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refractor best for doubles?

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#26 jrbarnett

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Posted 29 October 2012 - 10:42 PM

I observed doubles for more than a decade with a Newt. Still do from time to time (now with a Dob). I think Dob/Newts of moderate aperture work well for doubles. Bigger Dob/Newts, like bigger anything elses, are more subject to bad seeing. I suspect the reason so many folks conclude that refractors are great double star scopes has more to do with the rarity of refractors over 6".

I'd vote for seeing being the culprit. On a still night your 10-incher will likely eat your 4-incher for din-din on doubles.

Regards,

Jim

#27 azure1961p

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Posted 30 October 2012 - 09:35 AM

Some food for thought;

http://neilenglish.n...rious-achromat/

Regards,

Neil.


I thought the article was excellent RIGHT up to the point the detracting remarks were made about reflectors at the very end. The need for collimation and thermal issues and other things are all thrown together like some chaotic soup of troubles. At least for me, once the reflector is collimated, it stays that way for the night [or more], with regard to thermals, use a fan. Then theres the point raised of "mechanical flexure" which is absolutely false at least in my experience.

The whole article was well put together save for the biased note at the end.

Pete

#28 astroneil

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Posted 30 October 2012 - 11:17 AM

Thanks for the feedback gents.

PJ: Thoroughly enjoyed your CN article on the little Zeiss Telementor.

Post scriptum now added to the article for clarification.

Best wishes,

Neil. ;)

#29 fred1871

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Posted 30 October 2012 - 07:48 PM

Pete, I can't see the article as having a 'biased note' at the end, if you mean Christopher Taylor's summary of the issues with reflectors. Taylor uses a reflector himself, and I took his comments as based on his experience. It also matches well with my own experience with reflectors over decades. I don't currently own a Newtonian but I've had a few over the years and often used Newts owned by others.

Thermal issues can be a big problem with Newtonians - I had access to one (a 16-inch) that rarely if ever reached thermal equilibrium before midnight.

Another issue is collimation - less of a problem with longer Newtonians, but Taylor talks about super-fine collimation with his f/7 12.5-inch telescope and the difference it makes. That matches what I found with a 10-inch f/6 Newtonian I used to own.

Regarding Jim Barnett's comments - yes, the 10-inch will normally easily surpass any 4-inch. But I saw plenty of nights when the 16-inch I mentioned above was notably inferior to a 7-inch refractor, same night, same observing location. Pinpoint stars in the refractor, fuzzy blobs from thermal problems with the Newtonian.

Some of the worst star images I've seen were with 1980s "light-bucket" Dobs of the f/4.5 persuasion. Poor optics, inadequate mirror supports, poor collimation and flexure in truss systems, etc - fine for faint fuzzies but awful for double stars.

I think that was reflected (sorry, pun) in the Saguaro Double Star Database (version 2, back in 1991) where they excluded stars that a 4-inch refractor would show - to quote, "If the stars were closer than 2 arc seconds then the magnitudes had to match within one magnitude" or the pair was excluded.

So a double of mags, say 6.0 and 7.3, at 1.8", would be dropped out - too difficult. Never mind that it'd be within reach of a 4-inch refractor. I think that says a lot about the older generation of short Dobs. Thankfully, these days, there are short Newtonians that are well-designed and well built, and with a Paracorr added they can be useful double star machines. But some of the older Dobs are still around. And there are plenty of observers who don't collimate their scopes well enough. Refractors are pretty easy in that regard, as they don't usually get out of collimation and they show less of a problem if they are slightly out. And thermal issues are much less.

Most of us can't get access to larger refractors, so for the fainter pairs, and the closest ones, we have to find alternative optical styles. As Christopher Taylor shows, a good Newtonian can be a fine double star machine.

Neil's interest was to defend achromat refractors. I fully agree with his thesis, as the big refractors, though suffering great gobs of false colour, still do remarkable things for double star observing. In the smaller sizes the market offers, apochromats are great, and I've had extensive experience with them, but for doubles their benefits are in my experience overstated. In typical amateur sizes, say 8-18cm, apos show only modest improvements compared to equal size achromats for splitting doubles. The biggest gain is in colour fidelity.

#30 Jon Isaacs

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Posted 02 November 2012 - 05:15 AM

Regarding Jim Barnett's comments - yes, the 10-inch will normally easily surpass any 4-inch. But I saw plenty of nights when the 16-inch I mentioned above was notably inferior to a 7-inch refractor, same night, same observing location. Pinpoint stars in the refractor, fuzzy blobs from thermal problems with the Newtonian.


- In my experience, Newtonians require more care and attention to make them perform. This is not only in the collimation and thermal management but also in the design and even the mounting. Certainly a large scope that has not thermally equilibrated will not perform, this doesn't really depend on what type of scope it is...

Yeah, there is no doubt that a 16 inch operating near the Dawes limit is going the not only require careful preparation, decent optics but also excellent seeing... Such a stable atmosphere is rare enough that one rarely chooses a large Newtonian for observing doubles... And too, there are better things to do with a large well cooled scope when the seeing is excellent.

On the other hand... when it all does come together... :whee: :whee: :whee:

- Location, Location, Location.. Different scopes for different folks, at latitude 50N one is facing a much different situation than someone at 30N. Someone observing on the lee side of a mountain range faces a much different situation than someone living along a peaceful coast with gentle winds wafting overhead. How good is the seeing? How often is it excellent? (half arc-second or better) What is the diurnal temperature change, how often do the jet streams present a problem?

- Scopes that are operating near the Rayleigh Criteria (1.36 arc-seconds for a 4 inch) under perform compared to a larger scope that is just "idling along." Doubles that are barely doable in a 4 inch are easy pickings in a reasonably well prepped 10 inch.

- The original question here was about Iota Cassiopeia... For me, it's a challenge in an 80mm, easy pickings in a 4 inch and up. The brightness difference is enough that a 10 inch shows it better than a 4 inch.

- If I were a skier, I would not choose San Diego for a home base. Good skiing requires long drives to get to the good conditions. On the other, for splitting doubles, San Diego makes a good home base. Temperate climate, mild weather near a calm ocean most commonly south of the jet streams... I wouldn't be picking a large Newtonian to split doubles if I lived most other places but here, it makes good sense.

Jon Isaacs

#31 drollere

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Posted 02 November 2012 - 01:26 PM

and the timing is perfect for me to saunter back into the lounge, hitch my suspenders, crack some phlegm, scratch my auricular wax with the long nail on my little finger, and observe that it really does come down to personal preference.

all the substantive and factual statements made above are certainly substantive and factual. no doubt, big newts are bigger, calm skies are calmer, and star colors are colorful. however:

i claim for refutation that there is not much of a correlation between these issues and personal choice of telescope. line up 50 folks living under bad seeing with a refractor, and i could probably find as many living under the same conditions with a reflector. line up 20 folks that use a newt because they love star color, and i'll find 20 that use a refractor because they love star color. (hi, sissy!)

"Doubles that are barely doable in a 4 inch are easy pickings in a reasonably well prepped 10 inch." this is literally true, but elides the fact that double stars form a fractal (self similar) landscape: double the resolution and magnitude limit, and you're just splitting pairs that look like the pairs you could already split -- there are just more of them, and most of them are going to be very faint. you're also going to double your sensitivity to, and hassles with, cool down and seeing and collimation and so on.

the practicable visual limit appears to be fixed around v.mag. 10: my experience puts it at around 9.5 with my 12" SCT, and in paul couteau's opinion "experience shows that, whatever the aperture, magnitude 10 is a barrier. In a large instrument the images lose their sharpness and break up. Light is lost in the diffraction rings, and the eye does not receive very much more illumination." he may just be describing a limit imposed by atmospheric dispersion even when there is absolutely zero thermal turbulence. point is, a 150mm aperture is already reaching well beyond this limit.

i also don't know of, but suspect there is, a limit in enjoyable star color that is probably around v.mag. 7 or 8. i do not know of any binary star where the lore says "colorless in small apertures, this pair shows a lovely color contrast above an aperture of X." all the famous color pairs are quite colorful in apertures of 150mm at most.

there is a huge diversity of instrument ownership and observing experience on CN, and sifting through it all one can seem to come to conclusions about equipment, good or bad, up or down, regardless of observing experience.

i am in the minority: i have come to conclusions about observing experience, regardless of equipment ... and my main conclusion is: diff'rent folks, diff'rent strokes. each of us has to try equipment for ourselves. "refractors best for doubles?" is a question that can only be answered by personal experience. some of us hate achromat color, and some of us don't even notice it. some of us can discover that about ourselves after owning 2 telescopes, and others will want to own 10, or 20 to definitively define their prejudice. and that process, too, is just a personal preference ... nothing "objective" about it.

#32 WRAK

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Posted 02 November 2012 - 05:25 PM

... the practicable visual limit appears to be fixed around v.mag. 10: my experience puts it at around 9.5 with my 12" SCT...

Despite a far smaller aperture of 140mm (5.5") I came to a similar conclusion - while I can despite light pollution split doubles up to +9.5mag for the primary and up to +10.5mag for the secondary the visual pleasure is certainly less than for brighter pairs because the image quality is no longer this satisfying.
Wilfried

#33 jrbarnett

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Posted 02 November 2012 - 05:28 PM

I dunno, bruce. To my eye, color saturation improves linearly with aperture. My 16" Dob always shows much more vivid star color than any of my smaller apertures, several of which you've sampled. Agree about the rest (best scope design for doubles being largely personal preference), but not about color intensity. Bigger scope of any design, more color than smaller scope of any design.

Case in point. I met a couple at Calstar who had just purchased an 8" Orion XT Intelliscope, but were too intimidated to bring it to their first star party. They arrived kind of late, and ended up camping out in the hinterlands with me and the other hermits. It was dusk when they rolled in, so they missed my "Keep Out", "Visitors not Welcome", "Touch it and die!" "No Trespassing" and similar signage. I'm glad they did, too. I took down the signage before dawn so that they would think better of me than I deserved.

Anyway, I had two scopes set up and two impressionable minds to mold and co-opt to refractor weeniedom; an AT111EDT and the 16" f/5.1 Dob. Early in a session it's great having newer observers around. The stuff you look at waiting for it to get *really* dark is the stuff that feeds the imaginations of folks just starting out. I think I showed them about fifteen deep sky objects in each of the two scopes, with some overlap to illustrate what aperture does and does not do.

The the hubby asks "the question". You know the one. "Why do people bother looking at double stars?" I thought about decking him, but changed my mind. Instead, I put my "other" to bed and decided to do a little teaching. First I showed them a few bright stars in the AT111EDT. Vega, we agreed, without coaching from me was bluish white. Arcturus, by comparison, was yellowish, etc. So then we looked at Alberio. "Wow, you really can see the color difference!" To which I replied: "Nah, not really. Come over here." In the 16" the colors that they thought were vivid and unmistakable in the 4.3-incher really were vivid, technicolor objects in the 16-incher. I find the same to be true as magnification drops, too.

Regards,

Jim

#34 azure1961p

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Posted 02 November 2012 - 06:38 PM

Hi Jim,

My take and this isn't concrete, is that color, particularly red stars show best at half the limuting magnitude of the scope. That right there puts a preconditiin on it since to have

#35 azure1961p

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Posted 02 November 2012 - 06:41 PM

Hi Jim,

My take and this isn't concrete, is that color, particularly red stars show best at half the limuting magnitude of the scope. That right there puts a precondition on the claim aince it assumes a specific limiting mag and that varies wildly. For me though it seems to fit my scopes. Briggter yhan that and the reds turn orange or worse, yellow.

My finds anyway.

Pete

#36 WRAK

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Posted 03 November 2012 - 04:06 AM

I...To my eye, color saturation improves linearly with aperture...
Jim

Interesting - my experience is quite contrary: When decreasing aperture with masks I get more color saturation.
Wilfried

#37 jrbarnett

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Posted 03 November 2012 - 01:35 PM

Ever see colored stars in globulars with smaller apertures? Color in nebulae? :question:

"In general, larger aperture will resolve more, because its effective point-source (which can be also seen as image pixel) is, as mentioned, inversely proportional to the aperture size. Also, it will have better color saturation."

http://www.telescope..._resolution.htm

- Jim

#38 drollere

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Posted 03 November 2012 - 02:12 PM

I dunno, bruce. To my eye, color saturation improves linearly with aperture. My 16" Dob always shows much more vivid star color than any of my smaller apertures, several of which you've sampled. Agree about the rest (best scope design for doubles being largely personal preference), but not about color intensity. Bigger scope of any design, more color than smaller scope of any design.

well, yes and no. what i was pointing to was (my experience) that increasing aperture does not uncover a new perception of color in stars that in smaller aperture appear colorless, and this "colorless" threshold may be fixed by factors independent of aperture. that is separate from the changes in apparent color that occur with changes in aperture in stars that are already perceived as colorful in small apertures, including binoculars.

i'd conjecture that how much a star color appears to change with aperture depends on the visual brightness and the hue of the star. i'd guess there will be a stronger perceived effect for "red" (K and M) stars than for "blue" stars, because the eye is inherently more sensitive to luminance differences as color differences in the long wavelength part of the spectrum. (in fact, the D65 illuminant, which is vaguely blue in terms of its spectral profile, appears "white" at almost any luminance level.) in addition, as pete observed, changes in the brightness of yellow to red hues produce distinct color shifts -- darkening toward red, and brightening toward yellow.

but i'd need to share these comparisons at the eyepiece in order to make sensible judgments. that's the only way to determine how much of this is individual differences in color vision.

all i have to offer on vlad's site, other than my extreme admiration for his scholarly diligence in optical theory, is that many of his declarations about color vision are based on theoretical conjectures with limited bearing on actual (and individual) color experience.

#39 JIMZ7

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Posted 03 November 2012 - 10:27 PM

I had a 8" f/6.9 Coulter reflector that provided maybe the best images of double stars when I masked it down to roughly 3.75". It gave me refractor type images. I also had a Criterion RV-6 with a small secondary along with a 3-vane curved spider which gave refractor type images without any false color. I'm happy with my present refractors,but there are ways to make sharper images in reflectors with the little things I mentioned.

Jim :refractor:

#40 Daniel Mounsey

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Posted 07 November 2012 - 09:49 AM

More commonly, this is what the reflector will look like for the average observer unless they take some cooling measures.

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#41 Daniel Mounsey

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Posted 07 November 2012 - 09:50 AM

And more often, this is what the refractor will look like.

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#42 Daniel Mounsey

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Posted 07 November 2012 - 10:25 AM

The solution is an aperture mask, which will greatly clean things up to make the image look like #2. Two important things will happen though.

1) reduced angular resolution.
2) light loss for seeing fainter stars.

The solution? Make a list of double stars that are within the power range of the aperture and enjoy, problem solved.

Sure, there will be some nights where you'll wish you had a bit extra, but that mask is a great solution.

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#43 dpwoos

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Posted 18 November 2012 - 01:38 PM

Here in Vermont (not thought to have very good seeing) I regularly get clean, beautiful splits of iota Cass in my homemade 10" f/6 dob at 120X. I accept that a top-flight refractor can split doubles at lower magnifications than a significantly larger reflector, but it ain't a huge difference - more like 20-30% less mag at the most. I think that any 10" reflector that can't keep up with a smaller refractor within that range has some problem(s).

#44 Daniel Mounsey

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Posted 19 November 2012 - 08:28 PM

Your point is well taken. Actually what the images mainly imply are thermals that many amateurs encounter. I remember using my 14.5" dob(which I sold) on doubles with and without the off axis mask at a regular event which rarely encountered good seeing. The results with the smaller off axis aperture mask were much better. Where the aperture comes in handy though are on fainter doubles and tighter doubles but that's only if the circumstances allowed. My 10" DK easily outperformed my 6" apo by splitting finer and tighter doubles, no if ands whats or bts about it.

The general issue though is that aperture is over rated, mainly because the average observer will most likely adopt a reflective design. Because of the inherent issues they encounter, it requires special attention that most observers would not be aware of in fact I know several veteran observers here on CN who still don't even know.

#45 dpwoos

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Posted 20 November 2012 - 12:59 AM

I know that my 10" dob is excellent, and yet I also know that it is not as up-to-snuff as a $4,000 apo. My collimation is imperfect, and my coatings are imperfect, and my thermal control is imperfect, and my stray light control is imperfect, and the larger exit pupil means that my imperfect eyesight also comes into play. However, I also know that the views of Jupiter that I had this evening surpass anything that a much smaller (but more perfect) apo can provide, and the fact that I can easily split 1 arcsecond doubles means that, in my mind, my homemade reflector is one heck of an instrument. I can live with the fact that the apo can split a 2 arcsecond double at 70x whereas my 10" requires 90x!

#46 Asbytec

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Posted 20 November 2012 - 01:44 AM

Well, I promised Mike Cee I'd promote his 10" refractor as the best scope on doubles. :)

#47 Ziggy943

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Posted 21 November 2012 - 11:51 AM

I don't need perfect seeing to detect the dark space in 72 Pegasi.....but it sure would make a perty picture! :lol: Mike :brick:


Awesome view of your scope.

#48 Asbytec

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Posted 21 November 2012 - 12:14 PM

Daniel, that's zeta Cancri. (Edit: just realized it even says so in the corner...LOL)

More often than not, the obstructed views of zeta Cancri I get are more refractor like according to your image comparison. The difference you illustrate is a bit harsh toward obstructed scopes. I understand you were driving home a point, but that comparison is scary for reflector owners. The message is refractors are pretty, reflectors are UGLY. That's just too biased.

No doubt refractors have a very aesthetic view and beautiful diffraction patterns. It's probably (and sadly) true many reflectors operating out there need proper collimation and cooling. But, that's no fault of a superb design. If the aperture debate is off the table, well the argument ends here. There are a lot of variables to cover, including the nature of seeing. I disagree the solution is to stop down the aperture and settle for a wider Dawes limit. However, doing so when conditions suggest it will help is fine. An aperture mask is a tool, not a solution.

But an obstructed scope can put up nice images with nice diffraction patterns, too. Cooled and collimated, of course, as you correctly imply. And if so, they will look like your second image, more often than not.

#49 Daniel Mounsey

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Posted 21 November 2012 - 09:27 PM

Norme, after looking at it some more, I actually agree, it is a bit too harsh. Let me see if I can find something a bit more accurate. Thanks for mentioning this. I don't want to mislead anyone too much.

#50 Daniel Mounsey

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Posted 21 November 2012 - 09:56 PM

How about this for on Albireo instead. Hard finding accurate images hmmm...

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