APO telescopes for imaging, does aperture make a difference?

[feras281]

I was hoping to take the plunge into astrophotography and noticed some amazing images taken with the skywatcher 80ED in the sky at night magazine .

I was thinking of getting a small APO to have a go but was thinking of investing and was wonder if getting the larger apertures makes any difference when taking long exposures? Am I right in thinking the advanage of a large aperture APO is mainly just visual or does it help with the images too? otherwise I can save investing in the 120 sizes and start with an 80mm size 

[spaceman_spiff]

Hi I am also interested in this thread - I am tempted by the SW 100ED, with the focal reducer/field flattener it is quite a fast scope. I wonder how it compares with the ED80.

[kalasinman]

Seems like the number photons captured per unit of time is entirely dependent on the aperture area.

[johnrt]

A larger aperture OTA will allow you to achieve a longer focal length and still maintain reasonable exposure times. Imagers don't think about aperture like an observer would, but rather it's value in relation to focal length, or the focal ratio of the telescope.

To pick the right OTA think more about the field of view you would like to have with your camera, and then match an OTA size to that. Have a look at this handy tool to get an idea of different fov's OTA & camera combinations will give. http://www.12dstring.me.uk/fovcalc.php

[Zakalwe]

Focal speed and length is what is important, not aperture.

[A40farinagolf]

It's more about what you do with it rather than the size.

My APO130 with reducer has the potential to create superb images but doesn't do (at the moment), the reason for this is my inability to use if proficiently, my images are nowhere near as good as some taken through a 80mm scope by more experienced AP.

I'd start with the best 80mm one you can afford and then upgrade when you've exhausted it's potential.

[feras281]

Okay so lets compare two telescopes , Skywatcher ED 80mm and ED 120mm. Both of these have the same focal ratio of 7.5

On the simulation programe http://www.12dstring.me.uk/fovcalc.php i put in the dumbell nebulae M27. The 120mm produced a larger sized image than the 80mm. 

So.....does this mean for the same image you still have to wait for the same exposure times for the same object but in the larger aperture telescope because its larger in size at the end the quality may be a bit higher because it will take more space on the camera sensor ?

[ollypenrice]

Forget aperture in isolation because in isolation it doesn't tell you anything that you need to know.

As Zakalwe says, what matters are focal length and focal ratio.

Focal length decides whether you take a wider field image or one which is 'closer up.' Be aware that many nebulae are very large and need a short focal length to fit them on the chip.

Focal ratio determines the exposure length.

Now look at you pixel scale. http://www.12dstring.me.uk/fov.htm

The fewer arcsecs per pixel you have the better your guiding needs to be and the better the seeing has to be as well. There is no point in trying to resolve details if the guiding and/or seeing simply blur them out.

If you want to make life easier for yourself go for a short focal length and a fast focal ratio. This will give you easier guiding and faster exposures. So go for short focal length and relatively large aperture for that focal length.

Olly

https://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

[CraigT82]

There is a chap on YouTube who does some good videos (on planetary imaging), but they're helpful to understand how focal ratio and aperture impact on photography.

https://m.youtube.com/watch?v=4CEJVSkayYw

[kalasinman]

It seems counter intuitive to me to make a statement like forget aperture. Then say it's FR that's important. Considering aperture is half of the equation for FR, I'd say on the face of it that aperture is equally important to FL. Sorry, I just don't like sweeping generalities, which is a sweeping generality.

[Alien 13]

For AP there are many questions to be answered the most important is "what do i want to image" this fixes the required focal length and also determins the required mount, long FL heavy and expensive.

The focal ratio you choose at a particular FL is dependent on how deep your pockets are and what type of camera you want to use Fast FR is ok with a DSLR Slow FR requires CCD.

This would seem to suggest that aperature by itself is less important but remember a 80mm F5 and a 80mm F8 gather exactly the same amount of photons so for example if you where imaging a single faint star with both with a  suitable sensor there should be no change in real speed, of course we image the whole field which is different.

Alan

[Knight of Clear Skies]

For AP there are many questions to be answered the most important is "what do i want to image" this fixes the required focal length and also determins the required mount, long FL heavy and expensive.

The focal ratio you choose at a particular FL is dependent on how deep your pockets are and what type of camera you want to use Fast FR is ok with a DSLR Slow FR requires CCD.

This would seem to suggest that aperature by itself is less important but remember a 80mm F5 and a 80mm F8 gather exactly the same amount of photons so for example if you where imaging a single faint star with both with a  suitable sensor there should be no change in real speed, of course we image the whole field which is different.

Amen to the first part of the post.

I struggle to get my head around optics but I don't think the last paragraph is quite right. Certainly the same number of photons are hitting the front element but in the case of the f8 scope fewer of them will reach the other end. The elements in the f5 scope have a greater index of refraction so they gather photons from a larger area of sky then the f8 scope, resulting in greater brightness and lower magnification.

[Alien 13]

Amen to the first part of the post.

I struggle to get my head around optics but I don't think the last paragraph is quite right. Certainly the same number of photons are hitting the front element but in the case of the f8 scope fewer of them will reach the other end. The elements in the f5 scope have a greater index of refraction so they gather photons from a larger area of sky then the f8 scope, resulting in greater brightness and lower magnification.

I think the confusion comes from the difference between a single point source and an objects overall surface brightness If there where say 1000 photons captured by an 80mm objective from a single point source it would make no difference what the focal length is provided they focussed at a single point on the chip.

This is the reason for the FR myth.

Alan

P.S in reality sky conditions and optics might mean that the photons get spread over a few pixels reducing the speed of capture.

[Zakalwe]

There is a chap on YouTube who does some good videos (on planetary imaging), but they're helpful to understand how focal ratio and aperture impact on photography.

https://m.youtube.com/watch?v=4CEJVSkayYw

Planetary imaging is different to DSO imaging though. In planetary imaging, aperture is king. As is long focal length.

[ollypenrice]

It seems counter intuitive to me to make a statement like forget aperture. Then say it's FR that's important. Considering aperture is half of the equation for FR, I'd say on the face of it that aperture is equally important to FL. Sorry, I just don't like sweeping generalities, which is a sweeping generality.

I didn't say forget aperture, I said forget aperture in isolation, by which I meant 'in isolation from focal length and focal ratio.' I'll stand by that because many people coming into AP imagine that you simply must get a better result from a bigger scope and this is an assumption which has no validity.

There are two critical numbers, focal length and aperture, which give rise to a third critical number, focal ratio. In explaning the situation to a beginner I think it's a bad idea to start with three variables when there are two which operate directly on the imaging experience. The two that do are FL and FR.  Aperture does not operate directly on the experience but indirectly via its effect on FR. It's for that reason that I always set it aside in giving an explanation.

(Aperture does operate directly on the experience in that it impacts directly on resolution if all things are equal - but they rarely are. In the amateur world optical quality inevitably tends - I stress tends - to diminish with increasing aperture for reasons of cost, quite simply. And focal ratio does not necessarily reduce exposure times either. Using a focal reducer on a target which will fit on the chip unreduced will not speed up capture. And so on and on. Aaaargghh!!!)

So to the poor old beginner I say,

- Focal length says what fits on the chip.

- Focal ratio says how long it will take.

I think that's a reasonable starting point.

Olly

[dph1nm]

It is really aperture and pixel scale (on the sky) which matter. After all, you can have two identical aperture telescopes with the same f-ratio , but if you stick cameras with different pixel sizes on the back they will be different "speeds" for imaging.

One thing to remember in all this talk of fast and slow is the fact that only aperture (mostly) determines how faint an object you can detect. Which is why professional astronomers keep building larger and larger scopes.

NigelM

[ollypenrice]

It is really aperture and pixel scale (on the sky) which matter. After all, you can have two identical aperture telescopes with the same f-ratio , but if you stick cameras with different pixel sizes on the back they will be different "speeds" for imaging.

One thing to remember in all this talk of fast and slow is the fact that only aperture (mostly) determines how faint an object you can detect. Which is why professional astronomers keep building larger and larger scopes.

NigelM

This is the professional to professional explanation. It's correct. But it isn't nececessarily the explanation to give to a beginner wanting to set up a small astrophotographic rig within a context of known products on budget.

Olly

[ollypenrice]

By the way, I don't think the F ratio myth arises from the difference between point sources and extended ones. As has been said, there are no point sources in amateur sized scopes anyway.

The F ratio myth may well arise from focal reducers which speed up F ratio by reducing focal length rather than reducing it by adding aperture. The former brings no new object photons while the latter does bring new object photons. Once you start comparing scopes of different focal length and different focal ratio you go straight up a gum tree.

Olly

[Jonk]

I'm also thinking of moving from a newt to a frac for imaging.

I am interested in both galaxies and nebulae and don't mind having a go a mosiac-ing on nebula as some are huge, so my question is this:

Can anyone recommend a used triplet (or doublet if my expectations are all wrong) in the region of £350-400?

What focal ratio should I be looking at F5? Faster?

Bearing in mind I'll be using a modded 1100D with cls filter for now, but hoping to move to ccd in the future.

[Scorpius]

After all, you can have two identical aperture telescopes with the same f-ratio , but if you stick cameras with different pixel sizes on the back they will be different "speeds" for imaging.

NigelM

This is something I hadn't heard before. So are you saying the size of a camera's photosites influences the f/ratio "speed" for imaging and if so - in what way? Not sure the more experienced experts on here need to worry so much about over simplifying things for us newbies - after all we do still have a brain and need to learn this stuff somehow just like the experts did when they were first starting out...

[kalasinman]

This is something I hadn't heard before. So are you saying the size of a camera's photosites influences the f/ratio "speed" for imaging and if so - in what way? Not sure the more experienced experts on here need to worry so much about over simplifying things for us newbies - after all we do still have a brain and need to learn this stuff somehow just like the experts did when they were first starting out...

Binning pixels is a great example of this. When you join pixels into a super pixel, the speed increases as the resolution fall.

[Steve 1962]

The aperture of a telescope determines its resolving power (R or Dawes Limit). The equation for this is R=11.6/D in cm.  

The resolving power of a telescope in DSO imaging is also affected by atmospheric seeing and the mount's guiding performance.

It makes sense to match the pixel size of a camera to the resolving power of a telescope so that any image is neither under sampled (blocky stars and loss of fine detail) or over sampled (detail being limited by a small pixel size). The "sweet spot" for us amateurs imaging with commercial mounts which aren't located on a mountain top in Chile seems to be to image at about 2 arc seconds per pixel.

If we take this figure and apply the maths of Harry Nyquist and his chums (who basically said that we should sample at least at twice the frequency of what we're looking at), we get a optimal sampling rate of  about 1 arc second per pixel.

Plug this figure into the Dawes Limit equation and you get the optimal diameter of an imaging scope D = 11.6/1 cm = 11.6cm. 

This is, I believe, why you see so many 4" / 5" class refractors in use by amateur imagers out there.....they strike the right balance between cost and real world imaging conditions.

Moving onto focal ratio - I'd like to use some real world experience here:

I bought my first scope - a SW100ED f9 on the basis of of SaN review before I had any idea that I wanted to get into imaging. When I plugged my DSLR into it, I found that I really struggled to get anything decent out of it, so on the advice of well known and respected dealer I "upgraded" to a WO 70mm f6 scope - and voila! Much brighter images! This is because even though the new scope had 25% less aperture, it had a shorter focal length and was focussing light from a bigger portion of the sky onto my camera chip.

I now have a 106mm f5 scope, which gives me even brighter images and (this is the proof of the pudding coming up) - I've just added a 0.73 reducer to it - making it a 106mm f3.7 scope.

Yesterday, I took a load of flat frames using the same flat panel as I've always used and found that (with an Ha filter as an example) while I had to use exposure times of about 3.6 seconds to get a good flat frame with the scope at f5, I found I only needed 1.8 seconds get the same ADU value flat frame using the same aperture scope with the same camera from the same light source.

I was staggered by this decrease in exposure time until it dawned on me that the area of aperture had increased as a square of decrease in focal ratio (i.e. (5/3.7) squared) = 2.04.

The f3.7 scope was therefore collecting light from twice the area of the flat panel, focussing this double amount of light onto the camera chip - and therefore halving the exposure time from that of the f5 scope. This is reality - not a myth.

Translated out under the sky, the f3.7 scope will provide a FOV of twice the area that the f5 scope would provide, therefore individual objects of any given brightness in the FOV will illuminate half their previous area on the chip - and will therefore appear twice as bright as on the same chip in the f5 scope.

I've just posted a first light from the new set up of the Pelican Nebula in Ha in the DSO imaging thread, and there is no difference in the average ADU of a 15min sub at f3.7 and a 30min sub at f5...just a wider field of view and a sampling rate of 2.4 instead of 1.76 arc secs per pixel.

Using this in the OP's situation - personally, given the choice between the ED80 and the ED100 - I'd save my money and get the ED80 - and if you get the reducer as well, I'm sure you'll be delighted with the f6.4 over the reduced 100ED's f7.7.

Hope that hasn't confused things!

Steve

[kalasinman]

Well risking foolishness: the aperture determines THE LIMIT of resolving power. The actual resolving power being a function of pixel scale among other things.

While there is no question of your result, the conclusion of no myth is less than accurate. The myth part is the conclusion that there is any other way of capturing more photons from a source than larger aperture and /or more time.

[Knight of Clear Skies]

Good explanation above Steve.

I tend to think of the speed of an optical system as being a combination of how much sky it harvests photons from and how efficiently it does so.

[Steve 1962]

I tend to think of the speed of an optical system as being a combination of how much sky it harvests photons from and how efficiently it does so.

Exactly - well put sir!

The myth part is the conclusion that there is any other way of capturing more photons from a source than larger aperture and /or more time

The key words there are "a source". My point is that you CAN collect more photons by looking at more sky - this is not a myth..

The other thing that I didn't touch upon is that as the aperture increases, the focal length gets longer (unless the focal ratio decreases) ...and this places greater demands on the guiding. Hence - another reason short focal length fracs are so popular.