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Ajohn

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  1. I was thinking in terms of using a shelf going back into the case to support the horizontal gears Gina. Backing paper running down from that an also in the "pocket" formed by the shelf leaving all of the gearing visible. I suggested making the clock face a bit smaller to get round the lack of visibility in the area you indicated with arrow A. That might mean changes to the size of the case or it might not. As it's a visual thing hard to say. The shelf gets around having a slot and leaves everything visible. I'm no artist though. Just a humble engineer. Mmmm you would have to support the prawl from the top of the case. Maybe something printed ? John -
  2. Maybe a smaller dial just showing the teeth of the larger gear and the rest of the drive sitting on a shelf hidden by the background. The case would probably look better oblong then rather than square. I nearly suggested larger gears to keep the dial the same but the case would still probably look better oblong. John -
  3. 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 -
  4. 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 -
  5. 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 -
  6. I've found this thread interesting and wonder if the earlier post ( which I hope was on here ) concerning noise against temperature is the important aspect. John -
  7. Not sure if it will be of interest to you Herra but full frame has been reckoned to offer 2 to 3 stops better noise performance than APS. There is a problem though. As their pixel count gets higher and higher that becomes less true. Where is was true is cameras such as the canon 6D but at the 3 stop level even that one may be dubious. In the case of levels like the D800 one of the aims seems to be to do away with the anti aliasing filter as the lenses do that anyway. It looks like this has been absent on compact cameras for some time. All very confusing though. Sensors have improved but so has noise removal software. I own an interesting example of the effect. A Nkon V1 with CX sensor. They did a cheaper Nikon 1 with less pixels and lo it has less noise too. My most used "dslr's" now are olympus m 4/3. I haven't touched aps for some time now. The 6D isn't very well rated by serious photographers - af and etc is too simple. My last ff was a 5D and no interest any more so no idea of used prices. John -
  8. Gunson have made an automatic batter charger with a float setting for permanent connection for a long long time. It works rather well. John -
  9. I've not tried to do this but have a feeling that the lens needs to be focused onto the mirror not infinity. The reason for changing the lens on a web cam is to get a bigger image as the standard ones are fairly wide angle. The feeling comes from eyeball testing and also messing about with taking photo's from what comes out of an eyepiece where the camera does have to be focused on infinity. I'd be interested to know if I am correct as the idea of a nice big image on a PC screen appeals. So basically place the camera with the lens a little outside the centre of curvature, focus the camera onto the mirror. Having the knife edge close to the camera is mentioned here for instance. I'm sure I have seen the same thing mentioned elsewhere. http://foucault.sourceforge.net/#docs That test might be worth a go but I would cross check with a mask the usual way. A mask on the mirror would help to see if the web cam can be focused. John -
  10. I found the attached file on the web for download so no problems attaching it. It covers just about everything I have ever seen mentioned on dental stone but avoids using epoxy for sticking tiles. I suspect that is what people will have to look for in the UK not plaster. Some have used hydra stone over here, or a name very like it. It's a much stronger type of the usual modelling plaster. Some one on the youtube uses a mixer to mix dental stone up quickly (Gordon ??). It sets pretty quickly so probably best to mix a little to see what happens rather than just diving in. I've attached the file in pdf format. I saved it in odt which is a Linux word format so used something else to convert to pdf. If Adobe wont accept I can post in some other format if needed. Tex casts squares, heats them over a low flame and plonks them down on a warm mirror. From other sources I have seen this seems to be the pro way even on very large mirrors. He mentions that Ritchey ( as in Chretien telescope) painted hot bees wax on poor quality pitch. I've used Howard's method. Let it cool somewhat, pour in a spiral on the warm mirror, cover everything with rouge and rub around to achieve full contact, then cut slots with a wet saw when it's fully set. Messy so next time I'm going to cast strips, cut squares and stick them down tex style. Bet it sticks to what ever I cast it on. Whoops One thing to add. For sub diameter Gordon ??? again seems to have decent method. He uses an annular ring of wood of some sort to act a former with what ever is used to form the sides running down onto the mirror. Looks like he uses the thin plastic film (fablon) that can be stuck to shelves etc in the kitchen to stop the plaster from sticking to the mirror. John - tiletool.pdf
  11. I wish I had one Damian but I wont spend what they usually cost plus some only have 1/2" travel. Some people who test thin mirrors test vertically to avoid mirror flex problems. That makes an easy build x-y jig difficult but it's possible to manage without the 2nd axis testing in the normal fashion. Many do. It's also ideal for that caustic test I mentioned but if your having the mirror checked some other way that's over the top. That mic spindle on mine has a 1/10000" vernier on it and that has real meaning on that test. The only woolly part is the actual mirror rad. I tried push pull and a rule as a scale for rough figuring but found I prefer the mic spindle. That could even be a chopped up mic. Also pin pricks on paper measured afterwards. Stub mentioned the bolt. Knobs can be found with the graduations on them. There are cheaper digital machine scales about. ArcEuro maybe. There are all sorts of options really. Once some sort of stage is made it's pretty easy to try a number of things out including stationary source, slits, different ways of moving and measuring etc all part of the "fun" - ? if that's the right thing to call it. My feeling is that it is worth spending some time on the stage what ever type it is. Sometimes it's just a straight piece of wood with the moving part pressed against it. I simply thought that a piece of rod and a couple of brass V's was just as easy and it would definitely slide well. The tilt bolt could also run on brass or glass even. The main problem with mine was the knife tilt bolt - the thread is too coarse so I should have added a big knob or use something finer and probably a knob as well. Texereau gets round that by making the platform a lot wider and uses a finer bolt than I did. I'm might just soak the rusty bits in a mix of 25% molasses and water for a few days to get rid of the rust and more or less use the stage as it is. It's time for an upgrade on it anyway to try a big moving source but I'll probably still make another slit. John -
  12. I think Howard's book is on the archive. I did try drilling holes in callipers but found I needed a carbide drill and went of the idea. Lots of people like the idea of using a dti. Some move by hand some move mechanically. A lot depends on what is around. A spare focuser can be used as has been shown. Used micrometer spindles can be cheap. DTI's too. The metal parts I used don't cost much either and where metal is best used really easy to make. John -
  13. Thanks for mentioning off setting the knife. I've been webbed. That tester crops up all over the place and is what I intend to make but it can't work on axis, the mirror will return the image of the source right back above it and the knife wont cut it at all so it needs offsetting to cause the return beam to hit the knife. Leaves me wondering if it would be better to leave the led unobstructed and just have the knife central too it and above it. Minimising the offset does improve the accuracy. This is what I have used. It was in a loft room when we had the roof done. Covered in dust and got damp. It a fixed source version but the idea for the sliding table can be used with any type. It's easy to make. MDF etc would do but the sliding table needs some weight added even in aluminium. The tilt screws just run on a bit of metal plate under them. I added my own thoughts. As I used a slit I tilted the source rather than having a separate piece that can be tilted mounted on the knife. I also added a zero facility for the mic spindle - set mic to zero, slide table about to pick up the centre and then slide the bar up against the mic spindle and lock it in position. I didn't add a spring to pull back the table onto the mic, just used finger pressure on the table. Most people don't mount the bar the table runs on in blocks and just screw it down onto a board of some sort. In that case the mic spindle can be arranged to press on something on top of the table. It is best to make the V's that rest on the bar out of brass. It slides very smoothly. The slide going across has the V's made of aluminium and seemed ok. The other gubins that moves the knife across is for a version of the caustic test. For that a wire is mounted on the knife and I just held a 10x eye cup type loupe in my eye focused on the wire by moving my head. This test does need a slit. I've lost that but it was made as per Texereau. His way does work. No shadow reading at all with this test as it uses diffraction. I used it as a final check. The all thread - for playing with a Dall null test. I might play with that again and get a better lens for doing it. Abuse of the Ross tests looks easier though. Just move the lens and tester around until the mirror nulls flat and look for ripple and small zonal errors etc. John -
  14. That's odd really. The camera should work just the same way as the telescope does in the photo of the test rig I pinched of the web. It doesn't care that the frame of the tester and the knife edge is blocking the view. The knife edge has to for the test to work. I'd guess that getting the camera on axis and square to it would be a bit tricky though and some zoom lenses can do odd things used on systems like this. There was a mention of the Ross test. This can tempt people but on faster and larger mirrors the measurement accuracy needed form tester to lens and lens to mirror to make it worth while is very extreme. John -
  15. Trying to help again. The problems with Foucault testing are likely to be down to the tester used. There are lots of nvg info about however there is a sensible one shown on this page especially for some one with an x-y stage but that can be made up in all sorts of ways. http://www.stathis-firstlight.de/atm/foucault_tester.htm?sa=X&ved=0CDwQ9QEwEzgoahUKEwiw0eCp_frGAhUOKtsKHa0IBFg This one The telescope is optional. Note the comment on the web page about sanding the led or using a diffuser. I would add a flat ended led, If it's round ended just sand it flat. Some sort of plastic diffuser as mentioned isn't a bad idea anyway. What he doesn't mention is alignment. He's done many mirrors so will have some sort of bench set up so that the mirror can be placed on a stand and align well with the mirror. What I would do is fix the knife edge more securely and have a piece of cardboard with 2 holes in it. One to let the light through and the other smaller one to indicate where the knife edge is. If the centre of the led was say 10mm down from the top of the mounting this small hole should be 10mm above it. Best make that distance as small as possible but the return image or the led must clear the woodwork so the distance depends on the size of the led in some respects. If the cardboard is place in front of the tester it will catch the return image of the led from the mirror. At the ROC the image of the led and knife edge will be sharp and the set up can be manoeuvred around to place the image on the knife edge hole. Fiddle with the knobs and it should be fairly easy to null a near sphere now but the adjustments will be very sensitive. That can be helped by mounting say 50mm dia disks some how on the end of micrometer spindles. When a mask is used for measurements the shadows have to move equi spaced around the centre of the mirror as the knife is moved back and forth. For initial rough measurement the shadows can be evened up side to side with the other axis but that needs to remain fixed for final figuring. Old time testing using a fixed source will already have taken care of that and people without an x-y stand can use this sort of tester in the same way to set it up - alter it's angle until the shadows move evenly. This can also be done roughly with the cardboard in front of the tester as the return image will move from side to side. To use Ronchi just remove the knife edge and clip the screen on but set up in the same way. Setting up is a pain. A simple solution for most people if they haven't got a bench that is long enough and things can't be left in place, lack packing to raise the tester up etc might be a shelf on a wall. Maybe 2 one for the mirror and another for the tester so that mirror sizes and tester height can be at least part adjusted. I have seen a professional who made lots of mirrors daily work on a shelf. It seems to work well but he was mostly making 10in F6.2 mirrors so his tester was built for that size. Me I had a bench and a bookcase which made up the lengths The bookcase was taller than the bench so height adjustment wasn't a problem, just adjusted the height of the mirror. Once set up the mirror could then be just dropped in place, the tester checked for axial alignment and measurements taken. I haven't got that any more so am looking for a wall for shelves that wont upset the wife. One warning about bright sources. After a long session I found the pupil in my right eye was a tiny pin [removed word] and the other a lot larger. It took a while to settle down. The telescope on the tester in the link - just used to make the mirror look bigger. A converse view is to use one backwards to improve the view of the shadows. Pass never tried either. Whoops - People need to be able to get their eye behind the tester. Might not be easy as shown. Remove one side / turn it around etc but turning it around would need some slots in the cardboard to clear the frame or what ever is used for that. The knife will probably need moving forwards by 1/2 the thickness of the cardboard if the return image has been sharply focused. John -
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