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Used 10" Dobs are fairly rare, eh?


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I've found no matter what the base level, there is an improvement in either direction but with conflictions? Go lower and the image can be sharper but smaller, yet go higher and the image does get larger but less detail. 

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8 minutes ago, JOC said:

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.

I may be wrong, but I believe John's point above is the opposite i.e. The objects were easier to see at higher magnification?

I find these two sites useful, particularly the second one. The first allows you to calculate approximate (i.e. average) surface brightness for objects of different shapes. The second one helps understand object visibility based upon telescope, sky brightness, exit pupil and object surface brightness. Have a play around with it, it's very interesting.

http://www.users.on.net/~dbenn/ECMAScript/surface_brightness.html

http://www.bbastrodesigns.com/VisualDetectionCalculator.htm

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Exit pupil and magnification are inextricably linked. Double eyepiece focal length and magnification halves and exit pupil doubles -  all other things being equal.

That means it is meaningless to claim one and not the other is the reason behind any change in the view.

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14 minutes ago, Stu said:

I may be wrong, but I believe John's point above is the opposite i.e. The objects were easier to see at higher magnification?....

Yes, on this occasion, the higher magnification provided clearer views of these objects. That is generally what I find when using my 12" F/5.3 scope from my back yard which is where I do most of my observing from.

If the sky is really dark (which here is around NELM 5.5 or so) I find the lowest magnification more useful.

 

 

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4 minutes ago, John said:

If the sky is really dark (which here is around NELM 5.5 or so) I find the lowest magnification more useful.

Agreed, although at the star party the SQM reading was 20.5 at best, supposedly around 5.8 NELM. Because the transparency was poor I still found higher mag was better to cut down the sky brightness.

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56 minutes ago, Stub Mandrel said:

Exit pupil and magnification are inextricably linked. Double eyepiece focal length and magnification halves and exit pupil doubles -  all other things being equal.

That means it is meaningless to claim one and not the other is the reason behind any change in the view.

I wasn't claiming anything - just reporting the observation :smiley:

 

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9 hours ago, John said:

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

 

Acey's paper is another great source for the same subject, suitable exit pupil and magnification

https://arxiv.org/pdf/1405.4209v1.pdf

Heres's one of the figures:

suitable_exitpupil_mag.jpg.1542b337d0b2a7d6780d8e98020a0104.jpg

The DSO has the size about 10^0.7 arcmin^2, i.e. about 5 arcmin^2, or 2.5"x2", it'll be visible with that 6" scope around 75x, but invisible in 20x because of too small size(too small magnification), and invisible in 200x because of too dim image(=too small exit pupil):wink:

 

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Acey's paper has all the information that I am am looking for but It is too in depth for me to comprehend.

I have a 10" aND 16" reflector and would like to know how to determine the conditions that one would be better than another.

Can someone summarize it in 3 paragraphs?:icon_biggrin:

 

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14 minutes ago, Singlin said:

Acey's paper has all the information that I am am looking for but It is too in depth for me to comprehend.

I have a 10" aND 16" reflector and would like to know how to determine the conditions that one would be better than another.

Can someone summarize it in 3 paragraphs?:icon_biggrin:

 

For big objects use the small scope and for small objects use the big scope. ;) 

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26 minutes ago, Singlin said:

Acey's paper has all the information that I am am looking for but It is too in depth for me to comprehend.

I have a 10" aND 16" reflector and would like to know how to determine the conditions that one would be better than another.

Can someone summarize it in 3 paragraphs?:icon_biggrin:

 

If we are talking about nebulae and galaxies, I think you can assume that the 16" will always be better, unless the object will not fit in the field of view in which case the 10" is better (that's a slightly longer way of repeating what Steve said ;))

It will then be a matter of working out the optimum eyepiece to use in the 16" to get best perceived contrast.

The calculator I linked to in my previous post is very useful for this.

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2 hours ago, Stub Mandrel said:

Exit pupil and magnification are inextricably linked. Double eyepiece focal length and magnification halves and exit pupil doubles -  all other things being equal.

That means it is meaningless to claim one and not the other is the reason behind any change in the view.

Except that the exit pupil is what explains the brightness of the image.

Using a 100mm and 200mm scope at the same mag will lead to a brighter image in the larger scope. This shows the effect of exit pupil rather than magnification, so there is a consideration to be made there.

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I sold my 10" (manual) about 12 months ago to upgrade to a 12" GoTo, must say it was a good move although I did like the 10", I'd happily have another one again. I only sold it on to free up the storage space for the 12.  

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Well said Steve.

Whilst I sort of understand how it all works. ie Big & Bright vs Small & Same Brightness, the DSO sure look brighter in my biggish Dob  vs the little Dobs!

I'll give the article a good read later.

Paul

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When the jeststream or thermic currents  interfere with the seeing as I understand the article bigger is not always better.

I would like to understand better under what conditions it is better to step down in aperture in order to get better views?

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I would have thought that any thermal disturbance above will affect every telescope!
The important thing to remember ( here goes!) the primary task of the astronomical telescope is to gather and focus light, therefore the bigger the objective/mirror/aperture the more light you can gather, therefore it stands to reason that the bigger it is, the better it is, but no, there are limits, caused by the scope itself and  the atmosphere your looking through, and if those atmospheric limits were removed, then the design of the scope itself would still have a limit as to what it can achieve.
The telescopes ability to gather and resolve is all about aperture, and having more light to play with, you can/should be able to observe finer details when using an eyepiece, again one suitable to/for the task.

Next, the focal length of the telescope is important, as it controls the power of the telescope, How! The focal length of the telescope divided by the eyepiece focal length determines the power of the scope. You could have say 5 telescopes with different focal lengths and apertures, but with careful consideration to the eyepiece in use, they could all produce the same power, but invariably, the views may look different  in contrast and field of view, dependent of the design of the eyepiece, but all producing say 100x power.

As to when to step down in aperture, thats a personal choice only you can observe. I use an 8" aperture (200mm) yet I can downsize the same scope to a 2" (50mm) aperture, and all I can detect is a slight dimming of the target, which in this case is when I observe the Moon due to its brightness. I have never tried to lower the aperture for/on any other target, as I want the most my scope can capture. 

 

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3 minutes ago, Singlin said:

Thanks for the help.

My question was in reference to Acey's article specifically.

 

I'm not sure there is a formulaic approach to that question. I think it is more case of trying full aperture and if the seeing is too variable then try stopping down. It may even be worth running the scopes side by side to get an understanding for when the 10" is better than the 16" either full aperture or stopped down to 6 or 7".

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1 hour ago, Singlin said:

The whole article is Formulaic.

But as you say it's best just to use a practical approach.

I was just curious to read a summary of the article by someone who understood it.

Thanks.

Apologies, I have not read the article so I was making some assumptions. I believe that under good seeing conditions, the 16" will always out perform the 10". If you use the visibility calculator then you will see this.

I also believe that the only merit in aperture masking a 16" would be to improve its performance at high power under poor seeing. Masking to 6 or 7" would help stabilize the planetary, lunar and doubles performance if the seeing is poor, or perhaps the 10" would be better than the unmasked 16" under certain seeing conditions. But for faint deep sky objects, the full aperture 16" should always win.

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20 hours ago, JOC said:

it is quite counter-intuitive in places

Thats astronomy for you, felt the same when I was learning about 35mm photography, again, counter-intuitive in certain areas that baffled me at the time!

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5 hours ago, Stu said:

Except that the exit pupil is what explains the brightness of the image.

Using a 100mm and 200mm scope at the same mag will lead to a brighter image in the larger scope. This shows the effect of exit pupil rather than magnification, so there is a consideration to be made there.

I'm not trying to be awkward for the sake of it, but i could rephrase your statement as:

"Using a 100mm and 200mm scope at the same exit pupil will lead to a brighter image in the larger scope. This shows the effect of magbnification rather than exit pupil, so there is a consideration to be made there".

This is because

1) Exit pupil = (EP focal length/ objective focal length) * aperture

2) Magnification = Objective focal length / EP focal length

Rearranging:

3) EP focal length / objective focal length =  1/magnification

Substituting 3 in 1

4) Exit pupil = aperture / magnification

Rearranging:

4) Magnification = Aperture / exit pupil

 

This why I keep stressing 'all things being equal'. There are really only three basic  things you can change:

Aperture, focal length, eyepiece focal length (there's also field of view but that doesn't affect contrast or brightness).

All other measures, exit pupil, magnification, focal ratio are 'emergent properties' derived from two or more of the above three.

Exit pupil and magnification are (when aperture and focal length are constant) just different ways of measuring the same thing, in fact they are inversely proportional to each other.

Fit a zoom EP and exit pupil will get bigger as mag decreases and vice versa.

 

Now magnification is dimensionless, but aperture is a length. This makes Exit Pupil attractive as you can have a rule of thumb like 'a 2mm exit pupil is ideal for faint objects'.

 

This may seem like angels dancing on the head of a pin, but as the focal length and aperture of most scopes is fixed and leaning heavily on you and other people's comments above, varying EP focal length (and hence both magnification and exit pupil) is the main thing people can do to change their view. We can have a few rules of thumb:

  • Magnification beyond aperture in mm won't show extra detail (roughly speaking)
  • Reducing magnification below the point where the exit pupil is larger than your eye's pupil won't make the image brighter

<these two effectively set useful upper and lower limits on eyepiece focal length for a scope>

  • Increasing magnification will make the exit pupil smaller and the image less bright but make extended objects more contrasty
  • An exit pupil of about 2mm appears to be optimal for viewing extended objects

 

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33 minutes ago, Stub Mandrel said:

"Using a 100mm and 200mm scope at the same exit pupil will lead to a brighter image in the larger scope. This shows the effect of magbnification rather than exit pupil, so there is a consideration to be made there".

I'm equally not trying to be awkward, but without going through the rest of your post, your statement above is simply wrong.

i should add that we are talking about the same surface brightness for the object, not the overall brightness which will of course be higher due to the larger perceived area of the object at the higher mag of the larger scope.

EDIT The above point is key. A larger scope allows the same object to be seen at the same surface brightness but at a larger size. At the larger size, perceived contrast tends to be higher so the object becomes easier to see. The total brightness of the object does increase with aperture, but surface brightness depends upon exit pupil.

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12 hours ago, Singlin said:

When the jeststream or thermic currents  interfere with the seeing as I understand the article bigger is not always better.

I would like to understand better under what conditions it is better to step down in aperture in order to get better views?

 

10 hours ago, Singlin said:

Thanks for the help.

My question was in reference to Acey's article specifically.

 

Unless I missed something (which could easily have happened) the only discussion on those lines was that a small telescope at a dark site can outperform a large telescope at a light polluted site for the purpose of detecting low contrast objects (ie galaxies). 

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