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Astrophotography forum => ATM, Optics and DIY Forum => Topic started by: Todd Vann on December 31, 2017, 04:59:40 AM

Title: Optical experiment with a 102 mm achromatic objective
Post by: Todd Vann on December 31, 2017, 04:59:40 AM
Since this concerns a Polarex/Unitron lens, I doubted whether to post this here or in the classic scopes section. But it has more to do with ATM'ing, so here's the story. About 2 years ago forum member Xavier (more active on the classics section) handed me over a terribly abused 102 mm FD 15 Polarex lens. Coatings were badly scratched and a Ronchi bench test revealed a short edge. Also the CA on this lens was very obtrusive. I could only use the objective on the sun, in combination with a narrowband solar continuum filter. Since I wanted a more versatile instrument I decided to look for ways to improve the condition and performance of the lens. First of al I polished off the defective coatings on a pitch lap with cerium oxide. After about 10 minutes on each side, the glass looked like new.

Without the coatings, I was afraid of excessive internal reflections between the two elements. One way to cure this was to replace the air gap with some kind of immersion oil. I tried different sorts of oil, from kitchen oil to industrial silicone based oil. Even when the edges of the lens were sealed with teflon tape, these rather thin oils leaked into the lens cell after a few days. Both spherical aberration and CA however looked improved. The best results I obtained with a more consistent clear body gel. The first tests were quite promising. This gel however started to dry out at the edges after a few days and air got between the lens elements. Further experimenting lead me to another substance, a clear lubricant gel used for medical purposes. Bingo! This solution seems perfectly stable.

From ugly duckling to a superb performer rather in the ED- than in the achromatic class! The scope delivers razor sharp images at powers up to 400X (4 diameters!). CA is as good as non-existent. Repeated star tests during daylight on a bright reflection of the sun revealed nearly identical intra- and extra focal rings. Thickness of the outer ring was identical on both sides of focus, the inner rings were symmetrical and identical with only slightly lesser contrast on the extra focal side. In the expanded star discs the intra-focal image was color neutral, in the extra-focal image the space between the rings looks a bit purplish, but exactly in focus color only seemed detectable at the highest power of 400X. In the mean time I got rid of the short edge by masking the outer edge of the lens.

This objective continues to amaze me. Razor sharp images of the moon, with no color between the brightly lit surface and the dark shadows of the craters, also no colored rim on the moon. This little scope has put my excellent 200 mm Schiefspiegler out of work for a few weeks. It's ready to observe within minutes and it doesn't suffer as much from bad seeing as it's bigger unobstructed brother. There definitely is something magic about these long focus refractors. In the coming weeks I will further continue testing and keep an eye on the eventual leaking of the gel. In any case, the initial purpose of this experiment to avoid internal reflections, turned out to be beneficial on the CA as a bonus. Pure luck I guess, since I have no clue of the diffraction index of the gel.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Danny Cruz on December 31, 2017, 05:13:44 PM
Awesome! I`d have thought that the air space index of refraction could not be departed so far from...
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: John Sanchez on January 01, 2018, 03:31:43 AM
I wonder if your polishing off of the coating went just far enough into the glass to tweak the figure by a small amount in the right direction, maybe introduce a non-spherical surface or two. Anyway, a nice story to hear.

Fred
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Kyle Montes on January 03, 2018, 02:00:26 PM
Quote
Awesome! I`d have thought that the air space index of refraction could not be departed so far from...

Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: stalafovkith on January 09, 2018, 07:19:05 AM
Philip, congratulations on your success. I wish we could see some results, and some science behind it, re: Glenn's post. It can be easily shown by raytracing that placing oil where once was air would change the optical characteristics of the correction.

I am also curious about your Schiefspiegler. You have twice the aperture, yet you say it's outperformed by the 100 mm refractor. Why do you think that is?

Mladen
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Mario Carpenter on January 10, 2018, 02:52:59 AM
@Glenn
By tightening or loosening the screws on the lens retaining ring, thus by changing the thickness of the gel layer, I could easily see the difference in color correction.

@Fred
I don't think that I changed the figure during such short controlled polishing sessions

@Mladen
I think I've used the wrong English expression. The little refractor did not surpass in any way the Schiefspiegler. The Schiefspiegler essentially starts where the refractor ends, at 400 X, also resolution is beyond comparison. But the refractor gave such sharp, contrasty essentially color neutral images that I didn't mind leaving the Schiefspiegler inside for a while. Also, the view of the entire moon at 50, 75 and 100 X in the refractor has something mesmerizing. In the Schiefspiegler I can only see part of the moon. A few years ago I made a focal reducer for the Schief with a binocular objective. It changes the focal ratio from F/D 20 to F/D 8 and allows me to use lower powers and see the moon and the sun in it's entirety. Two problems however: field plane tilt of the Schief starts to become visible and I have to swap focusers in order to use the reducer.

I can only imagine what a 200 mm F/D 20 refractor (folded of course) with this gel would be capable of! I was planning the construction of a Schupmann but now I may try my hand at the fabrication of an achromatic objective. I doubt however if the use of a gel between the elements of a 150 mm F/D 8 would produce noticeable effects. Let's not forget that the 102 mm lens is F/D 15 to start with.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: creasseinicomp on January 10, 2018, 11:57:42 AM
Thank you, Philip. I get it now.

Mladen
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: zajusima on January 11, 2018, 03:23:16 AM
The subtleties of the English language, Mladen. In Dutch, the expression "put something to rest" literally means "let it rest aside for while" and by no means "kill it"
I will monitor the stability of the gel and the CA-improvement over the next couple of weeks and further report on it. I remember a thread here: 'adventures in lens oiling' where the OP reported similar improvements. There was no science behind my experiment, only the intention of eliminating internal reflections in the objective caused by the removal of the coatings. I guess I was lucky in stumbling upon the right substance (viscosity, transparency, refractive index...) leading to this very much improved CA-behavior.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Mark Patterson on January 11, 2018, 07:26:59 AM
Nice project, Philip! Enjoyed reading about it. Also, about using a binocular objective for a focal reducer. I wonder if the lens assembly could be tilted to change the focal plane tilt?
M.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Artavius Murphy on January 13, 2018, 03:23:34 PM
The lens was indeed slightly tilted Mark. But I could not eliminate the field tilt entirely, because it was starting introducing CA from a certain point. Anyway, I didn't use it that often, since a Schiefspiegler is by definition a high power telescope. Starting with the native F/D 20 and then using long focal eyepieces was better than the other way around: using a reducer and then pumping up the power.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: tmasnilypho on January 15, 2018, 10:39:28 PM
Quote
The subtleties of the English language, Mladen. In Dutch, the expression "put something to rest" literally means "let it rest aside for while" and by no means "kill it"


Yes, English is an idiomatic language. I imagine Dutch is no less. Idiomatic expressions mean something different from their literal meaning. Instead of using "put it to rest", one can also say "let it go", but a stubborn person may reply "No, I'll stick it out!"

Mladen
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: chirafepes on January 17, 2018, 12:42:50 AM
Quote
Quote

The subtleties of the English language, Mladen. In Dutch, the expression "put something to rest" literally means "let it rest aside for while" and by no means "kill it"


Yes, English is an idiomatic language. I imagine Dutch is no less. Idiomatic expressions mean something different from their literal meaning. Instead of using "put it to rest", one can also say "let it go", but a stubborn person may reply "No, I'll stick it out!"

Mladen
LOL
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Brandon Garrido on January 21, 2018, 12:18:38 PM
Update on the 102 mm 'oiled' lens: The gel and the image forming properties of the lens have not changed up to now. To better asses the quality of the lens, I did a shootout between the Polarex and the 200 mm Kutter schiefspiegler stopped down to 100 mm on terrestrial objects. The Schiefspiegler was equipped with a Baader T2 non-Zeiss prism, the refractor with a Televue enhanced aluminum diagonal mirror. Here are my first impressions:

-there is a pronounced difference in color temperature of both images. The refractor has a grey/bluish tone, the reflector a more yellowish tone. This is perhaps due to the fact that I specifically asked for a very thin aluminum coating on the mirrors in order not to influence the surface accuracy. When you look at the mirrors they actually do look yellowish. I did not dislike the bluish tone at all in the refractor.

-the refractor seems to give somewhat harder images in the dark tones

-in the mid-tones, the Schiefspiegler threw up more contrast and markedly better resolution (perhaps because the Schiefspiegler was now working at F/D 40) or due to a better surface accuracy of the long focus mirrors

-Airy discs were a bit tighter in the Schiefspiegler

-Intra- and extrafocal rings looked identical in both scopes

-in focus there was no clear sign in the refractor of residual color (at X 200)

-surprisingly, images in the Schiefspiegler (stopped down to 100 mm) seemed more stable and less influenced by seeing than in the refractor

What I've learned from this comparison:

-the refractor will probably get more use where aesthetics and casual observing of the moon and planets are concerned, or lower power star fields for that matter (lighter, easier to set up, less troubled by seeing...)

-when it comes to high resolution planetary work at high power the Schiefspiegler obviously is the clear winner, even when stopped down to 100 mm

I am extremely pleased with the performance of both scopes!
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Charles Marin on January 21, 2018, 02:59:33 PM
"-surprisingly, images in the Schiefspiegler (stopped down to 100 mm) seemed more stable and less influenced by seeing than in the refractor"

As far as I understand it, that's the benefit coming from increased focus depth.

Seeing will also try to defocus the image shifting the focal point and since the focus depth is proportional to square of the focal ratio ( Ld=4.13λF^2, from here: http://www.telescope...corresponding), (http://www.telescope-optics.net/defocus1.htm#corresponding),)
f/20 will have almost double the focal depth than f/15, thus having more stable image in any given seeing.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Chuck Klem on January 31, 2018, 06:31:43 AM
Aleksandar,

When comparing the 10 cm refractor to the Kutter Schiefspiegler at full aperture (f/20), the refractor gives the more stable images. Only stopped down and at f/40, advantage went to the Schiefspiegler. You could be right it has to do with depth of focus. I considered seeing-dependence mostly as a function of aperture, design (refractor versus reflector) and construction (open tube versus closed tube) and not so much as a function of depth of focus.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: John Pfister on January 31, 2018, 07:45:24 AM
Philip,

yes, I mixed up the ratios of stopped down 200mm. Still, the point being that in same aperture, the greater f/ratio helps with seeing induced focus error.

Neil English wrote about that: http://neilenglish.n...-stable-images/ (http://neilenglish.net/why-the-classical-achromat-produces-the-most-stable-images/)

I'm not sure how to weigh the aperture diameter vs. seeing error and focus depth vs. seeing error. as seeing error has both lateral shift and defocus...
It also depends on the type of seeing, too, but it must be better to have greater focus depth on the same aperture.

Cheers!
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: juskemenbo on January 31, 2018, 01:02:25 PM
Thanks for clarifying Aleksandar. I think I might investigate this a little further. I also had the impression that the brightness in the stopped down Schief was a bit higher than in the refractor. Strange, since I eliminated internal reflexions in the lens by using the gel and especially since the Schief has 2 mirrors, one correction lens and a diagonal prism in the optical train. But the difference, if there was any, was quite subtle. When I compared the chief to several 8" SCT's in the past, the Schief always lost as far as image brightness was concerned, even though it has no central obstruction. I think it has to do with the high reflectivity coatings of modern SCT's versus the standard aluminum coatings of the Schief.
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Seth Mamidi on February 03, 2018, 09:00:15 AM
One explanation for the possibly slightly brighter image in the Schief. I did not put a mask on the primary mirror, but instead I've put a stop at the correction lens, so maybe I had an effective opening of perhaps a bit more than 10 cm...
Title: Re: Optical experiment with a 102 mm achromatic objective
Post by: Michael Washington on February 09, 2018, 10:30:53 AM
I'm not sure what you mean by "stability". You're dealing here with two issues.

(1) Smaller apertures have more "stable" images because of the size of atmospheric thermal cells. The Schief is a 200 mm aperture and the cells are usually between 3 and 4 inches (about 75 to 100 mm to round it off). The bigger the aperture the more "dancing" the image will exhibit.

(2) Depth of focus simply means that an image will remain within the 1/4 wave Rayleigh limit over a certain range of focal shift. The definition of the depth of focus (DoF) is the amount of focal aberration corresponding to 1/4 OPD =± λ/(2Nsin<sup>2</sup>U<sub>m</sub>), where λ = wavelength (usually 0.00055 mm) , N is the ref. index of the medium (in this case the air N= 1.0), and sin<sup>2</sup>U<sub>m</sub>is the sine of the angle subtended by the marginal (peripheral) ray and the paraxial focal distance. The expression can be re-written in terms of thefocal ratio, F#,as OPD =±2λ(F#)<sup>2</sup>. Doubling the focal ratio quadruples the DoF distance. If you go from f/10 to f/20, doubling the focal ratio the DoF tolerance will increase 2<sup>2</sup> or four-fold. In case of an f/10, the depth of focus OPD tolerance =±0.11 mm. For anf/20 it's 0.44 mm.

Clearly, the slower the system the more it's going to stay "in focus", thereby appearing more stable. If this is coupled with an aperture not exceeding 4 inches or so the image will also be more stable because of the thermal cell size of small air pockets, so the performance will be judged "superior" to that of a bigger, and/or faster system.

Mladen