Guest peepshow Posted July 27, 2013 Share Posted July 27, 2013 Take for example the small Meade EXT 90 telescope. This has a clear diameter of 3.5 inches and a f number of f13.8 I have an old magic lantern lens with a dia of 4 inches but because the focal length is only 6 inches that means it has an f number of 1.5 SO, accepting that the magic lens may not be as optically correct as the Meade, and would only be used with a narrowish FOV anyway, will the magic lens produce an image over 5 times brighter on a CCD camera chip as the Meade or be equal to the light gathering capacity of a very much larger telescope with a smallish f number?.......... ( f1.5 / f2.8 / f4 / f5.6 / f8 / f11 / f16 ) This can't be a magical as it seems but I cannot see why not. :-) Link to comment Share on other sites More sharing options...
Tweedledee Posted July 27, 2013 Share Posted July 27, 2013 This thread might help... http://www.eastmidlandsstargazers.org.uk/topic/1252-appature-and-astro-photography/ But I'm sure an imager will explain it better than me. Link to comment Share on other sites More sharing options...
Guest Tweedledum Posted July 27, 2013 Share Posted July 27, 2013 Hi Richard, ITs probably not that simple !.. In a camera lens the f stop increases or decreases the amount of light that reaches the film /ccd. It also has the effect of varying the amount of image that is in focus in front and behind your actual point of focus. (depth of field) In a telescope there is no direct effect on the depth of field, instead the focal ratio indicates the field of view you can expect . Have a look here;-- https://en.wikipedia.org/wiki/F-number Link to comment Share on other sites More sharing options...
dawson Posted July 27, 2013 Share Posted July 27, 2013 I'd nor seen that old thread before; i'd previously assumed imaging at f/10 would take twice as long as f/5 for a given object to capture the same amount of light of the main subject (though it will be framed differently). James Link to comment Share on other sites More sharing options...
Guest Tweedledum Posted July 27, 2013 Share Posted July 27, 2013 With telescopes the only way of increasing light grasp is to increase apperture. Link to comment Share on other sites More sharing options...
dawson Posted July 28, 2013 Share Posted July 28, 2013 Or increase exposure time... If you are counting the number of photons hitting the sensor of the ccd/cmos Link to comment Share on other sites More sharing options...
Guest Tweedledum Posted July 28, 2013 Share Posted July 28, 2013 Exposure time does not influence the light grasp of a telescope. You have moved to the sensor / film.. Link to comment Share on other sites More sharing options...
dawson Posted July 28, 2013 Share Posted July 28, 2013 Sorry, yes, i was referring back to the original question about brightness on an imaging sensor. On a related point which the old tread or wiki link don't explain, why, physically, does a lower f/ number result in shorter exposure times compared to higher f/ numbered scopes, given the have the same aperture? I may have missed several points here. James Link to comment Share on other sites More sharing options...
Guest peepshow Posted July 28, 2013 Share Posted July 28, 2013 Thanks for all that and the links. I am still musing over it all. But my f1.5 4 inch dia lens produces a very bright landscape image when it's projected onto white paper, Far brighter than any other lens that I have. I still have my old 8 inch dia Newtonian mirror and although somewhat tarnished now, produces a very dim landscape image when projected onto a white wall. It's 5 foot focal length so is about f8 So it seems to me, (and I daresay I'm wrong ), that when the image from the f1.5 lens is projected onto my small CCD SPC900 webcam it should produce more of those desirable photons than any equivalent dia telescope. Link to comment Share on other sites More sharing options...
Tweedledee Posted July 28, 2013 Share Posted July 28, 2013 Sorry, yes, i was referring back to the original question about brightness on an imaging sensor. On a related point which the old tread or wiki link don't explain, why, physically, does a lower f/ number result in shorter exposure times compared to higher f/ numbered scopes, given the have the same aperture? I may have missed several points here. James With a short f/number, the all the light from the lens or mirror is focused into a fairly small area. The same lens or mirror with say double the f/number spreads out the same amount of light over twice the diameter or four times the area. Because the same amount of light is spread out, it is dimmer. You also see a larger image, but need a correspondingly greater exposure time to see the same brightness. Link to comment Share on other sites More sharing options...
dawson Posted July 28, 2013 Share Posted July 28, 2013 Pete, thanks for that, I've never heard it described so clearly before. Thank you muchly. James Link to comment Share on other sites More sharing options...
Tweedledee Posted July 28, 2013 Share Posted July 28, 2013 You're welcome James. Thought it best to leave out the formulas and calculations on a Sunday But the maths and physics of it all does work very nicely, but only if you like that sort of thing, and magic is only involved before you get your head round it Link to comment Share on other sites More sharing options...
Guest Tweedledum Posted July 28, 2013 Share Posted July 28, 2013 The maths is lovely, highlighting the relationship to the inverse square law. It falls flat on its face though with film, thank goodness its rarely used as you hit the reciprocity limits of film. Link to comment Share on other sites More sharing options...
dawson Posted July 28, 2013 Share Posted July 28, 2013 Is it: Tan (angular size of object in degrees) x focal length James (Just stumbled on this equation in my new book) Link to comment Share on other sites More sharing options...
Guest devil74 Posted July 29, 2013 Share Posted July 29, 2013 Geeks !! Just get as dob, swiz it roaund and stop all this f rubbish Link to comment Share on other sites More sharing options...
Tibbz2 Posted July 29, 2013 Share Posted July 29, 2013 Geeks !! Just get as dob, swiz it roaund and stop all this f rubbish Link to comment Share on other sites More sharing options...
dawson Posted July 29, 2013 Share Posted July 29, 2013 You dob owners have 'f-issues' too you know, it's not just for geeks Anyway, Tibbz is one of the biggest geeks on here! He wants to study astrophysics, haha JD Link to comment Share on other sites More sharing options...
Tibbz2 Posted July 29, 2013 Share Posted July 29, 2013 (edited) Just because I have an interest in Physics doesn't mean I'm any good at it... Edited July 29, 2013 by Tibbz2 Link to comment Share on other sites More sharing options...
dawson Posted July 31, 2013 Share Posted July 31, 2013 The first chapter in this book (this chapter is available free on Google Books) covers some of these issues: http://books.google.co.uk/books?id=ecSOxMTszJsC&printsec=frontcover&dq=astronomical+equipment+amateurs&hl=en&sa=X&ei=Vjv5UYD0L4PoPNSbgIgD&redir_esc=y#v=onepage&q=astronomical%20equipment%20amateurs&f=false James Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now