Should we be using light meters?

[vlaiv]

You know - this thing:

often seen used by professional photographers on photo shoots.

"What is he on about now..." I hear you say

Well, not long ago, I tried to explain on my local forum that moon is not very bright in telescope and that there is no need for ND filters. In fact - moon is not brighter in telescope than naked eye (this is true for everything except point sources like stars), it can only be dimmer (surface brightness) as we bump up magnification and exit pupil goes down from 7mm to smaller size.

But why do people get "blinded" by low power views of the moon? For same reason people get blinded by car headlights at night. Go outside during the day and look at those same headlights - you'll feel no discomfort of any kind, no blinding at all. In fact - they will look rather dim in a sunny day.

Similarly, if we observe moon in daylight, close to sunset - there is no chance you'll feel any discomfort even at low power, yet moon is shining with same amount of light as always.

Then I ran across a post by @Louis D - showing difference between achromatic refractor and reflector on planets. I felt that what has been shown is somewhat unjust to achromatic refractor. There was too much purple halo around the Jupiter.

I knew that camera sensors are more sensitive in short wavelengths than human eye and this is one of the reasons. While visually we can hardly see purple halo around stars in fast ED doublets (unless they are very bright) - in images, those halos are clearly visible. This is of course due to different sensitivity. I just did not know what is the actual difference in sensitivity between the two, so I went online to look it up.

What I've found is very interesting:

Sensitivity to shorter wavelengths (below say 480nm or there about) dramatically changes with vision mode.

I mentioned above issue with lunar observing - because it also heavily depends on Photopic vs Scotopic vision. In Scotopic vision we are far more sensitive to light levels and are easily blinded because different receptor cells are active.

Not only that - but there is added sensitivity at lower wavelengths and also - there is issue of visual acuity - that is reduced in Scotopic vision.

I also found out that there is "middle ground" - Mesopic vision - where there is transition between Photopic and Scotopic - and this transition is gradual and effects above are "interpolated" between the two:

https://en.wikipedia.org/wiki/Mesopic_vision

Quote

Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions.^[1] Mesopic levels range approximately from 0.01 to 3.0 cd/m² in luminance. Most nighttime outdoor and street lighting conditions are in the mesopic range.^[2]

This means that we start to adapt to dark already at street lighting condition levels - and certainly get Mesopic vision in our back yard at night time.

It will impact things like:

- color perception

- sensitivity to light levels

- sharpness of our vision

- sensitivity to chromatic aberration

I'm saying all of this because conclusion is very obvious - if you want to get good planetary and lunar views - don't get into Mesopic and Scotopic vision regimes. Stay in Photopic.

Observe with bright light around you.

Wiki states that Photopic vision is above ~10 cm/m2, Mesopic vision is between 0.01 to 3 cd/m2 and Scotopic occurs below 10⁻⁶ cd/m²

I have no idea how to judge certain conditions to figure out cd/m2 - but apparently there is device that will do that for us - hence the title of this post.

If you have one of these - measure light levels on yourself and if EV number is 6 or greater - you are good to go.

 

[globular]

I've been 'breaking my dark adaptation' when viewing planets and the moon for a good while.
I find there are clear benefits of doing this on the moon and with no downside. 
With the planets it seems to me that some surface details are better without dark adaption and others are better with it.... so I try and do a bit of both.

Nice to hear there is some science behind it.

[Louis D]

The Orion Nebula is also bright enough to benefit from photopic vision.  I've seen a greenish hue to it after staring at brightly lit white paper and then quickly looking in the eyepiece.  It doesn't always work, but when it does, it's quite rewarding.

[JAC51]

This is very interesting. I have for a while now left the normal outside light on in my observatory for observing the moon to reduce the perceived brightness but never thought of it improving detail seen. I must try it on the planets now. I may take up the suggestion above off shining torch on a white piece of paper.

It feels a little awkward after persuading my kind neighbours to turn their outside lights off while I'm observing to then turn one on myself.

[Alan White]

Interesting posting Vlaiv, thank you.

I love Lunar observing and planetary when I can see them from home.

Lunar observing I often leave the observing area light on and the outside lights on as well,
I perceived this as having no down side, rather than an improvement.
Perhaps your explanation now clarifies the reason for this, so thank you.

 

[vlaiv]

23 minutes ago, JAC51 said:

This is very interesting. I have for a while now left the normal outside light on in my observatory for observing the moon to reduce the perceived brightness but never thought of it improving detail seen. I must try it on the planets now. I may take up the suggestion above off shining torch on a white piece of paper.

It feels a little awkward after persuading my kind neighbours to turn their outside lights off while I'm observing to then turn one on myself.

To be honest, I have no idea how much our acuity is reduced in mesopic / scotopic conditions, and since there is no real way of determining "percentage" of photopic / scotopic attributions to mesopic vision.

Even for scotopic vision - just vague accounts on lowering ones visual acuity are given - like this excerpt from wiki on scotopic vision:

Quote

Scotopic vision occurs at luminance levels of 10^−3[2] to 10^{−6[citation needed]} cd/m². Other species are not universally color blind in low-light conditions. The elephant hawk-moth (Deilephila elpenor) displays advanced color discrimination even in dim starlight.^[3]

Mesopic vision occurs in intermediate lighting conditions (luminance level 10⁻³ to 10^0.5 cd/m²)^{[citation needed]} and is effectively a combination of scotopic and photopic vision. This gives inaccurate visual acuity and color discrimination.

In normal light (luminance level 10 to 10⁸ cd/m²), the vision of cone cells dominates and is photopic vision. There is good visual acuity (VA) and color discrimination.

and on the end of the page:

Quote

For adaption to occur at very low levels, the human eye needs to have a large sample of light across the signal in order to get a reliable image. This leads to the human eye being unable to resolve high spatial frequencies in low light since the observer is spatially averaging the light signal.^[5]

and

Quote

Another reason that vision is poor under scotopic vision is that rods, which are the only cells active under scotopic vision, converge to a smaller number of neurons in the retina. This many-to-one ratio leads to poor spatial frequency sensitivity.^[5]

It looks like our brain and construction of our eye perform sort of "binning" of signal in order to boost sensitivity in low light conditions thus reducing how much we can resolve.

As for neighbors - here is a "trick" that you can perform.

Record sounds of someone working with tools - like hammer and saw and play that with the lights on, giving your neighbors explanation that since moon is very bright there is not much you can do except do some upkeep on observatory