Jump to content


Photo

refractor best for doubles?

  • Please log in to reply
63 replies to this topic

#1 jhfenimore

jhfenimore

    Sputnik

  • -----
  • Posts: 40
  • Joined: 30 Nov 2008
  • Loc: Upstate New York

Posted 24 September 2012 - 08:36 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.

#2 simpleisbetter

simpleisbetter

    Surveyor 1

  • *****
  • Posts: 1848
  • Joined: 18 Apr 2011

Posted 24 September 2012 - 08:54 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.

#3 drollere

drollere

    Surveyor 1

  • *****
  • Posts: 1581
  • Joined: 02 Feb 2010
  • Loc: sebastopol, california

Posted 24 September 2012 - 11:32 PM

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 ƒ/20 dall kirkham that can get me up to 1500x ... i just love popping really close pairs under mediocre seeing.

#4 simpleisbetter

simpleisbetter

    Surveyor 1

  • *****
  • Posts: 1848
  • Joined: 18 Apr 2011

Posted 25 September 2012 - 10:05 AM

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

#5 jhfenimore

jhfenimore

    Sputnik

  • -----
  • Posts: 40
  • Joined: 30 Nov 2008
  • Loc: Upstate New York

Posted 25 September 2012 - 11:29 AM

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

#6 Ed Wiley

Ed Wiley

    Apollo

  • -----
  • Posts: 1013
  • Joined: 18 May 2005
  • Loc: Kansas, USA

Posted 25 September 2012 - 07:10 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

#7 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10354
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 25 September 2012 - 09:56 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

#8 WRAK

WRAK

    Apollo

  • -----
  • Posts: 1151
  • Joined: 18 Feb 2012
  • Loc: Vienna, Austria, Europe

Posted 26 September 2012 - 01:47 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

#9 Erik Bakker

Erik Bakker

    Gemini

  • *****
  • Posts: 3152
  • Joined: 10 Aug 2006
  • Loc: The Netherlands, Europe

Posted 26 September 2012 - 02:50 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.

#10 Ed Whitney

Ed Whitney

    Messenger

  • -----
  • Posts: 483
  • Joined: 08 Jul 2010
  • Loc: Palm Coast, Florida

Posted 30 September 2012 - 12:03 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.

#11 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43894
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 30 September 2012 - 06:48 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

#12 tomharri

tomharri

    Messenger

  • *****
  • Posts: 489
  • Joined: 19 Sep 2008
  • Loc: along the I-10 corridor

Posted 30 September 2012 - 06:53 PM

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

#13 jhfenimore

jhfenimore

    Sputnik

  • -----
  • Posts: 40
  • Joined: 30 Nov 2008
  • Loc: Upstate New York

Posted 04 October 2012 - 08:08 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

#14 fred1871

fred1871

    Viking 1

  • -----
  • Posts: 870
  • Joined: 22 Mar 2009
  • Loc: Australia

Posted 04 October 2012 - 08:32 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".

#15 Rich (RLTYS)

Rich (RLTYS)

    Fly Me to the Moon

  • *****
  • Moderators
  • Posts: 5167
  • Joined: 18 Dec 2004
  • Loc: New York (Long Island)

Posted 05 October 2012 - 05:10 AM

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

Rich (RLTYS)

#16 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10354
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 05 October 2012 - 11:29 PM

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


Pete

#17 mikey cee

mikey cee

    Cosmos

  • *****
  • Posts: 8038
  • Joined: 18 Jan 2007
  • Loc: bellevue ne.

Posted 06 October 2012 - 11:31 AM

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! :lol: :lol: :lol: Mike

#18 WRAK

WRAK

    Apollo

  • -----
  • Posts: 1151
  • Joined: 18 Feb 2012
  • Loc: Vienna, Austria, Europe

Posted 08 October 2012 - 01:20 AM

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! :lol: :lol: :lol: Mike


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

#19 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10354
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 08 October 2012 - 06:19 AM

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! :lol: :lol: :lol: Mike


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

Pete

#20 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4812
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 08 October 2012 - 03:46 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! :lol: :lol: :lol: 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? :grin:

- Dave

#21 drollere

drollere

    Surveyor 1

  • *****
  • Posts: 1581
  • Joined: 02 Feb 2010
  • Loc: sebastopol, california

Posted 08 October 2012 - 07:59 PM

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.

#22 fred1871

fred1871

    Viking 1

  • -----
  • Posts: 870
  • Joined: 22 Mar 2009
  • Loc: Australia

Posted 09 October 2012 - 07:27 AM

Dave, I think I'd put my money on the 10" refractor. :grin:
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.

#23 mikey cee

mikey cee

    Cosmos

  • *****
  • Posts: 8038
  • Joined: 18 Jan 2007
  • Loc: bellevue ne.

Posted 11 October 2012 - 04:44 PM

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:

Attached Files



#24 astroneil

astroneil

    Surveyor 1

  • *****
  • Posts: 1806
  • Joined: 28 Jul 2009
  • Loc: res publica caledoniae

Posted 29 October 2012 - 08:56 AM

Some food for thought;

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

Regards,

Neil.

#25 PJ Anway

PJ Anway

    Double-Star Observer

  • *****
  • Moderators
  • Posts: 2641
  • Joined: 04 Jun 2003
  • Loc: North Coast

Posted 29 October 2012 - 07:20 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!






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics