Focal ratio

[adamsp123]

This may seem daft but I am curious to know the effect of focal ratio on image quality.

To elaborate, there have been discussions on ISO numbers and the effect on image "quality" .... ie lower ISO - longer subs - more photons - more detail etc.

So if I image at F4 for example say 5 min subs then on a F5.8 image for 10 mins will the image "quality" improve, ok I understand the F4 puts more photons on each pixel but does the longer imaging time at F5.8 actually improve the image? or it just brings it up to the same lightness as the F4 version.

Specifically if I was to image with my F4 200mm Newt and then my 120ED at F6.5 but take significantly longer subs with the refractor will the result actually be better?

Pete

[brianb]

Photons per pixel is what determines signal to noise, and focal length (for a fixed pixel size) determines resolution. These factors work against each other (smaller pixels = more resolution but less light per pixel). But there is a limit to the maximum useful resolution, in practice for DSO imaging this is set by the steadiness of the seeing and the guiding accuracy of the mount rather than by aperture.

Digital sensors have no reciprocity failure & (apart from guiding errors & the increasing chance of a long exposure being ruined by cloud or a passing aircraft) x minutes at f/4 and 2x minutes at f/5.6 (and the same focal length) will produce exactly the same result.

[dph1nm]

Specifically if I was to image with my F4 200mm Newt and then my 120ED at F6.5 but take significantly longer subs with the refractor will the result actually be better?

Both scopes have roughly the same focal length, so the image scale will be the same, hence (even ignoring all arguments about the F-ratio myth!) the 8" image will be brighter (2.8x as many photons). Or to put it another way, it will take you about 2.8x as long to get the identical image on the 120.

NigelM

[ollypenrice]

What's the f-ratio myth? I'm curious! (I mean I genuinely don't know.)

There are some issues not connected with the f ratio/exposure time here, though. Most notably things like collimation, seeing, contrast and colour correction so the images will be most unlikely to end up identical. Try them both and see what really happens - then show us!!

Olly

[Doc]

Olly take a look here:

f-ratio myth

[barkis]

Yep, I'm incined to agree.

Experience is gained by doing, sucking it and see what results ensue. If they are good, then try to refine them by tweaking your own setup a bit, in whichever way you think will do it.

Your M13 image was terrific Pete. I don't know how much better you could have gotten that one..

Good luck at Lucksall, I hope for pristine skies for you guys. Oh! and some great laughs.

Ron.

[themos]

Yes, let's call the f4 200 newt and the f6.5 120 frac equivalent as far as focal length goes. The photon flux will be determined by the relative apertures (make sure you subtract the obstructed part of the newt). To gather the same number of photons (on average), you'll need to expose the frac for longer. But that's not all!

1) Contrast: will suffer in the newt because of the extra diffraction created by the obstruction.

2) Star shape: will suffer in the frac because the mount has to work for longer in each exposure, opening up more oppotunities for wobbles.

3) star size: Which primary/objective is better shaped will determine star size. Things like Strehl ratios are important here.

4) Colour aberration: If your frac is not well corrected then it will spread the colours.

5) Field flatness/coma: Parabolic mirrors produce distorted star shapes far from the optical centre and fracs tend to have a focal surface that is not flat so your edge stars will be out-of-focus while your central stars will be in-focus.

6) Hell knows what else.

[Whippy]

Whether or not you choose to believe that focal ratio makes a difference to your imaging (I certainly do BTW) one thing that would make a difference to the quality of an image is exposure time. The more of it, the better the image.

Tony..