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Ajohn

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Everything posted by Ajohn

  1. By the way some might think that I'm advocating refractors. I'm not because there are serious cost implications in getting one where a 5ins is likely to perform as well or better than a decent 8ins reflector. It has to be an apo. The old longer focus ones are often very cheap on the 2nd hand market though and well worth buying. A newt has a lot going for it that can be improved even further with a coma corrector. People should also realise that they can make the mirror themselves. Personally I would go for an 8ins F6 7 or 8 but F5 can be done. You can take a look at Oldham Optical if you want to buy a 1/10th wave mirror. He uses a thing called a Dall null tester on his mirrors and makes some comments about other methods of testing. Having made one I would be inclined to agree. John
  2. Thanks Kastern. I am aware of that but feel that the strehl ratio gives a much better feel for the performance of the scope than MTF. I also under stand that Pentax for instance quote strehl ratios over the field of the scope. It's a pity more manufacturers do not do the same. I spend some time playing with oslo looking for that perfect scope. The results are interesting. On the collimation problem it sound like your focuser isn't square to the axis of the beam coming off the 2ndry. I advocate spending a lot of time making sure that the 2ndry is accurately aligned centrally in the focuser and the it's also in the centre of the tube. Lots of time before bringing the main mirror into play. I usually take that out or mask it. And ensure that I can see the entire bottom of the tube central in the 2ndry too. This is a very important step. There is a catch though. Many moons ago a man wrote to sky and telescope pointing out that on fast scopes and f5 was really fast then the 2ndry mirror should be below the axis of the focuser and tilted at slightly less than 45 degrees. The idea being to account for the fact that the beam is wider as it gets nearer to the mirror. ie The top of the 2ndry is further away from the mirror than the bottom. Some manufacturers seem to have taken this on board. Set such a scope up as is usually suggested and there is a fair chance that the 2ndry will not intercept all of the beam from the main mirror. I came across a case where it was missing 30% of it. I think the article also advocated positioning the 2ndry slightly away from the centre of the scope and away from the focuser. If you have such a scope and have collimated it in the normal way the 2ndry holder is likely to look rather tilted when it's done. I would suggest you go back to setting the 2ndry as outlined and then set up the main mirror. If you still have a problem the optical axis is not square to the tube of the scope and the focuser needs packing to make that square. If you have the sky and telescope type set up then concentrate on being able to see the whole of the bottom of the tube with the 2ndry will have to be below the axis of the focuser. Then the focuser needs packing out to suit and the 2ndry will be central in the focuser again. It shouldn't need much packing and is likely to take several attempt. The key is being able to see the bottom of the tube evenly central in the 2ndry just as before. I fail to under stand why certain people thought this was a wonderful NEW idea. Must think our ancestors were stupid. My answer was to remove the 2ndry holder and remachine it. It still slightly out other wise I would have sold the scope by now. I'm too honest to a fault. I note kasterns comments on a better scope. May be able to put that into perspective but need to talk about the airy disc again. That's the little dot a star forms hopefully surrounded by nice round rings of light under high magnification. Lord Rayleigh came up with a limit for optical accuracy many many years ago but it's still valid today. He basically showed that to produce an acceptable image there must not be more tha 1/4 of the wave length of light error in the wavefront. What that means in respect to a mirror is that the maximum error mustn't exceed 1/8 wave. At this point 20% of the light that should be in the airy disc will mostly be in the dark area surrounding it. This is regarded as a virtually perfect star image. This assumes that the mirror is smooth to a much finer degree too. Moving on - an image of a planet can be considered as a lot of airy discs so even at this level there is a loss of contrast. Some say that 1/15 or 1/20 wave maximum error is needed for planetary observation. That's what might be called research grade. The only major manufacturer that I'm aware of who quotes figure is Orion UK. Oddly they use a so and so wave PV which stands for peak to valley. A more meaningful figure is maximum slope error and another for ripple. PV is much better than RMS that some used some time ago. That's 1/2 the square root of the true figure. These days hardly anybody quotes anything at all and I can't agree that 8ins F5 mirrors are easy to make that well in volume commercially. Tests of major manufactures products by private individuals crop up from time to time that show a whole wave error. There are a number of people about who will make 1/10 wave mirrors but search the web and check the cost. That's is a worth while limit. Then comes the 2ndry. The optimum on that score is 20% of the diameter of the main mirror. I have never ever come across a commercial compound telescope that meets this. Those Japanese 0.965 or whatever eyepieces make a lot of sense in this respect. It can and may be met on a newt. Again the central obstruction takes more light out of the airy disc what ever it's size but does get truly negligible at about 10%. Many people quote % area. It makes the telescope look a lot better on paper providing the reader is ill informed. John
  3. Hi I have to go through it on a canon 300d at some point but haven't tried as yet. Texereau goes through the motions for film. Basically a fine grained film can just about record a 0.025mms spot. (Film might do better than that these days.) Against that the is the airy disc of an F5 scope for instance is going to be one hell of a lot less. For a ccd the obvious thing to try would be an F ratio that gives an airy spot some what smaller than the pixel but in practical terms may turn out to need to be a bit bigger to account for ccd variations etc. I would try 2,3 or 4 etc times as big favouring higher numbers. This approach is for hi res imaging. The aim being to fully utilise the resolution of the scope. A high res image of the moon might need many mosaiced shots with some telescopes and or ccd sizes. Like most thing with scopes there isn't a hard and fast answer but if the resolution of the scope is to be fully utilised it needs to be thought about. On a planet for instance if there was a decent exposure time, image size and lots of pixels under it's airy disc size the scope will definitely be fully utilised. Sorry I used the word focal length befor. It may been better to say what focal ratio a given telescope needs to be used at to get the best possible result using barlows, reducers and eyepiece projection etc. There is another factor too and that's off axis aberations. An 8ins F6 newtonian spot size has gone up to 0.1 mms at a 1deg 30 min field and 10ins at a degree and 16mins. Go to F5 and the useable field in that respect on an 10ins scope is just over a degree. 0.1mms is going to be a fair number of pixels as well as being a lot over the on axis resolving power of the scope. It's shape is a sort of radial blob due to coma. On star fields and nebulae the aim has to be to just get the largest image possible on the ccd giving reasonable exposure times or number of exposures etc bearing the above in mind. For extensive fields it's probably best to forget the scope and use a camera lens. One of my lenses is rather interesting bought because I have an interest in getting into wild life photography. It's a long sigma apo zoom. I also have a 2 times converter for it. If any one else goes that way they will find that only the jessops converter will fit the camera. It's 35mms stuff and the dslr's have less room between the mirror and the front face of the camera. Remember to check. In many way a dslr and a decent couple of lenses is a much cheaper way of getting into what might be called stellar imaging. Wife might like having the camera about too Trouble is I only use a compact for social photography. She's a very understanding lady though. In any case a dslr is definitely the most economic way of obtaining a large ccd camera. Ok it won't be as good as a cooled severl £1K camera in terms of the exposure times it will take. I strongly suspect conditions and mount alignment will limit that in any case. John
  4. Those 2 scopes are a different kettle of fish. Few people image with achro's although it is possible with colour filters in exactly the same way as many large ccd black and white camera owners do. One takes a red,blue etc and combines them. The problem with getting the kit is that there is no guarantee that an achro scope will give a good sharp focus with blue light. The difference in exposure times with f10 and f11.8 is hardly worth worrying about. I would say that a change is a good idea. The mac has an advantage over a straight reflector. All the optical surfaces are spherical. This makes it much easier to manufacture a truly high quality scope. Your still likely to find that you want a focal reducer at some point if you intend to get into imaging nebulae. That also needs a stable well set up mount. Planets and the moon are a lot easier in that respect as the exposure times are a lot lot shorter. I would be a bit wary about reviews like the one listed. Using that sort of magnification on a planet with a scope of that size is simply blowing up detail that isn't there. Having said that though one of the tests of a very good scope is that is will still easily focus when used like that. Many will not. The other thing of course is that the same sort of magnification should give good diffraction rings round bright stars. You mentioned a C8. They are nice scopes I had one some years ago. I sold it to some one else who also used to own one and regretted selling it. I sometimes feel like that too even though I own larger scopes even the 10ins version but by meade. If you do get one I would strongly advice going for the dedicated fork mount. You will then get a good stable mount and drive and quickly wonder why anybody uses those silly german things. Out of interest I've managed to largely stay away from imaging for a long time but am now getting the things together to have a go. I always go into things thoroughly before I spend money as it's far to easy to buy the wrong thing. I'm after a set up that will image anything in the sky. I own a very high end meade german equ mount, a 5ins ED apo, a 12 ins F4.3 dob, a 10ins sc, a 10ins 4.5 equ mounted newt. I could use the 5ins ed apo and the meade mount but have decided and have bought for a number of reasons a williams optic megrez 110 F6 and a vixen gpdx mount. At some point I will add an auto guider and will image with a canon dslr that I have had for some time. Back up kit includes 2x and 5x barlows and a field flattener. The barlows are by Tely Vue and may not be of much use as they aren't apo's but some people seem to use them so I will give them a try. Then there are photographic extension etc At some point the whole lot will be remotely controlled via a pc. Doing that is likely to involve replacing the goto and drive controller that came with the gpdx. I've gone that way because I'm after a smaller lighter set up. The gpdx is also on a pier that I can easily level and locate exactly in the same place each time I take it out into the garden. I'm not listing this stuff to boast. I'm just illustrating that imaging can involve a lot of rather expensive kit. Thinking about it a fork mounted C8 makes a lot of sense - apart from the tripod. My main reason for going this way is that the first time I used the 5ins apo I was amazed by the views and the just take it out and use it aspect just like the C8. That meade mount is rather heavy though as is the scope and not so easy to set up on it's tripod. No problem for visual use but -. John
  5. The size of the diffraction or airy disk as it is more usually called is all down to the nature of light. The simple view is that it's wave like. The figure I quoted for lambda is for green light and is the distance between the peaks of the light wave. Red would be longer and blue shorter. The human eye is most sensitive to green light which is why it's generally used. The diameter of the central spot (airy disc) produced by a star is purely determined by the F ratio. The size of the mirror has nothing what so ever to do with it. The main message here is that the telescope can not produce a spot of light smaller than this. The reason for this is down to interference between light waves. Lasers which produce what is called coherent light are an entirely different kettle of fish. Normal light is lots of waves that are all over the place which is why the interference occurs. Some cancel out others is a nice simple way of looking at it. The F or focal ratio is the focal length/diameter of the telescope. Longer focal lengths give bigger images so an 200mms F5 mirror has a focal length of 1000mms a 400mms F5 mirror has a focal length of 2000mms so the image of two stars will be further apart in the larger F5 scope. Move the stars closer together and they will eventually merge as far as the smaller telescope is concerned. Edge diffraction which has been mentioned is another matter. Take a decent telescope and look at a very bright star at high magnification and a central dot can be seen surrounded by rings. The central spot is the airy disc that effectively sets the theoretical resolving power of the scope. It's possible to calculate how much of the light captured by the scope should be in the central disc but unfortunately some of it goes into the rings. Even into the dark bit round the airy disc. Take 2 scopes one with a central obstruction(2ndry mirror for instance) and one without and it will be found that more of the light will be lost to the rings as a result of the obstruction. Defects in the optics can have the same sort of effect. The result is lack of contrast especially on fine detail near the limits of the scope. The large 30% obstruction used on some scopes does really have a visible effect on images. That's 30% by diameter by the way. Many manufacturers quote the figure by area because they know that ideally the the diameter of the 2ndry mirror should be no more than 20%. For a casual user these things are mostly just of interest. I only posted because of the loose screws in the 1st post. If some one wants to work out just what focal length telescope is needed to get the most out of the pixel size of their ccd camera or the other way round it's a lot more interesting. Those tiny numbers associated with lambda also illustrate just why optics have to be insanely accurately made to give the most perfect result. There is an old book by a bloke called Texereau still in print called "how to make a telescope". For anybody interested in this general subject it's well worth a read. Might even persuade them to make one too. All of the general contents on this subject are factual and easy to read. I would argue about one or two aspects of his mirror making techniques though but at least all of that content is sound and also contains some info that I have never seen anywhere else. All that's changed really is the introduction of ccd's and short stacked exposures. He is actually famous. Blame him for plossl eyepieces. Nobody had ever heard of them until he mentioned them in the English version of his French book. John
  6. Maybe I should clear the points made at the start of this thread. All I can say is Oh dear and hope that beginners will read this too. The resolving power of a telescope is set by it's diameter. The bigger the better - wish there weren't many other problems in that direction. The actual brightness of the image is set by the focal ratio and diameter. Take 2 telescopes of the same diameter one at f5 and the other at f10 looking at the same object. The same amount of light will enter both telescopes but the image formed by the f10 scope will be twice as big as the one formed by the f5. The same amount of light will be spread over a larger area - it just has to be dimmer. The same thing applies to a star as the defraction spot of an f10 scope will be twice the size of the f5. Increasing magnification via eyepiece will also make the object dimmer for the same reason. The human eye and brain is an amazing thing but colour vision goes as the brightness diminishes and averted vision eventually comes into play. On the contrast comment on longer focal length scopes a lot depends on just how well the scope is made and central obstructions etc. There is some sense in the argument that the longer focal length scope is more likely to be well made. The confusing thing I found in this area is the resolution aspects in relation to the size of diffraction disk. As all f5 for instance have the same sized diffraction discs why does resolving power go up with diameter. I've never come across an adequate explanation. Only comments that the actual calculation of the size of the diffraction disk is rather complicated so Aries rather famous formula of dia = 1.2 lambda*f/d is usually quoted where lambda equals 0.00056 mms. The reason is simple the image scale of a 10ins f5 scope will be twice that of 5ins F5 as the focal length will be twice as large too. The image of the larger scope will be brighter because area of the 10ins scope is more than twice that of the 5ins one. John
  7. Ajohn

    Hi from Ajohn

    Firstly congrats on having an active astronomy board. Most are like grave yards. I'll have to watch it or I will spend far too much time on here. I'm a getting on a bit at 59 and am a retired engineer with a rather varied background ranging from mechanical things to electronics and software mostly of the frimware type. My interest in telescopes started at the age of 11 maybe even earlier but I never managed to own one until I was in my 20's. Built it myself. I have also made a mirror. I own too many scopes and am have very widely read on telescope design and atm in search of that illusive perfect all purpose telescope. I try to restrict any comments I make to things I've verified with personal experience. Observing wise things are now more patchy than the used to be. These days we own 2 propertied (and have lower levels of cash). The main one is about 2 miles from B'ham city centre. The other is in Pembrokeshire and has much darker skies. Trouble is that having "seen" there B'ham becomes a lot less attractive. Observing has to co inside with the inclination, the weather and being there. My current interest has evolved to a remote control telescope and ccd's. No idea how long it will take me to achieve it but that's what I'm setting out to do based around a refractor. I'm also likely to make another telescope from scratch so have spent some time evaluating various designs with the free version of oslo lt. It looks like that will turn out to be a reflector. Some may say why make but I don't think I can buy exactly what is needed. Me thinks I can do better - a problem that comes about from having spent 40 years working in new product development mostly on completely new things. Trouble is I often can do better. John
  8. Your focus problem is down to the magnification you are using - it's too much and the barlow may not be much good either if it's the one that they sold with it. Try a mag of about 150. That gives an exit pupil of 0.8mms (diameter of scope / magnification). That's in the ideal area for planets. You may want a bigger image but unfortunately you will just be blowing up detail that isn't there and lens aberrations and will not get a decent focus. Any detail will also be harder to see. If things aren't too good at 150 try an exit pupil of 1mm. My advice would be to spend your money on some decent eyepieces too. Anything by Vixen (even the zoom) or ebay ED's should be a good bet. To check the scope look at a bright star with the magnification you are using. At 220 you should be able to see nice round defraction rings. The barlow may well mess that up though. An exit pupil of 0.5mms is about right for that and can be used to adjust the lens cell. Can't remember if the helios used an adjustable cell though. The skywatcher does. John
  9. I had the same problem when I bought my son a telescope a few years ago. I went for the 120 for 2 reasons. One as has already been pointed out the 150 is a bit of a beast size wise and 2 the 120 at f8.3 is really stretching the technology as far as it can realistically go. Maybe even a bit too far. That's the other problem larger scopes need longer focal lengths or better technology. This is probably one of the other reasons that there isn't a 150 skywatcher apo. I don't think that even the most highly developed glasses available are capable of giving really good results with a doublet at the usual short focal lengths in that size. The only other thing that I would add is that I think you are wise going for a refractor rather than a cheap maybe even larger reflector. Please don't get upset mod - the following contains some useful information. Can't help adding the sales plug . Edit it if you like. As an aside I have my sons scope for sale on uk astro buy an sell for £150 as he just isn't interested. It's on an eq4 mount. From what I can see the eq4 is now the eq5 but that has a sturdier tripod. Some say the mount is now the eq3-2 but I don't think so. Anyway it carries the scope easily even at high magnifications. I should also add that I haven't stated the above because I want to sell it. The statement is based on my personal experience and cases I know of where people have sold the 150 for the 120 because it's simply too big. John
  10. Not wishing to start a "heated argument" as Karsten mentioned but: The mtf argument in relation to reflectors and refractors is clearly miss leading and not born out in practice for VISUAL observing. This can only be down to one of 2 things. MTF is usually a purely software analysis unless it's backed up with actual observations of a test card with the correct range of spatial frequencies on it. The other is contrast. Anyone who is interested in photography and buys say a nikon lens capable of resolving 200 lines per mm on the film and then actually tests it will know just what I mean. For astro scopes strehl ratios over the field are far more important. (bet I've miss spelled that.) It's a sad fact that all conventional reflecting telescopes aren't too good off axis in that respect. More so with lower F numbers. Poorly made mirrors will not be much good in that respect on axis either. Me I use both but have to conclude that the refractor even a straight achromat has much to offer over typical reflectors especially at the cheaper end of the market. Few people emphasise the problems associated with making accurate parabolic mirrors. The Fraunhofer refractor as already mentioned has all spherical surfaces and an air space. Easy to manufacture reliably and well corrected across a reasonably sized field providing it's f8 or more. Even skywatcher use them. Ok bright objects have a bluish purple haze but that isn't really a problem for visual observing. John
  11. The problem you have Phil is that most of the scopes on offer are designed to be short and convenient. Cynically speaking more can be packed on a container ship in china and the smaller mounts that are needed too. There is also the fact that very few manufacturers actually specify just how accurate their mirrors are. You could rate your scope by choosing to view a planet at certain exit pupils. That's the diameter of the scope divided by the magnification the eyepiece gives. It may seem an odd way to do it but it takes care of resolution too. Basically a more or less perfect scope will give a good image of a planet with an exit pupil in the range of 0.7 to 0.8 mms or so providing the seeing conditions are reasonable eg cold clear winter night. If you find that the image is fuzzy and much clearer at 1mms or above the scope aint so good for one or more of the reasons mentioned. Also with an exit pupil in the range of 0.4 to 0.6 you should see good defraction rings around stars. There's plenty of info on the web about how those should look. Oddly these figures are stiff but ok on an 8ins or smaller scope but may be too much for a 10ins. The magnification needed on a 10ins scope is starting to be a problem due to atmospheric effects. These may even mean that there are only brief instance where the image is clear on smaller scopes too. On eyepieces I can use cheap ed's at high magnifications on the f9 apo. Naglers don't make any difference. On another f6 scope I have a nagler is clearly better. Much better. Cheapest solution here is the nagler zoom. Still expensive though. Vixen do a longer focus zoom that works well on short focus scopes and the moonfish group do a cheap 2ins wide field eyepiece that also works well on short focus scopes. There is also a modified erfle about that may even be better. The moonfish was difficult to use on the f9 scope as my eye needed to be dead central not so on the f6. Don't bother with the bent etc spiders - they will just remove defraction spikes from around stars. They do that by smearing them all over the image - not good for planets or anything else really. You can get a the same effect by cutting oval holes between the vanes in a cardboard mask and placing it on the end of your telescope. You could also check the size of your 2ndry mirror but fixing that problem usually means moving the focuser too. John
  12. :nono: My F numbers come from personal experience. Could this be something you lack? :wave: John
  13. This sort of thing has interested me for a long time. Finally went out and bought an early 5ins meade f9 ed apo. (A couple of years ago.) Most of my experience had been with an 8ins and 10ins SC and a 10ins newt before that. I still have the 10ins sc but haven't had it out for years. The apo settles down instantly and gives very fine views of all objects. The 10ins SC is a pain in the neck in comparison but would probably out perform it in some respects when it's collimated correctly and has more importantly settled down - it won't match the apo for contrast though. That's the draw back with reflectors. The central obstruction and spider vanes of any form cause undesirable diffraction effects. These lower contrast and effectively reduce visible detail in any object including nebulae. This is a fact pointed out in many of the older amateur astro books - it takes at least a 6ins newtonian to match the performance of a 4ins achromatic refractor. I and at least one other source reckon 8ins is a safer bet. That matches up with my experiences with a 10ins sc and a 5ins refractor. The sc has a huge central obstruction compared with the optimum 20% by diameter and no spider. Even 20% causes a marked change in diffraction effects. Like most things concerning telescopes it's a compromise. This all ties in with my 1st decent scope a 10ins F6.8 newtonian with a central obstruction a little over 20%. The person who bought it off me fitted it with an even smaller mirror sized and positioned so that the focal plain was only just available for eyepieces or a ccd camera. I'm assuming that anyone reading this can apply all of that to using a reflector to studying the moon and planets. There are a couple of other factors. One is that a parabola only produces a perfect image of a point source exactly one axis and at infinity. Planets and the moon especially are not point sources. Not at infinity either but that effect isn't significant. The 2nd point is that off axis resolution drops of rapidly very rapidly on shorter focus mirrors. That can be somewhat problematic both visually and photographically. On film for instance one needs f20 in order to obtain the maximum resolution available from a 10ins scope. That's where cassegrainians come in but unfortunately these tend to have oversized 2ndry mirrors to avoid light baffling problems unless they are designed purely for planetary use. Optical quality has already been mentioned. All I would add is that is much easier to obtain on longer focal length mirrors. Even more easier on refractors too. Putting it all together an 8ins f8 newtonian with a quality 1/10 wave mirror and a small central obstruction is probably the best lower cost way to go what ever it's used for especially if a larger 2ndry mirror is also available of photography and viewing certain nebulae. There is even an argument that states that there isn't any point in an average amateur owning a telescope any larger than that. It's not as daft as it sounds. John
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