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Found 5 results

  1. Does Focal Ratio affect brightness or is it all about field of view. Will an f/5 scope show the Andromeda galaxy more brightly in the eyepiece than an f/12 scope, or is it just about fitting Andromeda in there and seeing more of the sky? Also does the Focal Ratio of a scope have any effect on the amount or length of exposures taken in astrophotography? Thanks.
  2. Hi All, I wonder if someone can please help, I'm fairly new to astrophotography and confused about aperture. Basically I'm looking at a Canon EF-S 55-250mm lens. It has the specs f4-f5.6 does this mean that the lowest point of aperture possible is F5.6 or will it go lower. What I find confusing is that I have a lens that's the EF-S 18-55mm lens which has the specs of f/3.5/5.6 however My camera does allow me to set the aperture at f/8 The reason I ask cause it is advised to use a low aperture of f/8 to capture images of the moon. Any help and guidance would be greatly appreciated!
  3. I read a comment made on here the other day about the maximum magnification you can realistically use in UK skies (250x apparently). Therefore I was wondering what everyone else thinks this is, especially for you guys with 16" plus, apertures. I'm still fairly new to this game (only being observing for about 7 months), and only have a 5" Newt at the moment (but am about to replace with a 10" Dob) so I don't get any higher than 159x (6.3 mm).
  4. Hello All! Currently, I have a Meade ETX 90 telescope. I really like it and get great views of the moon and planets out of it. However, I am hoping to upgrade to a large reflector. I am looking at various scopes ranging from the Orion SkyQuest XT8 to the forbiddingly pricey Orion SkyQuest XT12i Intelliscope. I know that aperture is one of the most important things to consider in a telescope, but I also realize that people can get "aperture fever" and go for scopes that are unnecessarily large. I am wondering; Is a bigger aperture worth the price jump from 8'' to 10'' or from 10'' to 12''? How much more will I be able to see? I have heard that the best telescope for a person is dependent on the kinds of things they want to observe. I don't really look at deep sky objects (though I am getting increasingly interested in them), and mostly enjoy the moon, planets, and a few double stars. I want a telescope that will accommodate this, but is also able to have a great grasp on deep sky objects. Honestly I think I am on the right track with the scopes I am looking at, but I really want some advice on which size is best for me. What do you think? Thanks for the advice!
  5. OK so just as I was writing my original and confused question I had a thought and think I might have worked it out. I've attached a drawing to try and show what I mean and help others if they're ever as confused as I was. I could only find sources that quoted brightness reduces four-fold for a two-fold increase in magnification, but I just couldn't wrap my head around and visualise it (too little mental exercise these days!!) - I knew it was something to do with the area of a circle but that's about it. Then I had a thought and made a little drawing: The blue circle represents what the telescope can see - for example it's maximum field of view (i.e. the most amount of sky it could ever possibly see: I worked out a theoretical 18.92 degrees for the SW200P - the second image shows how I came to this conclusion). The grey object (of no particular shape) in the middle of these blue circles is the same size because as far as the telescope is concerned the sky is the same scale (the scope still "see's" the same circle of sky). The red circle represents a lower power eyepiece's view and encircles a larger [field of view] area of the sky (the lower red circle show's what we'd see in the eyepiece: a smaller, brighter object) The green line represents a higher power eyepiece's view and encircles a narrower [field of view] area of the sky (the lower green circle show's what we'd see in the eyepiece: a larger, dimmer object) Am I right in thinking that the higher power eyepiece takes the light from a smaller [field of view] 'circle' in the sky than the lower power, but 'blows it up' to the same size in the eyepiece for us to see, as in the lower green and red circles? And if so is that what explains why the brightness goes down four-fold for a two-times magnification (because the light from a smaller [field of view] area is being shown at the same/similar size to the eye)? And essentially is this right: the scope always see's the same [field of view] 'circle' of the sky but the different eyepieces pick out different sized [field of view] 'circles' of this? Before I realised this I was under the delusional and confused impression that a smaller object in the sky might be brighter because the whole of the objective aperture could be used for that one object!!
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