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Sfarndell

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Everything posted by Sfarndell

  1. Hi Nigel Thank you for your guidance. I'll try your suggesstions and post feedback. I'm working tool on top for sure as my mirror is a bit thin. Scott
  2. Thanks John. Function over form. No-one is really going to see the call and as long as it works well, it doesn't matter.
  3. Looking good John! What did your neigbour use to cut 6mm aly with? I'm making my cell too and I'm not convinced about a jigsaw. Yet.
  4. Hi David I second what John says about the Foucault vs Ronchi. The results I get from the Ronchi test match what the experienced guys see at my telescope class using a Foucault test. However, I can't come close to replicating their results - it's a steep learning curve and requires a lot of patience and time. You're in good hands at CATS. Terry is a master. Simon was also making 3 secondaries of +-100mm minor axis a few years ago and can give you lots of advice (or sell you one if he finished them ). Good luck! I'm looking forward to following your progress. Scott
  5. Found some time this weekend to work on the mirror. In the end I decided to correct the centre a bit and catch up with the middle zones before addressing the edge. I did 2x 10 minutes with the 6" lap and a 1" offset followed by 2x 15 minute sessions with the 8" lap (narrow W) and finished off with a 3 minute (single trip around the barrel) with the 14" to smooth things off. I tested after each session, to make sure I wasn't causing major issues. The centre-deepening technique seems to have added a fair amount of correction, but it wasn't uniform - there is a funny zone betweel 4" and 6" radius. The 70% zone has a big kink which, according to Mel Bartels' matching ronchi calculator, is indicative of an over-corrected/deep 70% zone. This is in line with the readings from the Foucault test and would not have been affected much by the work with the 8" and 6" laps. To correct the edge, I intend to take the 14" lap and work with a narrow "W" and about a 25% overhang in 10 minute intervals. Hopefully this will push the correction in the outer 2" out over the edge and reduce the high (rolled?) edge. If this works and everything else stays the same, then I will try address the zone at +- 5" and add correction with the 8" lap. I won't work on it until Tuesday in the hope of giving time for more experienced members to comment/ share a better alternative. Herewith the ronchigram on decent camera, compared to the ideal.
  6. John - plywood cutouts work well. I did that in my 16" Dob (90mm MA secondary) and it works really well. I used silicon to glue the mirror directly on to the wood and have had no issues. (I also epoxied a safety wire to the mirror which is independently connected to the spider in case of failure!) Here's the link to a photo before I neatened it up and painted it. http://stargazerslounge.com/uploads/monthly_06_2013/post-1508-0-70225500-1372244669_thumb.jpg I made a wire spider similar to Mel Bartels' method. The wires are lightweight and very strong, but are thin enough that the diffraction effect is so minimal, I don't notice it even on bright objects. Scott
  7. Agree with that! John has a wealth of knowledge and experience and is generous in sharing it. Can't go wrong there. If budget is tight, I saw a 16" aluminised mirror on another forum for £400. It's a collection so perhaps worth a look so you could do a quick qualitative test before you buy with a ronchi? (I don't know the chap, but pm me if interested and I'll send you the link). It could be hit/miss in terms of quality, but at the worst case, it's still a bargain as the cost of having it refigured by a pro would still make it comparably cheaper. Or...if you're looking for some additional DIY fun... you could refigure yourself Whatever your choice, I'm looking forward to following your build.
  8. Lol...it will take patience to follow. Work gets done in week-long fits of inspiration followed by long periods of "aaarrrggghhhhh why am I doing this!!!".
  9. Agree with Nigel! You can make a decent scope with just a jigsaw and a drill (preferably get a router too). There is a lot of personal satisfaction in making your own telescope. The nice thing is that if something doesn't work out quite the way it should, you can fix it easily. There are a lot of designs on the Web which can give you inspiration, but your personal circumstances will dictate the type of scope you want (small/large, transportability, available storage space, weight...etc). From my experience, I made a 16" ultralight 1st time round, which worked well as a home-based scope, but was difficult to transport and keep the secondary protected. The truss poles were too long to easily fit in my car, so I rebuilt it in a more conventional manner and it is now my dark-site scope. good luck with your build! Scott
  10. I did some work on the centre for about 20 mins, but other commitments have prevented much progress since my last post. I am also worried about overworking the mirror without more quantitative analysis. As I am part of a telescope-making club, I took the mirror to class on Saturday for its 1st "professional" Foucault test from guys who've been testing mirrors for 15+ years. The results were interesting. The centre is still undercorrected, but i was surprised by the level of overcorrection at the 60%-80% zone (see diagram below). Any advice from as to how to approach this? tackle the edge then work the centre, or reduce the centre with a small (6" lap) and then work on the edge?
  11. Don't visit the forum for a few weeks and someone sneakily does a full build! I should not visit more often . Excellent work Simon. It's always a beautiful thing to watch a master at work!
  12. Returning to sphere is depressing! Been there done that. Had so much "fun" i did it again. Twice. Hopefully for the last time too... The reward is worth it though, so persevere!
  13. It's been a while since my last post, but a lot happened. A tragic accident left the face of the mirror deeply scored and I had to go back to fine grinding and motivation suffered so the project was paused for a few months. In the mean-while I made a spin-polisher, which saved a lot of time and now the mirror is back on track. The ronchigram below is progress as of last night. the edge is quite good - perhaps a little turned at the last 2 mm, but it could be diffraction. Either way, the current bevel is <1mm so any edge issue will be ground off after figuring. to my eye, the outer 40% is close to being parabolised and the inner 60% needs some deepening (compare with ideal ronchigram - 2nd from right at the bottom). The pic is rather grainy, but the actual surface of the mirror is smooth with no zones and shows no astigmatism. Plan is to do narrow "W" strokes (1-2" swing) with an 8" lap starting at a 25% (2") overhang and reducing that to a -2" overhang in 20 minute sessions to bring correction to the centre followed by a short smoothing session with a 12" lap.
  14. Well done Rick - excellent progress! If you can, do a Ronchi test initially just to check the general shape until you start seeing some parabolisation. It's much quicker and easier than doing a foucalt test, but at the cost of quantitative accuracy. Stellaphane has a link to print a 2"x2" 133 lpi ronchigram, which I printed at high resolution on an old Overhead projector film and it works really well! Looking forward to your test report. Scott
  15. Sounds like you're making great progress Rick! keep the posts coming!
  16. Chris a.k.a. "Yesyes" has done a lot of work on modifying his camera - perhaps try PM'ing him
  17. Thanks Nigel. Will try your suggestion and post the result. What is your recommended overhang?
  18. I worked some more on the hill in the centre using the 8" lap COC on a 6" stroke i.e. the stroke ended 2" inside the edge for 40 mins. The 1st pic was just after the 8" lap work and the centre hill looks mostly removed. This was followed by another 40 mins of smoothing with the 12" lap with a 3" overhang and a 2" offset (chordal stroke, not a "W"). The results were promising and the hill was mostly removed. The edge seemed to look worse though. I then continued with a chordal stroke for 30 mins then a W stroke for another 30 mins, using a 2" overhang and the results are in the pic below: the central hill is returning and the edge looks bad and rolled. I press my lap overnight using the weight of the mirror, so it is in good contact. There does seem to be some roughness as the lap passes over grooves where the coring came through the face of the mirror, but it is minor and likely the reason for the slight roughness. Figuring +-5.5 hours - inside focus Figuring +-5.5 hours - outside focus Mirror making gurus please chip in and correct my logic if it is off: My plan is to address the edge while monitoring the centre so I intend to use a "W" stroke with 1-2" max side swing and a negative 1/2" overhang (i.e. stop short of the edge) for an hour first to deepen the mirror uniformly to just inside where the rolloff starts so that it forms a hill, then shave that off by eventually extending the strokes to a 1" overhang to take the curvature over the edge...
  19. I worked some more on the hill in the centre using the 8" lap COC on a 6" stroke i.e. the stroke ended 2" inside the edge for 40 mins. The 1st pic was just after the 8" lap work and the centre hill looks mostly removed. This was followed by another 40 mins of smoothing with the 12" lap with a 3" overhang and a 2" offset (chordal stroke, not a "W"). The results were promising and the hill was mostly removed. The edge seemed to look worse though. I then continued with a chordal stroke for 30 mins then a W stroke for another 30 mins, using a 2" overhang and the results are in the pic below: the central hill is returning and the edge looks bad and rolled. I press my lap overnight using the weight of the mirror, so it is in good contact. There does seem to be some roughness as the lap passes over grooves where the coring came through the face of the mirror, but it is minor and likely the reason for the slight roughness. Figuring +-5.5 hours - inside focus Figuring +-5.5 hours - outside focus Mirror making gurus please chip in and correct my logic if it is off: My plan is to address the edge while monitoring the centre so I intend to use a "W" stroke with 1-2" max side swing and a negative 1/2" overhang (i.e. stop short of the edge) for an hour first to deepen the mirror uniformly to just inside where the rolloff starts so that it forms a hill, then shave that off by eventually extending the strokes to a 1" overhang to take the curvature over the edge...
  20. Jake - I downloaded a ronchi template from the Stellafane website and printed it on some old overhead projector film using the highest DPI setting. It works a treat and outlay was minimal.
  21. After another 60 mins with the 8" tool and a further 10 minutes with the 12" to smooth things out the hill is looking a lot better. Still a lot more work to do here. The TDE hasn't changed too much so should still be manageable once the centre is better. I also seem to have removed some correction at about the 60% zone. Any input from the pros? Glasspusher? Neil? I also unfortunately picked up a horrible 4cm long scratch from the small tool when it was first used which will not likely be polished out. Sigh. Still, if that's all the mirror has then it won't be too bad. Herewith a pic of the current status. Quality is suffering as it was taken on my phone and enlarged.
  22. I recall reading somewhere that it doesn't matter having a slightly concave rear, as long as the shape was uniform (marginally spherical??). Wish I could find the post though. That being said, when in doubt, take the longer, more conservative route that you know works! Pizzas are hard enough to work as it is without overcomplicating it.
  23. Wow that's big...looks like landing lights on top of the USS Enterprise's saucer section . My 2c...your comment indicates that you haven't tested the face for flatness yet. If not, I would test that for flatness and use the flattest side as the back. Hopefully you'll get lucky and it is much flatter. Otherwise, you seem to be doing the right thing.
  24. 22"!! Big gulp....preferably enhanced by whisky while you grind away . That really is HUGE. Manual or machine grind? If manual, your shoulders and arms will showing very improved definition by the end of it . Just spotted a typo ... I have a small turned down - not up - edge (only really noticeable in the ronchi outside focus to the untrained eye) about 3-4 mm wide. I'm hoping that it will be fixed with the hill zone issue, but if it's only reduced then I'll take a short-cut and just bevel the edge more (currently <1mm bevel). Watch this space...I think that if a mirror has had an interesting issue, it was this one so the fix will be informative. Never give up, never surrender!!
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