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About Kyriakos

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    Nicosia, Cyprus
  1. Kyriakos


    Hello Marie my friend!!
  2. One of those situations when having spent ages thinking about something, you then write it down and you instantly have your 'eureka' moment. The fact that the camera's focus point is lower means that the focal plane is actually lower than I originally thought right? So in terms of focal plane illumination this should mean that there is in fact a 100% illumination area. I'll have to double check the camera focus point now that its significance has occured to me, but would this mean that even though eypieces need more outfocus they nonetheless receive full illumination? K
  3. Hello all, I have been on SGL many, many times in the past but oddly enough this is my first post. Possibly a bit of an academic question, regarding the significance in terms of optical performance, of the size of the fully illuminated area (100% illumination) as a percentage of the focal plane area. Let me explain: I own a 10" f4.8 Newtonian telescope and I have recently replaced the old 1.25" stock focuser with a 2" crayford. I will not delve into too much detail here (mainly because I am not sure exactly what happened during the mod), but I believe that as a result of the above modification the focuser and hence the diagonal mirror have been shifted about an inch closer to the primary. I initially noticed this while collimating when I realized that at the approximate point where focus is achieved, I could no longer see the entire primary mirror's reflection in the secondary. Marginal, but noticeable. After a lot of research, I ended up plugging all the relevant measurements into NEWT (http://stellafane.org/tm/newt-web/newt-web.html) and sure enough, it appears that I have effectively reduced the size of the secondary in a way that it fails (again marginally) to capture the primary's 100% light cone (see attached image for NEWT Inputs & Outputs). As a result, there is no point on the focal plane that gets 100% illumination whereas most resources seem to indicate an optimal 100% illumination diameter of approx. 0,5inch. The 75% illumination diameter in my case is calculated at 1,8 inch. My question is this: What would the resulting effect be for Visual observing (my camera focuses a lot lower than my EPs so for imaging I do actually get a decent 100% illumination size)? My understanding is that I am basically reducing my aperture by a few mm on each side so instead of a 250mm primary it would be like using, say, a 240mm primary? Is this correct and how would this translate into the actual brightness of an object? I could potentially do some more DIY and shift the focuser and secondary further out, but would this be worth it? Note: I have not had a single opportunity to actually take the scope out and test it since the mod so this is all speculative at this point. Any insight would be much appreciated Thanks for reading K
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