Guest Altair Posted February 7, 2014 Share Posted February 7, 2014 80mm has faster focal ratio (F6.25?) hence shorter exposure time needed to capture faint details. Wider field but more field curvature than 100mm. 100mm f9 has a higher focal ratio so exposure time is longer but the size of the object on the chip is consequently bigger. Less field curvature than the 80. Better colour correction and more of a visual observing scope. You can use a 0.8x reducer to speed it up though. Longer exposures and longer focal lengths take more skill to get right and place more emphasis on guiding accuracy because any wobble shows up more easily on the sensor. I'd recommend 80 for imaging as easier to get on with.. Hope summary makes sense... I disagree that you can't image with an f9 scope though as I've seen it done, its just more challenging. Link to comment Share on other sites More sharing options...
dawson Posted March 27, 2014 Share Posted March 27, 2014 Just stumbled again over an article I read a year or more ago, which is where I'd got the idea stuck in my head that it is the f/ratio which determines the field of view: "A 10†long, 16.6†diameter tube willhave the same FOV as the 1†long [,1" diameter]tube. Optical systems of the samefocal ratio will have exactly the samefield of view at prime focus." From: http://www.danlessmann.com/articles/telescope%20optics%20101-part1.pdf JD Link to comment Share on other sites More sharing options...
Perkil8r Posted March 27, 2014 Share Posted March 27, 2014 (edited) You need to read the last sentence of that section Either that or I have a space time bending physics altering 200p. The FOV with the same camera on my 200p is smaller than the FOV with my ED80. The 200p being f5, the ED80 being f6.25 Edited March 27, 2014 by Perkil8r Link to comment Share on other sites More sharing options...
dawson Posted March 27, 2014 Share Posted March 27, 2014 Jd Link to comment Share on other sites More sharing options...
dawson Posted March 27, 2014 Share Posted March 27, 2014 Mike, Do you mean: "I should also note that other thingscan reduce the FOV. The field ofview allowed by the focal ratio atprime focus is thegreatest field ofview I can achieve with this tube" I presume this means any narrowing of the FoV by a narrow focus tube or using a 1.25" adaptor rather than a 2" one? The FoV in your ED80 is wider than the 200P because the focal length is only 500mm compared to the 1000mm of of the 200P, no? and independent of them being f/5 and f/6.25. Am I missing the point yet again?! I find that article is confusing as I read it that all scopes with a f/ratio of say 5, have the same FoV as long as nothing else impinges into the light path.... JD Link to comment Share on other sites More sharing options...
Perkil8r Posted March 27, 2014 Share Posted March 27, 2014 Mike, Do you mean: "I should also note that other things can reduce the FOV. The field of view allowed by the focal ratio at prime focus is the greatest field of view I can achieve with this tube" I presume this means any narrowing of the FoV by a narrow focus tube or using a 1.25" adaptor rather than a 2" one? The FoV in your ED80 is wider than the 200P because the focal length is only 500mm compared to the 1000mm of of the 200P, no? and independent of them being f/5 and f/6.25. Am I missing the point yet again?! I find that article is confusing as I read it that all scopes with a f/ratio of say 5, have the same FoV as long as nothing else impinges into the light path.... JD And that is why I would question the validity of this piece. I am not saying he is wrong, just that it seems to contrdict direct evidence. As you say, FOV is dependent on focal length primarily. Link to comment Share on other sites More sharing options...
dawson Posted March 27, 2014 Share Posted March 27, 2014 Ah, sorry, i misinterpreted what you meant. At least now i know where i got those duff neuronal connections about F/ratio and FoV. Jd Link to comment Share on other sites More sharing options...
Orion Posted March 28, 2014 Share Posted March 28, 2014 I'm thinking about buying an apochromatic refractor - 4 inch. Having read the topic, I'm a bit put off a 4 inch now because of the relative slowness wrt ed80. I did favour 4" for dual use as visual, and AP - both dslr and web cam imaging. But wouldn't a focal reducer make the 4" faster and therefore closer in fastness to the ed80? Why is a larger scope worse than an ed80 for AP ? I think I partly understood some of the answers that erred in favour of the smaller ed80 (easier to guide and control on a mount due to less weight). But are there some advantages of the 4 inch over the 80 ed? I'm wondering now whether I should step down to an ed80 size aperture? I was originally thinking of going for a goto HEQ5 as the mount plus 4" apochromatic refractor together with a refractor guidescope and loadstar guide camera. But I'm uncertain of the imaging refractor size now. Derek. Link to comment Share on other sites More sharing options...
+RonC Posted March 28, 2014 Author Share Posted March 28, 2014 Hi Derek, I have an ED80 and a HEQ5 although I use the frac on an EQ5 Pro without a problem. I don't understand some of this thread but got the general idea! Also the ED80 is a semi-apo as opposed to an achro so better colour and no fringing etc.. Worth thinking about?? Link to comment Share on other sites More sharing options...
dawson Posted March 28, 2014 Share Posted March 28, 2014 I don't understand some of this thread but got the general idea! I'm glad you get the general idea, i'm still lagging behind Jd Link to comment Share on other sites More sharing options...
Guest NadeemShah Posted May 5, 2014 Share Posted May 5, 2014 I remember sometime back, I was imaging with a 120ed scope @f7.5 .Even though it did not have the speed, the images were nice in contrast over faster scopes & better corrected to the edge with a Atik 383L+ without a reducer/flattener. There's always going to be some argument over the Focal Ratio Myth. On my website, I took two images of the flaming star nebula, one with the C80ED @f6.37 and the other with a 130PDS F5 Newt, same imaging camera being a 1100D, I found the detail and contrast better on the slower scope, which makes me wonder how good would be the image with the slower scope with a sensitive ccd camera over a DSLR. http://nadeemshah.com/Image_Gallery/Pages/Deep_Sky_Objects.html Ian King imaging introduce me to this website: http://www.stanmooreastro.com/f_ratio_myth.htm Over the years imaging I'm starting to tilt towards Agreeing with Stan Moore. Link to comment Share on other sites More sharing options...
+RonC Posted May 5, 2014 Author Share Posted May 5, 2014 Interesting reading Nadeem, I decided on a SW ED80 pro which I like very much now I've added a Moonlite focuser too. It is a nice contrast to the MN190 which I also like! As I hope to retire this years I'm hoping these two scopes will let me see/image everything I'm interested in for some time to come! Cheers Ron Link to comment Share on other sites More sharing options...
Guest NadeemShah Posted May 5, 2014 Share Posted May 5, 2014 (edited) The ED80 will compliment the MN190 very well, as both image well with good correction. That's one thing Skywatcher have got right ! Edited May 5, 2014 by NadeemShah Link to comment Share on other sites More sharing options...
Guest ollypenrice Posted June 23, 2014 Share Posted June 23, 2014 (edited) Having been mentioned in despatches earlier on in the thread, could I make my position clear? I do image down to F7 and would go slower under certain circumstances. Here's what I think; 1) A fast F ratio is good for imaging. 2) How do you get a fast F ratio? a) You increase the aperture. b ) you use a focal reducer. If you follow path a) you win. Simple as that. If you follow path b ) you may or may not win. If the object of your desires will fit on the chip with or without the reducer then you do not win. You have no more photons from the object with the reducer. All you do is squeeze the photons you have onto fewer pixels. More signal, less resolution. Why not save yourself the cost of the reducer and just squeeze more camera pixels onto fewer screen pixels by reducing the size of the image you present? This is the F ratio myth. If you actually want the wider field in your image then the reducer makes sense. I prefer to pin the focal length in any comparison of two scopes. Never compare two scopes of different focal length because they don't take the same picture. Reducing the F ratio at 'x' focal length is good but there is only one way to do it; add aperture. Reality check; we are amateurs. We would like to image distant galaxies and make them fill the frame and be nicely and excitngly resolved. For this we need, say, 2.5 metres of focal length. Can we afford 2.5 metres FL at F4, meaning an aperture of 0.625 metres? No, we cannot. Do we give up on the idea? No we do not. We take longer and image at the kind of F ratio we can afford, which might be a tedious F10. I'm an astrophotography provider and having folks here for shortish stays means that this is not the best idea in the world because they might not go home with an image. If you are up for it then do it. If I just worked for my own amusement then I dare say I might consider an Edge at F10 and take yonks on an image. The fact that I have a mental cut off at around F7 doesn't mean everyone has to feel the same way. Olly Edited June 23, 2014 by ollypenrice Link to comment Share on other sites More sharing options...
+RonC Posted June 24, 2014 Author Share Posted June 24, 2014 Well said Olly, and thanks for your valuable input and examples! I got the ED80 in the end to complement my MN190 and am pleased with both! Just need some dark sky's to use them!! Link to comment Share on other sites More sharing options...
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