[ngcles]

Hi All,

Having just re-read this thread, I think there are a couple of misconceptions that need to be commented upon

Originally Posted by mental4astro

Fast scopes are considered better for seeing 'faint fuzzies' like galaxies, and other 'extended objects', that is faint objects that have a large surface area.

I'm sorry but I believe this is incorrect. The only disadvantage a slow f/ ratio has is that it will have a smaller Field of View (FOV) and you won't be able to see everything of a very large object (like say Eta Carinae (NGC 3372) in one frame.

Example:

10" f/5 Newtonian with 32mm 50 deg AFOV eyepiece gives x39 magnification and 1.28 degree diameter FOV.

10" f/8 Newtonian with 32mm 50 deg AFOV eyepiece gives x62 magnification and a 0.8 degree diameter field.

But they both gather exactly the same amount of light ... a 10" aperture 'scope is a 10" aperture 'scope. If they are both used at the same magnification (obviously with different fl eyepieces in each ‘scope) they gather exactly the same amount of light and will have exactly the same exit-pupil. The image will essentially be the same if the eyepiece design is identical.

Originally Posted by mental4astro

Slow scopes tend to provide better planetary images by innately reducing their glare.

Well, no. How can “slowness” reduce glare? As I said, a 10" 'scope is a 10" 'scope. All 10" 'scopes gather exactly the same amount of light. Visually, they do not produce "dimmer" or "brighter" images according to how long the focal length is. This obviously is a consideration for imaging and photography, but not for visual use.

The reasons why the slower f/ratio is preferable to some extent on visual observing of planets is because they have a very small secondary mirror, reducing diffraction effects to negligible levels. In addition, instead of having to use a teensy-tiny focal length eyepiece with an eye-lens the size of the pin-hole to achieve say x200, the "slow" 'scope can use a 10-12mm focal length eyepiece with a wider eye-lens which is more comfortable on the eye and produces less eye-strain. The “slower” ‘scope will also appear to suffer less from off-axis astigmatism and coma because of the more “relaxed” light-cone and will have a larger percentage of the FOV will be coma-free and sharp. The faster ‘scope can regain “lost ground” by using modern advanced design eyepieces designed to cope with f/4.5 and also with a Paracorr – that corrects coma and produces a much larger “sweet-spot” within the FOV.

Leaving aside the issue of coma, A10" f/4.5 will probably have/need a 25% by diameter sized secondary for all-round visual use. This will produce a less contrasty visual image in perfect or near perfect conditions than a "slower" telescope that uses a secondary mirror <20% by diameter.

However, 9/10 nights, assuming similarly good optics and being used at the same magnification, there will be nothing to choose between say a 10" f/4.5 and a 10" f/7 on this contrast issue. On night 10 when the seeing is very, very good or near perfect, the increased contrast the slower Newtonian produces will become evident to experienced observers.

The BIG down-side to that of course is the physical-size thing and portability.

Originally Posted by mental4astro

Fast scopes can be made 'slower' by placing a mask over the mirror or objective, there by increasing the f/ratio.

Well, no. Actually all you are doing is reducing the aperture. Because the aperture is reduced and the focal length stays constant, the f/ ratio as a consequence increases. But you are throwing away light-gathering power and resolution (both of which are a function of aperture diameter).

Originally Posted by Afro Boy

I can purchase a 340mm (13.4") f/5 mirror (for my first build of a dob I'm thinking of). This will mean it has a focal length of 1700mm so I'll need to also build a ladder!

Probably not. Just one step needed at Zenith I’d think. With my 18” f/4.9 I spend the vast majority of my observing with either both feet on the ground or one step up a four-step ladder. It’s only when the ‘scope is pointed close to zenith in “Dobson’s Hole” that I need the second step up the ladder.

Originally Posted by Afro Boy

The smaller (faster) the f/ratio, the more "curve" there is in the mirror and therefore the smaller the focal length. So why do all these people build dobs that are 2+ metres long? Why not just get a faster f/ration mirror to reduce the length of the tube?

For the reasons outlined above. Longer fl Newtonians are better optical performers and the aberrations are minimised – but not eliminated. The "faster" you go, the more aberrations will intrude and you will need expensive, exotic eyepieces to correct and also a Paracorr. In addition, with fast 'scopes you’ll need an increasingly large secondary mirror that blocks more and more of the incoming light and produces undesirable diffraction effects. It’s all a compromise …

Originally Posted by Afro Boy

So let's test my understanding. A 1cm square bundle of light beams come through the scope and hits the mirror. [LIST][*]On a perfectly flat mirror, this will take up 1cm square of mirror surface exactly. [*]On a slower less curved mirror, this will take up a little more than 1cm square of mirror surface.[*]On a fast highly curved mirror, this will take up more than 1cm square of mirror surface = more light reflected to the ocular = better seeing.

No. a 10” mirror is a 10” mirror. They don’t gather more light with shorter focal lengths or less with longer focal lengths for the reasons above.

Originally Posted by mental4astro

There are several reasons why not to build too fast a scope:

1- The faster the mirror, the harder it is to produce the correct figure (shape). Not just a little harder, a bloody lot harder.

Absolutely correct. The degree of difficulty in getting the figure correct faster than f/5 sliding upward to say f/ 3.5 almost increases exponentially.

Originally Posted by mental4astro

2- Fast scopes suffer from an artifact called 'coma'. This is seen in a long focal length eyepiece as a flaring of the stars towards the edge of the field of view, getting worse the closer to the edge. This is a result of the eyepiece not being able to deal with the 'shape' of the light cone the mirror produces to correctly focus all the light to the one point. It is not a defect. Better quality eyepieces deal with this better, but the extremely wide field of view EPs will still show this. There are 'coma correctors' which are produced to deal with a range of focal ratios, but these are not suitable to all situations. .

All telescopes with a paraboloid suffer Coma, but the size of the coma-free field gets smaller and smaller as f/ ratio decreases.

Originally Posted by mental4astro

3- The size of the secondary mirror starts to get very large to deal with the shape of this light cone. It effectively reduces the f/ratio. So a balance is necessary to be found.

How can the size of the secondary reduce the f/ ratio? It is a flat mirror.

Originally Posted by mental4astro

The surface area of the mirror is only necessary to consider as a flat surface, but keep in mind that its shape is parabolic. This shape causes the light to be reflected to a singe point called the 'focus' -it is a mathematical term!

What is important here is the relative shape of the 'cone of light' the mirror produces. A slow mirror produces a cone that is long and relatively skiny. A fast mirror produces a short cone, quite stumpy in comparison. From this you can begin to see that to get this cone to be reflected at right angles to place the focus outside the path of the incoming light, a slow scope needs a smaller secondary than a fast to get all the light out. It is not a case of one-size-fits-all with secondaries.

Spot-on!


Best,

Les D