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THE TELESCOPE'S F/number

loron91423

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David Galvin

David Galvin

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Posted

I must apologise for having bit my tongue for this long but I am wondering if I am the only person who is actually finding the post(s) involving loron91423 and the 'concerned' replies a little embarrassing.

I would suggest that the moderators should step in and sort this out before the replies become acidic.

Personally, I am not sure if loron91423 is genuine or a spammer although the website does exist and somebody is sending in the post.

I am rather confused by it all. Please make it all better. :?

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Macavity

Macavity

Posted
Posted
I am rather confused by it all. Please make it all better. :?

[O.K. but somewhat tongue in cheek] But I genuinely see (and indeed relate to) what you mean. "Internet Mobbings" are never pretty, even if justified. But, as Prof. Macavity observes, people "deal with anxiety" in different ways? It was VERY difficult to devine if our friend was "legit" or not? My idea is always, if in doubt, say precisely what you mean? But it seems (on a local forums - So.Ca etc.) s/he was equally reluctant to engage in ANY dialogue with others? :?

Moderation is a difficult task, thoughbut. I have managed "international internet facilities" and even sat next to a lady who tried to "moderate" internet news (LOL)! I look on the positive side: Moderators here don't moan about how they are "unpaid and giving up their time...". Many "spats" are allowed to continue to (ahem) natural resolution. On a personal level, my "somewhat ironic humour", hasn't lead to "[insert party] bell-curve theorists" speculating about my inate / national lack of intelligence - Above all, we still seem to have the "right to reply" - VISIBLY! :(

Personally, I hoped "Loron" would adjust to that ways of SGL - Sadly, I recognise that my instinct was wrong in this case? But I do hope that it won't "put off" possible contributors... I, for one, welcome our "colourful" (more human?) characters... :mrgreen:

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dweller25

dweller25

Posted
Posted

Loron actually pm'd me when I asked him a question about the comparative performance of his reflector v his refractor.

He avoided answering my question and just advised me that Brandon eyepieces were the best.

Almost certainly this guy was a dealer and perhaps a tad patronising ???

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Xunil

Xunil

Posted
Posted

Well now, he was one of life's special little soldiers wasn't he?

I thought he was hilarious tbh, I couldn't work out what the heck he was on about half the time, but it was fun watching everyone else's comments while replying. I think given the normal (excellent) level of posting for this forum, we all restrained ourselves very well - loron could have had a far worse mugging.

I don't think if he *is* the resident astronomer at george lutz observatory (is it a "real" observatory, eg run by the scientific community?) that he does them any favours. I found other posts on other forums by him. Seems he has a habit of annoying people:

http://cs.astronomy.com/asycs/forums/p/32308/374831.aspx

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geoff_k

geoff_k

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I see in the other forum he states that "he is correct". Strange that, I thought that only my teenage daughter was right about everything.

I'm all for variety of opinions and dialogue, but not when its a dialogue of the deaf.

Let's get back to some real astronomy and he can carry on using his Branstons or whatever they are.

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David Galvin

David Galvin

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It's hard to know how to relate to people who advocate nuclear annihilation. Oh, you didn't see his "political page"?

and

I thought his political page was rather scary and makes me wonder how many more people out there share his view.

Any way he has gone off the forum and we can now return to learning astronomy again.

Now, I must buy some new eyepieces, but what brand should I go for?

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Naz

Naz

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It's hard to know how to relate to people who advocate nuclear annihilation. Oh, you didn't see his "political page"?

and

I thought his political page was rather scary and makes me wonder how many more people out there share his view.

Any way he has gone off the forum and we can now return to learning astronomy again.

Now, I must buy some new eyepieces, but what brand should I go for?

Surely you have learnt nothing David, asking a question like that, don't you know what brand(on) to go for :(

Sure is quiet around here, is some one missing? :D

Naz

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Ajohn

Ajohn

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Maybe I should clear the points made at the start of this thread. All I can say is Oh dear and hope that beginners will read this too.

The resolving power of a telescope is set by it's diameter. The bigger the better - wish there weren't many other problems in that direction. The actual brightness of the image is set by the focal ratio and diameter. Take 2 telescopes of the same diameter one at f5 and the other at f10 looking at the same object. The same amount of light will enter both telescopes but the image formed by the f10 scope will be twice as big as the one formed by the f5. The same amount of light will be spread over a larger area - it just has to be dimmer. The same thing applies to a star as the defraction spot of an f10 scope will be twice the size of the f5. Increasing magnification via eyepiece will also make the object dimmer for the same reason. The human eye and brain is an amazing thing but colour vision goes as the brightness diminishes and averted vision eventually comes into play.

On the contrast comment on longer focal length scopes a lot depends on just how well the scope is made and central obstructions etc. There is some sense in the argument that the longer focal length scope is more likely to be well made.

The confusing thing I found in this area is the resolution aspects in relation to the size of diffraction disk. As all f5 for instance have the same sized diffraction discs why does resolving power go up with diameter. I've never come across an adequate explanation. Only comments that the actual calculation of the size of the diffraction disk is rather complicated so Aries rather famous formula of dia = 1.2 lambda*f/d is usually quoted where lambda equals 0.00056 mms. The reason is simple the image scale of a 10ins f5 scope will be twice that of 5ins F5 as the focal length will be twice as large too. The image of the larger scope will be brighter because area of the 10ins scope is more than twice that of the 5ins one.

John

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Kaptain Klevtsov

Kaptain Klevtsov

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The confusing thing I found in this area is the resolution aspects in relation to the size of diffraction disk. As all f5 for instance have the same sized diffraction discs why does resolving power go up with diameter. I've never come across an adequate explanation. Only comments that the actual calculation of the size of the diffraction disk is rather complicated so Aries rather famous formula of dia = 1.2 lambda*f/d is usually quoted where lambda equals 0.00056 mms. The reason is simple the image scale of a 10ins f5 scope will be twice that of 5ins F5 as the focal length will be twice as large too. The image of the larger scope will be brighter because area of the 10ins scope is more than twice that of the 5ins one.

John

The way it works is simpler if you are imaging at prime focus.

Lets say we have my setup from last night, an f/5 200mm 'scope and a Nikon DSLR. The Nikon gives a nice scale for the target, M101, at the focal length of 1000mm. The resolution, according to your formula would be 1.2 lamda * 5 (as its a f/5 'scope).

If I switch to the 120mm f/8.33 refractor, then the resolution drops to 1.2 lambda * 8.33, as the spot size is proportional to the f/ ratio. Big f/ number = big spot = lower resolution.

Now, if I get my hands on a 500mm f/2 monster 'scope, the resolution goes to 1.2 lambda * 2.

The diffraction effect comes from the edges of the aperture, so as the area goes up as the square of the aperture, but the edge length goes up in proportion to the aperture, a doubling of the aperture means double the edge length, but 4X the area. Double the aperture (for the same focal length system) and the spot size is halved.

The good bit (aperture) goes towards getting a small spot, but the bad bit (edge diffraction) goes towards getting a bigger spot. As the aperture has gone up 4X, it beats the edge diffraction which has only doubled, giving an increase in resolution of 2X overall.

With a Newtonian 'scope, you get much more in the way of edge due to the spider vanes and secondary, but the same principal applies. By going bigger with the Newt. you can match the resolution of the APO refractor, and as Newts are easier to make and therefore cheaper, you get more resolution for the same pile of cash with a Newt.

The lambda bit BTW is the wavelength of the light.

Kaptain Klevtsov

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philsail1

philsail1

Posted
Posted

"Ajohn" & "Kaptain Klevstov" What a team!!

I had to read your posts several times over to try and understand - but I still don't understand what this "lambda" formula is, but perhaps I don't need too!

I fully "get" the "gist" of what you both are saying though - "The bigger the scope, the better the image." Yes, and I now see why Newtonians are perhaps the best choice for the money.

(It's reassuring to recieve sensible answers, instead of the "cryptic" ones we've been receiving lately!)

Regards,

philsail1

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