Posts Made By: Thomas Dey

Amusing for sure. In the absence of distortion, the linear field diam would be 2 x tan(50) x f. So 21mm f gives 50.05mm = 1.97 in. and 24mm f gives 2.25 in. Hence Al's would just clear a 2-inch "hole" if like the end of the focuser tube were collocated or inboard from the virtual field stop (which optically is at least POSSIBLE for a 2-inch) and the 24mm obviously violates that by a LOT. If a 2-in adapter were plugged into an even BIGGER focuser...it might actually not violate anything. I'm certainly suspect too. There is another possibility though that involves linear vs angular distortions but would be pretty much in the 'cheating" category. An apparent field could indeed appear to be 100 deg if pincushion distortion stretches it out toward the edges. But this is by decreasing the focal length of the eyep out in the field. So only the axial focal length would be as stated. There IS no free lunch! Tom Dey

Yes, starting with the 8-inch is sensible and will get you to your ultimate goal faster. I assume you will do that mirror by hand - good to see what proper grinding and especially polishing and figuring "feel" like. I would also recommend making it into a telescope - a old-fashioned Dobsonian would be just fine, easy, fast and economical - and work wonderfully! My own personal history went from 3 to 4.25 to 8 (dropped and broke it before done) to 6 to 6 to 8 to 12 to 13 to 18 to 16 to 16 to 29 to 36. The last four I didn't make...bought them, but did build the mounts. The 36 I still need to build the mount! I plan to use that and then...die. Tom Dey

Hi Matthew et al, I've been an obsessive avocational astronomer my whole life and fortunate to also work in Aerospace optics...so the combo is ideal. The community seems to be populated by dreamers, buyers, builders and observers. A good approach is to only reward yourself with equipment after USING what you have to its capability and beyond. And to spend at least HALF your time observing if you are an otherwise obsessive builder. I've seen the whole spectrum of obsession in friends and myself. One lens designer spent DECADES designing his ideal giant refractor and ... Never built, bought or used ANYTHING! Other friends built ambitions observatories and scopes and then rarely used them! So my default is "When in doubt - OBSERVE." The hero/mentor in this would be John Dobson, who built scopes from 6 to 24 inches from the cheapest materials using hand tools and then used the hell out of them for his own pleasure and as outreach to hundreds of thousands adults and kids. Image-capture seems all the craze now (been there, done that, burned out), but there is nothing more satisfying than looking at the photons, up close and personal. Enjoy the Universe and don't feel obliged to continually outdo yourself and others. This is just as much a reminder to myself as to others! Tom Dey

Great descriptions, Rod, That same night I trekked over to my big dobsonian and looked at the usual suspects and always at least one obscure one that I never visited before. Globulars M9, 12, 10, 14, 13, 92 all spectacular or course. Omega (Swan M17) glorious with the NPB filter. NGC6888 Crescent filled the field and showed same structure same as the picture in Burnham’s p796. M27 great using the NPB - that filter is unusually pleasing in that he intentionally allows just enough white leakage to have the stars look good without getting snuffed too badly. Great balance between enhancing the nebl yet seeing the stars too. Veil 6992, 6960 absolutely spectacular and glorious twisted filaments all over. Central star in Ring (no filter) very easy (it IS a huge Dob). My "new" object was NGC 6567, a bright but very small planetary in Sagg "Nice!, Small, dead-reckoning essential to have found it though." Tom Dey

That's an admirable job, Charles! Few observations: A pinhole source (poked hole in Al foil) is less subject to problems because its orientation has no preference. I put a tiny piece of Opal Glass just behind the pinhole. I also added a tiny condensed alignment source just below the pinhole so I could align (and then even USE) the gizmo in normal room light. Completely eliminates the difficulty of fishing around for the feeble return. At work we developed QKE "Quantitative Knife Edge" that is every bit as accurate as interferometry. So the old adage that Focault is NOT quantitative has become...untrue. Now the Knife can quantify ALL the aberrations! Ummm...with your source so far to the side, you will get quite a lot of field astigmatism in your signature. You will notice that its axis is vertical and it does not rotate with the mirror. Can mitigate that by getting the source much closer to the viewing knife. Tom Dey ADDITION: I just realized that the X-astig will NOT manifest overtly with the vertical knife because of the knife orientation...which is GOOD, because it is artifactual anyway. Only error will be a slight difference in focus AKA you are testing at one of the focal lines...the "wrong" one... So the test becomes blissfully unaware of the astig induced by the test set. QKE proves its presence because two orthogonal, coplanar knives are used. The software is simple and retrieves wavefront. In this case it would of course (ideally) be the spherical ab characteristic of testing a parab at CC. Anyway - just go with the shadowgram signatures and zonal radii as the books suggest --- and you will end up with a GREAT mirror! Tom Dey

Markus Ludes has ALL his APM APOs aligned and tuned by a specialty shop before shipping. I have the certificate for mine here someplace and they are indeed superb! I don't know that the semi-apos get that though. I used to work at B&L and Ralph Dakin (Yeah, the Dakin Barlow guy) used to service binos as a side-line (decades ago - RIP). I recall his stating that the more expensive binos are serviceable but the cheaper ones often are not because stuff is just glued in place with no adjustments - when they break - THEY BREAK! I have no idea whether this applies to your binos though. Markus would of course be able to advise. Tom Dey

Cary refigured a 16" for me and coated two. Work was superb throughout. I have heard that he doesn't check emails often but not to worry - he is probably busy DOING the work rather than talking about it. Refiguring especially is VERY touchy, requiring a lot of effort and rare skill. I'd back off and just let the progress flow. A great excavation guy is doing major work on my 16 acres, coordinated between other jobs. He’s the same way. But his work is better and more reasonably priced than anyone else. Ditto positive comment regarding the surgeon who fixed my eyes and my mistress. Like Donald Trump, they are DOERS, not debaters! Tom Dey

I've been thinking the same thing and coming to the same, depressing conclusion: I belong to a writers' site that has seen paralleled decline and the pattern is identical. Indeed "user-friendly" fora like Facebook and now Twitter "tweets" encourage people to input more and more quantity with less and less content - just because of the convenience. Just look at (most?) people doing the zombie-like gorilla-walk informing their friends and followers that they just bought a loaf of pumpernickel rather than rye. You can be right at the dinner table or on a DATE and same thing! My tactic is to ramble and then interject, "... AND that's what SHE said about you!" The subject (male or female) snaps out of their daze and demands, "WHAT did you say?!" I respond, "never mind, it wasn't important and Ummm just as well you didn't catch it because I really shouldn't have told you anyway." And then I refuse to expound. If interrogated further, I act embarrassed that I ratted on the person who hates them and WHEW so glad they were distracted rather than get all mopy and hurt feelings. I also notice that the reviews section is hopelessly out dated with precious little input. It's like a desert out there! Tom Dey

That is a common problem at the finest abrasive. Keep it pretty wet. If the bubbles prefer to stay near the center that would be a depression in the mirror there and it acts just like a big stiff suction cup when it hits center-over-center in the stroke. Quick fix to that is short c/c strokes. When both are good matched spheres, the bubbles have no preference and are real little and evenly distributed (and harmless). You can really tell a lot about the shape of the mirror just on how the bubbles behave. Tom

Almost always, the short answer is bigger is better. For a Dob, 10" is still on the easily portable side and you will be able to see a lot more. People who have an 8 will often dream of having a 10; those with a 10 won't dream of an 8 (but maybe a 12!). Tom Dey