at what size does

[gliderpilot]

At what size aperture do you start to see significant detail in galaxies and nebulae?

To put this question into context, I can easily find numerous DSO in my 4" refractor and 8.5" reflector. Apart from the orion nebula and the larger globulars, none of them show detail within their structure. Contrast this with using an excellent 16 inch dob at Kielder which readily showed the spiral arms and mottling in M51. Through this large aperture scope, M51 looked like a short duration photograph - a wonderful image that is still in my minds eye now.

Thanks,

Mark

[John]

I reckon this depends on the observing conditions and location just as much as the aperture of the scope. I've recently moved from an 8" dobsonian to a 12" dobsonian but I can't honestly say that I have been knocked over by the increased performance of the larger scope on DSO's from my usual observing site of my back garden which is moderately affected by light pollution of one sort or another. From reports I have read of how my larger scope performs under a truly dark sky I'm sure I would be much more impressed and I think details of DSO's would start to emerge much more readilly if I could get away to a dark site.

John

[Ben Ritchie]

I'd agree with this (in fact, i'd say conditions and location are probably considerably more important). You get people like O'Meara who can cover the whole Messier catalogue with a 3" refractor in superb conditions 10,000ft up Mauna Kea, while there are certainly Messiers that are so-far invisible to me with my 12" dob under moderate light-pollution.

[acey]

There are 3 crucial factors, in this order:

Sky quality (i.e. limiting magnitude).

Visual perception

Aperture

O'Meara observes from a site that reaches limiting magnitude 8. My site goes down to 6. A magnitude difference of 2 is worth the same as an aperture increase of 150% - to see from my site what O'Meara sees from his would require a 10-inch, where he needs only 4. My back-garden has limiting magnitude 4.5. Compared with viewing from a dark site, that's like cutting my aperture by half. So number one is sky darkness.

Then there's perception. When you look at O'Meara's drawings you see something he has built up over time, observing every little detail he can. The object didn't just look like that as soon as he put his eye to the eyepiece. It's almost like a long-exposure image. You have to actively look for details, like the way the brightness varies over the surface of the object, whether there is any mottling or speckling, etc. You can see spiral arms in M51 with an 8-inch from a mag-6 site, if you know what to look for.

Third thing, aperture. How much difference it makes depends on how much experience you've had with smaller aperture. If you use a 4" scope for a year and look at 10 objects, then move up to a 6", you probably won't notice any difference at all. Use it for 10 years and view several hundred objects, then move up, and you'll see a difference.

An aperture increase of 50% gives you roughly an extra magnitude of visibility. I recently moved from an 8-inch to a 12-inch and I have to say the difference just blows me away. But that's after several years of looking at mostly very small and faint galaxies with the 8-inch. When I now look at Messiers with the 12-inch it's astonishing. Mostly, though, I still look at faint stuff - the difference is that where before I was tracking down NGCs, now I'm seeing UGCs in Abell clusters. I love it!

Andrew

[GazOC]

I noticed a fairly significant improvement moving from 8" to 12", I think it gives you one magnitudes difference. IIRC to get another magnitude after 12" you need to go to a 20" scope. Thats the theory, as Acey says there are other factors to consider.

I have all of O'Mearas books and love them but you can't help feeling a little jealous as you read though them. I have neither the skies, skill or the time (he advocates spending hours over mulitple seesions on just one object, that equates to seeing about 4 objects a year in the UK!!) to get anywhere near his observations.

[gliderpilot]

Agreed, sky conditions and LP are crucial. I used to live in Farnborough under an orange sky and loved visiting my folks 15m outside Bristol under a stunning sky. Now I live near Salisbury Plain, I find that their sky is pretty mediocre with the glow from nearby Bristol!

O'Meara is certainly in a location that most of us would die for and that's before you started observing. Salisbury to Hawaii would get my vote!

[Doc]

I have yet to try my 16" under dark skies but under light polluted skies you still do not see alot of detail with a 16".

Granted they are esasier to find but basically they look like a grey smudge. I have seen detail in M51 but the skies were good and very transparant. So seeing is very important.

Also important is how much time you spend looking at an object, it takes time for those fleeting moments that bring details out.

What does inprove with aperture is seeing colour. M42 for example is almost all green with a tint of blue. planetary nebs are also blue or green.

[John]

....Also important is how much time you spend looking at an object, it takes time for those fleeting moments that bring details out....

Thats a very good point Mick

I think spending time on an object and observing it regularly does bring dividends. As you "get to know it" you do start to percieve details that were not obvious on first viewing.

I'm not dissapointed in my 12" dob - I realised when I upgraded that I was going to need to give it time and patience to get the best out of it.

John

[The Warthog]

The resolution of the scope is directly proportional to the diameter, so going from an 8" scope to a 12" scope gives you a 50% increase in resolution.This will help with those pesky tiny details.

[Ben Ritchie]

The resolution of the scope is directly proportional to the diameter, so going from an 8" scope to a 12" scope gives you a 50% increase in resolution

Good point - in fact i've just come in from looking at Saturn using the 12" f/5 (now well-collimated) and a 5mm XW under reasonable seeing, and the 12" is certainly strutting its stuff.

What does inprove with aperture is seeing colour. M42 for example is almost all green with a tint of blue. planetary nebs are also blue or green.

Another good point, I also like the way that red giants really pop out of clusters with larger apertures