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GTom

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About GTom

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  1. GTom

    Slumped mirrors?

    Thank you for your thoughts John. The ultimate goal is a portable big Dob like Mel Bartels. Portability is an absolute must for me, a conventional 1.5" thick ("thin") 24" is already too heavy.
  2. Thank you, I need something more South, around 28°N.
  3. Hi! What latitude is it optimized for? Is it dual-axis drive?
  4. I am thinking on grinding my own lightweight mirror (first f4 16", later f4 24"). The lightest and cheapest option is to get a thin blank and slump it in a decent kiln. Anyone has longer term experiene with slumped mirrors? Overall doesn't seem to be more work than a normal (not pregenerated) blank. Do I have to grind the backside as in case of normal flat back mirrors to avoid astigmatism? How do people support a convex back while grinding? Does it make sense to grind a hole in the middle for additional support? I am remotely considering a convertible Newtonian/Cassegrain system anyway...
  5. Back to the mirror topic, I'd be interested on blank suppliers. The €1000++ per blank doesn't sound very encouraging.
  6. No rage from my side at all, use that EEVA with a 24" Dob! I am actually seriously thinking on 1-2 minute subs deep sky photography. Quotes started to arrive in my mailbox, apparently the basic budget for 24" mirror making will be closer to €2500 than to 2000, main expense being the €1500ish blank.
  7. Full magnitude jumps are of course nice. It boils down to the additional investment you have to put in both work and $$$. My position is easier, the biggest thing I got is an unused GEM mounted 12" in a steel tube on a dead mount - not going to return to that scope. Transportability and total weight is worse than a modern 20-24" truss Dob or a c14... I need to determine the right entry point in this world. This would be the largest scope which breaks down to less than 25kg/55lbs components, and a carefully designed, thin-mirror 24" seems to sit right on that limit. 20" is readily available off the shelf "cheap" from mass production (ES, Skywatcher), but I don't like the reviews I keep seeing on them AND of course the affordable ones weigh as much as a CF construction 24". Latter is a steep, 4x jump in price.
  8. Indeed, 20"+ pricing is mostly guesswork/custom request the €3k/20" starting point doesn't even work there, because no mass producers like Orion will give you even a 22" mirror. E.g. Zambuto costs 50% more. There is a maker in Bulgaria, Astroreflect, their mirrors are sold by TS below Orion pricing but even a 20" is a custom order for them, I wouldn't wait months until they figure a 24" amidst piles of routine work - and pay a €5k-ish price... Afraid so... I don't count the research/reading/ordering part - I can do that quietly anytime, only grinding-polishing with the aid of a pre generated curve. I am a bit flexible, but have to be realistic, 100+hours on the glass would mean no scope for 3 years. I'll go trough Rustypsplit's thread on his very first grind, a 14".
  9. Went with the mirror discussion to your mirror topic
  10. Joining the club with big-mirror ambitions too :), unfortunately with no mirror making experience. Watching a dozen mirror making videos and reading reports, I am tempted to grind a 20-24" piece. I see the cheapest decent 20" ready made mirrors this side of the pond sell for about €3k (TS), a grand more if you want (and I do want!) a thin design. On the other hand, grinding has costs too. I am asking around for blank costs, this one seems very affordable, about half the price I've seen elsewhere! Problem also with bigger mirrors, that you might want to build your own Mirror-o-matic, which directly adds to the costs as this is a single-mirror project. Doing my homework on the figures, I'll go ahead with home grinding if I see the total cost for a 24" mirror will stay below €1500 AND 50 working hours starting with a pregenerated blank. This includes the grinding of a 12-16" practice mirror if it's absolutely necessary...
  11. Found this German source, prices listed until 20" which costs €360, they do more on order.
  12. Interesting! I wonder how much work a pre-generated blank would save to justify a 4x higher price compared to a normal blank? Maybe could get away even without making a turntable...
  13. No problem at all :). Wish the family the best of health and good chances of observing. I am doing a little research at the moment on mirror making, but 20"ish is huge. Got many cloudy days here though, but still, likely a second hand scope would make the most sense. Fortunately more and more big ones are sold in Europe, Taurus, Explore Scientific, etc...
  14. Thank you for the reassuring report! I am also looking forward to get a second hand scope (or build one myself). I will give it some thinking and of course careful testing why it is being sold. Haven't saw bad mirror reports on the 20" so far.
  15. You don't have to go as far as Tak or AP to get a mirror that doesn't fall out of collimation, doesn't delaminate and at least diffraction limited. Apparently vendors are shying away from giving replacement units on customer complains, they rather refund... I am not 100% sure about that 10x details question in very good seeing (where it is worth to do planetary work anyway) between a decollimated, lambda/2 optics vs a decent lambda/6 orion kept in a good cell. I am not a quality snob (alright, a little bit), but there are some BASIC expectations towards a product which costs multiple thousands of $/€. Or just turn the question around: apparently the optics is about lambda/2, mechanical quality is a weak beta version. As an absolute NOOB in mirror ginding and with little (but existing) experience in OTA building, could I produce the same quality? Very likely yes and the overall budget doesn't seem to reach $2k.
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