[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