Needing clarification!

[ashenlight]

Hello everyone,

As a keen reader of BBC's Sky at Night and Astronomy Now magazine, I often look out for sights that will be worth finding with my 5" reflector. Alot of the time, clusters, galaxies etc are said to be visible in a 'small' telescope, or a 'medium' telescope. Alot of the time, the magazine doesn't tend to give that much guidance as to what size scope fits into what category. I am under the impression that small is up to 4", medium 5" to 8" and large ones everything above? I'd appreciate it if someone would put me straight!

Clear skies and excellent seeing,

Amanda

[Doc]

I would have came up with the same impression Amanda

[RobH]

That sounds about right, but the worse your light pollution is, the smaller your scope is effectively.

When I had a 10 inch Meade and took it right out into the country to a pitch black site, I could see stuff that my 14 inch at home couldn't get anywhere near.

Cheers

Rob

[moocher]

I think and i could be wrong but when they say about small medium and large apertures they mean as used from a "dark sky location". Like Rob said LP in effect makes your scope "smaller". HTH

Alan:)

[barkis]

Hi. Amanda,

I took this table from Nortons Star Atlas.

It shows the Approximate Magnitude limit for different Apertures.

and the theoretical resolving of Double stars. But, we all know how other factors can play havoc with theoretical values.

Also, a nebula may have a magnitude of x, but that light is spread over it's surface, and the magnitude given, would be if it was a point source.

See how little gain in magnitude is achieved between a 6" and a 12".

Ron.

[Beyond_Vision]

Hi. Amanda,

I took this table from Nortons Star Atlas.

It shows the Approximate Magnitude limit for different Apertures.

and the theoretical resolving of Double stars. But, we all know how other factors can play havoc with theoretical values.

Also, a nebula may have a magnitude of x, but that light is spread over it's surface, and the magnitude given, would be if it was a point source.

See how little gain in magnitude is achieved between a 6" and a 12".

Ron.

The magnitude scale is non-linear so although the difference in numbers is small the brightness difference will be large.

Regards

Kevin

[ashenlight]

Thankyou everyone for your informative responses. Ron, that table made things clear to me, very helpful. I didn't know that light pollution effectively makes your scope 'smaller', that will be really helpful for me - that's one of the best things about this forum, you really do learn something new every day

Ron - "Also, a nebula may have a magnitude of x, but that light is spread over it's surface, and the magnitude given, would be if it was a point source." - really didn't know that, although it makes sense now!

Thankyou all again for your fast and helpful responses, really appreciate it.

Amanda

[Beyond_Vision]

On page 50 of the April mag is a key to the telescope sizes.

Regards

Kevin

[barkis]

The magnitude scale is non-linear so although the difference in numbers is small the brightness difference will be large.

Regards

Kevin

Very true Kevin.

Ron.

[scotastro]

Ron - "Also, a nebula may have a magnitude of x, but that light is spread over it's surface, and the magnitude given, would be if it was a point source." - really didn't know that, although it makes sense now!

You can turn this the other way around. Find a faint star which you can easily identify in Stellarium (or similar) and then defocus the scope. It will give a good indication of how a galaxy of that brightness would look in your sky.

I have used a Explorer 130P in my dark skies and can easily see objects such as M97 and M108 (both mag +9.9) using the supplied 25mm EP.

S&N have said on several occasions that a medium telescope is needed to see Saturn's rings when they are edge. Not so I could see them even in a Bresser 70mm refractor (the Lidl one) - something I would consider as a small scope.

Mike

[barkis]

Small scopes can be plagued by crappy mounts. The can become decent little instruments on a substantial one.

Ron.

[Stars in Their Eyes]

Great question Ashenlight and one that I have been pondering about recently after reading the same magazines!

[ashenlight]

Thanks for the responses everyone. Mike - it is always great to what fellow SW 130P users can see through their scope

Stars in Their Eyes - I thought someone else must be makes things alot clearer for me now I know.

Amanda

[Stars in Their Eyes]

Ashenlight I've noticed you've got the same scope as me, the Skywatcher 130PM. Got mine a couple of months ago. Great scope! Haven't had enough clear nights to get alot of use though

And I still haven't mastered the setting circles yet!

[ashenlight]

I got mine last week, been out for a couple of hours at a time, looking at the moon and saturn mainly. Managed to get it on Pollux and Arcturus last time too.... but yes, i do often stare blankly at the setting circles! I don't even know if i'm moving it the right way, but hey, it is moving.

Have you seen M42 through the scope yet? I was hoping to get a look tonight seen as Orion is barely hanging above the horizon, but some horrible orangey clouds rolled in so I packed up.

Amanda

[Stars in Their Eyes]

Hi Amanda,

Yes I was looking at M42 the other night but like you, the clouds rolled in! I was out last night looking at the moon and Saturn, both looked fantastic. I was also trying to look at the galaxies near to the Plough (as shown in this months Astronomy Now) however I couldn't find them. Not sure if the scope is too small for those or whether it was because the moon was shining so brightly, but I won't give up looking for them just yet!!!

[acey]

As others have pointed out, visibility of objects depends on aperture and sky darkness (i.e. light pollution). And because the magnitude scale is nonlinear, people often tend not to realise just how important the latter really is.

A dark sky in Britain has faintest naked eye stars of around magnitude 6. A typical suburban back garden will have limiting magnitude about 4.5. That difference of 1.5 magnitude is equivalent to halving your scope's aperture. So if an object is described as being "visible in a 100mm scope", and you've got 1.5 mags worth of light pollution, then you'll need at least a 200mm scope to see it.

Looking at it more positively, if you observe from a light-polluted site, you can effectively increase your aperture by 100% or more, just by taking your scope somewhere really dark. Then the objects supposedly "visible in a medium scope" really will be.

[Edstronomy]

I think the easiest way around the problem of LP is to just buy a huge light bucket Dobsonian or a set of filters.

[acey]

I disagree. If you could really beat light pollution with aperture then you could make a scope big enough to see stars in daytime.

In a light polluted sky you lose sky contrast, you get increased light scattering within the optics, and your eye is never able to dilate adequately. I've tried doing astronomy both in my light polluted back garden and at dark sites and as far as I'm concerned, from the garden it just ain't worth it. Just MHO but there you go. If I can't see the Milky Way with my naked eye then I'm not going to bother looking for DSO's with anything (not even my 12"), because the view is so badly compromised. If I could only do astronomy from my garden then I'd get a top-notch 4" refractor and look at planets and double stars.

But if you want to see lots of DOS's in glorious detail the answer is indeed to get a big dob - and take it to a dark sky.

Andrew

[Edstronomy]

Yeah i was joking really and of course it would be a great thing indeed to have a telescope that viewed the stars in the day! Be more convenient as well

You never know technology may one day deliver such a thing! or not