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alpal

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Everything posted by alpal

  1. Nice image but I think it's a little over smoothed which has lost some detail. cheers Allan
  2. I have done a safety tether string modification. see pic. I used part of a new shoe lace. It's glued on at 2 spots just for extra security. The lace could not be glued to the paint on the back of the mirror as the glue would attach to the paint and silicone remnants and be easily ripped off. It had to be glued directly to the glass. I gently scraped off the paint with a fine chisel and used metho but finished off with Acetone for a super clean surface. I used 5 minute epoxy resin. When I touched up some areas on the inside of the tube using a tiny brush - with Krylon 1602 Ultra Flat Black they show up against the TS "Black" anti-reflective paint. Which is better? The Krylon looks darker. The next time the spider is out I might need to spray paint as much of the inside of the tube as possible especially opposite the focuser. cheers Allan
  3. For the record I attach the before and after image via the Cheshire eyepiece with added circles. I am now ready to take some pictures outside when the weather is clear. cheers Allan
  4. Latest news, I fitted a 15 mm spacer and used a longer central socket head bolt - I also rounded the 3 screw ends a little bit. After a lot of collimation help from another forum I achieved a better result. cheers Allan
  5. What should I do about the dimple plate for the 3 adjustment screws? So - the dimples didn't line up so they just reversed the plate - or is that the normal setup?
  6. I have some pics after I pulled the hub and spider out. I think I have enough to work it out now.
  7. Thanks Jason, Yes I may have to go for - partially offset collimation. as per the post further back. cheers Allan
  8. Thanks Spile, I re-collimated the scope and - took a clearer image using my mobile phone through a Cheshire cap. and a person on another forum made this image and agrees that there is an offset problem. he says - The current secondary mirror (dashed green circle) needs to be moved to the solid green circle (about 1/6 of the secondary mirror diameter, so ~15mm). I am about to pull the secondary out for a closer look. cheers Allan
  9. I made my own Cheshire eyepiece by drilling a 1.5 mm hole in the center of a 1.25" focuser end plug. I post the picture of what I found. I can see the 3 mirror holding clips so you'd think it would be OK as is? How important is offset and collimation? cheers Allan
  10. I think I have some good answers here: https://catseyecolli...m/mccluney.html Offset Away from the Focuser - Unless the spider assembly or secondary mirror holder is specifically designed to include it, offsetting in this direction can be difficult. For a typical 4-vane spider the mounting holes may be drilled slightly shifted in the tube wall in order to accommodate the offset. It may also be possible to offset the spider using the spider leg mounting hardware by loosening the leg(s) nearest the focuser and tightening the farthest leg(s). This solution is less desirable as it may tend to increase the width of diffraction spikes seen around bright objects, or even make each existing spike 'branch' into two spikes. The primary reason for offsetting the diagonal away from the focuser is to keep the optical centerline and the telescope tube centerline coincident and prevent vignetting at the front entrance of the telescope. If this could be a problem, or if the most perfectly possible aligned system is desired, then include this offset dimension. When offset in this direction is not included, the optical centerline will be reflected by the secondary mirror by slightly more than 90 degrees. This will be compensated for by primary and secondary mirror tilt with no detriment to the telescope's performance. And here where there is no offset outwards from the focuser so it's done via tilt: https://skyandtelesc...condary-mirror/ But what if you want the fully illuminated field centered in the eyepiece, but must leave the secondary mirror centered in the telescope tube? It can be done, as shown in diagram C, by slightly adjusting the tilt of both mirrors. Now the optical axis is slightly tilted within the telescope tube. In practice, this is not a problem because the tilt is never more than a small fraction of a degree. Since the secondary is offset down the tube, this is known as partially offset collimation. It is no doubt the most common situation, even among telescope owners who may not even realize that their scope's secondary is offset at all.
  11. Thanks Mark again. I don't know. So TS in Germany got both offsets in the wrong direction and both of them wrong in value. Can you believe that? – they are master opticians ! LOL Allan
  12. Thanks Mark, You finally gave me the best idea I could find to measure the offset away from the focuser. I am glad I didn't disassemble my Newt before measuring it. I used 2 rolled up pieces paper with masking tape around them and kept trimming them until they would sit by themselves between the secondary and the other side of the tube. I was expecting to find that the offset was away from the focuser by 5.5 mm but instead the focuser side was shorter by 1.3 mm. The offset is wrong in value and it's wrong in direction as set up by TS in Germany. Now I'm going to have to work out how I will do that offset myself when the spider is reinstalled. see pic.
  13. Here is a good explanation of the offset and a picture too. It's from here: https://skyandtelescope.org/astronomy-equipment/offsetting-your-secondary-mirror/
  14. Thanks - I'm sure you're right.
  15. thanks, hopefully this is the last iteration I'll need to go through and the Newt. will be right. I want every penny's worth of the light entering my telescope tube so it will be working the best that I can make it. I have been checking the deign here: https://www.bbastrodesigns.com/NewtDesigner.html and here: https://stellafane.org/tm/newt-web/newt-web.html cheers Allan
  16. The spider needs to be rotated. Mike Sidonio's Newt. has the spider offset by 45 degrees to the focuser: https://pbase.com/strongmanmike2002/collimating_with_the_catseye It's obvious now that it's a better spot for the spider as it's not placing direct strain on the focuser which could cause tilt. Why didn't TS in Germany think of that too? Also - that if I move the secondary down 15mm I’ll need to move primary further down 15mm too. It’s not so easy: Gee wizz - moving the focuser tube up 15 mm could be a problem. To move the primary mirror down 15 mm would mean having to make new extension spacers for the mirror cell. I have only just added anti-lateral movement springs which work very well to stabilise the mirror cell. The gap left over would be wide enough for those wooden blocks as they could be placed partially over the metal ring. This is starting to get very complicated. see pictures. cheers Allan
  17. see here: https://www.bbastrodesigns.com/NewtDesigner.html click on "diagonal" then click on "Diagonal Offset study". A Newtonian telescope's diagonal must be offset in order to center the cone of illumination on the field of view. Because of geometry, there it is not possible to center the cone of illumination on the center of the field of view while also satisfying the desire for even illumination at the edge of the field. The diagonal is offset in relation to the focuser, so either the diagonal can be moved or the focuser can be moved. I assume a fixed focuser. Let's see why an offset is desired. etc
  18. That's a good idea - thanks.
  19. Thanks David, Yes - I don't see any other way out of this but drilling 4 new holes and re-installing the spider further down the tube 5.5 + 9.4 = 14.9 mm. I could fill the old holes with Araldite and black dye to cover them. 14.9 mm is too far for spacers or longer bolts to work. Once I get the secondary out I'll have a closer look. cheers Allan
  20. Embarrassing problem as I should have noticed this before. I am putting my 10" f4 Newt back into service after 6 years and this is where I'm up to: I hadn't cleaned the secondary mirror yet because I wanted to check it for offset - just in case it might have a small error in it. When I used my digital vernier – after about an hour of making difficult measurements - I came up with a result that I thought must be wrong – It tells me that the secondary mirror is offset in the wrong direction by 9.4 mm. An online calculator says the offset should be 5.5 mm towards the primary mirror – offset is approximated by: M/(4*F) where M is the minor axis of the secondary mirror and F is the F ratio. So for my 10 inch f/4 with secondary minor axis diameter of 88mm, the offset will be 88/(4*4) = 5.5mm. but it’s offset the other way by 9.4 mm towards the front opening of the telescope! Then I looked in the focuser and sure enough I can see that it’s wrong without even measuring it. See pic attached. ( I placed a red shirt on the opposite side of the tube to make a photo more obvious and I focused on the edge of the secondary mirror using my DSLR on a tripod. ) I don’t know how this could have happened. It was bought from TS in Germany in 2015 and they set up the position of the secondary mirror by installing the spider and focuser. How could professionals who charge a lot of money and make 100s of telescopes make such an error? How did my telescope even work at all like that? There is also the question of the other offset which would be away from the focuser by the same amount 5.5mm. The center of the spider is centralised so it may not have any offset away from the focuser. I'm not sure how to measure that. Any comments? cheers Allan
  21. Hi Dave, I'm not kidding. With a 12 panel mosaic you have enough detail to make a very large poster. Maybe not 5 meters wide but maybe at least 3 meters? It would look very interesting on a wall in a house or even in a restaurant or library. Allan
  22. That's amazing - you could make a poster 5 meters wide and it would still be sharp. cheers Allan
  23. What an amazing picture to get in only 90 minutes. That shows the power of f2. How could it be better? - 3 minute exposure have most likely hit the well depth on most of the stars - maybe a short exposure of say 30 seconds used and then the stars copied and pasted in to get more colour in the stars? cheers Allan
  24. Yes - I agree. It seems to be the answer to all the problems.
  25. You've done well. I even mentioned you on 2 threads on other sites: I think you are already there as Dan_I ? https://www.cloudynights.com/topic/861155-is-there-demand-for-premium-imaging-newts/page-8 and here: my post 03-02-2023, 12:07 AM https://www.iceinspace.com.au/forum/showthread.php?t=203108&page=2 I think will copy your mirror cell design one day - it's excellent. cheers Allan
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