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Equipment Discussions >> ATM, Optics and DIY Forum

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kfrederick
Carpal Tunnel
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Reged: 02/01/08

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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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
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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Mike I. Jones
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Reged: 07/02/06

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

+1.
Mike


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kfrederick
Carpal Tunnel
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Reged: 02/01/08

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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DAVIDG
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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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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DAVIDG
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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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MKV
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Reged: 01/20/11

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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Mike I. Jones
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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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kfrederick
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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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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
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
sage
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Reged: 12/21/11

Loc: Sri Lanka
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
Carpal Tunnel
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Reged: 01/20/11

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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Reged: 12/21/11

Loc: Sri Lanka
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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Dave O
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Reged: 12/21/11

Loc: Sri Lanka
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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Reged: 01/20/11

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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Reged: 12/21/11

Loc: Sri Lanka
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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kfrederick
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Reged: 02/01/08

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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