Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.



  • Content Count

  • Joined

  • Last visited

Community Reputation

25 Excellent

About davidc135

  • Rank

Profile Information

  • Interests
    Astronomy and ATM, science and natural history, trad archery
  • Location
    Hay on Wye
  1. I'd contact Stathis Firstlight in Germany regarding Borofloat glass. His web page says he can supply discs of low expansion glass in thicknesses from 5mm to 57mm with order times up to 2 months for the big stuff. Up to 20 ins in stock but larger blanks need to be ordered. David PS I should add that Stathis seems a person very much after your own heart and I should think will be very keen to help your project.
  2. Stathis Firstlight in Germany offer a complete package inc low expansion glass discs and send to the UK. Also Vacuum Coatings offer kits. David
  3. I don't know why 3mm bevels are essential. More hours of work to go? I'd maintain what you have now that you're down to 400 grit. !00 grit diamond Swiss files are cheap followed by a smoothing 400 grit file or your stone. Will the bevel reduce that much during the finer powders? Does the little chip matter that much? David
  4. Try also to take out of focus photos both inside and outside of focus and ensure both distances are exactly the same. (use shims) The distance you have in the photo looks OK. The relative size of the secondary shadows can be compared and will give a useful idea although it's rough compared to a Ronchi or Foucault test. David
  5. Those pits and possible abrasive particles are asking for trouble. Best scrap it and start again along the lines described above. You have to have complete faith in freedom from contamination. David
  6. As has been said above, as a wave front moves through a telescope parts of it are advanced or retarded by errors encountered in the optics. The overall range is given by the peak to valley term (pv). More useful is the RMS (root mean square) figure which averages the errors over the whole aperture. The Strehl figure refers to the amount of light that finds its way into the Airy disc compared to a perfect optic. As the Strehl goes down more light spills out of the disc into the surrounding rings reducing contrast. The PSF (point spread function) illustrate this. The MTF (modulation transfer function) curve compares the image contrast of the instrument with the ideal for widely separated points at left through to very close, at the limit of resolution points, at right. These two objectives are both close to perfect, the curves being not far off the ideal and show why high quality refractors give such beautiful planetary images. With poorly figured objectives or with sizeable secondary obstructions the MTF curves fall well below the ideal especially in the middle section, reducing contrast. Hope I got that right. David
  7. www.ukglasscentre.co.uk sell 10mm float in 50x50cm for 24.00 plus shipping plus vat maybe but your local glazier might be a bit cheaper. Enough for a number of tools. David
  8. Your local glazier might be able to supply 10mm ordinary float (plate) but there may be a minimum size of sheet. For some 'white' float I ordered it was 50cm square. Would you be able to cut it up? There's something to be said for using that 21mm thick tool, especially on a first project. David
  9. I make the spherical aberration of a 130/900 sphere equal .34 waves; acceptable at this price although I think the extra performance of 1/6 wave optics would tell in good seeing. One point that hasn't come up is the smoothness or otherwise of the mirror surface and which could be more relevant than whether it's a sphere or not. For example, Ben's Ronchigram appears to show zone(s) or it could just be turbulence as he says. Reducing the Ronchi lines to three will increase sensitivity as would a knife edge test. David
  10. I used a weatherproof 'Cassini' plaster at a 28 (water):100 mix. Hard, strong and not affected by water, although I did coat it. Loses very little weight on drying at 40 to 50 degrees. I'd have thought 'herculite' would be better than ordinary plaster. David
  11. Good luck with your project. Have you decided on the speed of the mirror? David
  12. Congrats on this project. Where did you get the lens surfaces coated? Ah, scientific mirrors I've just noticed. David
  13. Midges, hedgehogs and a hornet's nest in my garden. Hmm, will I regret my live and let live good intentions? Another evening of fine seeing with the grs very colourful and clearly separate from the seb. David
  14. Yes, great seeing here as well in Wales allowing detailed views of Mars in a Meade 8'' sct. Above the S. Polar cap was a dark band with some structure and above that a featureless equatorial band. Maybe a dust storm? In the Northern hemisphere a smaller dark patch less conspicuous than in the South. A bright crescent along one limb connected the S. ice cap with the North pole. I was joined by a pair of hedgehogs and, less welcome, by the sound of hornets from a nearby nest up and about their business even before dawn broke. David
  15. Had a look through my Meade 8'' sct on the mornings of the 5th and 6th. It was more worthwhile than I expected with the South ice cap prominent, also southern mid latitude dark areas and Syrtis major showing. Northern part all red and featureless. Very good seeing especially on Tuesday allowed some good views in spite of Mars' low position. David
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.