Sky 'darkness'

[patomlin76]

Managed to get out tonight for 20 minutes viewing between wispy cloud belts, but was wondering why some skies appear darker on certain nights. Would the wispy cloud tonight reflect a bit more light pollution back up? I had magnificent views of Orion and nebula, but the sky just appeared it as black as it can be - m82/81 in particular were disappointing, no better than my 5" scope but I felt as if I were viewing through a very faint cloud though it's hard to tell at night...

one more question, to view m82 m81 and other distant galaxies am I best looking through my 25mm or 30mm 80 degree eyepiece? The 9mm was useless on all but Orion as that's so bright.

Thanks!

[Charic]

.....dark  skies and low power is best for your DSO's.

M31 from my observatory is a miserable grey, barely visible  patch that most folk see the first time they come across M31 ( unless their skies are perfect )  however from a darker site ( no man made  light pollution) with good seeing, M31 comes alive, and its HUGE! so much so that I can't fit in what I wanted to see in my eyepieces, so I bought  the 25mm 60° Starguider to replace my Sky-watcher, and still not enough field of view, so next I purchased the 32mm 70°, now its visible, but alas, maybe a little more would be achievable, however it looks like I will maintain my Panaview EP.

If you use high power, the image will  tend be smaller, and darker, possibly missing some easy DSO hits.

[Paul73]

Glad that you got some good viewing.

Light needs something to reflect off. If there are particles in the air, be it dust or water the light from ground level and moon will make the sky appear a little milky. Still lots to see though.

Paul

[jetstream]

The low cloud will definitely reflect LP and make things worse and any cloud kills transparency. Taking NELM of your area a few times will help you figure out your skies.

If I had an f6 scope and a 25mm and 2x barlow I wouldn't be shy about trying it on M81/M82...

[patomlin76]

Ok I thought the light clouds may have had something to do with the light, just need to be patient! It also doesn't help that at this time of the year m82 and the plough are sitting right above Yorks light pollution in the east. When it's more northerly (or nearer the zenith) I think I get better views...

[John]

55 minutes ago, patomlin76 said:

Ok I thought the light clouds may have had something to do with the light, just need to be patient! It also doesn't help that at this time of the year m82 and the plough are sitting right above Yorks light pollution in the east. When it's more northerly (or nearer the zenith) I think I get better views...

DSO's almost always look better when they are higher in the sky. Sometimes the improvement is dramatic. In fact this goes for all objects really.

 

[Mr Spock]

I notice here on some clear nights the sky isn't dark. I put it down to slight haze picking up light pollution.

M31 is to big and diffuse to fit in larger scopes. I find the smaller galaxies look best at around x90 or so. There's pleanty of space for them in the eyepiece and the magnification darkens the background giving more contrast.

[patomlin76]

Must be slight haze as you say, it doesn't actually affect viewing of stars and planets through the telescope, in fact it appears invisible through the eyepiece, but it definitely shows an effect on DSOs!

Thanks for the help.

[swamp thing]

20 hours ago, patomlin76 said:

 

one more question, to view m82 m81 and other distant galaxies am I best looking through my 25mm or 30mm 80 degree eyepiece? The 9mm was useless on all but Orion as that's so bright.

Thanks!

Depends on the galaxy. 
We are constantly told "Use low power for DSO's". I'm going to go against this and say "Ignore that advice" most Galaxies are small and faint. A medium power works best. We have to make them big enough that we stand a chance of detecting them. You won't do this at low power in a mid sized scope. There is no point in trying to keep the surface brightness as high as you can if the object is to darn small to see. Don't be scared to push a bit of power. Remember our eyes see big bright things best. We cant have bright so big will have to do  
For a medium sized scope the image scale offered at its low minimum power is not enough for most galaxies. As Gerry say's above, your 25mm + 2x Barlow will get you there.  
 

[ollypenrice]

I live at a dark site. When the sky is dry the distant LP stays very low to the horizon. The more moisture there is, the higher it rises.

When the sky is at its very best, dry and clear, you can move around outside quite easily when adapted. Starlight is light. When its foggy and dark, though, you can see absolutely nothing at all!

Olly

[Size9Hex]

17 hours ago, swamp thing said:

Depends on the galaxy. 
We are constantly told "Use low power for DSO's". I'm going to go against this and say "Ignore that advice" most Galaxies are small and faint. A medium power works best. We have to make them big enough that we stand a chance of detecting them. You won't do this at low power in a mid sized scope. There is no point in trying to keep the surface brightness as high as you can if the object is to darn small to see. Don't be scared to push a bit of power. Remember our eyes see big bright things best. We cant have bright so big will have to do  
For a medium sized scope the image scale offered at its low minimum power is not enough for most galaxies. As Gerry say's above, your 25mm + 2x Barlow will get you there.  
 

I'd second this. Always worth listening to the convential advice, but never assume it's correct :-)

You've got a barlow and a 9mm to give some higher power options that are worth experimenting with. You might be surprised with better views, and at worst you'll learn why the conventional wisdom is right! My experience is that low power can leave the sky too bright, and the DSO seems to get lost. Medium power darkens the sky and improves things for me. On the smaller DSOs, I find that the high power can bring out some great details (dust lanes, notches, filaments, spirals, although sometimes at the expense of the overall big picture) and I get the impression I use high power on DSOs more frequently than other folks might. Mileage varies from night to night, from target to target and no doubt from person to person. Experimenting with different approaches to see what works for you is key I think though :-)

I'd also say a dark sky beats any combination of kit. My binoculars pick up more detail in some targets from a slightly darker site than my 10 inch scope manages from my slightly less dark back yard. I was more than stunned when I realised this...

[Linda]

When I look at deep sky objects, I just use the 24mm first to find them, or at least to locate the stars in the vicinity. Then I try out a medium eyepiece (15mm) to see if it can get better. Sometimes I try also a high power eyepiece (10mm or 8mm). When circumstances are not good, the higher power doesn't give satisfaction and I stay with the 15 or 24mm.

[DaveS]

I know that where I live local weather conditions can make a *huge* difference to the sky "darkness", any clag in the air can drop the NELM from ~4.25 (This is London after all) to no better than 2.5, or even lower. Especially in "summer" with winds from the continent the first clear night after rain / wind can be good, with NELM > 4, but will then slowly deteriorate until the whole sky is sickly orange with nothing visible below 2.0.

[Linda]

The funny thing is that if you are in a truly dark place where you obviously see the milkyway, then that milkyway can give the impression that it is quite lite, especially when there is snow on the ground. But that is your eye tricking you, because you can notice the enormous amount of stars.

[Piero]

My regular telescope (TV60) is not a galaxy hunter, and probably an infant compared to the big boys previous members owns. 

From my location, I can see M81 and m82 with my 13mm (2.2mm exit pupil) quite well. Of course they do not appear like in astro pictures, but you can clearly see that M82 is elongated and M81 is an oval. Regarding M32 and M110, increasing the magnification to a 7mm (1.2mm exit pupil - which is certainly not a low power!) makes them well distinct. If you go to a darker place, M32/M110 are even well distinguishable at 15x.

So, the old saying is to start off at low power, then increase until things improve. Once you found a magnification which does not work well, you go back to your previous magnification and that's your optimum for that target and for that night !

 

Of course proper eye dark adaptation, patience and dedication are also necessary ingredients for the cake!

Good luck!  

[Special K]

Too true, start low and build up to find what works for you. Brighter galaxies can stand up to some magnification and I think it's easier to work those theories out in the field.  The variables of atmosphere, surface brightness, light pollution, etc are so great that each observation is a unique case worth experimentation.

Those thin clouds are a mystery to me as well!  I've often wondered about this when it seems clear but results are less than expected. It's pretty obvious when you do a session before dawn and later see what is up there as it gets light.  Can be a surprise!

[timwetherell]

On 2/4/2016 at 17:05, Linda said:

The funny thing is that if you are in a truly dark place where you obviously see the milkyway, then that milkyway can give the impression that it is quite lite, especially when there is snow on the ground. But that is your eye tricking you, because you can notice the enormous amount of stars.

Yes that's exactly right! The first time I visited the Anglo Australian Observatory (1000 miles out in Woop Woop) It almost looked cloudy but with stars. The cloud of course being the Milky way. It's way wider at a true dark site than one sees looking at it from a darkish site. And looks like grey mass of boiling cloud. I never really understood why the coal sack was called the coal sack until I saw it against the milky way under that sky! The funny think is that it's hard to recognise the constellations because it's a ocean of stars. 

[Paul73]

Tim 

What a splendid description of a truely dark site. Concise. No flowery language needed.

I need to go walkabout (with a scope).

Paul