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Equipment Discussions >> ATM, Optics and DIY Forum

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MKV
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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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kfrederick
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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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DAVIDG
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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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Ed Jones
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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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Dave O
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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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Dave O
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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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kfrederick
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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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Dave O
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Reged: 12/21/11

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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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kfrederick
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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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ed_turco
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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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kfrederick
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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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MKV
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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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Dave O
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Reged: 12/21/11

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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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MKV
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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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Dave O
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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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Dave O
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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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kfrederick
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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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kfrederick
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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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Dave O
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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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