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Resolution VS aperture- again...

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I was wondering at what point (magnification) the increased resolution from larger aperture becomes noticeable? (seeing not considered). I realize as resolution increases that smaller detail can be seen- but is magnification a factor?

I'm curious if I took my 10" at 200x and a 14" at 200x if there would be a noticeable increase in resolution from the 14". For arguments sake lets say on the moon and Jupiter. Of course were are assuming equal optical quality and that conditions are not a factor.

Thanks

Gerry

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At apertures beyond roughly 8" seeing limits resolution more than aperture, in many cases. Even in good seeing, you would probably see little difference in terms of resolution until you went beyond 250x with the 19" and 14" in the example, as below that the resolution of the retina would be a limiting factor (depending on the visual acuity of the observer).

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Hi Gerry,

Both the 10" and the 14" will have the same image scale but the 14" will be noticeably brighter. The point at which you will notice depends on the sizes (or at least the size of the smaller aperture you are comparing to) of the comparison telescopes.

Doubling the aperture of a telescope makes the image four times brighter at the same magnification, or allows twice the magnification to be used while retaining the same image brightness. As aperture is increased, the light grasp increases by the square of the aperture. So a 6" telescope will be 4x the brightness of a 3" at the same mag, whereas the 14" will 'only' be x2 brighter than a 10" at the same mag with all other factors equal. You will pick out more detail at the same magnification with only a 2" upgrade (x1.4) from the 10" example you give.

What the larger aperture will also do is reveal fainter magnitude background stars and dso's at the same magnification.

The increased resolution from larger aperture as a result of a brighter image becomes noticeable at once. The greatest change in resolution depends on the ratio of your lower comparison scope to your higher one.

Regards

Dannae

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Thanks Michael.

I read that 3mm to 1mm exit pupil gains "eye resolution" and is diffraction limited- less than 1mm the resolution gains are there but quite smaller. I also read (just) that 34x aperture in inches will provide the 5 arcmin size needed to resolve well http://www.telescope-optics.net/telescope_magnification.htm which results in a .73mm exit pupil and 340x in my 10" f4.8 scope.

Backing off to a 1mm exit pupil would give me 250x with the the 10" f4.8.

The 14 " f4.5 @ 1mm exit pupil would give 355x I believe.

Under these conditions above the VX14 would best my VX10 355 vs 250x. If I upped my VX10 magnification to 355x would it equal the VX14's @ 355 as well? I'm thinking it might be close but with a hard to use exit pupil.

My seeing on the moon is generally 300x-340x under good conditions with the VX10 and 250x+ on Jupiter. I am making some assumptions about eyes here, mine are fine with .83mm-1mm exit pupils and my lunar seeing will go 400x under the best conditions and that is quite a sight.

I'm just pondering things about, any and all comments appreciated. :smiley:

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A good test of resolution (and of course all the other factors) are the craterlets of Plato on the moon. This can allow easy comparison between two scopes.

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Hi Gerry,

Both the 10" and the 14" will have the same image scale but the 14" will be noticeably brighter. The point at which you will notice depends on the sizes (or at least the size of the smaller aperture you are comparing to) of the comparison telescopes.

Doubling the aperture of a telescope makes the image four times brighter at the same magnification, or allows twice the magnification to be used while retaining the same image brightness. As aperture is increased, the light grasp increases by the square of the aperture. So a 6" telescope will be 4x the brightness of a 3" at the same mag, whereas the 14" will 'only' be x2 brighter than a 10" at the same mag with all other factors equal. You will pick out more detail at the same magnification with only a 2" upgrade (x1.4) from the 10" example you give.

What the larger aperture will also do is reveal fainter magnitude background stars and dso's at the same magnification.

The increased resolution from larger aperture as a result of a brighter image becomes noticeable at once. The greatest change in resolution depends on the ratio of your lower comparison scope to your higher one.

Regards

Dannae

Thanks Dannae, I sure type slow here and saw your reply come in ahead of me slogging away lol!

Well I have another question, recently I've heard and read that planets are considered a form of extended object and that their brightness stays the same regardless of aperture-thoughts? I'm not sure about this and there is some material on the planets as extended objects in the link in the reply to Michael. Is that true we need to magnify the wanted detail to about 5 arcmins in size to be able to resolve them? I think they used equal brightness source points as an example and may be different for different objects. Thanks again.

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A good test of resolution (and of course all the other factors) are the craterlets of Plato on the moon. This can allow easy comparison between two scopes.

This would be a great method Shane as it would take every factor into account, I don't have access to a VX14 though unfortunately. Some friends with the 14's and I with the 10" may make some filter comparisons and I would like to know if the tests can be an valid using these 2 scopes. I am looking at a VX14 myself, down the road, maybe. My thoughts are that my 10" may be "just under" and the 14" " just over" with regards to resolution, seeing considered, but it will be close I think.

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I have a 12" f4 and a 16" f4. Will try and make a comparison at some point with similar magnifications. This should give a decent representation of 10" and 14" I suppose.

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That would be great, thanks Shane! :smiley:  I know the 14" would be better than the 10" on DSO's, but would it match or exceed my 10" on the moon and planets in the range of seeing I have here.... I really love my VX10 and I'm not sure I could part with it, it gives fantastic view and is easy to carry. A new VX14 shipped to here would be costly epsc with my current exchange rate.

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A good test of resolution (and of course all the other factors) are the craterlets of Plato on the moon. This can allow easy comparison between two scopes.

Yep, thats one of the benchmarks I use. And the central rille in the Alpine Valley.

The only comparison of this type I can do is between my ED120 and my 12" dob. While the dob will show more lunar and planetary detail most of the time and show the fine stuff that bit more easily, I've often thought how well the ED120 does on such things given the pretty massive aperture difference between the scopes.

I'll be interested to see what Shane's comparison comes up with :smiley:

My understanding is that a scope can't make an object brighter but larger aperture does deliver a larger image scale with more "information" in it.

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I've noted similar between my 12" Dobs and the AR127L. On a really good night (probably less of those here than in Ontario) the magnification you can bang into the Dob seems to defy belief. However, that is not most nights. On moon/planets the AR127L tends to give a surprising account of itself and is certainly more tolerant of less stable viewing conditions. It accesses views on an average night, with greater ease and thus, more regularly.

I'd be surprised if the lower aperture differential between a 10 & 14" revealed a similar increase in tolerance of seeing conditions.

Russell

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Well I have another question, recently I've heard and read that planets are considered a form of extended object and that their brightness stays the same regardless of aperture-thoughts? I'm not sure about this and there is some material on the planets as extended objects in the link in the reply to Michael. Is that true we need to magnify the wanted detail to about 5 arcmins in size to be able to resolve them? I think they used equal brightness source points as an example and may be different for different objects. Thanks again.

Theoretically the brightness of an extended object doesn't change with aperture the object just gets bigger. However, in my experience human vision is a bit more complicated than say a CCD and in my experience extended objects do appear brighter (at least perceptually) in bigger scopes. With planets brightness is not really the issue but image scale is everything. Mars the size of golf ball will show way more detail than the size of a pea. Sadly the atmosphere will usually kill any real advantage above about 8" aperture. that can work against big scopes because you tend to wind the magnification down to the seeing which can make the planets too bright and glaring. Of course you can always stop down a big scope - it's a lot harder to stop up a small one :)  

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Except, to use Shane's and John's resolution tests on the moon in my 8" I'm pushing towards the max limits of the 'scope.

Last week, the moon was awesome and the steadiest I've seen it in a long time, I put a 2x barlow on my 6mm and although it was bonkers to be able to see detail at that magnification, the eye lashes, floaters, dust on the EP and general dimness, did rather detract from the view! :D

I relish the prospect of being able to wind the magnification down to the seeing. ;)

Pipe dreams at the mo.

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Theoretically the brightness of an extended object doesn't change with aperture the object just gets bigger. However, in my experience human vision is a bit more complicated than say a CCD and in my experience extended objects do appear brighter (at least perceptually) in bigger scopes. 

I understand where you are coming from Tim. It often seems that way when using bigger scopes doesn't it, but it is an optical illusion. 

I know this as I have experimented, using my two scopes next to one another. Both my regular scopes operate at f/4.5 so with identical eyepieces they offer the same exit pupil. 

When I use the same eyepiece in both scopes, its noticeable that image surface brightness is the same, but, the image scale favours the larger scope considerably. 

However; when observing normally the big scope uses its image scale to great advantage and it does indeed seem that it gives brighter images. It kinda pulls away as we observe.

The way to try and explain it is if we start with both scopes using  max exit pupil on a DSO and see what happens.

The 10" gives a view of said object at the maximum possible surface brightness. When I walk over to the 20" with the same eyepiece its not the brightness that blows one away, its the size difference of the object in the eyepiece, its huge, twice the size in fact through the larger scope.

Now, I try and match the image size (say the object is bright enough to allow this without too much problem) by doubling the mag with the 10". I now have identical magnification in both scopes but the object is now four times brighter in the larger scope.

If the object is still of sufficient brightness the amount of detail shown will be similar. However, if this is so, I can now "change up" in the 20" and double the image scale for the same surface brightness again. The 20" kinda pulls away. 

As I repeat this process the 20" keeps pulling away and the 10" just cannot keep up. 

This is the best way I can think of to try and explain what  extra aperture allows.

Most DSO's require magnification, the minute you need to magnify an object a larger scope will be of benefit as it already has a head start.

One is able to get objects like NGC891 stretching right across the eyepiece in a big scope with sufficient brightness for photographic like views. 

As always though, dark skies permitting :)

This is also why so many are disappointed by their views of large objects (like M31) through big scopes. Here we have an object so large that no mag is needed. The big scope can't even fit it in the FOV. Yes a large scope will pick out detail within the dust lanes if the site is good and dark, but It won't help with tracing the outer reaches of the spiral arms, bins do this brilliantly a 20" dob doesn't.

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One other point about resolution (in the formal sense of resolving limit of optics).  This is not something that would ever have an effect you'd see, but I find it interesting at a theoretical level:

Resolution is a function of diameter, not aperture.  What's the distinction?  Aperture (in the sense of light-gathering capability) determines the surface area of the light-gathering component (mirror or lens).  Area = pi*r2.  That's why doubling aperture increases light gathering by 4 times - it's that square in the area function.  Resolution, being a function of diameter not area, goes up linearly with increase, not as a square.
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I wonder if the greater volume of light collected by a larger aperture has something to do with the appearance of a brighter planet in the larger scope? If we limit the magnification to a certain size exit pupil, say 1mm then the longer focal length scope will show a larger image ( more magnification) and may appear brighter due to the larger image (?) ie a tiny light bulb of X amount of lumens vs a big bulb of the same output, the larger bulb would appear brighter (I think).

I have eyepieces that will "ghost" in the 10", but not the 90mm or 120mm, I've guessed that this might be the volume of light thing- but the 10" is widely different in spec than the fracs.

My guess is the 14" would show more detail above about 300x mag, the question for me is how many times does this occur and how much will the extra aperture actually hurt me during normal seeing.

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.....I have eyepieces that will "ghost" in the 10", but not the 90mm or 120mm, I've guessed that this might be the volume of light thing- but the 10" is widely different in spec than the fracs...

It could be the sharper angle of the light cone in the 10" challenging the optics of the eyepiece.

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hi gerry, from my experiences. before i bought the vx14 i had a cpc 9.25 when viewing say jupiter at 300x yes you could see great detail, but at 300x in the dob jupiter was cosiderably bigger and more detail. the most outstanding thing with the 14 was when viewing one of my most favourite dso,s m27. with the 9.25 yes it was lovely but in the 14 you could see all the stars through the nebula in the background or maybe foreground but never in the 9.25. as you also said gerry how often can you use more power in the 14 it seems that since i have had it only a couple of times that i havnt been able to use over 250x so yes quite often. best thing to do is allow dobfest to your house then you can try lots of scopes  :sad:  :grin: 

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No problem Mike! We have a house and separate cabin on Rainy Lake that sits in 21.7 mag skies, viewing from the ice is a great experience too.... so you guys are going to fly over with those great dobs?! :grin: Anytime fellas. I'll get you to sneak a brand new VX14 from OOUK on the plane for me too :grin:  :grin:

Great to know that you use 250x or more often Faulksy, first hand accounts are so valuable. Have you had it go 350x (1mm exit pupil)? My best seeing so far has allowed 350x-400x on Mars and the moon, but the tight exit pupil makes it hard for me. How long to cool the scope in the cold Mike?

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No problem Mike! We have a house and separate cabin on Rainy Lake that sits in 21.7 mag skies, viewing from the ice is a great experience too.... so you guys are going to fly over with those great dobs?! :grin: Anytime fellas. I'll get you to sneak a brand new VX14 from OOUK on the plane for me too :grin:  :grin:

Great to know that you use 250x or more often Faulksy, first hand accounts are so valuable. Have you had it go 350x (1mm exit pupil)? My best seeing so far has allowed 350x-400x on Mars and the moon, but the tight exit pupil makes it hard for me. How long to cool the scope in the cold Mike?

320x is the most at the moment gerry, 5mm bgo, but will try the pentax tonight, power mated my 6mm delos a couple of times during great seeing and that was on jupiter last year, image was big but no more detail than 300x. seem to be able to use 320x almost every night i view to be honest gerry and i still havnt learned the art with the catseye colimation kit. when i use the auto the radiation symbol is everywere, so maybe theres more to go yet with the little 14"

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I understand where you are coming from Tim. It often seems that way when using bigger scopes doesn't it, but it is an optical illusion. 

Hi Steve. Yes you're absolutely right,  it has to be illusional as your experiments bear out  and besides, the maths doesn't lie:) I think the source of the illusion is that the central vision of the human eye has excellent resolution and poor light sensitivity whereas the periphery has the exact opposite. So a typical nebula or galaxy in a small scope tends to subtend a small angle. If you look straight at it it's appallingly dim and with averted vision its completely unresolved. Same object with a big scope subtends such a huge angle, that even with the poor resolution of one's peripheral vision, one sees structure and brightness at the same time. When I was young i used to imagine that the andromeda galaxy would be an awesome sight if we were much closer, but of course in reality it would be no more bright than our own Milky Way or the Magellanic clouds. I've often wondered what those look like to animals with excellent night vision like possums?

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