Mirrors, faster, slower, f, etc.

[CarlJoseph (Carl)]

Being a newbie to this, I'd love some help understanding some of these terms. I hope this is the right place to post this. If not, mods, feel free to move it.

I now know that magnification is calculated by dividing the focal length by the mm of the eyepiece. M = FL/EP

However I'm a little confused about the 'f' people mention. I know that 'f' in photography has to do with the focal length and aperture. I assume it's the same for astronomy - f/5, f/3, etc. referring to the focal length of the primary mirror. Would this be correct?

In the same sentence, I also see people mentioning faster/slower mirrors. What does this mean?

Thanks,
Af.

[astro744]

Focal Ratio = Focal Length / Diameter of primary mirror or lens.

eg. Celestron C8 = 203.2mm (8") diameter, 2032mm focal length. F ratio = 2032/203.2 = f10. (usually expressed as 200mm dia and 2000mm f.l).

Newtonian 25cm (10"), f5 has a focal length of 25 x 5 = 125cm = 1250mm.

A refractor of 600mm focal length and f ratio of 7.5 has an aperture diameter of 600/7.5 = 80mm.

I have shown three examples with the unkown a different parameter in each case.

Fast is generally f5 and less.

Moderate is f6 to f8.

Slow is generally f10 and over.

The speed of the optical system will determine photographic exposure times but has little impact on visual use. However, a slow system allows the use of less corrected (simpler and less expensive) eyepieces than a faster system. Nagler eyepieces are corrected to f4 but work well in fast and slow systems.

[Bassnut (Fred)]

It is generally also understood that a low f ratio is "easy" to image with and definitley "woose" territory. That is, "slow" f ratios, 9 and more, are what astrophograhpy is all about, hard and interesting. Zoomed in, is where its at.........man. I mean, aint that what telescopes are for , no brainer.

Beware charitans that use "fast" f6 or less scopes, you will lose your self in boring done-that widefield rubbish that pollutes our LCDs and dillutes the pure pursuit that is true "deep sky" astrophotography

[multiweb (Marc)]

Originally Posted by Bassnut

Beware charitans that use "fast" f6 or less scopes, you will lose your self in boring done-that widefield rubbish that pollutes our LCDs and dillutes the pure pursuit that is true "deep sky" astrophotography

Imaging at F/1.8 where does that leave me in your book? "Illegal imaging" or "mega wuss"?

[Bassnut (Fred)]

Well, thats an exception Marc, extremes are always excepted, more extreme the better. F**K, who does the hell else does f 1.8, I love it

[mental4astro (Alexander)]

There is also a visual application for the f/ratio.

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.

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

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

Some slow scopes can be made a little faster by employing specialized lenses called focal reducer, however these tend to be used with astrophotography. These focal reducers are mainly used with Schmidt-Cassegrains.

[CarlJoseph (Carl)]

Thanks for the info folks. I'm starting to understand. I have a 115mm aperture x 900mm newtonian. This means the mirror in that is f/7.8 (900 / 115 = 7.8). Right?

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!

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?

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.

So let's test my understanding. A 1cm square bundle of light beams come through the scope and hits the mirror.

• 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.

I'm asking these questions because I'm thinking of building my first dobsonian. I'm pretty handy (woodworker as a hobby) so feel pretty confident that I can pull it all together. Just got to get a handle on the science behind it.

Thanks,
Af.

[mental4astro (Alexander)]

You are right about the f/ratio being 7.8 on your 114.

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.

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.

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.

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.

There is one book I would recommend you read before you begin. It is considered the 'Holy Grail' of scope building, though a little dated for the design, the reasons, maths, physics and material science it explores has not changed and is important to understand. Armed with this you can build what ever scope shape you like confident that you will still be obeying the principles of how and why the dob design works:

http://www.obsessiontelescopes.com/p...ons/index.html

Bintel sells this book.

I got this book and read it cover to cover before I started any cutting or even planning of my current project. Even though I had a shape in mind, it was important to know what the books professes to purchase the best suited materials and in the correct quantities once I had made up the plans following the book's guidelines. I'm still using the book as I go to double check my calculations and review my understanding.

[Wavytone]

Another thing - f/7 or 8 is IMHO about optimal in a couple of respects:

- it will accommodate a 40mm eyepiece at the low power scale, yet for high power it could use a 4mm on the best of nights - a 10:1 range of magnification.

- the secondary will be small enough not to degrade the image significantly.

Admittedly given the aperture of your scope, its more likely to have a 1.25" focusser and your lowest power eyepiece will be 25mm - it won't fill the field of view of a big 40mm widefield eyepiece, let alone a 2" focusser.

[sasup (Stacey)]

Im not sure I understand what your saying here. Is it that anything faster then f6 is rubish ? Im Kind of new to the astro world and this is the first time I have heard that f6 or faster is sh^t. Wish I would have known that before I bought an f5 dob. Thanks for leting us know. What scope would you buy?
P.S
I have a 16 inch full thickness primary that is going to start polishing soon. I may look in to this abit more.

Originally Posted by Bassnut

It is generally also understood that a low f ratio is "easy" to image with and definitley "woose" territory. That is, "slow" f ratios, 9 and more, are what astrophograhpy is all about, hard and interesting. Zoomed in, is where its at.........man. I mean, aint that what telescopes are for , no brainer.

Beware charitans that use "fast" f6 or less scopes, you will lose your self in boring done-that widefield rubbish that pollutes our LCDs and dillutes the pure pursuit that is true "deep sky" astrophotography

[Bassnut (Fred)]

Im kidding really, just kidding . Lower (faster) F ratios for a given apature give shorter exposure times, so your fine. Its a cranky hobby horse of mine that the gear trend is towards wider field, long FL is much harder to handle for many reasons. Youve made a good choice, just ignore me

[Wavytone]

Sasup,

The reasons people don't like longer focal ratio Newtonians are:

a) the big tube and length makes them awkward for an amateur using home tools to make a stable mount such that it is reasonably steady in even a light breeze. A Dob is the easiest; on anything else (german equatorial) a long Newtonian will be VERY sensitive to wind and the only real solution is to house it in a dome to keep the wind off.

There is a rule-of-thumb that for a Newt on a German equatorial, the diameter of the worm drives must be at least half the aperture of the scope otherwise the geartrain won't be sufficiently rigid to stand the torque applied by a large tube in any breeze, and hold it steady.

For this reason once you get up to say 20 cm aperture, you'll find some using a 20cm f/4 or f/5 for imaging, but no-one would attempt imaging on a portable Newtonian at f/7 or f/8 - it would be impossible without a dome for shelter.

By way of example, a german equatorial mount suitable for imaging with say a 12" f/7 would have to be one of the larger Astrophysics or Mathis mounts and the cost will exceed $15,000 just for the mount. For this reason the larger SCT's from Meade/Celestron start to look quite attractive if you want a long focal length.

b) On an equatorial mount the eyepiece is often in very awkward places unless you have some way of rotating the tube. Hence tube rings... Dobs are by far the most convenient if you're a visual observer.

c) Once you get up to 25-30 cm aperture, anything longer than f/7 means climbing ladders to reach the eyepiece. Dangerous in the dark and even more so on uneven ground.

[ngcles]

Hi Afro & All,

Interesting thread. If you intend on building a Newtonian telescope (irrespective of how it will be mounted) , then I'd strongly suggest you get a hold of an older publication called "Newtonian Notes" by Kenneth Novak which discusses a lot of optical theory in layman's terms and provide quite a bit of guidance on the implications of f/ratio, secondary sizes, coma, off-axis astigmatism and the size of the illuminated field etc etc.

There is no real right or wrong on f/ ratio for a visual-use Newtonian, but there are consequences that have to be lived with either way you go. You simply need to decide what you can and cannot live with or live without.

Just dealing with photography/imaging, a short f/ ratio will provide a wider field on the chip/film need shorter exposures etc. A long f/ ratio will generally provide narrower fields with larger image scale but also need longer exposures meaning a better (more expensive) mount is needed and is more demanding on the setup (and the operator).

Visually however, in terms of "image brightness" the f/ratio has no effect at all provided the telescope is used at the same magnification. In other words, for example a 10" f/9 used at x150 will have an identically "bright" image to a 10" f/5 used at x150. The longer focal length telescope will however use a longer focal length eyepiece. It will also, provided the seeing conditions are almost perfect, produce a slightly more contrasty image because the diffraction effects of the tiny secondary mirror will be negligible compared to the f/5 with a larger secondary.

The optical defect called "coma" affects all telescopes containing a paraboloidal surface. This includes all Newtonians and classical cassegrainians and some others.

The shorter the f/ ratio, the "stronger" (ie deeper) the parabola, the more coma intrudes and a larger portion of the outer areas of the field are seen to be affected with imperfect star-images. In slow telescopes, coma is hardly noticeable. In "slow" small telescopes (say an 8" f/10 Newtonian) the difference between a spheroid and a paraboloid is so small it does not bear considering and with so many "slow" commercial 'scopes of this sort, the manufacturer doesn’t bother to parabolise the mirror at all. No problem with that – it just isn’t really necessary on say a 4½” f/9 Newtonian. Slow optics are also more forgiving on slight mis-collimation and use/need very small secondary mirrors (ie less than 15% by diameter compared to the primary) so that diffraction caused by the secondary has no real practical effect on visual image quality and aesthetics.

If you intend the telescope to be a pretty much dedicated planetary Newtonian, a long f/ ratio is preferable for these reasons. It also means you can achieve high magnifications with longer focal length eyepieces that are more comfortable on the eye in use. Simple eyepieces of older design also perform much better with slow f/ratios because the light-cone does not converge so steeply. Long f/ ratios also have many practical cons for visual use once you reach 8” +.

The tube will be a monster and need a very substantial, heavy mount making portability a serious problem. Long focal length telescopes find it hard to achieve wide fields for things like Eta Carinae (NGC 3372) M42 etc etc as well. Many years ago I had a friend with an 8” f/11 Newtonian which was a very nice and sharp ‘scope that produced lovely images, but was a pain in the you-know-what to use.

“Faster” f/ ratios necessitate much more care when fabricating the mirror and coma will become an increasingly serious issue once you go faster than f/5. The simple solution is to use premium quality advanced design eyepieces that cope much better with short f/ratios and to use a Paracorr that will all but eliminate it. In the US a whole generation of new super-fast Newtonians are being born – like say 28” f/3.6. But they need a very, very experienced and dedicated optician to figure the optic correctly and are extremely demanding on collimation. They also need very large secondary mirrors (in the order of 30% obstruction by diameter) which I personally do not like for visual use, but they do allow you to own a 28” ‘scope and only need 1 step up to observe at zenith – a decided advantage. They are also therefore lighter and therefore need less “engineering” to be rigid.

In my opinion, the sensible thing of course is to compromise and pick a middling f/stop (depending on the diameter of the aperture) that will not be too expensive or too hard to make, be very usable for most or all visual purposes, keep the secondary mirror under 20% by diameter and have a 100% illuminated field big enough to fully illuminate an eyepiece field at say the 2.5mm exit-pupil size while maintaining an at least 75% illuminated field at the edges of your lowest power eyepiece. This is why personally I chose f/4.9 for my 18”. For me it was the best compromise and it meets all the above criterion.

On that, a 13.5” (let’s not split hairs) at about the f/5 mark, I think would be a very good compromise if it uses a 66mm secondary. Faster than f/4.5 and you’ll need a parracor, slower than f/6.5 will be a very long telescope that will be much less “portable”-friendly.

Of course if you are going to image with a Newtonian, there are all sorts of other considerations that need to be taken into account when choosing f/ stop and secondary mirror size.

All telescopes are a compromise of one sort or another. We trade money (ie cost per inch af aperture) against aperture against portability against image quality against width of field against aberration control.

I’m not sure where you would get “Newtonian Notes” nowadays. The last time I saw it was at BINTEL. Others may still carry it. It used to be only about $20-odd, about 150 A4 pages, cheap type-face, hand-drawn illustrations but had a wealth of good, practical advice and wisdom on how to design a Newtonian. The other book mentioned on building dobsonians is also mandatory reading but tends to concentrate more on building the thing and engineering it.


Best,

Les D

[sasup (Stacey)]

super info guys, thanks.

[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

[barx1963 (Malcolm)]

Hey Les
One of the best explanations in less than a book of Newtonion optics,
"a 10” mirror is a 10” mirror" seems obviouys but we sometimes get caught up in fast and slow and which is best for what.

Enjoyed reading it, very informative!

Malcolm

[mental4astro (Alexander)]

Les, shall we step outside and settle this like gentlemen?

I understand what you say about a 10" mirror is a 10" mirror. No argument from me there, but maybe I didn't make myself understood.

*I seen through both 'fast' and 'slow' scopes, of the same size and different. I much prefer the image of a planet through a slow scope, 8" f/6, than a fast scope 10" f4.5, both newtonians and side by side comparison, very similar focal length. The image is clearer and not as glarry in the 8". Don't even go there with collimation and optical quality! Don't! But if you must know, 8" GSO primary, 10" Parks primary.

Conversly, DSOs are better through the 10", though the sky glow is brighter than in the 8".

I don't believe it is the relative size difference of the secondaries. The percentage difference I don't think if preceivable by out eyes, except to "experienced observers". Maybe.

I think there may be a further misunderstanding of the effects of the f/ratio, .

*Well of cause you reduce the apeture with the mask, how else do you increase the effective f/ratio. That is the purpose of the mask. Yes you are reducing light gathering capability, but the planets are bright enough not to need 'light buckets' to observe well. The atmosphere is more a limiting factor to resolution than inches.

Comparing with my 17.5" f/4.5, bright planets are a better visual experience in an 8" f/6, with the same magnification. Too much light in the 17.5".

*The size of a secondary in a slow scope is smaller than that in a fast scope- you said this yourself. So what is there not to understand with a large obstruction not increasing f/ratio? It is reducing the effective diameter of the scope. The central obstruction in an SCT is huge compared to a newt. In the extreme, no way is a 10" f/10 SCT equivalent in light gathering to a 10 f/5 newt, magnification for magnification.


Now, of cause if we were able to do a like-for-like with magnification comparison between a fast scope and a slow one, both same size, we may we see the same image brightness. However, I must say that most comparisons done in the field is by swapping the same EP between scopes, .

The purpose of this thread, and my post, is to offer practical suggestions on how to improve planetary viewing with given gear.

Ta for the bits I did get 'right', .
Now, where can we share a brew?

[ngcles]

Hi Alex,

Originally Posted by mental4astro

Les, shall we step outside and settle this like gentlemen?

Absolutely, it's the only way I'd do it ... only we don't have to actually step outside, do we ??

Originally Posted by mental4astro

I understand what you say about a 10" mirror is a 10" mirror. No argument from me there, but maybe I didn't make myself understood.

That may well have been the problem.

Originally Posted by mental4astro

*I seen through both 'fast' and 'slow' scopes, of the same size and different. I much prefer the image of a planet through a slow scope, 8" f/6, than a fast scope 10" f4.5, both newtonians and side by side comparison, very similar focal length. The image is clearer and not as glarry in the 8". Don't even go there with collimation and optical quality! Don't! But if you must know, 8" GSO primary, 10" Parks primary.

Planets are bright and they have very high surface-brightnesses. The larger aperture telescope will of course produce a brighter image of a planet which is maybe what you are interpreting as "glare" . The image through the 10" will be 56% brighter than the 8". 10 x 10 = 100. 8 x 8 = 64. Perhaps the other reason why you prefer the 8" f/6 is because it has a much larger % of the focal plane that is coma and off-axis astigmatism free -- making a higher quality image because the "sweet-spot" is bigger. Add in the effect of the larger % secondary also lowering contrast in the 10" f/4.5 a bit probably accounts completely for the difference. And I didn't even mention collimation or optical quality ...

Let me tell you Jupiter through a 40" R/C 'scope feels like it's burning a hole in the back of your retina -- at f/8 and over x300 (the f/ratio is not the difference -- even if the 40" was an f/5 Jupiter would be no brighter). F/ratio does not affect light-gathering power.

Originally Posted by mental4astro

Conversly, DSOs are better through the 10", though the sky glow is brighter than in the 8".

Yes, of course. There's an extra 56% more light in the 10" over the 8".
The bright background is likely a function of low magnification/large exit-pupil more than anything else. I'll bet the 8" was being used at a smaller exit-pupil.

Originally Posted by mental4astro

*Well of cause you reduce the apeture with the mask, how else do you increase the effective f/ratio. That is the purpose of the mask. Yes you are reducing light gathering capability, but the planets are bright enough not to need 'light buckets' to observe well. The atmosphere is more a limiting factor to resolution than inches.

But there is little to achieve by reducing (slowing) the f/ratio. The purpose of using a mask is to make the aperture smaller because the chances of finding a relatively steady column of air say 6" diameter is greater than finding one 10, 12 or 17.5" diameter. That's the theory, some people (many in fact) are firm believers in it. I prefer to use the full aperture and wait for those fleeting moments when the seeing will allow the 'scope to show what it can -- but that is purely a matter of opinion.

Originally Posted by mental4astro

Comparing with my 17.5" f/4.5, bright planets are a better visual experience in an 8" f/6, with the same magnification. Too much light in the 17.5".

And this amplifies what I said before. The brightness of the image interferes with being able to see it well. I think this is what you are talking about when you say "glare". But the brighness won't change between a 17.5" f/4.5 and a 17.5" F/9 provided they are used at the same magnification (which means the same exit-pupil too). f/ ratio does not alter image brightness in when used visually . Try perhaps using a neutral-density filter on the 17.5" to make the image a little dimmer but you'll still get the resolution benefits of 17.5". Photogaphically, is a completely different bucket of fish ...

Originally Posted by mental4astro

*The size of a secondary in a slow scope is smaller than that in a fast scope- you said this yourself. So what is there not to understand with a large obstruction not increasing f/ratio?

But it doesn't. I think you ahve the diameter of the aperture and the effective area of the aperture confused.

Let's be silly for a moment. Take two 10" f/6 Newtonians. Each has a fl of 1500mm yes? One has a 2" secondary, and the other has a 6" secondary. Now with the different secondary mirrors has the f/ ratio changed. No not at all. They are both still 10" diameter apertures with a fl of 1500mm -- f/6.

Of course the one with the 6" secondary will have compromised light-gathering :

(10 x 10) - (2 x 2) = 96

(10 x 10) - (6 x 6) = 64

... but it does not change the f/ratio. It will also suffer from severe contrast depletion, but that doesn't change the f/ratio.

Originally Posted by mental4astro

It is reducing the effective diameter of the scope. The central obstruction in an SCT is huge compared to a newt. In the extreme, no way is a 10" f/10 SCT equivalent in light gathering to a 10 f/5 newt, magnification for magnification.

Exactly -- the light-gathering ability is slightly changed, but not really by much. The typical 10" S/C has a 3.5" diameter central obstruction.

So ...

(10 x 10) - (2 x 2) = 96

(10 x 10) - (3.5 x 3.5) = 87.75.

Sure there is a difference in light-gathering, but not that huge really. Where the S/C looses is in visual contrast but only on the steadiest nights where the extra diffraction effects can be seen.

Originally Posted by mental4astro

Now, of cause if we were able to do a like-for-like with magnification comparison between a fast scope and a slow one, both same size, we may we see the same image brightness. However, I must say that most comparisons done in the field is by swapping the same EP between scopes, .

Yep, and this is the problem. You need same diameter, different f/ ratio but same magnification and you will see essentially, same image. F/ ratio does not of itself affect image brightness.

See:

http://www.televue.com/engine/page.asp?ID=140

and to extract a quote:

"F/number is of little importance visually. A "fast" telescope implies a short focal length and a large field. Fast, however, is a term borrowed from photography (an f/5 telescope can take a photograph with one-fourth the exposure time of an f/10 instrument). Visually, well made fast and slow telescopes of the same aperture have no difference in image brightness or resolution.

Many binocular users know this already. While aperture, magnification, and exit pupil are the key specifications for binoculars, manufacturers never give the f/number of the objective as a specification. It means nothing as far as visual image brightness is concerned! I find that photographers have the most difficulty understanding this concept, because their experience that a faster f/number means brighter images on film and in the viewfinder is so ingrained.
"

Originally Posted by mental4astro

Ta for the bits I did get 'right', .
Now, where can we share a brew?

Any time mate, any time. You seem like a bosca fellow to me -- very enthusiastic, in love with the sky and willing to share it with others -- I like that a great, great deal.

When you've done your re-build, why don't you pop-down to Bargo one night for a "nearly-dark" sky experience?


Best,

Les D

[mental4astro (Alexander)]

Hey Les,

I thank you for the fisty-cuffs, mate.

I do think I've been confusing photographic aspects with visual, and even a few misconceptions too.

I hadn't thought of using an ND filter with the planets with the 17.5". I always took them for use with the moon.

I'll take you up on the invite to Bargo, Les. Progress is slower than anticipated with the 17.5". I was warned on this by Blue Skies. It's also a bit intimidating I'm finding.

Hasn't this turned into a great thread. Ta afro boy!

Mental

[barx1963 (Malcolm)]

I have found that for viewing any really bright object the ND filter is very useful. A few months back when Jupiter was high and bright, I used it to great effect to get clearer images of the cloud bands, very useful. Will also try it on Saturn and Mars soon.