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Sporadic Dobstronomer

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About Sporadic Dobstronomer

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  1. The difference between a sphere and a paraboloid is given by r^4 / 8.R^3 where r is the radius of the mirror and R is the radius of curvature for 130/900 this works out as 0.7 wave so it does matter which shape the mirror is
  2. Not unreasonable! Can you tell me what the blue things at the back are?
  3. Here is my 2p worth:? One way of looking at it is that if you take out the eyepiece and look into the focuser without the Barlow, the objective is quite a long way away. When you insert the eyepiece it, being a lens, creates an image of the objective. That image is the exit pupil. The exit pupil is quite close to the eyepiece. If you now take the eyepiece out, put the Barlow in and peer in again, the objective looks much nearer (and much smaller). If you put the eyepiece back in, the image of the objective it is then further away than before. In other words, the eye relief increases -sometimes by quite a lot. What I think happens with modern eyepieces is that they have light paths that have been carefully designed around an intended location for the exit pupil. If you move the exit pupil further away by sticking a Barlow in the path, then the light ends up not going where the designer intended so you can get vignetting. The TeleVue Powermate is essentially a Galilean telescope shoved backwards into the focuser so the objective still looks a long way away and does not cause the same effect.
  4. There exist services that make up prototype PCBs in small quantities. It costs about £80-£150 to have two or three boards made up and takes about a week (or less if you pay more). You still have to design the board and put the components on but that would also be true if you had one of these machines. The advantage of that machine is the absense of the 1-week delay.
  5. Focal ratio of telescope = focal length / width of beam entering the objective Focal ratio of eyepiece = focal length / width of beam through the eyepiece from any one star It so happens that these are measurements of the two sides of the same cone of light. The "width" of the beam here would only be simple to measure if you have a telescope with one thin lens for its eyepiece but the principle is valid for any eyepiece. In that simple case, the "width" is the diameter of the spot of light hitting the eye lens from any one star. It is also equal to the exit pupil. The beam from any one star does not pass through the whole width of the eye lens so for any given focal length the width of the beam through the eyepiece is determined by the f/number of the objective and not by the eyepiece itself. The beams from different stars pass through different parts of the eye lens so that lens needs to be much bigger than is implied by its effective f/number. Hopefully I have now found words that work...
  6. That is one way of looking at it. I wrote in terms of the width of the beam within the eyepiece because I think it is easier to visualise.
  7. Yes. The light from a star is only a small almost-point at the focal plane. It converges from the objective to that point and diverges again as it continues on its way into the eyepiece before being focused to a point again by your eye
  8. 1. Think refractors because they are easy to visualise. The type of telescope makes no difference in this matter. 2. If the eyepiece has a focal length equal to the objective's f/ratio you will get a magnification of about 25x per inch in any telescope. If you have typical eyes, this is about the maximum physically possible before over-magnification starts to make the image visibly blurred. But this is a different matter from blurring caused by using an eyepiece in a too-fast telescope. Suppose you consider two telescopes with equal focal lengths but one has an objective lens twice the size and therefore has half the f/ratio. In that faster telescope the beam of light from any one star will be twice as wide as it passes through the eyepiece. The width of that beam dictates the effective f/ratio of the eyepiece which turns out to be equal to the f/ratio of the objective. The beam being wider means any optical aberrations in the eyepiece will do more damage (I read somewhere that it goes with the cube meaning the aberrations will be 8x as bad but I don't know for certain). Because this worsening of aberrations seen in an eyepiece increases quite sharply with decreasing f/ratio there will be, almost suddenly, an f/ratio below which the image gets noticeably blurred. This must be what they mean by "critical" f/number. The precise value of minimum f/number that is acceptable will be a matter of personal taste. Modern eyepieces such as the newest TeleVue designs have less aberration and therefore can be used in very fast telescopes before all this starts to matter.
  9. I can't resist the temptation to put an explanation in my words... The difference between the two ends of a telescope is that light from, say, a star passes through the whole surface of the objective but passes through only a small part of the eyepiece -a different part for each star visible. That means the eyepiece inherits its f/number from the objective (as long as the exit pupil is no bigger than your eye pupil).
  10. I would agree with all of that up to a point! The unusual thing that you do get with the fast Newtonian, and definitely not with a Mac, is a wide field of view even with 1 1/4" eyepieces...
  11. No. I tried some glass-pushing several years ago and learned that it is a risky thing to try unless you have an expert to guide you. The mirror was refigured by Terry Pearce at VCSM. It turned out to be more trouble than he initially expected. I then added a centre ring. I think that starting from scratch this would not be a good way to get a telescope but I was starting from a sunk cost, having already bought the telescope. I have also modified it to make it collimateable in a way that is not very good but does work if you are careful enough! And added a bit of flocking. The next stage is a usable travel tripod which is not yet ready...
  12. I've got one! 99mm f/4 Newtonian. Mine is a Skywatcher Heritage P100 but with a refigured mirror. The mirror as supplied was poor. With a 40% obstruction, it is clearly no match for a triplet refractor but it can give a nice, wide field of view even with a 1 1/4" eyepiece and is a pleasing bit of fun. It will just split epsilon Lyrae (the double-double) and can just show the great red spot. Having said that, it is really not worth pushing above 90x magnification because of the large diffraction spread and the lack of a two-speed focuser. I don't have a coma corrector. I don't get the impression that it would benefit from one. It does like decent eyepieces though -which therefore cost more than the telescope. Showing it to people on holiday, they were enchanted because even this telescope is enough to show Saturn's rings and the mountains of the moon. Like all fast Newtonians, it has a small sweet-spot and so is rather sensitive to collimation. This telescope is not all things to all men. It is a usable compromise of light weight, rich field, and some ability to magnify a bit more but it does not have great contrast on planets. For me it works because I intend to use it under dark skies. In the unmodified state, this kind of telescope is probably not good enough to please as a travel scope. If you want an off-the-shelf solution that is reasonably compact then you are probably stuck, as you thought, with choosing a small refractor...
  13. I don't see the problem. Surely the laces don't damage your night vision as long as they are red?
  14. Like I said above, I disagree. I think that if it does not have big altitude bearings then it is not a Dob.
  15. To my mind, the defining feature of the Dobsonian is the very large altitude bearings which sit on their carriers. There are plenty of telescopes that are, in my opinion, wrongly called Dobsonians presumably to cash in on the prestige that comes from many true Dobs being quite large or very large. In my opinion, the telescope shown above is not a Dob.
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