Telescope Size vs Darkness [Seeing] Conditions

[pevsfreedom]

I had a quick question, so I live in Los Angeles which is obviously light polluted.

Sometimes I make the trek out to a dark sky site about an hour away,

Now my question is,

How would a 12" dob (in my yard) with a limiting magnitude of say 4.5 [in my yard]

compare to a 8" dob (which is easier to transport) if I dragged it out to the desert with me [limiting 6 mag]

Does that make any sense? Would the 12" give me the same views as the 8" would at a darker site, or is that all irrelevant, and the 12" will just magnify further? 

I haven't ever bought a telescope, just trying to piece things together for when I do. 

Thanks

[rwilkey]

Hi there, with both these scopes it is not about magnification, it about the amount of light they gather.  If you are interested in DSO's then both scopes will do well in this department, however, the 12" will show more distant and feinter objects such as feint planetaries, which the 8" will struggle with.  Both scopes will hugely benefit from viewing at a dark sight.  At the end of the day, your mobility is important and in that respect I would consider the 8" a 'light-weight'.  On the 12", that all depends on you.  From my own point of view I am partially disabled and mobility is an important issue, but if you are fairly strong and mobile then I would say the 12" will give you more objects, but at the price of some mobility.  It will be cumbersome in my view so I recommend you think carefully about how far you want to take the hobby and how much free time you have to spend on it.  Good luck in your decision.

[Charic]

I have an 8" telescope that appears to perform better from darker sites, but a 12" would possibly still out perform my 8" from my edge of suburban skies. You should experience the same.

The bigger aperture gathers more light allowing for fainter targets and finer detail to be observed from either location?

Both scopes will be better at your drive away site, the 12" should be better at the light polluted site. Dont think that the bigger aperture just sucks in more light pollution, theres more to it than that!

Also note the small 4" difference between scopes affects weight, cost size and, eyepiece selection, this being very selective to reduce the effects of coma in the faster telescope?

[Scooot]

I would rather live on the dark site with the 8" than vice versa. The 12" will allow you to see more on either site but the quality of the view is much better with the better contrast from a dark site.

[NGC 1502]

You have some great advise above.

I'd just like to add - if you can get to a dealer with showroom stock, then you can see first hand the size and weight involved.

All the best with your decision, Ed.

[Ags]

It is not a small 4 inch difference... The larger scope can see a full magnitude fainter. It's worth bearing in mind that light gathering goes up by aperture squared, but unfortunately scope weight and bulk go up by aperture cubed :-(

[PeterW]

Do you like driving? If so then get the largest scope you can transport and setup. If not then consider video astronomy or as you live in the U.S. you could also consider an image intensifier and narrow band filter to filter out the light pollution. Both the latter options prefer a short focal ratio scope, cheap refractor or reflector, one designed for imaging should provide enough infocus to use focal reduction. Then when you do get to a dark site you will see a load more than you usually do, and still get some observing done at home.

Light pollution does not need to stop you seeing stuff, I live on the edge of London with streetlights so I can read in the garden at night!

Cheers

Peter

[YKSE]

Answer to your question is that 8" in the dark site will outperform the 12" in your backyard with margin!

Magnitude is defined as 100 times brightness difference between magnitute 1 star and a magnitute6 star, it means that 1 magnitute difference corresponds 2.51 times of bridghness differences.

For telescopes, the light grasp capacity is proportional to the area of telescopes, which is the Pi*(D/2)², therefore the difference between a 12" and 8" is (12/8)²=1,5²=2,25, which is hardly 1(one) magnitude difference as we can see above, and the mentioned 1.5 magnitude difference speaks for itself.

[pevsfreedom]

Thanks for the answers everyone. I think the difference between the 8" and 12" was about 40-50 pounds. I think I might end up going with the 8" from what I've read here. Still need to do some reading however between a dob and an apo before I decide which type I think I want. 

[PeterW]

8"... Dob or apo.... I think you'll find the dob easier to use and a bit cheaper.... ;-)

Peter

[Stu]

I would go 8" with the ability to get it to a dark site, I have a 12" scope but still mainly use it for planetary at home because my skies are similar to yours by the sound of it.

With low surface brightness objects, galaxies, nebulae etc sky brightness will have a big effect on their visibility. Dark skies rule in my book

[swamp thing]

Always pick dark skies over aperture. Never the other way round. 

LP skies even with giant apertures can be disappointing. Dark skies even with very modest kit are mind blowing. 

[Ags]

It's worth noting that bigger telescopes cannot make fuzzy objects brighter, just bigger for the same level of brightness. So if you have difficulty separating the fuzzy object from a bright background, more aperture won't help that much.

[Lonestar70]

8"... Dob or apo.... I think you'll find the dob easier to use and a bit cheaper.... ;-)

Peter

LOL...  I think you could be right there Peter.

200mm DOB with full goto complete £ 749.00

185mm APO (OTA only) £ 17,000... throw in a mount for another £1,500 or so... not much change from £20,000.

I dread to think what a full 200mm APO and mount would come out at... and I would need a lot of convincing that it would produce much better visual results.

Think I will stick to what I have got

Ho Hum

Keep happy.

[pevsfreedom]

I don't want an 8" apo  Well I do, but you have to pay for it. 

[pevsfreedom]

Seems a no brainer the 8" dob is the way to go for me.