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9.25 inch refractor project

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#51 Mike I. Jones

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Posted 13 February 2013 - 06:54 PM

Great news, Jim! Can't wait to see your picture(s).

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.

Standing by,
Mike

#52 jimegger

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Posted 13 February 2013 - 08:26 PM

Mike, I do have a glaze on my lap that I suspect is not doing a good job with the Cerium Oxide. I bought some Gugolz 64 pitch which I aim to make into new laps if the wire brushing of the present laps doesn't speed things up. On the first go around I made some laps from Burgundy pitch and got a real nice polish at 14 hours with the Cerium Oxide. At any rate , it will get done if it takes me 20 hours! It is almost there.

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.

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#53 Mike I. Jones

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Posted 13 February 2013 - 11:15 PM

Hmm, I'll take your word for it at this point - low contrast, no sense of scale, and unknown level of horizontal path turbulence, so it's hard to tell. Can't wait to see a clear day shot, followed by lunar terminator shots!

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!

Mike

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#54 Mike I. Jones

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Posted 13 February 2013 - 11:49 PM

I processed some on your picture to see if I could bring out any detail. Still can't tell scale, but you're right, it's still an RGB color picture, but I don't see much in the way of secondary spectrum in the fine details.

More pictures!
Mike

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#55 jimegger

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Posted 14 February 2013 - 02:10 AM

What i usually do Mike for comparison is look through my APO which I know to be excellent at the same power at the same time to get a better picture of how badly the atmosphere is behaving. I did not do it this time. The part of the peak you see here is about 5 miles distant and a Raven would be just about the size of that point you see at the very top. The lenses are still milky enough when used together to ruin the contrast as well as the sky being cloudy with a slight haze.

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.

#56 jimegger

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Posted 14 February 2013 - 11:15 PM

Okay, so I wire brushed the laps and guess what.... it polished like a fiend !! Right now they appear fully polished but just to be certain I will be putting in 1 more hour on each lens surface. By wire brushing the pitch surface it made the lap "bite" much better - one could feel the lap working its magic with more of a grabbing feel during the stroke. Now the lenses look clear. If I had done this in the beginning I would have been done quite a ways back. I was not sure wire brushing the laps would be something one could do without hurting the laps until you told me you did it with no trouble Mike. Thanks for the advise !

#57 Mike I. Jones

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Posted 15 February 2013 - 12:07 AM

Glad to help! Wire brushing shatters off the surface scab glaze and makes billions of little microchannels that the polishing slurry can more easily flow through. Be sure to brush lightly, and just enough to get rid of the glaze. And, you still have to press in a little afterwards, as the lap profile is definitely changed by brushing. Rinse the lap well in lukewarm to slightly warm water before pressing to get rid of the powderey brushed-off mess. Wire brushing is far superior to pressing petticoat material, scratching up with a razor blade, etc., but you have to develop a feel for how hard and long to brush, and how quickly the microchannels will press out while working the lap and it starts to get too "grippy" or "draggy".

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.

Mike

#58 jimegger

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Posted 15 February 2013 - 01:08 AM

The laps I have are pretty hard from use, being old and the fact that I added the beeswax in the very beginning. I cold press the laps overnight by letting the lenses sit on them with no weight to keep their shapes. The wire brushing was just enough to leave scratches on the hard pitch surface. It was just perfect for good polishing action. The other trick is to press a fine mesh into the pitch and get many small facets for increased action. The scratches were just enough to do a really good job in my case though. I use a fairly watered down slurry of Cerium Oxide for most polishing and at the end just use water as the lap leaves a smoother polish that way.

#59 MKV

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Posted 15 February 2013 - 02:38 AM

Mike you're right - wire brushing is so much better than microfacets. Also quicker. I use brass wire brush only, but I imagine a very hard tooth brush may do the job too. Worth investigating. The bag and clean water pressing is another good shop technique. You can't overemphasize the need for such methods, along with the final figuring slurry concentration of 1:100, 1-minute figuring spells, and cold pressing after each.

#60 jimegger

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Posted 15 February 2013 - 02:19 PM

Here are a couple of pics of the final polishing results. The crown is on the polishing stand in this one.

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#61 jimegger

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Posted 15 February 2013 - 02:19 PM

Here is the flint element.

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#62 jimegger

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Posted 16 February 2013 - 08:57 PM

I took the lenses and and put them in the cell then bolted it on the refractor tube for a look at Pioneer Peak with my ES 127 Apo at the same magnification for a visual comparison. The air was turbulent so neither instruments showed the best images. I can say with certainty however that the 9.25 inch and APO images differed little clarity-wise and the color in the 9.25 was excellent !

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.

#63 jimegger

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Posted 22 February 2013 - 01:18 AM

After trying to get some stable air to make a simple visual test of the objective today was the day. I had my ES 127 APO set up at the same magnification as the 9.25 inch scope looking at the same spot on the mountain 5 miles distant. I could actually see more detail in the 9.25 inch during moments of quiescence . There is definitely more range of focus in the bigger scope at f/12 but surprisingly tight for that f value. The color is excellent , not as good as the APO but not in the least bit distracting to my taste. I have yet to bench test the objective with a green LED in the Foucault test for fine tuning -if any is even necessary . From what it shows visually , it could very well be there now.

#64 kfrederick

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Posted 22 February 2013 - 10:18 AM

Hi Jim any plans to get the lens coated? I bet the views will be great Thanks for posting.

#65 Mike I. Jones

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Posted 22 February 2013 - 12:32 PM

The statement you made about focus being surpisingly tight is a very good sign! Double-pass autocollimation testing will be the true read on the lens figure. Can't wait to see the results.
Mike

#66 jimegger

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Posted 24 February 2013 - 03:34 AM

When the flint was put under the Foucault test for the concave side I got a close sphere but have a pesky hill in the center that does not seem to respond to various strokes designed to get rid of it. It is very slight but we are seeking a perfect sphere here where it can be seen directly. It could be this pitch lap is too hard at this juncture. I did modify the lap to work more on the center and will give it some more time. The hill is so slight that it probably won't affect the wavefront very much but again... it should be gone on this surface. It is much smaller than when I first looked at it so the image should be improved from the pre-testing visual checks.

#67 neo

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Posted 24 February 2013 - 05:48 AM

Jim, fantastic project you got there! Hats off :bow:
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.

#68 kfrederick

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Posted 24 February 2013 - 08:49 AM

Hi Jim putting the concave on top might help fix the center . . Congrats

#69 MKV

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Posted 24 February 2013 - 02:04 PM

Subdiamter tool, watery, thin polishing suspsnsion, light pressure, very short spells.

#70 Mark Harry

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Posted 24 February 2013 - 07:30 PM

-NO.-
M.

#71 jimegger

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Posted 25 February 2013 - 12:38 AM

What I did do was to try and get the normal tool to work instead of going to a sub-diameter tool as it is my experience that the sub diameter tools are a bit harder to blend in just right. I used those on my 26 inch mirrors with great success but have had better luck on the smaller stuff with modifying strokes or modifying the full sized lap. In this case I have had great success bringing down the hill by putting a couple of extra layers of paper between the backing tool and the center of the lens about the same size as the hill - 2 inches in this case. I have been using the lens with backing tool on top. Now the extra layers of paper have put a bit more pressure on the center of the relatively thin flint forcing the center of it to get more polishing action. I believe the flexure in the flint has allowed me now to get the bulk of the hill out. It is very nearly gone now with an excellent cutoff on the Foucault test. It can't be more than a small per cent of a wavelength off from a sphere now. Now I am merely trying to get a smoother figure by putting less pressure on the polishing strokes. Today should be the final day for figuring it. It will go back into the lens cell for another look see to see how much the image improved. Afterwards the lens cell will be put into the double pass auto-collimation test apparatus with the green LED for a look at the whole assemblies performance.

#72 Mark Harry

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Posted 25 February 2013 - 08:48 AM

"What I did do was to try and get the normal tool to work instead of going to a sub-diameter tool as it is my experience that the sub diameter tools are a bit harder to blend in just right."
********
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!
M.

#73 kfrederick

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Posted 25 February 2013 - 09:21 AM

The reason for having the concave on top of a full sized tool is that will work the center more . Like in fine grinding if you wont the curve deeper put the concave on top . Many good ways to happy .

#74 jimegger

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Posted 25 February 2013 - 02:03 PM

Yeah Kevin, I have been using the lens on top throughout the whole polishing process with the typical center over center 1/3 stroke as well as blending strokes towards the end. I believe my lap is a bit to hard for best results with the added bees wax. With the center of the flint shimmed out it is working better. It is almost there now.

#75 jimegger

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Posted 25 February 2013 - 04:29 PM

Here is the setup for getting rid of the hill on the flint concave side. The flint R4 is resting on the R4 tool with Saran Wrap and a paper towel between it. I put 2 more layers of paper towel in the center where the hill is to put added pressure on it during polishing. I have a collar of Mylar taped to the R4 tool so the flint can "float" on its paper towel bed. It is loose in there in that it can be taken in and out but does not move back and forth to any appreciable extent. IT WORKED !!! The hill is gone !

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