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acey

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About acey

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  • Birthday 10/12/61

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  1. A scope with aperture A and central obstruction diameter D has effective clear aperture sqrt(A^2-D^2). I don't know D for an 8" SCT but that's how you'd work out the aperture of a refractor with the same light grasp. Though of course there's far more to a scope's performance than light-grasp, as the comments in this thread illustrate. For example, if an 8" SCT had central obstruction 3" then the effective clear aperture would be sqrt(64-9) = 7.4 inches. A 7.4 inch refractor with the same optical quality as an 8" SCT would be expected to have the same limiting magnitude for stars, but slightly better contrast (e.g. on planets) because of the diffraction (light scattering) introduced by the central obstruction. No scope is perfect, and every scope deviates from perfection in its own way, so the question has no precise answer. But if the main aim is to view DSOs then light grasp is the most important factor, while contrast and resolution are important for planets, in which case a refractor of higher optical quality and significantly smaller aperture may well out-perform the SCT.
  2. I'd never heard of Orion's Belt being called the Three Sisters, but it's apparently a recognised name (on Wikipedia at any rate): https://en.wikipedia.org/wiki/Orion's_Belt
  3. Whoever said that was wrong. Gravitational waves interact with matter: that's how they were detected. When gravitational waves meet any matter they have an effect of stretching and squeezing it (quadrupole radiation) - this causes some heating so the waves lose some energy. The effect is tiny which is why it took about a century between predicting their existence and directly measuring them. Even then it had to be an incredibly powerful source: a pair of colliding black holes. Gravitational waves can refract, diffract etc, analogously to electromagnetic ones (with the proviso of quadrupole rather than dipole radiation, and a "charge" - mass - that can only be positive). Hence the excitement: being able to "see" gravitational waves potentially gives a whole new way of looking at the universe. The very early universe was opaque to electromagnetic radiation: the cosmic microwave background marks the era when it became transparent, and is a limit on how far back we can look with electromagnetic radiation. The early universe was not opaque to gravitational radiation: in theory we could look right back to the Big Bang. I suppose a rough shorthand might be to say that those waves can "cross the universe without being affected by anything." Insert "much" and it becomes more accurate.
  4. Was asked recently in another thread...
  5. Tighten the clips sufficiently to hold the mirror in place. Don't worry about pinched optics, thermal expansion etc, a slipping mirror will do far more to harm views. Don't test by rotating the mirror beneath the clips as you may scratch the mirror edge. To check that your mirror is sufficiently well held, tighten down the clips until they seem to be holding the mirror without unduly pressing, collimate the scope, and check that it maintains collimation at all angles of altitde. An overly loose mirror will go out of collimation when the scope is tilted, a sufficiently rigid mirror is one that maintains collimation. (It is of course possible for a tilted scope to miscollimate because of larger mechanical issues such as inadequate truss tubes, but let's leave that aside for now...)
  6. I wouldn't wash any mirror with a coating less than a year old. I made the mistake with my first scope and scratched the mirror (though the scratch was very fine and did nothing to the view). A couple of weeks ago I washed the primary and secondary of my 12" flextube - it was about 5 years since last time so I figured they could do with it. I cleaned them the same way I wash my specs every day: under the kitchen tap with washing up liquid, using my finger to rub clean. (Pauses for gasps of horror...) With a cotton wool ball you can probably feel grit down to about a tenth of a millimetre. With a fingertip you can feel hundreths or even thousandths. When I scratched my first mirror years ago it was with cotton wool. Using my fingertip I was left with an immaculate, scratch-free mirror. Some water spots didn't drain off: I used a hair dryer on them, which left a few visible spots when the mirror was held at the right (or rather wrong) angle under strong light. They make absolutely no difference to the views. I'll do the same thing again in another five years. I saw the fingertip approach on a youtube vid a while back and after the initial shock I realised it was the obvious way to do it - I can certainly vouch for it. In the past I've used isopropyl alcohol - and made myself giddy with the fumes. I'll stick to washing up liquid thanks. And by the way, don't forget that all the light from the primary comes to the eye by way of the secondary and eyepiece: the system is only as good as its dirtiest part. That's why I gave my secondary the same treatment. Eyepieces, needless to say, don't get dunked under the kitchen tap...
  7. Glad you liked my post about dark sites Yes, I pack my gear in my car and drive 25 miles to a remote spot down a minor road with almost no through traffic. I chose the spot a few years ago after my previous one started to be encroached upon too much by a light dome on the southern horizon. I found it by looking at the OS map for reachable rural spots likely to have a clear, unlit southern horizon, went in daytime to check it out, and saw there would be easy parking that wouldn't obstruct anyone. Sitting alone there in the middle of the night I feel far safer than I would walking down any city street (though I'm not scared of that either). Primal fear of darkness/ghosts/wolves/psychos soon goes away once you're doing something interesting in a familiar place. Only downside is the investment of time and petrol, and the times when I've driven all that way only to be clouded out. My philosophy is that no one expects to go skiing or fishing or bunjee jumping in their back garden. You could I suppose do snorkelling in a bath tub, but it wouldn't be worth it. Some hobbies necessitate a bit of travel, and I live with that. My one tip: keep all your gear packed and ready in one place, and keep it simple, taking only what you really need. I once got all the way to a dark site only to find I'd forgotten my stool to sit on. Never made that mistake again. I can think of only a handful of occasions when anyone has stopped to ask what I was doing, and of course I was happy to enlighten them. No one has ever given me any problems, and there's absolutely no reason why anyone ever should. If a place is safe and legal to visit in daytime then the same applies at night.
  8. Look through the focuser without eyepiece. Every area you can see should be blackened/shielded/flocked to stop stray light entering the eyepiece. Usually means the area opposite the focuser, top of secondary and interior of focuser tube (and part of the dewshield). I found it led to some reduction of stray light issues on bright targets at my dark site (diffraction spikes were slightly less prominent). No difference on faint targets.
  9. Some reasons why it can be hard to get a scope pointing at the right bit of sky: 1. Sky is too bright (light polluted), so few stars are visible and it's guesswork to point the scope initially at a bit of black. Far easier if you can start by aiming for a naked-eye star and working from there. 2. An inadequate map (too detailed or not detailed enough) so that once you start looking through the finder you can't be sure if you're looking at the star you thought. 3. Inadequately aligned finder. Check it in daytime or with a stationary object at night (Polaris or, *sigh*, distant streetlight). 4. Trying to view objects that are just too faint/difficult for the scope/conditions/observer. Go for things you know you have a real chance of seeing. 5. Failing to match the finder or eyepiece view with the map. Be sure to observe which way stars are drifting in the FOV: they exit at West (like sunset) which is on the RIGHT side of a star map with north at the top. 6. If your finder happens to involve an odd number of mirrors (e.g. 1 but not zero) then everything is left-right inverted compared to your map, which is basically a nightmare (been there and had the headache). A prism (on a right-angled finder) is fine. Here, for what it's worth, is how I do it: 1. Align finder on Polaris. (I use the stock 50mm that came with my 12" flextube and have used the stock 30/50mm straight-through finders on all the scopes I've owned in the last 17 years). 2. Choose target area, examine naked-eye stars and compare with S&T Pocket Atlas. Point scope at a naked-eye star. 3. Compare magnified view with map. Navigate around star, comparing view with map, until absolutely certain that it's correct, and the map is correctly oriented. 4. Get within shouting distance of the intended target. 5. With a low-power eyepiece in the main scope (I use a TeleVue 32mm plossl) and an appropriately detailed map (I use Great Atlas Of The Sky), navigate to the exact target area. 6. Raise power and wait. After about ten minutes a mag 15 galaxy comes into view. Description composed mentally, then dim red light on and write it down. Choose next target (the nearest thing to the one you just looked at, while you're still familiar with star patterns in that area). Alternatively keep buying new gear in the hope it might solve any of the problems listed above
  10. Male-female Newbie-veteran Amateur-professional Moon-haters, moon-lovers
  11. Your conditions are fairly typical for a suburban location. You should be able to see some of the most conspicuous DSOs (e.g. M42, M31, M13, M81/82, Double Cluster...), but less conspicuous Messiers (e.g. M1) will be a struggle while ones of low surface brightness (e.g. M33, M101) will most likely be impossible. To see M31 as anything more than a little fuzzy blob you'll need to get in your car. If you can get to a place where the Milky Way is easily visible with the naked eye then you can be confident of getting good views. That usually means a naked-eye limit of about 5.5 or better.
  12. The cross hairs are much closer to the eye than the virtual images from the mirrors - I can't focus on mine either. It doesn't matter to me since I never use them. For secondary alignment I use a laser, looking down the top of the tube and adjusting the secondary to get the red dot centred on the primary. If I were using the crosshairs I think I'd just tolerate a blurred view and get things reasonably centred: the primary is more critical. If the primary is correctly aligned with respect to the secondary then the only significant "risk" is that the secondary is rotated slightly with respect to the focuser, resulting in a tiny loss of illumination at the eyepiece. If in doubt then do things iteratively: secondary-primary-secondary-primary... until satisfaction, boredom or madness ensue. I wouldn't use a camera to do it for fear of misalignment but if it works for you the problem is already solved. If you really want to see your Cheshire crosshairs in focus then the only solution may be backing away from the viewing hole or else use reading glasses. We all need them eventually.... Incidentally, the "f-ratio" of a Cheshire (length to width) needs to be in reasonable correspondence with the f-ratio of the scope, in order that the whole primary will be visible through it. For a long-focus Newtonian you can use a long Cheshire, and the cross-hairs at the bottom end will be far enough from the eye to be seen in focus. Fast Newtonians (such as my f4.9 Skywatcher 12") necessitate short Cheshires, hence the eye strain.
  13. Possible if you really had a clear horizon but in this case it looks like the setting sun may have been seen through distant trees in which case the "flash" could have been a final glimmer of light through foliage. If it was a green flash it would have looked distinctly green - that's how you'd know.
  14. Why not just use one of those things you see in pound shops for mounting a smartphone on a car windscreen? Or even better, just get used to using the finder - far simpler, not affected by magnetic fields, and won't spoil your dark adaptation
  15. Have you ever wondered what astronomers did before electric lights were invented? A candle or lantern could hamper dark adaptation as much as an LED. William Herschel got round it by dictating observations to his sister who sat inside a lit hut. A later astronomer (whose name escapes me) used the smouldering end of a cigar as his red light.