16 inch Telescope

[Tiny Small]

I used to live in Kingston, so not far from you. To get a reasonable view of the milkyway, I used to have to travel down to Headley or Newlands Corner on the road to Guildford.

[swamp thing]

4 minutes ago, Tiny Small said:

 

 I'm not disagreeing with you, although saying it's not quite right is itself, not quite right. In my later posts I elaborated on this. Perhaps I should have said that as far as the optic is concerned, an f5 scope is an f5 scope, as demonstrated by the use of cameras on scopes and, of course the human eye adds compounding factors and so a larger scope will show the eye more. Although now I am iterating what I said in posts further on in the thread.

Fair enough mate  

 

[spaceboy]

I think (and correct me if I'm wrong Tiny) it was meant as you can see the same things in the 10" as you can in the 16" ?? which I agree to some degree! The difference being is you can see them easier and in more detail in the 16". OK the 10" may take a lot more skill but you'll still see M1 - M110 in a 10" "as you would" a 16" scope. I think where the same comment falls short is when you move away from the more commonly known brighter DSO like the Messier and NGC to other catalogues which are solely reserved for light buckets and dark skies where a 10" just simply doesn't have the aperture to compete.

[Tiny Small]

2 minutes ago, spaceboy said:

I think (and correct me if I'm wrong Tiny) it was meant as you can see the same things in the 10" as you can in the 16" ?? which I agree to some degree! The difference being is you can see them easier and in more detail in the 16" OK the 10" may take a lot more skill but you'll still see M1 - M110 in a 10" "as you would" a 16" scope.

Pretty much. But only in terms of limiting magnitude. And assuming that every eye is the same... which it is not. The added detail and image scale in a bigger scope will reveal more, unquestionably.

And going back to the earlier point about aperture... its measured in fractions of focal length... so technically a 16" f5 dob and a 3" f5 refractor both have the same aperture.

[Scott]

8 minutes ago, Stu said:

I'm not sure fairly light polluted and being able to see the Milky Way clearly go together Scott. My skies are around mag 4.5 to 4.8 ish, and I never see it from home, perhaps the faintest hint on a very transparent and dark night. I'm a few miles inside the M25 and about 6 or 7 miles south of Heathrow, not great in other words!  

I know if I invited the dob mob for a session I'd be stoned to death. I can almost never see all of ursa minor (4.95) and can see it. (I'm not saying it's lucksall bright or every night, but it's not averted imagination either ) . I'll take a pic of my eastern horizon (Stoke) one day

 

[estwing]

6 minutes ago, Scott said:

I know if I invited the dob mob for a session I'd be stoned to death.

we dont do drugs Scott...

[Stub Mandrel]

36 minutes ago, Scott said:

I really don't wish to be a wet blanket but my garden is fairly light polluted and I regularly see the MW very clearly. count the stars you see in ursa minor (not major) and if you can see ALL the main stars easily without averted vision then I'd say your sky is on the good side. 40 miles from London I'd say you still get some light spillage . 

That's not light pollution! I can see Polaris and Kochab, that's all. Under deal conditions and fully dark adapted I can see a whisp of the MW when Cygnus is overhead. Now the streetlights lights are out I could just see the MW in Perseus the other night, but North and East is still yellow fug.

[cloudsweeper]

A very informative thread!  A bigger aperture will indeed give a larger image of an object of a certain surface brightness, but only up to a point since pollution limits contrast.  So the benefit of a large aperture is only felt at  a dark site. BUT if we're talking brighter objects rather than just faint galaxies, then larger aperture is still a better experience.

I hope.

Doug.

[Chris]

1 hour ago, swamp thing said:

This is not quite right buddy.

I understand your thinking and the logic behind it however you are forgetting we are not using a camera, we are using our eye. The human eye sees objects better that are big and bright. Once objects get faint and small it struggles. Remember that no extended object will ever have more surface brightness than when it is seen with the naked eye alone. The reason we cannot see DSO's all over the place when we look up is...........

Image scale. 

The bigger the scope. The larger the image scale at every given exit pupil. This is why we will see more objects through a 16" f/5 than a 10" f/5 

Nice explanation, I've learnt something

[Stu]

47 minutes ago, Tiny Small said:

Pretty much. But only in terms of limiting magnitude. And assuming that every eye is the same... which it is not. The added detail and image scale in a bigger scope will reveal more, unquestionably.

And going back to the earlier point about aperture... its measured in fractions of focal length... so technically a 16" f5 dob and a 3" f5 refractor both have the same aperture.

I don't think I agree with this. It may be relevant for imaging, but what is important for is aperture, magnification and exit pupil.

An f5 scope may give the same exit pupil with the same eye piece, but the results speak for themselves in terms of image scale and brightness. The larger scope will give the same brightness at much larger image scale, or a brighter image at the same image scale.

[Timebandit]

1 hour ago, Tiny Small said:

Pretty much. But only in terms of limiting magnitude. And assuming that every eye is the same... which it is not. The added detail and image scale in a bigger scope will reveal more, unquestionably.

And going back to the earlier point about aperture... its measured in fractions of focal length... so technically a 16" f5 dob and a 3" f5 refractor both have the same aperture.

 

Sorry think I must disagree again ?

If aperture in a telescope is the diameter of its main light gathering mirror(or glass) 

How can a 16" and a 3"  both have the same aperture?(where is my Astro wiz kid). By my reckoning the 16 has 13 more inches than the 3"

And in practice my 4"apo can not touch my 14" dob on Dso. The 14"dob can resolve dso that the frac cannot even pick up. 

IMO aperture rules in DSO and I cannot see the Dob Mob chaps trading in their  20"   cannons for 3"  because of some of the posts on here. I certainly will not as in my experience aperture gets results when smaller aperture can not even pick up targets. Just my opinion ☺

[Tiny Small]

22 minutes ago, Stu said:

I don't think I agree with this. It may be relevant for imaging, but what is important for is aperture, magnification and exit pupil.

An f5 scope may give the same exit pupil with the same eye piece, but the results speak for themselves in terms of image scale and brightness. The larger scope will give the same brightness at much larger image scale, or a brighter image at the same image scale.

Whilst you have quoted one reply there, this is pretty much the argument I have been making... you just achieved it more succinctly. My whole point is that it will give you the same brightness with larger scale.

[Tiny Small]

15 minutes ago, Timebandit said:

 

Sorry think I must disagree again ?

If aperture in a telescope is the diameter of its main light gathering mirror(or glass) 

How can a 16" and a 3"  both have the same aperture?(where is my Astro wiz kid). By my reckoning the 16 has 13 more inches than the 3"

And in practice my 4"apo can not touch my 14" dob on Dso. The 14"dob can resolve dso that the frac cannot even pick up. 

IMO aperture rules in DSO and I cannot see the Dob Mob chaps trading in their  20"   cannons for 10" Dobs because of some of the posts on here. I certainly will not as in my experience aperture gets results when smaller aperture can not even pick up targets. Just my opinion ☺

Aperture is defined by focal length. An aperture is measured as a fraction of focal length: F stop: F ratio. It is dependant on objective size AND focal length. A telescope with an f5 focal ratio has an aperture defined by it's focal length and no matter the focal length, the object will be the same relative size. You may disagree with this, but in this instance you would be wrong. A bigger objective does not necessarily mean a bigger aperture, though loosely, aperture is used with reference to the size of a telescope's objective alone. This is an incorrect use of the word however.

I think that you are misunderstanding what I am saying... a 16" scope can and does collect more light than a 3" scope. This is self evident. If they are both f5, then they have the same focal ratio and as such, the objective is the same relative size. This has a direct consequence of limiting both scopes to a theoretical limiting magnitude that is the same for both scopes (ignoring the effect the eye has on the system... I'm talking about quantitative measurements of brightness magnitude)... HOWEVER... the larger scope will display the SAME brightness as the small scope at a much larger scale. If you were to try to increase the scale in the smaller scope, you have to do it by magnification which increases the theoretical focal length and therefore reduces the aperture, resulting in a dimmer image.

[Timebandit]

2 minutes ago, Tiny Small said:

Aperture is defined by focal length. An aperture is measured as a fraction of focal length: F stop: F ratio. It is dependant on objective size AND focal length. A telescope with an f5 focal ratio has an aperture defined by it's focal length and no matter the focal length, the object will be the same relative size. You may disagree with this, but in this instance you would be wrong. A bigger objective does not necessarily mean a bigger aperture, though loosely, aperture is used with reference to the size of a telescope's objective. This is an incorrect use of the word however.

I think that you are misunderstanding what I am saying... a 16" scope can and does collect more light than a 3" scope. This is self evident. If they are both f5, then they have the same focal ratio and as such, the objective is the same relative size. This has a direct consequence of limiting both scopes to a theoretical limiting magnitude that is the same for both scopes (ignoring the effect the eye has on the system... I'm talking about quantitative measurements of brightness magnitude)... HOWEVER... the larger scope will display the SAME brightness as the small scope at a much larger scale. If you were to try to increase the scale in the smaller scope, you have to do it by magnification which increases the theoretical focal length and therefore reduces the aperture, resulting in a dimmer image.

 

I may be wrong about the meaning of" aperture "when it comes to telescope, but everything I have read defines aperture in telescope as" the diameter of its main gathering mirror (or glass) . So there must be an awfully lot of wrong persons out there writing articles on such matters.  

As I have said my 4" apo will not compete with the 14" Dob on faint fuzzy. Earlier there was the matter of the 10" showing you the same as a 16" . The same condition under the same dark site aperture will resolve more detail and targets than a smaller aperture. We can go back and forward on this. But in my opinion and from my experience aperture with all other matters the same will win.

A good big telescope will always show more than a good small telescope IMO  

 

      

[cloudsweeper]

4 minutes ago, Timebandit said:

 

A good big telescope will always show more than a good small telescope IMO  

 

      

........especially in dark skies.  But anyway, it will show them at higher mag.

Doug.

[swamp thing]

17 minutes ago, Tiny Small said:

Aperture is defined by focal length. An aperture is measured as a fraction of focal length: F stop: F ratio. It is dependant on objective size AND focal length. A telescope with an f5 focal ratio has an aperture defined by it's focal length and no matter the focal length, the object will be the same relative size. You may disagree with this, but in this instance you would be wrong. A bigger objective does not necessarily mean a bigger aperture, though loosely, aperture is used with reference to the size of a telescope's objective. This is an incorrect use of the word however.

 

It's is not an incorrect use of the word. According to my dictionary both terms are correct. The astronomers use of the term aperture is not wrong and neither is the photographers.  The Aperture that astronomers refer to is the size of the opening the light goes through. This is an aperture according to my dictionary.
When using an astronomy forum I tend to use the astronomers way. It just saves a whole lot of confusion buddy  If you ask astronomers what aperture scope they are using they wont reply "2.8" any more than a tog would say "a 4 apo" when asked about t his long lens. 

[Tiny Small]

6 minutes ago, Timebandit said:

 

I may be wrong about the meaning of" aperture "when it comes to telescope, but everything I have read defines aperture in telescope as" the diameter of its main gathering mirror (or glass) .

 

      

Telescope aperture is measured in exactly the same way as with a lens... fraction of focal length. When scopes are advertised, they advertise objective diameter and focal length... not aperture.

[Tiny Small]

21 minutes ago, swamp thing said:

It's is not an incorrect use of the word. According to my dictionary both terms are correct. The astronomers use of the term aperture is not wrong and neither is the photographers.  The Aperture that astronomers refer to is the size of the opening the light goes through. This is an aperture according to my dictionary.
When using an astronomy forum I tend to use the astronomers way. It just saves a whole lot of confusion buddy  If you ask astronomers what aperture scope they are using they wont reply "2.8" any more than a tog would say "a 4 apo" when asked about t his long lens. 

There is no confusion. This is entirely correct and what I have been saying all along. The aperture is not the same as objective size. It is the size of the hole...

which just happens to be the same size as the objective.

[Tiny Small]

I think that people are getting a bit confused because it's a dob we're talking about. In a frac, it is literally a hole at the front of the scope, but with a dob, because the mirror is at the back and reflects the light, people are mistakenly referring to the mirror size as the aperture. The aperture is still the hole at the front.

We seem to be high jacking the thread so maybe getting back on to the OP's post. With a 16" dob and the right skies... you will see a hell of a lot.

[niallk]

Hi to the OP,

Best of luck with the 16" 

The only thing I'd strongly recommend is choosing a scope with easy tool-less assembly/ disembly in the dark, and that breaks down to manageable sections for transport. I think this is very important to ensure use ... if it is a chore you come to dread then it will fall out of favour.

You will just love how globs, general star colours, and planets (no 'zzzzz'!  ) look in the increased aperture.

With a transportable dob, getting out to dark skies around the new moon if the weather is good will be an amazing experience - unleashing the beast on DSOs and getting the benefit of increased image scale.

As for a 16" being the last scope you'll ever need to buy ... well after a week of getting my 15" (love it), I think I was already stealing glances at 20"-22" scopes on line  It's very important to have dreams!

[spaceboy]

45 minutes ago, Tiny Small said:

When scopes are advertised, they advertise objective diameter and focal length... not aperture.

This is a good point but as Steve pointed out it is just the norm to refer to it as aperture amongst astronomers. You are though correct as far as my head can get around it, to what you are saying. It seems to be a common thing in astronomy to quote photographic reference which I assume originated from using lenses in refractors ? But in truth it is not always accurate to do so just as it wouldn't be accurate to say a camera lens is a f/2.8 based on the size of its lens objective over focal length. (hope that's correct ?)

 

As you say though the thread has become somewhat distracted and I think the OP has gone to rest his spinning head after this thread

[Pippy]

If aperture don't matter, we wouldn't need it, but being as we do need it, aperture must matter

[Stub Mandrel]

The photographers are wrong. What they call aperture is f-ratio   (= focal ratio = ratio of focal length to aperture).

[swamp thing]

1 hour ago, Tiny Small said:

I think that people are getting a bit confused because it's a dob we're talking about. In a frac, it is literally a hole at the front of the scope, but with a dob, because the mirror is at the back and reflects the light, people are mistakenly referring to the mirror size as the aperture. The aperture is still the hole at the front.

 

Not quite. According to my dictionary.

The aperture refers to the hole through which light passes. The aperture stop on a telescope is taken to be edge of the primary objective not the hole in the tube. So the aperture is the diameter of the primary mirror or lens. 

Sorry I did not explain this very well in my last post.  

[Moonshane]

One thing a larger aperture (I.e. A bigger minor or objective) does give is more resolution.