Focal Ratio - Reflector Telescope

[Charic]

Focal Ratio - Reflector Only.

Reading a new thread here, and straight away, reports of "you need certain lenses due to the fast f/ratio.......WHY!

FACT: The reflector Telescope's light gathering power and its ability to resolve detail is related to the diameter (aperture) of its objective (primary lens or mirror). The larger the objective, the more light the telescope can collect and the finer detail it can resolve at the focal plane/point.

FACT: The focal length determines the telescopes magnification with a given eyepiece.

FACT: The f-number ( Written as a ratio f/# ) is the ratio of the lens' focal length to the diameter of the entrance pupil.

Why  state how faster a  10" reflector is over an 8" reflector, and the fact that you need `different lenses` / `better lenses` / `expensive lenses` ?

I understand f/ratios from 35mm and DSLR photography. The light has to pass through several lenses, along the length of the barrel, past an adjustable aperture ring to the film plane/ sensor  By adjusting the aperture in f/stops you increase/decrease the depth of field, and the amount of light that can enter. The speed setting controls the duration of the light.  That all makes sense.

On my relector telescope, there are no lenses for the light to pass through, only a parabolic primary mirror. The light is directionally reflected to the secondary mirror then redirected up the draw tube to a focal point, where the fitted eyepiece is moved towards/away ( up/down for some folk ? ) from the focal point in order to see the image in the eyepiece.

Why/where is f-ratio so important on the reflector telescope?

I`ve researched and cant find any reason to worry about f/ratios on a reflector telescope. I know CATS & FRACS  need the f/ratio. but not my  8" Skyliner?

Maybe I`m still missing something! theres always two sides to every story / debate ( by the way, Im comparing the Skyliner 8"- 10" which provides the same lenses at the point of sale ? ) so maybe someone might change my view on the subject, but Im certain, that the f/ratio (bought over from the photographic industry is not so important on  a  basic reflector telescope .

[AlexB67]

Fast and slow have benefits. You can argue both sides of the coin, but in a nutshell, and probably one of the most important factors is that it has all to do with optical aberrations, and they grow with a faster f ratio. This means the quicker away form the centre of the view in an eyepiece aberrations can be seen more quickly ( mainly due to coma being the more serious one ) , it also means the more difficult it becomes to collimate the scope for optimal performance. 

The net result is that in a faster scope you need more expensive eyepieces  and a coma corrector, if say faster than f/4 - f/4.5 or to retain a good quality view across the FOV in wider angle eyepieces. 

Much more that can be said on this topic, but my much too late dinner is ready and need to go, but this will hopefully wet the appetite as food for thought to do some reading around and no doubt others will chip in 

[John]

All scope designs have aberrations of one sort on another. With fast newtonians coma is one which comes into play, the faster the newtonian, the more coma it produces. Most eyepieces have an aberration called astigmatism to a greater or lesser extent and this becomes more apparent when eyepieces are used in faster scopes (of whatever design).

I think this web page covers this in more detail quite usefully:

http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

[Charic]

AlexB67......... M A N Y  T H A N K S,  for answering my question. Problem solved!

Reflecting telescopes only suffer from monochromatic aberrations, and out of the  five aberrations available: Coma, Spherical Aberration, Astigmatism, Field Curvature, Distortion, I would only really be interested in Coma, for my parabolic reflector. Coma is an off-axis aberration.  Stars in the center of the field wont be affected by coma, but the effect grows stronger toward the edge of the field. My eyes will probably correct for this, Ive not actually witnessed this yet (will need to check next time) so Its not much concern to me at present. I tend to set focus for the middle anyway, despite the fact, the image will passing through my field of view.

It would seem that most Newtonians suffer from Coma aberration at the field edges (again, I`ll have to check mine next time?) Coma is a function of both off-axis distance and focal ratio, meaning faster focal-ratio (smaller f-number) telescopes will have more coma than a similar size but slower telescope. So on paper then, an 8" f/4 Newtonian should more coma than an 8" f/6 Newtonian. I now know that Coma correctors are available that can minimize the amount of coma in a Newtonian design.  These lenses will fit into the focuser ahead of the eyepiece.  Since coma affects the edges of a field, getting larger with increasing distance from the optical axis, it is a significant aberration with regards to wide-field viewing and imaging. So unless I purchase some 2" super wide 30mm+ eyepieces, Ive nothing to worry about. And if i`m offering advice on here, I can now understand why any eyepiece may be of some concern, If esspecially super-wide, or the requirement of correctors. Thanks again.

John...... Thanks also, your text was added whilst I was away writing my story? Aberation it is, Coma. I have an answer.

[ronin]

Your post seems to expect that the edge of your image is created from light incident on the edge of the mirror. That is wrong. The light that creates the edge of your view will hit the mirror the same as the light that creates the centre of your image - everywhere on the mirror. The light is collimated and all light from your object will be equal across the mirror immaterial of where it came from. Getting your head around collimated light can be a bit odd.

One aspect people forget or don't realise on a scope is that your scope does not output an image, it takes collimated light in and puts out collimated light, no image.

Take a small circle of light, place a lens at a distance equal to the focal length from the light circle, what comes out the lens will be collimated light from the circle source, place another lens a little distance away to reimage the light and you get an image of the circle. Now put a piece of card to block off the lower (or upper) half of the beam bewteen the 2 lens and you get a complete image of the cirle, not half of it.

The edge of the image you see is not created by the edge of the mirror, it is created by all the mirror.

A fast lens will need greater curvature and this gives rise to spherical aberration, the same way as a fast parabolic mirror will give greater coma. Both are caused by the profile not being ideal. Hubble is not parabolic for a reason = it is not good enough.

If you design a lens then a fast lens will have more chromatic aberration then a slow lens, of the same glass types, this is true for achro, ED's, apos, then comes depth of field and depth of focus. All intertwined and all related to the f number in some way.

Is the lens thick or thin? Here the f number remains the same but other properties may alter. If you have achromat of 2 thick lens of F1 and F2 they will create an achromat different to 2 thin lens of F1 and F2 when combined. Same glass, same radaii, same focal lengths but 2 different lens at the end.

[Charic]

ronin......*Your post seems to expect that the edge of your image is created from light incident on the edge of the mirror. That is wrong*

I`m not sure I meant it to sound like that!  I`m aware that light from source travels in parallel lines (collimated) therefore hitting parts of the mirror not affected from shielding from the spider and secondary mirror!  and Ive  noted  through some  optics i`ve used in the past,  that the image was always better centrally (on axis) rather than towards the edges (off-axis) due  from the bending of the light  from the objective to the focal point.  I`m still learning. I like to try and delve deep into subjects to  try to further understand their function. I`ll take another look at collimated light  and  parabolic mirrors. Cheers.

[Charic]

ronin.......

It would seem that most Newtonians suffer from Coma aberration at the field edges

Your post seems to expect that the edge of your image is created from light incident on the edge of the mirror. That is wrong. The light that creates the edge of your view will hit the mirror the same as the light that creates the centre of your image - everywhere on the mirror.

Thanks for your guidance.

I`m assured that Parabolas (my parabolic mirror) has the ability (due to its reflective surface) to allow light, entering the mirrors axis of symmetry, to be reflected to its focus, regardless of where on the parabola the reflection occurs. And the points between  Vertex and Focus measured along the axis of symmetry, becomes the  focal length. i learn something every day/Night in this case.  Take care.