/crown triplet with no coatings; both go down to the Dawes limit with ease on the rare occurance of a good night. 

[rl]

And try John's test with 32 and 52 Orionis! That will be worth a whole bunch of armchair theorizing...

[markclaire50]

39 minutes ago, rl said:

I'm not sure the Rayleigh is more relevant; it is certainly better defined but the choice of definition was failry arbitrary in putting the centre of one disc over the first dark ring. It has stood the test of time very well but but Rayleigh could just as easily have chosen the first light ring which would have included a definate gap. The Dawes is totally empirical and more subjective but applies to what people actually perceive at the eyepiece which is maybe what matters. 

The moons of Jupiter are nonstellar even in a humble 6" newtonian; compare them in the same scope with any 6th magnitude star and you will see a difference given enough magnification. All four are about 2 arcsec diameter from memory and the disc you see is noticeably bigger than an Airy disc for a stellar point source. People with scopes 12" and upwards have managed to image very rough details on Jupiter's moons that with a lot of imagination bear a dubious resemblance to blurred voyager photos .... but with small/ medium scopes all you see is an obviously enlarged disc with no real detail. 

Slightly off subject, one of the things that strikes me on this subject is the way that technology has not really improved on the basic physics (why should it, you may ask...); the same rules that worked 150 years ago have stood the test of time and are still used to judge modern scopes.  Both Airy and Dawes used handmade long focus achromat scopes with simple crown/flint combinations, with no coatings. The results we get nowadays with ED glass and superb coatings are really just the same in terms of double star resolution. There are obviously other benefits of progress in terms of colour correction and contrast and scope length but from my own experience a well-made simple long focus achromat can deliver all the detail in a double star that can be seen with the best of modern ED triplets. Apart from the 5" I have a 4" cheapo skywatcher crown/flint achro amd a 6" uk made crown/flint/crown triplet with no coatings; both go down to the Dawes limit with ease on the rare occurance of a good night. 

Thank you RL. The moons of Jupiter at opposition tend to range from about 1" for Europa to around 1.75" for Ganymede. Europa would be hardest to resolve to a disc with a 5". I remember many many, many years ago, first time I looked at jupiter with a 8.75" Dob, expecting points of light, and seeing a huge disc for the planet (well, huge compared to the 10x50 binos I had used before then!) and tiny, but nonetheless discs. That was a wonderful sight as I really wasn't expecting that. 

[niallk]

One night I watched Europa transit across Jupiter's disc.  When it was on the limb, it appeared as a beautiful '3d ball'.  Shortly later as it continued in it's orbit, it went back to a very bright little disc + glare against the blackness of space: the distinct '3d ball' effect was lost.  This was a 15" dob, mag 330x.

[mikeDnight]

The first time i looked through my Takahashi FS128 the Galileans appeared as discs and not points. They each had slightly different colours. That was the first time i ever saw them as discs, but since then I've seen them as such many times, and even doing so when using a 102mm Vixen ED, following the tiny disc as it traversed Jupiter's globe. Attached is a sketch of how a moon appears through my 100mm Takahashi.