Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_30_second_exp_2_winners.thumb.jpg.b5430b40547c40d344fd4493776ab99f.jpg

Ajohn

Members
  • Content Count

    783
  • Joined

  • Last visited

Everything posted by Ajohn

  1. I'm one of the older astro people I mentioned - no hang ups though. I received a TS Optical UCF2 filter this morning. Initial impressions are disappointing but I can check this evening. All I have to do is look through a window at the front of the house with it. I may be wrong but I suspect that it will do little. The LP Sodium light filter makes a huge difference. Pitch black and all that can be seen is car head and tail lights. I've found that it increases the contrast of any objects rather noticeably but the broad spectrum street lighting is getting closer and closer. Strange really when the world is supposedly reducing power consumption as for watts in an light out it seems LP sodium is even better than LED or any other form of lighting. It's easy to check that a filter is of the interference type. They reflect the light that they reject and usually their apparent colour changes when viewed at different angles. The UCF2 is just coloured glass and reflects white light rather strongly. John -
  2. I would strongly advice people to disregard the comment "not much use" because if you happen to be able to find the right type of filter to suit your local street lighting they will more or less completely irradiate the problem. This is particularly true of low pressure sodium lighting as their light output spectrum is narrow and rather specific and easily removed with interference type filters along with little loss of light. It's a pity all street lighting isn't done this way but unfortunately more and more broad spectrum white light sources are being used despite that fact that they are a lot less efficient in terms of power needed for a particular light level. Some older astro people I have met don't rate filters at all. An opinion based on types that basically use what in real terms are just coloured glass. Interference filters are entirely different. They are made in much the same way as the anti reflection coatings applied to optics but designed to reflect specific wave lengths of light and let the rest through. Some info is even on the wiki http://en.wikipedia.org/wiki/Sodium-vapor_lamp Sadly a stretch of my local motor way is using the high pressure types now. Even discussed with the chairmen of a local astro club, No doubt he too thinks filters are useless - well in respect to the type of light used now they probably are. -
  3. There is still a lot of low pressure sodium lighting around where I live. I bought a sodium light interference filter years ago and it improves all view eg no sign of nebulae in orion but could be clearly seen with it. I went back to the same shop some time later to get a 2in version and was told that they couldn't obtain them any more. Actually I was looking around the web again today for one and found myself buying a TS Optical Contrast boost filter. Best bought direct as they are then cheaper than Amazon. How well that will work - pass. Much depends on the lighting in your area really but it may well be worth trying one of the broader band moon glow / light pollution filters or one of the ones that state normal sources of pollution - mercury lighting. The mercury lighting aspect doesn't make much sense to me. I suspect it must be a problem in the USA or somewhere other than the UK. John -
  4. Hi I haven't been around here for some time. Rather than lugging rather hefty mounts about I thought I would add something lighter and smaller so recently bought the above with the 127 mac on it. I'm wondering if anyone has tried any other scopes on these mounts that are out of spec weight wise. I'm think of a megrex 110mm in particular which weighs about 4.5kg. Possibly other scopes as well. Most of the flex in this mount seems to be in the region of the vertical bearing despite the rather flimsy looking tripod. At least this one is tubular steel of some sort. Have I had a clear night yet to try it - no. Bought used but the optics look to be at least fairly good going on terrestrial views. John -
  5. I tend to lean towards refractors these days too. Don't know how many early amateur astronomy books I've read that say it takes a good 6ins newtonian to equal the performance of a good 4ins refractor and now I've owned a couple I have to agree. 2nd in line would be a newtonian. I've owned a few sct's. They have a big compactness advantage but suffer for visual use. Have to admit that in many ways I'm more interested in pleasing views than resolution. In my experience SCT's just wont take the magnification a good refractor or newtonian will take. I've also tried mac cas but was left with an I'm not sure feeling. My next scope is likely to be a true cassegrain. Can't buy what I want so I'm going to have to make it. I used to lust after a ritchey type but having gone through it I think I would be better of with a normal type and a coma corrector. Much more flexible. John
  6. The light loss on plane glass is about 4% but it depends on the glass type. It's slightly less when the light exits the lens. Put them together and total transmission is about 92%. From memory standard coatings will half this and true multicoating will half it again. Making a comparison is a bit difficult because the figures quoted for plain glass and standard blue coatings refer to green light as our eyes are most sensitive to that colour. Multicoating can be arranged to do all sorts of things. As far as I'm aware the best varieties of that will have a brownish tinge. It's a bit difficult to say that an orthoscopic is better or worse than a plossl. All the terms mean is the general arrangement of lenses used in the eyepiece. All I would say on that score is that higher quality eyepieces are needed as the magnification goes up. It's also best in that respect to view high magnification at anything above and about 11 per inch diameter of the scope. (exit pupil of 2.3mms) That's the point where the scopes maximum resolution matches the human eye's resolution. It takes a really good scope and eyepiece to go 3 times that. There won't be any more detail in the view at that level but it may be easier to see. John
  7. It isn't possible to say without numbers so the best thing to do is try it terrestrially on something at least 40 to 50 times the focal length of the scope away. Further if you can manage it. 1 to 200yds will definitely be very close to the infinity focus. Your biggest expansion will be scope - barlow - extensions - web cam. The question is whether the scope can focus like that - or if you have enough extensions - or if the barlow sticks into the tube which isn't much of a problem anyway. If using the camera is a bit of a problem when trying it terrestrially it's possible to hold an eyepiece and position it so that the image is in focus. That can give you an idea where the focal plain is. On a plossl the focus of the eyepiece is very close to the outside of the inner lens. ie The one you can see if you look in the wrong end of it. :oops: Your right about the prose - I re arranged it several times. Maybe I should have googled. This is a half decent explanation. http://www.licha.de/astro_article_barlow_lens_projection.php You might find the link at the bottom of the page of interest too - eyepiece project. Usually the best route to high levels of magnification. John
  8. Just incase some one fancies modifying a barlow / taking a hack saw to there scope etc. The magnification a barlow gives is determined by how far it is inside the normal focus of the scope. The more inside the higher the magnification. The new focal plain tends to go in the opposite direction away from the normal focal plain. Extension tubes allow correct focus to be achieved with the barlow further inside the normal focus position. Just how far this can be pushed depends on the scopes focuser. Refractors often offer more leeway and the barlow can even be modified so that it will slide inside the focuser. John
  9. The bader coma corrector will work at the sort of f ratio on straight newtonians you are trying to use but I've no idea how much back focal length it needs so much would depend on how far the field was from the meade reducer. Bit expensive to find out. Field curvature should only enlarge what ever aberrations are there. A star point gets elongated slightly. Coma is a radial fan off the star. John
  10. Your ccd only has an 11mms diagonal chip and that's a big scope so something doesn't make sense. Is it a f6.3 scope or an f10 scope with the reducer on? I don't understand the "under f6.3" bit. Do you mean when you use it at f10? John
  11. One thing that puzzles me about radiation and black holes. Say some one is watching something fall into a black hole. To an observer it looks like it going slower and slower as it gets nearer the event horizon. To me that means that any electromagnetic radiation is getting more and more red shifted - in this case to zero - what ever that means. So just how can any from of radiation be detected coming out of a black hole? John
  12. Beamish take a look at Olber's paradox on wikipedia.org. Don't think that the introduction is very good so. If the universe if infinite and contains an infinite number of stars and is mostly empty why is the night sky dark? That's Olber's paradox. The maths is a bit like black hole maths. Stars give out light in a sphere so the intensity of the light drops of rapidly as the sphere gets bigger and bigger. As there are an infinite number of stars the night sky should still be light. That's Fred Hoyle's way of explaining it. It's infinity mathematics. These don't make much sense in some ways as one can always add another number to any other number. In this case though a rigorous calculation would say that the night sky shouldn't be dark. Hoyle basically says that there isn't an easy way of explaining it. There is another version based on the heat output of an infinite number of stars too. We should all be burnt to a crisp. Some web sources sometimes state that as Olber's paradox. I also wonder what a light ray would do if it passed an infinite number of stars. As the age of the universe has been revised several times I've always liked the thought that photons/light waves decay and loose energy over huge astronomical distances - hence red shift and the apparent expanding universe etc. Not that I'm qualified to have an opinion. I just like it. Every now I come across something that makes me like it even more - like jets that are travelling faster than light, needing dark matter etc. One of the problems with astro physics is that we can't actually measure the distances past a certain point. 1st it was trigonometry, then cephid variables, now it's super nova and red shift. John
  13. I would go along with kpax. Some of the explanations I've seen effectively say that gravity warps space - light follows the curvature of space etc. See gravity at wikipedia.org Gravity can also slow down light and I thought that physicists now accepted that light does have a mass/exerts a pressure. There is an interesting victorian toy called a radiometer. It has 4 vanes suspended in vacuum in a bulb. The vanes are painted black on one side white on the other. It was suspended in a glass bottle in vacuum. Light makes it turn. See for yourself If that link doesn't work or you want to buy one use http://www.gyroscope.com/d.asp?product=RADIOMETER# John
  14. As little as possible now NGC. I handed my notice in 2 years ago and they retired me in case they wanted me back. I'd had enough and didn't want the next task they had set. The pension is a bit of a strain but not too bad. Prior to that I spent around 15 years working on truck and trailer ABS and ASR. Electric vehicles before that. Mostly software with some electronics when needed. It's difficult to split that sort of work sometimes. On those super sharp images there is an iris tutorial on dithering that makes interesting reading. There was also a give away image processing package on the front of a UK magazine recently. It came with an example. Utterly amazing given the quality of the original shot. Problem - the guy who wrote the package and did the example earns his living enhancing images. It's a complex subject. I've stayed away from imaging and am now collecting the bits and pieces I will need. Some how think that the biggest learning curve will be image processing. Having said that though when the web was full of amazing planet shots taken with meade etx's I strongly suspect that some people literally looked at what they had got - thought about what it should look like and corrected with a paint brush ect. Having played around with photo's in the past it's surprising what can be done manually at a pixel level. John
  15. As some one that has done a lot of work on software that does analogue things I've found that a factor of 2 is usually a good idea - usually at least 2 times as good as one wants. With ccd's that would suggest 4 pixels at least in the diffraction spot. Since then I've read about under sampling and dithering with pixel level noise but still think oversampling is likely to give better results and can't see why that can't be improved with pixel level noise too. On your question. I just came across 2 untouched hubble images. One before the fix and one after. The one before the fix was awful but the one after they added the correction wasn't much better. The answer is getting to grips with image processing. If you want a truly much used professional package search imageJ but remember that some use a paintbrush. The best tuition I've found is around in the iris site. Another good package. John
  16. I didn't think I was say that you could do it ngc. I just pointed out the difference. There are several different resolution limits for telescopes. Raliegh's is 0.61*wave length/NA, Sparrow's is 0.51* wavelength/NA and Dawes was determined by eye and is a little larger than Sparrow's ( about 2%). Ralieghs is based on the radius of the 1st dark ring in the diffraction patterm and as an angle (arc secs) works out at infinity as 1.22*wavelength/Scope dia. If the scope dia is in inches that comes out as 5.5/Dia the 5.5 changes to 4.5 for Sparrows and 4.6 for Dawe's. That article I mentioned showed that is was indirectly possible to separate stars well below the Dawe's limit even visually. Raliegh's limit was aimed at splitting doubles clearly and Sparrow's was theoretical. I've always wondered about the 2 rivers. On that score at the Raliegh limit black and white lines would only show 10% contrast. Makes one wonder about planetary views. John
  17. Sticking my tongue out :mrgreen: try http://en.wikipedia.org/wiki/Gravitational_slingshot Can't help laughing. Excuse me John
  18. I think the sling shot effect can be explained like this. A planet has gravity. That sets the escape velocity. If a craft is approaching a planet at the right speed and trajectory the planets gravity will speed it up and or deflect it's path. That can be arranged to put it into an orbit that will take it to the next planet it's going to visit and so on. These things don't go in straight lines. I came across some time to mars numbers based on that approach as against going straight there. Lot more energy versus time and more acceleration - deceleration problems too. The usual approach as I understand it its to place the craft on the same orbit as the planet and allow the craft to slowly catch up with the planet or the planet to catch up with the craft. John
  19. I think what the book really meant was that the path of the bullet holes wouldn't make sense. Say the shot was at the lift and it was higher than the shooter. The bullet hole would show that it entered at an angle and that it exited at the same angle but lower than where it entered. The only path that would make that sensible is a sort of S shape on it's side. ie Not straight. Can't help wondering about what the lift passenger would think about the sonic boom as the bullet went through the lift too. It's a fairly famous thought experiment but I've never come across an adequate explanation. The web no only seems to mention people falling down lift shafts, violinists and etc. ie Doppler effects. John
  20. The 4 dimension thing is a bit misleading. They way I think it's referred to is a time line. 3d space moving in time. SEEMS it can be illustrated by the lift shaft thought experiment. Say your in a lift travelling at a constant velocity, no windows or external references so your completely un aware of it. Some one outside then shoot a high velocity riffle at the lift - high velocity so that the bullets trajectory is flat. The observer in the lift sees that the bullet entry and exit points don't line up and that the bullet appear to have followed a curved path. :? I used the word "seems" because I don't see how this explains a time line at all. Must have been a bad book. John
  21. I've tried to find out which sony chip the sac10 uses but no luck so far. Japanese semiconductor sites are often difficult in that respect. The super HADs are often more sensitive to blue and have depressed red sensitivity. There is also a super had 2 now. One thing I do know on the sac10 is that an over exposer definitely goes blue. I asked Graig Stark about the colour scaling and having default scaling for the camera and the ability to save several sets of settings for a camera. Seems the scaling problems are down to me, there is some default per camera scaling and he might add the save settings at some point. Looks like nebulosity is purely geared up for deep sky and no real interest in other things. Pity really - I asked about a straight twain interface for a straight philips web cam. Modded web cams are supported. Haven't got much interest in running 2 lots of capture software. Also asked about the cooler on this camera. There is a switch on the control box that I assumed switched cooling on and off. Problem was that the camera drew more or less the same current which ever way the switch was set. Seems the peltier cooler is always in and the switch just speeds up the fan. More fan and the cooler can cool more. It takes well under an amp which means I can easily run the mount and the camera on a 5amp 12volt ps. I was concerned about that because they were selling a 4amp supply to go with it and the dealers had no idea how much it needed. John
  22. I've got 2 of them ron and a number of others on a number of things :hello1: and even more on things not even relating to astronomy. Don't need book 1. I should open a book shop. One thing on texereau is that over time I've found him reliable if a little extreme. I bought his 1st book not long after it came out. Bought the 2nd version too. Bit disappointed in that one. There are better ways of final testing of a mirror but apart from that much of what he has to say is still relevant today. Just needs a bit of thought. John
  23. Yep ron that's correct but the spot diameter is just twice the radius. I think it's worth thinking about it another way too. Take to round coins of the same value. Lay them side by side. How far apart do they have to be before they don't overlap? The radius of the spot comes in with stars and is probably complicated by strange diffraction effects too. I am not to sure on this point. Years ago I came across something produced by an observatory that showed that stars closer than the limit can be split by looking at spider diffraction or the light curve across the 2 stars. Due to that I have some doubts about the radius aspects. One of the arguments put forwards was that spider diffraction was an aid in splitting stars at the limit. I also wonder about planets. Say there were 2 rivers on Mars - never know the some one may find some :mrgreen: . Say each was wide enough to just make up one diffraction spot so they were just resolvable. How far apart would they have to be before 2 rivers could be seen rather than one wide one? Pass but it clearly couldn't be just the radius of the diffraction spot. But the light intensity would change down the middle in anycase. These rivers are extended objects were as stars (most but not all of them) are more or less zero diameter points of light. Glad some one has bought the book. There's a lot in it and it's well explained unlike many. And he is right it does set the limit of resolution. What you should do now is work back the diffraction spot to the theoretical resolution. It just straight trigonometry with the base of the triangles formed by the diameter or radius of the mirror. John
  24. Ron it's best to look at it in terms of f ratio 1st and then focal length. The F ratio sets the size of the diffraction spot. Diameter of the scope doesn't matter. Take 2 scopes both F5 one 100mms diameter and the other 200mms diameter. The focal lengths would be 500mms and 1000mms. The focal length sets the size of an image. It gets bigger the longer it is so the F5 200mms scope has twice as much space to get diffraction spots in and will give a finer resolution. Now make the 200mms F5 an F10. The focal length is twice as big, 2000mms, so the image is twice as big again but the diffraction spot is also twice as big so the resolution is the same. It will also be the same what ever f ratio 200mms scope it happens to be. John
  25. What software can retain the settings? I'm not sure nebu lousity will do that. Might gather I'm not to impressed either. Scaling a colour seems to fill the 64bits rather than scaling and maintaining the range. Could do with and has space for a lot more icons but maybe short cut keys work. Also wonder if it the whole thing would be best done with image processing software. I suspect I will be a lot happier if I can judge the shot at the scope. I have had some luck getting a reasonable green in the garden shot but the white card seems easier. Couldn't remove all of the blue though. The card faced the window and the camera without lens faced the card without casting a shadow. When I get a uv/ir block filter I think I will try the same thing with an out of focus mac/cas telephoto lens pointing at the card and filling the frame - saves setting up the scope. True white paper is easy to get thanks to inkjets. The un removable blue is probably down to the ccd. Some of them go mad as the wavelengths get shorter and the software just says oh well that's blue. John
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.