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About Astrobits

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  1. Not negative at all, I was just merely pointing out that filters that perform the required function have been available for some time. The new one is just an improved version which will suit more light polluted skies. Regarding price, Image Optics started selling the filters to my spec for around £50. I was retailing them for less than that. Can't see them on their web site now. Nigel
  2. Those were just two that I pulled up from my files. 20+ years ago I commissioned a company in Essex ( Image Optics/Galvoptics ) to produce some LP/Nebula filters to my specification. ( I am sure that they could have done any specification had they been asked ). When I used these in the dark skies of Norfolk there was no advantage to the narrowest transmission filter over the broader one. In the more light polluted skies elsewhere then the narrowest transmission filter did perform better than the wider one. If you have a lot of light pollution then maybe line filters would be even better than the one you mentioned. I also agree with Louise that you might need to give different exposures to the two wavelengths which could not be done with that particular filter. Nigel
  3. A number of filters transmit both O111 and Ha and have been available for years. Most also transmit some longer IR which can be cut with a second suitable filter. For example: Though I must say that the filter referred to is very neatly done. Nigel
  4. The 8.75" f/6.7 Mirror Grind

    When you make up the test apparatus is up to you. Personally, I don't test until I am satisfied that all the pits have been polished out. The best way to check for pits is to examine the surface with a 10x loupe ( 25mm eyepiece ). I used a light box with a sheet of black paper/card on top from which holes had been cut. The mirror is put face up on this. Examine the area just into the dark edge and the pits stand out as bright specks. The first test that I do when starting figuring is to examine a pinhole image with the 10x loupe in and out of focus. This is a very quick check for astigmatism. Thereafter I use a knife edge for the normal Foucault set up. Moving the knife edge either side of focus gives the same info to me as a single edge of a Ronchi grating hence I have never owned or used a Ronchi grating. When I am getting nearer to the parabola I include a null test ( Dall Null ) which I find is much easier than using Foucault or Ronchi tests. There is info on using a the basic Foucault on the internet to finish mirrors. Nigel
  5. The 8.75" f/6.7 Mirror Grind

    It's go for polishing. I would always do about 5 minutes of polishing and then clean up and check the polish pattern appearing on the mirror. This just checks the lap contact and stroke pattern is good or bad. If good, the mirror will show polish all over, probably a bit more in the center than the edge with MOT, while if it is bad then there will be a big difference and you can do something the correct it like pressing for longer , changing the stroke or switching to TOT. Keep checking with only short polishing spells until you are happy that the mirror is polishing evenly all over. If I had to leave the polishing for a long time between sessions I would leave the lap in contact with the mirror and put them in a stout plastic bag or box with extra water to prevent drying out. That way the lap was ready pressed to restart polishing immediately. Nigel
  6. The 8.75" f/6.7 Mirror Grind

    As you are using a pyrex blank ( just had to look back to the start of this thread for that info) some of those will be bubbles that have broken through. If you try to eliminate them you will almost certainly break through into others. That is the problem with pyrex. I would ignore these defects ( as they will not significantly affect the performance of the finished mirror ) and continue with the next stage. Nigel
  7. The 8.75" f/6.7 Mirror Grind

    Your last post says going back to 600 grit?? I thought from a previous post that you were only ready to start the 600 grit. I would go on to the next grit size and see what happens, checking a selection of the larger pits as you go. Nigel
  8. The 8.75" f/6.7 Mirror Grind

    There are not many of them and they are small. Most, possibly all, can be removed with the 600 grit. Identify the exact location of a selection, 3 or 4, and check them after a couple of 600 wets. Then decide if they are likely to go with further 600 wets. If not then go back to 400 for a few wets. It is quite possible that they are caused by the largest particles in the 400 grit anyway so the 400 will remove some and produce some more. Nigel
  9. Filter technical data

    Now we know which filter you are talking about, have you looked here: http://www.schursastrophotography.com/ccdmoonplanets/JupiterFiltertests011616.html there is some interesting/useful info. Nigel
  10. Filter technical data

    This site has lots of info on optical filters including representative transmission curves if you follow the links: https://www.edmundoptics.com/resources/application-notes/optics/optical-filters/ Nigel
  11. M1 Tonight

    Out of interest, I managed to nab M1 with a pair of Canon's 10 x 30 binoculars. I was waiting for the 1999 Leonid meteor shower to get going at a dark sky site in Southern Portugal. I also remember coming across it some 20 years ago when idly trying out my newly made 16" dob. This big, bright cloud appeared in the field of view and I thought "What The Heck is THAT", shortly to be identified as M1. Ahhh!! Memories I wonder what it looks like in a 25" 'scope? Aperture fever rides again Nigel
  12. There are a limited number of materials that fulfil the requirements of lens coatings. ALL lenses use these materials, the most important is that the last ( outer ) coating is sufficiently hard to resist scratching. This applies to all lenses so any proprietary cleaning solution intended for "lenses" will be safe for the exposed surfaces of ALL lenses, camera, telescope, binoculars, and specs. Nigel
  13. Shim washers

    Would these be of any help to your build: https://uk.rs-online.com/web/p/locking-anti-vibration-washers/0428672/ other sizes are listed. Nigel
  14. The 8.75" f/6.7 Mirror Grind

    A 20" in the Kitchen? now there's a thought. Rather depends on the space and who else uses the kitchen Nigel
  15. Yup. Classical astigmatism. Make a pinhole light source and test the primary mirror. Leave it in the telescope, just view down the tube from twice the focal length. examine the image of the pinhole both sides of focus with a 25mm eyepiece. Keep the eyepiece and pinhole as close together as possible. If it's the primary that is astigmatic you will see the same effect as you see on the stars. Check that the mirror cell retaining clips and centring components are not tight as these can distort the mirror. Nigel