David the Ronchi tester look OK. That's a good idea with the Webcam. It looks much better than mine. Mine looks like something out of the Flinstones. As for the lap I use a hard pitch, and with deep channels. I don't have to repour or rechannel even after 10 hours of polishing and 2- 3 hours of figuring. The down side is that you have to press continuosly, which is not a bad thing. Keep going and remember to shave your lap a couple of mm smaller than your mirror.
Thanks Cristian, have had a play with the tester this afternoon and found that if I put a diffuser on the led to increase the size of the light source and use the Ronchi screen instead of a slit, the tester works as advertised with more lines both sides of the ROC.
Have also had some success getting rid of the turned up edge, but now have a central hill to remove. Probably this won't be too difficult.
I find having the Toucam mounted permanently on the tester a great help particularly when lining up the mirror, because you can see what is happening at the tester while standing at the mirror and turning the slow motion controls.
Looking good David. If the first picture is IRC, then you have a slight hole in the middle. You can try a little longer to try and remove it or you can parabolise from here, I would parabolise. If the first picture is ORC then you have a hill and I would try to correct before parabolising. Let me know which it is and I will give you a tip on parabolising. I get my stuff from my astronomy club but you can get it here too. http://www.telescopes-astronomy.com....escopes036.htm I have used their powders before and no problems at all. The secondary mirrors I get from Andrews and you can get the mirror silvered by C Quin Pty Ltd. Ph(0412957809), there are afew others in Sydney as well.
Regards Cristian
Last edited by cristian abarca; 04-02-2007 at 07:36 PM.
Thanks Cristian,
The order of images is IRC,at RC,and ORC. What sort of strokes should I be using to parabolise the mirror? Should I be using a set of computer generated Ronchi images from here or should I change to a Focault test? I think that this will be the fun bit.
Hi David. To parabolise a mirror you basically use the same technique as for polishing (W strokes and walking around), except that the up and down movement and the side to side movements are larger. Work with the mirror on top. Mark the centre of the mirror (the back). From the edge measure 6 cm and place a dot, do this all the way around and then join the dots to make a circle, use this circle as a guide. As you do your strokes the circle you have marked should reach the edge of the lap at every turn, (sideways and up/down). Where the circle reaches the edge of the lap, is the 80% zone which is where you need to be at every stroke. Do this for about 15-20 minutes and test the figure. Remember to press in between each session. Don’t do too much at a time, take it nice and slowly. Also make sure your strokes are smooth. Download this program http://members.aol.com/ronwin20/ and go to this site, scroll down to figureXP download and download another program, you will need it later. http://bi-staff.beckman.uiuc.edu/~me.../ATMlinks.html . For the moment use Ronchi for windows. I have also adde some drawings explaining parabolising. Sorry for the quality but I hope the make the explanation simpler. Good Luck
Thanks Cristian, I had used your method of drawing the circle on the back of the mirror later in my polishing. It definitely made it easier to get even strokes.
Have previously downloaded the two programs you mentioned. I was originally trying to use the Foucault test (with my own spreadsheet) to measure the mirror figure, as I could not get the Ronchi test to work for me. When I managed to get the Ronchi test working, I realised the extent of the TDE. This has been a very steep learning curve, but I am glad that I have had a go and am grateful for your expert advice amd patience.
You flatter me David, but I'm no expert. I've only made 6 mirrors, I just like to help people who are interested in making their own mirrors and to pass on any tricks that I have learned along the way. I find that some literature tends to confuse people a little and since mirror pushing seems to be a dying craft, the simpler it is explained the more people will be interested in taking it up. I also find that the Focault test is a bit difficult to interpret for first timers, the Ronchi test is easier. have a look at these two sites regarding the Ronchi test. In particular the one relating to the Lumley test. This is an excellent way of getting a good figure on a telescope mirror and it will only take a little effort from there to get better than 1/10 wave using FigureXP. Besides it was by an Australian E Lumley. http://www.turbofast.com.au/astrotel/ronchi1.html
Hi David you are in the right track, just do a lot of pressing and try to keep the strokes even. By this I mean don't do more strokes in the middle try to do the same amount of strokes in the middle as well as the edge so that you work the whole mirror evenly. at the moment you are working the centre more than the edge but that is OK at this stage. The edges will turn automatically if you keep the strokes even. Proceed slowly and get back to me after another 1/2 hr. I would be doing 10 - 15 minute sessions, with about 1/2 hr to 1 hr pressing in between.
Regards Cristian
Last edited by cristian abarca; 11-02-2007 at 09:34 PM.
Thanks Cristian, at this stage am trying to take it slowly, and hopefully not make too many errors, but things are happening comparitively quickly with this stroke.
Sorry to trouble you again, but looks like I may have made a bit of a mess here. Cannot seem to get the edge of the mirror to respond. Any idea what I should do from here?
Hi Cristian, I'll have to set my dial guage up on the tester to get the measurements. Haven't been using any measurements yet, but I guess its about time I did. At this point, the Roc may be a bit difficult to track down precisely, so it will have to be offsets from an arbitrary zero point I suppose.
Hi David I think that you have over corrected you mirror. Have a look at the picture. This is a ronchigram of an over corrected mirror. It's a hyperbolic cassegrain secondary test plate, that I made. You can fix it by doing 1/3 strokes for about 5 minutes and pressing for about 1/2 hr this will take a bit of time but It will keep you going for a day or two. I'll get back to you tomorrow on another course of action. I'm moving house and all my astro stuff is in boxes at the moment.
