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Alexandros

Can you see DSOs in colour just with your eyes?

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We have all seen in the internet and many of the people in the forum have taken themselves, beautiful cosmic photos of many deep sky objects, like the deep red lagoon nebula or the colourful eagle nebula. However, from what I have learned here, when we are observing with a telescope and are fully dark adapted, the objects are so faint that our eyes are not great dark colour cameras and are unable to use cones and therefore use rods to sense the light coming from them. So my question is, is it possible to see deep sky objects in colour, especially nebulas, like they appear in photos? I'm refering to a scenario where you are at a very dark site, with very little atmospheric disturbance and with a very good telescope. If such a scenario exists, what are the requirements?

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as a rule no but sometimes you can see pale shades like you get with the great Orion nebula, if you want stunning colour you have to get into imaging and staxing but that's a dark art 😀. charl.

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It's very difficult for reddish nebulas because the eye loses a lot of red sensitivity in the dark but bluish and greenish nebulas show some color under one or more of these conditions:

- sky is quite dark and transparent

- scope is real big

- scope has very high contrast

- scope operates at low power to concentrate the light on a smaller part of the eye, making the target seem more intense

- target is bright

- target is small and thus has higher brightness per unit of sky area (planetary nebulas are king in this)

- you're looking through an O-III filter which makes the background so black the deep-sky object's color is not as difficult to discern because of stronger contrast

That's for nebulas which have a large range of colors but galaxies and globular clusters have a smaller range of colors so the factors are mostly aperture and object brightness.

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I've seen a hint, perhaps, of blue in the snowball nebula, possibly a whisper of green on M27 but never anything else.

I've always wondered why Orion never shows colour considering how vividly it jumps out on photo's. But my skies are terrible - I've only just seen M51 for the first time and that's after I made the jump from 5 to 12 inch. And its almost directly overheard, and it still has no real shape even with AV.

If I need my colour fix I just go to Alberio!

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Our eyes cannot see colours at low light level.

On a rural road without street lighting (there are a few left) there are no colours, but turn on a car headlamp and the colours show.

A simple test is to look at the plain boring colours in a dark garden, then take a photo with the camera shutter open for a few seconds. The grass is once again green.

Or fix your camera, with a wide lens, to a tripod pointing to the sky. Leave the shutter open for 30 seconds and you get see some different colour stars.

Hope this helps, David.

 

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1 hour ago, Ben the Ignorant said:

you're looking through an O-III filter which makes the background so black the deep-sky object's color is not as difficult to discern because of stronger contrast

Given that an OIII filter only lets through a few nm of green light, anything viewed through an OIII filter will be green, no matter the spectrum emitted by the object. 

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The blue snowball looks blue to me. That's about it.

The colours of deep space objects are real, but very subtle.

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David has it as far as my experience goes. Quite ironic really, if you are at a really dark site and fully dark adapted, you are less likely to see colours in smallish scopes. From my fairly light polluted back garden, I often get quite strong hints of green in M42 because my cones are still stimulated by the ambient light.

In larger scopes, you might get this colour under dark skies, and in much larger scopes you will likely see hints of pinks and reds aswell as the greens in M42.

In the brighter, smaller planetary nebulae I have seen colour such as green in the Blinking Planetary and blue in the Blue Snowball.

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On a good night with a 16” scope, M42 is washed out green and purplish. The Ring nebula is definitely green to me, but no fireworks.

Paul

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1 hour ago, Ricochet said:

Given that an OIII filter only lets through a few nm of green light, anything viewed through an OIII filter will be green, no matter the spectrum emitted by the object. 

Alexandros doesn't ask what color things look, he asks how visible that color is; so the blacker background makes the color more likely to be seen if it's still intense enough after the filtering, if the eye is dark-adapted enough, if, if, if, and so on. 

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This thread has me thinking - I haven't used my 20" dob in anger yet, bought it in April. Does anyone know what to expect colour-wise  with a  scope  of  this size under dark Bortle 2 skies? I have an astronomik OIII filter as well, but no UHC or H-B yet... 

I can't wait to use it properly at my dark sky site with the synscan and GOTO set up, especially with the nice eyepieces I've miraculously managed to pick up this summer

For the first time in my life I'm waiting for summer to end! First true darkness here in NE Scotland begins third week in August, it's in my phone calendar already!

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4 minutes ago, Ships and Stars said:

This thread has me thinking - I haven't used my 20" dob in anger yet, bought it in April. Does anyone know what to expect colour-wise  with a  scope  of  this size under dark Bortle 2 skies? I have an astronomik OIII filter as well, but no UHC or H-B yet... 

I can't wait to use it properly at my dark sky site with the synscan and GOTO set up, especially with the nice eyepieces I've miraculously managed to pick up this summer

For the first time in my life I'm waiting for summer to end! First true darkness here in NE Scotland begins third week in August, it's in my phone calendar already!

@jetstream should be able to help!

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Not really, no. 

But it's not just black and white. Well yes it is, but hear me out, whenever I look at a nebula they don't just look black and white to me. When I actually try to see some colour I realise they are just that, black and white, but for some reason they seem as if they have the faintest colour. 

Maybe it's just my head colouring them in 😂

However on the Orion Nebula I have noticed the faintest colour during very good conditions from my garden here in the south west! 

Clear Skies!

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2 hours ago, Ships and Stars said:

Does anyone know what to expect colour-wise  with a  scope  of  this size under dark Bortle 2 skies?

Green is the predominant color I see, but in M42 the very rich mottled green is accompanied by a pinkish red color under the wings. Friends say the 24" under VG skies will begin to show blue as well. Planetary nebula are amazing including the Catseye NGC 6543 and the Magic Carpet NGC 7027. The Eskimo nebula NGC 2392 is superb and your scope will show a 2 toned green (stunning) and very fine radial filaments. Of course M57 is just so bright.....

You might be amazed at the views with no filter...

If you get well over 21mag skies objects take on a different nature and every bit over 21 matters...really matters, along with transparency.

 

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When I've been doing outreach sessions with my 12 inch dobsonian I've noticed that younger eyes seem generally more sensative to seeing colour in astro targets than mine are. Some older observers struggle to see even the colour different in the binary Albireo for example wheras the youngsters shout out the colour difference between the stars often without prompting.

 

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7 hours ago, Carbon Brush said:

Our eyes cannot see colours at low light level.

On a rural road without street lighting (there are a few left) there are no colours, but turn on a car headlamp and the colours show.

A simple test is to look at the plain boring colours in a dark garden, then take a photo with the camera shutter open for a few seconds. The grass is once again green.

Or fix your camera, with a wide lens, to a tripod pointing to the sky. Leave the shutter open for 30 seconds and you get see some different colour stars.

Hope this helps, David.

 

David makes a valid point here.  There is always human variability but the transition from night vision (using the rods in the retina which only see contrast) to the start of colour vision (using the cones which respond to colour) occurs somewhere between starlight and moonlight.  DSOs tend towards the dimmer end of starlight.

A good general article is here:  http://www.yorku.ca/eye/sensit.htm

A much more detailed article on the human eye and optics with some fascinating insights (pun unintended) is here:  https://www.olympus-lifescience.com/en/microscope-resource/primer/lightandcolor/humanvisionintro/

John

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A good test for the eyes on the brighter larger DSO is seeing if you lose dark adaptation- I do on M57 and M42. Also eye illumination is important ie I see pink in M42 with the 200mm f3.8 with 20mm Lunt even at a low X38 mag...

We can definitely see color in a few DSO, espc green.

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2 hours ago, jetstream said:

If you get well over 21mag skies objects take on a different nature and every bit over 21 matters...really matters, along with transparency.

Thanks to Alexandros for asking in the first place, and thanks Gerry for the information. According to the ol' light pollution map, 21.93-21.95 for my dark sky spot away from home, so I should be in business on a good night. I've had some really clear nights there camping. If a cottage nearby ever comes up for sale...I will be extremely jealous of whomever buys it! 😆

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I haven't read all the replies yet, but I can say that I am almost 100% sure i could see blueish/green colouring to the Cat's Eye Nebula in Draco last night...Every other DSO target was black and white and this was my last one for the night... so i was pleasantly surprised to get a colour from it. 

Unless I imagined it of course. But i don't think i did, it was pretty obvious. 

Cheers

 

Edit: Skimming the replies above, looks like I could have imagined it...bummer. 😕

Edited by MKHACHFE
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I'm going for the Blue Racquet ball NGC 6572 tonight, conditions look vg. I'll try the 12.5mm and zoom.

Edited by jetstream
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If you are looking for colour, try double stars.  
Their temperatures are generally indicated by their colour. Red are cooler, and example being
the giant star Betelgeuse in Orion. another in the same constellation is Rigel which is blue in colour,
BLue indicating very Hot stars. when you see doubles, either binaries, or optical, their colours 
are very beautiful when seen in a telescope. Good examples Albireo the Swan's head in The constellation Cygnus.
Gamma Andromeda is another lovely pairing, but there are more.
Ron. 

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With my 10" Dob., I have found that Uranus and Neptune are a good blue/green contrast to adjacent "white" stars.

Geoff

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