Not your typical 8" vs 10" Dob question...

[Timebandit]

Hi. A lot of interesting and informative information being given by the more advanced members on this topic. So as I understand things from what I have read. That a dark site is far more important than scope aperture. So could I ask the question then. If the dark site is the most important factor rather than aperture. If you have the same dark site with two scopes with equivalent quality mirrors say one scope a 8" and one a 14"" being side by side under the same conditions on the same night . You have the same equivalent eyepiece in the scopes giving the same magnification power. What differences would you see ? Would the 8" give the same sharpness , brightness , contrast as the 14" the images therefore being of equal. Or would there be a difference and what would it be ? As if my understanding is correct from what I am reading that increase aperture assists image scale. I hope this question makes sense and you don't mind me asking. Thanks 

[John]

Thats a very good question Timebandit. I'm looking forward to reading the responses

[cloudsweeper]

For a bigger aperture, you get more magnification for the same surface brightness.

Going back a bit - @YKSE - concerning contrast - " It's a bit more complicated than that. "

Complicated, indeed - and a very interesting topic.

As I understand it, a point source does not dim with mag.  The background (being huge) dims a lot with mag.  So is there not some correlation between size of object and degree of dimming (with mag)?  And thus, would a "smallish" object not dim just a little - and less so than the background - thus increasing contrast?

To complicate matters more, I've read (but don't necessarily just accept) that increased mag shows more detail, and this in turn can be perceived as an increase in contrast.

Doug.

 

[Soupy]

If I can add my two penny worth... the surface area difference between an 8" and a 14" is just over 3x. This means the 14" scope will gather over 3x the amount of light than the 8" this will equate to being able to see approx a magnitude deeper.  This opens up fainter stars DSOs etc. As well as being able to bring out fainter detail in more diffuse objects. 

[Louis D]

3 minutes ago, Soupy said:

If I can add my two penny worth... the surface area difference between an 8" and a 14" is just over 3x. This means the 14" scope will gather over 3x the amount of light than the 8" this will equate to being able to see approx a magnitude deeper.  This opens up fainter stars DSOs etc. As well as being able to bring out fainter detail in more diffuse objects. 

Exactly.  I went from an 8" solid tube dob to a 15" (with a Galaxy mirror) truss dob and the increase in detail in nebulae and planets was astounding.  Nebula filters work better because there's more light to work with at a given magnification.  Planets look better because decent image scale can be achieved without pushing the scope's limits.  It is loping along at 200X and takes 350X with ease.  Galaxies were better, but you really need over 20" of aperture to start seeing structure easily.  Now the downside.  Weight!  After my back injury, I couldn't heft the 65 pound mirror box around any longer, so I went back to the 8" and added a 72mm ED refractor and have been quite happy with the combo.

That fast 10" probably won't come with a Zambuto or other premium mirror, so it may not out-resolve the slower 8" on planets.  Fast mirrors are more difficult, and more expensive, to make compared to slower mirrors.

I'd say get the 8" due to weight, mirror quality, easiness on eyepieces, collimation forgiveness, and general availability on the used market for cheap.  If you like what you see, make the jump later to something 12" or larger to see a noticeable improvement.

[Stu]

1 hour ago, Timebandit said:

Hi. A lot of interesting and informative information being given by the more advanced members on this topic. So as I understand things from what I have read. That a dark site is far more important than scope aperture. So could I ask the question then. If the dark site is the most important factor rather than aperture. If you have the same dark site with two scopes with equivalent quality mirrors say one scope a 8" and one a 14"" being side by side under the same conditions on the same night . You have the same equivalent eyepiece in the scopes giving the same magnification power. What differences would you see ? Would the 8" give the same sharpness , brightness , contrast as the 14" the images therefore being of equal. Or would there be a difference and what would it be ? As if my understanding is correct from what I am reading that increase aperture assists image scale. I hope this question makes sense and you don't mind me asking. Thanks 

At the risk of incurring John's wrath by over complicating things, I can only refer back to some of the concepts already discussed rather than just saying 'it's a bigger scope so the image will be brighter'

It's not necessarily a simple situation, but let's assume that we have a 14" f4 scope which will have a focal length of 1400mm, vs a hypothetical 8" f7 scope which also has a focal length of 1400mm. Let's forget about any other differences between them in terms ie abberations like coma.

With a 21mm Ethos (why not ?) they would both give x66.6 magnification. The 8" scope would have an exit pupil of 21/7= 3mm whilst the 14" scope would have an exit pupil of 21/4 = 5.25mm

The basic result would be that the image in the large scope would be the same size but significantly brighter, but so would the sky background. Whether this gives a better view really depends upon the target (size, brightness and whether it is stellar as in open or globular clusters, or diffuse such as a galaxy or nebula), the sky brightness and whether a narrow and filter is being used.

If the target was say the Veil under good skies, with a OIII I would expect the result to be very nice in the 14" for instance, but a galaxy under LP skies would most likely be washed out.

As we keep saying, the benefit of the aperture would be to magnify smaller targets so they are larger (doh!), and the eye is able to percieve the contrast and detail better (even though the actual difference between target brightness and background brightness does not change).

Taking our two example scopes, the 14" will have an exit pupil of 3mm when used with a 12mm eyepiece. This would give a magnification of x116.6 vs the x66.6 in the 8", at the same brightness. Larger image scale, much better detectability of detail and a better result.

It is not a simple subject, but there is so much more to it than 'bigger scope gives brighter images'. The dob mob know all about this; big dobs under the darkest skies in the UK, detecting small faint galaxies by exploiting what I've said above.

EDIT I'm in no way claiming to be an expert on this, I just try to educate myself where I can to better understand how to get the best results. I'm learning all the time.

[YKSE]

6 hours ago, Timebandit said:

If you have the same dark site with two scopes with equivalent quality mirrors say one scope a 8" and one a 14"" being side by side under the same conditions on the same night . 

To my understanding, the answer is simple, 14" wins every time.

If you use eyepieces to get same magnification in both scopes, the image scale will be the same, but 14" gives 206% ((14/8)^2=3.06, brightness is proportional to area) larger exit pupil(EP)= 206% brighter, as

EP=aperture/magnification

If you use expieces to get exit pupil in both scopes, the brightness will be the same, but image scale will be 75% (14/8=1.75) larger in 14".

In both cases, you'll able to see more details in 14", either because of brighter image or larger image scale.

 

PS. exit pupil can be calculated by dividing eyepiece's focal ratio with scope's focal ratio too, you can do the algebra to get the above equation.