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Pls Explain Over & Under Correction in Refractors

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#1 spongebob@55

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Posted 19 September 2013 - 02:54 PM

A newbie refractor question: I've seen this in a lot of refractor posts, and I think it has to do with the focusing of CA, either the blue or the yellow?
I've also read that some people change the spacers in achromats in order to change the CA or SA?
How do you know if your scope is either and should you try to correct it?
I'm 'sponging' up all the information I can since I want to buy a larger achromat than the 102mm I have....perhaps 5" or a 6". Considering a ES f/6.5, Astrotelescope f/5.9 or a Synta f/8 so I need to know this stuff to help me decide.
Thanks
Sbob

#2 JimT

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Posted 19 September 2013 - 04:51 PM

These may help. Found these on a web search.

The in-focus image of a star in a good refractor at high power
should only show one faint diffraction ring around the central disc.
There may be another fainter ring outside this, but poor seeing
scrambles these rings together, so it's often hard to make this out.
The expanded rings you get when you defocus should be of equal
clarity on either side of focus. If the outermost ring looks sharper
inside of focus (move the eyepiece closer to the objective) you have
undercorrection. If it's sharper outside, overcorrection. Sounds
easier to see than it is.



http://en.wikipedia....ical_aberration

http://starizona.com...refractors.aspx


Hope this helps. I'm no expert for sure.

#3 T1R2

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Posted 19 September 2013 - 05:15 PM

I think that's about right, put another way view the defocused star image inside or outside of focus with med to high power, if the rings are sharper outside its over corrected, all the rings in my scope are evenly illuminated so I don't go by just the outside ring, but all my rings are sharper looking outside of focus, so mine is slightly over corrected, this rarely affects the views of both over or under corrected scopes.

#4 Eddgie

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Posted 19 September 2013 - 06:52 PM

It is very hard to diagnose the presence of spherical aberration in a refractory without resorting to using an obstruction, but I swore I would never go in to how to star test again.

What I will say though is the sharpness of the rings inside and outside of focus is not at all meaningful for testing for spherical aberration. No one should rely on this as an indicator of either a good or bad objective.

In refractors, color mixing out of focus can easily account for a difference in how the rings look.

#5 Eddgie

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Posted 19 September 2013 - 07:10 PM

First, spherical aberration.

Nice, clear explanation of spherical aberration

This is an aberration where the rays coming from the outside area of the lens fall at a diffent focus than the rays that pass though the center of the lens.

It is not specific to one color or two colors. When spherical aberration is present all of the light from the inside or outside of the caustic cone (which is what the converging rays form) will all either fall ahead or behind the light rays coming from the center of the lens.

There are two symptoms of severe spherical aberration. The first is that light is removed form the Airy disk and distributed in the rings, but most of the light that is refocused will fall into the first diffraction ring. An refractor with severe SA will show a brighter diffraction ring than one without.

The other symptom of severe SA is that a telescope with this condition will appear not to have a distinct best focus. This is because some of the rays coming from the outside of the lens cross the axis before or after rays from the center.

The result is that the caustic cone is stretched out so that the energy that would normally hit the Airy Disk at best focus is not passing though at different distances.. Either before where the center rays are converging, or after.

And this is where the terms under and overcorrected come from.


Re-spacing and why not to do it.

Did you know that an air spaced doublet can be completely free free of coma!

Did you know that a cemented doublet cannot be completely free of coma?

Now, hmmm. One is air spaced and has no coma, one is cemented and has coma.

Gee. What's different?

Well, of course it is the air spacing that gives the designer the ability to completely eliminate coma, and well, it is the exact distance between the curves that makes this possible.

If you change the spacing, you may introduce coma where there was none before.

And while you might change the chromatic aberration of the system, if the designer has done there job, the highest energy concentration in a doublet will be in the C-F lines where the eye is most sensitive. Changing the spacing may change the fringing, but if the system is already putting most of the energy into the green where the eye is most sensitive, what do you think the result would be if you changed the focus of two of the different wavelengths?

If you must have better color correction, get an APO or f/9 ED.

Or, play around with it if you want, but my bet is that the design program is a lot better at setting the system parameters than you will be by chance..

Here is a link that shows the caustic and how spherical aberration stretches out the focus point:

#6 tubehead999

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Posted 19 September 2013 - 07:49 PM

A picture is worth ....

Attached Thumbnails

  • 6090619-Star Test.jpg


#7 PowellAstro

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Posted 19 September 2013 - 08:22 PM

While respacing can and does change the correction for coma, the benefits can and most likely will be very good on overall performance. Also, the amount of change required for most lenses I have seen, will not really effect the coma corrections, as that usually requires a much larger change and only effects the last few percent of a very large field. After respacing mine, I have very tight stars all the way to the edge of a 2.7 degree field!

When I first got my 6", it was so over corrected, the planets were nothing but fuzz balls as best. Stars were surrounded by a bright halo 20 times the size of the airy disk and the halo was so bright you almost could not see the airy disk lost in the glare. Now the planets are razor sharp and the stars are text book with only the slightest halo of very pale blue around them. The blue halo around them is now no larger than 1 to 1.5 times the size of the airy disk!

If you get an achro that does not perform as it should, you can try the respacing. With care you will not do any damage and if you fail to improve your scope, you can always put it back to the original spacing. The only thing that matters is what you see at the eyepiece and not what some software said it should be. Remember, the software relies on humans to enter and receive the data and that can be entered or read wrong. Also, you have no guarantee the person assembling the unit did so correctly or that they picked up the correct spacers for a given unit when they were assembling it.

#8 T1R2

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Posted 19 September 2013 - 09:30 PM

Mr. Powell, was your's to any resemblance to the over corrected image in the above diagram, I'm thinking yours may have been worse than what is shown, and is the one shown acceptable?

#9 PowellAstro

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Posted 19 September 2013 - 10:22 PM

Mine was worse than what is shown. The ones shown above would all give a very good image and would most likely only show a difference between them on nights of the best seeing.

#10 T1R2

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Posted 19 September 2013 - 10:53 PM

Thanks, that gives me more perspective on judging mine, which is pretty good, from that info.

#11 De Lorme

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Posted 20 September 2013 - 01:21 AM

Eddgie,PowellAstro,and TubeHead999, Thanks for explaining{making it easy to understand}
over & under Correction and how to recognize through a star
test what we might have. I really appreciate it. You guys are just great! De Lorme

#12 De Lorme

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Posted 20 September 2013 - 01:24 AM

I didn't mean to forget you Jim. Your great also!
De Lorme

#13 spongebob@55

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Posted 20 September 2013 - 08:24 AM

Yes thanks to all you guys for the discussion. :bow: I'm going to print out these graphics and compare my test with them.
As to shimming, from where do people get these shims? I mean with these precise thicknesses, are they produced somewhere or does one have to make due with cuttings from thickness guage shims or what?
And from what material are they made?
Not that I'm doing this to my 102mm since its views are fabulous.
Regards,
Sbob
Sbob

#14 tubehead999

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Posted 20 September 2013 - 09:46 AM

If you really want to have some fun...get a hold of this book..

http://www.willbell.com/tm/tm5.htm

Some have dogged it , others swear by it. The allusions to audio theory get a wee bit irritating, at least to me; but the book was well worth the price of admission if only for the chapter on spherical aberration. I am either enlightened or even more confused :) every time I am forced to read the same chapter over and over and over.....

#15 PowellAstro

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Posted 20 September 2013 - 12:15 PM

You can use aluminum tape made for AC work. You can stack it for the right thickness then cut your three pieces from it.

#16 spongebob@55

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Posted 20 September 2013 - 04:18 PM

You can use aluminum tape made for AC work. You can stack it for the right thickness then cut your three pieces from it.


I have some of that tape. :grin: Ok, sorry, but it is paper backed.....do you fold it over on itself, or leave one pc with the paper backing on it and start sticking it on top, one by one until you get the thickness....

#17 Rutilus

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Posted 20 September 2013 - 05:33 PM

All my Synta made Achro's have shown some degree of Under-correction. I re-spaced all of them and the end result
was a better scope. I now have a 120mm f/8.3 and 150mm f/8 which after re-spacing are now very nice scopes.
When I viewed Jupiter and Mars with the 6 inch Achro I realised that the game was up for my 4 inch Tak APO
(which had stunning optics), the Tak simply could not match it for planetary detail observed.

#18 PowellAstro

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Posted 20 September 2013 - 06:17 PM

The tape I am speaking of, has no paper backing, it is just plain aluminum. You do not fold it over. Just stick one piece on top of the other until you are at the right thickness. Most of this tape is about .006 inch thick per piece. This way you do not have to use any glue on the lens, just stick it on the edge once you have it the right size and thickness.

#19 JimT

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Posted 20 September 2013 - 06:50 PM

I would love to see a video of someone doing this as they explain it. That would be a keeper vid for sure.

#20 PowellAstro

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Posted 20 September 2013 - 07:19 PM

I have a friend that is going to buy an AR152 and send it to me to do the spacing. When that happens, I will shoot a video of the process and post it.

#21 JimT

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Posted 21 September 2013 - 08:59 AM

PowellAstro

That would be great. Can't wait to see it.

#22 spongebob@55

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Posted 21 September 2013 - 09:12 AM

PowellAstro

That would be great. Can't wait to see it.


Absolutely! Me too! Thanks for your effort to help us all! :jump:

#23 Eddgie

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Posted 21 September 2013 - 01:11 PM

What I would like to see is an interferometer test to show that there is an improvement in Strehl performance.

There are a lot of people selling a lot of services today based on little more than anecdotal evidence that they are "improving" things.

Not my money being spent, so to me it matters not at all, but if the designer could make some meaningful improvement by something so simple as using a bigger spacer, well, logic would suggest that they would do so....

People are always trying to turn acromats into APOs.

People that want APO performance should just buy an APO.

Or a reflector... :roflmao:

#24 PowellAstro

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Posted 21 September 2013 - 05:30 PM

First off, I am not selling anything if your comment is directed to me. Second off, what business is it of yours if a person wishes to spend less and then try to increase their performance? I had an interferometer report provided with my scope showing it was almost a 1/8 wavefront and the image was trash. So, i am just to believe the paper? No thanks. I know optics and what the image should look like and I don't need a piece of paper to guide me. So, I guess you think all the older scopes before this new test equipment was developed was junk, is that what you are saying. Interferometer test are most times done on lenses out of the lens cell and tells you nothing about what it does once it is installed in the scope.

#25 De Lorme

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Posted 21 September 2013 - 07:00 PM

Thanks PowellAstro for doing this. I was going to buy the book but was still apprehensive about adjusting the lens.
Seeing it done would make a world of difference. I belive mine is a little over corrected. The rings are clearly seen
past focus but not quite as defined before focus. Still
you can see that the rings are there. The moon was really great using my 12mm AT Paradigm barlowed. I hope this is not breaking any rules BUT I think we should all chip in and buy you an eyepiece! Your going through a lot of trouble
for us. Let us turn you on to! If you don't mind of course.
De Lorme


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