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Posts posted by Jessun

  1. vlaiv, I think it's easy to do away with numbers and pixels to grasp the basic concept of F ratio. The number is a fraction with no end to the decimal places, and it has no unit. 

    Then we put something tricky to observe in front of the lens and something even trickier (a sensor) at the focal plane and practical matters kick in in abundance.

    I enjoy these discussions but I don't want to be the broken record on the subject...

    It's often the case that your OTA slips, the sensor ices over, the laptop crashes or clouds gathering above etc, way before you really have to lose sleep over the F ratio of the rig...


  2. I agree vlaiv. It's just optics and the 'sensor' should be though of as a white postage stamp rather than something that is segmented.

    Well done on the train question. It's just one of those riddles that only has a ratio at heart. 4 people ever solved it during the day I flew with them. One guy put in the speed of sound in a series of equations and came up with a number of miles. The simple ratio never clicked.


  3. You are right vlaiv as far as I'm educated. All practical considerations when it comes to taking an actual image are as we all know making it an almost impossible task.

    It is just so fundamental to understand F ratio at its core. Noise, pixel size, well depth etc are merely the constraints we have to deal with and don't quite play a part in understanding F ratio. There is for instance no F ratio myth as often proposed. 

    As a side note I have attached an pic that I have drawn for most of the pilots I ever flew with. Some 3000 to date.


    A man is fishing on a bridge. He is positioned one third in on the bridge. When he detects a train approaching he knows he has to vacate the bridge. He can either go to the left and Juuuuust miss being hit by the train or he can go to the right and Juuuuuust miss being hit.
    The man will walk at a pace of 6km per hour as he departs.
    Question, what is the speed of the train and how far away is it when he starts walking? You can answer in a unit of your choice if you don't like the metric system.



    • Like 1

  4. vlaiv, I really like your thinking and you have the skill to formulate a model that works very well for you.

    I gathered that the OP wanted to go down to the logical basics, and for a more logical approach I think it's important to just accept it for what it is. A simple ratio between two measurements. The only other ratio I deal with daily is Mach number. It also has no unit. Mach is not a unit, although Americans often say something like "2 Mach". Don't ask me why this is. It's Mach 2. Ratio 2. Mach 2 is the same for a Lockheed aircraft or a Northrop aircraft. It's an absolute. Just like all F x telescopes are all equal.

    F ratio is just the same. It does away with any maths anyone cares to throw at it. 80/10=8. No number is more important than the other. 80 is no more magic than 10 etc. The troubles only begin when we put a finite pixel at the business end and start drawing conclusions, or go down the route that astrophotography is somehow different from normal photography just because we have some very bright, small stars in the mix.




  5. Things go funny when we include the mystical spot target or the (almost as confusing) baffling choice of sensors and pixel sizes.

    In essence F ratio is simply dealing with the theory of optics. It has only a number, and no frills to it. F5 is F5. Not F5 Special because of this or that.

    Draw a simple telescope on a paper, one lens, one tube, one focal plane with an image circle. No measurements needed. Imagine it's a small telescope then the image circle is small. And vice versa for a big one. A simple division is all the maths needed. Point what ever sized telescope you envision to the mother of all flat panels and then put a photon counter, the size of a plank length (or half lol) and for any sized version of your design the photons will come in at the same rate at the counter wherever you count, lets say smack in the middle or 10% off centre or anywhere, proportionally.

    There is no real mystery to this. Confusion kicks in when there is a large fix sized pixel at the end.

    Aperture determines the max theoretical resolution and surely doesn't dictate the faintest object you can detect. A photon is a photon. It won't aim squarely at only large telescopes if they come from afar...

    A lower f-ratio is per definition always faster than a higher one.  There is no way around this. One only has to do away with the idea of finite sampling points in the image circle. Imagine an infinite number instead to better understand F ratio.


  6. Wow, what a blast from the past!

    I'm happy that it cleared up some issues. The basics of optics is never determined by one parameter alone, they are all linked.

    Think about this: The Hubble Space Telescope is as far as I can gather f24.

    The focal length of the HST is something like 60 meters vs a 2.5m mirror, give or take. You can build the same f-ratio telescope at home, scaled down to a few lengths of toilet rolls and if you achieve same f-ratio, (A two inch toilet roll would have to be stacked to a length 24 times the aperture  meaning a 48 inch telescope for that tiny aperture). 

    If you built this and put it in orbit, or indeed kept it in the back yard, it would fill the wells of a given CCD or CMOS at the same rate as the Hubble. OK, perhaps only for the very central pixels as the light cone fades rapidly towards the edges, but the f-ratio does not take this into the equation...

    When that fundamental penny drops, there is no longer a mystery concerning f-ratio ever again.


  7. Is it not simply easier to align something with three points of contact? Much like a three legged stool self aligns without wiggling? 

    The roundness in general puzzled me somewhat though. Sure things screw on and off with ease but just as easily do they unscrew for various reasons... 

    I'd be all for a non rotating assembly system that clicks in place. 

    I'll buy your first focuser Olly 🙂


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  8. Very interesting. I remember a colleague who bought a 40" Plasma for £10.000 back in the days.

    Early adopters will always pay the price. I'll stick with CCDs for a while longer, because I'm used to them and don't mind spending some more time on a target to make up for lower QE.

    It's interesting though how the rise of CMOS blows life in the age old argument about sub length - short vs long, which was often simply dealt with on a theoretical level.


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  9. I ran three ED80s simply bolted down. It's easy to adjust one ring 'sideways' (Dec) to match the FOV of the other. Then I used tinfoil to make shimmy bits for 'up and down' (RA) if that makes sense.

    I got fairly close. I got some adjustable saddles from FLO but never got to try them out before breaking the rig down. May or may not be better.


  10. I'm with Olly on this. You certainly have a whole arsenal to experiment with.

    I think you'd be hard pressed to tell the difference. I used ED80s with some success at 1.8 arcsec/pixel. I print 6000*4800 after a PS enlargement and they look great. Well, to me at least.

    Aperture is great for max theoretical resolution if you have seeing and a camera to match, but f-ratio makes for quicker exposures of course. You're comparing 5.9 vs 7 native? Fairly similar, insofar as conditions on the night will have bigger influence.

    NB is great for big structures of course so I'd go for max FOV. (Which is what I'm working on myself at the expense of resolution)


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