View Full Version here: : Does using a focla reducer reduce exposure time?

13-10-2017, 06:57 AM
Does a focal reducer allow more light to hit a sensor or is that only controlled by the aperture?

It would seem it would focus more light from a wider field of view onto the sensor thus more flux. Or is that only controlled by the aperture?


13-10-2017, 07:06 AM
It’s the same light entering the scope, spread over a smaller area so the image is brighter but smaller and the exposure time for extended objects (eg nebulae, moon) is correspondingly reduced as this is inversely proportional to image brightness.

The focal reducer reduces the effective focal length of the telescope, but the aperture remains the same.

The effective focal ratio e = R x F/D where R is the reduction of the reducer (eg 0.8x), F is the focal lengrh of the telescope and D is the aperture. Or e = R x f, where f is the focal ratio of the telescope (ie f=F/D)

Image diameter is proportional to R x F.

Image brightness is proportional to DxD/(FxR) or D/e.

13-10-2017, 07:30 AM
The answer is both yes and no.aperture determines the amount of photons entering the telescope BUT a focal reducer increases the amount of photons hitting each pixel.

To illustrate. A 16” F/8 RC with KAF-16803 and a flattened gives a pixel scale of 0.57”/pixel. With a 0.7x reducer it is reduced to F/5.6 so 0.86”/pixel.

The 16” of aperture determines the flux; the rate of photons entering the system to square arcsec of sky. The focal ratio determines the how that flux is spread across each pixel. So let’s say one shock front delivers 10 photons per hour per square arcsec with 16” aperture.
At F/8 you’re getting 3.25 photons per pixel per hour but at F/5.6 that increases to 7.34 photons per hour per pixel.

14-10-2017, 09:27 PM
Both great replies and pretty much what I thought and have experienced imaging.

Gotta watch some people on other sites trying to sound smart and giving out false data or 1/2 truths.


14-10-2017, 10:12 PM
Amen to that :thumbsup:
(+1 for appreciation of the great explanations too)