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Used 10" Dobs are fairly rare, eh?


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On 3/23/2017 at 08:19, JOC said:

 as a non-owner I assumed that the bigger the scope the bigger the view of the object you saw

You were correct in your assumption.  
This is the exact reason we use larger scopes.  

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1 hour ago, swamp thing said:

You were correct in your assumption.  
This is the exact reason we use larger scopes.

Yes, I get that the bigger the scope ultimately the bigger the view of the object must be, but it seems to me that in real terms this difference is secondary to the EP that you use - If I put my 10mm Plossl in a 8" Skywatcher Skyliner 1200mm F6 and a 12" 1500mm (f/4.9) the apparent size of Jupiter isn't going to be much changed is it? 

As far as I understand the maths I'm going to get 120X magnification out of the 8" and 150x magnification out of the 12" with the same 10mm EP - a difference that sounds scarcely noticeable given the size of the object to begin with. 

Yet if I multiply up with the EP and go to a 6mm I get 200x out of the 8" and 250x out of the 12" again possibly not hugely different to the eye.

However, looking at the EP changes, at 10mm I get 120X magnification out of the 8" with the 6mm we get 200X out of the 8" (with the 12" its 150X out of the 10mm and 250x out of the 6mm).  Therefore, it is not necessarily the aperture that makes the images bigger, the biggest differences seem to come from the EP's and before I got a telescope I didn't realise this - I didn't even realise that you needed separate EP's until I logged onto SGL.  It is quite possible that the person using the biggest mirror with a larger mm EP might be seeing an image literally no bigger than the smaller mirror owner with a smaller mm EP.

Hence the comment:

as a non-owner I assumed that the bigger the scope the bigger the view of the object you saw - this was why I got the biggest I could.  As I understand things yes, someone might get more light from a 10" version of what I have, but IMO the same persons overall view on many objects wouldn't be hugely different to an 8"  - at least as I understand things.[\quote]

The EP's seem much more important if you are looking at magnification than the actual size of your telescope.  At least this is my current thinking.....

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4 minutes ago, JOC said:

 It is quite possible that the person using the biggest mirror with a larger mm EP might be seeing an image literally no bigger than the smaller mirror owner with a smaller mm EP.

No bigger maybe but brighter and with more detail, and at greater viewing comfort (more eye relief from longer EP)

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13 minutes ago, CraigT82 said:

No bigger maybe but brighter and with more detail, and at greater viewing comfort (more eye relief from longer EP)

Yes, that makes perfect sense - bigger mirrors more light and detail gathered :-D

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28 minutes ago, CraigT82 said:

No bigger maybe but brighter and with more detail, and at greater viewing comfort (more eye relief from longer EP)

 

14 minutes ago, JOC said:

Yes, that makes perfect sense - bigger mirrors more light and detail gathered :-D

bigger mirror don't make the image brighter, they increase the image scale, not the brightness.

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51 minutes ago, estwing said:

 

bigger mirror don't make the image brighter, they increase the image scale, not the brightness.

If using different EPs in each scope to give the same magnification, which is what JOC was kind of talking about, the bigger mirror would give the brighter image would it not? 

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I get so frustrated by posts about this sort of thing!

Three variables:

  • Focal length (FL)
  • Eyepiece focal length (EP)
  • Aperture

Keep two of the three the same but:

  • Increase FL and the image gets darker and greater magnification.
  • Increase EP and the image gets brighter, but less magnification.
  • Increase aperture and the image gets brighter but magnification stays the same.

Focal Ratio is the ratio of FL to Aperture and is fixed for a certain scope.

  • You can only vary the focal length of the eyepiece (EP)

Within limits, two scopes of different focal length can give the same magnification by using different focal length eyepieces.

Two scopes of the same aperture set up with eyepieces to give the same magnification will have the same brightness regardless of their focal length.

5 minutes ago, estwing said:

Think of it this way....a 40w bulb at 10ft away is still a 40w bulb 2ft away it's still giving off the same light but now it's closer

But the image of the bulb on your retina is spread out as the 40W bulb is not a point source like a star. This means the much greater (25x) light is spread over much greater area (25x) so it doesn't look brighter.

Take a star at 10 light years and move it to 2 light years, the amount of light is 25 times greater amount of light still only affects the same time spot on your retina, so it looks 25 times brighter.

 

With a scope, higher magnification has the opposite effect (as the light collected by the aperture remains constant) now magnification means a nebula covers a greater area, so it becomes less bright. On the other hand stars remain points and just look further apart. A bigger aperture and both stars and nebula look brighter.

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1 hour ago, JOC said:

Yes, I get that the bigger the scope ultimately the bigger the view of the object must be, but it seems to me that in real terms this difference is secondary to the EP that you use - If I put my 10mm Plossl in a 8" Skywatcher Skyliner 1200mm F6 and a 12" 1500mm (f/4.9) the apparent size of Jupiter isn't going to be much changed is it? 

As far as I understand the maths I'm going to get 120X magnification out of the 8" and 150x magnification out of the 12" with the same 10mm EP - a difference that sounds scarcely noticeable given the size of the object to begin with. 

Yet if I multiply up with the EP and go to a 6mm I get 200x out of the 8" and 250x out of the 12" again possibly not hugely different to the eye.

However, looking at the EP changes, at 10mm I get 120X magnification out of the 8" with the 6mm we get 200X out of the 8" (with the 12" its 150X out of the 10mm and 250x out of the 6mm).  Therefore, it is not necessarily the aperture that makes the images bigger, the biggest differences seem to come from the EP's and before I got a telescope I didn't realise this - I didn't even realise that you needed separate EP's until I logged onto SGL.  It is quite possible that the person using the biggest mirror with a larger mm EP might be seeing an image literally no bigger than the smaller mirror owner with a smaller mm EP.

Hence the comment:

 

As the guys have mentioned, the assumption here is wrong. The bigger dobs will maintain surface brightness whilst showing a larger image. They can't make things brighter, but do make objects easier to see by giving the larger image. Your eye is better able to detect the contrast and detail at larger size.

This was quite evident to me at the weekend, observing side by side with a 4" Tak and C925. The view in the 925 was  not necessarily bright, but the objects were dramatically bigger and easier to see.

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24 minutes ago, estwing said:

Think of it this way....a 40w bulb at 10ft away is still a 40w bulb 2ft away it's still giving off the same light but now it's closer

I prefer to think of it like this:

If you look at a 40w bulb with your eyes squeezed tight, the image is dim. If you look at a 40w bulb with your eyes wide open, the image is bright. The bulb output doesn't change but our perception of that output does.

The truth is... There is no spoon. 

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It's easy to get hung up on the size of the image in the eyepiece, ie: the magnification. What I've come to value from the larger aperture scopes is the richness of the image in terms of detail. Features that you have to work hard to see with smaller apertures become more obvious with larger ones and you then set forth on trying to see new levels of detail in those targets.

Well, thats the way that I've found it anyway :smiley:

 

 

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9 hours ago, John said:

It's easy to get hung up on the size of the image in the eyepiece, ie: the magnification. What I've come to value from the larger aperture scopes is the richness of the image in terms of detail. Features that you have to work hard to see with smaller apertures become more obvious with larger ones and you then set forth on trying to see new levels of detail in those targets.

Well, thats the way that I've found it anyway :smiley:

 

 

I think that certainly applies to planetary and lunar observing John, and getting the right balance of magnification applies in any scope. It's just the bigger scopes allow you to see small faint objects so much easier by magnifying them whilst maintaining the brightness. The big dob boys are chasing fainter and fainter small galaxies, whereas I guess we are normally looking for more detail in brighter targets?

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34 minutes ago, Stu said:

As the guys have mentioned, the assumption here is wrong. The bigger dobs will maintain surface brightness whilst showing a larger image. They can't make things brighter, but do make objects easier to see by giving the larger image. Your eye is better able to detect the contrast and detail at larger size.

This was quite evident to me at the weekend, observing side by side with a 4" Tak and C925. The view in the 925 was  not necessarily bright, but the objects were dramatically bigger and easier to see.

Yes, that's because the increased magnification given by the C925's focal length of nearly 2.5 metres offsets the increased aperture. If you put a really long EP in the C925 so it had the same magnification as the Tak, then the image would be much brighter. Or put a really short EP+barlow in the Tak to get greater magnification and the image will become much dimmer.

That's why i emphasised that to understand how changing one variable has an effect you must keep the variables the same.

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If it's any consolation, I find this hard to get a handle on as well :dontknow:

And I've been in the hobby for 30+ years !

I've owned and compared loads of scopes so I've a lot of experience of the differences I've seen between them but understanding and explaining the "whys" always leaves me more in the dark (ha ha !) than it ought to :rolleyes2:

Fortunately there are members here who do understand and can explain much better than I can :smiley:

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2 hours ago, Stub Mandrel said:

Yes, that's because the increased magnification given by the C925's focal length of nearly 2.5 metres offsets the increased aperture. If you put a really long EP in the C925 so it had the same magnification as the Tak, then the image would be much brighter. Or put a really short EP+barlow in the Tak to get greater magnification and the image will become much dimmer.

That's why i emphasised that to understand how changing one variable has an effect you must keep the variables the same.

Fully aware of that Neil.

Ultimately this comes down to that little factor called exit pupil. John will tell me off for going down this route, but at the same exit pupil the images will have the same surface brightness but different scales.

Roughly speaking, the Tak with a 24mm Panoptic has the same exit pupil (3mm) as the C925 with a 30mm ES. With these combinations you have x31 in the Tak and x78 in the C925. Same brightness but more than double the image scale in the C925.

M42 in the C925 was spectacular, but then the Double Double and M44 were stunning in the Tak. Horses for courses.

Sorry if this is off topic! :(

Back to the 10" question....

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Stu has put it very clearly, the brightness of extended DSO depends solely on the exit pupil size, larger aperture gives only larger image scale at the same brightness(=exit pupil), this larger image scale gives more detail in our retina.

A practical example is comet 41P lately dicussed in the observation section, there were no big differences when observing with 15x70 binoculars than 120ED, or even 12" dobs, simply because people with a scope, more than often, used too high power (=small exit pupil=dimmer image), they mostly ended only seen the brighter core of the comet, the fainter outer parts of the comet just disappeared under the higher magnification.

On the other hand, if one wanted to see both 41P and M108, then higher power was need under ordinary backyard sky, because M108 needed some more magnification to be seen. Our eyes can only see extended DSO of certain brightness (not too dim) AND a certain size(not too small).

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52 minutes ago, YKSE said:

Stu has put it very clearly, the brightness of extended DSO depends solely on the exit pupil size, larger aperture gives only larger image scale at the same brightness(=exit pupil), this larger image scale gives more detail in our retina.

A practical example is comet 41P lately dicussed in the observation section, there were no big differences when observing with 15x70 binoculars than 120ED, or even 12" dobs, simply because people with a scope, more than often, used too high power (=small exit pupil=dimmer image), they mostly ended only seen the brighter core of the comet, the fainter outer parts of the comet just disappeared under the higher magnification.

On the other hand, if one wanted to see both 41P and M108, then higher power was need under ordinary backyard sky, because M108 needed some more magnification to be seen. Our eyes can only see extended DSO of certain brightness (not too dim) AND a certain size(not too small).

That makes some sense, I think :icon_scratch:

When I observed comet 41P, M108 and M97 togther in the sky a few days back, I used my 12" F/5.3 dobsonian. With the 31mm Nagler (51x, 5.85mm exit pupil) the 3 objects could be seen but were not exactly jumping out at me. With the 21mm Ethos (76x, 3.96mm exit pupil) the objects were more distinctly defined (I've deliberately not used the term brighter) against the background sky although the amount of sky shown (the true field) had decreased a little. The objects also appeared larger because of the additional magnification of course.

I've been searching around for further information on the topic of exit pupils, magnification, aperture and observing deep sky objects and I've come across the website below which, despite it's slightly odd title, I think provides some very interesting information. I'm going to study it in some detail and hopefully my brain won't melt :smiley:

http://www.clarkvision.com/visastro/omva1/index.html

 

 

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Yes, Clarvision is an excellent guide:thumbsup:, a favorite of mine when I started out some years ago. a few data might be not very up-to-date, an great fundmental understanding to DSO hunting nonetheless:smiley:

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6 hours ago, John said:

That makes some sense, I think :icon_scratch:

When I observed comet 41P, M108 and M97 togther in the sky a few days back, I used my 12" F/5.3 dobsonian. With the 31mm Nagler (51x, 5.85mm exit pupil) the 3 objects could be seen but were not exactly jumping out at me. With the 21mm Ethos (76x, 3.96mm exit pupil) the objects were more distinctly defined (I've deliberately not used the term brighter) against the background sky although the amount of sky shown (the true field) had decreased a little. The objects also appeared larger because of the additional magnification of course.

I've been searching around for further information on the topic of exit pupils, magnification, aperture and observing deep sky objects and I've come across the website below which, despite it's slightly odd title, I think provides some very interesting information. I'm going to study it in some detail and hopefully my brain won't melt :smiley:

http://www.clarkvision.com/visastro/omva1/index.html

 

 

At DIYPSP the skies were around magnitude 20.5 on an SQM, and the transparency was not brilliant. On extended objects, applying more power darkens the sky backgraound but it also darkens the object itself. By making it larger however, the visibility is improved.

I found (confirmed what I knew) that at lower powers in the C925 i.e. With the 40mm giving x59 and exit pupil 4mm, the sky background was quite bright and faint objects quite indistinct. The only time I really used this was when trying to view the Leo triplet in the same fov.

Upping the power to x118 with the 20mm, I had a 2mm exit pupil which is renowned for giving best contrast on fainter DSOs, and sure enough, darker background, larger object and easier detection. It's sometimes a game of experimentation though, trying different mags to see just where the point of maximum visibility is.

I can't wait to get the C925 under a properly dark and transparent sky, it will perform beautifully I'm sure.

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8 hours ago, John said:

When I observed comet 41P, M108 and M97 togther in the sky a few days back, I used my 12" F/5.3 dobsonian. With the 31mm Nagler (51x, 5.85mm exit pupil) the 3 objects could be seen but were not exactly jumping out at me. With the 21mm Ethos (76x, 3.96mm exit pupil) the objects were more distinctly defined (I've deliberately not used the term brighter) against the background sky although the amount of sky shown (the true field) had decreased a little. The objects also appeared larger because of the additional magnification of course

Apologies to the 10" vs 8" notion of this threa, but I find John's comment above really interesting - I'm still not 100% certain that I understand the 'why', but it is interesting that it would appear that some objects are easier to see at lower apparent magnification.

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