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  1. Ahh! O.K. It's just that your image seemed to show a pivot point in the centre of the disc and is explained as line of sight effect by AlexK. Thanks for the clarifications. Nigel
  2. Your Boxmount is a most interesting design, Chriske. I am currently planning to make an equatorial platform and will investigate this option for my 16" scope. However, with the rotation axis in the centre of the platform I think that the scope will need a very low COG to be safe at +/- 15 degrees of tilt ( 2 hours) unless it is firmly attached to the platform. Nigel
  3. You can also try SRB Photographic: https://www.srb-photographic.co.uk They do specials, you just have to find the link on their web site. Nigel
  4. For me the best description of the "Star of Bethlehem" is given by Michael R Molnar in his book: https://www.amazon.co.uk/Star-Bethlehem-Legacy-Magi/dp/0813527015. It turns out to be Jupiter. The circumstances fit pretty much all the details from the Biblical stories provided we view these from the society of 2000 years ago and not from our current times. It doesn't need any unusual/unexplained events in the heavens. None of the other "obvious events" to our eyes (Novae, comets, conjunctions etc) can be regarded as fitting anywhere nearly as well - hence the continuous debates. Ni
  5. Every angle of lines on the mask will produce two spikes at 180 degrees to each other. Therefore if there are three different line angles you will get 6 spikes and with 4 line angles there will be 8 spikes. Nigel
  6. I think you will have to have a longer "back working distance" than that in your Cass calculation as it has to accommodate the radius of the tube plus the working distance outside the tube for focuser and camera. Nigel
  7. I think that one of the telling comments in that announcement is that we only know, on Earth, of anaerobic micro-organisms that produce Phosphine and HUMANS. I am sure that it was not suggested that we produce the gas via our bodies but in chemical reactions in labs. This process requires that we bring together the right chemicals at the right conditions and we get as much Phosphine as we want. Chemistry does the rest. I, personally, believe that chemical reactions will be found on Venus that explains this result despite the offered analysis that says it can't. Nigel
  8. If I was designing something like this I would make sure that it was easy to maintain/repair using the simplest, readily available, components possible as a first requirement. Any friction implies that there will be wear that will eventually cause the component to malfunction so the easier it is to replace the better. Nigel
  9. While it is fun/instructive to design the telescope structure and analyse it's performance on a computer, I would have thought that the fundamental question in this project is "can you make a mirror to the specifications desired?" Personally that is the first thing that I would do.....make the mirror. If it works out fine then it will wait for the rest of the structure to be made. If you make the structure first, or alongside the mirror making, and the mirror is not up to your specs then it could be a big disappointment. Nigel
  10. I'd scrub option 2. If you are going to slump it then low expansion will be a safer bet than float. Low expansion can be cooled a bit more quickly than float. With the development of toughened and laminated glass the thickest float glass now commonly produced is 19mm although 25mm is listed by some Chinese suppliers. Really thick stuff can still be had when aquaria replace their very big glass tanks with acrylic and occasionally when old ships are broken up. Unfortunately most of these sources have already gone, so these are not very easy to come by but there might be some available from some
  11. Thinking about it I have a local secondhand wood yard who often have damaged worktops. Now, Formica is not damaged by water but the chipboard it is stuck to is. Can you see where I am going with this? I will have to see what I can do when the yard is open for business, or perhaps speak to my local builder for some from refurbished kitchens when he gets going again. Nigel
  12. If you can locate a local worktop manufacturer they might be able to supply offcuts at a reasonable price ( possibly even free from their rubbish bin). Nigel
  13. Individual lenses made by these top end companies always have a ridiculous price tag. If your requirements are very specific then there is nowhere else to go. At the cheaper end is Edmond Optics which will sell you a 50mm achromat for £120 +vat However if your requirements can fit around particular specifications then you can get achromatic lenses 50mm or larger for much less. A pair of 50mm achromats with about 250mm focal length can be had for way less than £100. You will get two eyepieces thrown in for free. They are called binoculars. Nigel
  14. The edge bevel definitely should be more than 1.5mm, you will loose a little more in the finer grits which could leave you with 1mm or less as the final bevel if not enlarged. A shrinking bevel size is a common problem in many mirror making efforts especially when more coarse grinding than expected has been needed to get to the curve. You can try to only grind the edge on the stroke away from the surface, lifting the grindstone away from the glass for the backstroke. This will hopefully leave chips only on the outer edge of the bevel but be prepared to go back to coarser grinding if there
  15. As the May issue of S & T is current I will not scan the article to post here. That said, I will try to give the flavour of the test report by Dennis di Cicco. Overall I think he was impressed. He tried it with the following telescopes: 2.4, 4 and 6 inch refractors 12" f/5 and 18" f/4 Dobsonians 8" and 16" Schmidt Cassegrains 6" f/12 Mak-cass 8"f/3 Newtonian and with and without a coma corrector on the reflectors "It worked well on all of them." He also used a pair of 26mm Plossls with 2x, 3x and 5x barlows without any problem. There w
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