12 v 8

[PSALM19.1 (Shaun)]

Hi all,
Wondering, what difference in "visual" terms is there between an 8" dob and a 12"? I realise that the 12 would give a more detailed view, but just HOW much more? Is it a no contest or is the difference not worth the price difference?

[tonybarry (Tony)]

Hi Shaun,

What are you interested in ?

If you want wide field, the 8" will have a better field of view.

If you want something you can move, the 8" will weigh less and fit in the car much more easily.

If you want galaxies and faint stuff, the 12" will deliver more galaxies.

If you want planets, the 12" will have a better resolution and will handle higher magnification.

Now... how much better ?

One stellar magnitude in brightness is about 2.5 times more light. So an 8" dob will have about 50 square inches of surface area (ignoring central obstruction).
A 12" Dob will have about 113 square inches of surface area. That's about 2.26 times light collecting area - which translates to about 1 magnitude.

So you should be able to see stuff that is 1 magnitude dimmer in the 12" than what you can see in the 8".

If that sounds like a minimal amount of improvement for a fair bit of cost ... well it is not a huge increase. But it might be the difference between seeing something and not seeing it.

In terms of resolution, well the 12" will resolve smaller features, if the seeing permits. Most of the time the seeing will be your limit, not the scope's optics. So that may not translate to a great improvement except on those odd nights when things are All Good In The Heavens.

My feeling is that the 12" dobs are the sweet spot in terms of price to aperture, and weight to aperture, right now.

But the improvement is what I would call incremental rather than revolutionary.

Hope this helps.
Regards,
Tony Barry
WSAAG

[OzStarGazer]

I wish I knew from experience...
But all I have for "comparisons" is this website I once bookmarked:
http://telescope-simulator.com/telescope-calculator/
http://telescope-simulator.com/home/...pe-comparison/
It doesn't show DSOs for comparison though (apart from the Pleiades), which is what you are probably after.

[N1 (Mirko)]

Hi Shaun,

This subject can be as easy or complicated as you like it to be. I asked myself the same thing some time ago and read a lot here and elsewhere about how many times more light a bigger scope would gather (a misleading approach because 4 times more light - just as an example- isn't 4 times brighter), to buy the biggest aperture you could afford (an unergonomic approach because weight increases faster than aperture as you go bigger) etc. etc.

To me, the simplest way to compare two scopes in practice is this (using the numbers of your candiates): 12"/8"=1.5

This means the bigger scope will show the same object at the same apparent brightness in the eyepiece 1.5 times bigger.

Yes, the bigger scope resolves finer detail.
Yes, the bigger scope sees fainter objects.
Yes, the bigger scope looks more impressive.

But the first consideration above I found by far the most tangible. I know it's simplistic.
I have a 2.5", a 3" and a 10" scope. I can make all of them show the same object at the same brightness

[PSALM19.1 (Shaun)]

I guess, when I buy one, I'll still have my 8" and can road test for a while before I give it to my nephew! But all very helpful info...! Thanks guys

[brian nordstrom (As avatar)]

Sweet thread Shaun .
My best is 806x on Saturn on the best of nights in my now sold Takahashi M210 , do a search here ' Saturn at 806x' hand held .
1 more vote for the 8 inch telescope / 806x is hard using a 12 inch Dob .

Brian.

ps. my C9.25 performs as well .

[Allan_L (Allan)]

Originally Posted by PSALM19.1

Hi all,
Wondering, what difference in "visual" terms is there between an 8" dob and a 12"? I realise that the 12 would give a more detailed view, but just HOW much more? Is it a no contest or is the difference not worth the price difference?

Having just this year buying a 12" Skywatcher Flex Dob goto, and selling a 10" Skywatcher Flex Dob (manual), I was able to do a side by side comparison at our dark sky site while I had both.

There is not an immediate noticeable huge difference to the eye.
However, on close attention I noted that on that night I could clearly make out six stars in the Trapezium with the 12" while the 10" yielded the 4 main stars clearly with a hint of the "E" star.

That was enough for me to be happy with my purchase.

Hope that helps.

[OzStarGazer]

Originally Posted by PSALM19.1

I guess, when I buy one, I'll still have my 8" and can road test for a while before I give it to my nephew! But all very helpful info...! Thanks guys

If your nephew reads this he will probably shoot me... but I don't think you should give away your 8". To only have a 12" scope can also be a challenge. Most people here who have a big scope also have a smaller one (or even more than one)!

[SkyWatch (Dean)]

The best scope is the one you use the most, which is generally the easiest to set up, carry around etc: unless you are viewing from a fixed site.
There is no doubt a 12" is a lot heavier etc than an 8", so you need to factor that in to your thinking.
However, a 12" will gather 2 1/2 times more light into your eyes, so objects will be obviously brighter at the same magnification, and you will pick up fainter objects as people have already pointed out. On a good night a 12" will also offer better resolution- but it will take a bit longer to reach ambient temperature (a built in fan is a good idea).
One way to pump up an 8" is to use a deep sky filter on emission nebulae: this will really improve the view and make it seem like you have a much larger scope, especially if there is a bit of light pollution around. (I prefer one of these to an OIII for general viewing: the deep-sky doesn't wash out the stars so much and gives a more "natural view".)
However, I agree with the comment that a 12" hits a pretty good "sweet spot" in general viewing: and my personal home-made 12" packs up to smaller than my 4" refractor, with a 3-5 minute set-up time (depending on how clumsy I am feeling)- so it is a great option.
If you are thinking 12", then a collapsible one (like the Skywatcher) is worth looking at in terms of portability and ease of set-up.

I would suggest you go to a star party or a club night and have a look through both options before you commit; but both options will give you many years of great views.

- Dean

[speach (Simon)]

Originally Posted by OzStarGazer

I wish I knew from experience...
But all I have for "comparisons" is this website I once bookmarked:
http://telescope-simulator.com/telescope-calculator/
http://telescope-simulator.com/home/...pe-comparison/
It doesn't show DSOs for comparison though (apart from the Pleiades), which is what you are probably after.

Thanks for that site very informative.

[rustigsmed (Russell)]

when I was looking for a dob, I started comparing the pics people were taking on the different sized dobs (8", 10", 12" and 16"), most notably the planets. my searching came to the conclusion was that 12" and up seemed to produce the best images (although I did see some awesome pics taken with a few 10" newts but they were less common). that has got to be somewhat representative of the view at the eyepiece.

[OzStarGazer]

Originally Posted by speach

Thanks for that site very informative.

It is a useful site. I was just surprised that the moon is not brighter with big scopes that let in more light?

[barx1963 (Malcolm)]

Originally Posted by OzStarGazer

It is a useful site. I was just surprised that the moon is not brighter with big scopes that let in more light?

I think those simulators are more about the size of the moon, not the brightness!
Getting back to the original question. Having used an 8", 12" and 20" dob, I can tell you that the main difference in the abilities of a 12" versus an 8" is the ability to detect the fainter objects. You will see more of the fainter details in the 12" and the ability to use more power helps in doing things like detecting the E and F stars n the Trapezium in M42 or the lobes of Eta Carina.
That said there is big change in weight and convenience going to the bigger scope which must be weighed into the equation.

Cheers

Malcolm

[CJ (Chris)]

Originally Posted by OzStarGazer

I wish I knew from experience...
But all I have for "comparisons" is this website I once bookmarked:
http://telescope-simulator.com/telescope-calculator/
http://telescope-simulator.com/home/...pe-comparison/
It doesn't show DSOs for comparison though (apart from the Pleiades), which is what you are probably after.

Stellarium can also show the size of objects via the Viewing config, Plugins, Oculars menu. Takes a bit of setting up but once done it's quite interesting. And it can do any part of the sky.
Cheers

[OzStarGazer]

Originally Posted by CJ

Stellarium can also show the size of objects via the Viewing config, Plugins, Oculars menu. Takes a bit of setting up but once done it's quite interesting. And it can do any part of the sky.
Cheers

Starry Night actually does that too, but that site is faster... However I tried to save the page and it is funny as "select object" works, but "select telescope" and "select eyepiece" don't if you are not online, and I saved the "complete" page...

@ Malcolm - Yes, you wouldn't expect it to be accurate with brightness, but as you can even choose the brand of the eyepiece I hoped it would... I thought they had possibly used real photos with those scopes and eyepieces? Cool anyway. Now I know what I am missing.

This is nice too: http://astro.unl.edu/classaction/ani...lescope10.html
(the 4-inch scope results seem to be far too good though)

[PSALM19.1 (Shaun)]

Originally Posted by barx1963

I think those simulators are more about the size of the moon, not the brightness!
Getting back to the original question. Having used an 8", 12" and 20" dob, I can tell you that the main difference in the abilities of a 12" versus an 8" is the ability to detect the fainter objects. You will see more of the fainter details in the 12" and the ability to use more power helps in doing things like detecting the E and F stars n the Trapezium in M42 or the lobes of Eta Carina.
That said there is big change in weight and convenience going to the bigger scope which must be weighed into the equation.

Cheers

Malcolm

Thanks Malcolm; yes, I can see why a 12" Dob would pick up fainter objects. I assume that looking at Nebula etc would be quite superior than an 8"..(given conditions, EP's and all those variables)..am I correct in assuming that if I was looking at Jupiter (for eg) that I would see significant difference in details of cloud belts and GRS? This would seem to make sense to me given more light via aperture? As far as weight etc; this is not really an issue for me as I would have the scope on the same trolley I now use for my 8" and wheel it a few metres out the back garage door...so it would still get used alot...