8" and 14" Telescopes not much difference?

[MarsG76]

13 hours ago, cloudsweeper said:

Good point Stu.  This theme is one that seems to polarise opinion.

A bigger aperture gives you more mag for the same SB.

Some then say this makes the extended object seem brighter just because it is larger, and contrast plays a part in this.

Others further say that since the image occupies a bigger area, then the total or integrated brightness increases, which is after all to be expected since greater light flux has been captured.

Hmmmm........

Doug.

 

But through a telescope you can see stars that a much fainter than with the naked eye, ie 6 mag vs the neutron star n the middle of the helix nebula that mag 13.5, so surely the scope does make certain objects brighter... pin points of light, perhaps.

 

[cloudsweeper]

17 minutes ago, MarsG76 said:

But through a telescope you can see stars that a much fainter than with the naked eye, ie 6 mag vs the neutron star n the middle of the helix nebula that mag 13.5, so surely the scope does make certain objects brighter... pin points of light, perhaps.

 

Definitely so in the case of point sources, brightness being a function of aperture.

The situation with extended objects however is different.  (And has been known to promote lively debate!)

Doug.

 

[estwing]

Not brighter just possible to see.....

[MarsG76]

1 minute ago, estwing said:

Not brighter just possible to see.....

Seems strange... bigger objective/mirror but the brightness is the same. I know it's must be true, that's why terrestrial objects are no brighter than normal when viewed through a telescope, but still more and fainter detail/objects are visible... so I thought that perhaps point sources are the exception.

 

[Stu]

39 minutes ago, MarsG76 said:

But through a telescope you can see stars that a much fainter than with the naked eye, ie 6 mag vs the neutron star n the middle of the helix nebula that mag 13.5, so surely the scope does make certain objects brighter... pin points of light, perhaps.

 

Totally. As Doug says, point sources behave differently from extended sources as effectively they do not spread out as you magnify them

[MarsG76]

1 minute ago, Stu said:

Totally. As Doug says, point sources behave differently from extended sources as effectively they do not spread out as you magnify them

Yeah, that does make sense.

 

[kev]

On planetary you are going to be seeing limited, especially with visual.  Having said that, i found that the extra light from a 14 inch made details on planets much easier and colours really easy to see

 

[MarsG76]

1 hour ago, kev said:

On planetary you are going to be seeing limited, especially with visual.  Having said that, i found that the extra light from a 14 inch made details on planets much easier and colours really easy to see

 

I'm starting to realize that... still last time I was observing I thought I was close, 450X was very good, albeit with a bit of a vibration due to the astigmatism (I assume). I think once I sort out the astigmatism issue I'll have some amazing views of planets, but until than and during the less than perfect nights, the 8SE will be the weapon of choice, leaving the 14 for DSO.

 

[YKSE]

As Stu and others have pointed out, the brightness of the extended DSO doesn't increase with larger aperture, compare to a samall scope, it's the bigger image size at the same brigtness (assuming same exit pupil) making the smaller details visible, as the anology here.

 

[MarsG76]

Back to the astigmatism issue... I'm sitting here at work, wasting a perfect observation night or sorting out my astigmatism time, thinking about the oval defocussed astigmatised stars...

Could rotation in the secondary mirror cause the "astigmatism effect"... thinking about it, the secondary mirror can be slightly tilted and rotated away from the perfect 45 degree in one plane but still reflect the primary and look collimated. Eventhough it can look like it's collimated but the slight rotation on the axis could bounce the primary mirror reflected light slightly squashed? Thoughts?

 

[MarsG76]

53 minutes ago, YKSE said:

As Stu and others have pointed out, the brightness of the extended DSO doesn't increase with larger aperture, compare to a samall scope, it's the bigger image size at the same brigtness (assuming same exit pupil) making the smaller details visible, as the anology here.

 

Good analogy. That makes perfect sense.

 

[Peter Drew]

1 hour ago, MarsG76 said:

Back to the astigmatism issue... I'm sitting here at work, wasting a perfect observation night or sorting out my astigmatism time, thinking about the oval defocussed astigmatised stars...

Could rotation in the secondary mirror cause the "astigmatism effect"... thinking about it, the secondary mirror can be slightly tilted and rotated away from the perfect 45 degree in one plane but still reflect the primary and look collimated. Eventhough it can look like it's collimated but the slight rotation on the axis could bounce the primary mirror reflected light slightly squashed? Thoughts?

 

Rotaion of the secondary about its mount axis usually results in on axis coma.  

[MarsG76]

13 hours ago, Peter Drew said:

Rotaion of the secondary about its mount axis usually results in on axis coma.  

Right.... what about cleaning the secondary where there would be microscopic streaks left on it in straight parallel lines?

[MarsG76]

13 hours ago, Peter Drew said:

Rotaion of the secondary about its mount axis usually results in on axis coma.  

I'm trying to think back what I did that might have caused the astigmatism since skywatcher has a good reputation so I doubt they would have a astigmatism problem be mirror.

[Peter Drew]

Streaks and scratches on mirrors will cause scatter rather than other obvious effects. Astigmatism is often caused by the manufacturers, or the owners after cleaning, tightening the primary mirrors too tightly. However the mirror is held it must be free enough to rotate or move with light finger pressure. 

[MarsG76]

15 minutes ago, Peter Drew said:

Streaks and scratches on mirrors will cause scatter rather than other obvious effects. Astigmatism is often caused by the manufacturers, or the owners after cleaning, tightening the primary mirrors too tightly. However the mirror is held it must be free enough to rotate or move with light finger pressure. 

That's what is a bit worrying, did I introduce the astigmatism?? 

[Peter Drew]

1 hour ago, MarsG76 said:

That's what is a bit worrying, did I introduce the astigmatism?? 

If you did, no worries, it's not permanent and the situation is easily reversed.  

[MarsG76]

Just now, Peter Drew said:

If you did, no worries, it's not permanent and the situation is easily reversed.  

certainly hope so

 

[Charic]

On 04/08/2017 at 06:26, MarsG76 said:

But through a telescope you can see stars that a much fainter than with the naked eye, ie 6 mag vs the neutron star n the middle of the helix nebula that mag 13.5, so surely the scope does make certain objects brighter... pin points of light, perhaps.

 

Can I add to what some folk have already stated!

The function of the telescope is to gather photons (light) and bring them to focus, allowing the image to be studied by the operator using an eyepiece.
The Telescopes total brightness or its light grasp is based on the size of its aperture, and the larger the aperture, the greater the light grasp.

That said, its the telescopes magnification which determines the  surface  brightness of whatever your studying, which is why you don't always see things brighter, even though they  appear brighter..........no wonder confusion abounds?

The reason!...........Everything up there has its own surface brightness, lets say that's level 1. This is also the level you would see with the naked eye alone, if the target was in range. 

Now although the eye and your mind appears to think that the extended source your looking at is much brighter, the fact is, when you magnify anything through the  telescope, not only does the image appear closer, but  due to any increase in magnification beyond the level of your own eye, the photons are affected too, by spreading them further apart, in effect,  diluting their brightness,, so you now have a situation where the surface brightness of whatever your looking at is less than the original level 1. This is the reason that they say 'no telescope can increase the surface brightness beyond the level of brightness beyond your own naked eye view. It may appear brighter, but physically its not possible.

Simple really! No its not.......it does take a while to fully understand some of the many aspects within our hobby, especially subjects like this one.

 

[estwing]

There should be a sticky on this subject...

[MarsG76]

18 hours ago, Charic said:

Can I add to what some folk have already stated!

The function of the telescope is to gather photons (light) and bring them to focus, allowing the image to be studied by the operator using an eyepiece.
The Telescopes total brightness or its light grasp is based on the size of its aperture, and the larger the aperture, the greater the light grasp.

That said, its the telescopes magnification which determines the  surface  brightness of whatever your studying, which is why you don't always see things brighter, even though they  appear brighter..........no wonder confusion abounds?

The reason!...........Everything up there has its own surface brightness, lets say that's level 1. This is also the level you would see with the naked eye alone, if the target was in range. 

Now although the eye and your mind appears to think that the extended source your looking at is much brighter, the fact is, when you magnify anything through the  telescope, not only does the image appear closer, but  due to any increase in magnification beyond the level of your own eye, the photons are affected too, by spreading them further apart, in effect,  diluting their brightness,, so you now have a situation where the surface brightness of whatever your looking at is less than the original level 1. This is the reason that they say 'no telescope can increase the surface brightness beyond the level of brightness beyond your own naked eye view. It may appear brighter, but physically its not possible.

Simple really! No its not.......it does take a while to fully understand some of the many aspects within our hobby, especially subjects like this one.

 

Yeah, what? huh...

No... I get it....

I think

 

sort of

 

[MarsG76]

15 hours ago, estwing said:

There should be a sticky on this subject...

What is a "sticky"?

[Charic]

12 hours ago, MarsG76 said:

What is a "sticky"?

It's where a post is 'Stuck' or to be more precise 'pinned' to the top level of a forum page.
Look at the start of some of the forum pages, the green 'pins' are stickies. Their pinned for various reasons, but there at the forefront of all the other threads for that forum, making them easier to find, but quite often, its because the advice is sought after more than once, and that particular sticky will have enough answers to cover the topic in question.

Even so, a sticky can still confuse, so don't be afraid to ask, if in doubt, give a shout!

 

[MarsG76]

15 minutes ago, Charic said:

It's where a post is 'Stuck' or to be more precise 'pinned' to the top level of any forum page you view.
Look at the start of each page here the SGL, The green 'pins' are stickies. Their pinned for various reasons, most of them informative/helpful.

 

Right, Of course... I'll give it a go

 

[Charic]

Not every forum has a sticky, their special! and their only pinned by the team! The Administrators