Am I missing something here?

[vlaiv]

Here is another way to look at all of this - fast vs slow.

Let's not fix aperture, let's fix magnification. We have certain eyepiece - for example 28mm 68 degrees. We want to use it at certain magnification so it gives us certain true field of view.

Let that be x20.

For this - we need telescope with 560mm focal length.

We can use 80 f/7 scope for that or we can use 130mm F/4.3 scope for that. Faster one will provide us with brighter target as it has more of light gathering.

[andrew s]

1 hour ago, parallaxerr said:

Maybe that's the bit I'm missing then John - that regardless of exit pupil, the sky background brightness is a function of the eyepiece focal length only?

No it's a function of magnification,  as far a I know.

Regards Andrew 

[jetstream]

ie my 24" f4.1/PCII @1.15x gives f4.7 which is close enough to compare my nominal f4.8 15" to. Using the same 20mm eyepiece for a ~4mm exit pupil the 24" gives 125x mag the 15" 91x mag,

the difference in views is huge.

Edited June 24, 2020 by jetstream

[jetstream]

2 hours ago, John said:

Also certain exit pupils are more effective when using O-III and UHC filters.

100% agree and confirmed.

[andrew s]

3 minutes ago, jetstream said:

ie my 24" f4.1/PCII @1.15x gives f4.7 which is close enough to compare my nominal f4.8 15" too. Using the same 20mm eyepiece for a ~4mm exit pupil the 24" gives 125x mag the 15" 91x mag,

the difference in views is huge.

Yes but that's because you are comparing different focal lengths. Regards Andrew 

[parallaxerr]

What is becoming readily apparent is that different people have vastly differing views on the subject and we may have strayed a little!

For me, it's simple now. When considering two scopes of equal aperture, in most cases the view at the eyepiece can be matched (notwithstanding minor optical differences) by the use of different focal length eyepieces, such that I can make a decision based on physical practicalities alone, i.e shorter faster scope is easier to handle and mount.

Thanks everyone for your input, it has definitely helped!

Edited June 24, 2020 by parallaxerr

[andrew s]

2 minutes ago, jetstream said:

100% agree and confirmed.

Do you know why? I can't think of an obvious reason. 

Regards Andrew 

[vlaiv]

4 minutes ago, parallaxerr said:

What is becoming readily apparent is that different people have vastly differing views on the subject and we may have strayed a little!

For me, it's simple now. When considering two scopes of equal aperture, in most cases the view at the eyepiece can be matched (notwithstanding minor optical differences) by the use of different focal length eyepieces, such that I can make a decision based on physical practicalities alone, i.e shorter faster scope is easier to handle and mount.

Thanks everyone for your input, it has definitely helped!

Do consider that faster scopes require more expensive eyepieces to produce equally pleasing images and that longer FL scopes make it easier to achieve high mags while short FL scope makes it easier to achieve low mags. If you primary interest is high power viewing - slower scope. If you enjoy wide field view more - faster scope.

[vlaiv]

5 minutes ago, andrew s said:

Do you know why? I can't think of an obvious reason. 

Regards Andrew 

That is because of things that I listed - but primarily because of non linearity of brightness response.

I don't mean log based nature of human perception - but the fact that even expressed in magnitude scale our vision at low intensity becomes non linear.

This is the reason why we have preferred exit pupil for DSO observing depending on LP levels, and also reason why filters work best at certain exit pupil.

Because it creates the most perceived contrast - both sky and target are darkened by same amount - but to our perception sky gets darker than target - or rather perceived contrast increases.

[parallaxerr]

8 minutes ago, vlaiv said:

Do consider that faster scopes require more expensive eyepieces to produce equally pleasing images and that longer FL scopes make it easier to achieve high mags while short FL scope makes it easier to achieve low mags. If you primary interest is high power viewing - slower scope. If you enjoy wide field view more - faster scope.

Agreed, I can make those more my primary considerations now.

To be fair the scopes I am considering are not vastly different, F7-F8 mostly so my reasonable quality EP's should cope OK. It's the resultant tube length that makes a difference to me at 130-150mm objective diameter sizes.

Edited June 24, 2020 by parallaxerr

[jetstream]

13 minutes ago, andrew s said:

Do you know why? I can't think of an obvious reason. 

Regards Andrew 

I believe there is a "range" of exit pupils that work, somewhat based on the individual.  I can't give a mathematical reason or physics reason but it boils down to the tradeoff between magnification and image apparent brightness through the eyepiece. Too much mag will kill a filtered view, way too dark.

The wider pass UHC lets more light through so a bit more mag can be used, the Hb less light so less mag and is much more fussy.

Hbs have a narrow exit pupil range around 5mm-6mm- my VX10 shows the HH easily at 5.2mm exot pupil, stray a small amount from this and it disappears. Go up and the HH gets small in the EP, go down too dark. Blackwell and Clarks ideas at play.

For others: contrast is a fixed between the brightness of the sky and the brightness of the object. The only variable in our control is the darkness from which we observe from.

Edited June 24, 2020 by jetstream

[vlaiv]

7 minutes ago, jetstream said:

For others: contrast is a fixed between the brightness of the sky and the brightness of the object. The only variable in our control is the darkness from which we observe from.

This is true only for light intensity - measurable / physical quantity of number of photons or energy per unit time. That is not the same as perceived brightness.

[jetstream]

3 minutes ago, vlaiv said:

perceived

yes, perceived by us. The actual contrast is fixed between the brightness of the sky and the object. As we use filters, up mag lower mag it changes how we see things through the system.We are trying to "please" the brain so to speak.

[John]

Clear night forecast again. A chance to try experimenting perhaps ?

 

[parallaxerr]

41 minutes ago, John said:

Clear night forecast again. A chance to try experimenting perhaps ?

 

Definitely!