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Equipment Discussions >> ATM, Optics and DIY Forum

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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5763545 - 03/29/13 02:10 AM

Jim,
First, thanks for the compliment, you are most welcome!

Second, and just a thought: you could try DPAC-testing your lens with your 12.5" scope if you put a small high-precision flat mirror (like a Newt diagonal) at the focus of your lens and ran the light backwards through the lens and 12.5" scope. Worth a try anyway.

Mike


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5763642 - 03/29/13 04:53 AM Attachment (38 downloads)

Why do you need a diagonal mirror? Could you draw a picture of that setup? Is this what you are proposing:

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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: MKV]
      #5764139 - 03/29/13 10:27 AM

I will, but read what I wrote again and try thinking it through.
Mike


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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5764570 - 03/29/13 12:41 PM

What I do Mike when I use the 12.5 inch mirror is to put the pinhole at the focus of the 12.5 inch mirror turning the mirror into a searchlight of parallel light. The way I determine the pinhole is at exact focus is to use a telescope focused at infinity to observe the pinhole image until it is sharp by looking into the mirror and even doing a Foucault test if necessary. After seeing that the pinhole is dead on focus I put the objective I want to test in the parallel light path and do both a visual and Foucault test on the objective. It is not any different than looking at parallel light from a star except that the star in this case is a pinhole and has some angular size.

For testing of the six inch lens it worked excellent as the results show in its use on the stars. It is truly diffraction limited. I also noticed that using a very high power eyepiece in the telescope when set up this way gave a microscope type view of the pinhole and you could evaluate the qualitative value of the pinhole as well. I was surprised how sensitive this was and how little polishing action could dramatically change the quality. I actually made a knife edge tool that inserts into the focuser of the telescope to do the Foucault test. Once I did that I'd take the knife edge tool out and put in the eyepiece and examine the image at best focus for the quality.

Being a single pass test it did not show the sensitivity of the DPAC but it is entirely sufficient for getting an objective to work at its theoretical limit. For those people who have a larger Newtonian mirror of excellent quality than the objective they want to test it is an effective substitute for using an optical flat they probably do not have handy.


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Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5764684 - 03/29/13 01:25 PM Attachment (31 downloads)

This is the test I was proposing that uses your 12.5" and a small return flat to give a DPAC test. The flat can be a small Newtonian diagonal, placed at the focus of your refractor.

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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5764690 - 03/29/13 01:27 PM Attachment (21 downloads)

and this is the DPAC test result for either interferometry or Foucault testing at 0.578m. Pretty much perfect double-pass test, assuming your Newtonian primary has the figure smoothness needed for DPAC testing.
Mike


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Gord
Carpal Tunnel
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Reged: 01/06/04

Loc: Toronto, ON, Canada
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5765278 - 03/29/13 04:58 PM

Mike,

Would it be possible to replace the Newt secondary with the small flat and perform the DPAC test on the refractor just as if the newtonian primary and small flat were working as a flat? There would be the same limitations on the quality of the mirror(s) and it would require a modified setup versus a newtonian on it's own.

One benefit I could see to this alternate approach (if it would work...) is that you could use a smaller secondary (flat) for less obstruction. For example, there are high quality round flats available from Surplus Shed that are only about 1" diameter and 1/20th wave.

Thanks,


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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5765294 - 03/29/13 05:02 PM

Now I see how you set it up Mike, I was under then impression that you were somehow using only 1 small flat but in fact you are utilizing the Newtonian's normal flat as well. It makes sense.

When I did the single pass testing the Newtonian's diagonal was removed and a small led light with a pinhole was placed out in front of the mirror at its focus. It was supported on a thin blade so the whole assembly does not take up much central area making a much smaller silhouette there.

The setup for DPAC using the mirror is pretty slick Mike but a bit more involved than doing a single pass test.


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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project new [Re: jimegger]
      #5765674 - 03/29/13 08:21 PM

Gord, yes, you could reverse the test by putting an even smaller flat, like 1/2" diameter, at the Newt focal point. The slitless KE tester would be located at the lens focal point, using a green LED for illumination. Again, the test is only as good as the Newt primary.

The angle of the flat is critical; if it is tilted, the expanding cone of light reflected from it is tilted twice as much. This causes vignetting of the lens pupil. Fortunately, it's not hard to get good alignment. The flat surface needs to be clean, as any dust on it is sharply imaged at the DPAC focal plane.

DPAC with a truly smooth, flat mirror is clearly THE best method. But in Jim's case, the figure quality of his return flat is highly suspect, and the Newt collimator test setup is his cheapest and quickest fallback method.

Mike


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kfrederick
Carpal Tunnel
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Reged: 02/01/08

Re: 9.25 inch refractor project new [Re: Mike I. Jones]
      #5765692 - 03/29/13 08:32 PM

Mike I sold Jim a surplushed flat that was to be 1/10wave big enough to test it . But I do not know if it is good enough . Jim if the telescope works good and you are happy then that is the best test .Congrads . What is next a 12inch??

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saemark30
Pooh-Bah


Reged: 02/21/12

Re: 9.25 inch refractor project new [Re: kfrederick]
      #5770632 - 04/01/13 11:54 AM

Jim, do you recall the size of the BK7 and F2 blanks you started with in your great 9.25" refractor including the thickness for ordering? (In case someone wants to duplicate this project).

Edited by saemark30 (04/01/13 11:54 AM)


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: saemark30]
      #5772948 - 04/02/13 01:20 PM

The blanks started out at 1.1 inches thick for both the flint and crown. I had them made to the maximum diameter they had in stock at 9.25 inches. They were ground flat on both sides as well. I remember the crown being around $275.00 and the flint about $450.00. I got one set from Schott and one from Newport glass works. I agree with Mark Harry in getting the curves generated beforehand , it will save glass thickness and a lot of time. Newport Glass actually has 6 and 8 inch sets they sell with curves generated and all the tooling ready for fine grinding. The last time I looked they had an oiled set and an air spaced set with different glass types for the lens materials and for different f values. They list BK7 and F4 glass types with f/10, f/14.1 and f/15 kits in the astronomy glass section.

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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]
      #5775613 - 04/03/13 01:42 PM Attachment (47 downloads)

After some more figuring on R1 using the convexing lap to bring the center up, the star images are looking nearly perfect and the inside focus and outside focus diffraction patterns are very close in having identical appearance. I picked a bright star and looked at it with about 70X magnification. The colors look excellent. I do not see a turned edge at all. The only thing I did notice was the star image is slightly out of alignment (collimation) but not bad enough to get a good feel for the lens performance.

Then I looked at Jupiter just as one of its moons was coming into view from either in front of or behind it. It was obvious in the refractor appearing as a small dot looking very star like. The air was fairly steady so seeing was good. I did notice a ghosting of Jupiter due it seems to the internal reflections of the lens and being off collimation it was in the same direction as the star images when out of focus but it would go away when I slightly de-centered my eye at the eyepiece. The image of Jupiter itself was sharp with clearly defined bands that had excellent contrast. I put my 12.5 inch reflector on Jupiter at the same time for comparison of image quality and could see little difference other than the ghosting. This all leads me to believe that it is on the verge of being finished. Today I did one more 15 minute figuring session with the convexing lap on R1 since that amount of time seems to take the objective in baby steps towards ideal diffraction images. It was as described above very very slightly under corrected.

Here is a shot with my Canon 20 da through it at the mountain. I did my best to focus but with my old man eyes it is hard to get it right on. At any rate the color looks excellent and with the eyepiece in it everything looks razor sharp ! Here's the pic.

The point at the very top had a Magpie sitting on it one time and it was about one fourth the size of that pointy rock. This is 5 plus miles distant for an idea of scale.

Edited by jimegger (04/03/13 02:09 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 new [Re: jimegger]
      #5775956 - 04/03/13 04:42 PM

Not too bad- shoot the 'crow' off that rock!
M.


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kfrederick
Carpal Tunnel
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Reged: 02/01/08

Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5775976 - 04/03/13 04:50 PM

Looks like Mikeys numbers were good . Congrats Jim Great thread .

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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: kfrederick]
      #5779700 - 04/05/13 02:04 PM

I found the spacer I had machined into the lens cell was not of uniform thickness by putting a dial gauge on it while chucked up in the lathe. Total variation was .014 inches from one side to the other so I machined off the difference to make the R2 and R3 surfaces parallel to each other within .001 inch. Last night I did more star testing after machining the cell and after doing 10 minutes of corrective polishing on the crown. Results were , the ghosting disappeared and the colors look even better. The inside and outside focus star images are even closer in looking alike but still not exactly the same. There is still remaining a tiny bit of under correction. The objective was able to quite easily split Caster in spite of moderate seeing conditions. Today it gets another round of corrective polishing action and more star testing tonight if it stays clear.

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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project [Re: jimegger]
      #5779712 - 04/05/13 02:12 PM

Good on you for getting the airspace wedge down to 0.001", that is critical. You are getting so close that you might also try marking the sides of the lenses and rotating the crown with respect to the flint to minimize the effects of any residual wedge in the elements themselves.
Mike


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Old Will
sage


Reged: 02/12/10

Loc: Utah
Re: 9.25 inch refractor project [Re: Mike I. Jones]
      #5780850 - 04/05/13 11:07 PM

How much does the objective weigh?

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [Re: Old Will]
      #5780942 - 04/06/13 12:14 AM Attachment (33 downloads)

I'll have to weigh that yet - as of now I am not sure how much it weighs.

After doing a 10 minute correction session this morning and assembling the objective back into its cell, back on the scope, I took this picture through it. I hand held my Canon camera up to the eyepiece looking at the mountain after the air had settled down enough to give decent images without all the boiling air. This is what I got at around 53X magnification. This is the sharpest the mountain has looked yet when using my eyes at the eyepiece. The glinting of the sun off the snow gave tiny sharp star-like glows. It was a type of star test and if it is any indication of the objectives state I would say it is right where it should be. Anyway, here is the pic.....


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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 9.25 inch refractor project [Re: jimegger]
      #5780970 - 04/06/13 12:34 AM Attachment (24 downloads)

The effect of having a 0.014" wedge across your airspacer ring diameter is pretty bad, as shown in the plots. The airspace wedge angle is ATAN(0.014/9.25) = 0.08672. The image should have shown at least some perceptible coma, which I think it had and you saw, but you mis-interpreted as collimation error. Good that you got it machined down to 0.001" or so, for obvious reasons.
Mike


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