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Equipment Discussions >> ATM, Optics and DIY Forum

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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: saemark30]
      #5759631 - 03/27/13 06:42 AM

Short answer, no; not if you don't want to saddle yourself with a lot of work.
M.


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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]
      #5759639 - 03/27/13 06:58 AM

I can add one more thing to Dicks excellent posts. If the radii in the IF are not equal, and show a bullseye pattern,

DO NOT VIEW SUCH A PATTERN THRU THE BACK SIDE OF THE FLINT!!!
A round pattern will most likely show up as being flattened somewhat on the side approaching the bevel, IIRC; giving an indication of a ripped edge. Whenever making an IF test, an effort should be made to look thru a positive power; ideally at the focus of the piece.
********
A lot of the angst can be alleviated, if you make a reasonable testplate that's around 1/2D or a bit more. Such a plate, if ground along with the full size surfaces only takes a few minutes of additional effort. And it doesn't have to be completely polished; only spherical and precisely measured.
My 2 millicents worth,
M.


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759776 - 03/27/13 08:53 AM Attachment (32 downloads)

Quote:

Finally here is Selsi 80mm f/11 objective, also of excellent quality from the nice straight Ronchi bands.




Very telling and indicatuive of the value of autocllimation. Even though these are Ronchi an not IF fringes, the test is a double precision test at the focus, so perfect null even with a 133 lpi Rinchi is smomwhere around 1/8 to 1/10 wave PV if not better.

I would like to add something DAVIDG alerted me to previously, namely that it is very important the lens is properly oriented front to back when installed in a cell. It can inadvertently happen.

This image illustrates the effect of reversing the front to back as opposed to the correct orientation. The reversed orientation showed the lens to be severely undercorrected, but when it was flipped around the lens showed clean straight bands indicating a reasonably well corrected optic (this was a $30 Surplus Shed 80 mm, f/5 air spaced lens)

Mladen

NB The light source was not fully diffused

Edited by MKV (03/27/13 08:55 AM)


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DAVIDG
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Reged: 12/02/04

Loc: Hockessin, De
Re: 9.25 inch refractor project new [Re: MKV]
      #5760069 - 03/27/13 11:50 AM Attachment (24 downloads)

Autocollimation can be used to check the color correction of an objective as well. As Dick pointed out the color correction is a function of the radii and thickness of the elements along with the refractive index. In the design for most classic achromats the red and blue wavelengths come to focus at the same or nearly the same point. So to check the color correction you set up the lens in double pass mode and use a narrow band light source to produce either red or blue light and find the exact focal position. Then without moving the knife edge or ronchi screen switch to other wavelength. If the color correction is right, there should be no are very little difference in were the focal plane is located at the other wavelength. I use narrow band interference filters and look thru them to selection the wavelength I want to check the color correction with a white light source. I also have a selection of fairly narrow wavelength LEDs that I can quickly change on my tester.
Here is a graph of the longitudinal spherical aberration of a typical achromat. It's hard to see but the green image is perfectly straight and on the y-axis which is the focal plane. The red and blue images focus behind the green but nearly on top of each other. So again one can take advantage of this and use double pass autocollimation to check this. If a radius is off, you still can get the green image to show a null but the red and blue images will no longer focus at the same position, they will move apart from each other. So as Dick pointed out it is critical to get the radii and thickness as close as possible to the design parameters.

- Dave


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5760203 - 03/27/13 12:56 PM

All of these discussions are very informative so thanks to all of you. I am certainly no professional optician and my knowledge base is from what I have read and experienced over the years. All of these discussions have pointed out some gaps in that for me to which I am most appreciative.

That being said, My crown has one heck of a polish ! Also after star testing it last night I have found that it has now gone ever so slightly under corrected again. I had polished on R1 for 45 minutes with the regular lap and it did the job of removing glass from the center but just a hair too much - and I mean a hair ! I was using Sirius as the test star and I was impressed by the color correction as well as the closeness of the optics to putting out an excellent star test. The images are round as well so good news on the astigmatism front as well. Today I am going to put 10 - 15 minutes on the convexing lap again to try and push it back towards a shorter center focus. I believe this may just do it.

Apart from the star test I used the scope to look at detail on the mountain as before. The sharpness of the image it provided was much better than previously noticed. Even with a burbling air mass I could clearly see details on the small pointed rock I have been using for comparison purposes all along. Furthermore there was a diminution of color noticeable along the edges of objects to where it seemed there wasn't any against a bright blue sky.

One other thing I noticed was the lens cell had shrunk up in diameter to where it was starting to squeeze the crown a bit too tight due to pretty cold temperature at around 10 above. So the calculations I did for differential thermal properties was off by a hair as well and need to be remedied by machining the cell diameter out a bit more.


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DAVIDG
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Reged: 12/02/04

Loc: Hockessin, De
Re: 9.25 inch refractor project new [Re: jimegger]
      #5760295 - 03/27/13 01:31 PM

Jim,
Can you discribe what your looking for when you do a star test and how you do it ?

- Dave


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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: DAVIDG]
      #5761047 - 03/27/13 06:56 PM

I find a fairly bright star and look at the image inside focus and outside focus by racking in and out. The images should show identical diffraction patterns on both side of focus with even spaced and same width diffraction rings.

If the diffraction pattern on the inside of focus shows a hole in the center but the outside focus image is full of light with a bright spot in the center then the objective is under corrected / the center has a longer focus than the edge.

If the diffraction pattern shows inside focus, a bright center and flooded with light in the diffraction rings but the outside focus image shows a diffraction pattern of rings surrounding a hole in the center then the objective is over corrected. The over corrected objective has a shorter center focus than edge focus.

My objective now shows a very small hole in the middle of the inside focus but the center with a bright spot outside of focus. It is now ever so slightly under corrected. The rings are all round and symmetrical for good signs of little if any astigmatism.

The diffraction rings should be round and symmetrical to show no astigmatism. They should be of even brightness and width to show no zonal errors.

The in focus star image should be a small bright dot with a couple of diffraction rings the first one brightest and the second quite a bit dimmer on a steady seeing night. My six inch shows perfect diffraction patterns on both sides of focus to a textbook "T". This 9.25 inch is very close to that now.

Edited by jimegger (03/27/13 06:59 PM)


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DAVIDG
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Reged: 12/02/04

Loc: Hockessin, De
Re: 9.25 inch refractor project new [Re: jimegger]
      #5762236 - 03/28/13 11:37 AM

Jim,
Sound good. If your not already doing so I would sugguest that you use a green filter and also an eyepiece whose focal length is same or close to the f-ratio of your scope.
Refractor can be difficult to star test in white light because of the chromatic aberration. A great book on the subject is "Testing and Adjusting Telescopic Objectives " by Cooke. It is an old book but the theory is the same today. You can read an on-line copy from here. http://archive.org/details/onadjustmentand00taylgoog

I also highly recommend that you test your fully assembled lens by double pass autocollimation with the objective facing the flat just like it would in the telescope. It will be a cross check of your star test results. If your lens is as close as you think to the proper figure then it should show a smooth figure using double pass and very close to a null. If it doesn't I would recommend investaging both the star test result and the double pass result to find out why they differ. As I have said, the flat would have to be many waves off before it adds any real amount of error to the test results and it is double pass test so it is very sensitive to errors in the objective. I have seen many amateurs believe they are doing the star test correctly but in reality they were not and getting a false impression that their optics were much better then they really were. I personnally know of a number of refractors were the image was bragged about as being excellent but when I tested them I found the objective was backwards in the cell!


Both Dick and I teach optical fabrication and we have won multiple awards for our optics. I'm not bragging, just saying that their is independent data from others to back our test procedures and methods. We were just at the 13th annual Delmarva mirror making class were we helped null figure around 20 mirrors, one as large as 20 f/6 in a few days. Over the years we both have seen optics that the owners stated were "1/10 wave" or better turn out to be no better then 1/4 wave or worse when tested correctly. The more data you have the better the finally results will be.

- Dave


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saemark30
Pooh-Bah


Reged: 02/21/12

Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5762315 - 03/28/13 12:30 PM

Seems like you are grinding and polishing by hand?!

May I ask where you got the glass blanks? I would consider such a project in the future.


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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: saemark30]
      #5762366 - 03/28/13 01:04 PM

David, I do have that book "Testing and Adjusting Telescope Objectives" by Cooke. It is excellent !
Right now I do not trust my flat so I am going to have it tested that is why the star tests are being done. When I think I have it in the bag the 12.5 inch reflector will be used for a single pass auto collimation test for a null in the Foucault test and a look with the Ronchi grating as well.

When I did the six inch lens some years back I used the 12.5 inch reflector as a single pass collimator and got excellent results to the point of having a nice null in the Foucault test and the star test confirmed the objective was good. Star images are text book and Mars at 500X shows exquisite detail. That being said , until I know how good the flat is I must rely on what I know works. The images now are really sharp. It clouded over last night so the objective was not able to be star tested after the last polishing run. Tonight may clear up and if so I'll test it again.

I read a neat story about a fellow in "Telescope Making Magazine" some years back when it was still being published about a fellow I believe was in Colorado who was making an 11 inch refractor. He had no flat and would literally test by driving to the side of a mountain where he could look at a light some 3 miles distant. He would do a star / Foucault test after polishing using that light. It took a bit but in the end he got a really good objective. There are many ways to skin a cat !

These blanks were obtained from 2 sources, Newport Glass works in California and Schott Glass works back east. They are F2 flint and BK7 crown. The color correction is really good from what I am experiencing.


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Mr. Bill
Postmaster
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Reged: 02/09/05

Loc: Northeastern Cal
Re: 9.25 inch refractor project new [Re: jimegger]
      #5762493 - 03/28/13 02:17 PM

WOW....great read. Don't understand everything but feel like I'm taking a crash course using the Socratic teaching method.

Thanks...


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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: Mr. Bill]
      #5762523 - 03/28/13 02:45 PM

Agreed, this is an awesome thread, truly showcasing CloudyNights at its best! Learning so much and really appreciate the detailed explanations, images, and commentary.

It's making me want to go a try it!

Clear skies!


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Ed Jones
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5762532 - 03/28/13 02:51 PM

Since both internal radii are the same you could oil and tape them together and autocollimate them. This would largely desensitize these internal surfaces. This will introduce overcorrection, however, but you can use a Ross lens to correct this SA.

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Mr. Bill
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Reged: 02/09/05

Loc: Northeastern Cal
Re: 9.25 inch refractor project new [Re: Gord]
      #5762534 - 03/28/13 02:51 PM

Quote:


It's making me want to go a try it!

Clear skies!




Uhhh...I wouldn't go that far. I'm happy to build my stuff with finished optics but it's nice to have some understanding of what's going on.



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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5762572 - 03/28/13 03:12 PM Attachment (20 downloads)

Jim, you can test your flat by autocllimation using the 12 inch mirror, which you say is a good parabola. If you are absolutely sure the 12 inch is a good parabola then you can test your flat with it, as shown below.

You should be able to observe straight Ronchi bands or a clean Foucualt null with the knife edge. Anything else (i.e. bowed Ronchi bands) will confirm that the flat is unacceptbale.

Double pass autcollimation is a superior test to asingle-pass method you're 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: jimegger]
      #5762875 - 03/28/13 06:19 PM Attachment (15 downloads)

Jim,
FYI I ran a quick ZEMAX macro to scan across the green spectrum, optimize BFL at each wavelength increment, and plot P-V, RMS wavefront error, and Strehl ratio. Looks like your best DPAC test LED spectrum would be in the 0.55-0.57um range.
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: MKV]
      #5763119 - 03/28/13 08:23 PM

The bottom line in all this is to have a scope that does what you want it to do - period. People who are perfectionists can be anal about having everything just perfect when in reality nothing we make ever is, there are just varying degrees of acceptable imperfection the levels of which depend on your own personal taste. For me perfection is having a scope that will show me the belts on Jupiter in good detail as well as the markings on Mars etc. The challenge of crafting my own objective with my own hands to that point is invigorating to the max. To look through a large refractor with unobstructed optics is a thrill in itself as all who have done so will testify. All of you have been a great deal of help through all of this and I thank you all from the bottom of my heart for it. My advise to anyone who wants to give it a try is - go for it ! At the very least you will learn an awful lot and with all this good help failure would be unlikely. When I did my 6 inch lens about 16 years ago I was pretty hesitant because I thought it was harder than it actually is. It is more work because there are 4 surfaces and you cannot directly test them all but there are ways to get it done.

I am right there at the finish line with this objective for what I want with it. There will be more coming as I have 4 more sets of blanks. That will be down the road some since this one still needs a mount etc.

Not having been able to star test it since the last polishing session makes me uncertain as to whether it is there yet but I can say this, Looking at the mountain today the image is every bit as sharp as the 6 inch with even more details visible in the 9.25 inch probably due to its greater resolving power. For my experience , the color correction is excellent as well ! Kudos to you Mike for a great design !!!


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DAVIDG
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Reged: 12/02/04

Loc: Hockessin, De
Re: 9.25 inch refractor project [Re: jimegger]
      #5763188 - 03/28/13 09:03 PM

Jim,
I'm glad your happy with your results but those that make optics both amateurs and professionals and use Double Pass understand that this is one of the best ways to test with the least amount of errors.
You referenced an article about the construction of a 11" refractor, were the author tested his lens using distant a street light. I believe the article your referring to is in the March 1978 Sky and Tel "A California Amateur's 11 inch Refractor". Here is a quote from that article about the author testing his lens, "A large optical flat was unavailable for autocollimation so for a source of nearly parallel light I settled on a distant streetlight". So clearly the author would have preferred to use the much better test of autocollimation but settled for one that wasn't as good. He goes on to say that his lens turned out well but doesn't perform at the Dawes limit.
So I believe that what all of us are saying is that if one is lucky enough to have a much better test available one should take advantage of it and not dismiss it just because one is more comfortable with other test means. The out come could be a much better performing lens for a lot less work and a new found understanding in optical testing.

All the Best,
- Dave


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Dick Parker
scholastic sledgehammer
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: 9.25 inch refractor project [Re: DAVIDG]
      #5763363 - 03/28/13 10:58 PM

And, Jim, We wish for your wonderful project to be the best it can be, that's all. Good luck and thank you for sharing your project with us. We chimed in because we love advanced optical projects and the old long focus achromats. Please keep us posted on your progress and we sure would like to see a photo or two of your completed system.

cheers
Dick Parker


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [Re: Dick Parker]
      #5763535 - 03/29/13 01:50 AM

I certainly did not have any intonations that double pass auto collimation , laser interferometry or any other methods of lens testing should be in any way shape or form discounted or dismissed. I absolutely understand how double pass auto collimation is better than single pass or star testing or any other form of testing. If I knew my flat was good I would be using it now for the DPAC. I am sending it off to get it checked. In the meantime the alternative methods of old such as single pass testing with the 12.5 inch mirror plus star testing will have to suffice. The six inch lens made this way on a night of good seeing easily reaches Dawes limit and was tested this way so I know it can be done with this lens. I'm not done yet and will keep posting. There is more to do such as yet getting this objective right on the money and painting the tube and fitting it up on a mount etc. I'll be posting pics as time goes by.As for the mount , I run Aeroquest Machining so will be making my own drives and the whole mount assembly probably over summer.
When this objective is done it will be giving results at Dawes limit (or it won't be done). I sure hope you guys with all the testing advise didn't think I was knocking any of it because I certainly didn't mean it that way. What I was getting at was some people need 1/20th wave to be happy when 1/8 wave would be more than sufficient. 1/20th wave would certainly be better than 1/8th wave but hardly noticeable is what I was getting at.

I love the views through these doublet refractors and it has been a fun time so far building this one. When I can I'll take some pictures through it and post them. I did buy a Baader IR-UV filter that might help in some shooting with the DSLR once I get the scope mounted.


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