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Ajohn

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Everything posted by Ajohn

  1. I take it you mean this type of split ring mount? http://www.saao.ac.za/~wpk/scope/scope.html John -
  2. They give the sizes of the guide camera here as DxH 65x50mm in the product overview http://www.lvi-cameras.com/download_eng.asp Interesting product and well priced compared with others I have noticed - and can do more too. John -
  3. What I have done is this - hasn't Rick's shot changed since yesterday ?? It really needs a bench drill and vice to match the tapped holes at the front. I did it with a hand drill for some reason, the drill wandered a bit and I had to file a little. I did the back one with bench drill but as the tapped holes needed in the dovetail bar can be spotted through it might be ok with a hand drill. I'll probably add a camera rail to the top at some point Taken with a Philips CCD web cam. Looks like something a little better would be a good idea. John -
  4. I had problems with that set up loosening some time ago - eq some low number and a refractor that was really too heavy for it. It really needs something to prevent the rings from turning in my view. They could do that with a small change to the rings. I'd still be worried about them loosening though. Looks to me like they could do that and make use of the 2 M5 holes using the screws they provide for those. I don't think the hex drive heads on these will foul the rings, Anyway I concluded it must be that way and suspect I will use it to hold the things together while I mark through the 2 M5 holes in the dovetail. I just did that by hand with a drill that was a nice fit in the tapped M5 holes and found that the core size on one side is distinctly different to the other, What quality. Hope I am right about the screw heads not fouling. I want the set up to extend the rail on something as it barely balances with 2in eyepiece gear on the end so I will need to drill for a ring that has to be positioned part way along the rail anyway. Thanks, I must have had the past problems with this set up in my mind when I looked at what was supplied. ( And yes if some one in particular is reading this I still want a smaller refractor - just hope the car bill is the lower one.) John -
  5. Don't know if I am being stupid and have missed something but I bought 90mm tube rings and a 210cm dovetail. I'm sure I have read that the dovetails are drilled to accept the rings but it doesn't look that way to me. The base of the rings have a flat thin protrusion that would sit on the rims on the dovetail plus there are no threaded holes to use at the ends of the dovetail bar for this anyway. There are 2 M5 holes at each end but nothing to match on the riings, There are a couple of 1/4 whit holes near the centre of the bar but all of the supplied screws seem to be M5 and M6. Looks to me that I am going to have to drill the rings to match the 2 M5 holes at each end of the bar. Bar http://www.optcorp.com/media/catalog/product/cache/1/image/400x400/9df78eab33525d08d6e5fb8d27136e95/1/3/13280-l_1.jpg Rings http://www.365astronomy.com/SkyWatcher-76mm-Tube-Mounting-Rings.html The centre holes at the end of the bar aren't tapped and if those were used to hold the ring in place they would break or bend the tube ring. ?????? Arggggggggggg. John -
  6. You should be able to get the adhesive style clear silicon rubber from a builders merchant rather than the one that is intended to use as a sealant. well you could at one of our local ones, not sure now as they have been taken over by a national chain. John -
  7. I believe that the change of RF frequency has helped interference a lot. Your over all aims are interesting. I sometimes feel that telescopes have remained in the past and have also got worse in some respects. The worse aspects are probably down to doing a number of things with PC's early on so they have stuck around. Worse - this for instance runs on a few AA cells http://www.opticsplanet.com/meade-starfinder-eq-telescope.html I have the 10in version of it and often wish I had got my hands on the early Magellan goto for it. Things are changing though as rather a lot of computing power is available that doesn't use a lot juice now but no signs of the manufacturers making use of it. People like ASCOM don't help either. Windows and PC's are assumed and I for one can't find any low level information on the protocol they use. Even Meade are better than that and some others fortunately. John -
  8. I wonder how they know the bearings are of inferior quality ? Or for that matter if fitting alternatives will be any better. It is possible to buy super precision bearings but the cost is surprisingly painful. I sometimes think there is too much of this sort of thing about on the web. Sealed bearings are prelubricated and aimed to last for the life of the product at a faster speed than they well ever see on the end of either parts of a worm set up. A really good clean out using say petrol and a regrease can extend life but scope bearings rotate so slowly that shouldn't be of much concern. Bearings are lightly loaded with grease because packing them with too much can cause problems when they are running at speed - probably not of much concern in this case but in general little is better. Certain types of grease shouldn't be mixed and the old or original stuff is hard to get rid of completely. They generally have some form of light oil in the packing as well to prevent rust. Another aspect which probably doesn't apply is that the originals might be stainless steel types. Easy to tell with a magnet. In some ways these may be better than hardened steel replacements even in terms of accuracy. John -
  9. Neat Boyd. UK weather encourages DIY in some fields but to be honest I see high mist these days where ever I go. I have been wondering about another aspect of model servo's etc. They can be controlled via an RC transmitter and receiver, range something over 100m. The transmitter controls would probably need modifying and that would just leave some aspects of the camera and getting images back. Cameras are beginning to offer wifi and android based remote control. Also while on holiday in Turkey I saw a film crew working. A lot of the shots were taken with a flying platform all controlled from an ipad complete with live viewing. Not sure what cameras they were using but there was some concern that TV companies were beginning to ask for 4K video so I'd guess they were just HD. John -
  10. You asked about epoxy and aluminium early on. Looks like you have changed your mind but might be of interest. Epoxy goes a long way towards holding aircraft together. It sticks to it rather well. With skins as in aircraft they also rivet as with most bonded joints they are weaker when peal forces are applied. I would suggest a pocket a little deeper than the diameter of a truss if you wanted to do it that way and sufficient clearance to leave space for a thin film of adhesive. It would need jigging while it set. You could also have a pin of some sort going through the set up. In short if the load is a shear force on the joint it will be strong. Peal forces wont be so good. I recollect that epoxies were being sold n tonnes etc and that will be in relationship to shear forces. I run Linux too. A 3D cad package like that must be a bit addictive. John -
  11. I've done a lot of this sort of thing at work at times. Reading the comments about solid core wire or even tinned copper wire which is what I use mainly if using strip board there will be a lot less problems if the correct electronic tools are used. Because links and components should be trimmed to length before they are soldered I generally bend one end of link over severely so that it goes hard down on the pad. That allows me to pull on the other end with a pair of pliers and do the same thing. Then trim and solder usually after I have done the lot with the pliers. Prior to soldering and trimming the leads go part across the pad and then have a vertical part that has to be trimmed off. The reason this should be done before soldering is that trimming then will stress the joint and they have been known to fail eventually because of this. The same sort of thing can be done with components but not so much force as the leads might pull out. With these it's more a case of an angled portion over the pad and the rest sticking up at a lesser angle. I did super clue components down for a while but it makes replacing one rather difficult. This sort of wire really needs wire strippers. There are 2 main types. Both are a little like pliers. One has a series of holes for various sizes of wire. The other is more plier like and has 2 V notches on the end and a screw arrangement to set how far they close. These are fine if adjusted correctly. They should close just short of clipping the wire core. They will nick the wire if this isn't set correctly. They will also break very fine wire. The other type are better for that. There are also automatic strippers but these tend to cost far more if they are any good. Electronic snips and pliers can cost rather a lot of money. RScomponents did do a cheap set that are well up to the sort of work they are intended to do - copper. Cut anything else with them and the snips will probably be ruined. Liddl have sold a set from time to time that is nearly as good - I use the snips from one of those sets for trimming my toe nails. This is the RS set I use at home http://uk.rs-online.com/web/p/plier-sets/0536458/ They are probably not up to intensive daily use but single items in that class cost more than the entire set. The correct cable stripper for solid wire is one of these http://uk.rs-online.com/web/p/cable-strippers/5401509/ I find remembering which hole to use a pain while I'm working so use some like these but they do need careful setting and I think my current pair came off ebay or more likely a local electricians supplier. http://uk.rs-online.com/web/p/cable-strippers/1585518/?origin=PSF_412527|alt Soldering irons are a bit difficult. Ideally one is needed that can take 2 tips, one long cone pointed one and anther similar shape but a flat on the side of it for heavier joints. I use an ancient Weller soldering station ie comes with a stand. Their tips do wet out with solder, some don't. I don't know what to suggest for that. I did use Antex at home once and they were ok but the tips don't last long. A piece of soldering iron type sponge is needed as well. It's used wet to clean the tip while working. The cheapest station RS do now is £66 plus vat. That is here. Oddly I use a non digital version of this one at work after a Weller transformer went phut as a cost saving experiment and it was fine. http://uk.rs-online.com/web/p/soldering-stations/7998939/ The trouble with soldering irons is that ideally they need a lot of watts for what they have to do but that means they need some sort of temperature control. Weller build that into all of there kit. Not sure about Antex. The one I used didn't it just settled at what ever temperature it reached flat out. If some one wants to solder fine leaded surface mount parts such as micro's etc the size of the solder is important - it needs to be really fine. A flux pen can be handy as can silver loaded solder. The technique is to tack one corner down getting things as correct as you can and then align all of the pads and tack the opposite corned down. Then do the rest. If the first one goes too wrong heat it up again and move it. All best done under a stereo microscope so that the leads look like they are 2.5 mm or so apart. I feel a decent iron that keeps it's temperature up is essential for this. If pins are bridged desolder braid might help providing you put a bit of solder on it first. A solder sucker can also be useful but they need a lot of solder to suck in order to work. If something has to be removed it's often better to snip the leads and then remove one by one. Another way best for people who do a lot of that sort of thing is to flood the lot with solder, get the part off and then clean up the excess solder. All comes from having to try the latest micro and it's software in an earlier controller and might include adding bits and "tracks" with wire wrap wire. All good fun if only done occasionally. I never was much good at removing quad flat packs so got the rework man to do that. John -
  12. I needed some machining I couldn't do myself - a bit of lathe restoration. I just looked in yellow pages and phoned one. When the work was done the people that actually did said next time see us rather than using the official channel - it would have been cheaper. Suspect that would only apply to one offs though. Suitable headings in yellow pages, tool makers or maybe machine tool renovation etc. I'd suspect that there are still a few about in Coventry. There are probably some small companies in Redditch as well. Also CNC machining. That might pay for 50 off. Not so good for one offs as the machine has to be programmed. You may then need to take / send them to an anodiser yourself. I suspect you will have to forget cheap - hourly rates for this sort of thing are fairly high. John -
  13. One thing about 4in F12 is that it should work well even as a sphere. I did a similar tube on a dobson. Hexagonal tube just using butt jointed cheap 6mm exterior ply. I cut the correct angles with a jig saw and a straight edge. I took some care to get the angle of the blade correct and used a good quality exterior wood glue. After the glue had dried I concluded that it didn't need and internal bracing - just the mirror box on one end. One problem - I didn't ensure that the end on the mirror box was dead square and rested that on the mirror box rather than "letting it into it" squaring up and fixing etc. John -
  14. If some one want to get a reasonable figure for current carrying capacity it could be worked out for the twin and earth cable used for wiring houses in relationship to the ratings in the fuse box but remember that these are ring mains but they still insist of the same size cable for spurs to a 13 amp socket so 2.5mm^2 is conservatively rated for 13 amps. Having seen work done on an electric vehicle charger though few sockets and plugs are really capable of taking 13 amps for long. They get hot and fail. The cable is ok though. At the time MK plugs and sockets had the best chance of surviving. Cable ratings are a rather woolly subject because it depends on their resistance and heat loss characteristics also it seems what they will be used for. I had to design a piece of equipment to discharge batteries at rather high currents and was told by a fellow of the IEEE to use 100 amps per inch^2 for solid copper bar parts of fairly short lengths. John -
  15. If you compare the diameter with fuse wire you will know how much current it would take to blow it. Seems to be about 15 amps according to the wiki. That might sound like a silly comment but it will probably take more current than the contacts in your breadboard are likely to. Solid wire has better high frequency characteristics than stranded so can take more current for the same diameter if any of that is involved. Really the size you need comes down to the bread board. They are usually designed to make good contract with wire of the same sort of diameter as leaded components have. I have solder 0.55mm tinned copper wire to the end of stranded wire and that was ok. I would guess you would have no real problems running a couple of amps through it over short distances but I doubt if the contacts in your breadboard are really suitable for that. John -
  16. The site that seems to have started extreme servo hacks linked to here may have changed it's name and gone completely commercial but even so the bare bones of a linear actuator are shown based round a model servo http://www.instructables.com/community/Hack-your-Servo-V100-Make-a-powerful-linear-actu/ The extreme hack mods seems to involve using open servo code in a replacement board and even a magnetic rotary encoder. They claim 10 bit positional precision with the encoder fitted but the chips used can have max errors that aren't as good as that would indicate, worst case is probably +/- 1 degree. John -
  17. I suspect they have more to offer than steppers but need a different approach. What started me wondering was this http://www.indilib.org/support/tutorials/143-diy-remote-focuser-with-indi.html Connected directly to an arduino pin as far as control goes. Then I started think why not use them in there normal form via the arm - via a push rod, or put a pulley on the end etc. A focus for instance doesn't need a lot of travel and something of the order of 1/500 accuracy on a turn of a main focus knob is likely to good enough for any scope - i would have thought so don't quote me. I like analogue too but things have moved on PC wise. It looks like the INDI set up can be driven with a netbook and a 'pi at the telescope end with an arduino to help focusing. Some one else has gone further. The whole lot is presented as a web page so can be driven from a tablet and still use INDI items via a tablet utility. That included plate solving. It's not fast but........ There are some links in a painless remote control post I started on here Since that one has also appeared on the INDI pages too. I wasn't knocking INDI just mentioning an alternative. Maybe just maybe some one will look at presenting INDI as web page opening the whole thing up. If some one wants to play with model servo's the testers are cheap and just need a 4 AA cell holder for power. I am not sure if these are stable enough to form the basis of an electric focus. That's the only question really. The same question has to be asked about the servo's too. Some encoders are cheap, ebay is full of them but is suspect when they say things like 24 pulse they are really 2 channels with 12 pulses per rev each in quadrature so that direction can be sensed. I believe people have printed their own encoder discs as well. John -
  18. That's neat piece of work Boyd I had my tongue in my cheek when I asked the question. Positional feedback isn't needed for focusing. There are various way of using them. The normal type are driven by a 50hz square wave form and the arms will usually move through about 120 degrees but I suspect are really meant to move over 90 degrees. The mark space ratio controls the position the arm takes up. The digital types have a dead band of 2 uSec so for say a 1 mSec total variation in the mark space ratio they have " 500 steps ". Might not sound useful but they can apply a lot of torque to the end of the arm that is usually fitted to them. The other type that can be bought as standard uses the same drive waveform but the mark space ratio controls rotational speed, There are details about on the web concerning modifying the ordinary ones to work this way. I believe programmable ones are available as well. The above operate in servo mode - they will apply power to keep in position or maintain speed. Some people just take them apart and disconnect the motor leads and drive it directly. The positional information is obtained with a variable resistor with end stops so that needs taking care of if rotational motion is needed. There are various type of construction. Brushless motors, currently rather expensive and then combination of metal gears, plastic gears and ball races. The most powerful ones I have seen without looking hard are just short of 20kg/cm but will go over that with a 6v drive. They are usually spec'd at 4.8v and 6v. The digital types are usually rated to work in sub zero conditions. John -
  19. Why is a position encoder needed ? John -
  20. That's better than I expected. A 50mm should be ok with 10 sec exposures according to DSLR pundits without guiding. I did try that at ISO 200 F2.8 just on a tripod hoping to catch some milky way at a reasonably dark site but it only caught brighter stars. So far the conditions haven't been suitable to try again at higher iso - i'm not in the same area very often. John -
  21. i've been tempted to build a mount many times. One thought that struck me was that worm wheels are usually gashed rather than having "rounded teeth" hobbed into them so why not use a gear and angle the worm. It's sometimes possible to get a gearbox lathe to cut worms of some pitch with suitable gearing added. A spread sheet can be handy to see what can be done that way as the ideal setting might crop up at any gearbox setting. i also thought about making a rotary table like that but went from a small hobby miller to a dore westbury and the guy that sold that to me threw in a small table and dividing head that had been built from kits thinking cheap Chinese stuff would be better, If a super accurate gear / worm wheel is needed they can be lapped. The idea is to think prime numbers and several sizes of lapping gears so that the lapping is even. Astrophysics didn't even worry about that when they worked on and rebadged Meade mounts. Toothpaste might be ideal for that sort of thing. A gear would need something a bit coarser initially. Fork mounts tend to be lighter and more stable on a decent tripod. An SC fork mount would just need some sort of tube with the scope rings in the centre. The bigger ones that would leave clearance for a small scope plus camera aren't that light though. Encoding - an optical mouse will resolve around 1000 dpi, not sure if they will work against curved surfaces such as a gear or what ever. They might against a flat portion of a belt but I would worry about give in the belt with varying load. Sort of extension of the old idea of using optical encoder out of a mouse. I'm fairly sure Mel what ever he was called came up with an idea to fix up alt az mounts so that they will track correctly - the camera has to be rotated as well. The info might still be about some where. Now there's a project for and Arduino or two. A pi might be a better idea probably with plenty to spare. I have no idea how well this might work. Worst of all if some one gets tempted - there is plenty of info about on the web concerning casting aluminium at home. I've managed to avoid the temptation so far but did ask one person about their propane burner design and how they got on with it. There is also a special sand available for use round the pattern that leaves a much better finish than ordinary sand. John -
  22. I have just bought an Ioptron mount - no steppers at all. DC motor plus rotary encoder and feedback to control the speed. Not sure what the max slew rate is but I prefer this arrangement - has any one ever looked at the angular step error that type of motor usually has? I have. Also a Vixen GP DX with SkySenor - the noise. That in itself has put me off but mounts do vary in that respect. The ultimate answer to fast slewing is direct shaft encoding - just spin it around as fast as you like by hand - the cost hurts, even the encoders on their own. Just looked the slew rate is 4.5 degrees per sec 1 min 20 secs for 360 degrees, 512x. Could be faster with more a more powerful motor but the gearing and probably more problematic the worm and wheel would have to be up to it as well. John -
  23. I believe once in days of old when knights were bold scopes usually specified the % diameter of the 2ndry mirror. People were or often are aware that increasing diameter degrades contrast so when compound scope started appearing they started specifying it area wise which oddly enough on most designs brings the number down into the 20 - 25 % range which in terms of the diameter way makes the number look better. Like most things though it's swings and roundabouts. Some one might say I don't want to have an SC for instance because calculated the right way the central obstruction will come out at about 30% and a bit. All it means in practice is that the scope will have less contrast than one without or with a significantly smaller central obstruction. The resolving power will be more or less the same as a scope without an obstruction. That's fine for splitting stars but resolving power expressed that way isn't so good on things like planets. At Rayliegh's limit contrast is only about 7 1/2 % of what enters the scope. What is more important for that sort of thing is when the contrast has dropped by say 50% or some such figure and that is where central obstructions have more effect, Mirror errors too up to a point as well. There is a decent page here that elaborates on the effects. It needs MTF plots to show it. The MTF curve is the same for all perfect scopes and shows resolution across the bottom and contrast on the vertical axis. That way fractions (coarser to the left) of any maximum resolution can be shown on the bottom scale for any scope. So the 1 for instance relates to when contrast is zero and the actual resolution that this implies is set by the diameter of the scope. http://www.damianpeach.com/simulation.htm When looking at pages like that it's best to bear in mind that people use equipment that on that basis is NVG but they still get decent or even spectacular results. For that reason I wont try to find one showing what happens off axis on a Newtonian. It's pretty bad and gets worse as the size goes up and as the F ratio goes down. A compound scope can do something about that - more swings and roundabouts. Looses out in one area and gains in another. That's life with telescopes. John -
  24. That's been done but it can get expensive and really needs coating. Texereau did it on his own cassegrain but it was a narrow field scope with a long focal length to get the image scale up to a size to suit film. I'd guess that a plain flat plate would mess up the optics on anything with a wide field of view. Even something just wide enough to cover the moon might have problems. John -
  25. Sounds great but I would check the 2ndry sizes you have. It's % diameter that matters not area. As the % diameter goes up contrast drops. Both mirrors are pretty good in that respect even the 60mm which comes out at 20%, a figure that some one might just choose to use as it will give acceptable results with very little difference to the smaller one which comes out at 17%. People sometimes choose a size that just covers the moon, bit more than 1/2 degree and use that for planets as well. For wider fields the main limitation is the quality of the image further and further off axis. Your scope for instance will show 0.1 mm radial fanning on stars at the edge of a field of 1.35in covering just over a degree. There is a decent article and little program for working out 2ndry mirror sizes at the bottom of this page not sure I would get too excited by the bent mirror program unless some one wants to try doing that for fun. http://www.skyandtelescope.com/astronomy-resources/freeware-from-sky-telescope/6/?c=y The program works out 2ndry sizes that will dim the edge of the field by 30%, a sort of pro standard. An even bigger 2ndry wont dim at all but will drop contrast more so it's a good compromise. It also helps the off axis image problems a little as well because the light at the end of the fanning is very weak. The drop off in light levels isn't too important visually as we look round big fields and our eyes adjust. I suspect some manufacturers take liberties at time due to that. I'm thinking of a certain Meade newtonian that I might sort out one day. The 2ndry is so small that it has problems covering the whole mirror even on axis. It's an F4.5 scope. I've no idea how common this is. I'd guess that the approach can mask mirror figure problems to some extent. Maximum sizes on compound designs such as SC's often go up to over 30%. Specified as an area they come down to ideal sizes % diameter wise which I feel is misleading. Major observatory scopes too. This size is another compromise. The airy disk centre has around 82% of the light it should have in it. The missing bits go into the rings spoiling contrast. It's noticeable visually on some objects especially weak low contrast detail in planets. John -
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