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Synta (Skywatcher) 6" f5 color shift by oilspacing

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#1 Stipe Vladova

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Posted 12 October 2013 - 09:59 AM

Hi to all!

I've some experience by optimizing achromatic objectives (Fraunhofer type) with success. It is about 80mm f5 and 60mm f11 eBay achromats. They were actually slight undercorrected in green and since 80mm is C-F corrected it showed a lot purple-reddish fringing around bright objects since it is only f5. It had thin tape spacers between the elements which I had removed and put oil instead of air. Slight spherical undercorrection vanished and the color correction changed. Red wasn't focused together with blue anymore, but moved somewhat towards the green line. That completely improved image sharpness because purple fringing vanished and remained only dark blue. I observed Jupiter and it was surprisingly sharp with much more details than before, and that made this little shorty usable for planet observation. I read something about it on the net and found that 19th century achromats were corrected like this and they called that a "planetary color shift". What is refractive index of the oil? Since Fraunhofer objective has R2 curve slightly smaller radius than R3, is then any chance that slightly negative thin "oil meniscus" caused all this new features. And what about Synta 6" f5 Fraunhofer achtomat, I read some reviews that show these large lenses slightly undercorrected with purple fringing and this would be worth trying to optimize at the same way. I don't know is the airspace also thin on 6" f5.

#2 skywatcher101

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Posted 12 October 2013 - 03:07 PM

This is very interesting. Have a synta f8, how do you go about converting to an "oil space" objective?

Paul

#3 Stipe Vladova

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Posted 12 October 2013 - 04:55 PM

If your f8 has a large airspace (spacing ring like that in some Jaegers objectives) oilspace is not recommended. Only if refractor has tiny airspace and is slightly spherical undercorrected (brighter ring inside focus, and softer outside) then it is possible and thin oil meniscus corrects remained spherical and moves colorcorrection from pure C-F to partially C-e correction. Probably on f8 that is not necessary, but at f5 refractor shows not only dark blue or dark violet, but average purple-reddish bright halo destroying lunar and planetary details.

I ordered 5" f5 Jaegers seconds some days ago over 50% off sale for $75 and I'll try to oilspace it to see if it works on that lens.

#4 skywatcher101

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Posted 12 October 2013 - 07:30 PM

Yes, my spacing on the objective is around .010 of an inch.

#5 Ed D

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Posted 13 October 2013 - 12:47 AM

Very interesting and useful knowledge. Thanks for sharing.

Ed D

#6 Stipe Vladova

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Posted 13 October 2013 - 05:42 AM

Is the Synta slightly spherical undercorrected? Check intra/extra focal images. If yes then you may try, if not then it will turn overcorrected when put the oil. The objective acts like old Zeiss when you improve it like this. For 6" you need about 8 oil drops. Lens then doesn't need to be taped at the edge, just put it in the cell.

#7 t.r.

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Posted 13 October 2013 - 06:47 AM

The shift to a C-e achromat was discussed some time ago by Roland Christen (link). It indeed is a better planetary scope shifting red and green to a common focus, where most planetary detail information is contained, and allowing the blue to go further out of focus. Therein is the problem. While this correction gives good planetary views, the stellar capability has been degraded. This type of correction will not be a good all-around scope, but would indeed be a very good planetary scope. Have you specifically observed some blue, blue-white stars with this scope since the oil-spacing? You may be disappointed.

In fact, I approached RF Royce Optical about making a custom C-e oilspaced achromat specialized for planetary for me. Robert asked why would I want such a thing? I told him that Roland had confirmed that it would be a great planetary scope but, Robert said it would be good at nothing else. He refused to make it, saying he would only stick with the C-F design which he does oilspace. He said everything else in the universe beyond the planets needs the blue end and making a scope solely for a handful of objects wasn't cost effective for the slight benefit.

Link to C-e Discussion



#8 skywatcher101

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Posted 13 October 2013 - 08:14 AM

@Stipe,

Yes, the synta is under-corrected. I have been playing around with the spacing, range is from .005 to .015. I want to see if it changed the image at all.

So what I do is leave the spacers in place and add oil. I guess I could use some good tape on the outside of the objective so the oil doesn't leak out.

What kind of oil to use? What would be the spacing of the objective when using oil? Can or should I leave the spacers in place when using the oil? Like I said, my spacing would be at low end .005.


Paul

#9 Stipe Vladova

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Posted 13 October 2013 - 04:33 PM

@Stipe,

Yes, the synta is under-corrected. I have been playing around with the spacing, range is from .005 to .015. I want to see if it changed the image at all.

So what I do is leave the spacers in place and add oil. I guess I could use some good tape on the outside of the objective so the oil doesn't leak out.

What kind of oil to use? What would be the spacing of the objective when using oil? Can or should I leave the spacers in place when using the oil? Like I said, my spacing would be at low end .005.


Paul


No, remove the spacers. It will do real mess if oil gets in with spacers. Excellent, when it is undercorrected this will do a great job by oil spacing. Clean the glue marks in the place where the spacers were, clean the flint well and put the oil. No tape required.

#10 Stipe Vladova

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Posted 13 October 2013 - 04:45 PM

The shift to a C-e achromat was discussed some time ago by Roland Christen (link). It indeed is a better planetary scope shifting red and green to a common focus, where most planetary detail information is contained, and allowing the blue to go further out of focus. Therein is the problem. While this correction gives good planetary views, the stellar capability has been degraded. This type of correction will not be a good all-around scope, but would indeed be a very good planetary scope. Have you specifically observed some blue, blue-white stars with this scope since the oil-spacing? You may be disappointed.

In fact, I approached RF Royce Optical about making a custom C-e oilspaced achromat specialized for planetary for me. Robert asked why would I want such a thing? I told him that Roland had confirmed that it would be a great planetary scope but, Robert said it would be good at nothing else. He refused to make it, saying he would only stick with the C-F design which he does oilspace. He said everything else in the universe beyond the planets needs the blue end and making a scope solely for a handful of objects wasn't cost effective for the slight benefit.

Link to C-e Discussion


This thread I wrote about fast achromats where bright red/purple annoying fringe occures, not about f10-15 achros. F10-15 achro may have C-F correction. This what I done actually isn't fully C-e correction, but somewhere between C-F and C-e, just the red to dissapear. I tried fully C-e correction and there was a lot blue fringing and the Saturn was much more orange-yellow colored. Intrafocal star image was blue at the edge and out of focus was orange. At C-F correction it is violet inside focus and yellow-green outside focus. I found best correction somewhere between C-e and C-F and the bright red-purple fringe was removed.

#11 skywatcher101

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Posted 13 October 2013 - 07:12 PM

@Stipe,

So no spacers, just oil. What if the glass is not ground perfectly the same(a higher spot on the lens)?

What oil to use and how much is needed?

Paul

#12 Stipe Vladova

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Posted 13 October 2013 - 08:12 PM

No spacers and 8 drops of oil should be enough. If it is undercorrected then it is better that inner surfaces are not equal. Actually on Fraunhofer lens the second curve of the front lens is slightly shorter radius than concave on the back lens. That will help to remove undercorrection. Vegetable oil can also be used.

Ooooh, I'll help a lot of people around the world, but never have enough money to buy even cheapest 6" lens..... unles it is not over $75 :crazy: Currently on SS 80/400 just now dealing with bad 0.87mm paper tape spacers and results are nothing but terrible coma on the axis...

#13 De Lorme

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Posted 13 October 2013 - 09:06 PM

Would you consider making a video or putting up pictures?
De Lorme

#14 skywatcher101

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Posted 14 October 2013 - 06:54 AM

I will be tying this out on my 6" f8 synta this week. Probably use vegatable oil or mineral oil.

Will use no spacers, just oil.

Anyother concern is when I tighten down the objective in the cell. Would this not squeeze out the oil between the two lenses?

I will just snug the objective assembly for now and see what happens.

Paul

#15 Stipe Vladova

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Posted 14 October 2013 - 01:00 PM

Do not tighten your objective cell, just screw it untill it stops.

Here are some images I took on my 4" f15 airspaced and 60mm f15 oilspaced scope with EOS1100D and 2x shorty barlow.

Posted Image
4" f15 Jaegers. It is C-F but f15 and has good spherical correction and there is nothing to change since there is no red-purple bright as in f5

Posted Image
60mm f15 after spherical undercorrection removal by oil. If the lens, especially chinese one has turned edge, it also can be removed by putting oil between, only if inner surfaces have turned edge (usually concave flint surface). This little lens also has some turned down edge (had).

Posted Image
This is my favorite image of nice tack sharp Airy disks at 60mm f15. This sharpness and color correction reminds me on old good Zeiss Telementor.

These images are taken in autocollimation against my 12" f5.7 paraboloidal mirror. The LED light source is covered with one old FS mirror with tiny pinholes and scratches on it used as an artificial sky.

#16 stevew

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Posted 14 October 2013 - 01:36 PM

Anyother concern is when I tighten down the objective in the cell. Would this not squeeze out the oil between the two lenses?


Paul

I believe you should tape the the two lenses together to seal the oil inside.
Let us know how it works out.

#17 Stipe Vladova

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Posted 14 October 2013 - 04:35 PM

Yes if there is enough space between the lens edge and cell, but if is tight then it can't be done. We will see the progress if skywatcher101 puts some photos of intra/extra focal images.

#18 skywatcher101

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Posted 14 October 2013 - 08:04 PM

Okay, first I used mineral oil with no spacers. It looked like the glass was touching its self. Gave a ring like glare through the glass. I was worried so I used a .004 spacer. Well, by trying to separate the glass by sliding it away from each other. I got some very fine scratches on the glass.

Is there a way to clean up the glass ?

Anyway, I did a look through the scope with the spacers, had nothing to loose now. It basically reversed the in and out of focuse rings. Before it was under-corrected it is now over-corrected.

Other issue was the oil started to drain out because of the .004 space. So, with fine line scratches in the glass, I will try with no spacers next time.

My concern, like I said, is the glass making contact with its self because it is not perfectly matched.

Paul

#19 T1R2

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Posted 14 October 2013 - 08:25 PM

Paul if you mean clean up the glass by removing oil residue then a wash under the sink with some dish soap will do, just rinse before washing with soapy fingers, you may have to wash and rinse a few times, sometimes those fine little scratches will come off if they are just streaks on the lens, if its scratched then nothing can be done

#20 skywatcher101

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Posted 14 October 2013 - 08:33 PM

It's fine scratches, no doubt it sad to say.
Now what about if the glass is going to make contact with its self. When I first used no spaces, I noticed rings in one area of the glass. With the .004 spacers in place no such rings appear.

Now, is it actually making contact or is it the space is so close it will make those rings. Will it affect the optics if I go ahead and use no spacers and the oil?

What shall I do?


Paul

#21 PowellAstro

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Posted 14 October 2013 - 10:51 PM

You do not want them to touch. The two types of glass expand at different rates and it can cause a lot of stress.

#22 skywatcher101

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Posted 15 October 2013 - 05:03 AM

That is what I thought. Since my lens set are not a matching surface I have to use some kind of spacing between them.

I had a feeling I did not have a great objective. Stars are nice, but looking at Jupiter in good conditions I did not receive a sharp contrast image. Like I said in another discussion, the image was "soft" to me.

So....I guess I need to find a other 6" f8 double lens set. Any ideas?

Can I fix the set I have now?

I know reading your post PowellAstro about the AR152. I know I will be getting one of them some time.

Paul

#23 Stipe Vladova

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Posted 15 October 2013 - 07:37 AM

Okay, first I used mineral oil with no spacers. It looked like the glass was touching its self. Gave a ring like glare through the glass. I was worried so I used a .004 spacer. Well, by trying to separate the glass by sliding it away from each other. I got some very fine scratches on the glass.

Is there a way to clean up the glass ?

Anyway, I did a look through the scope with the spacers, had nothing to loose now. It basically reversed the in and out of focuse rings. Before it was under-corrected it is now over-corrected.

Other issue was the oil started to drain out because of the .004 space. So, with fine line scratches in the glass, I will try with no spacers next time.

My concern, like I said, is the glass making contact with its self because it is not perfectly matched.

Paul


Spacer caused thicker oil layer and therefore overcorrection. You must try with no spacers. Any scratch will not be visible if oil is present. Post some intra/extra focal images when oilspaced with no spacers.

#24 Stipe Vladova

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Posted 15 October 2013 - 07:45 AM

It's fine scratches, no doubt it sad to say.
Now what about if the glass is going to make contact with its self. When I first used no spaces, I noticed rings in one area of the glass. With the .004 spacers in place no such rings appear.

Now, is it actually making contact or is it the space is so close it will make those rings. Will it affect the optics if I go ahead and use no spacers and the oil?

What shall I do?


Paul


Mine shows centered rings too and they are visible only under sunlight or halogen lamp light. They are centered with small circle at the center. They do not interfere with the image. Oil can be easily cleaned.

#25 skywatcher101

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Posted 15 October 2013 - 07:50 AM

@Stipe,

My thoughts as well using the spacers, this caused it to be over-corrected to far.

But, my concern is the lens do not seem to be "matched". The first time I tried the glass looked like it was making contact. It have a faint ring impresion when looking through it at a certain angle.

Now I did not have this when I used the spacers. I guess I have nothing to loose now since I have hair line marks on the lens.

I have looked online for a do it yourself fix. People have used a fine polish to remove marks from glass. Watched a guy on you tube on his glasses and it removed the scratches. Takes some time but it did work. May give it a try. Did a test run on a spare finder scope objective. It did not scratch the lens.

Paul






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