15 replies to this topic

[Stipe Vladova]

As you could see, there is a lot of various chinese achromatic lenses on eBay these last few years. I decided to buy one of these and play around with it. Since I have my own lathe at home bought especially for ATM only, I play with it all the time and make telescopes! It is a real and great fun! I bought a 90/600mm achromat from seller named lens-store. Lens came last week and I would like to share my experience with you guys. 

 

These sellers or the workshops from where they get that stuff are really playing trying to be opticians by designing really weird optical designs. This particular lens is made from K9 crown and F2 flint, but instead to make the standard Fraunhofer aplanatic lenses which you can normally find in every Synta achromat, this one has a lot of off axis coma when tilted, because it is not designed aplanatic, obviously not like the standard achromat which shows off axis astigmatism only, since standard Fraunhofers are aplanats by design. On the other hand, it came with a thin metal spacer .5mm thick and in that configuration this lens was overcorrected spherically. Then I machined a 2.5mm thick plastic spacer ring and the lens achieved good spherical correction in green light. This is very weird, it was as these glass elements were not designed properly at all! After I increased the lens spacing to reduce overcorrected spherical aberration, it also changed the color correction also and instead of slight blue halo around stars in focus, now I have more purple halo since the blue color now shifts more towards green and the red moves more away from these two. It is now chromatically undercorrected. Now I consider to aspherize the inner crown surface which also has no coating at all! By deepening the center it should correct the overcorrected spherical aberration in the position of the minimal lens spacing, and therefore regaining more optimal color correction. With minimal spacing the color correction is somewhat between C-e and standard C-F correction which gives nicely dark violet chromatic aberration, and when optimally separated for SA correction, it is somewhat in e-F color correction which gives ugly red/purple haze around bright objects, since the red color is much far out from the common blue/green focus. That chromatic undercorrection is more common to Zeiss C80/500 lenses from Asembi binoculars that needed around 130mm glass path of prisms to balance the secondary spectrum properly. 

 

These eBay lenses are therefore very risky to buy, and I do not recommend them for those who are not enough skilled ATM-ers. 

 

Here are some pictures of the collimatable machined aluminum lens cell and the finished OTA. Enjoy!

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Edited by Stipe Vladova, 20 November 2023 - 03:23 PM.

[Stipe Vladova]

The OTA.

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[duck]

too much fun!

[Stipe Vladova]

Now I removed the plastic ring that I machined, put three .25mm thickness foil shims between the lens elements and now there is a spherical overcorrection, but only a slight bluish halo in the best focus. Inside focus the defocused image has bluish edge and outside is yellow. With thicker spacer there was no SA, but inside focus disk was red/purple at the edge and outside was bluish/green. Very different chromatic correction order. I finally decided to repolish the uncoated R2 curve of the crown lens to deepen the center. It should remove the SA overcorrection. I hope not to induce any surface roughness... For now this lens is very smooth polished!

Edited by Stipe Vladova, 20 November 2023 - 02:43 PM.

[Diego]

Wow that was a lot of awesome machining for a so so lens. Hope you can refigure it. I also have a lathe and it's my favorite machine.

Looking forward to see how this project evolves.

[ccaissie]

I refigured a 4" unitron that was chromatically ok, but undercorrected spherically.  Spacing changes did not help..it was way off. Because R1 was uncoated, I was advised by DAVIDG to work that surface.

 

It took me a while to rethink the surfacces and correction needed.  To increase the correction I had to put an "Oblate Spheroid' shape in the lens.polishng the 70% zone... like a Schmidt corrector. 

I had to stop thinking like a mirror maker and do the opposite.

 

This did not change the chromatic correction but clearly changed the SA and it came out very well. 

 

I wonder if you could get the spacing right to correct the chromatic, then change a surface figure...not the radius/power, to get the SA right.

 

Bench testing, Ronchi,  against a flat and yelow-green light was the method that enabled me to hit the correction.

Edited by ccaissie, 20 November 2023 - 07:59 PM.

[Alrakis]

"But in case you need to or want to, here's the secret: the radius on R4 has a profound effect on the blue focus. If you make R4 stronger by about 100mm in radius, it will roughly halve the distance from the green to blue focus and somewhat increase the distance from green to red in our Fraunhofer design. It will also shorten the lens's backfocus by 30mm or so and slightly affect the system's spherical aberration. On the other hand, if you weaken R4 by the same amount, it will extend the blue focus, draw in the red focus and lengthen the backfocus by corresponding amounts. This is the key to fixing any residual color error in your lens."

 

http://bobmay.astron.../Refrchroma.htm

Chris 

Edited by Alrakis, 20 November 2023 - 10:38 PM.

[Stipe Vladova]

Thanks for your thoughts and sugestions. I cannot grind the R4 because it has coating on it. Only inner surfaces (R2 and R3) are uncoated. The lens is not extremely overcorrected spherically when I put the minimal spacing, it is something like 1 to 1/2 lambda overcorrected. I think the aspherization of R2 in the direction of hyperbola will quickly reduce overcorrected spherical aberration. As a matter of chromatic correction, I like slightly overcorrected secondary spectrum, something between C-e and the standard C-F. C-F in shorter f ratios still gives some purple fringe.

Edited by Stipe Vladova, 21 November 2023 - 12:00 AM.

[davidc135]

The plan to control chromatic correction with spacing and then correct SA with figuring should be straightforward. Slower objectives are insensitive to spacing changes.

 

I think adding over-correction would be easier but, in this case, maybe a long and wide parabolising stroke should do the trick if it doesn't turn the edge.

 

David

[Stipe Vladova]

Thanks. This lens is 90/600, f6.3 and it should be corrected that way. Maybe the best way to do that is by putting the lens on top, lap on the bottom and use medium to long "W" strokes and mix both of them to uniformly parabolize the surface by deepening the center. Maybe it will be necessary even to hyperbolize it depending of the correction needed.

Edited by Stipe Vladova, 21 November 2023 - 05:05 AM.

[Stipe Vladova]

I made a small lap today and put the front crown lens down and tool on top. The center got deepen, but the edge remained slightly overcorrected. The 70% zone remained also high. Now I will make a small ring lap with no center and no edge to work at 70% zone only. Lens got no roughness.

Edited by Stipe Vladova, 21 November 2023 - 05:19 PM.

[ccaissie]

Here's a link to the thread that I started, and in which DAVIDG gives clear instruction.  It is not the same case, but you can see his thinking and technique.  Should be helpful.

 

https://www.cloudyni...nhofer +unitron

[Stipe Vladova]

Correction completed! In and out disks are now equal with minimal spacing and there is no ugly red or purple fringing! Process went slowly since the lens shape changes 4x slower than on mirrors. This came out beautifully indeed! On the attached image there are CeO bottle, plastic lens holder, the mentioned ring lap, tiny central deepening lap and the full sized lap. The lens was tested on far distsnt lamps. It is not necessary to do DPAC, it can be done if you have it, but if you don't see any zones, turned edge or spherical aberration, then you acomplished your goal. I will take some star test images soon as the weather turns better.

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Edited by Stipe Vladova, 21 November 2023 - 08:23 PM.

[Stipe Vladova]

At 200x the lens showed some astigmatism in the flint. I thought to refigure it, but since the non-aplanatic lenses can't be corrected for on-axis astigmatism by tilting the whole cell, I came on idea to tilt the flint, and crown let to be centered. I managed to do it by putting the paper sticker spacer shims between the lens elements, in that order that two of them have 2 and one of them to have 3 layers of stickers put on the crown. On the lens cell lip where lies the flint I put two of the stickers with 3 layers and one with 2 (opposite order than the lens spacers). So the crown is not tilted and the flint is. You then must rotate the flint alone in the cell untill you see round stars outside focus... Man, this is so weird and I never did such modification before! :-D There are 3 collimation grub screws for collimating the crown and one for locking the retaining ring. I was in great doubt will I refigure the flint or not... So I am very happy to learn something new! Hope this will help someone else to fix possible antique non-aplanatic objective lens! I managed to catch the Moon between the clouds and was stunned by the crisp lunar craters! One thing is for sure: -Never buy these non-standard lens kits from eBay or Aliexpress, unless these which are salvages from damaged tubes from Orion, Celestron or other standard well known brands!

 

Here are the images of the lens cell and objective weird shims! :-D

 

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Edited by Stipe Vladova, 23 November 2023 - 07:18 PM.

[klr]

Nice work!

 

Thanks for sharing your experience with these lenses.  I thought about buying a set, but I don't have the skills that you do, so I'm going to pass now. 

[Stipe Vladova]

Your choice is much better if you live in US because there is much more stuff to get, a lot of vintage japanese stuff sometimes at very good prices, very good lenses at yard sales, or garage sales, so you can buy the lens, or any telescope part for which you know about that is good. We here in EU have nothing, only Zeiss, but it is very pricey and not for everyone, and if some scope parts are available here, they are much more expensive than in US. On the other hand, the shipping cost from US to EU is ridiculously high. Yesterday came to me the chinese all metal 2" focuser from eBay, it has nice black anodized finish, but it had a lot of play and needed to repair it. Now it is excellent. It is now on this 90/600 tube. I made some really nice scopes and would like to get some nice vintage eyepieces and stuff to use on them, like vintage diagonals, barlows, Parks volcano top kellners, circle T vintage Celestron volcano top orthos, or Meade research grade ones... All of them can be bought mainly from US.

Edited by Stipe Vladova, 25 November 2023 - 12:21 PM.