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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5597307 - 12/31/12 02:06 AM

Thank you, Daniel. Once thermal were raised again, granted an Achilles heel of some designs (including mine), I am trying to bow out of the argument traveling in circles. You noted this very well in one of your reviews of some fine reflectors. Thermals can often be and must be controlled, just as you noted in your energetic review. Which was the main thrust of my argument: those variables cannot exist across samples otherwise the challenge is biased.

I would agree that a 43% CO being "anything less than very good" is remarkable in itself. But, it might make some sense. If the optical figure is nearly perfect, then the overall degradation to the contrast transfer can be very good (defined as >80% peak intensity normalized to 1) with a larger CO. The contribution of the wavefront is essentially nil leaving only the contribution of the CO. In fact, a CO of 45% can be optimized for the entire range of the MTF (which is what SCTs attempt to do) preserving 80% nominal Strehl. In other words, very good according to diffraction limited criteria but not perfect. Of course, in a refractor, however, nominal Strehl nearly equals the actual Strehl so they have a much larger advantage in terms of peak intensity (and dimmer rings.)

So, while a nearly perfect scope will still be "very good" with 80% peak intensity (at the diffraction limit), a nearly perfect refractor will stomp it at 99% peak intensity. My own scope falls into the very good 80% range with a 28% CO because it's Strehl is a bit less than perfect. So comparing it to an equal aperture 6" refractor would be crazy. Maybe a 4 to 4.5" scope would be more like it (and MTF suggests that equivalent aperture and suggests the advantage of more aperture when obstructed.) Now, a 15% CO allows a Newt's peak intensity to approach to it's aberrant Strehl much like the zero obstructed refractor. So, the challenge becomes possible according to the science (provided the "real world" collimation, cooling and scatter - cleanliness, ripple and coatings - can be controlled or normalized across both samples pretty much equally.)

No doubt your 14" only exceeds the 6" APO when both instruments are not limited by seeing. Any amount of seeing can quickly thwart the larger scope's higher resolution even though that level of seeing might not affect the 6." (This is one real world advantage I benefit from, too) You mentioned this beautifully when you said the 8" TMB showed everything the 24" reflector showed. All the same details were present. If both scopes were working in 0.7" arc seeing, that would make sense (to me.) Seeing favors smaller apertures. Until, though, seeing affects both. At this point, the refractor is limited by seeing, too, but remains more aesthetically pleasing while the Dob's images turn into a speckled circus, if I understand it correctly. In an equal aperture challenge, though, seeing might not matter, but it should be very good to make the comparison easier.

I'd dare anyone to notice the difference in your well corrected and slightly obstructed Ceravolo, too. That is what Ed is saying, he can meet that same challenge in the real world by controlling the variables (optimizing) and delivering a scope in accordance with the science. At least, it will achieve the same observational results you mentioned above with even larger COs studied. In other words, it will be very difficult to tell a difference especially at 15% CO. Why? Because the rings brighten by only about 4% (if my number crunching is correct and estimating the optical quality surely Ed or his students are capable of.) That's pretty close to refractor-like, becoming increasingly difficult to tell a difference as the CO approaches zero in a very well made, optimized reflector. If the CO could actually be zero, there would very little difference between two well made scopes.

Bottom line, IMO, this challenge boils down to optimizing a Newt's real real world flaws achieving an excellent nominal operating Strehl of about 94% and comparing that with a nearly perfect refractor with an actual Strehl of about 98%. Then to detect that 4% difference in real world seeing.

Anyway, thank you Daniel for a great discussion. Always illuminating especially Mark Harry's post a few pages back on the virtues of a good APO and Vahe's discussion of scatter. Very informative.

But, time to bail. Happy new year to one and all.

Edited by Asbytec (12/31/12 03:29 AM)


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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Optimized Newts vs APOs new [Re: wh48gs]
      #5597428 - 12/31/12 06:04 AM

Vlad, a question:
"More important is larger-scale roughness, which can easily get to 1/30 wave RMS (roughly 1/10 wave p-v) on mirror surface, resulting in 4-5% contrast loss (i.e. 0.96-0.95 Strehl). For two reflecting surfaces, it is 8-9% contrast loss. With a doublet, this same surface quality would generate only half as large wavefront error as in a single reflecting surface, or 1/60 wave RMS, causing little more than 1% contrast loss."
----
Is there a way to quantify/qualify this type of surface? What method produces it? Particular brand of cerium or rouge; or quality of same?
Cerium polishing marks can be seen on the glass with an ROC test; in particular, coarse grades. Quite easy, especially with moderate to long Newt mirrors.
Personal experiments on several 8" mirrors that rated around 1/8pvw and .95 strehl seemed to be a tipping point whether end results to improve a cerium polish made any difference. (Sort of agrees with what you quote as an example above.)
Since it's rather dirt-simple to accomplish, and doesn't use much for resources/expense. I have been doing just so for the last 7-8 years by switching to a rouge/fine cerium mix. Feedback and IME tend to bear out the effort is worthwhile.
fwiw,
M.


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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5597456 - 12/31/12 06:42 AM

"Are these images on par with what some of you are seeing at the eyepiece?"
******

Seen the gamut; anything from far better in the smaller aps, to much worse in the larger ones. Most times the larger stuff suffers in particular in the way they're set up, collimated, surrounding viewing area, etc. But I would not view the youtube selections as last word on account of limited selection, different photographers, and likely none being done from the same area, with identical atmospheric conditions, etc.
Daniel, love that G/P scope!
***
I saw a 6" F/4 MN that was built for one of the guys who made some of the corrective optics for Hubble. The maker used a 10" tube to surround the optical train...did make a difference with all that space!
M.


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tomharri
sage
*****

Reged: 09/19/08

Loc: USA
Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5597497 - 12/31/12 08:03 AM

In the mid 90's found an unused Edmund 6" f/10 mirror set, used the period 7" aluminum tube, only optimization was using stainless strips .020 thick, 3/8" wide for the 4 vanes.

And it was LIKE WOW!!! The crispest-sharpest-images you could not even imagine. Background sky was pitch black, stars pinpoint sharp. Mars was so good, planet edges so distinct-sharp. Alum. tube gave 15 min. cool down times.

BUT... The old Coulter 10" showed more overall finer details even though they were kind of fuzzy.

My conclusion: If you don't start with a quality main mirror you are better off getting that overpriced apo.


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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5597610 - 12/31/12 09:46 AM Attachment (11 downloads)

Quote:

Quote:

CONCLUSIONS

Every dob owner should have a refractor and every refractor owner should have a dob. To disregard either design is a closed minded approach to the hobby








do you have any pics of the back end of the ceravolo?




The back is just a flat metal disc. To cool it, we just use a fan in the focuser while the HD216 f6 is entirely different.


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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5597663 - 12/31/12 10:18 AM

Quote:

The back is just a flat metal disc. To cool it, we just use a fan in the focuser while the HD216 f6 is entirely different.



Looks great painted. Just curious, is the secondary holder glued? If so, how do you collimate - the whole window?

Mladen


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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: MKV]
      #5597879 - 12/31/12 12:23 PM Attachment (12 downloads)

And here is, for a parting shot, an idea how increasingly bigger central obstruction (CO) diminishes contrast (but actually slightly increases resolution).

Keeping in mind that surface finish, coatings, collimation, etc., as mentioned by Vla and others thus far, are present and contributory in the final image quality, it is clear that the 32% CO (which corresponds to a 1/4 wave wavefront error) is the watershed. The effects become clearly dramatic at about 60% and extreme after 80%.

Thank God for science!

Wishing everyone a Happy New 2013,

Mladen


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Gary Fuchs
Pooh-Bah
*****

Reged: 05/22/06

Loc: Easton, PA, USA
Re: Optimized Newts vs APOs new [Re: MKV]
      #5597910 - 12/31/12 12:44 PM

Quote:

Thank God for science!




Which God are you thanking?

Well, no matter, nice to see God given credit...

I wonder which scope God would choose?

Gary


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Optimized Newts vs APOs new [Re: Gary Fuchs]
      #5597928 - 12/31/12 12:54 PM

Well since god isn't bound to time/space his vision is omni-"whatever you call it."

It would be cool if he did dispense the knowledge to fold spcace/time.


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David PavlichAdministrator
Transmographied
*****

Reged: 05/18/05

Loc: Mandeville, LA USA
Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5597995 - 12/31/12 01:38 PM

This has run well beyond it's course.

David


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