Noob telescope aperture question Generic equipment discussions · Lightbringer3 · ... · 8 · 410 · 0

I am new to AP, but have a lot of photography experience and I think that helped me get a good kit to start with, but understanding scopes is infinitely more confusing than lenses.

I have an askar 71f as my starter scope.  I am very pleased with it and for doing this for a a little over a month now I can’t complain at my images considering I’m still fumbling through learning Pixnisight and how to really commit to an object and when and why and what to shoot and how.  Which leads me to my question.

if I shot the same exposure count/duration of the same nebula with a 70mm scope and a 140mm scope, at a similar FL, would the 140 scope show me parts of the nebula that the 70mm doesn’t pick up, or would the sensor get the same “pattern” of light, but just in greater amount and detail in less time with the 140 vs the 70?   I have gone to some objects that the ASIAIR shows me all over that I just can’t get anything but stars to show (Witches head for example).  Would a larger aperture scope reveal things not revealed with a smaller one?

There are 2 characteristics of a bigger aperture: Resolution and light gathering power.

Resolution: A bigger telescope can resolve finer details, but your seeing needs to be good enough to use its full potential. 
As a rule of thumb, the resolving power is roughly 138/aperture. In your case, the 140mm would resolve details half the size of the finest details with the 70mm.

Light gathering power: Of course also depends on the f-ratio, but as you asked about similar FL, it certainly collects more light with the bigger aperture. In your case, 4x more. (140^2 / 70^2)

Johannes

Tony Gondola:
Your Askar has a focal length of 489mm at F/6.9. If you double the aperture to 142mm and keep the focal length the same you'll be shooting at F/3.5 As a photographer, the changes will be as you expect. The image will be much brighter so that means the same amount of detail with a shorter exposure time or fainter detail if the exposure time is the same. The larger aperture scope will deliver twice the resolution in both cases.

I understand all this, but I am not used to photographing things I can’t see.  I guess my specific question is I feel like no amount of integration time will reveal some parts of what I am shooting on with current gear and I don’t know enough to know if this is a B7 situation or one that can be solved with gear like how a nb helps to see more as where rgb so far for me has been worthless.  Forget about time and light and double the detail and all that.   Would the bigger aperture “see” things the 70 doesn’t even under any exposure time/count combo situations?

If I understand you correctly, then you try to shoot dark or reflection nebula under B7 sky and you are not getting enough signal from the faint object? If so, there are many threads here on AB or CN that go into detail on the topic of signal to noise ratio. In your case, the sky background noise is very high and your target signal is very low. I am not an expert on that, but if you read on this topic you will get a qualified answer. In your case under B7 sky you would need to spend a lot of time (many pictures, not long exposures) to get a signal from your object. Not sure if this helps. 
Good luck,
Rob

Would the bigger aperture “see” things the 70 doesn’t even under any exposure time/count combo situations?

As Tony said - the larger aperture will simply show a certain faint object at a certain noise level faster than  the 70mm would. If you don't care about resolution, you can simply think of the 140mm as "seeing" the object you care about at a certain noise level 4x faster, i.e., 4x less integration time than the 70mm.

Conceptually, what you are trying to do is collect enough photons so that the Signal from the object of interest exceeds the Noise from the sky. Let's take a concrete example. Let us say the 70mm can collect, from the Sky, 100 photons/area of interest and 5 photons from the faint object of interest per minute. The noise from the Sky is 10 photons, which is greater than the signal from the object. So the object is not seen - it is drowned by noise. But if you integrate for 10 minutes, you've collected 1000 photons from the sky, which gives a noise of 32 photons. But in the same time, you've collected 50 photons from the faint object, which exceeds the noise from the sky. So, when you subtract out the sky background, the faint object reveals itself.

What happens if we use the 140mm? From the same sky area, we collect 400 photons per minute from the sky and 20 from the faint object. So the noise from the sky is 20 photons, which is about the same level as the signal from the faint object. Let us integrate for 2.5 minutes. Then we have collected 1000 photons from the sky and 50 from the faint object. This is exactly the same situation as the 70mm scope achieved, but in only 2.5 minutes, or 4x faster. This is what aperture buys you.


In the context of light pollution, the higher the Bortle, the greater the signal from the sky, which means the longer you have to integrate to collect enough photons from the faint object to exceed the noise from the sky. The object is still sending the same photons as from a dark site - it is just that the sky is much brighter, and takes much more time to modell at a low enough noise level that the object reveals itself when you subtract out the sky. Aperture helps by getting you there faster. Theoretically, with enough time or large enough aperture, you can "see" anything from a Light Polluted site as from a dark site, but it very rapidly becomes impractical so people end up traveling to dark sites or setting up remote telescopes.