Why is the FOV so much better?

[dazzystar]

Hi All,

I was comparing a Startravel 102 with a WO Redcat 51, both with the ASI183MC fitted on the astronomy.tools website and got this.

They are both f4.9 scopes. Why is there such a huge difference in the FOV of the redcat compared to the ST102?

 

Cheers
Daz

[teoria_del_big_bang]

Th Focal ratio may be the same but the aperture on the Startravel 102 is 102 mm and on the Redcat is 51 mm (exactly twice that of the Redcat).
Also the Focal Length is also twice as much at 500mm as opposed to 250 on the Redcat, .

The f value is just a focal ratio so the relationship between the focal length and aperture, its the focal length that will determine the FOV.

So the focal ratios of the two scopes is the same.
Startravel 102 = 500 / 102 = 4.9
Redcat = 250 / 51 = 4.9

But, this is  why the FOV of the Startravel 102 is half that of the redcat.

Steve 

Edited March 10, 2022 by teoria_del_big_bang

[dazzystar]

Thanks Steve but I'm still confused. The ST102 has twice the aperture size so logic would say the FOV would be greater but it's half!

[vlaiv]

4 minutes ago, dazzystar said:

Thanks Steve but I'm still confused. The ST102 has twice the aperture size so logic would say the FOV would be greater but it's half!

Only in case that FOV depends on aperture - and it does not. It depends on focal length (think "zoom" - although it is not quite the same - longer FL, more zoom there is).

ST102 has longer FL than Redcat (twice as long).

[Richard_]

The focal ratio is calculated by dividing the focal length by the aperture. In order for the focal ratio to remain constant for a larger aperture, the focal length must increase. The higher the focal length is, the more zoom you have. In your example, the telescopes have the following specifications:

Redcat aperture = 51mm

ST102 aperture = 102mm

Based on the f-ratio of 4.9 then the focal length (rounded up) must be:

Redcat = 51mm x 4.9 = 250mm

ST102 = 102 x 4.9 = 500mm

Therefore, given that the ST102 has twice the zoom than the Redcat, you should expect to see half the field of view of the Redcat. If the ST102 had the same field of view as the Redcat (focal length of 250mm) the telescope would have a focal ratio of 2.45. 

[teoria_del_big_bang]

13 minutes ago, dazzystar said:

Thanks Steve but I'm still confused. The ST102 has twice the aperture size so logic would say the FOV would be greater but it's half!

Sorry. maybe not explained that well, maybe I should have mentioned first that the focal length of the Startraveler was twice that of the Redcat, as @vlaiv says it is the focal length that determines the zoom, for want of a better word. And the FOV will be a factor of the "Zoom" and the camera chip size.

I just mentioned that the aperture was also twice that of the Redcat to explain why the focal ratio ended up being the same at 4.9.

Steve

[teoria_del_big_bang]

Regarding your confusion over aperture, think about the main principle of a lens, it focusses all light from any one single point into a single point on the focal plane (the camera chip), that is to say all light from a any one point on the target will hit the same pixels on the camera chip.
So, no matter how big a diameter the lens at the front of the scope is all light will still get focussed at the same places, or pixels, on the chip, so the size of the image will be the same no matter what the aperture is, but, the greater the aperture the more light it will gather for any given time.

Steve

Edited March 10, 2022 by teoria_del_big_bang

[dazzystar]

Perfect. I understand now. Thanks very much!

[Louis D]

Having a big objective lens at the front of a telescope does not generally equate to a wide field of view.  It does equate to more light gathering ability as compared to a smaller front objective.

On camera lenses, sometimes a large front objective lens does equate to both a wide field of view and large light gathering ability as with the old Nikon 6mm f/2.8 fisheye lens:

[Toltec]

3 hours ago, teoria_del_big_bang said:

Regarding your confusion over aperture, think about the main principle of a lens, it focusses all light from any one single point into a single point on the focal plane (the camera chip), that is to say all light from a any one point on the target will hit the same pixels on the camera chip.
So, no matter how big a diameter the lens at the front of the scope is all light will still get focussed at the same places, or pixels, on the chip, so the size of the image will be the same no matter what the aperture is, but, the greater the aperture the more light it will gather for any given time.

Steve

I consider it in the other direction as well. If you think of the surface of the camera sensor as projecting a collimated point of light or beam from any given pixel that passes through the centre of the objective lens then the shorter the focal length the greater the angle subtended from one edge of the sensor to the other. The diameter of the lens makes no difference because the beam is always passing through the centre.

 

[vlaiv]

1 hour ago, Louis D said:

Nikon 6mm f/2.8 fisheye lens

That is some serious blue ring of fire!

[Louis D]

6 hours ago, vlaiv said:

That is some serious blue ring of fire!

I saw that as well, but it doesn't prevent me from wanting to collect one someday if I won the lottery. 🤪