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About Astrobits

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  1. 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
  2. Places to visit - Astronomy related

    Herstmonceux, East Sussex. Castle, grounds/gardens, Telescopes and interactive science stuff . Something for everyone in the family. Nigel
  3. Where would you live?

    It has to be near the equator. The possibilities of 10+ hours of darkness EVERY night of the year and the Southern skies which are, in my opinion, much better than the Northern sky seals it. At the equator you don't loose much sky, just bits around the poles where there are few gems anyway so all the Northern gems are there as well as those even better Southern sky gems. The Centre of the Galaxy with all those Novae and all comets will be well placed for your perusal. Work? Find a local job. Nigel
  4. Cheat!?

    In my view, GoTo is fine if you want to look at/image particular objects, or are doing some sort of visual marathon against the clock/sun. It also helps greatly if doing outreach. Your queue of 'customers' don't want to watch you struggling to find something for them to look at. For just viewing the heavens star hopping can spring surprises you didn't expect. The odd unmarked star cluster/asterism, novae can appear without your GoTo knowing about them. Just remember Messrs Hale and Bopp were star hopping to a particular DSO when they found their comet Nigel
  5. Low profile Allen head UNC bolts..?

    If you cannot find a replacement easily, see if a larger headed screw could be cut down on a lathe. Alternatively, does it have to have a hex fitting, would a slot not be a suitable alternative? In which case anyone with a lathe and 1/4 whit/UNC die could make one from scratch. A nice brass one perhaps. I am sure that someone on SGL will be happy to help ( I've got a lathe and 1/4 whit die, so have a number of others). Nigel
  6. It depends on just how good your optician is. The usual test for infinity in their rooms is really only about 18 meters or so. I always have to have my glasses adjusted by 1/4 dioptre from the test result to get true infinity focus. My optician is aware of this discrepancy and does the adjustment automatically. What I have done in the past is after the best has been done in their room, I went out and checked the view in the street while still wearing those frames with the slot-in lenses. At least that way you can get more than 18 meters viewing distance. As far as the astigmatism goes they can only adjust for one axis and if you have two axes of error then only the strongest one will be corrected leaving the weaker one that you might be seeing. Nigel
  7. 16" mirror recoating

    Try putting 'vacuum coating companies' or similar search terms into Google. There are other companies that do this but are aimed at industrial users so are likely to be much more expensive than the ones you already know. Perhaps you will find one nearer to you than the U.K. Nigel
  8. Galvoptics http://www.galvoptics.co.uk/ VCSM ( Vacuum Coatings Scientific Mirrors ) http://www.scientificmirrors.co.uk and as mentioned above Orion Optics Nigel
  9. Ebony Star alternatives

    Find a Kitchen worktop manufacturer. They will have offcuts, probably for free. Nigel
  10. Comin' along - JWT

    I am sure we will get some interesting pics but not the same type as Hubble. The camera on the JWST is Infrared, 0.6 microns is the shortest wavelength it will record. It won't be able to record the blue-green portion of the spectrum from our own galaxy and those very nearby. Once the galaxies are red shifted enough then those wavelengths will fall within it's grasp. Nigel
  11. Where is the Delaware Diamond?

    They, or someone, bought it from the unofficial " Star Registry", hence it's faintness. Nigel
  12. Ed: Polythene ( polyethylene) is a naturally flexible polymer so no plasticisers are needed. However, as the B-BQ cover is made for outside use it almost certainly has other ingredients such as UV stabilizers and, presumably, pigments that do have the potential to bleed to the surface. They are not as volatile as the plasticisers used in PVC so as long as the cover is not touching any delicate optical bits it should be O.K. PVC in it's 'natural' form is very rigid, think rainwater goods like guttering, and needs plasticisers to make it flexible. Dave: Yes we also have the triangular identifiers but these only appear on containers, things like polythene sheeting and cling film don't get the mark. The marks are primarily to aid recycling as mixing plastics is not a good idea if you want a usable product out of the end. For example, Polystyrene and Perspex (aka Plexiglass ) are both clear rigid plastics in their natural state but they do not blend together, they are, like oil and water, not miscible. By the way, Ammonium Fluoride is used to etch glass as well as HF. It is at least less volatile although still pretty dangerous if not handled properly. When I had an old enamel bath re-coated with a polymer the operators used it to etch the enamel ( =glass) coating so that the bonding was as good as possible. Nigel
  13. There has been a lot of concern for many tears about the plasticisers from cling film entering the food chain. Steak and Phthalates anyone? Nigel
  14. If your plastic food wrap is based on PVC then it is full of plasticisers which will leach out. These make the plastic flexible and tacky. Just Google "PVC plasticisers" to get some of them, mainly long chain phthalates. If that is the case then water will not clean your mirror and you will have to try other, more vigorous, materials. I would start with soap solutions and if that doesn't do it then some commercial lens cleaner or organic solvent such as Isopropyl Alcohol. ( nearly used the abbreviation IPA there but here in the U.K. that is a beer and while it may wash a mirror it is much better being drunk) Nigel---hic!
  15. The 8.75" f/6.7 Mirror Grind

    Yes, pits can seem to be very persistent, but they do, eventually, disappear. I like to think of the grinding action in this way: As a large particle of grit rolls over the glass it strikes the glass with one of it's points something like hitting a big lump of concrete with a pickaxe. This causes a crack down into the glass. At a later time another particle creates a crack that intercepts the first crack and a lump of glass is freed and a pit is formed. This second crack does not necessarily meet the first crack at the bottom, the first crack going deeper. At the end of using a coarser grit the surface is covered with pits and there are sub-surface cracks left over. On starting a finer grit these create finer/smaller cracks, some of which meet the existing cracks creating a somewhat larger pit at the location of the deeper cracks. Again, the smaller cracks do not meet the remnant cracks at the bottom so the pits tend to be repeated at a particular location until they do, at last remove the traces of the coarser cracks. This action is repeated with each grade of grit. Have fun Nigel