Big versus small setup

[FrancoRodriguez]

Greetings!
Suppose you have a big telescope with long focal length and pair it with a big sensor. Then you shrink all the equipment to get a smaller, short focal length telescope with an equally smaller sensor and pixel size. The field of view will be the same in both scopes.
My question is, why do people get the larger setup? There must be an intrinsic increase in quality, no? (Otherwise people wouldn't buy 2500mm scopes and pair them with large sensors). I understand the idea of getting a big telescope and small sensor to look at planetary nebulae etc, but I don't understand the seemingly self defeating notion of the former setup.
This question is bugging me quite a bit.
Any comments would be most appreciated
Clear skies!

[xelasnave]

Even small takes heaps of time setting up. For me setting up the 80mm refractor is not that much differrent to setting up the eight inch ...ones heavier but that the only difference really.
But do like the smaller scope and find its results satisfying.
I plan in the near future setting up both rigs heq5 and eq6 ...need another mono camera but the dslr will have to do on one scope☺.
I think there is a great deal of merit in the skyadventurer approach...and just use camera lens and a modded dslr is probably a rig you would use a lot.
I find with my widefield set up (BWM☺) you can carry it all on and out which means you can fire on those nights not worth setting up for with a big or small set up..both work...anyways you can take out for a couple of hour gap that magically appears or even say you wake up at 3 am and notice its clear..drag it out set the thing ruffly south...have a marked spot to drop it. .zwitch it on and it bangs away until you wake up☺...
I am finding I still have so much to adjust and make better ...cables a never ending story...
So my answer is...get whatever rig you like its still big set up time which ever way you go really.
Alex

[Slawomir]

With DSO imaging in mind, these are the reasons that I can think of at the moment, in no particular order:

1. Diffraction. Larger aperture allows to resolve finer details, and seeing rather than aperture become the limiting factor.

2. Larger aperture allows for imaging fainter stars. This does not directly translate to extended objects.

3. Generally high quality astronomical cameras with deep cooling and reliable electronics come with larger pixels, and such larger pixels must be paired up with longer focal length in order to avoid under-sampling. The last point is gradually becoming less significant due to the recent developments in the sensor area.

4. Large aperture with short focal length = speed.

Hope it helps a bit.

[Startrek]

Simple Answer
Larger aperture means more light gathering ability (more photons ) which means more ability to resolve dimmer objects
You can’t expect a 4” scope to resolve a magnitude 11 galaxy ( very dim ) to the same level of detail and contrast that a 10” scope can do
Recommend you buy some astronomy and astrophotography books which will help explain the above in more detail

[Stefan Buda]

Hi Francisco and welcome to IIS,

I won't give you a comprehensive answer but I'll try to point you in the right direction.
In the past, sensors with large pixels were much better than the ones with small ones, therefore one needed a large scope to get the optimum sampling.
These days the sensors have improved to the point that it is possible to achieve the same, or similar result with a smaller setup.
One important thing to keep in mind is that, if you shrink the scope, the star sizes don't shrink. Star size is determined by the f/ratio and not the focal length alone. Therefore the stars will be bigger/blobby with the smaller setup due to the fact that the pixels are smaller. That effect is somewhat compensated by the fact that the stars are not as bright in the smaller setup.
Hope that helps.

[Ukastronomer]

However.........................

Smaller scopes have many advantages over larger ones

1. Large scopes 99% of the time need a fixed location
2. Smaller scopes can be put up/taken down moved about
3. Smaller scopes (refractors) can double as terrestrial scopes, birding
4. You can easily take a small scope in the car for a weekend

I have large and small, yet the most used is my 72mm ED, it has been used for bird watching at nature reserves, at the beach, Astronomy and what it was intended for, Solar

[Wavytone]

Small scopes don’t cut it for the planets. A big scope with exquisite optics and long focal length are required. Resolving jupiters moons as disks is a fair start.

[FrancoRodriguez]

Thanks for replying everyone. I see people saying that big telescopes collect more photons, and although this is true, it's the f ratio that the sensor really 'cares' about. In my example, both setups have the same f ratio and hence the same photon flux per unit sensor area. Doesn't that make the 'bigger light bucket' argument invalid? Now I'm really confused!

[Stefan Buda]

Don't be confused, just read my answer above once more. Your argument is valid. Forget the light gathering arguments as those apply only to faint point sources, while most APs chase extended objects.

Let people explain the advantage of a 16" RC with 10um pixels over an 8" RC with 5um pixels. Apples with apples.

[Camelopardalis]

The bottom line is signal to noise ratio (SNR).

If you have two telescopes and cameras which result in comparable angular resolution per pixel, the larger scope will collect more photons per pixel. More photons means more signal.

Of course, it’s not quite as simple as that. Modern CMOS sensors have inherent advantages in that their total noise contribution is significantly less than the old dinosaur CCDs with huge pixels. But it’s not quite as simple as that either ;)

Fundamentally, you can’t break the laws of physics/mathematics, and to get good signal with the smaller scope you will need to expose for longer (in total exposure time) to reach the goal. Whatever that may be...

There is also somewhat of an equaliser....the atmosphere. Atmospheric disturbances, especially along the east coast of Australia, ultimately limit the resolution you can achieve during any “long” exposure. Larger telescopes aren’t magic in that they can see through this turbulence, just the opposite...they are more susceptible to he disturbances that cause blurring, whereas a smaller scope may be blind to it because it lacks the aperture to resolve any difference.

What has changed significantly with the CMOS revolution (by coincidence or otherwise) is the accessibility of acceptable quality astro photography gear. You no longer need to be minted to have a good go :D

[Slawomir]

I thought this only applies to stars, not extended objects, for which the faster the f-ratio the stronger the signal regardless of aperture given the same arcseconds per pixel and same RN and same QE and... :question:

I believe the main advantage of a larger aperture when doing pretty DSO pictures is smaller spot size.

[gregbradley]

Smaller scopes are very good at widefield imaging of which there are quite a few wide objects to image. They are generally very bright and you are not trying to get the smallest resolution possible.

Larger aperture scopes can image fainter objects like galaxies, even faint ones if you have a dark site.

So a typical reason to have both smaller aperture refractors of shortish focal length and a longer focal length larger aperture compound scope is to be able to do both types of images.

I don't think that has changed because CMOS cameras are available. But I do see may wonderful CMOS images and those are typically on a 10 inch Newt or something similar like a 200mm RC scope.

Not all CCDs are large pixels either. The popular KAF8300 sensor is 5.45 microns and the popular KAF16200 is 6 microns. A lot of full frame camera sensors have pixel sizes around the 6 micron range.

Greg.

[Camelopardalis]

Photons are photons...they’re all the same to the pixels (ignoring their QE curve which is just their response to different wavelengths). An example of another extended object, which is easy to test, is the Moon...which is noticeably brighter in a larger scope.

Signal only gets stronger with fast f-ratio because the light cone gradient is steeper which results in more photons being focused in the same area of sensor.

All things considered, a larger scope is always going to collect more photons than a smaller one.

[Wavytone]

Franco,

The exposure of extended objects is essentially determined by the f-ratio of the scope, as you have recognised. However that ignores the question of their (absolute) angular size in the sky vs the actual field of view in the camera.

In this respect you are right - a cheap f/4 camera lens will (potentially) image nebulae such as M42 as fast as an F/4 20cm newtonian. But what is totally different is the focal length - image scale, and the resulting magnification.

What is totally beyond small lenses - and small scopes - is serious lunar & planetary stuff.

This is where you need scopes with resolution better than 0.5 arc second (aperture > 25 cm) and focal length > 4 metres. And the mount to track precisely during an exposure; this is far from trivial.

[Camelopardalis]

Indeed, a pleasing, low noise (however defined) image is going to need a certain level of signal / noise.

The technological advancements that have resulted in improved QE and lower added noise have just improved the odds a bit for faint targets with smaller scopes.

[Slawomir]

My understating is that Nick is correct - for extended objects f-ratio determines the speed, not aperture.

If we take 10" f/5 and 4" f/5, the 4" will collect only 16% of the photons that 10" does. However, while keeping aperture the same, 500mm focal length will put 6.25 as many photons as 1250mm focal length (keeping aperture the same).

So if we combine both factors, aperture and the focal length: 0.16 x 6.25 = 1 On no! How can this be? 10" f/5 will be as fast for extended objects as 4" f/5? Actually, 4" f/5 will be somehow faster due to the lack of central obstruction and loss of light at the mirrors with the 10" :P

But for point sources, 10" will easily detect many many more stars than 4" and will most likely allow for a sharper image with tighter stars :thumbsup:

[Camelopardalis]

Indeed, they may appear equally bright, but not equally well resolved. We can’t have it both ways in this game :shrug:

[Stefan Buda]

Don't know why everyone is going off on a tangent, muddying the waters.
Francisco talked about shrinking the setup and I don't think he was thinking about a factor of ten or such. How do you shrink a 10 micron pixel by a factor of 10 anyway? Or more exactly where do you get such a camera.

So please someone explain how a 16" setup is better than an 8"? But please respect the scaling factor of 2x, do not just apply it to one thing or another.

Is signal to noise ratio better, on extended objects, for the bigger setup?
Is resolution of extended objects better for the larger scope?
Where is the advantage for the 4x more photons with the larger one?

[Stefan Buda]

Why not? Providing you can get a sensor with 2.5x smaller pixels and the same performance. You can't? Well that is why I think the comparison of the setups should be kept in a practical size domain where external limits, like seeing or sensor performance, don't change. Otherwise all this discussion is a waste of time.

[Camelopardalis]

Define practical size domain?

[Stefan Buda]

I have.
At least twice.
8" to 16" to make it easy.

[FrancoRodriguez]

I think you've given me exactly what I needed--thanks! "...sharper image with tighter stars." That's got to be it.
It's such a seemingly simple question, trivial in fact, but actually a little harder to answer when it comes down to it.
The awful truth is that I'm trying to convince myself THAT I COULD DO WITH A BIGGER TELESCOPE! Terrible I know, and rather embarrassing...

[Slawomir]

Glad my waffle was useful :thumbsup:

To cheer you up - I have a very nice 4” refractor and I’m also trying to rationalise the need (read: want) for a larger aperture :lol:

[Camelopardalis]

Indeed, as you can see, surprisingly hard ;)

I recommend you read up on Airy disk (even Wikipedia has a good page on this). The mathematics of optics means that the Airy disk size is related to the f-ratio of the scope, not the aperture.

Of course, the mathematics assumes perfect optics and steady (no?) atmosphere. In practice, we don’t get either, so practice is always worse than theory.

The bottom line is, a fast f-ratio scope will give you tighter stars than a slower scope. Getting a larger, faster scope is (sort of) win-win. To a point. With the increasing focal length you are more likely to notice the turbulence of the atmosphere that degrades the view/image - this is “seeing limited”.

Btw, you’re not alone in wanting more bigger...it’s called aperture fever :lol:

[Slawomir]

̶O̶h̶ ̶d̶e̶a̶r̶.

Let’s quote a few paragraphs from above mentioned Wikipedia article:

Mathematically, the diffraction pattern is characterized by the wavelength of light illuminating the circular aperture, and the aperture's size.

An optical system with resolution performance at the instrument's theoretical limit is said to be diffraction-limited.

The diffraction-limited angular resolution of a telescopic instrument is proportional to the wavelength of the light being observed, and inversely proportional to the diameter of its objective's entrance aperture. For telescopes with circular apertures, the size of the smallest feature in an image that is diffraction limited is the size of the Airy disk.

In short - small aperture = DSO data is always diffraction limited (unless seeing is horrible) = less detail, while large aperture = data is seeing limited = more detail.

[Camelopardalis]

Of COURSE the f-ratio is related to the aperture, but did you read down as far as "Mathematical formulation", where you conveniently missed out:

The radius q1 of the first dark ring on a screen is related to theta and to the f-number by q1 = 1.22 lambda N (where N is the f-number)

(End quote). This is a commonly quoted formula involved in calculating the spot size. For the mathematically challenged, at the same wavelength of light, the Airy disk size increases linearly with f-ratio.

In practice, I agree, a small scope will be diffraction limited, but a larger one is less likely to be, especially along the east coast of Australia. I think we're essentially saying the same thing but just not seeing it that way ;)

And FWIW, your condescending tone is not appreciated. This used to be a friendly forum and I might suggest you pull your head in.

[Slawomir]

I hope our discussion has answered Francisco's original question. I sincerely apologise for coming across as rude geek :hi:

[N1]

Crikey I didn't know the line between futile amateurism and serious (planetary) astronomy was that well defined, so thanks for clearing that up. Seriously ;) :thumbsup:

[FrancoRodriguez]

Thanks, you've all been most helpful. So, in a nutshell, would I get any tangable benefit of a 12.5 inch f7.5 planewave corrected dall kirkham paired with a 16803 chip? The field of view with this setup is not very different to my current setup using a 100mg APO triplet.

[Stefan Buda]

Now that you provided some numbers, it is much easier to answer your question.

In a nutshell, yes the larger setup would produce better results in some respects but not all.

As you said, the field of view would be the same. The required exposure for extended objects would also be the same. The main advantage would be in resolving fine detail in nebula and the stars will be smaller relative to the larger pixels. That is a big advantage if you are going after a globular cluster, but not big if you are chasing the Running Man nebula.

EDIT: I got it wrong. Exposure time would be the same if you used the same sensor, but with the scaled up sensor the bigger scope would be better at catching up with the Running Man nebula too.

[Peter Ward]

How about we reduce the argument to its absurd conclusion and apply some physics on top of the math?

Lots of luck collecting *any* photons down a 1mm aperture from a dim flux source...the shot noise will be woeful.

An aperture of 10 metres , or 10000mm *will* collect more photons. Yes there are moot points over pixel size...but without flux down the pipe, there is nothing to measure.

P.S. Infinite numbers of photons do not rain down from dim sources in the sky....bigger telescopes catch more over a finite period, which is hardly a surprise to those proposing the OWL.

[Stefan Buda]

Peter you are absolutely correct about the physics.
That is why I was talking about "practical size domains" in earlies comments.
That is why I tried to restrict the discussion to comparing an 8" to a 16".
Francisco is talking about a scaling factor of more than 3x, involving an increase in light gathering of 9x, and that is pushing it a bit. But even so the difference between the two setups will not be very obvious when it comes to S/N ratio in extended objects.

EDIT: I got it wrong.
Exposure time would be the same if we used the same sensor on both scopes. Because we are scaling up the sensor as well, as Francisco suggested, the exposure needed to achieve the same "depth", becomes significantly shorter.

The bottom line: The larger setup, despite of having the same field of view, would have somewhat better resolution and would record faint nebulosity significantly faster.

[Sunfish]

So simply, without confusing the mathematics and physics which makes my head spin, with the mechanics: the suggestion is that if I buy a much bigger aperture , I had better be prepared for a much larger sensor with larger pixels?

It would seem that small refractors up to 800 are well paired with apsc size 4.3micron pixel sensors at 1.1 arcsecond but a much larger aperture can benefit from a full frame with 6 micron pixels ?

Where is the useful limit of tolerance , 0.8 to 1.2 “ ?

[Stefan Buda]

You are on the right track. If you don't match your pixel size with the scope's focal length you may waste your money. For example if Francisco decided to get the 12.5" Planewave and use his present sensor, which I presume is one with pixels of around 4 micron, he would end up with a setup that not only has a much smaller field of view, but it would require the same exposure times and the only gain, not very dramatic, would be in resolution.
The limit for sampling rate is set by atmospheric seeing and I don't think going to less than 0.8 is beneficial, it would just increase exposure time without gaining resolution.
So basically the size of your pixels set the upper limit of your focal length and from there it is just a matter of choosing the largest available aperture for that focal length.

[Camelopardalis]

So, I understand the physics behind it, even while my attempts to articulate it may have been lacking previously :shrug:

But I have a question - and just to be clear - because I’m uncertain and would like to understand further, not because I’m trying to be funny :help:

What’s the deal with “extended” objects, and why are they treated differently? Surely they are photon sources subject to the same mathematics?

FWIW, I have example images of a popular galaxy taken with my 4” and with my 8” (with different cameras, so resolution +/- 10%), where the 8” shows better detail, comparable noise levels, but in 1/4 the exposure time. It’s not night and day, but I thought it was a worthwhile experiment. If anyone wishes to seem them, I will post...

[Stefan Buda]

The quick and simple answer is that star sizes on your sensor are determined by the f ratio alone and the size of an extended object, by the focal length (let's ignore the effect of central obstruction). That has implications regarding resolution and S/N ratio.

[AstroApprentice]

Hi Franco,
In addition to the sage comments posted already, Doug from Diffraction Limited summarises the argument pretty well here:
http://diffractionlimited.com/matching-camera-optics/

[gregbradley]

Bottomline you need both a widefield APO 4 inch or so or less even and a larger aperture scope for the fainter things and for greater depth in less time.

Nothing beats aperture in this game. My Honders 305mm F3.8 gets the image way faster than any other scope I have used. My CDK17 gets the galaxies and other smaller objects looking good that is not really the ideal target for the Honders even though it can do it.

One scope does not do it all. A few have come close.

Greg.

[Stefan Buda]

I have to amend my answer to Francisco because I forgot to consider narrow band imaging, which is a rather dumb thing to do considering how popular it has become. It must be because I never did and don't intend to do NB in the foreseeable future.

When we put a NB filter in the light path, we severely cut down the photon flux and we really get into shot noise dominated territory. And that is where aperture really matters (as Peter so nicely pointed out). Of course RGB is subject to the same laws of physics but NB is really pushing the limits.

So, Francisco if your game is NB imaging then you will have a much more capable setup if you fork out the money for the scaled up one.

[Sunfish]

Hmm.

Is that second chart correct or are the column annotations incorrect.?
Are we looking at image scale or pixel size in microns? Not good to show tables without units.

The top table is useful but so is this with better annotation

https://astronomy.tools/calculators/ccd