Reduction of F#

[bmitchell82]

howdie all, question for the more knowledgable.

having a big F ratio allows a narrow field view aka taking a photo you get more aparent resolution and more detail but at the expense of FOV.

having a low F ratio allows wide field shots although you can zoom in on things nothing beats Optical zoom over Digital zooming.

My question is if i where to stop down a 10" to say a 6" apart from the obvious factor that this will change the F ratio and subsiquently decrease the light capture, will this serve the same as having a high F ratio to do galaxy imaging or am i just reducing the light capturing ability with no gain.

Im just looking to confirm what CCD calc says. because the Camera FOV didnt change when the aperature decreased and F number where increased for a 40d.

Look forward to hearing what has to be said:thumbsup:

[Hagar]

High Brendan, To my way of thinking absolutely nothing to do with the F ratio or magnification will change due to the fact that the mirror is configured and ground as a 10". The curvature and focal distance will not change. The amount of, or ability to grasp light will however change and the image circle will also be reduced giving a black or at least false border to the image. (vignetting)

Just my thoughts, no science involved.

[mswhin63]

This was talked about in the later part of another discussion. I was considering reducing my aperture on my 12" DOB to change my F/L.

http://www.iceinspace.com.au/forum/showthread.php?t=45365

[Alchemy]

f15 gives narrow field of view, resolution is dependant on aperture and quality of optics




what you get in a telescope is what you get , if you want to get in and tight longer Focal length is required. NOTE--- you will get to a point where you are fighting seeing and sampling rates theres not uch point in having 100 pixels per arc sec if the seeing is poor.

:screwy: if you cut of a leg will you run faster ?

[bmitchell82]

mmm im guessing so, but then again you look at camera lens's, they don't change the glass, they only change the aperature and focal length. but i guess thats the key, to change the focal length. oh well. :) ill just have to wait till curtin gets the polar wedge and a guide scope to run high res photos.

[Terry B]

Yes you need to change the focal length. Reducing the aperture of the scope just reduces the light capture and also the resolution as resolution is proportional to diameter.(with a few caveates as seeing becomes involved as well)

[bmitchell82]

Yes i do understand that if i want to do it properly i need to get sorted with a slower f number. and with the resolution in photography yes there is the limiting resolution with the scope. but think of it this way, take a photo of a bug on the ground, its small clear but small. if you where to digitally increase the size of the bug there is a point where it is no longer clear. though using a macro lens so the bug is initally bigger (optically zoomed) you bug is now clear and big.

Im using a 10" f4.7 newt with a dslr my field of views are quite a size aka 3800*2500 taking photos of say M83 it ends up being a 50c piece size in the middle, the picture is clear, but zooming it in much more you start to see the effects of blurring, and no amount of "sharpening" will make it better.

If i where able to reduce my aperature to gain a few F# then for the time being this would favorable untill i have the funds to purchase a cassigrain maksutof or other form of di optical system.
:rolleyes:

"He who asks a dumb question looks dumb for 5 minutes... He who doesn't ask the dumb question stays dumb for life!"

[mswhin63]

I am not sure about the final outcome of this method of reducing apeture which is why I am willing to try as it seems like a relatively in-expensive experiment to try.

If it works, I may only need to buy a decent EQ6 mount.

[Alchemy]

no no and no



yes yes yes

your analogy of the bug is understood,

the focal length is independant of anything else 1000mm will be 1000mm always

the F RATIO in simple practical photo terms is really describing the amount of light per unit of area hitting your chip ie lower f number = more photons

Aperture.... this is MOST important..... the larger you go the greater capability you have of getting resolution

note a 6 inch refractor has a better resolution than a 6 inch reflector. various scopes have differing abilities.



were working to fix that :D

chuckle chuckle

clive

[TrevorW]

I read somewhere that the F ratio when describing telescopes should not be compared to the F ratio when referring to camera lens

Telescope Attributes - Telescopes are designed to gather light and bring it to focus so that the image can be examined in detail with an eyepiece, or recorded on film or with a digital camera. Telescopes reveal fainter objects than can be seen with the eye because they gather more photons than the eye can gather. Smaller details can be seen because they also magnify objects. The nomenclature used to describe telescopes and camera lenses can sometimes be confusing. Telescopes are usually talked about in terms of aperture, while camera lenses are usually talked about in terms of focal length. Most people will say they have an 8 inch telescope (meaning aperture), but they will also say them have a 300 millimeter camera lens (meaning 300mm of focal length). No wonder it's confusing! But we can easily sort this out.
Telescopes and camera lenses have three main numerical attributes that we are concerned with in describing them:



Aperture - The aperture is the size of opening in the telescope through which the lens or mirror gathers light. It is the most important attribute of a telescope because light gathering is what telescopes are all about. In astrophotography, the larger the aperture, the more photons can be collected. Aperture, however, is not the only criteria for judging a telescope. Optical quality is just as important. You can have a gigantic aperture and if the optical quality of the telescope is not good, the light won't be very well focused, and the images produced won't be very good. Aperture is the main determinant in how faint of a star you can see with a telescope. The down side to aperture is that as the size of the aperture goes up, so does the cost and complexity of making the optical system, as well as the weight and size. Bigger apertures also usually mean more focal length, and this makes mounting them, carrying them around and using them more difficult, especially for astrophotography. Aperture is measured in inches or millimeters (mm). There are 25.4 mm in an inch, so a 4-inch aperture telescope has an aperture of 101.6 mm.

Focal Length - The focal length of a telescope is the distance from the objective lens or mirror at which the light comes to focus. The longer the focal length, the larger the image is that forms at the focal plane, and the higher the magnification of the telescope. Increased magnification with longer focal lengths is a good thing for small objects like planets and double stars, but undesirable things also get magnified, like poor atmospheric seeing, and imperfections in the telescopes drive and wobble in the mounting.
Focal length is also measured in inches or millimeters. Camera lenses usually give the focal length in millimeters. A simple lens with a focal length of 300 mm will form the image 300 mm behind the lens. Some telescopes have a secondary mirror that bends the light path, sometimes even folding it back on itself, making the physical length of the instrument much shorter than the focal length would imply.

Focal Ratio - The focal ratio is the relationship between the aperture and focal length. The focal ratio is defined as the focal length divided by the aperture. For example, a refractor with a focal length of 800mm and an aperture of 100mm has a focal ratio of 800/100 = 8 or f/8. The focal ration gives the relative "speed" of the optical system. This is important for recording extended objects such as nebulae and galaxies. A faster focal ratio will record an image faster (with a shorter exposure).
Focal ratio is also known as the f/ratio, and is described by the f/number.
For example, a 4 inch refractor has an aperture of about 100 millimeters. If the focal length of this scope is 500 millimeters, then we can determine the f/number by dividing the focal length by the aperture, which in this case is 500 / 100 = 5. So we say this scope has an f/ratio, or focal ratio, or f/number of f/5.
F/5 is a mid-range f/number. Mid-range f/ratios are usually about f/5 to f/8. "Fast" f/ratios are usually considered about f/4 or lower, such as f/2.8 or f/2. You won't usually find f/ratios this fast in a telescope, but you definitely will in camera lenses. Slow f/ratios are anything bigger than f/9 or so.
F/ratios are also known as f/stops in photography. Each f/stop is equal to a doubling or halving of the amount of light. For example, an f/ratio of f/4 lets in twice the amount of light as an f/ratio of f/5.6 and requires half the exposure.
The full f/stop series, in one stop increments is:
f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/64
These numbers continue on each end of the scale, but these are the practical working range of f/stops.
Each of these f/stops is equal to a one-stop difference in light getting through. So every time you change the f/stop by one full increment, you also have to change the shutter speed, or exposure time, by doubling or halving the exposure to compensate.
For example, at the same ISO (Film speed or digital camera sensitivity), a 1 second exposure at f/5.6 would equal a 2 second exposure at f/8, or a 1/2 second exposure at f/4. All would be equivalent.
Here is a list of equivalent exposures, all allowing the same amount of light to reach the sensor:
f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/45 f/64 1/1024
sec 1/512
sec 1/256
sec 1/128
sec 1/64
sec 1/32
sec 1/16
sec 1/8
sec 1/4
sec 1/2
sec 1
sec 2
sec 4
sec
For simplicity in the short exposures, the higher shutter speeds are rounded off, such as 1/32nd sec is rounded to 1/30th sec, 1/64th to 1/60th, 1/128th to 1/125, 1/256th to 1/250th, 1/512th to 1/500th and 1/1024th to 1/1000th. The differences are so small as to be inconsequential.
If you take a camera lens with a fixed focal length, and stop down the lens, and look at the lens from the front, into the camera, you will see the size of the hole made by the diaphragm blades gets smaller as the f/number gets bigger. f/32 is a very small hole compared to f/2.8. f/32 is a "slow" aperture because the small hole does not let a lot of light get in over the same time exposure as a larger hole. It's "slow" because it requires a longer exposure.
Long focal length instruments with slow focal ratios will work well for bright objects like the Sun, Moon and planets. You can get by with scopes with high f/numbers because the exposures will still be reasonably short. Long focal length instruments also have small fields of view.
Short focal length instruments have wider fields of view and usually have faster focal ratios and can record faint extended objects faster.

[Robert9]

Hi Brendan, Haven't seen you for a while.
Contrary to earlier posts, I consider there is no difference between camera lens and telescopes. They are the same. The ratio of the diameter to the focal defines the f stop or focal ratio - they are the same figure. The aperture control on a camera effectively cuts the diameter of the lens, thus reducing the amount of light entering the camera. Same as for a telescope. If you stop it down, you will reduce the amount of light entering which amounts to effectively reducing the focal ratio. You will not change the focal length. That is a physical dimension ground into the lens or mirror. Only the focal ratio can change and that depends as said above, on the ratio of the focal length to the effective diameter. Change the effective diameter and you change the focal ratio - camera or telescope.
Robert

[sheeny]

FOV is not a function of f/ ratio. The FOV of a telephoto zoom lens does not change with changing aperture... it changes with change in focal length.:)

FOV is a function of the focal length of your scope and size of your camera chip (or characteristics of your EP). So stopping down a newt won't change FOV - but it will reduce resolution, light gathering power, etc.

Adding a focal reducer to a cassegrain type scope does change the f/ ratio, but it leaves the aperture the same... so it changes the all important focal length, and so changes the FOV while maintaining the light gathering power and resolution.;) Perhaps this is where the confusion comes from?

If you want to do higher magnification imaging, increase your focal length i.e. add a barlow / Powermate.

Al.

[Starkler]

For photography is simplest to think of it this way..

focal ratio determines the light density hitting any unit area (pixel). -> exposure.

focal length determines the field of view.

[scopemankit]

If you want to change the focal length of you system use a Barlow or a focal reducer.

[bmitchell82]

Ahhh now to figure out how to attach my 2x barlow to my dslr.. :) off for another tinkering adventure.!



University has that kind of effect :( hehehe, don't worry i have been getting out and about using my scope in anger against the sky, im still getting there with the photography, but its getting better. check the web site :)