# How much faster is F5.3 compared to F7.5

## Recommended Posts

Simple questions really, or maybe not.

120mm F7.5 scope 900mm My old ED120

compared to a

190mm F5.3 1000mm scope My new MN190 Mak Newt

What is the percentage difference in light gathered between the two over a nomimal period say 5 minutes ???

• Replies 25
• Created

#### Popular Days

Well, your Mak gathers (190/120)^2 = 2.5x more light (ignoring the central obstruction). However, it also spreads it out over ((190*5.3)/(120*7.5))^2 = 1.25x more area. So, if you haven't changed you camera, it is putting 2.5/1.25 = 2x more light into each pixel than before.

(though it's 4am, so I absolve myself of stupid maths errors )

##### Share on other sites

Well, your Mak gathers (190/120)^2 = 2.5x more light (ignoring the central obstruction). However, it also spreads it out over ((190*5.3)/(120*7.5))^2 = 1.25x more area. So, if you haven't changed you camera, it is putting 2.5/1.25 = 2x more light into each pixel than before.

(though it's 4am, so I absolve myself of stupid maths errors )

Wow : Nice work mate and makes sense.

So a 5min sub now on the MN190 is almost the equivalent of a 10min sub on my old ED120....

##### Share on other sites

Take the ratio of the focal ratios & square it ... (7.5/5.3)^2 = 2, as near as damn it ....

##### Share on other sites

... which is pretty spectacular!

Folks are incredibly hard to convince, sometimes, that f ratio is what determines exposure time. I had an enquiry from a guy with a C14 who could just not believe that his enormous telescope was going to be very slow.

Olly

##### Share on other sites

I had an enquiry from a guy with a C14 who could just not believe that his enormous telescope was going to be very slow.

Fastar it to f/2.1, though...

##### Share on other sites

I had an enquiry from a guy with a C14 who could just not believe that his enormous telescope was going to be very slow.

But only for extended objects. For stars, so long as they remain points, speed is proportional to the square of the aperture: a 14" f/10 is 16 times as fast as a 3.5" f/5.

##### Share on other sites

Olly, I want to understand this from a different angle. 14 inches of aperture is a great advantage, right? But the focal length is huge, almost 4 meters. This can produce an image that is perhaps 8 times scaled up (in linear dimension) from what a small APO might give you. If we were using film, we could use grain 8 times as big (which would be a lot "faster") and if the film dimension was also scaled up then we would end up with the same resolution as the APO. I am trying to think what the digital equivalent would be... larger pixels and larger sensor, less noise?

##### Share on other sites

Coming from a photographic background I always think of each full F stop being x 1.4142 (square root of 2) apart, therefore 5.3 x 1.4142 = 7.495, your MN190 is a stop faster than your refractor.

I've been thinking about Brian’s point that this only applies to extended objects and that for point light sources it's all about aperture, am I right in believing then that if you had two telescopes, say a 100mm F5 and a 200mm F5 they would both produce the same brightness for an extended object, say M42. If you view the 2 images side by side on a monitor the nebula will look the same in both cases (bigger image from the 200 of course) but the stars in the image from the 200mm F5 will be twice as bright from those on the 100mm F5 ?

Mel

##### Share on other sites

Olly, I want to understand this from a different angle. 14 inches of aperture is a great advantage, right? But the focal length is huge, almost 4 meters. This can produce an image that is perhaps 8 times scaled up (in linear dimension) from what a small APO might give you. If we were using film, we could use grain 8 times as big (which would be a lot "faster") and if the film dimension was also scaled up then we would end up with the same resolution as the APO. I am trying to think what the digital equivalent would be... larger pixels and larger sensor, less noise?

Yes, yes! This the whole point which people miss. If you use larger pixels for longer focal length scopes (and these can be software created pixels, they don't have to be hardware) they they are just as fast as their shorter focal length cousins. There is nothing magic about focal length - it is the aperture which determines how many photons you get. A 14" scope WILL blow a 4" away if you image correctly with it, whatever the focal length.

NigelM

##### Share on other sites

Yes, yes! This the whole point which people miss. If you use larger pixels for longer focal length scopes (and these can be software created pixels, they don't have to be hardware) they they are just as fast as their shorter focal length cousins. There is nothing magic about focal length - it is the aperture which determines how many photons you get. A 14" scope WILL blow a 4" away if you image correctly with it, whatever the focal length.

NigelM

Yes, and people imaging with C14s will often bin even the luminance on large chip CCD cameras. There is your equivalent of the coarser, faster grain. But you also have to accept that you can't just go on using a bigger and bigger chip. For one thing you can't afford to and for another the scope won't cover it!!

No doubt about it though, a well fettled F2 Hyperstar C14 is going to take a bit of beating.

Olly

##### Share on other sites

So my MN190 is gathering twice as much light in the same time on the same object as my ED120 ?

They are roughly the same FL 1000mm compared to 900mm the only difference is 190xF5.3 compared to 120xF7.5

##### Share on other sites

So my MN190 is gathering twice as much light in the same time on the same object as my ED120 ?

They are roughly the same FL 1000mm compared to 900mm the only difference is 190xF5.3 compared to 120xF7.5

If the focal length is (more or less) the same you can just directly compare the areas of the objectives, Pi R squared. That is your light grasp and it is concentrated onto the same area because the f length is the same. OK, central obstruction, but in round terms you are indeed twice as fast.

Olly

##### Share on other sites

If the focal length is (more or less) the same you can just directly compare the areas of the objectives, Pi R squared. That is your light grasp and it is concentrated onto the same area because the f length is the same. OK, central obstruction, but in round terms you are indeed twice as fast.

Olly

Sweeter than a sweet thing that has been molested by the big sister of all things sweet

##### Share on other sites

Folks are incredibly hard to convince, sometimes, that f ratio is what determines exposure time.

It took me over a year to work that one out! I was fairly convinced that a huge aperture F10 SCT would image faster than a small aperture, fast scope. But the penny finally dropped a few weeks back.

Here's how it makes sense to me, please correct me if I am wrong:

Ignoring things such as differences in contrast, take these two scopes:

1) 80mm refractor, focal length 500mm, fairly fast at F6.25.

2) 280mm SCT, focal length 2800mm, slow at F10.

Penny drop #1:

At prime focus - no barlows, reducers, ... - how zoomed in you are on an object depends on the telescope's focal length, nothing else. A bit like on a camera's 50-300mm zoom lens, 300 gets you in way closer than 50.

So at prime focus, objects look 5.6 times bigger in the SCT (2800mm / 500mm).

Penny drop #2:

When you switch from prime focus to using a 2x barlow to double up the image size, the amount of time you need to get the same brightness of image is not double, it is FOUR times as long an exposure.

A bit like if you have an image in Photoshop, such as 100x100 pixels, if you double it up in size, it actually takes up four times as many pixels (100x100 = 1000, versus 200x200 = 4000).

Penny drop #3:

Although the SCT has over 12 times the light grasp of the 80mm refractor (280mm squared / 80mm squared), objects are 5.6 times bigger in the SCT, which works out that they cover over 30 times as many pixels (5.6 * 5.6).

12 times more light versus sharing the light over 30 times as many pixels - that's why the SCT needs a longer exposure - 12 times more light in, but spread over 30 times the number of pixels.

Final penny drop:

Of course, while the refractor is faster, it is nowhere near as zoomed in at prime focus. Imagine if your target needs the zoom of the SCT. To get the equivalent zoom in the 80mm, you'd need to use something like a 5x barlow,which would take 5x5 = 25 times longer to expose than the usual prime focus exposure. So if you are using an SLR in particular and want to get in close on small targets, the SCT could come into its own then, this is presumably why some imagers use a refractor for bigger targets and the SCT for the small, faint things.

Edited by Luke
• 2
##### Share on other sites

• 12 years later...
On 19/01/2011 at 23:16, Luke said:

It took me over a year to work that one out! I was fairly convinced that a huge aperture F10 SCT would image faster than a small aperture, fast scope. But the penny finally dropped a few weeks back.

Here's how it makes sense to me, please correct me if I am wrong:

Ignoring things such as differences in contrast, take these two scopes:

1) 80mm refractor, focal length 500mm, fairly fast at F6.25.

2) 280mm SCT, focal length 2800mm, slow at F10.

Penny drop #1:

At prime focus - no barlows, reducers, ... - how zoomed in you are on an object depends on the telescope's focal length, nothing else. A bit like on a camera's 50-300mm zoom lens, 300 gets you in way closer than 50.

So at prime focus, objects look 5.6 times bigger in the SCT (2800mm / 500mm).

Penny drop #2:

When you switch from prime focus to using a 2x barlow to double up the image size, the amount of time you need to get the same brightness of image is not double, it is FOUR times as long an exposure.

A bit like if you have an image in Photoshop, such as 100x100 pixels, if you double it up in size, it actually takes up four times as many pixels (100x100 = 1000, versus 200x200 = 4000).

Penny drop #3:

Although the SCT has over 12 times the light grasp of the 80mm refractor (280mm squared / 80mm squared), objects are 5.6 times bigger in the SCT, which works out that they cover over 30 times as many pixels (5.6 * 5.6).

12 times more light versus sharing the light over 30 times as many pixels - that's why the SCT needs a longer exposure - 12 times more light in, but spread over 30 times the number of pixels.

Final penny drop:

Of course, while the refractor is faster, it is nowhere near as zoomed in at prime focus. Imagine if your target needs the zoom of the SCT. To get the equivalent zoom in the 80mm, you'd need to use something like a 5x barlow,which would take 5x5 = 25 times longer to expose than the usual prime focus exposure. So if you are using an SLR in particular and want to get in close on small targets, the SCT could come into its own then, this is presumably why some imagers use a refractor for bigger targets and the SCT for the small, faint things.

I have been trying to get my head around this myself for some time, finally I have found an explanation that tells it in my language that I can relate to !

##### Share on other sites

7 minutes ago, bomberbaz said:

I have been trying to get my head around this myself for some time, finally I have found an explanation that tells it in my language that I can relate to !

And is unfortunately incomplete explanation.

F/ratio is not measure of the speed of the system (as it omits one more component that can change and that is pixel size).

##### Share on other sites

15 minutes ago, vlaiv said:

And is unfortunately incomplete explanation.

F/ratio is not measure of the speed of the system (as it omits one more component that can change and that is pixel size).

Right but it does explain to me the barlow 2x/4x issue which for a long time, had me confused. No longer.

TBH I find much of it confusing, I guess I have not got a great head for maths.

##### Share on other sites

1 minute ago, bomberbaz said:

Right but it does explain to me the barlow 2x/4x issue which for a long time, had me confused. No longer.

Ah, right - well that bit is the essence of it all - telescope projects onto a surface (focal plane) for given "magnification" - and surface grows as square (like volume grows as cube ...).

• 2
##### Share on other sites

12 years is a long time. I wouldn't reply now as I did then. What I would say now is that...

1) The number of photons from the object of interest (read that twice) depends only on aperture.

2) How bright that is going to make your image depends on how many of these 'object photons' land on each pixel.  (Read that twice as well.) The more light, the brighter each pixel.

3) How much detail you capture (AKA how big your image will be) depends on how many pixels you place under the projected image of the object in your scope.

Points 2 and 3 can be adjusted in two ways. You can put more light on each pixel by using a shorter F ratio or you can do so by using larger pixels (or by binning them or by resampling.)

The useful image size or pixel count of your object of interest is ultimately limited by a) the seeing b) the guiding c) the resolution your optics d) the sampling rate of your system. The ideal system is one in which you sample at the finest scale your seeing/guiding/optical resolution will support. In a nutshell, if that limit is 1.5 arcseconds, use the biggest aperture that will give you 1.5 arcseconds per pixel.

Olly

• 1
##### Share on other sites

5 minutes ago, ollypenrice said:

In a nutshell, if that limit is 1.5 arcseconds, use the biggest aperture that will give you 1.5 arcseconds per pixel.

I love that you used those numbers as example .

• 1
##### Share on other sites

3 minutes ago, vlaiv said:

I love that you used those numbers as example .

Yes, you talked me up a bit from my earlier preference for 0.9. Actually, I was discussing this with my collaborator Paul Kummer with regard to imaging at higher resolution. I'm in favour of a C11 RASA at 1.25"PP.

Olly

##### Share on other sites

3 minutes ago, ollypenrice said:

Yes, you talked me up a bit from my earlier preference for 0.9. Actually, I was discussing this with my collaborator Paul Kummer with regard to imaging at higher resolution. I'm in favour of a C11 RASA at 1.25"PP.

Olly

1.25"/px is quite reasonable resolution with 11" of aperture in good conditions. Not sure if RASA11 can deliver that sharpness in terms of spot diagram - but as far as diffraction limited systems go - quite sound resolution.

##### Share on other sites

1 hour ago, vlaiv said:

1.25"/px is quite reasonable resolution with 11" of aperture in good conditions. Not sure if RASA11 can deliver that sharpness in terms of spot diagram - but as far as diffraction limited systems go - quite sound resolution.

My impression of the RASA 8 is that it operates on two levels. Stars are not particularly good but extended fine detail is excellent.

Olly

##### Share on other sites

Posted (edited)

I wish I had the head for the higher math, but this has been an interesting discussion. I'm trying to get back in to astronomy after a 3 year hiatus due partially to the pandemic.  We're doing comparisons here to telescopes with a disparity of f/stop and aperture. How does the math work in this instance, with these conditions:

8" Edge HD with a F/7 reducer, compared to a 80mm refractor using a 1.5x Barlow (in this case a William Optic GT81 which is f/5.9  in its native focal length). Given that the imaging camera is the same for either setup (pick whatever pixel size you want), which would be the better one to image with for deep space objects?

Edited by Luna-tic

## Create an account

Register a new account