Big versus small setup

[FrancoRodriguez (Franco)]

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 (Suavi)]

Originally Posted by FrancoRodriguez

My question is, why do people get the larger setup?

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 (Martin)]

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 (Jeremy)]

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 (Franco)]

Originally Posted by 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

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]

Originally Posted by 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!

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 (Dunk)]

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

[Slawomir (Suavi)]

Originally Posted by 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.

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...

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 (Dunk)]

Originally Posted by 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...

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

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 (Dunk)]

Originally Posted by gregbradley

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.

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 (Suavi)]

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"

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

[Camelopardalis (Dunk)]

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

[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]

Originally Posted by Camelopardalis

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

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 (Dunk)]

Define practical size domain?