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  1. Advice for making a parabolic mirror

    Most ATM'ers make the primary mirror but buy in the flat from a commercial source. Unless you are making something special you will not save any money over the commercially available mirrors from sources such as SkyWatcher. Making your own gives you the satisfaction of having made it and the learning experience for future mirror making. Contact your local astro societies as I expect you will find members that have made their own mirrors and can guide you if you decide to go that route. They might even have mirror making classes where you can make mirrors alongside others and benefit from the bulk purchasing of materials. Nigel
  2. Best wood for a wooden box-tube?

    Plywood. Solid wood expands sideways with the humidity and is likely to warp as well. Warping can be avoided in solid wood by laminating several strips together with the orientation of the grain of each piece opposite to it's neighbours. It still expands and contracts sideways though. Whatever you use, treat the outside similarly to the inside of the "tube" otherwise the sheet will bend with differential response to the humidity. Nigel
  3. The 8.75" f/6.7 Mirror Grind

    You will find that Al Oxide is much softer than SiC and gives a much finer surface. I would typically polish after 12 micron Alox so I would recommend that you go with the 400 and then the 600 and then polish. The finer you try to grind the greater the chance of scratches which often take longer to grind out than the time possibly saved with a finer surface. If memory serves, the numbers of the smoothing grits are the particle sizes with a decimal point missing, 125 being 12.5 micron etc. If that it correct the 600 Alox is finer than the three smoothing grits in your list, so they would be a retrograde step. Nigel
  4. Best Silicone for secondary

    I use any Silicone that specifies it's use for bonding to glass. They usually mention greenhouse applications. So the one you link to should be satisfactory. The secondary only needs one blob in the centre of the mirror, extending to no more than half the minor axis diameter, assuming that you have a solid base, and not a cut tube for the holder. Make sure that the final thickness of the silicone is approx 3mm thick to prevent astigmatism. Nigel
  5. The 8.75" f/6.7 Mirror Grind

    It doesn't really matter, MOT or TOT for smoothness of curve. My preference would be to keep TOT, the longer the ROC the easier it will be to figure. The top piece gets more action in the centre and the bottom piece the edge. Therefore with MOT the ROC will get shorter while with TOT the ROC will get longer. The longer the stroke the more severe this action will be, probably leading to non-spherical surfaces one way or another. At 400 grit this action will be small but noticeable. The reason for the small W stroke is to smooth out any tendency to produce zones caused by some regularity in the tile tool ( even if you can't see any obvious regularity ). Nigel
  6. The 8.75" f/6.7 Mirror Grind

    As you've discovered, each grade of abrasive leaves it's own variety of pits. The reason for using shorter strokes is to prevent flattening the edge of the mirror at the expense of leaving the centre too deep (with tool-on-top). Once you have generated the curve with the 80 grit then all you are wanting to do is to remove an even layer of glass over the whole surface thus removing the pits. Whatever length of stroke you use the piece on top always gets more work at the centre and the bottom piece, the edge. The longer the stroke the more pronounced this effect, thus keeping the strokes short is beneficial. I would avoid continuous COC and move the tool about 1/2-1 inch left and right of the centre as in a W stroke. As far as the grits to use now I would say, yes, go 400 and as I said before follow the biggest pit you can find until it has disappeared then that grit is finished. I am a bit concerned that I can see no cushioning between the mirror and the wood. That can ( very possibly ) lead to astigmatism as the wood will not be supporting the mirror evenly over the whole area. Try to get some carpet between them and rotate everything frequently. Nigel
  7. According to my Stellarium, and the IOTA web site, disappearance of alpha TAU will be about 1:15 ( 1:12 from my west country location ) and re-appearance about 2:00 on the morning of 31st Dec. This is a weekend event ( Sat/Sun) so should not be too difficult to stay up for. Nigel
  8. You will have another chance on 30th Dec. Early evening, moon nice and high, both disappearance and re-appearance. Stellarium view: Nigel
  9. Mirror Cell base diameter ?

    150mm. you don't need much extra diameter ( if any ) to fix side clips. Personally, I would not use side clips nor a circular disc at all but a triangular one with the mirror bonded with Silicone at three points only, making sure that there was about 2-3mm gap between the mirror and base board. That gap prevents any strain transferring from the baseboard to the mirror and makes it easy to remove the mirror when the time comes to re-coat the mirror. Even a triangular board is more than necessary, all that is needed is a Y shaped base with the arms about 1" wide. They will provide the points for the Silicone and the collimation bolts/screws. Such a shape will be the lightest possible and give the best airflow to keep the mirror temperature at ambient. Nigel
  10. NB filters (historical query)

    I listed a Miralux broadband filter in early 1998 for a retail price of £59, so they must have come in before then. Nigel
  11. My first reaction to this is that it might be more difficult than plain dirt judging by how difficult it is to get snail/slug slime off ones fingers. I would go for soap and water but I have no idea whether the Photonic solution would work. Worth a try first if you really don't want to remove the mirror from the tube as I can't see you using soap and water with the mirror in place. It will be interesting to see how you solve this. Nigel
  12. ES12'' Gen II Dobsonian

    Nylon IS plastic. Not surprising for a 12" ( 300mm ) mirror to show more than an 8" ( 200mm ) mirror. Some interesting observations on this telescope that might help other owners. Nigel
  13. Observatory anyone?

    get 'em while they're young Seen in Bristol Hobbycraft. Nigel
  14. The 8.75" f/6.7 Mirror Grind

    Assuming that the pit you identified, as per my previous reply, disappeared then you can assume that any further pits are from the abrasive you are using and it is therefore pointless to continue with that grade grit. In your current situation I would continue with the 220 grit with the tool on top to lengthen the ROC. Keep the strokes very short ( much less than 1/3D ) or you will end up with a difficult shape to sort out. As before, select the largest pit you can find ( near the centre of the mirror with tool-on-top working ) and when that one is gone check the curve and correct if necessary. Then move on to the next grit. Note that the quoted grit sizes are average mesh sizes ( usually holes per inch ) that allow the grit through. There is a range of particle sizes within a batch. For example, 180 grit has a maximum grit particle of 114 microns, an average of 76 microns and the finest 25 microns. 220 grit has max 102, average 63 and finest 20 microns. It is those biggest particles that give the bigger pits. Thus the biggest pits from 220 grit will be bigger than the average pits from 180grit. These are figures from NEWPORT GLASS who supply mirror grinding materials. There is no guarantee that grits from other suppliers will conform to these specifications. There is also the possibility that whoever packages the grits from bulk doesn't clean up effectively when going from a coarse grit to a finer grit contaminating the finer grade. Nigel
  15. Places to visit - Astronomy related

    Herstmonceux, East Sussex. Castle, grounds/gardens, Telescopes and interactive science stuff . Something for everyone in the family. Nigel