9.25 inch refractor project
Posted 13 February 2013 - 06:54 PM
Just curious: what pitch and polishing compound are you using? Could either be slowing you down? I've been working for years out of a gallon jug of ZOX and really like it. CeO2 is fast but can sleek unless you ball-mill it. CeO2 is better for dummy shining mirror backs, sides, etc. Red rouge is SLOW!
Using Acculap or Gugolz 64, I find that if I slop on too much slurry the lap surface gets overcharged and the polishing also slows down, along with the figure getting hinky. Taking a wire brush to the lap occasionally to scrub the hard surface glaze off gets you back to native pitch.
Whatever, your perseverence is certainly paying off!
When you get to double-pass testing, test in green light like from a green LED, that spectrum should be narrow enough, and a properly corrected lens will give you nice straight fringes. Just a suggestion: before going to a figuring session, try a few different airspace shims first - the airspace I gave you assumes all measured data is exact.
Posted 13 February 2013 - 08:26 PM
Here is a picture I just took of Pioneer Peak on a windy cloudy day with my Canon 20 da. Focus may not be perfect but to my eye the images look good. Color correction seems very good as well.
Posted 13 February 2013 - 11:15 PM
Here's a test suggestion for double-pass autocollimation testing (assuming you have like a 10" optical flat?). Get a Radio Shack 5mm Green LED, Model 276-022 , Catalog #: 276-022, for a whopping $1.99. Website is here. Spec sheets says its spectral centerline is 570nm, with half-power points at 550nm and 590nm. Perfect spectrum for double-pass testing with Ronchi and slitless knife-edge (do both!). I simulated your lens in double pass with these wavelengths and weightings.
It says the forward voltage is 3VDC, and operating current is 20 milliamps. You have to put a resistor in series with the LED or you'll burn it up. If you use a 9V battery, the resistor should be (9-3)/0.02 = 300 Ohms. A quarter-watt fixed resistor is plenty. To vary the LED brightness you can also put a little 0-1K pot in series with it as well. But the 300 ohm resistor must stay in the circuit path.
If you're a master electronics guru, I apologize!
Posted 13 February 2013 - 11:49 PM
Posted 14 February 2013 - 02:10 AM
Tomorrow comes more polishing possibly with new laps. I am trying to weigh the time spent making a new lap to just using the old ones again for a bit. The glass is so close to being polished out another run of a couple hours on each side just may do it.
Posted 14 February 2013 - 11:15 PM
Posted 15 February 2013 - 12:07 AM
You do know about pressing in after brushing with just clean water, no slurry, and a single layer of a polyethylene trash bag stretched between lap and glass, right? The freshly brushed pitch will re-form to the glass shape almost perfectly, and the two parts will never stick together. Using only water keeps the lap from charging up too much and needing another scrubbing. Another old cool optics shop trick I picked up along the way.
Posted 15 February 2013 - 01:08 AM
Posted 15 February 2013 - 02:38 AM
Posted 15 February 2013 - 02:19 PM
Posted 16 February 2013 - 08:57 PM
Before the 9.25 inch can be star tested a mounting must be made for it. Until I can make a German equatorial later on , the plan is to mount it on my 26 inch reflector with a dovetail for good tracking. In the meantime the objective will be undergoing the double pass auto collimation sometime later this week. It has not been Foucault tested yet and I imagine it needs some correcting. This time the image quality far surpasses what it gave the first time around. The focus actually seems pretty crisp for a long focus Achromat - I am pleased !
For the lunar limb shots , it will be a bit of time to get the scope setup.
Posted 22 February 2013 - 01:18 AM
Posted 22 February 2013 - 10:18 AM
Posted 22 February 2013 - 12:32 PM
Posted 24 February 2013 - 03:34 AM
Posted 24 February 2013 - 05:48 AM
Can't wait to see it pointing the sky.
Have you tryied 'W' strokes, 1/3D of course as Texereau describes? Those should get you a nice even surface.
Posted 24 February 2013 - 08:49 AM
Posted 24 February 2013 - 02:04 PM
Posted 25 February 2013 - 12:38 AM
Posted 25 February 2013 - 08:48 AM
Precisely! from the tryout earlier, you're really close. A sub-tool at this close to being finished on a flint can make a lot of trouble, and resultant do-overs. I use sub-tools only when they -AREN'T- the last step, if at all.(particularly with a softer glass) Nice way to shim the center, btw!
Posted 25 February 2013 - 09:21 AM
Posted 25 February 2013 - 02:03 PM
Posted 25 February 2013 - 04:29 PM