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Baker Reflector Corrector help

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#1 Ajohn

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Posted 04 February 2013 - 01:06 PM

It sounds like this could correct an F3 parabolic mirror over a much wider field than I need. The design is in ATM III but uses old glasses. Modelling them is too inaccurate and leaves far too much for me anyway to correct.

It seems that some one called ?? Gregory sorted this out for modern glasses but I can't find any info on the web. Maybe some one has details?

Failing that full 6fig 3 colour RI's for these glasses would help.

Rolled ophthalmic crown C-1 523586
Dense flint DF-2 617366
Crown BSC-2 517645

It seems that the extreme blue with these glasses is not too good so visual wavelengths would be best.

:question: With these figures I may be able to work out modern glasses for one element at a time.

The field angle seems to be huge - a plate covering a 50% obstruction.

John
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#2 DAVIDG

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Posted 04 February 2013 - 01:22 PM

The glass is Bausch and Lomb material. You can find the data at Bob May's website http://www.webring.o...http://bobma...
By the way we have one of these correctors for use on 24" Tinsley Cass. The Cassegrain front end assembly is removed and the Baker Corrector swapped into place.

- Dave

#3 Ajohn

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Posted 05 February 2013 - 06:27 AM

Thanks for the link.
I missed a comment in the article - optimised for blue light - but he also implies that it's ok for white. So far either r g or b can be diffraction limited over a 5x4in plate film on a 20in mirrored scope with a 17in corrector. It covers a semi angle of 2 degrees. I thought that this was rather spectacular for 3 pieces of glass and a mirror.

Going to play some more though as the focal length doesn't come out correctly so may have made a mistake working out the spacings. Also I suspect that the corrector needs more orders than olso's sliders can give. Unfortunately he only give a sag for the base curve rather than a direct radius. That's +0.00174in and variations in the other digits may have an effect.

John
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#4 DAVIDG

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Posted 05 February 2013 - 09:39 AM

We used the Baker Corrector with 7" x 7" Kodak 103 glass plates.The last time it was used was for a NASA project for Halley's comet back '85-'86 The images were sharp because the sensitivity of the glass plate matched that of the color correction of the Baker design so the unfocused light wasn't being recorded. With modern day CCD's one would have to image thru a filter since the chromatic aberration would be recorded and the star images would be bloated badly.

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#5 MKV

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Posted 05 February 2013 - 11:09 AM

Also I suspect that the corrector needs more orders than olso's sliders can give.

Please post the design. I don't have time to look for my ATM books.

#6 Ajohn

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Posted 05 February 2013 - 03:28 PM

This is the design as near as I can get it to the aricle. Oslo doesn't come up with the correct focal length without the rad on the 1st surface. :confused: Only problem there is if I play around after setting that rad and then reset everything back to how it was the focal length sometimes doesn't go back to what it was.

Somehow I think that there is something amiss with the numbers in the article but I have managed to get distinct r g b focuses that can be bought onto the focal plain one at a time via the corrector. There seems to be a colour focal length problem with it - spots can be ok but can't all come together.

He mentions the nearest curve to the aspheric having a 0.00174in sag and hand figured from a sag of 0.002in. The aspheric should be on surface 1. I've had similar results with it on 2.

If anyone can get it fine in white same or ordinary cheap glasses please post the len file.

I'm switching to a windows laptop as I'm bothered by things not resetting as they should.

John
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#7 Ajohn

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Posted 06 February 2013 - 03:26 PM

There seems to be errors in the rad's he gives for the achromat. Having a better idea now of using the sliders on the schmidt type plate I managed this. Who has a ccd camera with a 5in diagonal chip?

One problem though. The vacpan spread sheet runs perfectly on my libreoffice spreadsheet program but it uses a different type of aspheric constants to olso edu. How do I convert from one type to the other?
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#8 MKV

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Posted 07 February 2013 - 01:24 AM

John, you got as close as possible I believe with the original design. I simply departed form it, split the lens, changed some spacing and the second radius of curvature and optimized the aspheric surface. You can remove the small residual of coma by refiguring the primay to a conic = -1.019, reducing the corrector to mirror distance to 80.2 instead of 81 inches, and refocusing by -0.0017 inches.

Mladen

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#9 MKV

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Posted 07 February 2013 - 01:43 AM

and here is the configuration adjusted for coma as described above.

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#10 Ajohn

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Posted 07 February 2013 - 08:56 AM

The trick to balancing it seems to be the basic curve of the corrector and the achromat. The achromat must have some positive power to flatten the field. It's all a lot to play with.

I stuck with the cemented achromat as 2 less glass surfaces. If I have done my sums correctly 0.001in on the focal plane is just under 0.05 seconds. Pretty impressive at 1.75 degrees.

He points out it's most suitable for 12in plus primary mirrors. I quickly looked at 220mm using BK7 and F4 but the back focal length is too short to get it to the outside of the tube for a dslr. I'm going to try reducing the back focal length from the lens and using a diagonal.

:) If it works out even more encouragement to try and make an F3 to F15 cassegrainian.

The oslo file is attached.

John
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#11 MKV

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Posted 07 February 2013 - 10:20 AM

With the glass Baker used you can't really get much improvement in the polyshcormatic spectrum, save for cosmetic touch-ups. Lateral chromatic aberration is huge, which is why Baker intended the scope to work in the blue and violet end fo the spectrum only, using 103 F film (which was the standard astronomical emulsion).

As DavidG mentioned, that emulsion recorded only a narrow spectral range of colors and was "blind" to all others. It was essentially monochromatic photography. With slight readjustment in the aperture stop to mirror distance to 82.4 inches, practically all residual coma at 0.4863 µm wavelength disappears, and you have an essentially perfect photographic telescope covering 3.5° in the Baker's original configuration. So, there's no need to change anything as long as oyu stay in the intended blue-violet spectrum.

The real problem is not in the design but in recording such a huge field with today's technology. As you already observed, who's got a 5 inch CCD sensor to cover this field? Even a 12-inch version would required a 3-inch sensor. Baker's reflector Corrector was designed to work with large professional astronomical glass plate emulsions of yore on systems that are simply not feasible anymore. Today's most expensive 2x2 inch processors will cover only 16% of the FOV area. Is that "progress"?

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#12 Ajohn

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Posted 07 February 2013 - 02:46 PM

Playing around I had it so that any of the 3 colours could bought onto the focal plain while playing with the corrector plate constants the wrong way. Setting for sa seems easier than trying to use the ray intercepts. I need to find a tutorial on the meaning of the intercepts. There was one on the web but it seems to have gone.

One thing I would like to know MKV is your view on the ideal base rad for the corrector. I notice you have used one that doesn't have the sag mentioned in the article. He describes the lens as a weak plano convex with a focal length of about a mile that doesn't have much effect. Me well I find it does and it's best to get that right before attempting to do anything with the achromat.

:grin: People can still buy film for plate cameras. Roll film too.

John
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#13 MKV

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Posted 07 February 2013 - 07:14 PM

John, the lens as you have it has a focal length of about 1/2 mile. I assumed the NZ to be at the 86.6% of the radius and he actually wants his to be at 70%. That would certainly flatten the first radius quite a bit.

But if you want a pcx lens of 1 mile focal length (1 mile = 63,360 inches) you'll need a radius of approximately 39,000 inches.

I don't do slides, John. I let OSLO optimize the design for me. I just pick parameters. You need to review some of the optimization videos and articles. It will save you a lot of time. I let the program optimize the vertex radius and the deformation coefficients, and that's how I ended up with a much shorter radius on the corrector.

#14 Ajohn

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Posted 08 February 2013 - 05:40 AM

I've used opic a lot MKV. Not GENII though. I'm presented with a box of options with that one and no info so far on what each one does.

Sliders have there place for seeing trends but are very cumbersome to use as oslo comes. I recently read a comment by Ross ( I think ) "Missed because of the extensive use of computerised optimisation which just wouldn't do this".

I would love some links to good videos and web pages on this general subject but if they are there then they are rather hard to find. Oslo optimisation for instance brings up a page on the atmsite on edu and others on LT and premium. I have 1/2 hr every now and again looking. I'm surprised there isn't a list of links somewhere on the web. The atmsite has some but I have looked at those. One I found recently on there points out that the author lacks the experience to change genii parameters! So where are these web pages? I even tried youtube.

It's amazing to think that Baker probably came up with that initially using a SLIDE rule and experience that only a full time optical designer can acquire. ;) I had my time with a slide rule but not on optics.

Also can you tell me what the correct rad would be for a 70% neutral zone? I spent some time with 6fig log tables at home in my youth so using the 70% doesn't surprise me. Less rays. Trouble is that I couldn't entirely get my head round what I was doing. :bawling: More recently I tried reading Conrady.

John
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#15 MKV

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Posted 08 February 2013 - 11:15 AM

John, there is a saying you can lead a horse to the water but you can't make him drink it. I can't do your homework for you. You've been given enough hints to get to the water, but you'll have to do the drinking part.

First, you should have no problems finding help for OSLO optimization online. I just don't buy that you can't find anything.

Second, you need to learn some fundamentals about optics. Open a book such as Kinglake's Lens Design Fudamentals, or Warren Smith's Basic Optical Engineering, or even Rutten and van Venrooij's Telescope Optics, and read. If you don't understand the math then open calculus and geometry books.

Conrady is not light reading, so I don't recommend it. As for Baker, I doubt that he used a slide rule. Probably Conrady's equations. But even if he didn't, he could have retro-engineered the corrector by raytracing.

#16 Mike I. Jones  Happy Birthday!

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Posted 08 February 2013 - 11:57 AM

I would also recommend downloading and giving Don Dilworth's "Synopsys" optical code a try. The fully functional version is available at http://www.osdoptics.com/, with the free version limited to 12 surfaces (compared to only 10 with OSLO). The interactive design tools in Synopsys are the best in the business, MUCH easier than OSLO, Zemax or CODE-V.
Mike

#17 Ajohn

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Posted 08 February 2013 - 03:45 PM

I have been thinking of looking at that Mike mainly as Olso's software standards don't seem to be too good. Being able to write code for it is useful though but I expect there may well be some hidden problems in edu otherwise some one with the right skills could make it work more or less as premium.

Telescope optics is too noddy to be of much use. One of the problems with the "next step up" is notation. There seems to be more than one standard and even where it's actually mentioned. There can be peculiarities eg something I looked at earlier today used a small p in an equation. Curious thing in this instance the text then talks about a P rather an a p, the P is then defined in an equation, not p. In some branches of mathematics capitalisation has very distinct meaning.

Examples in books always work as they have often been "predone" one way or the other. Conrady was a well known source of initial designs to modify. There are others. His books are tough reading but do give some insight into the basics that lie behind lens design as pressing a button on a PC on a design from scratch in all probability wont achieve anything other than layout a scope to an existing design.

Calculus - taking the corrector plate as an interesting instance there is lots on the web but taking the coefficients oslo uses no mention of how they achieve the shape. I'd guess that they represent dimensional changes in the radius of the base curve but they could be dimensional changes that are always along the axis of the system, changes in slope or any number of things. That leaves me and probably others using functions parrot fashion without any real basic understanding. Personally I generally wont do anything parrot fashion. Mathematical abilities do not even relate to that aspect.

I asked for links because of a comment some one else made relating to horses to water. I don't think there are any really just poorly described examples. Actually I used oslo a lot around 10 - 15 years ago and there seems to be a lot less useful info about now than there was then. Might be down to the knowledge = money aspect but more likely down to the lack of useful help which often causes people to loose interest. Also the general fact that many telescopes can be bough for far less than they would cost to make.

John
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#18 Ajohn

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Posted 10 February 2013 - 09:21 AM

:o Must admit I'm impressed by GENII now I know I can edit the operands but can't find a list of what they influence. Some are obvious. Also would like to be able to get rid of genii operands and use the direct normal operands without exiting oslo and starting it up again. These only seem to work axially though. I can't see any way of say pointing them at the 70% zone of the image. Is there one?

Maybe some on can help?

John
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#19 MKV

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Posted 10 February 2013 - 05:54 PM

Must admit I'm impressed by GENII now I know I can edit the operands but can't find a list of what they influence. Some are obvious. Also would like to be able to get rid of genii operands and use the direct normal operands without exiting oslo and starting it up again. These only seem to work axially though. I can't see any way of say pointing them at the 70% zone of the image. Is there one?
Maybe some on can help?

You seem to be the only one who seems to have insurmountable problems with OSLO.edu, even suggesting it's flawed. I gave you the titles of several books by optical experts which would make the concepts involved comprehensible, especially Warren Smith's Modern Optical Engineering (Kingslake's Lends Design Fundamentals is much more manual raytrace oriented).

What you don't seem to understand is that your vertex radius detemrines where the "neutral zone" will be. You have to understand the nature of the optical path difference and how it is structured and only then you can proceed with OSLO. That's not the kind of subject you pick up on a forum's short answer list. OSLO is a bare-bones freebie but it does require some basic optical understanding to be used to its fullest extent.

The alternative is to simply raytrace Baker's system (primary plus field corrector), but without the aspheric corrector, determine the amount of uncorrected spherical aberration and, based on that, obtain solutions for the desired corrector. OSLO can do the rest without any sliders.

#20 Ajohn

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Posted 10 February 2013 - 05:57 PM

This is as near as I can get to what Baker seems to have intended - coma a slight problem at a semi angle of 3 degrees covering an 8in dia circle. Going on the MTF curve it looks like it even nearly meets Rayliegh's 1/4 wave limit at 3 degrees. Astigmatism seems to be the main problem there.20in mirror and 17in aperture.

John
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#21 MKV

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Posted 11 February 2013 - 12:35 AM

This is as near as I can get to what Baker seems to have intended - coma a slight problem at a semi angle of 3 degrees covering an 8in dia circle. Going on the MTF curve it looks like it even nearly meets Rayliegh's 1/4 wave limit at 3 degrees. Astigmatism seems to be the main problem there.20in mirror and 17in aperture.

John, what are you doing??? The spots look good consideirng the angle, but in order to cover a 6 degree field your corrector will be 14 inches in diamater and the clear aperture is 17 inches! That's an 82% central obstruction! That leaves 32% of the clear aperture or equivalent to 5.6 inches inch telescope in light gathering power. You have a 20inch f/3 mirror and end up with the light of a 5.6 inch equivalent aperture, working at an effective f/14 or 15, and an 82% CO. Are actually thinking of making one of these? :confused:

#22 Ajohn

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Posted 11 February 2013 - 09:39 AM

His basic suggestion for any design is to follow the 17in corrector 20in F5 mirror example with a 50% obstruction = 8.5in in this case and the system comes out at F4.5. The 50% obstruction is a constant for any design as he sees it. He shows a spot diagram over 3 degrees but the scale is too small to compare really. The spots for an F4.5 parabola are shown above it. He points out that there is vignetting at both the mirror and the corrector. The recommended plate size for this one is 4x5in = 6.4in diagonal. All I have done is reproduce his spot diagram.

In a way the spot diagram isn't as it should be for photography. As he mentions the lens spacing has to be adjusted for the most even circles of confusion across the field.

John
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PS Yes if I can make it work in colour with a 220mm mirror. Field angle TBD. I need some info on Olso's schmidt constants in relationship to actual glass deformation and their relationship to the formulae the vac pan spread sheet uses. That one isn't provided in EDU.
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#23 Ed Jones

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Posted 11 February 2013 - 06:51 PM

Did you consider a Honders/Busack? I'm working on one myself; all spherical optics using ordinary crown glass.

#24 MKV

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Posted 12 February 2013 - 12:00 AM

I second that Ed, a Busack or a Honders-Riccardi f/3.7 or thereabouts is ideal.

John, here's a Honders-Riccardi 220 mm f/3.7 configuration. The corrector is a simple biconvex BK7 lens, and the primary is a Mangin mirror. The field corrector is an almost a pcx lens. All surfaces are spherical.

It covers a fully illuminated 44 mm FOV or 3.1 degrees.

I would rather do that than mess with aspheric plates and achromatic doublets. Baker's design was intended for monochromatic photography and it can be corrected extremely well for that purpose, but not for polychromatic applications.

I have no shop experience with aspheric plates, but my understanding is that they are not easy. Thank God there are viable alternatives.

As for your questions about the actual glass deformation for various zones as related to deformation coefficients, OSLO provides that information. The Schmidt vacuum method is well covered in literature and tells you exactly how much of a sag you will need for a desired vertex radius plate.

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#25 Ajohn

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Posted 12 February 2013 - 08:30 AM

The whole idea is a design based round a conventional parabolic mirror in my case but that one may be of interest at some point. I suspect that it wouldn't be as good at F3 as the Baker anyway and no need for a high quality chunk of BK7 for the mirror. Interesting though as I think Bk7 normal mirrors can be bought. I can also get the image out of the side of the tube. Not entirely happy with the way I can do that at the moment though.

On correctors MKV the vacpan info that is available to me is based round a corrector curve of Z=Ar^2+Br^4 where Z seems to be the actual numeric deformation of the glass. Oslo EDU doesn't offer that is uses a 4,6,8, and 10th power series and so far I haven't seen any indication of what this means physically on the glass. On none Schmidt scopes people get round the problem by using factors of a Schmidt plate's power. I'm not surprised some have problems. They would also have problems if they tried to put the correction on both sides as per the design you posted. Baker's F4.5 systems plate deviates from a sphere by about 5 waves. Trying to split that and centre it on both sides would be a joke. It usually is on all schmidts. There is also the possibility that Baker's plate deviates from a Schmidt plate. Basically it seems to form part of the colour correction. In blue light actually a very obvious choice of Schott glasses hardly makes any differences at all. Chromatically I'm 90% sure it can still be better than the spot diagram.

I still await an answer relating to the meaning of the olso edu coefficients.

As this is the Baker corrector thread I may as well post the spot diagram again. Pointing out that it still isn't as it probably can be. That's part of the other changes I want to make mentioned in the other thread.

John
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