EntropyStar Posted October 10, 2016 Share Posted October 10, 2016 Hi, Slightly embarrassing question I should probably know better..... My collimation is spot on with my F4.7 Dob (use a combination of cheshire and laser to get everything spot on) and I wondering if i'm missing something: For bright stars such as Vega, Altair etc: Despite achieving perfect focus (visually or in conjunction with a Bahtinov mask), bright stars always look slightly squiffy and I'm wondering if the stars magnitude is the critical factor in all this. Fainter stars (Mag 3+) appear as perfect points of light. Whereas brighter stars I can focus them down to almost a perfect point but when approaching 'perfect' focus the star always develops a very small arc (think of the Wembley Stadium Arch...random I know) along one axis, which too me looks off. It only happens on really bright stars. I'm about to delve into AP soon and know a coma corrector is a must with my scope, but even when centred in the eyepiece I'd expect the bright stars to look like bright symmetrical points as per the fainter ones. Any ideas where I'm going wrong here? Is this the limitation of a reflector telescope? Is it simple physics at play? (ie a facet of wave-particle duality?) For background info I've rechecked collimation and focusing when observing, I've carried out numerous defocused star tests and my defocused star 'light rings' are perfectly uniform. Thanks Link to comment Share on other sites More sharing options...
andyboy1970 Posted October 10, 2016 Share Posted October 10, 2016 Might be stray light, is the focuser squared in the tube? Link to comment Share on other sites More sharing options...
Dave S Posted October 10, 2016 Share Posted October 10, 2016 Do your "sqiffy" stars look like the image on the left? If so it's definitely coma distortion. Dave... Link to comment Share on other sites More sharing options...
Ben the Ignorant Posted October 10, 2016 Share Posted October 10, 2016 When a defect appears only on bright stars it's probably astigmatism in your eye. So: - rotate your head to see if the defect rotates, too - use higher power, the smaller exit pupil lets light go through less eye material, thus less astigmatism - use the other eye, if the defect disappears or changes, the scope is innocent - don't worry: eyeballs are not shaped by Zeiss machines, they are naturally more or less astigmatic depending on people Link to comment Share on other sites More sharing options...
EntropyStar Posted October 10, 2016 Author Share Posted October 10, 2016 Hi all, thanks for the replies.... Is the focuser square in the tube? Yes it's dead straight in the tube. Although I wonder if my secondary is too close to the primary as I can't see the mirror clips despite it being perfectly centred. I've tried moving the secondary a few mm away/towards the primary and it make next to no difference. The collimation is spot on though, i think just means I'm not getting all 10in of aperture into the EP. It looks a little like the picture on the left but the semi minor axis is much smaller than the major axis, it's a really stretched ellipse of light. It might be my eyes, at my age I should be pushing a 7mm exit pupil quite comfortably though (not sure if that helps/hinders). I've been mainly looking through the 28 mm so will ramp up the mag later and see if it helps. Thanks for the responses. Link to comment Share on other sites More sharing options...
ronin Posted October 10, 2016 Share Posted October 10, 2016 Stars will never be points in a scope, at this time the laws of optics forbid this. So points are not going to happen. Next I am not sure that a parabolic mirror is the best, I think that hyperbolic is better but more difficult to produce, Hubble is hyperbolic and that must have been for a reason. Equally on a refractor a spherical surface is not the best. So I think that immaterial of the primary objective we are not quite right. A reflector has the secondary and that is said to "soften" the final image. Owing to the sometime presence of significant coma I am not sure how good the edges of a parabolic mirror are. From the DIY section I find it "odd" that they parabolize the centre of a mirror, I would have though that it would have been the edges that had to be parabolised. Never looked into it, never made a mirror but it just seems to me that it would be the edges that needed parabolizing. So I would say we are using as best we can within the confins of cost and mass production and the odd law of physics/optics. Link to comment Share on other sites More sharing options...
Hadron Posted October 10, 2016 Share Posted October 10, 2016 Hi. If you're certain your collimation is spot on (bearing in mind we're talking about a relatively fast scope here and hence it will be more sensitive to small errors) I'm wondering if your issues could be a result of a pinched primary mirror. It may be worth checking that you haven't over tightened the adjusters or lock screws and that the tube rings aren't excessively torqued up. A further thought is if you've had the scope apart for any reason could you have overtightened the mirror retaining clips. I'm a relative newcomer to newts so I'll happily bow to the knowledge of the many more experienced and knowledgable members that frequent SGL but I also know I'm grateful for all the advice I've gleaned here so if I can begin to give a little back all the better. Hopefully this may be of some help. All the best. James Link to comment Share on other sites More sharing options...
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