8" vs 10" dob telescope in the city

justink (Justin) · #1 02-07-2015, 12:34 PM

Hi All,

I know there has been multiple post in regards to the 8" vs 10" and have read that the 10" is better due to aperture so apologies if this has been asked over and over but is there much difference in the two if i will be mainly using this in a city or if the difference is mainly with very dark skies (outside of city)? Would the 10" also have more light pollution seeing as it has a larger aperture?

I live north of Sydney City and have a private rooftop terrace where my telescope will live and mainly be used however i will take it out to places with less light pollution every now and then.

The difference between the 8" and 10" sky watcher (full tube) is $200 which I'm happy to pay if it will make that much difference however if i get the 10" i wont be getting any accessories, etc. but if the 8" is suitable then I'm willing to get a barlow lens, filters, etc. straight away.

Also i might add that this will be my first telescope so I'm very new to all this.

Any help here is greatly appreciated and thank you for your time.

Cheers,

Justin

madwayne (Wayne) · #2 02-07-2015, 01:05 PM

Hi Justin - the surface area of your primary mirror is over 50% more in your 10" than an 8". 10 squared = 100, 8 squared = 64 (pi*r sq for the area of a circle). That alone would give you an indication as to the numerical logic.

One question I would ask though is do you have an intention of going to star parties, have a friend or relative with a farm or joining an astronomy club with access to dark skies. If you answered yes to one or all of these then your first question is portability. The second is then can you physically lift your shiny new telescope in to your car and will it then fit?

Depending on that answer is where your decision lies in my opinion. Not much point in having a 10" if you stuck under light polluted urban skies with no chance of taking it to a dark sky. I'd get the 8", very portable and will give you the opportunity of getting to star parties, looking through other peoples light buckets.

The very first thing you should do is get to your local astronomy club or star party (check the star party listing on IIS for your nearest) and ask your questions. The members are very friendly like any hobbyist who has someone interested in their passion. Alternatively your local astronomy retailer and actually see one of these things of beauty in the flesh. Read specialised astronomy retailer, eg Bintel, not shops that sell telescopes.

So while I haven't directly answered your question I hope my insight helps to clear things a bit for you.

All the best and keep asking questions.

Wayne

inertia8 (Australia) · #3 02-07-2015, 01:37 PM

I cannot answer your question, I live under moderately light polluted skies north of Melbourne and can just make out some clouds of the milky way when there is no moon present in the night sky. I've used a 5.11" dob (mine) and 8" dob (loan) side by side and the fainter objects appeared to pop out more in the 8".

You say that it will live on a rooftop terrace most of the time, so one question I would ask you is whether you have access to the terrace with an elevator or by stairs and if it is stairs then will you often have someone to help move the scope to your car?

As suggested above, I would recommend that you visit both a store and a star party so that you can see both sized scopes in person. From the store you will be able to handle the tube and base so that you can determine if you can comfortably move them about at home and to the car and at the star party you will, with clear skies, be able to compare the views presented by each.

Good Luck

justink (Justin) · #4 02-07-2015, 01:38 PM

Thanks you for the responses, Appreciate it.

At this stage i don't plan on going to any star parties (actually didn't know about these) and dont have any friends that live on a farm so i will be mainly using this in an urban environment and going out to dark skies on the odd occasion.

lifting and portability wont be an issue as i have a 4WD. I do however have quite a number of stairs to go up to the terrace but also up/down to the car but it is doable. The telescope will live just inside the door leading out to the terrace so will not be affected by weather.

Thank you for the advice. It would be good to check out some local clubs and go along for some information and sight seeing.

From what i could gather i would be better off with the 8" inch mainly due to the light pollution i currently get? would i be seeing the same stuff with the 8" compared to the 10" in an urban area?

Thanks again

inertia8 (Australia) · #5 02-07-2015, 02:56 PM

Here's a couple of write-up's on the subject.

https://tonyflanders.wordpress.com/a...ght-pollution/

http://uncle-rods.blogspot.com.au/20...y-how-big.html

I would agree with Uncle's statement about M13 and light pollution 6"v12"... Except that with my skies and 5.11" to 8" aperture test on Omega Centauri the 8" showed it brighter and resolved more of the stars.

justink (Justin) · #6 02-07-2015, 07:25 PM

Thank you! Great articles.

Still un decided as they both have their benefits. I guess it comes down to cost. Is the extra $200 for the 10" worth it under light polluted skies and will it make that much difference when viewing planets and nebulas or is it mainly deep space under dark skies where you start to notice the differences?

I read in the article that the higher the aperture the better with light polution but even with 2" difference will that still apply?

Apologies if it sounds like im repeating myself just want to get some thoughts and opinions so i can get a good scope from the start as im new to all of this.

Thanks again for all the help

astroboof (Steve McN) · #7 02-07-2015, 07:37 PM

If they are both f5, then many including I would tend to go for the 8" in such LP conditions.

Generally speaking, large aperture is not desirable in heavy LP. For instance my ed100 a 4" refractor gets more use than my 8,10,12" Newts under light polluted skies, depending on the targets.

I avoid these kind of threads like the plague usually.

Whatever you choose Justin, you sound like you are on the right track to making it work out, there are plenty of options for either scopes, 8, or 10".

justink (Justin) · #8 02-07-2015, 08:37 PM

Thanks Steve,

Really stupid question but what's an f5?

The 2 I'm tossing between are both sky watcher full tube telescopes.

N1 (Mirko) · #9 02-07-2015, 09:16 PM

After many hours at the eyepiece of telescopes of both sizes (and many others) under all sorts of skies, I've developed the strong view that the only thing that matters in answering a question such as the OP's is the exit pupil range that will be used most often. Forget f-ratio, forget aperture as absolute measures. A city dweller with great seeing might find a large aperture extremely useful if planets are their main interest. Likewise, a fast f/5 will show a very dark sky background even in the city when a 2.5mm eyepiece is attached to view small, bright targets.

Basically, when the exit pupil exceeds 4 or 5mm in the city, the sky background becomes unpleasantly bright. Below 0.5-1mm, your target becomes so dim in the eyepiece that you'll need to shield ambient light (of which you'll have plenty) to make it work. Between these values, you're sweet. Find out what type of target is your favourite, work out the power range needed, calculate the corresponding exit pupil range and select scope accordingly. Extended DSOs tend to look best at 2-3mm exit pupil, perhaps a bit less than 2mm if the sky is really bad.

Speaking of selecting - regarding 8" vs. 10", none of the above will matter much if that is all you are going to choose from. The difference in the field is so small that you need to see them side by side, or observe very specific features on your target, to know it's there. The 10" will show the same target, at the same brightness, 25% larger, or a factor of 1.25. Nothing more, nothing less.

Value for money: 8" wins. Want a little bit more reach, especially on marginal DSOs, none of which you'll want to observe from the city, get the 10".

$0.02

cometcatcher (Kevin) · #10 03-07-2015, 02:00 AM

Bigger aperture is going to be better for planets. Bigger aperture will work better with filters, like an OIII or UHC which would be very useful for nebula in the city.

ZeroID (Brent) · #11 03-07-2015, 06:08 AM

Regarding size and portability, an 8" F6 and an 10" F5 are almost the same length and the 10" only fractionally bigger in diameter and marginally in weight.
For LP situations in the city filters work better in bigger scopes. They dim the view so the more light you can collect the better.

N1 (Mirko) · #12 03-07-2015, 06:56 AM

Quote:

Originally Posted by ZeroID

Regarding size and portability, an 8" F6 and an 10" F5 are almost the same length and the 10" only fractionally bigger in diameter and marginally in weight.
For LP situations in the city filters work better in bigger scopes. They dim the view so the more light you can collect the better.

Yep, some of the brighter nebulae can look surprisingly good through an OIII from a city or suburban location. Agree, 8" f/6 and 10" f/5 very close length wise. Will fit across back seat of normal car. 10" weighs a few kgs more.

justink (Justin) · #13 03-07-2015, 07:31 AM

Thank you all so much for the knowledge and advice. So glad i found this forum.

From what i gather a couple of filters are a no brainer to help with the LP and the filters will work best with bigger apertures however I'm leaning more towards the 8 at this stage but will you also see the same benefits of filters in an 8"? I ask because with the lower cost of the 8" i can afford to get a few filters and a better eye piece (barlow lens?) otherwise for the time being ill have to go just with what comes stock with the 10".

But if this means that i wont be able to see planets/nebula and some DSO clearly with the 8" even with filters then ill go with the 10" and look at getting a couple of filters in a couple of months.

Allan_L (Allan) · #14 03-07-2015, 07:43 AM

Quote:

Originally Posted by justink

Thanks Steve,

Really stupid question but what's an f5?

The 2 I'm tossing between are both sky watcher full tube telescopes.

f5 refers to the ratio of focal Length to diameter
Skywatcher 10" is 1200 / 254 = f4.7
Skywatcher 8" is 1200 / 200 = f6

As both are focal length 1200 it appears both optical tubes will be about the same length (approx. 1200) , so similar to transport.

The cheapest price I could (quickly) see for the SW 10" full tube was $949.
Whereas, I could find (at another Sydney store) the SW 10" collapsible for little more ($999).

For $50 I would definitely consider the collapsible. It will make transport so much easier. (IMHO)

I remember moving an 8" SW Newtonian around and it was awkward.
My 10" SW collapsible was so much easier to move and store.

justink (Justin) · #15 03-07-2015, 07:51 AM

Thanks Allan,

From the place I'm getting the SW from the 10" is $695 and 8" $499 both full tube. Collapsible 8" $575 and 10" $869.

So with that price range in mind i could consider the 8" collapsible and still have wiggle room for filters and stuff but the 10" collapsible is a bit outside what im looking at.

Is it best to have a higher or lower ratio in that case?

Allan_L (Allan) · #16 03-07-2015, 07:58 AM

Quote:

Originally Posted by justink

Thanks Allan,

From the place I'm getting the SW from the 10" is $695 and 8" $499 both full tube. Collapsible 8" $575 and 10" $869.

So with that price range in mind i could consider the 8" collapsible and still have wiggle room for filters and stuff but the 10" collapsible is a bit outside what im looking at.

Is it best to have a higher or lower ratio in that case?

Those are good prices. (can I ask where?)

RE f ratio, google is your friend.
eg: http://www.oocities.org/capecanavera...ocalratio.html

justink (Justin) · #17 03-07-2015, 08:24 AM

Seems like it wouldn't make that much of a difference (f4 vs f6) but will still get a barlow lens anyway to make up for any differences it may have.

The place im getting it from is http://www.astropetes.com.au/telescopes.html

N1 (Mirko) · #18 03-07-2015, 08:45 AM

It's worth noting that the slower the f-ratio of the unit (i.e. higher f number), the easier it will be to collimate and keep collimated. Granted, this is less important if the scope stays in one place most of the time.

Also, a slower f ratio will allow cheaper eyepieces to be used (within reason), and without extra gear such as coma correctors, for quite good results. The faster the scope, the more coma it introduces.

Filters: again, it's the exit pupil that matters. I use an OIII on a 60mm refractor regularly for extended low-power targets such as the Carina Nebula, viewed from the city. The tradeoff with small scopes is not whether an OIII or other filter will work or not, but the power limitations of the small aperture.

Form what I've read here so far, 8" does not seem a bad choice.

justink (Justin) · #19 06-07-2015, 02:23 PM

Thank you all for your help with this.

I bought the 8" skywatcher collapsible scope. Cant wait to get this assembled and get the fun started.

Can anyone suggest a good site/app to help locate stars, planets, etc.?

inertia8 (Australia) · #20 06-07-2015, 05:34 PM

Congrats on the new scope!

Start off by grabbing stellarium for your pc and input your home GPS co ordinates. Then you can increase the time and see roughly where the objects will be in the sky later at night.

Sky safari is a good app. The free Google sky maps will be of some use too. Keep in mind that some objects require dark adaption of your eyes, sometimes over 30mins and so looking at a phone screen can set you back. Not really an issue for brighter objects but still even then more detail will be seen after you are adapted. You can use red cellophane over the phone screen and the apps mentioned above have night modes but the brightness can still affect your eyes.

I use a planisphere for general viewing and charts/app for specific targets.

Start off by setting up with the lowest power eyepiece and just cruise around the sky for a start. Near the scorpions tail and southern cross you will find objects of interest.

Roughly to find the planets at present.

Western sky early evening. brightest "star" is Venus and below it will be Jupiter, they just recently went thru a conjunction where they could be blocked out of view with a finger so they are still close together (couple of fingers width) but growing apart. Later in the evening you will see Saturn near the head of the scorpion, it's fairly bright and wont be twinkling.

After a while of observing you will get to know the basic constellations and recognize when a planet is nearby etc.

Clear skies.