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Equipment Discussions >> ATM, Optics and DIY Forum

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kfrederick
Carpal Tunnel


Reged: 02/01/08

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
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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ed_turco
Pooh-Bah
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Reged: 08/29/09

Loc: Lincoln, RI
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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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
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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Dave O
sage
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Reged: 12/21/11

Loc: Sri Lanka
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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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
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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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
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
Carpal Tunnel
*****

Reged: 01/20/11

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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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
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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kfrederick
Carpal Tunnel


Reged: 02/01/08

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
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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kfrederick
Carpal Tunnel


Reged: 02/01/08

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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kfrederick
Carpal Tunnel


Reged: 02/01/08

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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Ajohn
sage
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Reged: 12/03/07

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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Dave O
sage
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Reged: 12/21/11

Loc: Sri Lanka
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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kfrederick
Carpal Tunnel


Reged: 02/01/08

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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Ajohn
sage
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Reged: 12/03/07

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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Dave O
sage
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Reged: 12/21/11

Loc: Sri Lanka
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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