The benefit of more aperture? Esp- under LP skies.

[pipnina]

So my 5" has gotten me some good results so far: M42, M45, M31 et al, M33, the veil and the couble cluster look pretty good. But it's among the smallest newts you can get and I keep hearing that more " = better. From my current understanding, I can only understand how that's so for small targets (e.g. planetaries like the little gem neb or dark & small galaxies) and the extra " also helps for seeing fainter stars & planets...

But surely there's little point in getting anything bigger than my 5" for anything "normal sized"... Surely it just gives you more magnification on your existing view and possibly prevents you from fitting in the whole object?

Also, if you live under skies with some LP (say, mag 5.2- decent but not amazing) will more aperture still pull out fainter objects or will you ultimately be limited by the LP?

Really confusing for me... I'm considering getting a 10" or a 12" in the new year and wondered what more it would get me.

Cheers,

    ~pip

[John]

My skies are around the level you describe. A little worse sometimes, a touch better at best. Once or twice a year I take my scope to a darker site but the vast majority of my viewing is done from my back yard, on my small patio to be exact which is around 2 metres from the house.

I've found 12" of aperture extremely rewarding - my 12" dob is used more than either my 102mm or 120mm ED refractors.

Despite my less-than-perfect skies and a fair amount of obstructions including large conifers, a number of neighbouring houses and the odd security light and several streetlights I've managed to see many sights that my smaller scopes simply have not been able to show me over the past 24 months or so that I've owned the scope. These include:

- my first quasar

- dozens of galaxies down to mag 13.8

- H-II regions in M33

- 7 supernovae

- 10 craterlets on the floor of Plato and the central rille in the lunar Alpine Valley

- Neptune's moon Triton and Uranus's moons Titania and Oberon

- Many subtle details on Jupiter

- Splitting Sirus and seeing the "Pup" star

I had to work at these of course and some took a number of attempts and some patience to wait until the object was in a more favourable position (ie: not behind a tree or buried in the Bristol glare !) but I don't believe that smaller aperture scopes would have shown me them and I feel more fulfilled as a visual astronomer by having seen them.

I believe there is quite a bit more to come as well.

So, for me, having a 12" scope has been very well worthwhile

[Piero]

I don't have much experience with large telescopes and, among these, the one I looked through the most was a Celestron C8. It was the telescope of an old amateur astronomer who gave me my first good introduction in astronomy when I was your age.

There was a substantial increase in detail compared to my newton 114mm on all targets.

This said, be careful with aperture fever. All the enjoyment comes with balanced expectations. I would seriously advice you to see these telescopes (size, weight) and also consider your body strength at moving one of them.

Everyone of us would like to have a bigger telescope, but not many actually have one. This is not just about money, but it also concerns physical aspects, longer cooling time, storing space etc.

I believe there is nothing worse in this hobby than having a nice large scope stored inside and covered with dust.

Just a thought of course.

Piero

[jetstream]

^^^ like the boss (John) says

If you look at the table it will show you why the 12" works so well... from your 5" to 12" you gain 2 magnitudes...this will be very noticeable. I used Moores observed aperture vs magnitude difference, and this is on point sources (stars). On extended objects, John's scope gives a great magnification (focal length) for the f ratio-5.3- which also gives a great exit pupil with some wides and hyperwides ie 31mm  Nag, 21E.

I believe that any aperture jump should give at least a magnitude gain... hence my 15" from the 10".

John's OOUK will be the most portable 12" around.IMHO.

http://www.twcac.org/Tutorials/limiting_magnitude_table.htm

[Stu]

To pick up the point Gerry referred to in terms of ultra wide eyepieces, this becomes very relevant when using longer focal length, larger aperture scopes for the reasons you mention.

The first diagram shows two image circles, the largest using a 32mm Plossl in your scope, and the one just inside represents a 31mm Nagler (82 degree afov) in John's scope, 1.78 degrees vs 1.6 degrees so really not much in it. The Eastern Veil is framed nicely in both but will show dramatically more detail in the 12" under decent sky conditions.

To show the whole complex, you need to be looking at a widefield frac with a widefield eyepiece, giving a much larger fov (second diagram)

Ultimately a lot comes down to sky brightness, a large scope can't beat light pollution, if a galaxy is lower surface brightness than the sky background then you won't see it in any scope. Take a large dob to a dark site and it will really fly.

[Charic]

Your 5" (130mm) may have won your heart with some wonderful images, but having more aperture will reward you even further? The bigger the aperture the greater the amount of light the telescope gathers, it/you will see more under the right conditions.

Your 130 scope has a light grasp of 344x that of your own eyes?

The 10" scope will have 1275x that of your eye.

The 12" scope will have 1836x that of your own eye, so a bigger aperture is a bonus.

It's the whole purpose of an astronomical telescope, to gather as much light as possible, providing a more detailed image. 

In order to get the best results, even under light polluted skies, requires aperture?

I too had considered a bigger scope, a 300P, but after a lot of thought and some simulations, the jump in image scale does not warrant me to rush out and buy a new scope just yet.

From a 5" scope maybe, but owning the 8" IMHO, I have found a scope that is truly competent in its task.

If I can compare a 300P against my scope from my observatory, and the difference is notable by a long shot, I think I would upgrade, however, I think my 8" (200P)f/6 scope is more than adequate for my needs at present. It far excels the capability of my Celestron 127.

There is no getting away from it,  a darker sky itself is the best option for seeing just about everything! It feels like I have a better scope, especially when viewing M31, Andromeda. Just the core is visible from my observatory, but from the darker site, M31 fills my eyepiece. More aperture provides better detail, and having more aperture and light grasp will often allow higher magnifications, that in it self will help darken the sky increasing the contrast between target/sky.

The 8" is a very popular sized telescope ( has a light grasp of 816x your eyes?) and probably more manageable than its larger brothers ( or sisters )  the 10" & 12" ?

[jetstream]

A nice test for aperture are the bright clusters, M37 and M13 , Wild Duck and others come alive even in moonlit skies. With my 10" dob, under bright skies, M13 looks like mod Qualias sketch, with enough mag. It really comes alive under dark skies as do the others.

Pip, your descriptions of M33 etc indicate you would benefit from more aperture- from really LP skies the objects you see would be almost invisible...

[pipnina]

Cheers guys, I guess a 10/12" will be on my shopping list come late february. Maybe, if i get a 10/12" I should take my 5" with me as well and compare views... Or would that be too much hassle haha.

[Charic]

Strap the 5" onto the 12" ? Would make a decent finder, Lol.

My 5" Celestron never gets used now!

[Gazabone]

I've enjoyed reading this thread and concurs with my opinion and experience. I am a little puzzled though why big apature scopes aren't made with shorter focal lengths? Is it that there is a balance between focal length and apature outside which scopes don't function well?

Cheers all

[pipnina]

I've enjoyed reading this thread and concurs with my opinion and experience. I am a little puzzled though why big apature scopes aren't made with shorter focal lengths? Is it that there is a balance between focal length and apature outside which scopes don't function well?

Cheers all

I think faster focal ratios in newts (like f2.5), to a certain extent, is a bad thing as it means the secondary mirror needs to be larger and block more light... But if the focal ratio is too slow, (f12, say.) you'd need EPs with absurd focal lengths to reach a 5mm or more exit pupil.

It seems like f4 is the fastest most people go with newts/dobs. I guess lower and the scope becomes too demanding on EP quality or blocks too much light with the secondary.

[jetstream]

and money...those big fast mirrors can get very pricey. Any fast mirror is exponentially harder to make than slower ones, some of the best fast mirror specialists will make a 20" mirror 1 1/2" thick... sub f4...

[Moonshane]

Bear in mind that aperture gives more resolution resulting in a larger image at the same brightness. You just see more.

Newts tend to stop at f4 as a compromise. At f4 the coma free zone without correction is off the top of my head about 1-2 mm. At f11 it's about 22mm. Coma correctors and fast accurate mirrors are not easy to make or cheap.

I really enjoy my 12" f4 dob at home and my 16" f4 is great unleashed at dark sites.

[LukeSkywatcher]

I really dont worry my pretty little head about f numbers etc on scopes. If the scope is portable enough for me then thats good enough. More aperture is always good. I just enjoy the views no matter what scope i use under whatever skies. Filters are a god way to go if you cant escape LP.............but they dont solve the problem 100%.

[DevonSkies]

To pick up the point Gerry referred to in terms of ultra wide eyepieces, this becomes very relevant when using longer focal length, larger aperture scopes for the reasons you mention.

The first diagram shows two image circles, the largest using a 32mm Plossl in your scope, and the one just inside represents a 31mm Nagler (82 degree afov) in John's scope, 1.78 degrees vs 1.6 degrees so really not much in it. The Eastern Veil is framed nicely in both but will show dramatically more detail in the 12" under decent sky conditions.

To show the whole complex, you need to be looking at a widefield frac with a widefield eyepiece, giving a much larger fov (second diagram)

Ultimately a lot comes down to sky brightness, a large scope can't beat light pollution, if a galaxy is lower surface brightness than the sky background then you won't see it in any scope. Take a large dob to a dark site and it will really fly.

Interesting discussion. If you don't mind me asking, what software did you use to generate those diagrams? It looks like a very useful tool.

Regards,

Ed

Sent from my GT-N5110 using Tapatalk

[RikM]

I found the jump from 6" to 10" very worth while. My skies sound very similar, mag 5.2 ish, and I have managed to tease out some really nice objects. The Skyliner 250PX also fits very nicely in the car for trips out to darker sites. The one person manageability was the main reason why I chose the 10" over the 12". It also matched my personal 'views for money' ratio. A second hand OOUK 12" would be spot on if you can find one.

[Stu]

Interesting discussion. If you don't mind me asking, what software did you use to generate those diagrams? It looks like a very useful tool.

Regards,

Ed

Sent from my GT-N5110 using Tapatalk

Ed, the app is SkySafari Pro, running on iOS but available on Android too. There are various versions available, a standard and Plus which are cheaper. I believe they all have the same capability to display field of view circles but I'm not 100% sure.

I don't use it now, but for PC Stellarium is a free download and is very good. I believe there is an 'ocular plugin' which allows similar functionality.

Stu

[DevonSkies]

Ed, the app is SkySafari Pro, running on iOS but available on Android too. There are various versions available, a standard and Plus which are cheaper. I believe they all have the same capability to display field of view circles but I'm not 100% sure.

I don't use it now, but for PC Stellarium is a free download and is very good. I believe there is an 'ocular plugin' which allows similar functionality.

Stu

Thanks. I'm off to check out SkySafari Pro now!

Ed

Sent from my GT-N5110 using Tapatalk

[Bart]

Ed

Skysafari Pro is great, but relatively expensive. I got mine on a sale and I've seen sales of it relatively often, can't remember how often, maybe somneone here knows if there's a trend.

Barry