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Best Double Star Scope

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#1 mic1970

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Posted 08 April 2019 - 09:21 AM

I'm adding DS searching as a criteria for my next scope. 

 

Best scope for DS under $1K with mount or $700 without. 

Best scope under $2K

Bonus Question: favorite eyepiece.



#2 Astroman007

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Posted 08 April 2019 - 09:36 AM

Well, a refractor for sure. And Tele Vue eyepieces.


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#3 Astrojensen

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Posted 08 April 2019 - 09:48 AM

My Altair 102/1100mm ED certainly does extremely well on double stars. Price is around $600-$700. I use ES82 eyepieces and a 2mm Sky-Watcher LET eyepiece. 

 

gallery_55742_4772_351883.jpg

 

 

Clear skies!

Thomas, Denmark


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#4 mic1970

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Posted 08 April 2019 - 09:49 AM

Well, a refractor for sure. And Tele Vue eyepieces.

I'm struggling with focal length.  I have wide field, but seems to need to much magnification. 



#5 mic1970

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Posted 08 April 2019 - 09:53 AM

I'm thinking about Celestron's Mak or SCT. 

 

Astro... I want to grow up and be like you some day.  You have the best toys. 


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#6 CBert504

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Posted 08 April 2019 - 10:01 AM

I'm struggling with focal length.  I have wide field, but seems to need to much magnification.


Certainly not a DS aficionado, but have recently obtained a ES 102ED 700mm. Grabbed a second hand TV Type6 3.5mm and was amazed at the resolution i was getting on the handful of DS i went after a few nights ago...

#7 mic1970

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Posted 08 April 2019 - 10:28 AM

Certainly not a DS aficionado, but have recently obtained a ES 102ED 700mm. Grabbed a second hand TV Type6 3.5mm and was amazed at the resolution i was getting on the handful of DS i went after a few nights ago...

My buddy is an ES vendor and has that for a personal scope.  Man, that one is nice.  Out of my budget (for now).



#8 Hesiod

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Posted 08 April 2019 - 10:46 AM

For small apertures, a mid/slow focus refractor is the first thing to come into mind...however, mid/slow refractors are long, and call for rather sturdy mounts, so I would not go over a 4" one within your 1000$ limit (e.g. OMNI 102+cg4).

The good is that there is a lot of systems you can split with a 4", the bad is that, well, there are even more you could not...so it is really up to your personal aims.

The "humble" 8" f/6 Dob is well proficient even on DSs, and a simple tracking platform can easily add this useful feature, but of course you will notice spiders' diffraction and have to start to cope with largish telescopes' own quirks.

As for MCTs/SCTs, IMHO these have really 2 advantages: first, can use a largish aperture on a smallish mount (e.g. 5" MCT on eq5 mount), second the folded light path allows both for the use of rather simple eyepieces and good ergonomics/portability (assuming that these are meaningful to you).

 

With a 2000$ budget there are more options, but a nice Intes Micro MC or MN are IMHO better than most achromatic refractors (and some "apos" as well): beside the good correction of aberrations, the very smooth mirrors are an useful feature when observing strongly uneven pairs

 

As for me, I employ mostly a refractor, or a 5" MCT; as for the eyepieces, if the telescope focal is long enough, Baader Classic Orthos are a great budget option



#9 DAVIDG

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Posted 08 April 2019 - 12:49 PM

  An unobstructed scope with perfect color correction and a  long focal length with aperture of 4" or more.  With no obstruction a companion star  doesn't get lost in a diffraction spike. Perfect color correction mean you see the true colors of the stars. Long focal length allows  for high power with eyepieces with good eye relief and wider fields. So what scope fits those

criteria ?  My 4.25 F/27 Schiefspeigler.  Cost me under $100 to build, easily transportable in that it is  3 feet long and fits nicely on CG5 mount.

 

                     - Dave 

 

meandmyschief.jpg

 


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#10 Magnetic Field

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Posted 08 April 2019 - 01:10 PM

Well, a refractor for sure. And Tele Vue eyepieces.

 

You are saying a 5" refractor*** is the better double star splitter than say a 8" SCT?

 

Maybe I am wrong but aperture wins also for double stars (no different to observing the planets or globular clusters).

 

I posted in another thread that I would probably stay away from instruments with a secondary mirror that is being mounted by means of "spiders".

 

 

***Realistically there are not many 6 or 7" refractors out there; so I chose 5" as an illustration.

 

 



#11 Magnetic Field

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Posted 08 April 2019 - 01:14 PM

  An unobstructed scope with perfect color correction and a  long focal length with aperture of 4" or more.  With no obstruction a companion star  doesn't get lost in a diffraction spike. Perfect color correction mean you see the true colors of the stars. Long focal length allows  for high power with eyepieces with good eye relief and wider fields. So what scope fits those

criteria ?  My 4.25 F/27 Schiefspeigler.  Cost me under $100 to build, easily transportable in that it is  3 feet long and fits nicely on CG5 mount.

 

                     - Dave 

 

attachicon.gif meandmyschief.jpg

 

And your double star observing life is limited to 1".

 

What is wrong with 0.5" and a 8" SCT (barring a price comparison to your $100 built)?



#12 DAVIDG

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Posted 08 April 2019 - 01:44 PM

   Note that I said a scope of 4" aperture or more. My schief with it's aperture 4.25"  gives me all three of criteria I listed and performs at what theory show it can for that aperture. 

The large central obstruction in a 8 SCT moves energy out from the center part of the  Airy disk into the diffraction rings making them bright and larger.  For double star work this  can sometimes work to your advantage when the companion falls in the dark area between diffraction rings  but can also it work the other way and  can hide a close companion. Also having also bench tested many SCT and  their real optical quality can be fair from what is advertised. So you can  have spherical aberration reducing the resolution as well.  

   So if you have tested a SCT and it tests optically very well it will be a good double star scope but if  your looking for the best you want one that also doesn't have a central obstruction. 

   If you look at a long focus refractor, they have been the preferred scope for double star work because they meet two of the requirements I listed  and come very close on color correction.  

 

          - Dave 



#13 Magnetic Field

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Posted 08 April 2019 - 02:37 PM

   Note that I said a scope of 4" aperture or more. My schief with it's aperture 4.25"  gives me all three of criteria I listed and performs at what theory show it can for that aperture. 

The large central obstruction in a 8 SCT moves energy out from the center part of the  Airy disk into the diffraction rings making them bright and larger.  For double star work this  can sometimes work to your advantage when the companion falls in the dark area between diffraction rings  but can also it work the other way and  can hide a close companion. Also having also bench tested many SCT and  their real optical quality can be fair from what is advertised. So you can  have spherical aberration reducing the resolution as well.  

   So if you have tested a SCT and it tests optically very well it will be a good double star scope but if  your looking for the best you want one that also doesn't have a central obstruction. 

   If you look at a long focus refractor, they have been the preferred scope for double star work because they meet two of the requirements I listed  and come very close on color correction.  

 

          - Dave 

In and ideal world everyone would have an 8" apo.

 

I have no proof but would probably bet money on the following: every mediocre 8" SCT will have an easy ride on your 1" double star list for your 4" instrument; with all the additional benefits of the larger instrument (planets, deep sky observing etc).

 

A 4" apochromat (or a Schiefspiegler) is definitely a potent instrument but will never be in the same league than a 8" SCT.

 

My instrument is a 110mm modified Cassegrain from Vixen. There a reasons why I cannot go up in size.


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#14 Jon Isaacs

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Posted 12 April 2019 - 10:41 AM

The large central obstruction in a 8 SCT moves energy out from the center part of the  Airy disk into the diffraction rings making them bright and larger.

 

 

On the other hand, the larger aperture of the SCT means the Airy disk is smaller, the diffraction rings are smaller, the energy more concentrated.  In comparing a 4 inch apo to an 8 inch SCT, the first diffraction ring of the SCT fits inside the spurious disk of the 4 inch apo.

 

For my money, the best double star scope depends on your seeing and climate.  It is important not to use the Dawes limit as a measure of resolution, a Dawes limit split is a very difficult split with the centers of the airy disks nearly touching.  Double the aperture, the Dawes limit split becomes much easier.  With a 4 inch, an equal magnitude 1.2 arc-second split is very difficult, with a 10 inch, it's wide.

 

Climate is important because it thermal stability is very important, is it actually possible for the scope to truly reach thermal equilibrium?  

 

I like Newtonians for double star work. 

 

Large apertures with good quality optics are affordable, active cooling is easy, central obstructions are relatively small.  I observe the planets and double stars from our home in San Diego, it's about 4 miles from the ocean, nice light laminar flow breezes off the ocean. It's typically south of the jet streams.  And the climate is very mild, indoor temperatures and outdoor temperatures are typically about the same.  

 

1 arc-second seeing is very common, half arc-second is not uncommon.  Set the Dob out, start the fan cooling and hope for good seeing.  

 

I like my 10 inch, it's handy and quite capable.  The 13.1 inch F/5.5 has better optics, cools more slowly but will split tighter doubles if the seeing allows.

 

Jon


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#15 John Fitzgerald

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Posted 13 April 2019 - 02:01 PM

Jon,

Do you get much salt air at your place that would degrade coatings?

 

The best double star scope I have seen for its aperture was my friend's excellent 6" f/15 Jaegers lens homebuilt refractor.  He doesn't get it out much because the mount is so tall and heavy and the scope is so long.  I would love to have that scope in an 11 foot dome.



#16 Cotts

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Posted 13 April 2019 - 03:06 PM

  An unobstructed scope with perfect color correction and a  long focal length with aperture of 4" or more.  With no obstruction a companion star  doesn't get lost in a diffraction spike. Perfect color correction mean you see the true colors of the stars. Long focal length allows  for high power with eyepieces with good eye relief and wider fields. So what scope fits those

criteria ?  My 4.25 F/27 Schiefspeigler.  Cost me under $100 to build, easily transportable in that it is  3 feet long and fits nicely on CG5 mount.

 

                     - Dave 

 

attachicon.gif meandmyschief.jpg

Central obstructions less than 20% by diameter give diffraction patterns indistinguishable from completely unobstructed scopes.*   If you mount the secondary on an optical window or a corrector plate the problem of spider vanes disappears.

 

For double star work my 5.7” Mak Newt was the equal of a 1988 AP 150mm triplet APO and had better colour correction. 

 

*Suiter's book as well as my own side-by side comparisons.

 

Mak-newts are the poor man's APO refractor.  For instance, to get noticeably better resolution and contrast to my 145 Mak-Newt you would have to get into a 155mm triplet APO.  For at least 3 times the price....

 

Dave


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#17 John Fitzgerald

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Posted 13 April 2019 - 03:56 PM

Central obstructions less than 20% by diameter give diffraction patterns indistinguishable from completely unobstructed scopes.*   If you mount the secondary on an optical window or a corrector plate the problem of spider vanes disappears.

 

For double star work my 5.7” Mak Newt was the equal of a 1988 AP 150mm triplet APO and had better colour correction. 

 

*Suiter's book as well as my own side-by side comparisons.

 

Mak-newts are the poor man's APO refractor.  For instance, to get noticeably better resolution and contrast to my 145 Mak-Newt you would have to get into a 155mm triplet APO.  For at least 3 times the price....

 

Dave

Dave, I believe you.  Don't say that in the refractor forum.  You would be steamrolled, bulldozed, bludgeoned, and shot. (figuratively speaking of course) tongue2.gif



#18 coopman

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Posted 13 April 2019 - 05:57 PM

Shot AND sent to the Russian Front! (from an episode of Hogan's Heroes).



#19 fred1871

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Posted 14 April 2019 - 07:19 AM

Dave, would be helpful to describe a bit more detail regarding your Mak-Newt - presumably high optical quality. What maker? And what was the CO size, and focal ratio?

 

I have the SkyWatcher 190mm Mak-Newt, f5.3 and a lot more than 20% CO diameter - and it in no way is competition for the 6-inch f/12 AP apo triplet I used years ago. Yours I would expect to need more than reduced CO to match the refractor.



#20 Jon Isaacs

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Posted 14 April 2019 - 07:40 AM

Jon,

Do you get much salt air at your place that would degrade coatings?

 

The best double star scope I have seen for its aperture was my friend's excellent 6" f/15 Jaegers lens homebuilt refractor.  He doesn't get it out much because the mount is so tall and heavy and the scope is so long.  I would love to have that scope in an 11 foot dome.

 

John:

 

We are about 5 miles from the ocean, salt air doesn't seem to be a problem.  The ocean can really help stabilize the seeing and it makes for a moderate temperatures in the summer and the winter but it's a great source for clouds that roll in during the evening after a bright, sunny day.  

 

Jon



#21 Cotts

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Posted 14 April 2019 - 12:55 PM

Dave, would be helpful to describe a bit more detail regarding your Mak-Newt - presumably high optical quality. What maker? And what was the CO size, and focal ratio?

 

I have the SkyWatcher 190mm Mak-Newt, f5.3 and a lot more than 20% CO diameter - and it in no way is competition for the 6-inch f/12 AP apo triplet I used years ago. Yours I would expect to need more than reduced CO to match the refractor.

Made by Peter Ceravolo of Ottawa, ON.  His goal for the scope (and its 216mm big sister...) was to make a no-compromise visual-only instrument of the very highest quality.  I suspect the mirror and corrector optics are as perfect as can be.  Add to that a very low profile helical focuser allowing a 16% central obstruction in a n f/6 scope.  I have no strehl or other data on the 145 but my experience of comparing it side-by side with a TEC 140 and a 1988 f/8 AstroPhysics triple APO at VERY high power on numerous double stars was this:

 

  (The tests were done 145 vs. AP 6" and then 145 vs. TEC  a couple of years later... I did not own the two refractors at the same time)....

 

The first diffraction ring of the 145 was no brighter than that of either of the refractors on the same star, same night.  The central disc was the same size and brightness.  There were no "extra" artifacts of any sort in any of the three telescopes.  I was specifically looking for the tell-tale signs of a central obstruction: a brighter first ring and a smaller central spot.  I could not find them.  

 

I should add here that I have tried the same side-by side between my 12.5" Lockwood f/6.5  18% C.O. and my 11" Celestron Ultima 30% C.O. and the difference in the diffraction patterns was painfully obvious.  

 

The downside: The refractors had a much larger fully-illuminated field than the 145 - the tiny secondary mirror must be paid for somehow.  But in a double-star scope this is of no concern...

 

What the Physicists (and the guys over in the refractor forum) will mention:

 

(From Telescope Optics dot net...)

 

%CO        % of encircled light inside the first minimum      Remainder of light mostly in the first ring.

 

0%.......................................83.8...............................................................16.2

 

10%.....................................81.8...............................................................18.2

 

20%.....................................76.4...............................................................23.6

 

30%.....................................68.2...............................................................31.8

 

Roughly interpolating for a 15% CO the numbers are 79.1 and 20.9.

 

The question is whether a person can see the difference between 16.2% of the light in the rings and 20.9% of the light in the rings in the field under real conditions in very good seeing.  Suiter says 'no' and my experience is the same.

 

Another way of looking at it is to consider the cost of getting the 83.8/16.2 figures versus the corresponding numbers for a 15% or 20% C.O....

 

Dave


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#22 fred1871

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Posted 14 April 2019 - 09:25 PM

Dave, thanks for the detail. Very informative. I agree with you and Suiter about the lack of discernible effect of very small CO, under about 20%.

 

In the case of the Celestron vs Lockwood, there are likely two major differences - one obviously is the CO relative size, quite large in the C11 at 30+%; the other is that the Lockwood mirrors are to be expected better on optical quality, fabricated to a high rather than a sufficient standard. No surprise that the diffraction patterns differ notably.


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#23 jgardner

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Posted 15 April 2019 - 01:06 PM

I can confirm the Ceravolo HD145 is a superb double star performer.  It is capable of theoretical limits performance (I've had it near 1 arc second) and also reliably splits Antares.  I've never tried side-by-side with other scopes, though.


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#24 Sarkikos

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Posted 24 April 2019 - 08:44 AM

My best double star scope is the one with the largest aperture:  a 10" f/4.8 Dob.  But do I use it often for double stars?  No. 

 

For doubles, I take out a small APO/ED, like my C80ED or AT72ED.  Maybe if I was pushing it, I'd bring out the NP101 or SW120ED.

 

Double star astronomy is grab-n-go for me.  Light and easy and a quick cool-down.  If I take out a bigger, heavier scope, I'm probably going to be looking at deep sky or at least the planets.

 

Mike


Edited by Sarkikos, 24 April 2019 - 09:05 AM.

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#25 Tom Axelsen

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Posted 24 April 2019 - 09:54 AM

A Takahashi Mewlon 180 is also a great double star scope smile.gif

 

It splits stars to Dawes limit and shows nice oval Airy-discs if the separation is below Dawes-limit.

Long focal length (2160 mm) gives plenty of magnification if that is necessary e.g. for check of oval Airy-discs smile.gif

 

I used my Mewlon 180 this past Sunday to look at 30 UMa, with a magnification of 540x, the Airy-disc was obvious oval. The separation was 0.42", far below Dawes limit for an opening of 180 mm.


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