Posted 10 December 2012 - 09:21 AM
Posted 10 December 2012 - 09:35 AM
Posted 10 December 2012 - 09:54 AM
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.
Posted 10 December 2012 - 10:00 AM
Posted 10 December 2012 - 10:55 AM
Posted 10 December 2012 - 11:44 AM
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.
Posted 10 December 2012 - 12:26 PM
Posted 10 December 2012 - 12:55 PM
Posted 10 December 2012 - 01:15 PM
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?
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.
Posted 10 December 2012 - 01:25 PM
and on Dave Stevick's site at
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.
Posted 10 December 2012 - 03:21 PM
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.
Posted 10 December 2012 - 03:40 PM
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.
Posted 10 December 2012 - 05:03 PM
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...
Posted 10 December 2012 - 06:54 PM
Relative insensitivity to tolerances could make this a desirable design.
Posted 10 December 2012 - 07:52 PM
Posted 10 December 2012 - 07:57 PM
Great. And how did it perform? I can't open the "Examples" files.
on Dave Stevick's site at
Posted 10 December 2012 - 08:49 PM
Posted 10 December 2012 - 09:00 PM
Posted 10 December 2012 - 10:32 PM
Posted 10 December 2012 - 10:55 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.
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.
Posted 10 December 2012 - 11:03 PM
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 ran several scenarios of the file in OSLO. Relatively INsensitive to spacing, radii, and angles.
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.