Help regarding bigger scope please

[webboid]

Hello fellow astronomers.

I have been observing for a year now & have the itch for more aperture.

I mostly observe from my back garden. The sky suffers some sky glow but no direct street lights. Naked eye mag 4.5 sometimes a little better. With my 200P I have seen many DSOs. Probably around 100 NGC objects. My observational skills have improved quite significantly over the year. It is difficult to get to a dark site regularly so most of my 70 odd sessions so far have been at home. So my question is would I get a noticable improvement in what I can see with a 12 or 14 inch dob. Some sources say you amplify the sky brightness with more aperture so your gain is slight. Others say aperture always is better. Getting a bigger scope is a significant outlay for me so I do not want to rush in & find the difference is small. My thinking is that what I have seen with my 200P from a dark sky might be viewable from my back garden with a larger aperture.

Also which scopes ought I consider?

Hoping to hear from those of you who have first hand experience of urban stargazing with larger apertures.

Thanks

[nytecam]

A larger scope should help you to see 'deeper' and raising the EP power usually helps too as do LP filters. I confess I don't eyeball any more with my LP NELM ~3.0 as my simple imaging is virtually uneffected by LP and goes "desert deep" - it's satisfyingly weird

[swamp thing]

Hi

To understand how aperture actually helps us is probably the first thing to help you make a decision.

Brightness of image is dictated by focal ratio. A 4"f/4 with a 20mm eyepiece in the focuser will provide an image of the exact same brightness as a 20" f/4 with a 20mm eyepiece. The difference will be the image scale the 4" scope will be at 20x whilst the 20" at 100x, so the larger scopes image of objects will be much bigger for the same brightness. Larger objects are more easily seen by the eye and more detail can be seen.

When I had a 16" f/4.5 at the same time as my 10" f/4.5 it was like the 10" view was always just lagging behind the 16". The 16" would produce a much larger image with each subsequent eyepiece . To try and catch up, I would pump the mag up on the 10" only to find a view the same size but dimmer.

This is why large objects like the NA nebula are best seen in small rich field scopes. Their already big so making them bigger doesn't help. A big 20" scope will not improve the view over what's seen through bins or small RFT on objects like this.

How does this translate to LP skies though? Well if the object you are looking for is hidden in sky glow making it bigger wont drag it out. As we up the mag darkening the sky background unfortunately we darken our target too. This is also why dark skies count more than aperture. For aperture to really work in our favour the sky must be good and dark. Visual observing is all about contrast. In order to see fainter stuff the sky must be darker than our target.

Anything bright enough to be seen through the sky glow will be helped by aperture. Just don't expect to suddenly see a load of faint stuff. If, from your observing site, M101 can't be seen in your 8", a 12" or even a 20" won't see it either. For faint stuff we need dark skies.

[nytecam]

Thanks Steve for this excellent clarification and why I'm stuffed [visually] in suburbia. Thank goodness I've something to fall back on

[webboid]

Thanks for the explanation Steve.

Over head & down to about 30 degrees above the horizon is where I usually observe & the sky is pretty dark there. Seen NGC 891 for example. So would more aperture reveal more?

Any urban observers with larger telescopes than mine comment on their experiences please.

[NGC 1502]

Hi Paul, post #3 hits the nail very firmly on the head, worth reading a couple of times.

I'd like to add this - portability is relevant too. Now what's portable and convenient to one person, won't be to another. Physical strength, how keen you are should be considered.

My own situation is that my 10" Dob is stored in an upstairs room. That means three trips up & down the stairs for the mount and then the tube, then the EP box etc.

If you can store the scope in an outside shed or garage close to where it will be used, that makes a huge difference, but that's not an option for me.

Cooldown is a consideration too.

Not trying to put you off getting a larger scope, just trying to help with what needs to be thought about.

A larger scope will give better views wherever you are. Globular clusters are a fabulous ball of stars, galaxies invisible in a smaller scope can be seen, and if you are under a decent sky, galaxy structure can be seen in some.

HTH, Ed.