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Ajohn

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Everything posted by Ajohn

  1. Gina every time I see the name of a 3D printer mentioned I have a look at it on the web. Is the Titan you mentioned the one that uses a closed loop servo drive ? Price very ouch for me but the construction is interesting. My Weller is still going strong on it's original supply. It dates from the 70's. Just hope it remains like this. John -
  2. Talk about plumbing the depths Not sure what of though John -
  3. You can get pitch from Walsh. A reliable source told me that this can be used but I haven't used it yet. https://www.hswalsh.com/product/pitch-embossing-cement-black-tp57 It looks to be cheaper here https://www.goldschmiedebedarf.de/product_info.php?products_id=8096 Also from here along with the grits, blanks etc http://www.stathis-firstlight.de/spiegelschleifen/materialeng.htm When I contacted him about pitch he offered a softer grade than he usually sells as I am in the UK. Not sure about that as soft can help obtain turned edges according to some but hard can mean sleeks. If it's too hard it can be softened with real turps - but don't try white spirits like I did once. It works after a fashion but it tends to come to the surface of the lap. I believe the German supplier suggests vegi oil. Turps can be boiled off if too much is added. Oil ?????? I doubt it. John -
  4. That video I mentioned is this one. Slightly different to the methods used in the pdf I posted but similar and shows what may be needed to create grooves in it if the pdf method doesn't work out. Also how to make one suitable for rough grinding. In the UK it's dental stone rather than dental plaster. Some people have used rather large steel nuts rather than tiles for hogging out - pass but seems to work. He's good source of information, might help avoiding using too much polishing compound and problems like that. Also plenty of examples of how to use a fixed post machine. I prefer to press onion sacking into a lap rather than use the wire brush. Each to their own etc. I've also come across uk people using Hydrastone but believe this is a bit nearer ordinary plaster than the dental stone. John -
  5. Try this. It's very similar to what some one called Gordon Waits shows on youtube. I might have the name slightly wrong. He uses a former to make sub diameter tools. Best look at the video. Forgot to add. I have another some where that suggests that the best source of tiles is Homebase. Pass when I last did it I used tiles that were left over from some I had laid. Floor tiles so had to be cut into small squares. I feel it would be better to use ones of the correct sort of size. tiletool.pdf
  6. As I am slowly collecting bits for AP I bought one of the ZWO one's I linked to. I've a lot to sort out first. May as well mention here as using it will be a bit diy. Nice and light. Pleased about that as finders can be pretty heavy. Things lock up nicely. Having looked through a lot of optics I reckon I can get a good idea using them terrestrially. Nice and bright and very sharp even with a 10mm eyepiece and then a 22mm Vixen. 280mm fl. If the sky ever clears I'll see what stars look like but strongly suspect it will be ok. I was bit dubious when I saw that there were no baffles in it. It will focus with an eyepiece in it but a televue plossl wouldn't unless it was pulled out about 6mm. Distance around 30m. Another old Vixen was ok. So due to looking how much the focus was sticking out I thought I might get away with a 1 1/4 diagonal in it. No. Might be able to by modifying one but doubtful. There is some scope for modifying by making new bits for it where this could work out - with a lathe. The fact that this one has a rotating focus doesn't seem to be much of a problem to me. It certainly isn't with an eyepiece in it and I can't see it being much of a problem with a guide camera either. It's a nice holder with 3 adjusting screws that can be locked on each ring but it sits on a short Vixen dovetail. Small holders for these are available but making one or adapting would be a lot cheaper. So, wont focus with one eyepiece so I could send it back legit but o/all think I will keep it. I was surprised just how light it was given it's 60mm objective. Pity about the work needed. The focus adjustment could have been longer. May well not be a problem with a cross hair eyepiece but it would benefit from an eyepiece focal length that maximises the field of view, say 32mm giving a mag of just under 9x the edge of the field might be a bit colourful using a 1 1/4" eyepiece like this. It's only intended to be used over a small sensor. Then there is the work needed to make some sort of camera par focal with an eyepiece. John -
  7. Some people use a piece of hardwood with a slot in it plus bolt, penny washer and wing nut for the removable location. I've also seen 3 long ones being used for small mirrors on larger tables. I've not done any very thin blanks but one idea of getting rid of any uneven surface on the table is to cover it with several layers of wet news paper. This wont help with astig due to table wobble. I've used the same sort of idea to make it a bit easier to clean up when changing grits during ginding, dry though as the blanks have been fairly thick. Polythene may be way way better for that. John -
  8. I've had a set of coated drills for well over 10 years, so long I can't remember really. I have snapped a few fine ones off when using a hand drill, The 1mm one didn't last long. Eventually I managed to break others up to 2.5mm but I'd say it's more down to me and sillies using a hand held drill. I've replaced them with one of those cheap boxes of ebay that contain black drills in steps of 0.1mm up to 2.5mm with several of each size. These are more flexible also not so sharp. I have to measure them rather than take the stated size as gospel but as with many sets they are just a touch under size - that can matter a lot when drilling small holes for fine taps. So Dave could well be correct but I've never bothered using a lubricant on them. Jobbers drills shouldn't be brittle hard along their full length. Stuff from places like Lidl often is. Personally if I needed a large step drill I'd go for the screwfix uncoated ones in M2. Screwfix are pretty good for some things. The quality can be surprising at times for the prices they charge. Probably a bit mixed at times too though. John -
  9. Googling uk skywatcher guider conversion came up with this link http://www.rothervalleyoptics.co.uk/rvo-finderguider-adaptor-skywatcher-to-c.html However looking at prices selling yours and buying this might work out cheaper - not sure about mounting but it does come with a decent holder. It might mean sawing of the base of a bracket and attaching it some how. http://www.365astronomy.com/ZWO-60mm-Finder-and-Guide-Scope-60280.html They also do the adapter - probably cheaper than the other link John -
  10. I think that the titanium coating does help make things stay sharp and last longer but where it's available items like these that state HSE, M35 or 5% cobalt HSS are better but may well cost more. Screwfix for instance state M2 - the lowest grade of HSS available but it's still way way better than hardened tool steel. Sorry I should have read your post more thoroughly - I thought you had bought it off ebay. However if it states a shank size and has another take it back. John -
  11. The 5/8 chucks on the cheaper drills are really intended for woodwork and don't always last that long when used for metal. Mine has finally had it. I think drilling several 3/8" holes in 1/2" stainless finally finished it off. Small drills wobble about now but it has done a lot of work. Request a return - not as described. If you get no where raise a dispute. I think you will find that will happen anyway automatically. Seller might offer a partial refund - common these days. Refuse and mention that you are returning it on the basis of the EU rights to return mail order items and state why it's unsuitable. The fact that it's 14mm and not 10 is enough for a full refund including postage and your costs sending it back - if they want it back. I'd mention that too. I have a feeling Screwfix do these in HSS with the right shank size. It's always worth looking there and at Toolstation. RsComponents too but they will be more expensive - industrial stuff. John -
  12. I've managed to get Sketchup running on Linux. Now have to wait 30 days before I really know what I can do with it. This link helped get it running http://hrvooje.blogspot.co.uk/2015/11/how-to-install-sketchup-2016-in-ubuntu.html One problem, I have mentioned it on that page. So far the rest seems to work. Not that I know how to use it. To me these applications are a bit obscure when it comes to setting the dimensions of bits and pieces. No doubt it will be there some where. John -
  13. Thanks Gina. I take it that the free version is SketchUp Make ? ;-) I've been looking at pro. When I looked at the price I thought ouch. The Wanhoa is too expensive for my needs really. I will be inclined to try one of the cheap ones and see how I get on. Maybe even build from a kit myself. I have seen a twin extruder model that would allow soluble to be printed but I'd guess that complicates the software that will be needed. I have seen one package that is fairly affordable from asking the question some where else. Cubify. There is free add on about that adds scripts also prewritten ones to go with it, gears included it seems. It might be possible to run this one on Linux under wine - it will run on Vista but the only way to find out under wine is to try it. People have also mentioned Fusion but that looks like it's rented to me. By the month or for a month when needed. John -
  14. I have toyed with the idea of getting a 3D printer but when I look at the cost of the software I go right off the idea. I see sketchup mentioned for instance and gears and all sorts being printed. From what I seen cost wise it's more than I would be prepared to pay for the amount of use it would see but I have also seen comments that it's free and needs a plug in to produce the type of file needed. So the question is simple. What is about, what can it do and what is it likely to cost. A router might also suite my needs but again choice of cad software complicates things. John -
  15. Opticians have been fond of using microscope objectives to provide small pin holes also a wide beam for testing very fast optics such as Schmidts. Might be of interest to some. Most of the newer ones are designed for what is called an infinite tube others and older ones are used with a fixed tube length. Apart from Leitz most are marked with a tube length of 160mm. This mean that if a pin hole is positioned 150mm from the shoulder of the objective the size of the "hole" that comes out will be reduced by the magnification of the objective. 10x and 20x seem to have been popular but the higher the magnification the wider the beam that comes out. The objectives are corrected for working at this distance so it's a high quality beam. It's possible to buy drills easily down to 0.3mm / 0.012" dia. It's also possible to make neat pin holes in foil. Place it on a piece of lightly ground glass. Press a sowing needle down on it vertically and rotate it. The ground glass will stop it from skidding. People have been known to strop the needle on news paper to make it extremely sharp and obtain pin holes down near to 1 thou diameter. If some one has a microscope they can have a look and see. With a bit of care it seems to be fool proof anyway. It might be best to use a high powered green led for illumination as most optics are fully corrected for green. On the other hand if something else is used and there aren't any funny chromatic effects other than any due to the scope so what. John -
  16. Thanks for that. I've wondered about making a reticule using a milling machine to generate the co ordinate - but not entirely sure how I would make the marks - yet. Maybe rotate a sharp carbide scriber by hand, black the lot and then clean off leaving the dots. I may have a book on it's way from the USA with it all laid out. I have seen it some where but not sure where. I'll photo copy the details and post them if they are in it. Meade produced a right angled finder with the reticule in it. That may prove to be the easiest way to go in some ways but the finder needs to be aligned to the mount not the scope. I entirely agree about muddy knees. I have a mount now where even getting them wouldn't help. I can make something to easily attach a camera right angled finder to it but wonder why they don't provide one. Maybe Gina will start printing and selling them. Canon fitting would be favourite. John -
  17. Why not make something flexible so that you can try all of them. Texereau describes how to make a slit using bits of brass which is very easy to work and do. This link includes diagrams, also the same set up using razor blades or what ever for the slit drona.csa.iisc.ernet.in/~priti/HowToBuildTelescope.pdf It's been modified a little from the original - it uses metal angle to support the knife rather than wood. I'd use mdf for all of the wooden parts but it's no problem for me to use aluminium. Blades can be attached any old how really as they are in many designs. The main point is that it's a slit plate that is then attached to something. If one of the blades of the slit is extended and the other left off and a led is fitted as usual it's a slitless tester or the gap can be set with the other blade. It will still be slitless as far as aligning the blade with the slit is concerned. As it can be removed it's easy to set the slit parallel by viewing a bright light through it. If this is turned upside down, knife edge at the bottom a ronchi screen can be clipped to it. Or make a separate part for the ronchi screen without a knife edge. I would try various sized holes over the light source. Or even just clip the ronchi to the knife. The Texereau diagram gives a distance between the knife edge and source. Having any distance introduces errors. For an 8" F6 mirror he will have chosen one that doesn't introduce significant errors. When led's are used the distance can be a lot shorter. It would also be easy to convert his arrangement to both knife and source moving with these or come up with something that could do both arrangements. He describes a test for a sphere that needs a stationary source. No need for a ronchi screen really with that. Trying to null out a sphere needs very fine adjustments with the slit size he suggests. Some have used an x-y stage for moving the tester about. They might use both axis initially to balance out adjacent shadows and later align the tester so well that it only needs moving back and forth. Texereau's rocking idea should do something similar but for final testing it needs to be aligned. That's easy with a slit. The image of the slit can be seen via a cheap eye cup type loupe so both the knife edge and it are bought into focus and the stage slid back and forth and things adjusted until there is no relative movement between the two. Should work out on slitless too but to get shadows the knife edge will need to block part of the image, as it has to with a slit. The easiest way to initially align is to catch the return image on a card and move stuff around until it falls on the knife edge. A mechanism to adjust the tilt of the mirror is a good idea. That way if the source is initially dimensionally central to the mirror axis it can easily be adjusted to be square on as well. I've packed the tester up with books. Some people support the mirror on a strap that runs half way round the edge making it easy to adjust it's height. Some stick the tester on a camera tripod. It could shake around when touched. The web seems to have gone a bit astray when slitless testers are mentioned. They talk about the difficulty of aligning the knife with the slit. It's easy with Texereau's arrangement. The knife and slit image are viewed with the eyecup loupe and the knife angle adjusted to suit when it's half way through the image of the slit. The whole initial idea of using led's was to obtain a larger source to avoid very dark shadows and diffraction effects. Texereau's tester in that guise might just have a 2.5mm hole rather than a slit. Pass I've no idea but faster mirrors produce darker shadows so some hole size should be more ideal than others. Too big and sensitivity will be lost. On the face of it there is no need for a slitless tester's knife to pass half way over the hole the led is in. It could just be above and central to it. In some ways that makes more sense. However it's done the knife edge needs to cut into the return image otherwise no shadows - so if it's half way across things need to be tilted. LOL I'm 'issing into the wind bothering typing this out but one fact for sure people will spend some often a long time using their tester. It deserves a lot of attention especially to the stage the business end runs on. Once that is made various things can be stuck on it all pretty quick to make. The stage itself is likely to take longer. Texereau's is easy to make and works well, ok it need some bits of bar and brass plus something to weigh it down but it does work well. The spring isn't needed as pressure can be applied with a finger. A dial gauge or any of the other ideas for measurement can be used instead of a screw. If the rocking aspect is built in do use a very fine screw / put a large knob on it in case at some point you want to use a fine slit. It's also possible to mount another on top to get an x-y stage. I've done that to try the wire / caustic test. There is no ambiguity at all with that test. Ideally the x and y need to be perfectly square, the usual reason for people not using it. I decided to just set it as square as I could. The results tied in with other methods and very accurate readings can be taken easily. Cameras - well every source says close to the knife where the eye would usually be. Not something I have done but I'd bet alignment issues will prevent that from working out. The other problem is the focal length of the lens - ideally it needs to fill the image on the sensor in the camera or at least give a decent sized image on it. It should be simple trig to work out what that needs to be based on the focal length of the mirror. I have used a red led in a test set up. A Dall null test. As they are pretty narrow band the chromatic effects were dramatic. I don't think this matter for knife/ronchi testing but it might make any diffraction effects worse if fine slits are used - pass. Led's have optics in them. I'd file the end off going as close to the chip as I could and put a bit of slightly milky coloured kitchen food / sandwich bag over it as well. For all I know the optics in the led, reflectors and lens shape might mess up the test. Also initially play with slit size and also probably try a few sizes of hole. I wonder if figuring with Ronchi is a good idea especially for a first mirror. Using a faucault tester takes some practice. Maybe it's best to get stuck in first when it doesn't matter too much so that people will be ready for when it does. Arggggggggg no preview - that's when I usually spot my weird typo's. Some are really weird at times. John -
  18. I had been collecting bits to build one for a long time. Cheap pulleys seem to be a rarity in the uk especially as the size goes up so looking at the costs of those shaft's, bearings and belts the used 3 phase motor with gearbox worked out cheaper than anything I could buy especially new. A used inverter probably messes that up a bit but not by much. I happen to have a spare one so that aspect didn't matter much to me. Can't say that I am entirely happy about using an inverter. The motors make too much noise when driven with one. Having one with random switching helps but it's still a pain. Look forwards to seeing some photo's of the build. I'm sure they will help others who want to do the same. John -
  19. If I do it I intend to relieve the form of the worm at the base to avoid it bottoming there and if needed reduce it's od a bit as well so that it can only drive on the flanks. I'd hazard a guess that the theoretical perfect profile for a telescope drive would be a square thread but that wont work out with zero clearance for obvious reasons. I reckon that all wheels I have seen have been cut with ordinary hobs which I understand will be either an acme profile or a variation on it based on the pressure angle. ACME would give 14 1/2 degrees and the metric version of this 15. The reason 20 and things like stub form are used on gears is strength which also relates to pitch. The last aspect is the most important one in that area. I did know some one that came very near to cutting a worm wheel with whit tap - messed up because he didn't gash first. Really if things are sized correctly and they run on the flanks there isn't any real reason why these shouldn't be used either. The pressure angle will be up though 22 1/2 degrees. I don't think a rotary table is a sensible way of doing it. If some one wants to drive the wheel they would be better off using a gear arrangement as the hobbing machines do. CNC given a steppers typical step tolerance might turn out to be a no no. :-) One day I might get my head round how accurate a tooth spacing needs to be to achieve a certain degree of tracking error. It's not that difficult really. Say 10um error. It will give a certain amount of angular error on a 50mm dia worm wheel and a lot less on a 200mm dia one. It's just basic trig. Tying that in with auto guiding needs is a touch trickier. Those will be at fractions of a turn. John -
  20. His casting video's are probably the best gaj. He's helpful as well if people have problems. John -
  21. I think the point ChrisH made concerning the wheel being free running is rather important for good results. Lapping would clearly improve things too. That's what astrophysics used to do to a mount I own when they rebadged it. Personally I am going to try using a metric trapezoidal tap at some point. Some have a good lead in which means that there will be less of a pcd problem. It can initially be used where the teeth are very shallow and then further along to deepen the cut. ;-) That's the theory anyway. They are also available with various nice coarse pitches. Personally I thought Myfordboys video was nice a clear as all of his usually are. Simple too, no over complications except a better bearing arrangement would be a good idea. It shows how trivial relieving the cutter can be. He does gash at the lead angle of the worm. I'd shim up my dividing head to do that. The form by the way is correct as the worm will be rack form and that should generate the correct from on the wheel. Where it could get tricky is that the usual gear calculations usually assume a certain amount of clearance which could be too great for this application. Edit - I was a little dismayed by the way he uses his hands to face off the work. That surprised me actually. John -
  22. This person generally does good video's and he has recently done one on this subject He has also done a lot on casting at home. Especially in aluminium. John -
  23. Used 3 phase motors with a gearbox turn up pretty cheaply on ebay at times as do 3 phase converters that provide variable speed. Often these provide a right angle drive as well which means that the motor can be mounted on the base rather than the side with the spindle vertical. With a decent motor power it should be possible to safely get a 2 : 1 speed range by driving the motor from 40 to 80Hz. Running it more slowly for long periods might burn the motor out due to over heating so a couple of pulleys might be good idea as well. Depends on the gearing of the motor. Most that crop up will be too fast. I have wondered about removing the motor fan and adding a separate fan that always runs at the same speed - on your own head but it should help. It has been said that gears are a bad idea in this sort of equipment as they may cause patterns to appear on the glass. TBD but I reckon it is worth a go especially when pushing by hand. Pulleys could too unless they are running precisely concentric. John -
  24. They are coated Neil. It appears to be BBAR - broad band anti reflection. It's more or less colourless but if you look at it under a fluorescent strip light at various angles you will see some colours in the reflections. The colours are caused by the depth of coating. The reflections will be pretty weak compared with polished glass anyway. The usual thing with lenses is curved side toward "infinity" and flat side towards the focus - can't make you mind up same curve on each side. The weak curve on the back towards focus can be applied to plano convex lenses as it improves all sorts of things and is often used in better quality magnifying glasses even with a single piece of glass. John -
  25. It will be the most curved side to the sky Neil. These lenses sometimes have a longer radius on the back side which goes towards the focus rather than being flat. The 50mm one is close to flat on that side. Mine is still in it's packet but I can feel that one side is close to flat. John -
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