On the visibility of extended objects.

[Mr Spock]

I got myself thinking about this a couple of nights ago. Quite a few of us were out observing the comet in relatively clear skies. Some were saying it was well beyond naked eye visibility, yet others (including me) were able to see it.

In the moment between the sky getting completely dark and the moon interfering, it was there, seen with averted vision. This wasn't from home where my sky is listed as Bortle 6 (seems more like 8 these days), but a short drive out to the edge of the Peak District which is Bortle 4. I could see the milky way quite easily. From this location when completely dark and transparent, you can see M33.

For the comet, and the same applies to objects like M33, apart from sky conditions, I wonder what else might be a factor in observing these objects. There does seem to be a wide variability of who can and can't see them.

[Rusted]

Variability? Ahem. I think they call those things: "Clouds." 😉
I was able to see the comet naked eye at dusk before the moon lit up the sky.
On other occasions I needed my 8x42 binoculars to pinpoint it amongst all the clouds.
Image handheld One+ Nord 2T phone. ƒ/1.88 1/4s 5.59 mm ISO15866
The only one which came out without camera shake. 16/10/24 19.29CET
That may have been the time I borrowed a neighbour's fence post as a pier.

[Ratlet]

Sounds a lot like my back garden.  I've found for the faint fuzzys time and comfort are big factors.  Comfort especially.  The observing chair makes seeing detail in the fainter stuff a lot easier.  Some targets feel like one of those magic eye puzzles where you just can't see it until you can and being relaxed and comfortable certainly helps.

Time obviously is a big factor you need to spend the time on the target and not on other light sources.  Probably also would say 'observing not looking' falls into this camp.  Take in the full field, positions of stars around the target.  It'll often help being averted vision into play.

Giving the scope a wee wiggle also helps as you can quite often see the edge of extended objects move when you can't see it when it is stationary.  The brain is great even in low light at spotting movement.  Once it sees it though it'll tend to get locked in.

I'm not sure what it is, but I suspect it's hydration is also a factor.  When I'm really trying to get the detail I will sometimes get a static effect like on a detuned TV.  Again not really sure the exact cause but it can be a nightmare.

Playing with the magnification can play a big part too.  I can't remember the reasoning but someone far smarter than me explained it.  From memory for extended objects when you change power you are affecting the brightness of the object and the sky at basically the same rate and it's about finding the sweet spot where you can get the best contrast.  It also helps because some extended objects are bigger than you think and you might be seeing into the middle of it and not the edge.  

[Mr Spock]

It was very dark. I could barely see the scope until the moon came up.

[John]

From my back garden (where I do 99% of my observing) it depends which direction I look.

To the N and E there are light "domes" from Cardiff, Newport, Bristol and Bath that dominate those horizons and extend for around 30 degrees above the horizons. I need to wait until fainter objects are well above the horizon in those directions. To the S is much better and I've managed to see some quite faint targets, including the Horsehead Nebula, in that sector of the sky.

Overhead I can often see quite a lot of the milky way and the Andromeda Galaxy is generally visible to the naked eye once it rises above around 45 degrees.

I feel that my sky should get a "depends where you look" bortle / NELM rating 🙂

I reckon that sky transparency (the key to seeing extended DSO's) is gradually getting worse generally though, quite probably all over the country.

Then there are factors such as visual acuity of the observers eye (which I'm sure does vary between individuals and with age) and observing experience (which does help of course).

Not sure about eating carrots to help you see in the dark (as my Mum used to say) is really going to help. I do like them though !

 

Edited Monday at 10:23 by John

[vlaiv]

I think that several things contribute to final result:

1. surface brightness of the target

2. surface brightness of the sky at the time of observation - or more specifically "contrast ratio" - or ratio of two surface brightness

3. sky transparency in direction of target (which alters point 1 and "modulates" point 2).

4. skill, dark adaptation and general eyesight performance of the observer

[josefk]

Agree with everything above especially about comfort and experience (sp. knowing what a really dim object can look like) but I would add an exclamation mark to dark adaptation for emphasis. In the darker nights of the lunar cycle i've started pottering about in an eyepatch over my observing eye when not looking through the actual scope and I now try to avoid exposing my observing eye even to a dim red light when making notes or a quick sketch. i thought i was dark adapting successfully before these steps, but now i'm doing this i am finding a definite reward for going the extra mile. Not necessarily seeing things i wouldn't otherwise see i don't think but seeing dim things better.

[Swoop1]

For me it is the fact that I am moving on from being an apprentice grumpy old git to being a fully fledged grumpy old git with the degeneration of my eyesight that that entails.

As a young man in the army or early in my police career, where many a happy hour was spent in darkness waiting for the enemy or the misguided miscreant, my eyes were able to easily detect much more than they can today. A3 would be a dead cert naked eye object for me back then.

[Ratlet]

35 minutes ago, josefk said:

Agree with everything above especially about comfort and experience (sp. knowing what a really dim object can look like) but I would add an exclamation mark to dark adaptation for emphasis. In the darker nights of the lunar cycle i've started pottering about in an eyepatch over my observing eye when not looking through the actual scope and I now try to avoid exposing my observing eye even to a dim red light when making notes or a quick sketch. i thought i was dark adapting successfully before these steps, but now i'm doing this i am finding a definite reward for going the extra mile. Not necessarily seeing things i wouldn't otherwise see i don't think but seeing dim things better.

That's a good shout!

I've got a hat I wear for observing that has ear flaps.  I don't use them when observing to cover my ears though, when I look through the eyepiece I use one of the flaps to cover the other eye.  Not closing your off eye makes a big difference imho.  Feels more comfortable and I think your brain works better for processing the data (sort of, this eye is open and should be seeing the same but it isn't, let's do something about it!).

Going to order me an eye patch yarrr.

[Rusted]

I used to go back indoors to complain that there were too many stars to recognize anything.
The Milky Way had a hard edge and spoilt the contrast with the rest of the sky!
That was on moving from very rural detached N.Wales to middle of nowhere Denmark.

Light domes are appearing even here now. Late incomers are terrified of the dark!
LED lighting is far too cheap to run! They even light rural cycle paths now!
That's probably the reason everybody is moving to the cities. Lots of light.

My eyes are very different in their sensitivity. Because I have never used my left eye.
My left eye is a complete beginner's eye. Even at 77. With no skill at looking though an eyepiece.
Everything is too bright and fuzzy! 🫣 

 

[skyhog]

On 21/10/2024 at 16:06, Rusted said:

 

My eyes are very different in their sensitivity. Because I have never used my left eye.
My left eye is a complete beginner's eye. Even at 77. With no skill at looking though an eyepiece.
Everything is too bright and fuzzy! 🫣 

 

I would imagine the idea of a trained eye is completely lost on those who are not involved in the hobby. My observing eye happens to be my left, if I observe Jupiter with my right eye I see almost nothing of the detail. As you say, a bright and fuzzy disk.  Maybe we should have switched eyes occasionally.....😉

[josefk]

...by pure coincidence after typing in this thread yesterday i had the opportunity last night to explain my "it's not a pirate patch really" approach to a policeman cruising his police car through the woodland parking spot i use as my local dark site. He had the decency to turn his lights off once i explained. LOL.

I think in this case the patch did assist in my capture of this mag10.29 more or less face on spiral galaxy NGC 772 in Aries. I calculate the surface brightness to be 21.7 MPSAS, 1.8 magnitudes difference to the 19.88 MPSAS I measured on the sky but it felt much harder than that. Nowhere near as obvious as this pic of the sketch would infer. Patch assisted (and protected) dark adaptation + experience netted me this one i think.

[Ratlet]

That's a really nice sketch.  Im always disappointed with my own but doing the faint stuff is incredibly hard.  It is so difficult to keep it 'dim' enough.  Did you sketch this at the eyepiece?

[josefk]

1 hour ago, Ratlet said:

That's a really nice sketch.  Im always disappointed with my own but doing the faint stuff is incredibly hard.  It is so difficult to keep it 'dim' enough.  Did you sketch this at the eyepiece?

its actually the "morning after" version of this grey on white one done at the eye piece @Ratlet - hardly worth the bother in either case but part of the record nonetheless. i use cotton buds or cotton wool balls to pull nearly all the white pastel back off the black card if they are really dim.

Recording the bland faint ones puts the showier stuff into context and this pic of the field grey on white version is actually a more accurate rendering of how much contrast there wasn't 😂. for this particularly bland one. Uranometria describes it as a "faint, small anonymous galaxy". Funny old hobby this isn't it...

BTW since my post above I have checked Uranometria for this galaxy. It gives a larger size than i used to calculate 21.7, and therefore a correspondingly lower surface brightness at 22.8MPSAS, much closer to a genuinely quite difficult 3 magnitudes difference against my 19.88MSPAS sky last night.

Edited Wednesday at 14:59 by josefk

[John]

11 minutes ago, josefk said:

its actually the "morning after" version of this grey on white one done at the eye piece @Ratlet - hardly worth the bother in either case but part of the record nonetheless. i use cotton buds or cotton wool balls to pull nearly all the white pastel back off the black card if they are really dim.

Recording the bland faint ones puts the showier stuff into context and this pic of the field grey on white version is actually a more accurate rendering of how much contrast there wasn't 😂. for this particularly bland one. Uranometria describes it as a "faint, small anonymous galaxy". Funny old hobby this isn't it...

BTW since my post above I have checked Uranometria for this galaxy. It gives a larger size than i used to calculate 21.7, and therefore a correspondingly lower surface brightness at 22.8MPSAS, much closer to a genuinely quite difficult 3 magnitudes difference against my 19.88MSPAS sky last night.

You have done a good and delicate job there.

I think I would find it very difficult to illustrate some of the fainter DSO's that I've seen. Some are almost more "sensed" than actually seen 🙄

[josefk]

2 minutes ago, John said:

Some are almost more "sensed" than actually seen

Thanks @John - its tricky to make a record that has something on the paper but without exaggerating it sometimes!

I think the point you make about sensing the thing more than, or before, actually "seeing" the thing is really valid on these threshold objects (or at least objects that are threshold for our particular combination of kit and sky at that moment). For myself it's about building the experience for when to trust that sense and spend more time on the observation to a point where i feel i can call it "seen" and when to distrust it due to some inconsistency (e.g. what you think you sense moves, or you sweep away and come back and you can't "sense" it again). 

 

[ollypenrice]

There is, I think, a staggering variety in visual acuity between individuals. There is also a variation in types of acuity, so detection of faint signal, detection of detail, acuity in low light and in bright, etc etc. Alas, I think I come bottom in most of them! A bad eye trained, however, may beat a good eye untrained - especially in unfamiliar tests.

Olly