F5 vs f10

[lee g]

I currently have an f10 8”sct and observe under bortle 6 skies. My scope is great on bright objects but not so good at the dimmer dso’s such as galaxies. My question is would an 8” f5 reflector be a big improvement ie allow me to see more galaxies? 
I know dark skies would improve my view but I do all my observing from my back garden.

many thanks for any input 

[alacant]

Hi

I used to believe that aperture was the key to this and that the greater the aperture, the brighter. But in fact it isn't: f5 is indeed brighter than f10. Thanks to @vlaiv for explaining this.

To answer your question, I'd say the galaxies would appear smaller, but brighter.

Cheers

[M40]

Interesting subject, I think something like the 200P explorer has to be better as the primary mirror is basically at the bottom of a tube so any direct light source is diminished but I don't think anything will help with the skyglow itself. As for the difference between f10 and f5 and observing DSO's it will be interesting to see what others think. I suppose a screen or obs is not an option?

[andrew s]

27 minutes ago, lee g said:

I currently have an f10 8”sct and observe under bortle 6 skies. My scope is great on bright objects but not so good at the dimmer dso’s such as galaxies. My question is would an 8” f5 reflector be a big improvement ie allow me to see more galaxies? 
I know dark skies would improve my view but I do all my observing from my back garden.

many thanks for any input 

Do you mean visually or imaging? In both cases both collect the same amount of photons ( give or take few) so it depends on what else your doing. 

 For dim galaxies field of view is not an issue. @alacant point is specific to the circumstances. 

Regards Andrew 

[lee g]

16 minutes ago, M40 said:

Interesting subject, I think something like the 200P explorer has to be better as the primary mirror is basically at the bottom of a tube so any direct light source is diminished but I don't think anything will help with the skyglow itself. As for the difference between f10 and f5 and observing DSO's it will be interesting to see what others think. I suppose a screen or obs is not an option?

I already have a screen up but due to the weird shape of the garden an observatory is a no no

[lee g]

7 minutes ago, andrew s said:

Do you mean visually or imaging? In both cases both collect the same amount of photons ( give or take few) so it depends on what else your doing. 

 For dim galaxies field of view is not an issue. @alacant point is specific to the circumstances. 

Regards Andrew 

I mostly due visual 

[lee g]

Just now, lee g said:

I mostly due visual 

Or should I correct it to “ I mostly (dew) visual...😁

[andrew s]

25 minutes ago, lee g said:

Or should I correct it to “ I mostly (dew) visual...😁

I am not a visual expert  but, given good practice,  then magnification is probably the dominant effect on visibility.  Regards Andrew 

[vlaiv]

Not easy question to answer.

Depends on your eyes and on levels of light pollution.

F/5 scope will let you easily achieve maximum exit pupil - which is about 7mm in young people - but is highly individual.

You need 35mm eyepiece with F/5 scope to achieve this.

With F/10 scope - you can't achieve such large exit pupil and best you can hope for is about 5mm - 5.5mm (55mm plossl for example in 2" variant).

What does exit pupil have to do with anything? Well - it makes target smaller in size because it provides lower magnification - as long as you keep exit pupil smaller than your pupils - otherwise you'll be wasting light as not all light exiting eyepiece will be able to reach the eye.

Smaller target means more concentrated light - which is in principle easier to detect - if there is no light pollution involved. If there is light pollution involved - it behaves the same - it also gets more concentrated and background sky looks brighter at lower magnifications.

In light pollution it is all about contrast - but that does not change much with exit pupil.

Best way to get better views of galaxies is to go to a darker place. Really. Even stepping to next aperture size or one after that is not going to improve things as much as going to bortle 3 skies for example.

I saw more galaxies in 4" aperture in Bortle 4 than I ever managed with 8" aperture in bortle 7-8

[SiriusB]

On 10/02/2021 at 12:35, alacant said:

Hi

I used to believe that aperture was the key to this and that the greater the aperture, the brighter. But in fact it isn't: f5 is indeed brighter than f10. Thanks to @vlaiv for explaining this.

To answer your question, I'd say the galaxies would appear smaller, but brighter.

Cheers

I'm not sure this is so, i've seen that f ratio  thing  quoted in books by respected AA's. That dog don't hunt, as they say in the States.

It's highly relevant to photography (or imaging as it's called in these more modern times.)

An f15 scope @ a given exit pupil will give as bright an image as an f10 , f5 (or f1 for that matter)~ for visual use.

I've certainly not seen any difference in the field regarding f ratios.

As to aperture being they key, well it is....but not quite the whole story.

Contrast is important for DSO's too.

Hypothetically let's take two scopes, one has twice the light gathering of the other but the smaller one has twice the contast.

Now i'll grant you it's hard to quantify contast, but stick with me.

Ignoring any resolution advantage, & assuming the DSO magnitude is within the light grasp of both, what you see in both is likely to be about the same.

Actually, it may be aesthetically more pleasing in the smaller contrastier scope. (Think old school TV~ turn the brightness dial up full & the pictures bright but washed out without fiddling with the contrast dial also.)

Now real world, it's possible,(but unlikely) any scope will be twice the relative contast of another, so aperture is much more significant than contrast for practical purposes, but contrast is not to be ignored.

I don't claim to know everything, so may stand corrected, but f ratio has very little,if anything  to do with visual DSO brightness.

Time that old chestnut was laid to rest. IMO

 

[pregulla]

I would think it won't make much of a difference. Shorter focal length will allow you to reach lower magnifications and wider field. It will make the object brighter but also the sky background, so no much help on visibility. For most object optimal magnification will be within range of your SCT.

Smaller central obstruction of reflector over SCT may help somewhat with contrast, then F/6 or slower should theoretically have better contrast, but I am not sure how noticeable will it be in practice.