Affect of focal length on images?

[pipnina]

Say, for example, you had a 50mm lens at f6 and a 100mm lens at f6. If you pointed them at the same DSO (say orion neb down to witches head FOV) obviously you would expect sharper results from the 100mm, but is there an affect on other aspects of the image?

After all, the 100mm is gathering 4x the amount of light, but also trying to focus light from a smaller field of view, so my assumption would be that:

• Stars would be brighter in the 100mm (might lead to stars becoming more overcrowded due to wide fov?)
• Nebulae would be the same brightness in the 50mm & 100mm, but fainter detail in the nebulae will come out faster in the 100mm?

I can imagine there are many astrophotographers here who have used a wide array of apertures & focal lengths, and can shed light on this matter.

 

Thanks

 

P.S. I haven't been around here for a while, clouds & daylight savings have done my observing in (hopefully getting some observing done on the 12th next month if all is well!)

[wimvb]

Pixelscale (arcsecs per pixel) at 100 mm would be half that of 50 mm. So each pixel covers only 1/4 of the sky at 100 mm as compared to 50 mm.

The 100 mm at f/6 has the larger aperture and gathers more photons per unit of time. Object SNR will be higher.

There was a discussion about this recently in the astrolounge.

 

[pipnina]

15 minutes ago, wimvb said:

There was a discussion about this recently in the astrolounge.

I suppose the NGC 1964 post does answer the question. Thanks

[ronin]

As both f/6 the objects imaged should be the same, collects 4x as much light and produces an image of 4x the area. Not sure on a point source such as a star since optics will not produce another point source.

Pixel scale is altered and for imaging that is likely most relevant.

As said image is bigger owing to the longer focal length also the image formed is flatter. so if the sensor is smaller a flattener may not be required for the edge of the sensor.

[pete_l]

27 minutes ago, ronin said:

As said image is bigger owing to the longer focal length also the image formed is flatter. so if the sensor is smaller a flattener may not be required for the edge of the sensor.

My understanding is that for the same F-ratio, the angle of convergence of the light cone will be the same. So the flatness (unless I am misunderstanding the term) would be the same.