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View Full Version : Object of the Week, June 7, 2020 – Herschel’s Ring



Howard B
June 8th, 2020, 06:51 AM
Canes Venatici
M51, “The Ring Nebula in the Canes Venatici”
RA: 13 29 52.7
DEC: +47 11 43
Magnitude: 8.4 (v)
Size: 11.2’ x 6.9’

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(John Herschel's 1830 sketch of M51) (HST image of M51)

For a moment, let’s pretend the year is 1844. William Parsons hasn’t finished his 72-inch reflector yet, and the accepted appearance of M51, then called the “the ring nebula in Canes Venatici”, is John Herschel’s drawing from 1830.

Herschel described it as “a vB neb 1’ in diameter of a resolvable kind with a double ring or rather 1 ½ ring like an armillary sphere” using his 18 ¼-inch reflector. There are no hints of spiral structure. Although he prophetically speculated that M51 could be “like our Milky Way & perhaps this is our Brother System”, the main expectation of the time was that the core of M51 would be fully resolved into stars with the advent of larger telescopes.

A thought experiment:
1. You and your 2020 telescope and eyepieces are magically transported back to 1844.
2. You don’t know about M51’s spiral arms but are fully aware of Herschel’s Ring.
3. Your main observing goal for M51 is to resolve the core of NGC 5194 into stars.

Would you have seen the spiral arms?

There’s no way to know, and I swing between thinking most of us would see the spiral arms or none of us would. Could we look past our Herschel Ring bias to see the spiral arms, or would we focus on resolving the core of NGC 5194 and continue to see Herschel’s Ring surrounding it?

Herschel was using his 18 ¼-inch scope, and before Parsons finished the 72-inch he made a successful 36-inch reflector – both plenty large enough to clearly show the spiral arms - and they didn’t see them. If we were back in their time, and knew what they did, why would any of us see M51’s spiral arms?

While you’re chewing on that, let's discuss seeing Herschel’s Ring in the 21st century. Under what conditions and with what telescopes have you seen it?

I’ve seen it three times in three different telescopes. It takes a mental shift with my 28-inch scope but none with an 8-inch and only a little with a 13-inch. In fact, the first time I saw Herschel’s Ring was with the 13-inch, and I was caught completely by surprise by how evident it was. I most recently saw it in February 2020 with my 28-inch from my backyard under a 20.35 SQM sky. The moderately dark sky hid M51’s distinctive spiral structure just enough that I easily saw the ring. It’s a delight to see M51 as Herschel did.

3906 3904
(8-inch sketch of M51) (28-inch sketch of M51)

Herschel’s Ring consists of the brightest parts of the spiral arms. The beginning of the arms in the core, and the north and south tidal arms aren't bright enough to be part of the ring. That leaves the core of NGC 5194 as a fuzzy bullseye separated from the ring, and the core of 5195 as separate object outside the ring. It's similar to the appearance of Hoag's Ring (PGC 54559) in Jimi's 48-inch scope, but with a bright companion.

Seeing Herschel’s Ring is somewhat like the crater illusion – craters shifting between looking concave or convex – but Herschel’s Ring points toward something more subtle, and potentially troublesome. How do we know what we see is real? How do we separate what we expect to see from what we think we see? Or want to see?

Our biases are most powerful when we aren’t aware of them, and although awareness helps a great deal, like a clever optical illusion they can still be hard to see past even knowing the trick.

I contend that M51 is a prime example of photographic bias – we see spiral arms because we know they’re there and what they look like from photographs. Observer’s saw Herschel’s Ring between 1830 and 1845 because that’s what he saw, and being the pre-eminent observer of his day, his observation set the expectation – the bias. It took the massive 72-inch to transform Herschel’s Ring into spiral arms.

M51 is an interesting case in another way too – the visibility of its tidal bridge between NGC 5194 and NGC 5195. It’s easy enough to find online reports and sketches of M51 with small telescopes clearly showing the bridge, but I’ve always seen it as one of the fainter features of M51. I see no trace of it with my 8-inch even under excellent conditions.

We are subjective creatures by nature, and objectivity is difficult to attain simply because of how our brains are wired. For an extensively photographed and widely observed object like M51 it may be difficult to look past its well-known spiral arms and see Herschel’s Ring. However, the effort is worthwhile not only to see M51 as Herschel’s did, but to demonstrate how elastic our perceptions can be.

Give it go and let us know!

Howard B
June 8th, 2020, 07:03 AM
Sorry about the size of the images - I scaled them in Gimp to be the same size but for reasons I don't understand the HST image of M51 came out way too large!

kisspeter
June 10th, 2020, 12:27 PM
Very interesting thoughts and a great idea, Howard!

As for Herschel’s Ring I can't add too much. I've never tried to look at M 51 with that bias. I have a 4" drawing which shows the spiral structure. I remember I fought very hard for it. It was on the edge of the telescope's and my eyes' capabilities (under a really good sky). But I drew only what I believe I truly saw. I've even seen the bridge which was probably an illusion because it is really much fainter in bigger scopes. I think the small magnification (small angular size) might play a role in this illusion.
Next time I will try to look at M 51 with Herschel's Ring in mind.

As for the biases in general I can add a little story. I hope you don't mind. On the next-to-the-last night of our first trip to Namibia (Hakos Farm, 2012) I just finished my Omega Centauri drawing and I ran out of objectives for the second half of the night. I didn't want to waste the perfect sky and wanted to choose something very uniquely southern. The Small Magellanic Cloud was already up so I aimed my scope at it. I knew it had a few objects in it just like LMC, but didn't know anything about them. I picked the nicest looking field with my little 4" and started to draw it. As I got to the details of NGC 346 I slowly started to realize that what I am drawing looks like a barred spiral galaxy. I didn't see it directly in the scope - the details added up like that on the paper. I thought this is not possible. A diffuse nebula cannot look like that. But then I started to see it in the telescope as well. Then I stared to doubt and challenge myself. Look at it with my head tilted. With my other eye. Move the field. Pretend it is elogated in the other direction / faint where I see it to be bright / etc. and check how consistent it is. Nothing helped - it still looked like a barred spiral galaxy. I couldn't do anything but draw it that way. Next day I learned that I saw it correctly. (My drawing is here: https://deepeye.hu/en/drawings/ngc346.html ) Using a 4" scope the details are very far from being obvious.

Unfortunately it very rarely occurs to me that I draw something unbiased. (But I never look at photographs while drawing.) It was easier before I had access to DSS or internet from home in general.

So I think without Herschel's bias we would be able to see the spiral structure. But who knows?

j.gardavsky
June 10th, 2020, 03:55 PM
A very good idea of unbiased observing!

Most of my views on M51 have been through the 25x100 binoculars, and they have revealed just an egg shaped oval with a condensation due to the core of the M51.
My last telescopic observation has been on the 8th April 2018:
6"F/5 achromatic refractor, Pentax XW3.5mm, KK Abbe ortho 5mm: two spiral segments East and South, NELM 5.3mag
Unfortunately not the Herschel's ring illusion.

On the other hand, I can imagine that with a large aperture telescope (I don't have) and low magnification, the spiral arms may visually merge into a ring. There is also a sketch made by Roger Clark, Fig.1 in https://clarkvision.com/visastro/m51-apert/ which shows the outer disc split into 4 condensations. Given the low quality eyepieces during the Herschel's times, these condensations may have been interpreted as an outer ring.

Otherwise, I approach the DSOs without much knowledge what to expect, and without detailed star hopping charts. Either I find it, or I have had bad luck. And either I can see some form, or just a faint glow.

Best regards,
Jiri

MelBartels
June 12th, 2020, 11:41 PM
I have a sketch I did a long time ago with an 8 inch at low power. I wanted to find out how much my knowledge of M51's spiral influenced my sketch. So I pretended not to know and stared for a good time. I ended up seeing a ring with a little handle. With a 14 inch though it was impossible to not see that the ring was more spiral in nature. Maybe that says something about the effective aperture of scopes of Herschel's era. I'll look for that sketch...

Mel Bartels

MelBartels
June 13th, 2020, 12:40 AM
Here are three M51 sketches. The 6 inch is at very low power, where I saw a ring.

http://bbastrodesigns.com/drawings.html#M51

Mel Bartels

Ivan Maly
June 13th, 2020, 06:47 PM
Great thread. Actually, 2020 or 1844, we are biased toward spirals. First there is Archimedes, On Spirals, c. 3rd century BC. Second, visual processing in the brain is wired to see spirals whether they are there to be seen or not (spiral hallucinations).

Being only vaguely aware of this discussion, last night (SQM 21.85 stable overhead, beautiful transparency) in my 20" I was amazed how clear the Herschel ring was. My first thought was, "Do I see it because Howard named his OOTW Herschel's Ring?" Be that as it may, the galaxy was the split ring plus one spiral arm curving outwards on the SW side. The details of the dark lanes (dust and inter-arm gaps) of course allowed to trace out the arms as such, but that's how I would nonetheless describe the overall picture. All nicely decorated with blazing-bright star formation regions. Also at 150x there was the real bridge (not the arm projecting in front of the companion, which folks call "bridge" in amateur observing) around the E side, but it was considerably subdued despite the very promising conditions and was seen only as a thin outline of the NE (connecting to the companion) and SE (connecting more or less to the SW arm) segments. So I guess it was the ruins of the real bridge for me last night.

j.gardavsky
June 15th, 2020, 11:47 AM
Hello Mel,

the Ring may be an illusion at low magnifications, as your sketch of June 11-12, 2018, could be also interpreted.
Otherwise, my observings through the 6" F/5 achro at 150x show the dominating spiral arm pointing away, like in your sketch of August 4-5, 2018.

On a side line, I've got the three sides of the frame surrounding the M51 through the 15x85 binoculars with the blue(RGB)CCD filters, on May 19th, 2020. We have had in May those fantastic Covid clear nights for hunting the IFNs.

Best regards,
Jiri

Uwe Glahn
June 17th, 2020, 08:52 PM
Great threat and idea of the topic "bias" and "transformation". And a nice reminder of who was the first in seeing details within Messier 51.

In my mind Herschel did see the ring-shape because of two reasons.
1. The sky conditions, the effective telescope aperture, the used exit pupil, the experience and the observing time (did we know this?) exactly match to resolve the spirals as a ring.
2. Herschel did transform the observed details in a ring because he was not aware of any spiral structure but of ring structures in former discovered/observed objects.

I'm not a fan of the thesis that we see what we want to see or what our brain saves from seeing detailed photos. Most of our brains (not all) are simply not powerful to bookmark the exactly structure.
Of course, we are biased of general structures like spirals within Messier 51. But where do they sit and started exactly? Where are the fine structures exactly? Not many brains can fetch these details.

I more like the word "transform". We tend to create a suspected structure. One famous example in the historical and spiral content are the spiral nebulae (http://www.deepsky-visuell.de/Projekte/Rosse.htm) Lord Rosse described. He also saw spiral structure in the open cluster NGC 2506 (http://www.deepsky-visuell.de/Zeichnungen/NGC2506.htm). And when you search for it, you see them for real.

Back to Messier 51 and my experiences. I never saw a clear ring. But I do see spiral segments in different apertures which forms an interrupted ring. When apertures or/and condition were good enough for seeing spiral structure I saw it as spirals.
Last days I got the chance to use a 130mm Refractor telescope under soso conditions (bad transparency, NELM around 5,5mag). My buddy and I saw clearly two separated segments. We both suspected a fine direction of rotation without seeing any connection to the core or clearly resolved arms. I tried to imagine a ring without any success. I ask my friend (who was not aware of Herschels ring) if he could see a ring. But he also had no success.

I end up with a sketch which shows the achievable details with a 4"-5" to my experience.

sketch: 4", 88x, NELM 6m5+
3917

akarsh
March 14th, 2021, 09:00 AM
I was out observing last night in subpar skies, and decided to try sketch M 51 through my 18", without referring to any image to identify features. As I was working through the sketch, I did notice bits and pieces of the faint connecting arms, but most of the arms of the main member of M 51 wrapped around into a ring, indeed! The contrast on the "inside" of the ring was so poor, that I couldn't see the spiral structure -- just the ring and a few off-shoots and bits of the connecting arm. It's only after going half way through my sketch of the bright regions and saying to myself "this does not look like M 51", that I tried and found the lower-contrast regions where the arms forming the "ring" spiral out of the core. so when I tried to trace the arms going into the core, I marked them somewhat incorrectly, indicating that I wasn't really seeing exactly where they were going into but only had a general sense of which way they were going.

So I didn't exactly see Herschel's Ring, but I think I did pick up the central theme of it.