102 F/7 or 80 F/7 for White light?

[tico]

Hello. 

I Am looking for One refractor for White light observations, My question, there is too Much diferente between a 80 mm F/7 and 102 mm F/7 ? 

tico

[michael.h.f.wilkinson]

The 102 mm is bound to show more detail, especially when using a solar continuum filter, which hass a fairly narrow passband, so chromatic aberration is hardly an issue

[vlaiv]

Both are very close to limit of resolution in order to see granulation.

Solar granules are about 1500Km in size - which makes them about 2 arc seconds in size. Some are larger, some are smaller.  Airy disk size of 80mm scope is 3.2" while that of 102mm is 2.5".

I think you have better chance of seeing granulation in 100mm scope over 80mm one - so if that is interesting to you - get larger aperture scope.

In fact, this article:

https://skyandtelescope.org/observing/observing-the-sun/

says:

Quote

Granulation is visible only in excellent seeing with high power and at least a 4-inch aperture.

 

[michael.h.f.wilkinson]

4 minutes ago, vlaiv said:

Both are very close to limit of resolution in order to see granulation.

Solar granules are about 1500Km in size - which makes them about 2 arc seconds in size. Some are larger, some are smaller.  Airy disk size of 80mm scope is 3.2" while that of 102mm is 2.5".

I think you have better chance of seeing granulation in 100mm scope over 80mm one - so if that is interesting to you - get larger aperture scope.

In fact, this article:

https://skyandtelescope.org/observing/observing-the-sun/

says:

 

I regularly spot granulation in my APM 80mm, but more aperture certainly gets more detail. I have ordered a 6" F/5.9 achromat for solar, and with a solar continuum filter I don't worry about CA.

Big sunspots and granulation in 2017, using my APM 80 mm F/6, with 2x TeleXtender

[vlaiv]

43 minutes ago, michael.h.f.wilkinson said:

I regularly spot granulation in my APM 80mm

Spot as in visually see, or spot as in record with lucky imaging approach, stack and then sharpen up?

Lucky Imaging, stacking and then sharpening / frequency restoration will produce image with detail that can't be observed at eyepiece - our brain does not posses ability to sharpen up image like that.

[michael.h.f.wilkinson]

2 hours ago, vlaiv said:

Spot as in visually see, or spot as in record with lucky imaging approach, stack and then sharpen up?

Lucky Imaging, stacking and then sharpening / frequency restoration will produce image with detail that can't be observed at eyepiece - our brain does not posses ability to sharpen up image like that.

I can see granulation at the eyepiece without issue, unless seeing is very bad. The brain can memorise the moments of good seeing very well (which is why planetary sketches showed more detail than photographs managed). 

[Stu]

3 hours ago, vlaiv said:

Spot as in visually see, or spot as in record with lucky imaging approach, stack and then sharpen up?

Lucky Imaging, stacking and then sharpening / frequency restoration will produce image with detail that can't be observed at eyepiece - our brain does not posses ability to sharpen up image like that.

It is rare that I don’t see granulation in my FC100DC with Herschel wedge, and at times it can be startlingly vivid. I think many people consider they are seeing granulation when just detecting mottling ‘orange peel’ effects on the sun. Whilst I do normally see it, to catch it properly does require excellent seeing and high power. I often use x200 and when conditions allow you can see the individual granulation cells and watch as they change slowly.

I’m not sure I agree with your figures though. If I can split, say Pi Aquilae at 1.4” easily in the Tak, how can the airy disk size be 2.5”?

I would definitely agree that a 100mm scope should show more than an 80mm scope, for resolution reasons. I would caveats that with the fact that they need to be well corrected in spherical abberation as that can really rob you of the fine detail which you are looking for.

[Stu]

This post I made a while back may be relevant.

 

[michael.h.f.wilkinson]

I typically know I have hit perfect focus when granulation appears. The C8 can show more detail, for sure, but the grains are clearly visible in the 80 mm. 

[vlaiv]

13 minutes ago, Stu said:

I’m not sure I agree with your figures though. If I can split, say Pi Aquilae at 1.4” easily in the Tak, how can the airy disk size be 2.5”?

Diameter of airy disk is 2.5" but you can split doubles that are separated by radius of airy disk that is half that size:

Top image - Airy pattern of two stars separated by airy disk diameter (and having same intensity / magnitude). Middle image - this is often called Rayleigh criterion for resolution - we can still resolve two stars as being two separate entities - this is when their distance is that of airy disk radius (and not diameter). This happens when maximum of one star sits at first minima of other star.

Here is better graph of that:

This shows that although Airy disk diameter is 2.5" - you can split stars separated down to 1.25".

[Stu]

Just now, vlaiv said:

Diameter of airy disk is 2.5" but you can split doubles that are separated by radius of airy disk that is half that size:

Top image - Airy pattern of two stars separated by airy disk diameter (and having same intensity / magnitude). Middle image - this is often called Rayleigh criterion for resolution - we can still resolve two stars as being two separate entities - this is when their distance is that of airy disk radius (and not diameter). This happens when maximum of one star sits at first minima of other star.

Here is better graph of that:

This shows that although Airy disk diameter is 2.5" - you can split stars separated down to 1.25".

Fair enough, replied quickly without checking, but still, your post implied that you can’t see granulation cells which are sub 2” and that’s very much not the case.

Do you do solar white light observing?

[vlaiv]

1 minute ago, Stu said:

Fair enough, replied quickly without checking, but still, your post implied that you can’t see granulation cells which are sub 2” and that’s very much not the case.

Do you do solar white light observing?

No, not much - I observed on couple occasions - but never with aperture smaller than 4".

I based my claim that 80mm won't be sufficient - simply on difference between airy disk size of those two scopes and the fact that 4" is often recommended as a minimum needed to clearly show granulation (see link I posted - it says that granulation is to be seen in excellent conditions with at least 4" scope).

Size of granules according to wikipedia (https://en.wikipedia.org/wiki/Granule_(solar_physics)):

Quote

A typical granule has a diameter on the order of 1,500 kilometres (930 mi)^[1] and lasts 8 to 20 minutes before dissipating

and that of airy disk - support that.

[michael.h.f.wilkinson]

Note that lucky imaging cannot extract detail that isn't in the data, and the images clearly show the presence of granulation, albeit at a very fine level. It is also not noise, as you would then expect to see regions with the "wrong polarity", i.e. dark speckles on a lighter background, rather than the reverse (on occasion you see the odd pore, of course)

[Stu]

21 minutes ago, vlaiv said:

No, not much - I observed on couple occasions - but never with aperture smaller than 4".

I based my claim that 80mm won't be sufficient - simply on difference between airy disk size of those two scopes and the fact that 4" is often recommended as a minimum needed to clearly show granulation (see link I posted - it says that granulation is to be seen in excellent conditions with at least 4" scope).

Size of granules according to wikipedia (https://en.wikipedia.org/wiki/Granule_(solar_physics)):

and that of airy disk - support that.

I guess real world experience doesn’t always tie in exactly with theory, or necessarily all the commentary on the web. There is plenty of difference between say a fast 4” achro and a decent 4” apo as to what you can see in terms of fine detail.

Did you view granulation when you observed? What sort of powers did you use?

I’ve found over quite a few years of regular white light solar observing generally with 80 to 120mm scopes that, much like night time Astro, optimising all elements in the chain really does help pull out the detail.

I think also that, in the same way linear lunar features or planetary detail can be detected when theory says no, granulation is perhaps a different type of feature than does not directly relate to airy disk size being made up of linear features surrounding each cell.

I agree though that aperture helps, and there is a benefit to be had in using a 100mm over an 80mm in fine detail visible. I don’t agree that it is not visible in smaller scopes.

[vlaiv]

Just now, michael.h.f.wilkinson said:

Note that lucky imaging cannot extract detail that isn't in the data,

I agree 100%  - however,that does not mean that the eye at eyepiece can also see all there is in the same data.

There are couple of important points related to how we see and why processed images show more detail than can be observed at eyepiece.

1. Contrast enhancement.

Humans have something called JND - just noticeable difference. We can't tell difference between two levels of brightness if it is not as high as JND.

Processed image has no issues with this

2. Motion blur.

We observe in presence of astronomical seeing. In lucky imaging we use exposure times that are at least 5-6 times shorter than what our visual system uses as integration time. We can look at 30fps image and see no individual frames - our brain "integrates" for at least 30ms. We often use 5ms or shorter exposures for lucky imaging. In 30ms, atmosphere easily additionally blurs the image.

3. We use sharpening in processing.

Our brain can't do that. Here is perfect aperture MTF:

When we sharpen image up - we "raise" this line to be (almost) horizontal. When it is like above - it acts as low pass filter and image is blurred. In best of seeing - that is limit to our vision, but not to processed image - processed image can be sharper (which is again local/micro contrast that can be further enhanced)

[michael.h.f.wilkinson]

10 minutes ago, vlaiv said:

I agree 100%  - however,that does not mean that the eye at eyepiece can also see all there is in the same data.

There are couple of important points related to how we see and why processed images show more detail than can be observed at eyepiece.

1. Contrast enhancement.

Humans have something called JND - just noticeable difference. We can't tell difference between two levels of brightness if it is not as high as JND.

Processed image has no issues with this

2. Motion blur.

We observe in presence of astronomical seeing. In lucky imaging we use exposure times that are at least 5-6 times shorter than what our visual system uses as integration time. We can look at 30fps image and see no individual frames - our brain "integrates" for at least 30ms. We often use 5ms or shorter exposures for lucky imaging. In 30ms, atmosphere easily additionally blurs the image.

3. We use sharpening in processing.

Our brain can't do that. Here is perfect aperture MTF:

When we sharpen image up - we "raise" this line to be (almost) horizontal. When it is like above - it acts as low pass filter and image is blurred. In best of seeing - that is limit to our vision, but not to processed image - processed image can be sharper (which is again local/micro contrast that can be further enhanced)

Yes, I know all this, I teach computer vision at the university, and I include lectures on the human visual system, on which many computer vision methods are modelled. But as I said, an experienced observer can pick out the moments of good, steady seeing, and I can readily spot the appearance of granulation as the image snaps into view. This is also important, as the image is not stationary while focusing, and the human visual system is extremely adept at picking up temporal changes. This is why you can pick up very faint fuzzies by nudging the scope. The moment the blob moves in the FOV, other parts of the visual cortex kick in (notably in V3 and V5), and detect these slight changes that would go unnoticed if viewed as a stationay image.

Experience of the observer also plays a role: in variable star observing, you tend to get better and better at picking out slight differences between stars, as you gain more experience.

Regarding motion blur: this is why I advise people to take a lot of time observing planets: there are moments of prolonged steadiness in the atmosphere. These are rare, so you have to be patient enough to observe them

[Stu]

1 minute ago, michael.h.f.wilkinson said:

Yes, I know all this, I teach computer vision at the university, and I include lectures on the human visual system, on which many computer vision methods are modelled. But as I said, an experienced observer can pick out the moments of good, steady seeing, and I can readily spot the appearance of granulation as the image snaps into view. This is also important, as the image is not stationary while focusing, and the human visual system is extremely adept at picking up temporal changes. This is why you can pick up very faint fuzzies by nudging the scope. The moment the blob moves in the FOV, other parts of the visual cortex kick in (notably in V3 and V5), and detect these slight changes that would go unnoticed if viewed as a stationay image.

Experience of the observer also plays a role: in variable star observing, you tend to get better and better at picking out slight differences between stars, as you gain more experience.

Completely agree with this Michael. In many instances I see as much or more detail visually than I see in even stacked solar images. It only the really high powered, top notch close ups than show far more than is available visually.

[Stu]

17 minutes ago, vlaiv said:

I agree 100%  - however,that does not mean that the eye at eyepiece can also see all there is in the same data.

There are couple of important points related to how we see and why processed images show more detail than can be observed at eyepiece.

1. Contrast enhancement.

Humans have something called JND - just noticeable difference. We can't tell difference between two levels of brightness if it is not as high as JND.

Processed image has no issues with this

2. Motion blur.

We observe in presence of astronomical seeing. In lucky imaging we use exposure times that are at least 5-6 times shorter than what our visual system uses as integration time. We can look at 30fps image and see no individual frames - our brain "integrates" for at least 30ms. We often use 5ms or shorter exposures for lucky imaging. In 30ms, atmosphere easily additionally blurs the image.

3. We use sharpening in processing.

Our brain can't do that. Here is perfect aperture MTF:

When we sharpen image up - we "raise" this line to be (almost) horizontal. When it is like above - it acts as low pass filter and image is blurred. In best of seeing - that is limit to our vision, but not to processed image - processed image can be sharper (which is again local/micro contrast that can be further enhanced)

I’m conscious that we have strayed quite a way from the OPs question which I suspect related to observing as we are in the observing solar forum. Let’s keep the topic in the real world of solar observing and why can be seen practically with these scopes.

[vlaiv]

1 hour ago, Stu said:

Did you view granulation when you observed? What sort of powers did you use?

In most cases - I was seeing limited. I'm not experienced solar observer and most of the times I was actually looking at some phenomena - like eclipse or transit rather than actual solar observation.

I remember seeing granulation - but it was on one or maybe two occasions.

My current WL solar equipment is actually very suited to run experiment on this - I have both 80mm F/6 APO and 102mm F/10 achromat. Together with Lunt Herschel wedge and Baader solar continuum filter - it is decent WL setup.

1 hour ago, Stu said:

I don’t agree that it is not visible in smaller scopes.

I have no reason to disagree as you have first hand experience.

To the first approximation, it looks like 80mm is not able to resolve granulation, but that is not what theory is saying (and in fact - when theory disagrees with practice - it is not because theory is wrong - it is because it is not properly applied).

Best that I could do is make simulation of view in 80mm and 100mm scopes so we can see how much better the view would be in 100mm scope under perfect conditions.

Alternative to that would be for me to get out and make comparison in WL between two scopes and attempt to see granulation in 80mm one. That would require good seeing. Last time I observed in WL I had trouble even seeing faculae near the limb, and a friend that I invited over for a session - could not see them at all (that clearly shows that even limited observing experience helps in seeing detail).

[Stu]

25 minutes ago, vlaiv said:

Alternative to that would be for me to get out and make comparison in WL between two scopes and attempt to see granulation in 80mm one. That would require good seeing.

Do give it a go. Quite a few variables in there eg apo vs achro, focal length etc but if the 4” is well corrected it should still be worthwhile.

Even when seeing is not excellent you should be able to see it during the better periods. Fine focus is critical too.

Let us know how you get on. I’ve got 80mm scopes currently but not ones that will work with a CoolWedge.

[IB20]

I’m thoroughly enjoying white solar viewing with my f10 80mm achro & wedge. I can easily see faculae which are enhanced near the disc limbs with both continuum and Oiii filters. In good seeing at 80x I have seen dark lanes within the umbral regions of spots and have pushed this to 167x. I must admit I haven’t really observed granulation yet but I’d also say I haven’t really looked for it either, I’d probably need a higher mag eyepiece than 5mm too?

I’m not much of an imager and take the odd at the eyepiece iPhone shot, but I’d certainly disagree with that the images I get from that after processing surpass what I see visually. If anything the images I generate aren’t quite as detailed.

[michael.h.f.wilkinson]

12 hours ago, vlaiv said:

Diameter of airy disk is 2.5" but you can split doubles that are separated by radius of airy disk that is half that size:

Top image - Airy pattern of two stars separated by airy disk diameter (and having same intensity / magnitude). Middle image - this is often called Rayleigh criterion for resolution - we can still resolve two stars as being two separate entities - this is when their distance is that of airy disk radius (and not diameter). This happens when maximum of one star sits at first minima of other star.

Here is better graph of that:

This shows that although Airy disk diameter is 2.5" - you can split stars separated down to 1.25".

The final statement implies that an 80mm scope can see granulation, as the centroids of the convection cells are (at least) 2" apart, and the Rayleigh criterion for resolution at 540 nm (solar continuum band) is 1.7". The individual cells are not really resolved in terms of their shapes or sizes, just like none of our scopes can show a resolved image of any star apart from the sun. However, the texture is quite easily seen.

[Stu]

10 minutes ago, michael.h.f.wilkinson said:

The individual cells are not really resolved in terms of their shapes or sizes

Not sure I totally agree with this, when seeing is excellent then at high power I can discern the individual granulation cells. Different shapes and sizes, plus changing slowly over a period on tens of minutes. They are pretty vivid when conditions are excellent.

[michael.h.f.wilkinson]

47 minutes ago, Stu said:

Not sure I totally agree with this, when seeing is excellent then at high power I can discern the individual granulation cells. Different shapes and sizes, plus changing slowly over a period on tens of minutes. They are pretty vivid when conditions are excellent.

What I mean is that in an 80 mm scope, the smallest convection cells are not necessarily accurately imaged, because they are blurred by a point-spread function that somewhat bigger than their extent. I can distinguish the individual cells, and the bigger ones definitely stand out clearly. A four inch scope would show this a lot better. The problem with language is that the world "resolved" can be interpreted differently. Are the individual grains resolved by an 80 mm scope? According to the Rayleigh criterion they are. Can we accurately resolve the sizes and shapes of 2" grains with a 2" radius of the Airy disc? Not really.

[vlaiv]

Well, here is simulation so we can see what it looks like. I took this high resolution image of granulation (best I could find and in fact it's titled: "Highest_resolution_photo_of_Sun_(NSF)_as_of_January_20,_2020"):

this is actually scaled down image - original is huge 7320px x 7320px - which is plenty of resolution for simulation, individual cells are huge and nicely resolved:

I took two perfect 80 and 100 mm scopes, added Baader Solar Continuum filter (540nm simulation) and produced respective images. In reality, view is going to be worse than this because of imperfect optics and seeing effects:

80mm:

100mm:

Both of these squares represent full image posted above (first one scaled down to fit the screen).

Although you can see the granulation texture - you really can't resolve single cells in either 80 or 100mm scope. Most cells are visually simply joined into larger blob that we see and think is individual cell - but it's not.