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freestar8n
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Reged: 10/12/07

Re: Stranger than fiction! new [Re: jrbarnett]
      #4095041 - 10/05/10 05:36 PM

OK - I didn't know about that link and its theoretical assessment - but I'm focused on what actual humans claim about any differences they observe with real telescopes. If short apo people concede that their performance is inferior to long achro's - and long achro people also agree - then their conclusions are at odds with the theory at that link and the theory is suspect.

My point is that the only empirical data at hand come from human assessments at the eyepiece. Whenever you have a theoretical description of a very complex situation - it may be missing key elements that alter the result. I am not crazy about relying too much on MTF's, and I consider the way a human studies a scene in the presence of turbulence to be a very complex and dynamic perceptual process. So - if there are observations or opinions at odds with theory - they should be considered.

Frank


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johnnyha
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Re: Stranger than fiction! new [Re: freestar8n]
      #4095966 - 10/06/10 02:02 AM

My take on the article after two readings was that the advantages of the achro are only apparent when the ED doublet APO is either (A) slightly out of focus, or (B) badly corrected.

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Mr. Bill
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Re: Stranger than fiction! new [Re: astroneil]
      #4099776 - 10/07/10 07:04 PM

Thanks for the thoughtful article....though I spend most of my time in the far scotopic world observing dark nebulae with my refractors...binoculars, short focus achromats, and one lone apo.




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jrbarnett
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Reged: 02/28/06

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Re: Stranger than fiction! new [Re: johnnyha]
      #4100229 - 10/07/10 11:46 PM

Aaaaaaaaaauuuuuuuuuughhhhhhhhh!

No, no, no, no!

What the article says is that even a nigh on perfect f/6 apochromat will struggle to put up as steady an image under average seeing conditions as a mediocre long focus achromat, *and* one must nearly try deliberately to make a poor long focus optic.

Sell your Taks...quick...before news of this article spreads.

- Jim


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johnnyha
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Re: Stranger than fiction! new [Re: jrbarnett]
      #4100517 - 10/08/10 05:02 AM

Well I knew it said something like that...

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astroneil
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: johnnyha]
      #4139867 - 10/24/10 09:50 PM

Some 2003 wisdom from a most respected man.

http://www.cloudynights.com/item.php?item_id=410

Thanks,

Neil.


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astronomer2002
newbie


Reged: 09/01/08

Re: Stranger than fiction! new [Re: astroneil]
      #4167481 - 11/06/10 12:17 PM

Hmmm..... the neverending debate on which telescope is best.

Though I have a big newtonian and a couple of large SCT's for imaging I have always had a hankering for refractors.

I have a 4inch f15 achromat in a brass tube that dates from around 1970. I chose to build a refractor back then as the images with my 3inch were so crisp compared to 6inch reflectors and I particularly wanted it for observing Venus. Looking at a Mars opposition with the 8inch Fry at Mill Hill Observatory convinced me that refractors were really the best choice for planetary detail as well as splitting close binaries. The original lens was a Wildey, whose OG's were well regarded at the time. Despite several returns to him either it wasn't sharp or had far too much colour. I was put off refractors for many years. The replacement was a hand chosen air spaced pair made by Emerson optical which transformed the scope. I watched the Venus transit with this instrument whilst the Apo's were automatically recording the event. Residual colour annoys me, and whilst this scope is now pretty good, and I still have it, the FS102 is far more pleasing as a visual instrument. Ah, I hear you say - but the Tak cost many times more. Nope - I paid £680 on fleabay for my Tak and heaven knows how much more over the years on my f15 achro.

I regularly use a 6inch f14 Cooke, and this instrument certainly delivers good images from a fairly mediochre lens. It does have the advantage of being on a mount that must weigh a ton or more, and that is probably the key. Having a second Cooke mount with nothing on it I tried a £200 6inch f8 Chinese achromat on it. With the Baader colour fringe reducer this performs adequately for open evenings though is not something I would recommend to any real observer. Even with a 48 inch focal length you need a really heavy mount to ensure the optics are limiting the image quality rather than the mount.

Visually comparing the Ed100 with the FS102 and f15 4inch the conclusion I have come to is that both the APO's have superior dark fields making nebulae and clusters stand out more than in the achro. The Tak has slightly more colour than the ED100 (damn) but it has a slighly more velvety dark field and so is marginally preferable as a visual instrument. The Tak is mainly used for imaging whereas this is not practical with the f15 achromat. If asked to suggest a small refractor then I don't hesitate to recommend the ED100 as this has by far the best performance to cost ratio of my refractors and can be used on a sensibly sized mount.

Just my two penneth. I would really like to believe an achro is as good as an apo - but I'm afraid I have yet to be convinced.

Edited by astronomer2002 (11/06/10 01:41 PM)


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Alan French
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Reged: 01/28/05

Loc: Upstate NY
Re: Stranger than fiction! new [Re: astroneil]
      #4167677 - 11/06/10 02:02 PM

Quote:

You can also get a modest reduction (~18%) in secondary spectrum using Bak2-F1 glass at little additional cost to standard achromat optical glasses.

Some interesting reading here:

http://www.rfroyce.com/refractor%20spots.htm

Cheers,

Neil.




Neil,

Your post reminded me that I'd been meaning to look at BaK1/F2 doublets compared to BK7/F2 (I'm assuming your BaK2/F1 is a typo).

For a C-F corrected BK2/F2 doublet I get a variation of focus of 1 part in 1788, which would translate into 1.279mm in a 6" f/15.

For a C-F corrected BaK1/F2 doublet I get a variation of focus of 1 part in 2002, which would be a variation of focus of 1.142mm in a 6" f/15.

Clear skies, Alan


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Donnie
scholastic sledgehammer
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Reged: 03/15/04

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Re: Stranger than fiction! new [Re: Alan French]
      #4168306 - 11/06/10 08:12 PM

This may sound stupid. but what is the effect of the longer tube on long focal length refractors with regard to the column of air that they are looking through internally?

I read and understand that the defocus range is actually greater with the instrument that has the most focus range. I got that. So even if there were some defocusing caused by the air itself within the tube and not the atmosphere, it should be just as great with the longer tube.

Before anyone jumps on me, I am not attempting to refute any evidence within the article. Over the years I have gleaned a lot of knowledge from Vlad's postings here and elsewhere. His involvement in this at any level tells me that there is important data within. Just curious if anyone has ever done tests with regard to the effects of air stability as it relates to the length of a tube and how that impacts image quality, if at all.

Congrats to Neil. This was a large undertaking, hats off to you.

Edited by Donnie (11/06/10 08:17 PM)


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astroneil
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Re: Stranger than fiction! new [Re: Donnie]
      #4169955 - 11/07/10 04:43 PM

Alan,
Concerning your focus variation stuff, I think it would be better to talk to Bob Royce about that. Unfortunately, I do not have the means to verify your data.

Donnie, thank you very much for your comments. As ever greatly appreciated!
In response to your question; in earlier drafts of the work Vlad and I discussed more obscure tube-related effects relating specifically to close-tube instruments like refractors. One consequence of having a 'hermetically sealed' design is that the metal, which cools at a much faster rate than the enclosed air, causes the air layer close to it to cool down more quickly than the air in the middle of the tube. That would effectively form a very weak negative (concave) lens, with the outer rays travelling slower than those toward the centre of the tube. If the tube air temperature gradient changes so that a near-spherical wavefront shape forms, the effect could manifest itself as focus extension. However, this is a very minor effect IMO, but again, it causes defocus error approximately in proportion to the F-ratio.

With best wishes,

Neil.


Edited by astroneil (11/07/10 04:50 PM)


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Alan French
Night Owl
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Reged: 01/28/05

Loc: Upstate NY
Re: Stranger than fiction! new [Re: astroneil]
      #4170203 - 11/07/10 06:41 PM

Quote:

Alan,
Concerning your focus variation stuff, I think it would be better to talk to Bob Royce about that. Unfortunately, I do not have the means to verify your data.
[SNIP]

Neil.





Neil,

It's actually a simple calculation based on the data on the glasses from the catalog.

I also found I had designs using BK7/F2 and BaK1/F2. The first shows a focal variation from C to F (at the 70% zone) of 1.31mm, while for the second it is 1.15mm.

Clear skies, Alan

Edited by Alan French (11/07/10 07:00 PM)


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astroneil
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: Alan French]
      #4170226 - 11/07/10 07:00 PM

Alan,

Then, as I said, you need to talk to Bob Royce about that.

Cheers,

Neil.



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Alan French
Night Owl
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Reged: 01/28/05

Loc: Upstate NY
Re: Stranger than fiction! new [Re: astroneil]
      #4170286 - 11/07/10 07:27 PM

Neil,

About what?

Clear skies, Alan


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Donnie
scholastic sledgehammer
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Reged: 03/15/04

Loc: Western Ky
Re: Stranger than fiction! new [Re: astroneil]
      #4170531 - 11/07/10 09:34 PM

Thank you Neil. I was just curious as to the effects. I was not sure if they would have a correlation or not and even if they did if it was substantial or minor.

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Glassthrower
Vendor - Galactic Stone & Ironworks
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Reged: 04/07/05

Loc: Oort Cloud 9
Re: Stranger than fiction! new [Re: Donnie]
      #4174222 - 11/09/10 01:43 PM Attachment (58 downloads)

Great article and an informative read.

The age-old debate between the long-achro crowd and the short-apo crowd will never be settled to everyone's satisfaction. Both scope types excel in certain areas and have disadvantages in others.

I've owned both types, long achros and short apo. I liked both, and I would be hard-pressed to choose one over the other. Instead, in a perfect world, I would own both - a long achro for binary-splitting and planetary viewing, and a short-fast apo for richfield viewing.

Ultimately, to me, the debate is about true field size and not so much the lack of spurious color. Unless I am viewing planets, I'd prefer a wider TFOV that grabs more sky in a single eyepiece view.

But, I am attracted to the aesthetics of long achros, and prefer them whenever possible.

Best regards and clear dark skies,

MikeG


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astroneil
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Re: Stranger than fiction! new [Re: Glassthrower]
      #4181686 - 11/12/10 07:20 PM

Thanks Michael,

And a very happy birthday to you!

Best wishes,

Neil.


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Mentor
professor emeritus


Reged: 09/30/10

Loc: Ontario, Canada
Re: Stranger than fiction! new [Re: astroneil]
      #4212902 - 11/27/10 11:27 AM

Superb article! Thanks to the authors for all of their work on this.

I'm not sure if I'm going to regret stepping into this or not, but I always considered that the primary reasons that one bought short focal ratio refractors were not because of a perceived advantage in optical quality, but rather:

1. The shorter tube is easier to transport, handle, and mount; and
2. The short focal length allows for rich-field views.


If anything, the decision to choose a short focal ratio refractor comes with it the understanding that you will compromise optics in favour of these other parameters. Of course the slower scope will be superior optically, all else being held equal.

Many of the arguments here seem to be neglecting these considerations, or even assuming that optics are the only consideration when one chooses a telescope. They are not.


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Starman1
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Re: Stranger than fiction! new [Re: Mentor]
      #4212969 - 11/27/10 11:51 AM

Quote:

Superb article! Thanks to the authors for all of their work on this.

I'm not sure if I'm going to regret stepping into this or not, but I always considered that the primary reasons that one bought short focal ratio refractors were not because of a perceived advantage in optical quality, but rather:

1. The shorter tube is easier to transport, handle, and mount; and
2. The short focal length allows for rich-field views.


If anything, the decision to choose a short focal ratio refractor comes with it the understanding that you will compromise optics in favour of these other parameters. Of course the slower scope will be superior optically, all else being held equal.

Many of the arguments here seem to be neglecting these considerations, or even assuming that optics are the only consideration when one chooses a telescope. They are not.



True.
I have had a lot of fun with a 5" f/5 refractor (definitely a Jimi Hendrix "Purple Haze" scope) at low powers.
My 12.5" reflector, though, if a refractor, would be too long and too heavy to transport--even if (and I couldn't) I could afford one.
One of the reasons long focal lengths are optically superior (if/when they are, that is) is that the shallower curves require less time to make and are less likely to have serious errors in figure. It's the reason why telescope designs that use spheroidal surfaces (such as Maksutovs) can be optically excellent--the curves are easier to generate and polish than paraboloidal or hyperboloidal surfaces.
That's not to say that an experienced hand at making the optics won't be an important issue.

One thing, though, has appeared as I've gotten older--the small exit pupils that arise when using long f/ratios have become more problematical visually. And it's harder to get eyepieces that result in larger exit pupils when the f/ratios are long.


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Mentor
professor emeritus


Reged: 09/30/10

Loc: Ontario, Canada
Re: Stranger than fiction! new [Re: Starman1]
      #4213003 - 11/27/10 12:08 PM

Quote:

Quote:

Of course the slower scope will be superior optically, all else being held equal.



One of the reasons long focal lengths are optically superior (if/when they are, that is) is that the shallower curves require less time to make and are less likely to have serious errors in figure. It's the reason why telescope designs that use spheroidal surfaces (such as Maksutovs) can be optically excellent--the curves are easier to generate and polish than paraboloidal or hyperboloidal surfaces.
That's not to say that an experienced hand at making the optics won't be an important issue.




Agreed. The other advantage that I had in mind, and that the article touches on, is the depth of focus. The tight focus tolerance on a fast scope is a major issue. Not only does it lead one to install a micro-focuser, but it also makes the scope more sensitive to atmospheric aberrations.


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Mauro Da Lio
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Reged: 09/12/04

Re: Stranger than fiction! new [Re: astroneil]
      #5006765 - 01/08/12 07:10 AM

I read the article and found it interesting. This definitely puts the word "end" on the thesis that "seeing sensitivity" is caused by focal ratio per se.

I have some remarks. The first is that "linear" depth of focus is meaningless. You can stretch the x axis in figure 3 at your wish by using a different reduction ratio for the focuser. For focusing what matters is the ratio between turning the focuser knobs and the defocusing effect. A scope with half focus depth and doubled reduction will behave apparently the same way: producing the same amount of defocus per turn of the knobs. Of course shorter focus scopes need more reduction.

What is meaningful is the ratio between the ideal focus depth and the focus depth as reduced by spherochromatism. We know the shift in best focus caused by turbulence is locked to the focus depth of an IDEAL telescope. So perfect scopes behaves exactly the same with respect to seeing, regardless of their focus ratio. This is not a novelty: Fried already got this conclusion in his 1966 paper: Optical Resolution Through a Randomly Inhomogeneous Medium for Very Long and Very Short Exposures, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 56, NUMBER 10. So it is surprising that claims of long focus behaving different than short focus still survived.

However a focus with residual spherochromatism has reduced depth of focus. The ratio between this ACTUAL depth and the IDEAL depth can vary depending on the amount of residual aberrations.

This completely changes the perspectives, because it is now clear the the cause is not long focus but residual aberrations.
What you show is that telescopes with more spherochromatism are more sensitive to seeing. Incidentally short focus apos are more likely to have greater residual aberrations. But the causal link is with residual aberration not with length of focus. If a shorter focus scope had residual aberrations less than a longer one, the longer would be worse. Again, a Newtonian design, as well as any other scheme, perfectly made would have the same sensitivity to seeing as a IDEAL scope (all equal).

On the other hand it is not surprising that the cause is residual aberration. Seeing induced wavefront errors and residual aberrations stack together (see Suiter). Thus if a threshold is set for image quality, then this threshold is reached first if the scope already has some residual aberrations. In this form people (I would say mostly reflectors guys) know that bad figured scopes are less tolerant, because they already have "eaten" a fraction of the error budget.

To be clearer, we know that the expected value for the rms wavefront error induced by seeing is (roughness term only) http://www.telescope-optics.net/induced.htm

rsm_seeing = 0.058 (D/r0)^(5/6)

For a given aperture D, the rms_seeing increases with decreasing of the Fried scale r0.

However th quality of the image depends on the total rms wavefront error, which is the sum of the above and the rms error of the scope:

total_rms = rms_seeing + rms_scope.

Thus it is clear that to stay within the same total_rms budget, in case the scope is worse, we can tolerate less atmospheric seeing, which means that we get the same quality of the image only if r0 is smaller, or we get worse quality with the same r0. The greater the rms_scope the smaller the seeing that can be tolerated before exceeding a set image quality threshold. Should the scope be barely diffraction limited, it leaves no room for additional (environmental) wavefront error. Bad scopes looks like the seeing were worse.

In conclusions bad scopes are more sensitive to seeing that good ones. Whenever somebody thinks that his long focus scope is better than his short focus one, that only means that the shorter one is less corrected (or the focuser has not the proper reduction ratio).

Edited by Mauro Da Lio (01/08/12 07:20 AM)


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