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Equipment Discussions >> ATM, Optics and DIY Forum

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5681636 - 02/15/13 02:19 PM Attachment (67 downloads)

Here is the flint element.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5684227 - 02/16/13 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.


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5693897 - 02/22/13 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.

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kfrederick
Carpal Tunnel
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Reged: 02/01/08

Re: 9.25 inch refractor project new [Re: jimegger]
      #5694372 - 02/22/13 10:18 AM

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

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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: kfrederick]
      #5694620 - 02/22/13 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


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5697749 - 02/24/13 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.

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neo
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Reged: 02/12/08

Loc: Iasi, Romania
Re: 9.25 inch refractor project new [Re: jimegger]
      #5697811 - 02/24/13 05:48 AM

Jim, fantastic project you got there! Hats off
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.


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kfrederick
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Reged: 02/01/08

Re: 9.25 inch refractor project new [Re: neo]
      #5697899 - 02/24/13 08:49 AM

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

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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5698333 - 02/24/13 02:04 PM

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

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Mark Harry
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project new [Re: MKV]
      #5698815 - 02/24/13 07:30 PM

-NO.-
M.


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5699188 - 02/25/13 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.

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Mark Harry
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project new [Re: jimegger]
      #5699424 - 02/25/13 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.


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kfrederick
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Reged: 02/01/08

Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5699479 - 02/25/13 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 .

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: kfrederick]
      #5699905 - 02/25/13 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.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5700192 - 02/25/13 04:29 PM Attachment (43 downloads)

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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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [Re: jimegger]
      #5700196 - 02/25/13 04:30 PM Attachment (38 downloads)

Here is the lens in its cell ready for mounting on the tube assembly.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [Re: jimegger]
      #5700199 - 02/25/13 04:30 PM Attachment (41 downloads)

Mounted on the tube .

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [Re: jimegger]
      #5700204 - 02/25/13 04:31 PM Attachment (49 downloads)

here is the whole scope at about 106 inches focal length.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [Re: jimegger]
      #5700210 - 02/25/13 04:35 PM Attachment (45 downloads)

This is a shot with a hand held camera before the lens has cooled down through a 50 mm eyepiece at about 53 X of a mountainside 5 miles distant with a lot of heat waves from the sun causing image deterioration. Visually I noticed a distinct improvement from what it was before addressing the hill on the flint the focus is even more crisp and the image sharper in spite of the heat effects. Once it settles down this evening and the lens reaches equilibrium I'll give it another look see.

Edited by jimegger (02/25/13 04:41 PM)


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project [Re: jimegger]
      #5701110 - 02/26/13 07:44 AM

Plan to leave it uncoated, or coat?
M.


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