Am I right about how a reducer works?

[Swoop1]

Potentially looking at an Evostar 72ED which has a declared focal length of f5.8. I see the one I am looking at comes with a x0.85 reducer/ flattener.

Does the reducer basically mean that, by reducing the focal length to f4.93, I will effectively be imaging through a lens with a bigger hole so more light?

Thanks,

Matt.

[Victor Boesen]

Not entirely, since the aperture stays constant. Instead the focal length is reduced. On the topic of reducers for the Evostar 72ED, I've found the OVL field flattener great for around half the price compared to the dedicated skywatcher reducer.

[wimvb]

1 hour ago, Swoop1 said:

Potentially looking at an Evostar 72ED which has a declared focal length of f5.8. I see the one I am looking at comes with a x0.85 reducer/ flattener.

Does the reducer basically mean that, by reducing the focal length to f4.93, I will effectively be imaging through a lens with a bigger hole so more light?

Thanks,

Matt.

Absolutely not. Each pixel collects light from a larger section of the sky when you use a reducer. A focal reducer increases the pixel scale (arcsecs per pixel), by reducing the focal length, as the name says.

There's no magic bullet here, the only way to get a larger aperture is to buy it.

Edited June 28, 2021 by wimvb

[The Lazy Astronomer]

As has been mentioned, a focal reducer will not affect the amount of light your setup will gather, what it does do though is twofold:

1. It spreads the light over fewer pixels. This means that for a given scope and camera combination, it increases your signal to noise ratio. Theoretically, this allows you to create an image of a given 'quality' (for lack of a better word) in less time, at the cost of some resolution.

2. It widens your FOV, which can be useful if you are unable to fit a particular target nicely in the frame.

The same effect could also by achieved shooting a mosaic (without a reducer) and binning the data in post processing.

[Swoop1]

OK. Thanks for the explanation. I guess I'm still trying to get my head around the difference between astro f and photography f stops.

[BrendanC]

I have to say, the idea of a reducer somehow widening a field of view while keeping the aperture the same size has never made sense to me. I mean, I know it does. I just don't for the life of me understand how. 

[vlaiv]

24 minutes ago, Swoop1 said:

OK. Thanks for the explanation. I guess I'm still trying to get my head around the difference between astro f and photography f stops.

There is no difference really - they are the same thing - ratio of aperture to focal length.

It is the fact that:

a) there is plenty of light and noise is not as dominant

b) different F/ratio lenses are used on same camera body

that can simplify things so that "photography F/stop" can be be used as indicator of speed - the way it is used in regular photography.

In astrophotograpy - things can't be simplified like that because light is scarce and certain scope can be paired with many different cameras and pixel sizes.

For this reason in astrophotography it is better to think in terms of aperture at resolution rather than only F/ratio.

13 minutes ago, BrendanC said:

I have to say, the idea of a reducer somehow widening a field of view while keeping the aperture the same size has never made sense to me. I mean, I know it does. I just don't for the life of me understand how. 

That is because you need to think of FOV in terms of focal length rather than aperture. It is the focal length and sensor size that determine field of view.

Take any telescope / camera combination - and simply "stop down" aperture using aperture mask - FOV won't change. When you stop down regular lens by using slower F/stop - do you change the FOV? No you don't.

Why would then FOV be related to aperture size?

[BrendanC]

No idea. Beats me. 

 

[wimvb]

6 hours ago, BrendanC said:

No idea. Beats me. 

 

Another way to see how aperture, focal length and field of view are related is to compare to a zoom lens. When you zoom in, the lens diameter stays the same, but the focal length and the view change. So does the F-number.

[ollypenrice]

On 28/06/2021 at 23:23, Swoop1 said:

OK. Thanks for the explanation. I guess I'm still trying to get my head around the difference between astro f and photography f stops.

The difference is quite simple. When a daytime photographer compares F stops s/he is comparing different apertures at the same focal length with the same pixel size.  His or her camera lens is shooting through an iris which opens and closes, varying the effective size of the lens. When the iris is wide open it lets in more light. When stopped down it lets in less light. Obviously, when it is wide open it lets in more light and reduces exposure time. When stopped down, it lets in less light and exposure times increase. What matters is the area of the open aperture. Each reducing F stop equates to a doubling of the open area of aperture and so a halving of exposure time. 

When we put a focal reducer into a telescope we are not increasing or decreasing the size of the aperture. It remains the same, so we cannot expect to find the same simple relationship between F ratio and exposure time. What we need to do is consider an assortment of telescopes of the same focal length. If we do, we can easily put them in order of 'speed,' because the one with the largest aperture will be fastest and the one with the smallest aperture will be slowest. And, guess what, this will be borne out by their F ratios, just as on a camera lens, but only because the focal length is the same in all cases.

Below is a gum tree. If you compare  different focal ratios at different focal lengths you will go straight up it! Only compare F ratios at the same focal length and with the same size of pixel.

lly