Aperture to resolve 47Tuc or Omega C

yagon · #1 08-01-2007, 09:29 PM

I've only ever seen these globulars in an 80mm refractor, and they look like a ball of cotton wool.

How much aperture is required in a scope to make a prominent globular like 47 tuc or omega c look like a cluster of stars rather than a ball of cotton wool?

acropolite (Phil) · #2 08-01-2007, 10:28 PM

I haven't really looked that hard with my ED80 but my 200mm LX resolves the stars quite nicely in both globs at around 70-140x magnification. As with all DSO's, the more aperture the more detail can be seen.

matt · #3 08-01-2007, 11:16 PM

I agree with Phil.

My old 8" (200mm) Skywatcher (f5) used to resolve both these globs very nicely.

Fox · #4 09-01-2007, 12:01 AM

Hi Yagon
what magnification are you using to try and resolve globulars with your 80mm? In my TV Genesis (100mm), my 27mm Panoptic (giving 18x) shows globulars like blobs - ie. no resolution. But with a 6mm Radian (80x) upwards, resolution is no problem with the majority of globulars. I would hazard to guess that it would be a similar story with a good 80mm refractor?
Cheers, Fox.

yagon · #5 09-01-2007, 07:18 AM

I was using an ED80 with mags up to 150x in Sydney. Some of the stars in the outer areas of the glob could be resolved, but the main body of it was just a ball.

I'd like to buy a scope that, among other things, will show some more detail in the globular. I'm thinking about a 6" mak-cass.

rmcpb (Rob) · #6 09-01-2007, 10:58 AM

If you want to resolve stars in the centre of these clusters then you are going to need some more serious aperture. My 8" starts to resolve them but a 10" is noticably better and a 12" is really getting there with stunning views.

I don't think a 6" will do the job you are expecting of it in this case.

Cheers

iceman (Mike) · #7 09-01-2007, 11:51 AM

Also, don't forget the seeing plays a considerable role in being able to resolve the core of those 2 great globs.

When the seeing is not good, it doesn't matter how much aperture you have, all the stars star to blur together and do not resolve well.

When the seeing is very good, you can push the power up to 250x or more and start to resolve the inner core starts.

It's also much more difficult with 47Tuc due to the more condensed core than omega centauri.

Kal (Andrew) · #8 09-01-2007, 12:01 PM

I must admit the nicest view I've ever had of a globular was through an AP130EDFS - would have been hundreds of tiny speckles of light!

Omaroo (Chris Malikoff) · #9 09-01-2007, 12:46 PM

Quote:

Originally Posted by matt

I agree with Phil.

My old 8" (200mm) Skywatcher (f5) used to resolve both these globs very nicely.

And it still does Matt.

Fox · #10 09-01-2007, 03:35 PM

Quote:

Originally Posted by iceman

When the seeing is not good, it doesn't matter how much aperture you have, all the stars star to blur together and do not resolve well.

When the seeing is very good, you can push the power up to 250x or more and start to resolve the inner core starts.

Agreed, this is what I thought as well. My Genesis (100mm) is much less than 150 to 200mm, but it has no problem with globular resolution. It's not as bright as those bigger scopes, but then again contrast in the Genesis is superb. I don't think you necesarily need 150 to 200mm per se, I reckon you may have been limited by seeing with your 80mm, as IceMan says.

But its an interesting question, really what is the 'minimum' needed to resolve a typical globular. Cheers, Fox...

PS: Here's an interesting link to observing globulars. It mentions that 70mm can 'resolve' some globulars, a lot has to do with faintness of the cluster, so more aperture will help because of greater light gathering and not so much because the stars are 'close' together. Obviously with better seeing and dark skies, your 80mm will perform better.

http://www.ngcic.org/papers/gctext.htm

dhumpie · #11 09-01-2007, 10:27 PM

I agree. But for comfortable resolution you need a minimum of 6" of aperture. My Celestron/Vixen C6 resolves 47 Tuc and Omega to the core with anything above 150x magnification. I once had a 102mm refractor and while it did resolve some of the outer stars, I found I prefered the views in my 6".

Cheers,
Darren

JoeBlow · #12 13-01-2007, 02:32 PM

My 4.5" reflector was able to resolve the outer stars in both 47 Tuc and Omega C as long as I used enough magnification (75x), and with good seeing and transparency.

However your 80mm refractor should be able to easily resolve these globulars from a very dark site.

CoombellKid · #13 13-01-2007, 04:28 PM

It can be an ep thing to along with conditions. When I used to look
at NGC 104 & NGC 5139 with my 8" using the ep's that came with the
scope (cheap&nasty chung-wah ep's) they pretty much looked like
haze, with a scattering of stars on a good night. It wasn't until I ungrade
to U/O HD's that they started to resolve nicely. If you want a good
glob ep try a 10mm XW : )

regards,CS sunny days

AstroJunk (Jonathan) · #14 13-01-2007, 09:57 PM

Quote:

Originally Posted by iceman

Also, don't forget the seeing plays a considerable role in being able to resolve the core of those 2 great globs.

When the seeing is not good, it doesn't matter how much aperture you have, all the stars star to blur together and do not resolve well.

I don't really find this to be the case. Seeing destroys the detail on planets and nebulae for sure, but even in the very worst conditions I have observed in I can easily resolve these objects to the core with the 20". For sure there is plenty of movement and I can't see the tendrils of stars so clearly, but resolve them I can.

I suspect this is due to the sheer resolving power of a big scope - low power is 80 to 200X so can still do the trick even in bad conditions! I love my dob!!!!

Rodstar (Rod) · #15 14-01-2007, 08:19 AM

I have been doing quite a bit of visual observing with my ED80 in the last month or so. Twice in the last month I have tried to resolve 47 Tuc, without success. On one of those occasions it was a night of excellent seeing. I was not pushing magnification past 90x, though. The best I could do was start to resolve the outer edges.

I suspect that you will have great difficulty ever satisfactorily or consistently resolving 47 Tuc or Omega Centauri with 80mm of aperture.

A 6" dob would be a great minimal investment to be able resolve these southern beauties to pinpoint.

Starkler (Geoff) · #16 14-01-2007, 12:16 PM

Its looking like the answer is 6 inches.

With my 5" newt I see granularity in the core of 47Tuc, but not quite resolved to individual stars.

JoeBlow · #17 14-01-2007, 01:24 PM

Omega Centauri and 47 Tuc are the skies brightest and most spectacular globular clusters, however it does not necessarily mean they are the easiest to resolve out of all globulars. From memory, I think I found it easier to resolve stars in M4 and NGC 6397 in Ara with my 4.5" reflector.

PaulH · #18 14-01-2007, 02:02 PM

I'm just a beginner but I read up on buying scopes and went for the biggest aperture I could get. I mainly wanted to see what's up there in the greatest detail. I eventually purchased a 12" Dob (Meade Lightbridge).
My first big outing with it was at the Queensland Astrofest under dark skies. I was just amazed at what's up there. My most vivid recollection is using 200x in a wideangle eypiece and 2x barlow to view Omega C. It was just fantastic. Even in the centre I could resolve some the stars.
I'd recommend a 12" Dob - preferably with trusses so you can transport easily.