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jhfenimore
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Loc: Upstate New York
refractor best for doubles?
      #5438843 - 09/24/12 09:36 PM

Have recently discovered the pleasure of double star observing from my light polluted and poor-to-average seeing environment. Observing i CAS tonight, I was very surprised that I could see only two stars with my excellent 10 inch Dob, while my 4 inch APO clearly showed all three. The 6.9 companion was lost in the 4.6 glare in the 10 inch, but not in the 4. I'm new at this game. Is this refractor advantage a common experience with doubles, or was my Dob simply a victim of mediocre seeing. I'm just trying to determine which of the two scopes is the best all-around choice for doubles. Thanks in advance for your advice.

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simpleisbetter
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Re: refractor best for doubles? new [Re: jhfenimore]
      #5438856 - 09/24/12 09:54 PM

Only 2 factors ever hindered me on doubles when I had dobs.
1. Collimation, if that was off, dimmer stars sometimes couldn't be viewed, and the primary stars airy disk isn't as well controlled.
2. The components tended to hide from me in the diffraction spikes of the spider assy.

Other than that, if glare or separation was an issue, that was easily solved just by increasing magnification. Doing that increased separation to aid in getting dim components further away from the primary's glare.

Jon Isaacs gave a good summary on this a year or so back IIRC, in a thread about resolving Epsilon Lyra. He explained that the larger the scope, the higher the magnification needed for the same resolution. It's not about magnification so much but about exit pupil. To achieve the same exit pupil and resolution, a larger scope needs higher magnification than a smaller scope. Perhaps he or might pass by to help explain, especially if I'm misquoting him or got something wrong on the explanation.


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drollere
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Re: refractor best for doubles? new [Re: simpleisbetter]
      #5439038 - 09/25/12 12:32 AM

the basic answer is that you should use the scope you prefer or enjoy using more. use them both on the same night if possible, on the same targets, until you form a clear preference. (you may also form a preference depending on the target double: colored doubles may look prettier in the refractor, close doubles may split better in the reflector.)

there are considerations that cut both ways, and you will hear technical arguments for one side or the other. i've come to believe that it is mostly personal perferences and esthetics that determine these choices.

the problem you describe with iota cas illustrates the tradeoffs. because a reflector has a central obstruction, it pushes some of the light in the airy disk into the rings around the airy disk, making them brighter. as you found, this can obscure a faint companion if the companion falls within the rings. on the other hand, a smaller aperture produces an airy disk that is larger in angular width, which can overlap a very close companion.

the airy disk in a 4" (~100mm) scope is about 2.3 arcseconds in diameter, but in a 10" (250mm) scope is only 0.9 arcseconds in diameter. a matched double star that was separated by 1.5 arcseconds would appear as an elongated fat rod in the 100 but as two cleanly separated small discs in the 250, if viewed under equal magnification. the size of the airy discs is the reason that the rayleigh resolution in the 100 is about 1.4" but in the 250 is 0.56".

(digression on steve's version of jon's recommendation ... the basic principle is that the angular interval that can be resolved gets smaller with larger aperture. to see that smaller interval, you need greater magnification (because it's smaller). because the airy disc also gets smaller in larger apertures, you'll also need greater magnification to see the airy disk clearly in a larger aperture. but if two stars are separated by a specific interval (say 2 arcseconds), then the first issue is just the magnification that makes that separation visible to your eye; the second issue is whether you will see a rod or two small disks with the aperture.

(magnification depends on your personal naked eye resolution, looking at a dim image with a dark adapted pupil. the usual quoted dark adapted resolution is about 120 arcseconds; i've measured mine, and it's 115 arcseconds. the magnification required for an observer to clearly see a resolution interval R, given an eye resolution of X, is X/R. so assuming steve has a resolution of 120", he will need 120"/1.4" = 86x with the 4" but 120"/0.56" = 214x in the 10".

(exit pupil EP is the ratio of aperture over magnification. once you determine your resolution EP then that applies to all telescopes. so 100mm/86x = 1.17, and 250mm/214x = 1.17, and steve at 120" eye resolution will need an EP of about 1.17; i would need 1.22. the common recommendation is that you need an exit pupil of 1.0 to clearly see a split at the resolution limit of your telescope, although the airy disk can be visible at about twice that, or an EP of 2.0. i think jon was making the point that the necessary exit pupil for visual resolution is a constant across all telescopes, but that the necessary magnification depends on the aperture.)

to the extent that glare is part of the telescope image, then magnification does little to mitigate it. a good eyepiece does nothing to alter any aspect of the image per se; it only makes it visually larger. however this does make the image fainter, because detail is spread over a wider area of the retina; this makes the sky darker and therefore (paradoxically) faint stars more visible -- up to a point. it also shifts the image brightness in relation to the eye's contrast sensitivity, so that if Sirius B is just slightly brighter than the glare from the primary, there will be an optimal magnification to make that slight contrast easiest to see.

coming back to iota cas, i'd recommend you go back with the 10" and try piling on the magnification. bright double stars (those bright enough to create distinct airy discs) can tolerate pretty much all the magnification you can throw at them. and at high magnification the disks can actually appear through atmospheric turbulence, like headlights through a heavy rain. otherwise, a "just right" magnification somewhere between EP 1.0 to 0.5 can adapt the image brightness to your contrast sensitivity in a way that can make the star visible within the diffraction rings. it's the ability to achieve really high magnification that would make the 10" a much more attractive choice for me. in fact, i have a 250 /20 dall kirkham that can get me up to 1500x ... i just love popping really close pairs under mediocre seeing.


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simpleisbetter
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Re: refractor best for doubles? new [Re: drollere]
      #5439528 - 09/25/12 11:05 AM

Thanks Bruce, my memory's not what it used to be and doesn't seem to improve with age...

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jhfenimore
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Reged: 11/30/08

Loc: Upstate New York
Re: refractor best for doubles? new [Re: simpleisbetter]
      #5439692 - 09/25/12 12:29 PM

Thanks Bruce. I'll do as you say: crank up the magnification on the Royce 10" f/6 (home made)dob, and compare the two scopes on the same object the same night. I really appreciate the effort that went into that very comprehensive reply.

Jack


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Ed Wiley
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Re: refractor best for doubles? new [Re: jhfenimore]
      #5440551 - 09/25/12 08:10 PM

The "received view" is that refractors are better than reflectors for observing doubles. That said; an 8" refractor is huge and an 8" reflector can be small. An 8" reflector, well collimated and with a descent mirror, will split doubles that a 4" refractor will not. I have friends who simply like refractors. Fact is, at the Okie-Tex I looked through a 6" AstroPhysics refractor that blew my socks away. But for me, my 8" DK, well collimated and well focused is just fine simply because it fits in my SkyShed Pod.

Ed


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azure1961p
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Re: refractor best for doubles? new [Re: Ed Wiley]
      #5440812 - 09/25/12 10:56 PM

The short answer to the post title is that refractors are best technicaly, but reflectors and cassegrains are best practicaly.

The test you did with the 4" versus the 10" is really only comparable when the power per inch of scope is identical or nearly so. Under those conditions, provided the reflector is collimated, cooled and high quality, the little 4" won't have a chance. Horrendous seeing might temporarily favor the smaller aperture but in mediocre to good, the ten will show stars utterly impossible through the refractor u mention. Too, the list of doubles available to the ten swamps those available to the 4.

Pete


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WRAK
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Re: refractor best for doubles? new [Re: azure1961p]
      #5441012 - 09/26/12 02:47 AM

Within it's optical limits (1.2" separation and +8.5mag for the primary and +9.5 for the companion due to light pollution) your 4" refractor will do the job probably better than your 10" reflector and if only it is simpler to handle.
If you want to split doubles with less than 1.2" separation and fainter doubles as indicated above you will simply need the greater aperture.
Wilfried


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Erik Bakker
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Re: refractor best for doubles? new [Re: jhfenimore]
      #5441047 - 09/26/12 03:50 AM

Refractors are superb in showing a beautiful clean Airydisk with very little light-energy in the rings. Coupled with a good binoviewer, the views are just stunning. However, the refractor is limited by aperture. So for the first hour or two, I enjoy my apo. After that, my 16" Newt generally takes over on most nights. But never with the stability of the Airydisks of the little 4" apo.

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Ed Whitney
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Re: refractor best for doubles? new [Re: Erik Bakker]
      #5448422 - 09/30/12 01:03 PM

If you -must- have a refractor for doubles, then you might consider the ES152 acro.

I recently got the SW102AR acro and love it for doubles! Lots of CA in daylight, but none for night work with stars only views. It works right down to the Dawes Limit for resolution also, sharp clean images, pin-point stars and splits. The reason I got this over possibly the ES152 is that the 152 would be too heavy on the Voyager mount. You would need a stronger mount with the ES152, like the CG5-ASGT, etc.

However, when I want to get serious and split close doubles, then it's the C8-EdgeHD that does the job nicely. If I had to choose only one scope for doubles it would be the C8. And the Voyager mount does carry the C8 with less vibration settle time than the SW102 because the tube is shorter, weight being nearly identical.


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Jon Isaacs
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Re: refractor best for doubles? new [Re: Ed Whitney]
      #5449063 - 09/30/12 07:48 PM

My thinking/experience:

Sky Tools 3 says Iota Cassiopeiae looks like this:

AB: 4.53+7.09 mag PA 229 Sep 2.61"
AC: 4.53+8.5 mag, PA 116 Sep 7.40"

In my experience it's a relatively easy split in most scopes as long as the mag 8.5 companion is visible. There have been lots of great posts but I have not seen much said about the basics:

When it comes to splitting double stars, to get the most out of a Newtonian, it's got to be cooled down. I am not talking about setting a 10 inch outside for an hour and waiting, I am talking about setting it outside for an hour with a good fan cooling the mirror and then observing for another hour as it further cools. And this is in San Diego. If it's a Dob, take care with touching the OTA, your hands setup tube currents, wear gloves...

As Bruce and others mentioned, crank up the magnification, your eye just does not resolve Airy disks at 2mm exit pupils, those 4 inch refractor views look nice at 100x because it's a small 1mm exit pupil, in the 10 inch it's 2.5 mm and doesn't see it, crank it up to 250x and things begin to show...

I have a variety of scopes, from my backyard, my NP-101 gets in a lot of double star viewing but there is no doubt that my journeyman 10 inch F/5 GSO Dob does the number on the NP-101 when it comes to double stars if I have taken the time to prepare it... Stars that are at the limit of the NP-101 are easy splits in the 10 inch... if it's ready.

Jon


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tomharri
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Re: refractor best for doubles? new [Re: Jon Isaacs]
      #5449071 - 09/30/12 07:53 PM

Larger is better, my 10" average dob can see and split more doubles than just about any refractor under $10,000.

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jhfenimore
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Loc: Upstate New York
Re: refractor best for doubles? new [Re: tomharri]
      #5455732 - 10/04/12 09:08 PM

Bruce (and others)
First clear night in more than a week. Aimed at iota cas, increased the power on the 10" dob to 214x (never went above 175x before tonight)and the close companion came out of the primary's glare. At 250x, it was even better. Thanks for the optics lesson.
Jack


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fred1871
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Re: refractor best for doubles? new [Re: jhfenimore]
      #5455764 - 10/04/12 09:32 PM

Some good comments in this thread - and no clear answer, 'cos there isn't one. Personally I like refractors for their clean images, but a Newt, MakCass, SCT, etc - these can all be fine for double stars. And with bright doubles, as the OP found, more can be less until you add more magnification. Hence the 4-inch doing what the good 10-inch at first appeared not to do - but factors such as extra light and greater seeing sensitivity can make a big difference.

Back in the mid-90s I spent a period near San Diego, and observed many of the northern doubles I can't see from my normal south of the equator location. Regarding the star that started this thread, my notes indicate I had a very nice view of Iota Cass as a triple one night with a C8 - the wider pair obvious at 80x, with the closer companion just seen - at 135x clear separation, a beautiful triple. So, as Ed Whitney remarked, a C8 "does the job nicely".


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Rich (RLTYS)Moderator
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Re: refractor best for doubles? new [Re: fred1871]
      #5456124 - 10/05/12 06:10 AM

I've always prefered reflectors for double star observing because, in my opinion, they show more accurate star colors.

Rich (RLTYS)


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azure1961p
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Re: refractor best for doubles? new [Re: Rich (RLTYS)]
      #5457290 - 10/06/12 12:29 AM

And in the generous apertures they come in there's just do many more available that show color.


Pete


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mikey cee
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Re: refractor best for doubles? new [Re: azure1961p]
      #5457887 - 10/06/12 12:31 PM

If anyone cares to bring their big dobs or newts over to my place my 10" R30 Istar refractor is lookin' for an easy KO! Mike

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WRAK
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Re: refractor best for doubles? new [Re: mikey cee]
      #5460024 - 10/08/12 02:20 AM

Quote:

If anyone cares to bring their big dobs or newts over to my place my 10" R30 Istar refractor is lookin' for an easy KO! Mike




10" refraktor, wow. This is an achromat - right?
No problem with colors when splitting close doubles?
Wilfried


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azure1961p
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Re: refractor best for doubles? new [Re: mikey cee]
      #5460171 - 10/08/12 07:19 AM

Quote:

If anyone cares to bring their big dobs or newts over to my place my 10" R30 Istar refractor is lookin' for an easy KO! Mike




If somebody brings a big dob and the seeings good, your going to be sorry you made the offer.

Pete


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DAVIDG
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Re: refractor best for doubles? new [Re: mikey cee]
      #5460943 - 10/08/12 04:46 PM

Quote:

If anyone cares to bring their big dobs or newts over to my place my 10" R30 Istar refractor is lookin' for an easy KO! Mike




If I lived a bit closer I would be happy to bring over my 4.25" Schiefspeigler, or my 4" f/12 or 4.5" f/16 Schupmann or 3.6" off axis newtonian and see about that easy KO Remember the movie Rocky?

- Dave


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drollere
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Re: refractor best for doubles? new [Re: jhfenimore]
      #5461266 - 10/08/12 08:59 PM

Quote:

Aimed at iota cas, increased the power on the 10" dob to 214x (never went above 175x before tonight)and the close companion came out of the primary's glare. At 250x, it was even better. Thanks for the optics lesson.




glad it helped. magnification is very effective on many double stars, especially in mediocre seeing. it's remarkable that isn't more widely known ... maybe the planetary observer's rule ("no more magnification than the seeing allows") is inhibiting.


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fred1871
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Re: refractor best for doubles? new [Re: DAVIDG]
      #5461722 - 10/09/12 08:27 AM

Dave, I think I'd put my money on the 10" refractor.
When you've got various unobstructed apertures, I expect aperture to win, unless seeing is very very bad.

Wilfried - no problem with achromats - most of the classical observers, right up to quite recent years, used large achromats by preference for discovering and measuring doubles.

Main problem for those of us who like refractors is the unavailability of larger sizes for most amateur observers. So we go to other designs for bigger scopes - perfection isn't necessary. Though I do find, for close unequal pairs, the unobstructed scopes perform beyond their aperture compared to those with central obstructions. Given high-grade optics all round, of course.


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mikey cee
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Re: refractor best for doubles? new [Re: fred1871]
      #5465729 - 10/11/12 05:44 PM Attachment (71 downloads)

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

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astroneil
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Re: refractor best for doubles? new [Re: mikey cee]
      #5494334 - 10/29/12 09:56 AM

Some food for thought;

http://neilenglish.net/the-mysterious-achromat/

Regards,

Neil.


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PJ AnwayModerator
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Re: refractor best for doubles? new [Re: astroneil]
      #5495259 - 10/29/12 08:20 PM

Hi Neil,

Very nice article.

I think of the Dorpat 9.5" achromat used by Struve or the Bedford 5.75" used by Smyth, of which he said " It will bear, with distinctness, a magnifying power of 1200 " or my little Telementor for that matter and I know your argument is solid!


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jrbarnett
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Re: refractor best for doubles? new [Re: jhfenimore]
      #5495560 - 10/29/12 11:42 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


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azure1961p
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Re: refractor best for doubles? new [Re: astroneil]
      #5495996 - 10/30/12 10:35 AM

Quote:

Some food for thought;

http://neilenglish.net/the-mysterious-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


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astroneil
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Re: refractor best for doubles? new [Re: azure1961p]
      #5496100 - 10/30/12 12:17 PM

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.


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fred1871
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Re: refractor best for doubles? new [Re: azure1961p]
      #5496861 - 10/30/12 08:48 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.


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Jon Isaacs
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Re: refractor best for doubles? new [Re: fred1871]
      #5500444 - 11/02/12 06:15 AM

Quote:

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...

- 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


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drollere
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Re: refractor best for doubles? new [Re: Jon Isaacs]
      #5501052 - 11/02/12 02:26 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.


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WRAK
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Re: refractor best for doubles? new [Re: drollere]
      #5501317 - 11/02/12 06:25 PM

Quote:

... 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


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jrbarnett
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Re: refractor best for doubles? new [Re: drollere]
      #5501326 - 11/02/12 06:28 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


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azure1961p
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Re: refractor best for doubles? new [Re: jrbarnett]
      #5501447 - 11/02/12 07:38 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


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azure1961p
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Re: refractor best for doubles? new [Re: jrbarnett]
      #5501454 - 11/02/12 07:41 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


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WRAK
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Re: refractor best for doubles? new [Re: jrbarnett]
      #5501967 - 11/03/12 05:06 AM

Quote:

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


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jrbarnett
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Re: refractor best for doubles? new [Re: WRAK]
      #5502494 - 11/03/12 02:35 PM

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

"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-optics.net/telescope_resolution.htm

- Jim


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drollere
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Re: refractor best for doubles? new [Re: jrbarnett]
      #5502543 - 11/03/12 03:12 PM

Quote:

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.


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JIMZ7
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Re: refractor best for doubles? new [Re: drollere]
      #5503179 - 11/03/12 11:27 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


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: JIMZ7]
      #5508103 - 11/07/12 09:49 AM Attachment (38 downloads)

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

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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5508104 - 11/07/12 09:50 AM Attachment (61 downloads)

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

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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5508152 - 11/07/12 10:25 AM Attachment (46 downloads)

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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dpwoos
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5526166 - 11/18/12 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).

Edited by dpwoos (11/18/12 01:40 PM)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: dpwoos]
      #5528741 - 11/19/12 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.


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dpwoos
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5529249 - 11/20/12 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!

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Asbytec
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Re: refractor best for doubles? new [Re: dpwoos]
      #5529288 - 11/20/12 01:44 AM

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

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Ziggy943
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Re: refractor best for doubles? new [Re: mikey cee]
      #5532093 - 11/21/12 11:51 AM

Quote:

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




Awesome view of your scope.


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Asbytec
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Re: refractor best for doubles? new [Re: Ziggy943]
      #5532135 - 11/21/12 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.

Edited by Asbytec (11/21/12 12:21 PM)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Asbytec]
      #5532976 - 11/21/12 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.

Edited by Daniel Mounsey (11/21/12 09:35 PM)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5533011 - 11/21/12 09:56 PM Attachment (32 downloads)

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

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Asbytec
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5533031 - 11/21/12 10:11 PM

LOL, Daniel. Spikes!

Well, if a refractor was looking over a clothes line, eh?

(Chuckling silently to myself.)


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Daniel Mounsey
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Re: refractor best for doubles? new [Re: Asbytec]
      #5533043 - 11/21/12 10:19 PM

LOL!!! Maybe you could share something that gives an idea of the appearance in the eyepiece. The Albireo image is too saturated.

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Asbytec
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5533055 - 11/21/12 10:32 PM Attachment (35 downloads)

If you looking for something with spikes, I'd have to drop by the mall.

Well, cooled and perfectly collimated and obstructed, maybe something like...


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zvaragabor
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Re: refractor best for doubles? new [Re: Asbytec]
      #5546693 - 11/30/12 06:20 AM

Do you think a 90/1000 refractor is good for double star observing?

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azure1961p
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Re: refractor best for doubles? new [Re: zvaragabor]
      #5546733 - 11/30/12 07:30 AM

A 90mm is fine and some folks are forever content and even prefer it over larger though obstructed telescopes. I prefer my 6 and 8 over a smaller refractor as the available list of doubles resolvable in both seperation and color is substantially longer. The bigger you go the less u have to pick and choose but those beautiful BRIGHT doubles can then appear blown out and glaring through bigger apertures. Just some thoughts. My 70mm is dandy on doubles but I prefer my 8".

Pete


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zvaragabor
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Re: refractor best for doubles? new [Re: azure1961p]
      #5546737 - 11/30/12 07:35 AM

Thanks azure. Unfortunately 6" (and more) is out of my budget, and it even needs a larger mounting, which weighs and cost even more. I want a lightweight and low-budget scope. I even thought of a 4"/1000.

Edited by zvaragabor (11/30/12 07:37 AM)


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WRAK
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Re: refractor best for doubles? new [Re: zvaragabor]
      #5546951 - 11/30/12 10:37 AM

With a 4" refractor you have many 1000 interesting doubles within the range of your scope and it es very easy to use - certainly a good choice for DSO.
Wilfried


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azure1961p
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Re: refractor best for doubles? new [Re: WRAK]
      #5547003 - 11/30/12 11:25 AM

Youmight find Orion's 6" f/8 reflector dob very budget friendly. Probably more affordable than the refractor. I appreciate your budget concerns. At anyrate either system well made is at least good.

Pete


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zvaragabor
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Re: refractor best for doubles? new [Re: azure1961p]
      #5547220 - 11/30/12 01:23 PM

I own a 10" reflector dob, so a 90mm refractor would be a huge step back. But my dream is to observe doubles with a long focal length refractor. A refractor with aperture 90-100mm, and f/ratio f12-15 would be the best, but unfortunately it's a rarity in Hungary. The closest match is a Meade DS-90 (90/1000), a Soligor 3.7"/1000, or a Celestron 100/1000. All the tree are within budget, but the longest f/ratio is the meade. I know the bigger aperture means a slightly better resolution, but I symphatize with the meade.

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azure1961p
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Re: refractor best for doubles? new [Re: zvaragabor]
      #5547663 - 11/30/12 06:47 PM

In that case get the refractor by all means. It compliments what you have in the 10" and if ever you feel the need for more resolution reach for the ten. Both s opes would actually strike quite a fine balance. It IS fun going between my refractor and reflector. I thought you were looking for one to do it all. And a lot of folks who are seasoned observers actually are fine with four.

Good luck with your choice. You were fine anyway I just thru in my two cents.


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azure1961p
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Re: refractor best for doubles? new [Re: zvaragabor]
      #5547667 - 11/30/12 06:47 PM

In that case get the refractor by all means. It compliments what you have in the 10" and if ever you feel the need for more resolution reach for the ten. Both s opes would actually strike quite a fine balance. It IS fun going between my refractor and reflector. I thought you were looking for one to do it all. And a lot of folks who are seasoned observers actually are fine with four.

Good luck with your choice.

Pete


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rockethead26
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Re: refractor best for doubles? new [Re: Daniel Mounsey]
      #5548371 - 12/01/12 07:41 AM

I agree with Jim. Doubles like Albireo have much more saturated color in my 14.5" dob than in my 120 ED. It's not even close, at least from my light polluted front yard.

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azure1961p
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Re: refractor best for doubles? new [Re: rockethead26]
      #5548921 - 12/01/12 02:49 PM

Its interesting Albireo with different aperture:

The yellow blows out in my 8" and actually has better saturation in my 70mm and 150mm. Conversley the daphire companion is most vivid in the 8". My perceptions anyway.

Pete


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zvaragabor
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Re: refractor best for doubles? new [Re: azure1961p]
      #5551541 - 12/03/12 05:51 AM

Thanks guys, I'll let you know which one I choose.
Cheers


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