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kfrederick
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Herring
      #5563797 - 12/10/12 09:21 AM

Dave O sent me some OSLO designs on a Herrig 9.5inch f12 . Any one have a design to share ? Dave s is very good Just thought ask. I think my interlocking box with slots might work for the Herrig . One thing on this one is a 800inch RCcx

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kfrederick
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Re: Herring new [Re: kfrederick]
      #5563818 - 12/10/12 09:35 AM Attachment (80 downloads)

The 9.5 inch could be made with two 14.5 quartz blanks. Only two spherical surfaces . If a box could be made to hold them perfect . I tried one 10 years back but failed .math error . The .7 inch thick quartz is light and strong . The optical box could be light weight 4.5 ft long Maybe a box like this

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DAVIDG
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Re: Herrig new [Re: kfrederick]
      #5563850 - 12/10/12 09:54 AM Attachment (61 downloads)

Dave O and I have been playing around with this design for few months. There is a BASIC program that was written to calculate the design and one of the functions is to save the output in the format that WINSPOT can read. I modified the program a tiny bit so it it was easier to run. You need access to QBASIC to run the program. Luckly I was able to find and old complier.
For those that don't know what the Herrig design is, it uses a CONVEX primary and Conave secondary and the light bounce off each twice. It also unobstructed. The correction is excellent. The only issue is figuring the long focal length convex primary. My thought is to make it out of Quartz because the low thermal expansion but more importantly Quartz is pretty clear optically so you can test thru it. This would allow one to first grind and polish a concave sphere on the back surface, the purpose of which would be to cause a null condition when the long radius convex surface was also a perfect sphere and you tested thru the concave surface. Just like one does when testing a Schupmann corrector.
Here is a spot diagram for 7" f18 version for 0.5 degree field of view. The black circle is the size of the Airy disk. As one can see it is better then diffraction limited out the very edge of the field. There are very few optical designs that are this good. It definitely on the "bucket list" of designs I would like to make.

- Dave


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DAVIDG
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Re: Herrig new [Re: DAVIDG]
      #5563854 - 12/10/12 10:00 AM Attachment (83 downloads)

Here is the OLSO file for the above 7" version which could be made from 8" blanks.

- Dave


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DAVIDG
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Re: Herrig new [Re: DAVIDG]
      #5563900 - 12/10/12 10:34 AM

Here is a link to the BASIC programs to calculate the Herrig design. By the way the correct spelling of the design is HERRIG, no "N". http://bhs.broo.k12.wv.us/homepage/alumni/dstevick/software.htm

All the Best,
- Dave


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kfrederick
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Re: Herring new [Re: DAVIDG]
      #5563933 - 12/10/12 10:55 AM

Unobstructed /color free / Only two optical{ spherical} surfaces .Thanks Dave

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Mike I. Jones
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Re: Herring new [Re: kfrederick]
      #5564012 - 12/10/12 11:44 AM

"THEN, you must chop down the mightiest tree in the forest wiiiiiiith.... A HERRIG!"

Definitely on my retirement bucket list as well! Even though Cuzzin Ed rightfully questioned my sanity for using two 12.5" blanks to make an 8" telescope, I just GOTTA see it work! The Herrig is one of those "that CAN'T work but it does" designs, a truly weird, non-intuitive root to an aberration polynomial for sure.
Mike


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kfrederick
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Re: Herring new [Re: Mike I. Jones]
      #5564074 - 12/10/12 12:26 PM

Seeing EDs design he posted on Marks thread got my one brain cell to think that my box with the slots would hold all his optics in perfect 3d . Then I thought about Herring only two mirrors on center line and just tilted .

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tim53
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Re: Herring new [Re: kfrederick]
      #5564107 - 12/10/12 12:52 PM

What is this? A 'fractor or a 'flector?

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kfrederick
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Re: Herring new [Re: tim53]
      #5564115 - 12/10/12 12:55 PM

4 reflections and maybe more after the Dave s And Jones get done

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MKV
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Re: Herring new [Re: Mike I. Jones]
      #5564171 - 12/10/12 01:15 PM

Quote:

The Herrig is one of those "that CAN'T work but it does" designs, a truly weird, non-intuitive root to an aberration polynomial for sure.
Mike



Hmmm, for sure. Looks good 'on paper". With four reflections, I can just imagine what the tolerance are....good luck! Does anyone know of a Herrig that was actually made and objectively tested? Or is it all (OMG!) ancedotal?

Mladen

Edited by MKV (12/10/12 01:16 PM)


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Mike I. Jones
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Re: Herring new [Re: MKV]
      #5564191 - 12/10/12 01:25 PM

Oh yes, very real: November 1997 Sky & Telescope; p. 113,

and on Dave Stevick's site at

http://bhs.broo.k12.wv.us/homepage/alumni/dstevick/herrig/newtct.htm

I also put up the 8" design a LONG time ago here that I'm going to make here.

Although, I want to improve on Herrig's mailbox-like enclosure design. Seems very leaky from a stray light standpoint, and the aperture stop is steeply slanted rather than perpendicular to the optical axis, which is better.

Mike


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mark cowan
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Re: Herrig new [Re: DAVIDG]
      #5564370 - 12/10/12 03:21 PM

Quote:

My thought is to make it out of Quartz because the low thermal expansion but more importantly Quartz is pretty clear optically so you can test thru it.




This will only be true if the quartz (AKA fused silica) is of transmission grade. The majority of what I've seen from surplus sources isn't - it contains many tiny bubbles and won't pass a coherent wavefront. The only true test I know of to differentiate the two is to try it as a transmission piece.

Best,
Mark


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DAVIDG
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Re: Herrig new [Re: mark cowan]
      #5564395 - 12/10/12 03:40 PM

Mark,
I've work with Quartz up to 5" in diameter that was pretty good but I understand that the bigger stuff might have issues in transmission. If worse comes to worse then there is always BK-7. While it doesn't have the thermal expansion coeff. that Quartz does, there are plenty of large optics, that have elements made of the stuff. Just going to need to wait longer to test and to let the finished scope cool down.

- Dave


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mark cowan
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Re: Herrig new [Re: DAVIDG]
      #5564518 - 12/10/12 05:03 PM

Hi David,

If it's semiconductor surplus it just plain varies. You can spec it if you're buying new but the price goes nuts. The way it's made determines the bubbles.

The material from RecoLabs has a small % of transmission quality pieces and it's not hard to test for that as they typically are plane-parallel and polished (not optically though). If a laser shows no scatter I've just put them on the stand in front of a good mirror and check the Ronchi - scatter is immediately obvious.

Material from Five Star Optics (formerly Superior Optical) may or may not be - I haven't tested any of it yet as it requires working for transmission tests and SFAIK neither has anybody else...

Best,
Mark


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ccaissie
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Reged: 09/13/10

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Re: Herrig new [Re: mark cowan]
      #5564709 - 12/10/12 06:54 PM

There was some grumbling over the 4 surfaces, etc., in the earlier posts to Jones. Assuming the ability to make some nice spherical surfaces, how does this design tolerate angular misalignment? spacing? radius of curvature?

Relative insensitivity to tolerances could make this a desirable design.


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MKV
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Re: Herring new [Re: Mike I. Jones]
      #5564782 - 12/10/12 07:52 PM

Hi Mike, I think you and Ed Jones covered all the points on your archived thread - except one: the four reflections. I mean if autocollimation doubles the the wave error, I can just imagine what will four reflections do to A 1/10 wave P-V error!

Mladen


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MKV
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Re: Herring new [Re: Mike I. Jones]
      #5564792 - 12/10/12 07:57 PM

Quote:

on Dave Stevick's site at



Great. And how did it perform? I can't open the "Examples" files.

Mladen

Edited by MKV (12/10/12 10:32 PM)


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kfrederick
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Re: Herring new [Re: MKV]
      #5564879 - 12/10/12 08:49 PM

I think there are some that use a 3 mirror for the last reflection

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Mike I. Jones
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Re: Herrig new [Re: mark cowan]
      #5564894 - 12/10/12 09:00 PM

I'm not following the issue of transmission. The Herrig is all-reflective. It is true that bubbles that grind through will scatter some light. But the internal transmission of the mirror substrates is immaterial.
Mike


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kfrederick
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Re: Herrig *DELETED* new [Re: Mike I. Jones]
      #5564898 - 12/10/12 09:02 PM

Post deleted by kfrederick

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Mike I. Jones
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Re: Herrig new [Re: kfrederick]
      #5564899 - 12/10/12 09:03 PM

OHHH - "Never mind"!

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ccaissie
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Loc: Whitefield, Maine
Re: Herrig new [Re: MKV]
      #5565032 - 12/10/12 10:32 PM

I ran several scenarios of the file in OSLO. Relatively INsensitive to spacing, radii, and angles.

C


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ccaissie
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Re: Herrig new [Re: Mike I. Jones]
      #5565060 - 12/10/12 10:55 PM

Quote:

I'm not following the issue of transmission. The Herrig is all-reflective. It is true that bubbles that grind through will scatter some light. But the internal transmission of the mirror substrates is immaterial.
Mike




Testing the convex surface through the back as a concave?

A.Leonard wrote in ATMT 2 about this in making convex hyperboloids. pp57,58, and in making mak correctors.

Right, needs to be homogeneous to a high degree for testing through the back.


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MKV
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Re: Herrig new [Re: ccaissie]
      #5565066 - 12/10/12 11:03 PM

Quote:

I ran several scenarios of the file in OSLO. Relatively INsensitive to spacing, radii, and angles.

C



The spacing, angle and radii were never even mentioned. If you read Mike Jones' archived thread from 2006, he and Ed Jones mention the "cons", one being that you need a 12.6" and an 11.2" mirror for an 8-inch clear aperture and a substantial loss of transmission on each reflection x 4 Ed put is succinctly: "What do you like about it? Four reflections, 2 oversize optics, doesn't have much going for it."

I think four reflections will mulitply even the tiniest errors on the mirrors wavefront and make them stand out. It's a well know fact that in autocollimation testing you have doubling of the errors even with fewer reflections.

Mladen


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Dave O
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Reged: 12/21/11

Loc: Sri Lanka
Re: Herrig new [Re: MKV]
      #5565177 - 12/11/12 12:04 AM Attachment (29 downloads)

Well, let's be fair here ...

1. Yes, there are four reflections; but only two from each of two mirrors. (And they are offset from each other ...)

2. Surfaces are spherical, so easier to get smooth by most people's experience.

3. Many SCT's use 3-mirrors (including the diagonal); so really only ONE extra reflection ... and an even number of reflections will orientate the image the same as what you'd see in a Newtonian.

4. As there are only two mirrors, one could go with the enhanced coatings and improve the image brightness.

5. Yes, some glass is worked that is not used ... however, the quality of the image speaks for itself -- I think it is a good trade. (And one gets the 'option' of orientating the mirror to present its best face to the incoming light, so to speak.)

6. Four 'optical surfaces' -- but only two elements to align with each other (+ the focusser). As they are both spherical, there is some leeway on 'centering' -- just need to get the separations and tilts right.

7. In my mind, the real difficulty in the design is supporting that upper mirror ... but, other designs have gotten around this (Schupmanns, Yolos, etc.).

8. It does have some field tilt (generally <5 deg. from what I have seen) -- so, this will have to be accounted for as well.

Attached is the OSLO file I had sent to Kevin. It is larger than I'd build one, but as most of us know, Kevin likes big scopes.

The Herrig can be as fast as f/9; but aberrations do increase with faster focal ratios.

I'd also like to extend a big thanks to Dave G for getting those Basic routines up and running. The Herrig is a very easy scope to design using those routines -- enter your desired aperture and focal ratio, and it spits out a well-corrected design, which can be ported to Winspot for analysis. From there, it is an easy thing to enter into OSLO.

I had mentioned to Kevin that Dave G had some ideas for testing that long CX surface; was happy to see he mentioned it here ... Dave G is a pretty clever guy who knows his stuff.


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MKV
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Re: Herrig new [Re: Dave O]
      #5565270 - 12/11/12 12:57 AM

You make good points, Dave O, but we all know that each reflection doubles, or nearly doubles, the wavefront error.

The mirrors are spherical, but OSLO shows that conic constants of both mirrors, to a reasonable degree, do not really affect the image that much. One has to really go off the scale to begin to see deterioration that cannot be refocused.

I think the best and easiest way to test a convex surface is using a test plate. One easy way to do that is to use a full size glass tool as your test plate. Nulling a front surface through the back requires raytracing to determine the exact position of the light source. But it also requires polishing and figuring the back of the mirror flat (assuming ou have a quartz mirror). You might as well make a tests plate from cheaper glass instead. Testing through the mirror doesn't really save on effort, imo.

Mladen


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Mark Harry
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Re: Herrig new [Re: MKV]
      #5565434 - 12/11/12 06:11 AM

Making a convex spherical surface is not difficult. If certain accepted methods/parameters are used, very little change in ROC ocurrs, and the glass will readily adopt a smooth zone-free surface. Very little testing will be required; likely not until near the very end of the job. Preliminary curve could be done with a good spherometer. A 1/2 diameter testplate could be used if preliminary setup is done correctly as outlined above.
******
What I'd like to know, is what the variance of the ROC of the long convex surface could withstand- could it vary by 5% (+ -) without a bad effect?
******
This design has a particularly fascinating quality about it.
M.


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Mark Harry
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Re: Herrig new [Re: Mark Harry]
      #5565440 - 12/11/12 06:21 AM

I see that DMG's version is actual 6.5" diameter. I sized it for 6" and a 1 degree field- still very high quality spots, and smallish errors!

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Dave O
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Re: Herrig new [Re: Mark Harry]
      #5565470 - 12/11/12 07:22 AM

Quote:

What I'd like to know, is what the variance of the ROC of the long convex surface could withstand- could it vary by 5% (+ -) without a bad effect?



The ROC of the first mirror largely determines the system's final focal length, so if that 'spec' is loose, then one could fab the first (CX) mirror to get as smooth a sphere as possible, somewhat close to the desired ROC. With that mirror done, the system can be re-optimized to the measured ROC (with the resulting change in focal length) and the second (CV) mirror could be figured to match without any appreciable change to the system performance (other than focal length/ratio).


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DAVIDG
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Re: Herrig new [Re: Dave O]
      #5565504 - 12/11/12 08:09 AM

The issue with testing the convex surface with a test plate is the convex surface has a very very long radius so the concave test plate needs to match this. So you need to make that long radius concave test plate. So your back to the same problem of testing the tertary mirror in Tri Schief ie having the mirror many yards away and testing out doors. The accuracy of the concave test plate determines the accurary of the convex piece your testing with it. If you test the convex surface by testing thru the back and using a much short radius sphere to setup a null condition then the accuracy improves. To me there is nothing better then actually see the surface under a knife edge/ ronchi type null test.

- Dave


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kfrederick
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Re: Herrig new [Re: DAVIDG]
      #5565570 - 12/11/12 09:14 AM

The 6 inch f 12 looks like a easy risk .The concave is a 8 inch f 10 spherical [160 inches] .And the convex is 503 inches RC spherical . I think some times it will need a 90deg diagonal like a cass. does .A sloted interlocking CNC water jeted aluminum box will cost around 100 dollars. Big question the there are 4 tilts and two mirrors I need to know the tilt of each optic .Just talking. Told the wife the house will be finished before any more telescopes .

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MKV
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Re: Herrig new [Re: DAVIDG]
      #5565743 - 12/11/12 11:15 AM Attachment (16 downloads)

DAVIDG is right. For relatively long radius surfaces, testing through the back surface may be the only practical approach, which is by no means as simple as it seems. It involves grinding and finishing both surfaces to optical standards, and calculating exact position of the setup. It also requires a nulling lens and a diagonal mirror, etc.

The Herrig seems rather insensitive when it comes to the sphericity or asphericity of the mirrors, so a smooth surface rather than an exact figure appears to be a priority given that there are four relfections, imo.

Below is an example of the Herrig configuratuion provided by Mike Jones. The upper set of spots is as provided by Mike. Both mirrors are spherical (cc = 0). The lower set of spots represent an altered setup, where the convex primary is a rather strong prolate ellipse (cc = -0.5), and the secondary is a full parabola (cc = -1).

Mladen


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kfrederick
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Re: Herrig new [Re: MKV]
      #5565777 - 12/11/12 11:37 AM

Good to know that a long focus newt mirror can be used for the secondary .But keeping the surfaces spherical makes things easy.I saw one that had two mirrors instead of the one secondary .Guess that free it up for a different RC on the last surface..

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Mark Harry
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Re: Herrig new [Re: MKV]
      #5565851 - 12/11/12 12:26 PM

You guys are making this more complicated than it actually is to do. CXs are relatively easy; I've made thousands of them. They're easier than flats, or long CVs. (-IF- you have initial parameters correct when you start off, and prepare the polisher correctly.) If you spend too much time worrying whether you 'theorized' enough, it can literally stop any meaningful progress in making a (good) CX surface, period. A 1/2 diameter TP can be made, polished/dummy-shined in 15-20 minutes, and with reasonable common sense can ascertain whether the CX surface is smooth or not, has no edge issue,zones, etc. By the time you figure out a null test thru the back side to take a look, a lot of progress could have been made in the meantime- up to you. Dave is somewhat correct about TP determining the accuracy of the surface; but it doesn't necessarily have to be as precise as the final tolerance desired. Ideally it shouldn't have a convex fit to the optic, and be just reasonably smooth. If not, you can use the optic to 'test' the TP, but this might be a bit frightening for the inexperienced to contemplate and get a handle on.
BTW, CXs normally don't go prolate during normal polishing. -THAT- is rather difficult to accomplish even deliberately. This can be a big help trying to get a sphere.
*******
How sensitive is that CX first radius ??????????????????????????????????????????????????????????????? Can you get to within 5%???

MVK, it almost looks like if you deformed one or both of the optics somewhere between (less) than what you did above, it could make it better than an all-spherical scope....?
M.


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Dave O
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Re: Herrig new [Re: Mark Harry]
      #5565928 - 12/11/12 01:22 PM

Quote:

How sensitive is that CX first radius ??????????????????????????????????????????????????????????????? Can you get to within 5%???




Perhaps I am not understanding the question -- the answer depends on how much variance you can tolerate in the final focal length -- similar to a Newt. For any primary (CX) ROC, there is a matching secondary (CV) that will produce similar images -- but, the system focal length will change.


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Dave O
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Re: Herring new [Re: Mike I. Jones]
      #5565971 - 12/11/12 01:41 PM

Quote:

Although, I want to improve on Herrig's mailbox-like enclosure design. Seems very leaky from a stray light standpoint, and the aperture stop is steeply slanted rather than perpendicular to the optical axis, which is better.




When I saw that picture, I was thinking that what it really needs is a 'tilt up' top that when 'closed' covers the optics, but can be opened for use. It would be hinged back by the eyepiece end and could include baffles as well. I think it would be very neat -- folded up into a nice box for transport.


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DAVIDG
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Re: Herrig new [Re: kfrederick]
      #5565992 - 12/11/12 01:51 PM

Mladen,
When you have long radius optics, if you see any signs of an aspheric surface at all on the test stand, the conic constant will be very large, like on the order of 100 to 1000 or more. The conic on the back convex surface my Schupmann objectives is -1128 and the radius is 302" convex. I didn't have to chisel the glass away to achieve that conic. It only took some mild polishing and on the order of what is done to parabolize a much shorter radius surface. So your analysis is somewhat misleading to the insensitivity of the Herrig design to figuring errors. If you don't get to smooth and spherical surfaces you could have problems.

- Dave


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kfrederick
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Re: Herrig new [Re: DAVIDG]
      #5566080 - 12/11/12 02:42 PM

At 800 inch radius cx could a flat be used? Not much difference.

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DAVIDG
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Re: Herrig new [Re: kfrederick]
      #5566096 - 12/11/12 02:50 PM

Quote:

At 800 inch radius cx could a flat be used? Not much difference.



Kevin,
No you need that convex surface, how ever long it is.

- Dave


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MKV
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Re: Herrig new [Re: DAVIDG]
      #5566204 - 12/11/12 04:10 PM

Dave, yes of course your concic constant must be huge to be perceptible on a very long focus surface. The problem is that when the surface appears spherical it may be quite a ways off anything close to spherical. The reason is the limiting resolution of the test method.

Take, for example, an 8-inch mirror with a radius of curvature of 480 inches, and a focal length of 240. It's an f/30 mirror. If it is a parabola (cc = -1), its sagitta is 1/60 of an inch or 0.01666667 inches. If it is a perfect sphere, its sagitta is 0.016666957. The difference between a parabola and a spehere of that diameter and focal ratio is 0.0000003 inches (3^-7), or 1/74 wave, which means that the resulting wavefront error would be 1/37 wave.

Do you really think you can see that difference, given the air currents, and numbers of glass surfaces which themselves could have additional errors? Do you have a k-e stage that can record such difference?

Assuming autocollimation and testing at the focus (240 in) instead of RoC, you are still talking about a difference well beyond common mechanical measuring devices.

Now, let's assume the mirror is not really a paraboloid but a slight prolate ellipse (say, cc = -0.25). The sagittal difference between the two wouldbe even smaller, 0.0000001 (1^-7) or about 1/200 wave. Then on relfectionn the wavefront error difference would be 0.0000002 or 1/99 of a wave.

There is just simply no way I can see how such surface can be reliably figured to a sphere no matter what method you use Maybe I am missing something.

The good thing is that OSLO tells us that, even if the Herrig mirrors depart from sphericity by substantial amount, the system would still perform well.

Mladen


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Re: Herrig new [Re: Mark Harry]
      #5566239 - 12/11/12 04:28 PM Attachment (13 downloads)

Quote:

MVK, it almost looks like if you deformed one or both of the optics somewhere between (less) than what you did above, it could make it better than an all-spherical scope....?
M.



Not really, Mark. Here is one with the primary deformed to an oblate ellipsoid (cc = +0.2), and the secondary to a prolate ellipsoid (cc = -0.3)

The performance is the same as if they were spherical. For some reason, the Herrig seems to be resistant to any conic deviation beyond any reasonable expectation.

Mladen


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Re: Herrig new [Re: MKV]
      #5566244 - 12/11/12 04:32 PM Attachment (16 downloads)

And here is the same configation except both mirrors are extreme hyperboloids (cc = -3.0), and then refocused!

Mladen


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Re: Herrig new [Re: DAVIDG]
      #5566255 - 12/11/12 04:37 PM Attachment (15 downloads)

I worked out a way to test the convex primary mirror for the 166mm design I posted. Using this test method one can get a direct look at the surface of the convex primary. One would need to make an addition 8" f/7 concave mirror and then figure it to an ellipse that has a conic of -0.76. One be could off by +/-0.05 in the conic. You then set up the mirror like your doing a double pass autocollimation test but instead of a flat you use the convex primary mirror from the Herrig as the "flat". The spacing between the two mirrors is set to 80" but it can be off by a few inches and it doesn't make much difference.
One uses a small Newtonian diagonal placed at 45 degree that isn't shown in the drawing of the test layout. So the optical test layout looks like a type newtonian facing the convex primary. The Foucault/Ronchi tester is off to one side. So light from the tester is directed at the 45 degree mirror, then to the 8" f/7 concave mirror then to the long radius convex mirror, back to the 8" f/7 and back to the tester. Both the distance from the tester and too the 8" f/7 mirror and back from it the mirror to focus point are the same. So you use a tester were the light source and knife edge/Ronchi screen are coplanar. This forces them to always be the same and at the correct spacing. So you don't have to measure this. You just move the tester until you find the focal point. Now you figure the convex mirror until the system nulls like a sphere being tested and shows straight Ronchi lines. If the 8" f/7 is smooth, the conic is close on it's surface and one achieves a null condition by figuring the convex surface, the convex surface will be smooth and actual figure will be no worse then mildly aspheric with a conic value of 100 or less. If you plug a conic of 100 into OLSO for the convex primary one gets a PV value of 1/10 wave for the complete Herrig system. Too bad and now one has chance to make to optics so the actual telescope performs like the theorical one.
- Dave


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Re: Herrig new [Re: DAVIDG]
      #5566258 - 12/11/12 04:40 PM Attachment (28 downloads)

Here is the OLSO file.

- Dave


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Re: Herrig new [Re: DAVIDG]
      #5566492 - 12/11/12 07:40 PM Attachment (13 downloads)

DaveG, I love the null test, except that it is a null no matter what conic constant you assign to the f/51.4 primary. Here is your test except with the primary deformed to a hyperbola whose conic constant is -12! The off-axis performance actually improves the more hyperboloidal the mirror becomes! You don't even have to refocus.

I think this only confirms my skpeticism about knowing when your very very long radius mirror is actually a sphere.

Trying to get it to be spherical simply doesn't change anything, even if you could determine when such a mirror actually is spherical.

Added: Therefore, I'd say polish the convex mirror, center over center, until polished out - its final figure will surely fit the super generous conic tolerance envelope. I don't think any fancy tests are really necessary. What this boils down to is that the convex mirror really doesn't have to be spherical at all.

Regards,
Mladen

Edited by MKV (12/12/12 01:59 PM)


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Re: Herrig new [Re: MKV]
      #5567035 - 12/12/12 06:29 AM

Mladen, I think I see why there is a problem you are having understanding how to detect such a long sphere.
You figured the sag of the paraboloid as being .0167"- which also represents the movement of th KE stage with moving source. (rounded off) Fixed source would be .0334"
A perfect spherical surface would be 0.000" on the KE stage. (~1/32" difference, or about 3/4mm)

I have a good (fixed source) KE rig made with a lathe cross-slide, dial indicator, and KE with no slop or backlash. At 200" ROC, I can detect/repeat measurements to .002" on average, and often hit measurements well under that. A bit less than 480" is the limit of the length of my cellar, but occasionally I get contorted and make a reading or two at the major part of that length depending on what I'm doing. Never at any time has this sensitivity exceed + - .005"; within .010" when that far out there; depends on exactly what I'm testing, and how large it is. That puts the mirror you use above, being tested with around 1/100th wave sensitivity. These are conservative parameters. last mirror I tested that had a 200" radius, I was hitting the measurements to .001".
*******
I just made a rather large 8" convex surface, and used a 4.5" dummy-shined TP to check the regularity. The radius was 39.125" and is the front surface of a positive meniscus. The TP had a ripped edge, but the central half was a really close slightly concave match to the meniscus surface in question. I used this area to screen the surface. It took about 20 minutes to make the TP, and about 80-90 minutes to pitch polish the meniscus surface smooth and with no edge problem. (-ZERO- tde) The central area of the TP remained a pleasing and accurately round bullseye, anywhere on the meniscus, from center to edge. If there was any edge problem left, the round bullseye would have adopted the shape of a lightbulb for a moderate edge problem, and a horseshoe, or 'omega' symbol for a bad edge. The rest of the meniscus surface is beautiful.
The TP? I don't really care about being all that fussy with it. Later on, it will be turned into something else; so I'm not about to waste a bunch of time to dream up all kinds of tests, or spend a lot of time worrying about the TPs edge. I put the "horsepower" where it belongs- on the meniscus. This makes perfect sense considering I'm not new to contact IF testing- saves me a ton of needless work, most certainly!
M.


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Re: Herrig new [Re: Mark Harry]
      #5567538 - 12/12/12 12:52 PM

Mark Harry, my understanding is that a real difficulty with Foucualt testing a mirror at f/30 or slower, such as used in the Herrig configurations, is in the lack of intensity of the shadows. As you undoubtedly know, the intensity of the shadows increases with faster focal ratios, and decreaes with slower. This is proportional to the optical path difference (OPD) of the wavefront. The smaller the OPD the fainter the shadows.

In my example, I used an 8-inch mirror of RoC = 480 inches (40 ft), f30, and David G's configurations calls for an 8-inch mirrors of RoC ~ 823 inches (68.6 ft), f/51. Both mirros have extremely small OPD values even if parabolic in profile, and therefore would present very faint shadows, if not indsicernible shadows, even if we assume all other conditions, such as air currents, etc) were controlled during testing.

At conic constants closer to the sphere, say cc = -0.25 instead of -1 (parabola), the shawods would become even more difficult to discern. The only rational conclusion from this is that it would be difficult to know if your mirror is truly spherical or not. But, the good news is that in a Herrig it really doesn't matter. The configuration has a very generous tolerance range when it comes to that. Any well polished Herrig convex mirror would probably fall well inside the tolerance envelope.

You describe much faster surfaces, whose shadows could still be clearly discerned. At f/10, or f/12, the shadows can easily be seen under the k-e. At f/30 with great deal of diffculty, if at all, and at f/51 I seriously doubt it.

Regards,
Mladen

Edited by MKV (12/12/12 01:47 PM)


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Re: Herrig new [Re: MKV]
      #5567656 - 12/12/12 02:03 PM

All I can say, change your test, or modify it to get the -REPEATABILITY- down to the range where you can be sure what it's telling you about error and what type to ascertain with any reliability what you're saying or assuming is correct. I've told you what a reasonably done measurement in relation to ROC can reveal.
******
In the past you've advocated accepting 1/4-1/2" wide error as being acceptable for edge error. In IFgrams, you've "verified" reductions of .33 wave by XP and ZEMAX as 2 fringes (direct observed) error. Do you deny this????? Do we have to discuss this further????
I think that most comments by you should be taken in context with your actual capability and assessment of what you -THINK- is the "real world" so to speak.
*********
If you cannot get your mesurements reasonably close to what I state, then I think you should refrain from pontifying anything about test procedures, accuracies or conclusions; for they certainly are not legitimate, or correct in any sense.
Maybe you should stick to selling aspirin? Just a thought...
At any rate, I view your past posts as minimizing the accuracy necessary to obtain a respectable performing optic, or to minimize the accuracy required to be considered legitimate. What -IS- the case???
M.
**********
If I could get one of you guys to get me a -TOLERANCE- on the 5% I requested (now 3 times!) I am sure if you supplied me the glass and that tolerance is acceptable, I could get you an optical set within 1 week, and I could enjoy my cup of 'joe' in the morning, and not feel rushed, one bit; and it would be a shining example of what this scope design would offer. End of story.
********

The A36-(forerunner to the P51 Mustang) was designed, built and flown in 117 days. The rate this discussion is going, there would be no hope of that ocurring at all- a shame, and a bow to EGO- an expensive mistress!!!!
Cordially,
M.


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Re: Herrig new [Re: Mark Harry]
      #5567673 - 12/12/12 02:13 PM

I've got some glass sitting around. What version of this would you like to do? Somebody get him the 5% please!

I like the TP approach - was trying to figure out the easiest way to test one of these suckers but that didn't occur to me.

Best,
Mark


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Re: Herrig new [Re: mark cowan]
      #5567735 - 12/12/12 02:34 PM

TP verification has been in practice for over 100 years or so!
Hi there, Mark!
M.


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Re: Herrig new [Re: Mark Harry]
      #5567787 - 12/12/12 03:03 PM

Heaven forbid I ever think of a shortcut to lessen the work load with TP mfr, or testing procedure. I guess I should understand, most folks that haven't had to ever think of this stuff, might be a bit slow on the uptake on what I'm suggesting.
Sorry, please accept my appology.
M.


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Re: Herrig new [Re: Mark Harry]
      #5567895 - 12/12/12 04:18 PM

Mark (H),

Didn't Dave O. answer your tolerance questions in posts #5565470 & #5565928?

My take on what MKV said about testing the really long radius spheres needed for this design was that it was a matter of not being able to resolve the shadows sufficiently to tell if you really had a null or not (or I guess Ronchi line straightness); not anything to do with repeatability. I'm currently working on a 313" or so radius convex surface and it's not easy (for me) to accurately determine the null/line straightness at that distance--at least compared to something in the under 100" range.

(Darn, now I'm thinking about accuracy vs precision and wondering if I got it right?)

As a side note, I'm surprised to see you using the term "pontifying". I would have thought you would be more sensitive about that sort of thing. I'm assuming you mean 'pontificating' but if not and "pontifying" is actually a compliment I withdraw my comment...

Now the "aspirin" remark sure sounded like the dreaded snark to me...

Gary


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Re: Herrig new [Re: Mark Harry]
      #5568580 - 12/13/12 01:43 AM

Re: this is about Mark Harry's passionate (putting it mildly!) response to my skepticism about being able to test an f/30 or f/51 Herrig-type mirror using the Foucualt knife-edge method and obtaining "repeatable" results.

Mark Harry: As I sad to you privately, I am the old school, and the old school taught me that, the slower the mirror, the fainter the shadows. This is ATM 101.



Note: 200 mm diameter paraboloid (cc = -1), tested at RoC, compared to 200 mm sphere also tested at RoC.

Mark, you can talk the talk, but I believe the pictures, because there is good science behind them. Unfortunately, based on your post, you don't seem to have that science down. I find that odd.

Even under ideal conditions, no air currents, etc., at about f/20, the parabolic shadows are so faint they are difficult to discern. At f/30 and f/51 they are indistinguishable from a sphere.

So, then, Mark, what does your "repeatability" have to do with this?

Better yet, please explain, what measurements are there to "repeat" in a clean null? Your precision x-y table is not going to help you measure something you can't see!

This is so basic, so fundamental that it's actually embarrassing to me to have to teach you this, because you advertise yourself as an optical expert. Strange, isn't it?

Just how basic is this? Well, it can't get any more basic than this. Please, open Albert G. Ingalls' Amateur Telescope Making, Book One, the 1978 reprint edition (small red hardcover book), and turn to page 10 (yes page 10, it's that basic!).

There is a drawing of four shadowgrams. If you look at shadowgram F, listed as "Long Paraboloid", you seen barely perceptible shadows! Bingo! Anyone who has read ATM Book One knows this. So, then, please explain why you don't?

As I've already mentioned to you, the intensity of shadows is proportional to the optical path difference (OPD) between real (deformed) and ideal (perfectly spherical) wavefronts.

From this we know that the slower the focal ratio the smaller the OPD and therefore the fainter the shadows will appear. The actual and ideal wavefronts will approach a point of no distinction.

At this point the sphere and the parabola will look the same in a K-E test, i.e. a "flat" disk disk, with longitudinal travel of exactly 0.0000". There is NOTHING TO REPEAT, Mark!

So, based on this fact, I ask you, with regard to DAVIDG's f/51 Herrig convex mirror: Given the extremely small OPD of the surface at that focal ratio, how will you ever know the surface is spherical, oblate, prolate, parabolidal or even hyerbolodial? Simple: You won't (and can't)!

Mladen


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Re: Herrig new [Re: MKV]
      #5568603 - 12/13/12 02:19 AM Attachment (15 downloads)

The slowest mirror I've tested so far was a 10" CA f/18 mirror spec'd as a "paraboloid". I did a companion sphere at f/9 which was easy to test with a Foucault derived Strehl of 0.993 - and P/S IF supplied real Strehl of 0.994. The longer one was not so easy, but quite doable. The client (ITT Space Systems) kind of laughed over the paraboloid spec; indeed...IIRC the figuring time was about 3 minutes total.

In any case...interestingly enough these simulations show the shadows would be resolvable with imaging - I stretched the contrast to bring out the real detail in the bottom row.

Best,
Mark


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Re: Herrig new [Re: mark cowan]
      #5568648 - 12/13/12 03:52 AM Attachment (10 downloads)

Mark Cowan: Interesting indeed. Nice. I suppose a suitable camera could be mounted behind the K-E and push the contrast/brightness to maximum. Unfortunately, an 8-inch f/51 mirror has a radius of curvature of 68 feet. That represents a serious problem with air currents and even finding such a testing facility.

If the bottom row were resolvable for a parabolid, anything with a smaller conic constant then -1 would reach a null way before f/51. Say you have a 54% paraboloid, cc = -0.2916. IN which case you probably couldn't reoslve the anything even with a higher contrast. The picture below shows an 8-inch f/51 mirror cc = -0.2916, tested at RoC, "raw" and enhanced constrast and brightness.

It would also be necessary to make a Foucaugram of an actual f/51 or so mirror and see if the photograph could be enhanced the same was as the simulated image. In other words, the program may by itself form a pattern (artefact).

DAVIDG's solution is actually ideal as far as the long radius testing is concerned and all the problems with air currents are concerned. It cuts the whole test down to a neat 80-inch package, plus double precision, and a clean null.

Unfortunately, for a Herrig there really isn't any good reason to go through that trouble because the performance is not at all affected even if the mirror's figure is way off. That mirror would more likely then not be within the conic constant tolerance envelope without figuring, as soon as it was polished out.

Regards,
Mladen

Edited by MKV (12/13/12 04:06 AM)


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Re: Herrig new [Re: MKV]
      #5568663 - 12/13/12 04:28 AM

Quote:

Unfortunately, for a Herrig there really isn't any good reason to go through that trouble because the performance is not at all affected even if the mirror's figure is way off. That mirror would more likely then not be within the conic constant tolerance envelope without figuring, as soon as it was polished out.




Not sure that I'd call it 'unfortunate' ... perhaps I'm missing something?

Any rate, the 'problem' is still the one about "testing' that long ROC CX primary -- it does need to be smooth, it should be somewhat 'spherical', and it needs to be in the 'ball park' of the desired radius.

As to Mark H's question(s) on "tolerance"; for the 820" ROC primary of Dave G's 6.5" f/18, anything +/- 5% (41") is 'good enough' -- just plug in the high and low end of the range into OSLO, refocus, and you'll see very little changes -- even w/o re-optimizing the secondary ROC or tilts and separation.

Edited by Dave O (12/13/12 04:39 AM)


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Re: Herrig new [Re: Dave O]
      #5568715 - 12/13/12 06:23 AM

Quote:

Not sure that I'd call it 'unfortunate' ... perhaps I'm missing something? Any rate, the 'problem' is ... it does need to be smooth, it should be somewhat 'spherical', and it needs to be in the 'ball park' of the desired radius.



You got me there, Dave O, perhaps 'unfortunate' wasn't the best choice but I was trying to be sarcastic (considering how much fuss has arisen over a surface with extremely "fat" tolerances).

Generally speaking, proven polishing techniques should result in a smooth surface and if figuring is not critical then the mirror could be ready as soon as it polishes out.

The radius should not be problem, given that its sagitta is well within micrometric range (~0.01 inch).

I have also shown in the spot diagram above that even at cc = -12, the performance doesn't suffer, so I wonder what is meant by: the mirror needs to be "somewhat spherical". A mirror with cc = -12 is nowhere near the sphere, and yet the sport actually look better off axis then with a shpere.

Regards,
Mladen

Edited by MKV (12/13/12 06:29 AM)


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Re: Herrig new [Re: MKV]
      #5568916 - 12/13/12 10:06 AM

Quote:

I have also shown in the spot diagram above that even at cc = -12, the performance doesn't suffer, so I wonder what is meant by: the mirror needs to be "somewhat spherical". A mirror with cc = -12 is nowhere near the sphere, and yet the sport actually look better off axis then with a shpere.




Actually at the given radii and aperture, a CC of -12 is still pretty close to a sphere ... you could increase the CC of the primary on Dave G's 6.5" f/18 to -100 and still get diffraction limited spots .... By "somewhat spherical", I was simply implying a surface of revolution that is close enough to a sphere to produce a diffraction limited image -- whatever that CC tolerance happens to be for the particular design.


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Re: Herrig new [Re: Dave O]
      #5569190 - 12/13/12 12:43 PM

Quote:

By "somewhat spherical", I was simply implying a surface of revolution that is close enough to a sphere to produce a diffraction limited image -- whatever that CC tolerance happens to be for the particular design.



In this particular case that is absolutely true, Dave O.


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Re: Herrig new [Re: MKV]
      #5569240 - 12/13/12 01:08 PM

So the mirrors are easy to make and there are three ways shown to test the one cx surface . This is looking good .Bucket list telescope . Mine is a 50% scale of the Hooker 100 inch . Half sized Hooker

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Re: Herrig new [Re: kfrederick]
      #5569280 - 12/13/12 01:32 PM

One test mode is to make the optics the best as you can by what ever method you like, then jig them up and test the system by double pass autocollimation. If the ronchi lines are straight, then the wavefront has to be excellent and the telescope will perform.

- Dave


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Re: Herrig new [Re: DAVIDG]
      #5569296 - 12/13/12 01:39 PM

+1.
Mike


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Re: Herrig new [Re: Mike I. Jones]
      #5569480 - 12/13/12 03:40 PM

.The light has 9 reflections in Daves autocollimation setup . The mirrors would need coated to do the test ??

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Re: Herrig new [Re: kfrederick]
      #5569489 - 12/13/12 03:51 PM

Quote:

.The light has 9 reflections in Daves autocollimation setup.



Nope, just three.

Mladen


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Re: Herrig new [Re: kfrederick]
      #5569504 - 12/13/12 04:01 PM

Good point Kevin. I would make both surfaces in the Herrig the best I could and have the concave mirror coated. There are ultra bright LED's available for only a few dollars that one could use as a light source. One could also learn to silver glass chemically and coat the convex surface. I view the double pass autocollimation as a final check of the system and not so much as the primary test method.
One could also use a digital camera with one or both surfaces uncoated and take a few second exposure of the Ronchi lines to image the test results. You could even stack a couple of images together to average out the effects of air currents and improve the signal to noise ratio.
The digital imaging of a long focus mirror could be another way to detect faint zones and improve both the precision and accuracy of the knife edge/Ronchi test for these types of long radius spherical mirrors.

- Dave


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Re: Herrig new [Re: DAVIDG]
      #5569559 - 12/13/12 04:41 PM

DAVEG, your OSLO file shows three reflections in your Herrig test AC setup. It seem you agree there are 9. How so?

Mladen


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Re: Herrig new [Re: MKV]
      #5569589 - 12/13/12 05:06 PM

Mladen,
I believe Kevin is referring to my sugguestion of the autocollimation testing of the completed telescope and not of my AC setup for testing just the convex mirror.

- Dave


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Re: Herrig *DELETED* new [Re: DAVIDG]
      #5569628 - 12/13/12 05:37 PM

Of all the configurations I have seen on CN, the Herrig is most fascinating. I think it's worth a try.

Mladen

Edited by MKV (12/13/12 09:24 PM)


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Re: Herrig new [Re: MKV]
      #5569733 - 12/13/12 06:37 PM

Man, I can't wait to retire and get my Herrig built and get away from all this blibber-blabber! Dave and Kevin will probably get theirs built before I can get to it.
Mike


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Re: Herrig new [Re: Mike I. Jones]
      #5569883 - 12/13/12 08:22 PM

Thanks for all the great info I am not building one now . Happy with the 17

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Re: Herrig new [Re: MKV]
      #5570149 - 12/13/12 11:30 PM

Quote:

Unfortunately, an 8-inch f/51 mirror has a radius of curvature of 68 feet. That represents a serious problem with air currents and even finding such a testing facility.




I didn't say I was going there.

Best,
Mark


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kfrederick
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Re: Herrig new [Re: mark cowan]
      #5571108 - 12/14/12 04:30 PM

How fast could a 4 to 6 inch herrig be? How about f6 ?

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Dave O
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Re: Herrig new [Re: kfrederick]
      #5571791 - 12/14/12 11:41 PM

Kevin, according the 'literature', the Herrig design is quite good down to f/12. Designs between f/12 and f/9 will show more aberrations, but would likely be considered 'acceptable' for most applications. Faster than f/9 the aberrations would likely be considered 'unacceptable' for many applications.

However, as this design is a 'double bounce' (as Mike Jones once called it), it is VERY compact; so even an f/12 version is fairly short for the given aperture.


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MKV
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Re: Herrig new [Re: Dave O]
      #5571856 - 12/15/12 12:41 AM

Dave O: has anyone to your knowledge done an objective (quantitative, metric) comparative analysis of the Herrig's actual performance, against other configurations of the same aperture and focal ratio?

Mladen


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Dave O
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Re: Herrig new [Re: MKV]
      #5571878 - 12/15/12 12:54 AM

Mladen ... Examples of the Herrig do exist, they have been built ... but by very few it would seem. I am not aware of the existence of any performance analysis of the type you are inquiring about. I have never seen one, let alone looked through one. Perhaps someday, that will change.

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Re: Herrig new [Re: Dave O]
      #5571988 - 12/15/12 03:59 AM Attachment (34 downloads)

And here is a 6" (150mm) f/10 design, that has more 'moderate' curves and would be my personal choice for building. Dave G likes the longer focal lengths for the higher magnifications; I tend more for the 'faster' variety and use a Barlow ...

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MKV
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Re: Herrig new [Re: Dave O]
      #5572235 - 12/15/12 09:34 AM Attachment (13 downloads)

Quote:

And here is a 6" (150mm) f/10 design, that has more 'moderate' curves and would be my personal choice for building. Dave G likes the longer focal lengths for the higher magnifications; I tend more for the 'faster' variety and use a Barlow ...



Not bad, but the f/10 isn't all that much better than an ordinary 6-inch f/10 Newtonian. And all the work and two oversized mirrors that are required (in the f/10 Herrig you need an 11.5 and an 8.25), work, cost, etc. is not something easily discounted as insignificant.

Of course, an f/10 Newtonian covering a 1 degree (26 mm) field would require a secondary at least 25% of the linear CA, with concomitant diffraction which is. It's an individual decision which is more important. I can see and understand, though, why some are willing to go the extra step (more like three or five extra steps!) to have an unobstructed aperture.

I think Herrig seems to be a perfect planetary scope, compact like a Cassegrain, yet unobstructed, and fully achromatic, but easier to make than a Cassegrain. If I were making one now, I'd be shooting for something like f/20. Slower configuration would also require smaller mirrors.

Regards,
Mladen

Edited by MKV (12/15/12 09:38 AM)


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Dave O
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Re: Herrig new [Re: MKV]
      #5572359 - 12/15/12 11:00 AM

Quote:

Not bad, but the f/10 isn't all that much better than an ordinary 6-inch f/10 Newtonian.




But it is unobstructed (as you note later) and it is less than 3' long ... both desirable features in my mind. And for a 1 deg. field, it is definitely better than a 6" f/10 Newtonian.

Quote:

And all the work and two oversized mirrors that are required (in the f/10 Herrig you need an 11.5 and an 8.25), work, cost, etc. is not something easily discounted as insignificant.




Not sure how you determined the primary blank size -- the primary comes out to under 9.5", the secondary is less than 8.5". Two spherical mirrors ...

Quote:

Of course, an f/10 Newtonian covering a 1 degree (26 mm) field would require a secondary at least 25% of the linear CA, with concomitant diffraction which is. It's an individual decision which is more important. I can see and understand, though, why some are willing to go the extra step (more like three or five extra steps!) to have an unobstructed aperture.




Noted.

Quote:

I think Herrig seems to be a perfect planetary scope, compact like a Cassegrain, yet unobstructed, and fully achromatic, but easier to make than a Cassegrain. If I were making one now, I'd be shooting for something like f/20. Slower configuration would also require smaller mirrors.




I think that you will find that the 'savings' in glass for the longer focal length, is rather small, and of course the OTA is twice as long. The spots are definitely tiny though ...


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Re: Herrig new [Re: Dave O]
      #5575438 - 12/17/12 09:39 AM

Dave that is a great design compact .The 6 inch f10 .I have a 419.5 RC concave testplate . If carbon fiber and thin opticsThe weignt would be small. If I was to make one I would use a cnc waterjeted sloted box and try for perfect alignment of the two surfaces they are only 28inches appart How big can this design go before bad things creep in?

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Re: Herrig new [Re: Dave O]
      #5575761 - 12/17/12 01:51 PM

Dave O: no real disagreement on anything you said. A 6-inch f/10 Newtonian will not match the theoretical image of an f/12 Herrig, but will have a respectably small image over a 10 degree field / sufficiently small for photographic use, photographically, and visually it's not an issue, because you look at objects in the center of the field anyway.

A 6-inch f/10 will be cheaper, easier to make and lighter, for sure. It will lack the benefit of an unobstructed aperture, which is not an unimportant aspect for consideration, but it's not necessarily crucial, especially if the CO is kept under 20%.

The size of the mirrors is pretty much dependent on what size blanks are available. I estimated mirror diameters based on the ray heights. It was just a rough estimate.

Overall, the Herrig is an awesome solution. It's hard to believe it hasn't been discovered earlier. All spherical, two-mirror unobstructed reflector with such correction is simply unheard of. No torsion (in two or three mirror configurations), no off-axis paraboloids, no field lenses needed.

The mirror finish in the Herrig still has to be above and beyond to withstand four reflections without seruiously degrading the wavefront. It will also require nothing but the most reflective coating to minimize light loss and even then the loss will be at least 4%, which will affect the limiting magnitude of photographic images. The Herrig is certainly not light, either.

I think a realistic simulation would require that the mirrors have a built in random "roughness" of various RMS quantities to see how that would withstand four reflections and that without some sort of estimate of this type I wouldn't even entertain the idea of making one simply because it's a fact that each reflection nearly doubles the wavefront error.

The way I see it, Erwin Herrig invented this configuration (which also exists in three and four mirror versions, with or without torsion) in 1997 (actual "Application number is DE19961049841 19961202), and it has not really become a revolutionary scope its paper version seems to suggest it is in, almost 17 years after its indtroduction.

There must be more to this then meets the eye. A 1999 Harvard report didn't qualify it as stellar, although its design is definitely novel. The relative lack of any objective study, or comparative results of the Herrig since 1997 until now, and the odd scarcity on the Internet of links to it, suggests to me that something other than its theoretical attraction is at play when the theory meets reality.

Regards,
Mladen


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Re: Herrig new [Re: MKV]
      #5575815 - 12/17/12 02:30 PM

With computers programs like Zemax Odd designs become doable and better. This is the place for the ATM . New ways of doing things . Reading about this CCD camera how it could download a pic in under 1 second and could image anything the eye can see in that 1 second pic . A electronic eyepiece might be all you need . Just saying how one new thing makes other things better .Just chatter could have errors . http://www.sbig.com/astronomical-instruments-blog/stf-8300-electronic-eyepiece/

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Re: Herrig new [Re: kfrederick]
      #5576088 - 12/17/12 05:30 PM

When a new optical designs appears, what is needed to gain popularity is detailed instruction of how to make one. The roots of ATMing can be traced back to a few simple articles by Russell Porter in Scientific American detailing exactly how to grind a parabolic mirror and test it.
If one looks at a Maksutov, it is not an easy telescope to make. The reason why it became very popular in the 1960's was there were many articles and even a few books published on exactly how to make them. There was also a world wide club devoted to them. Another example is the Schupmann refractor, which is actually easier to make then a classic doublet achromat, but before Jim Daley's book on the subject was published little was available on how to make one.
So what the Herrig design needs is some very detailed instructions showing exactly how to make one. Once you have that, so the average ATM can make one, then it's popularity might increase.

- Dave


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Re: Herrig new [Re: DAVIDG]
      #5576347 - 12/17/12 08:10 PM

One of the issues I see with the Herrig is that baffeling will be a problem. It's like the Yolo in this regard.

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Re: Herrig new [Re: kfrederick]
      #5576628 - 12/17/12 11:35 PM

Quote:

If I was to make one I would use a cnc waterjeted sloted box and try for perfect alignment of the two surfaces they are only 28inches appart How big can this design go before bad things creep in?




Kevin, I don't think the CNC water-jetted box would be required for this design. I think the tried and true method of locating the two mirrors used by the Schiefbuilders would provide very precise alignment. They use a stud/pin to precisely locate the centers of the mirrors and then use three collimating bolts to adjust the tilts.

In the Herrig, you would position the locating stud/pin to precisely locate the centers of the first reflection from each mirror. Once these are properly aligned, the remaining two reflections should be in alignment as well (since they are part of the same mirror).

As far as size goes -- you saw a 9.5" f/10 earlier (which started this post) ... the real problem with size, is that your mirror blanks are 50% (or more) larger than your aperture ... things get pretty big, pretty quick.


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Re: Herrig new [Re: Ed Jones]
      #5576633 - 12/17/12 11:43 PM

Quote:

One of the issues I see with the Herrig is that baffeling will be a problem. It's like the Yolo in this regard.




Indeed, you are correct, Ed. One of the 'easy' fixes is the one used by the Yolo folks -- I think they call it a 'muffle' (?) -- basically an extension of the front of the OTA (along the incoming light path -- simple tube would work); which is long enough to prevent skylight from directly illuminating the field.

Other than 'painting it black' there really doesn't seem to be a lot one can do to baffle from internal reflections within the OTA, although there are a few 'strategic areas' were internal baffles may be useful ...


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Re: Herrig new [Re: Dave O]
      #5576832 - 12/18/12 05:55 AM

Dave what is a stud pin setup ?

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Re: Herrig new [Re: kfrederick]
      #5576849 - 12/18/12 06:40 AM

It was my (rather poor) attempt at describing the arrangement used by Oscar Knab for mounting the secondary mirror of his Schiefspeigler; which allowed for adjusting the tilt while maintaining the critical alignment with the optical axis. A drawing of this arrangement can be seen here:

http://www.gfphoto.com/atm/schief/sec-cell-detail-knab.jpg

In this particular arrangement a center bolt with a spherical head, under spring compression, pulls the aluminum mirror cell against three brass collimating bolts, which can be used to adjust the tilt. The spherical head of the center bolt (which I referred to as a stud/pin earlier) holds the back of the mirror cell at a fixed location; while allowing it to tilt up/down and side-to-side as determined by the three brass collimating bolts. A pretty simple arrangement, but rather ingenious.


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Re: Herrig new [Re: Dave O]
      #5576929 - 12/18/12 08:30 AM

If the correct numbers for the tilt and distance are known then a machined box could be made to hold the two mirrors and the focuser . This way fixed all my problems on the CHief. The Oslo has two distances and tilts for each mirror .Having only three things to line up by a adjustable setup should not be too hard if the non adjustable setup did not work.I still think a CHief makes a better telescope . If you wont unobstructed views . Thanks for Dave @ Dave designs

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Re: Herrig new [Re: MKV]
      #5577224 - 12/18/12 12:21 PM

Mladen, your statement about a 6" f/10 is basically correct, but a 6" f/10 can clearly be made with a 1" diagonal with little or no sweat, which makes for a 16% obstruction. And just possibly, it might be made with with less CO, but despite those APO guys, who never seem to look through such reflectors, at 16% you aren't going to be able to the the difference between 16 or 12%, if that might be possible, or 0%, seeing the APO fellows seem to care little about wavefront errors in their scopes.

Simplicity has a lot to say for it.


Ed


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Re: Herrig new [Re: ed_turco]
      #5577257 - 12/18/12 12:45 PM

ED I think some one who makes a Herrig has a shed full of newts . Sure for what it can do for the money and image a long focus newt cannot be topped. But a unobstructed can be better . I own a 20 and a 17 inch both unobstructed and can show you the difference at the eyepiece .It is not Just the stars it is the background too.a soft background with sharp stars .The light that is blocked by the secondary what part of the image is darker because of it . Ed glad you post do not be afraid to post more polishing tips .

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Re: Herrig new [Re: ed_turco]
      #5577343 - 12/18/12 01:47 PM Attachment (10 downloads)

Quote:

Mladen, your statement about a 6" f/10 is basically correct, but a 6" f/10 can clearly be made with a 1" diagonal with little or no sweat, which makes for a 16% obstruction. And just possibly, it might be made with with less CO, but despite those APO guys, who never seem to look through such reflectors, at 16% you aren't going to be able to the the difference between 16 or 12%, if that might be possible, or 0%, seeing the APO fellows seem to care little about wavefront errors in their scopes.



Ed, I couldn't agree more. It is perfectly doable to have no more than 15-16% CO in a 6-inch f/10 and have hardly perceptible loss of contrast at the eyepiece, as the PSFs show below, even though some observers claim to see the difference in contrast on extended objects.

Even if they do, I think it hardly justifies going through the trouble of making one of those exotic reflectors or ED APOs, but that's just my opinion.

Today's APOs are a lot better than the original Christen Triplet ("oiled") sandwich lens, which was really poorly corrected.

When Roland Christen first came out with his triplet, I wrote a letter to Sky & Telescope saying that something as simple and cheap as a $100 6-inch f/10 would actually beat Roland's $2,200 lens hands down, but it was never published.

I guess it wasn't politically correct enough (nothing new here), but the truth is that a 6-inch f/10 Newtonian is an excellent, cost-effective and easy to make "APO" that will never have any color or significant diffraction issues. It's not even that long, it's light and portable too.

Mladen


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Re: Herrig new [Re: ed_turco]
      #5578189 - 12/19/12 12:03 AM

@ Ed T. -- Nobody will build a Herrig because it is 'easy'. Rather, they will build one because it is a challenge, which offers an unobstructed field of view that is both fairly wide and well-corrected.

@ Mladen -- I fail to grasp what your rejected letter to S&T concerning Roland Christen's oiled triplet APO has to do with this thread???


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Re: Herrig new [Re: Dave O]
      #5578314 - 12/19/12 02:23 AM

Quote:

@ Mladen -- I fail to grasp what your rejected letter to S&T concerning Roland Christen's oiled triplet APO has to do with this thread???



Let's just say, it was tangential, touching on the effort and cost some people will go to for an unobstructed aperture, and, in the oiled triplet's case, choosing a very expensive, but inferior product.

Getting back to the topic, the thrill of new designs notwithstanding, I do wonder why would a promising configuration with geometrically superb results on paper, all spherical surfaces and generous tolerances, remain virtually unknown, untried, and without an objective review, 15 years after its invention. You don't find that odd?

On paper, the Herrig is almost irresistible; it's so perfect it makes my hands itch for some glass to push. I have an 8 and a 6 inch Pyrex blank and I can't believe I am already not half way through rough grinding my own Herrig telescope.

Before I do, I am trying to get to the bottom of the issue why did professional and amateur communities apparently shy away from it for years.

I think the four reflections may have something to do with it. After all, the reason why an autocllimation test is so good (double precision) is because multiple reflections magnify wavefront errors, Dave.

I am trying to figure out a way to simulate surface roughness in OSLO (using deformation coefficients) to see the degree of this effect in the Herrig. Perhaps there is some cancellation of error due to even number of reflections, which would be wonderful. Unfortunately, very little has been written about this design.

Mladen


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Re: Herrig new [Re: MKV]
      #5578343 - 12/19/12 03:35 AM

Quote:

I think the four reflections may have something to do with it. After all, the reason why an autocllimation test is so good (double precision) is because multiple reflections magnify wavefront errors, Dave.




The reason the double-pass autocollimation test is so good is because it reflects the rays back upon themselves; this is why the wavefront error is 'doubled', each ray is reflected from the same portion of the mirror twice. The Herrig does not reflect the rays back upon themselves -- each reflection is from a different portion of the mirror.

Yes, it is four reflections, which is only one more reflection than what is typically found on a SCT (normally used with a diagonal). As there are only two mirrors, enhanced coatings could be used to improve the brightness.


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MKV
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Re: Herrig new [Re: Dave O]
      #5578435 - 12/19/12 05:56 AM

Quote:

The Herrig does not reflect the rays back upon themselves -- each reflection is from a different portion of the mirror.



Dave, it is known that wavefront error doubles on orthogonal reflection, regardless if it's the same mirror section or not. The phenomenon is proportional to the angle of incidence, being the largest at 90 degrees.

More importantly (to me at least) is why has this otherwise very elegant configuration been collecting dust for 15 years? Unlike the dialytes of the Busack or Honders-Riccardi types, the Herrig is much more doable, yet it never did catch on.

Mladen


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Re: Herrig new [Re: MKV]
      #5578486 - 12/19/12 07:19 AM

Quote:

Quote:

The Herrig does not reflect the rays back upon themselves -- each reflection is from a different portion of the mirror.



Dave, it is known that wavefront error doubles on orthogonal reflection, regardless if it's the same mirror section or not. The phenomenon is proportional to the angle of incidence, being the largest at 90 degrees.




I think you are confused here. The wavefront error is double the surface error upon reflection .... reflection, in and of itself, does not double the wavefront error.

Edit. A second reflection can actually reduce the wavefront error of a previous reflection; multi-mirror systems are based upon this principle.

Edited by Dave O (12/19/12 08:44 AM)


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Re: Herrig new [Re: Dave O]
      #5578556 - 12/19/12 08:30 AM

It should be no problem having the secondary concave Spherical mirror to better than 1/40 wave The convex Not sure . I have a big test plate with a concave RC of 419.5 inches .And a concave mirror RC 138 those are close to what is needed for the 6 inch f10 .

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Re: Herrig new [Re: kfrederick]
      #5578608 - 12/19/12 09:16 AM

How big a Herrig could be made if the secondary mirror is a 6inch f 10.8? I have a very good one . . Make a nice finder for the 17

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Re: Herrig new [Re: MKV]
      #5578791 - 12/19/12 11:43 AM

Quote:

More importantly (to me at least) is why has this otherwise very elegant configuration been collecting dust for 15 years? Unlike the dialytes of the Busack or Honders-Riccardi types, the Herrig is much more doable, yet it never did catch on.




I suppose it has a lot to do with the fact for the same cost in time, material, and effort, you could have a 10" f/6 Newt? I mean seriously ...

Yes, it is 'cheaper' than a 6" APO ... but those refractor guys ain't gonna buy it ... literally or figuratively. And the reflector guys will opt for the maximum aperture for their money.

I expect it will remain on a lot of folks 'to do someday' list ... and some will actually get built ....

I think it would make a pretty neat, compact imaging scope ..., but that is just me.


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Re: Herrig new [Re: Dave O]
      #5578887 - 12/19/12 01:06 PM

The standard is the Schmidt-Cassegrain Easy to mass produce . Hard to build a Herrig that small and light . Great for ATMs to have the designs to try .

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Re: Herrig new [Re: Dave O]
      #5578957 - 12/19/12 01:46 PM

Quote:

I think it would make a pretty neat, compact imaging scope ..., but that is just me.



No doubt, but it still doesn't tell me why, since 1997, the Herrig remains obscure and unknown, not only in ATM literature but in professional as well. I'd hate to go through such a project and then find out for myself what should have been obvious from the start.

Why are there no quantitative analyses, or comperative studies available? How about some deep sky images? When it comes to optics, the Herrig promises superb correction with components that couldn't get any simpler - two long focus spheres, and the convex one can be just about any reasonable conic solid of revolution. The rest is just mechanical stuff. Yet, don't expect to find raving reviews, other than "it's cool".

Enough said.
Mladen


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Re: Herrig new [Re: MKV]
      #5579138 - 12/19/12 03:21 PM

I can think of a very good reason why it hasn't caught on.
M.


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Re: Herrig new [Re: MKV]
      #5579320 - 12/19/12 05:30 PM

Based on your pictures and your comments, I don't wish to hijack this thread so I'll start another...

Ed


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Re: Herrig new [Re: ed_turco]
      #5579420 - 12/19/12 06:42 PM

Intuitively, I suspect that the errors from those four reflections might not increase any faster than the good old statistical assumption of the square root of the sum of the individual errors squared... Two mirrors having wavefront errors of lambda/10 might, after four reflections, deliver a wavefront of lambda/5.

SQRT (0.1^2 + 0.1^2 + 0.1^2 + 0.1^2) = 0.2

Seem reasonable?


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Re: Herrig new [Re: GlennLeDrew]
      #5579804 - 12/19/12 10:59 PM

Quote:

Intuitively, I suspect that the errors from those four reflections might not increase any faster than the good old statistical assumption of the square root of the sum of the individual errors squared... Two mirrors having wavefront errors of lambda/10 might, after four reflections, deliver a wavefront of lambda/5.

SQRT (0.1^2 + 0.1^2 + 0.1^2 + 0.1^2) = 0.2

Seem reasonable?




That would be my 'gut' feeling as well Glenn.


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Re: Herrig new [Re: Dave O]
      #5580463 - 12/20/12 11:17 AM

A number of years ago I started building unobstructed telescopes that are also perfectly achromatic as well. These include Schiefspeigler, Schupmann refractors and off axis newtonians. I'm working an 6" f/10 off axis newtonian and a 6" f/23 Schief with a toroidal secondary. Before I built and used these scopes, I also believed that when the central obstruction was small, the effect would be too small to make a difference. All I can say is with my two eyes I can see a difference and that is why I continue to build these types of telescopes, really enjoy the images they produce and keep looking for other interesting designs of the same type to make. So the Herrig design on the list to make. Hopefully soon.

All the Best and Happy Holidays,
- Dave


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Re: Herrig new [Re: DAVIDG]
      #5581925 - 12/21/12 06:15 AM

Hi, Dave
I don't know how many times I've read where an obstruction of under 20% is inconsequential- whereas I've noticed as you say; the less, the better -REGARDLESS-.
****
+1.
M.


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MKV
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Re: Herrig new [Re: DAVIDG]
      #5583082 - 12/21/12 07:25 PM

Quote:

A number of years ago I started building unobstructed telescopes that are also perfectly achromatic as well. These include Schiefspeigler, Schupmann refractors and off axis newtonians. I'm working an 6" f/10 off axis newtonian and a 6" f/23 Schief with a toroidal secondary. Before I built and used these scopes, I also believed that when the central obstruction was small, the effect would be too small to make a difference. All I can say is with my two eyes I can see a difference and that is why I continue to build these types of telescopes, really enjoy the images they produce and keep looking for other interesting designs of the same type to make. So the Herrig design on the list to make. Hopefully soon.



Dave, I am not exactly sure how you can compare obstructed and unobstructed telescopes and come to your comparative conclusion. In order for you to judge obstructed vs unobstructed telescopes you'd have to view both diffraction images simultaneously - and you'd have to have some sort of a way to ascertain the difference objectively, not subjectively. As a research scientist, I'm sure you'd agree.

Aperture is the key to resolution. I can stop down a 6-inch refractor f/12 to a 3 inch f/24 scope and pretty much eliminate any perceptible residual of color, but by doing so I am reducing the aperture and the amount of detail and limiting magnitude I can see. So, there is no way a 3 inch will resolve finer details, or show more of it, than an aperture twice as big - assuming both are made to the same standards.

Mladen

Edited by MKV (12/22/12 04:23 AM)


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Re: Herrig new [Re: MKV]
      #5584117 - 12/22/12 01:37 PM

Mladen,
I have garage full of all different types of telescopes, from many of which have won the optical judging at Stellafane. I'm not trying to brag just saying that the optical quality has been confirmed by others and I have done the side by side test many times of comparing unobstructed vs obstructed and each time I can see the difference in the images that the unobstructed and also perfect achromatic telescope produce. If my eyes couldn't see the difference then I wouldn't have gone thru the trouble of designing and building the telescopes I have and I wouldn't be working on a 6" Schiefspeigler or 6" f/10 off axis Newtonian. The key point on both of these telescopes are that they are unobstructed and also perfectly color corrected.

Happy Holidays,
- Dave


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Re: Herrig new [Re: DAVIDG]
      #5585176 - 12/23/12 07:54 AM

Miaden [For me] ATM is not buying a mirror and putting it in a box per instructions in a book . Nothing wrong with that . If you are not selling and have a shed full then you mess around with the fun stuff. Color free image with out the obstruction . Guys like Dave Mike and ED are making this hobby very exciting with there new designs. I know EDs Chief design will become popular . With ATM .The Herrig should be a fun design. Thanks for the posts and designs

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Re: Herrig new [Re: kfrederick]
      #5585848 - 12/23/12 03:56 PM

Quote:

Miaden [sic] [For me] ATM is not buying a mirror and putting it in a box per instructions in a book.



Very true, and honest, Kfrederick. I was appealing to science, and the science says what it says. Here is a good article on this issue.

Airy disk and performance

The article deals with the worst-case central obstruction normally encountered, such as in the case of SCTs. One can easily draw a conclusion what would the effects be from a central obstruction half that size, or even smaller.

Some people prefer taste over science, others trust their perception. Nothing wrong with that, as long as we understand that taste and perception are not objective but subjective criteria, and shouldn't be used as an objective guide or even as advice.

I have no doubt that some leading ATMs see the difference in unobstructed and obstructed telescopes, no matter how small the central obstruction may be, and that's why they are avowed purists in this regard, namely that even the smallest possible central obstruction is noticeably prejudicial to performance.

Some people have a similar position on even the slightest trace of false color (chromatic aberration), even in the best corrected apochromats.

As the old adage says, we don't argue over taste, or perception for that matter. But when it comes to reliability and general truth, we depend on science.

In addition to following personal taste for things, some people will believe what they see no matter what. In Portugal, in 1917, in a place called Fatima, tens of thousands of people "saw" the Sun "dance" and "fall" towards earth. Some even "noticed" increase in heat!

According to some estimates between 30,000 and 100,000 people witnessed this event which reportedly lasted about 10 minutes. Here you can see the Fatima crowd.

The only problem is, film and photo cameras pointed at the Sun didn't record anything the crowd "saw". There simply was no dancing or falling evident on the film. But how can tens of thousands of "eyewitnesses" be wrong? Or were the cameras "blind"?

I'll stick with science, not anecdotes, and as an ATM myself, that's my choice. Others may do as they please.

Merry Christmas and Happy Holidays to all.

Mladen


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Re: Herrig new [Re: MKV]
      #5587248 - 12/24/12 01:34 PM

Absolutely there's a difference between an obstructed scope and an unobstructed (of otherwise equal quality), no question. It can't be argued away by appeal to authority or to simulations; it has everything to do with the central axis being pristine. If you can't see the difference in practice, well, then you can't see the difference.

Best,
Mark


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Re: Herrig new [Re: mark cowan]
      #5587826 - 12/24/12 10:21 PM

This Herrig might make a good sun telescope if you did not coat the mirrors would it be too bright?

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Re: Herrig new [Re: kfrederick]
      #5602542 - 01/03/13 08:48 AM

http://bhs.broo.k12.wv.us/homepage/alumni/dstevick/herrig/newtct1.htm They show the three mirror one here See how they list the mirrors spec in FL and not RC I made two 12 inch mirrors wrong for my herrig because I thought it was RC

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Re: Herrig new [Re: kfrederick]
      #5602988 - 01/03/13 01:17 PM

Interesting read on this subject. I have used an 8in sct and a 5in F9 APO and would say that the increased contrast of the apo makes it a better scope. Retaining contrast on many objects is very important. The link suggests resolution is better too. That's my impression. After all Newtonian were regarded as excellent for splitting double stars. Old boys own astro books also usually reckon that it takes a 6in newtonian to match a 4in F15 achromatic refractor.

On testing test plates for the convex mirror Texereau uses a method for checking for turned down edges on spheres using the knife test and a slit. It produces fringes ahead of the knife. It might be of use. The book How To Make A Telescope is on the internet archive.

Another way might be the wire test mentioned in the old ATM books but no wire just an eyeglass to view the image of the slit. A 10x eye glass has a short depth of field. Or maybe the wire could be centred on the beam as it returns and sums done in the usual way. Not sure about how effective diffraction effect would be from such a slow mirror though.

I quickly altered the radius of the F18 to 6 an 7 mtr on the main mirror. Oslo acted oddly maybe due to the double reflections. I had to focus twice. Changes to the rad of that size nearly fill the diffraction ring. Conics seem to make very little difference. I would suspect it might be best to make the convex 1st and then make the primary accordingly - assuming the convex can be measured with some certainty.

One thing confuses me on this. How is the tilt of the second reflection on a surface calculated? I would assume these tilts need changing when any other aspect is changed including to conics.

John
-

Edited by Ajohn (01/03/13 01:21 PM)


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Re: Herrig new [Re: Ajohn]
      #5604053 - 01/04/13 03:42 AM

Quote:

One thing confuses me on this. How is the tilt of the second reflection on a surface calculated? I would assume these tilts need changing when any other aspect is changed including to conics.




Correct. You can't just start changing numbers in the OSLO prescription as the 'matched' surfaces (1&3 and 2&4) will no longer be 'matched' ... they must be exactly the same surface or you will not be able to build the telescope.

I expect one could do the math to compute the new angles and separations ... a spreadsheet program would likely make it easier.

For me, it is easiest to go back to the HERDESG program and just let it recalculate the tilts and separations for any changes and then re-enter those values into OSLO ... of course it only really works for spherical surfaces ...


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Re: Herrig new [Re: Dave O]
      #5604309 - 01/04/13 09:04 AM

If the angle of the back of each of the two glass was known .Then a Box like I did For the Chief should work . That is how I would try first . I think the three mirror one might be easyr to make

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Re: Herrig new [Re: Dave O]
      #5604384 - 01/04/13 09:33 AM

I suppose it's just the tangent angle to the central ray where it strikes the mirror.

Thinking through this design though I think I will go back to my interest in the Stevick-Paul. More mirrors but I suspect they will be easier to make.

John
-


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Re: Herrig new [Re: Ajohn]
      #5604408 - 01/04/13 09:47 AM

Quote:

I suppose it's just the tangent angle to the central ray where it strikes the mirror.




What it boils down to, is the center and radii of surfaces 1&3 (and 2&4) must be the same point in space (for spherical surfaces). If you simply change the radii on the two surfaces (1&3 or 2&4), their centers will no longer coincide and they will no longer lie on the same surface. If you start messing with conic constants, then you also have to start looking at offsets, and things get pretty complex, pretty fast. The Herrig is simply a 'special case' of a four mirror system, where the 3rd and 4th reflection share a common (spherical) surface with the 1st and 2nd reflection. Only two mirrors to fab, and coat ... but not all of the mirror's surface is used ....


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Re: Herrig new [Re: Dave O]
      #5604425 - 01/04/13 10:02 AM

Thanks Dave. I would suspect that the design could be trimmed up based on how the radii finish up after they are made. From literature that seems to be possible with all of these types of design.

The tilts I referred to were based on playing with the design in oslo. To be correct the tilt angle differs the 2nd time the rays strike the same mirror. Unfortunately as far as I am aware oslo can't cope with that which would make optimising difficult.

Personally the shallow convex puts me off.

John
-


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Re: Herrig new [Re: Ajohn]
      #5608956 - 01/06/13 05:54 PM

Quote:

... I have used an 8in sct and a 5in F9 APO and would say that the increased contrast of the apo makes it a better scope. Retaining contrast on many objects is very important.

John
-



You compare an 8" SCT with close to a 35% CO with a 5" APO and say the APO is better? I wouldn't want to compare anyone's telescope with an SCT. Most SCTs will tend to make anything look better...


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Re: Herrig new [Re: ed_turco]
      #5609097 - 01/06/13 07:31 PM

Quote:

Quote:

... I have used an 8in sct and a 5in F9 APO and would say that the increased contrast of the apo makes it a better scope. Retaining contrast on many objects is very important.

John
-



You compare an 8" SCT with close to a 35% CO with a 5" APO and say the APO is better? I wouldn't want to compare anyone's telescope with an SCT. Most SCTs will tend to make anything look better...



+1! Talk about apples and oranges...


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