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Sfarndell

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About Sfarndell

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    Star Forming

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    Male
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    Johannesburg
  1. I'm looking for a pair of secondaries 3.75" - 4" (96mm-100mm really) for my binoscope which is currently in construction.
  2. Nabbed it - thanks Nigel. Will hopefully have a new travel scope to house it in (and to post in the DIY section) within the next few months.
  3. 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
  4. 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.
  5. 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.
  6. 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
  7. 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.
  8. 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
  9. 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.
  10. 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!!!".
  11. 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
  12. 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?
  13. 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!
  14. 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!
  15. 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.
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