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Eyepieces for double stars F/6 80mm ED

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#1 iKMN

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Posted 17 April 2019 - 09:05 AM

I’m mostly a DSO observer but lately I have been getting into double star observing. It’s something I can enjoy on moon nights. I have a set of ES100s and for the most part I just use my ED80 and the ES30mm to find them and then go straight to my 5.5mm. The thing is my 5.5mm is too tall to fit in my eyepiece tray on the TWI and there is a lot of back and forth from the case to the diagonal. Also the 5.5 only gives me x87 magnification in my 480fl scope.

So the goal is I would like a lightweight set of 1.25s that I can use for double star observing as well as some planetary.

I am thinking of getting the ES24mm/68 to use as a finder EP and then the 3mm 52, 4.5mm 52, 5.5mm 62. That should give me exit pupils from 0.5, .75 and .91 - which from what I have researched should be good for double stars. Just wanted opinions if I’m headed in the right direction with this.

K

#2 zohsix

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Posted 17 April 2019 - 09:14 AM

I look at doubles with an 85mm and find that I am limited by seeing conditions (Bortles 8-ish) to a 4mm eyepieces.  I tried a 3mm and it just wouldn't work well enough.  You might want to check the Brandon's, they're really clear and small.

 

Dan 



#3 iKMN

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Posted 17 April 2019 - 09:51 AM

I look at doubles with an 85mm and find that I am limited by seeing conditions (Bortles 8-ish) to a 4mm eyepieces.  I tried a 3mm and it just wouldn't work well enough.  You might want to check the Brandon's, they're really clear and small.
 
Dan


I guess I should have added, I live in rural part of FL Bortle 4 sky closer to 3 then 5. Seeing is mostly good, transparency not so much. I used to Barlow a 4.7 and 6.7 Es with good results but those have been sold off. I’ll look a those Brandon’s and see what FLs they have. I should have also mentioned maybe I should just try to pickup a used TV 3-6 zoom?

Sorry if I sound like asking a lot of questions I’m just not that much of a double star guru. I probably don’t even have an ideal scope for it I realize but it’s convenient for GnG and something to do. We are about to enter our rainy season in a few months so the clear nights are going to be far and few till Fall so I’m eager to get some eps for double stars Incase the moon is out. Thanks for the input.

#4 Astro-Master

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Posted 17 April 2019 - 10:05 AM

I use the TeleVue  3-6 Zoom for double stars, it works great.  You just dial in the right power for the separation you want or for the seeing that night.  One of the best features is it stays focused through out the whole range.

I use mine on my Stellarvue 105 APO.  It gives a power of 122x to 245x and with the barlow 250x to 500x


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#5 Spikey131

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Posted 17 April 2019 - 10:08 AM

A TV zoom is a great solution for high magnification in a short refractor. I use one in the NP101 almost every time I observe. Pricey, but worth it IMHO.

The 3mm end will give you 160x. With a 2x Barlow you would have 160-320x, pushing beyond the usable limit in your scope.
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#6 macdonjh

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Posted 17 April 2019 - 10:24 AM

One nice thing about Barlow-ing longer focal length eye pieces is preserving the eye relief of those eye pieces, if that's important to you.

 

If you're interested in the really high magnification end of the eye piece spectrum, I've read nothing but good things about those really short Vixen eye pieces, though I've never view through one.  They are available in 2.4mm, 2.0mm and 1.6mm.  They are pricey, though.

 

If you're feeling flush, Takahashi Abbe are available down to 6mm, LE down to 5mm and there is the TOE at 4mm.  Of course, if you've won the lottery, there are always the UW: 10, 7, 5.7 and 3.3mm (and $660 ea, yikes!).

 

I like the old Radians, but some don't like their color rendition.  If you're interested in those, the are available (used) in 3mm, 4mm, 5mm, 6mm, and 9mm (and longer if you like).  For current Tele Vue offerings there are the DeLites (3, 4, 5, 7 and 9mm) and Delos (3.5, 4.5, 6 and 8mm).

 

Meade HD-60?  Explore Scientific 52o?  Lots of choices out there.


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#7 rkelley8493

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Posted 17 April 2019 - 10:36 AM

The Delos 3.5 is pretty hefty for a 1.25" eyepiece. Weighs in over a pound, and it's very tall. Probably as tall as the 9mm ES 100so idk if that would fit your bill. It's a very nice eyepiece, don't get me wrong, but its features are not lightweight & compact. Anyways, one of the Cloudy Nights users had a lot of positive things to say about the 3mm ES 52and I think they posted a thread on here. Something like "My honest opinion on the 52eyepieces from Explore Sci" [paraphrasing]. 


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#8 Cotts

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Posted 17 April 2019 - 10:41 AM

A couple of things.   I don't think the darkness of your skies as measured by the Bortle scale is any indicator of how high a magnification you can use to examine double stars. The seeing/steadiness of the air is key. 

 

And do not be afraid to pour on the power, violating 'rules' which say things like 'no more than 50x aperture in inches' or 'such and such exit pupil'....   When trying to view pairs at and even a little below your scope's theoretical limit* you'll want to be able to clearly examine the Airy disc/s of the star/s as well as to be able to discern 'oval', 'peanut', 'snowman' or 'dark sky' splits.  150x  or even a bit more is entirely reasonable for this sort of effort...  

 

For an 80mm (3.15 inches) the Rayleigh limit for a true dark sky split is 1.73".  The Dawes limit which yields a 'snowman' is 1.45".   You should be able to 'notch' pairs even closer like 1.3".....

 

The advantage of an 80mm is that when the seeing is such that a 150mm or 200mm scope has the Airy disc smeared out to a fuzzy blob you can see the Airy disc quite clearly (because your scope cannot resolve the poor seeing)....

 

Dave


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#9 Cotts

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Posted 17 April 2019 - 10:56 AM

Here is a list of Test pairs for your 80mm....  From Stella Doppie, the best friend website a double star enthusiast has...

 

Screen Shot 2019-04-17 at 11.49.57 AM.jpg

 

The pairs were chosen to have separations from 1.1" to 2.0", have the primary be brighter than mag 7.0 and to have a magnitude difference of no more than 1.0 magnitude and be in the spring and summer sky via the excellent search function on the website...

 

Have fun!

 

Dave

 

 


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#10 coopman

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Posted 17 April 2019 - 11:00 AM

The Brandons are the most lightweight EPs that I've ever used.  Your scope won't even know they're there!   



#11 Sagitta

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Posted 17 April 2019 - 12:04 PM

Double star observing has been my favorite pastime during the last 5 years.

 

I own a TeleVue TV-85 (f7). I use the Explore Scientific16 mm 68* for separations as low as 10''. I have the Pentax XO 5mm and Pentax XO 2.5mm for tight binaries.

 

I have come to the conclusion that I don't like great magnification when observing double stars. This is why I mostly use my 16mm EP, preferring to observe the loose doubles over the tight ones...

 

Last year I bought the 9mm and 5.5mm ES 62* (but I haven't yet taken the time to use them!).


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#12 Jon Isaacs

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Posted 18 April 2019 - 01:51 AM

For an 80mm (3.15 inches) the Rayleigh limit for a true dark sky split is 1.73".  The Dawes limit which yields a 'snowman' is 1.45".   You should be able to 'notch' pairs even closer like 1.3".....

 

 

I do not believe the Rayleigh criterion yields a true "dark sky split", a Rayleigh split is when the first minimum of one star matches the center of the second star.  

 

http://hyperphysics....opt/Raylei.html

 

 

And do not be afraid to pour on the power, violating 'rules' which say things like 'no more than 50x aperture in inches' or 'such and such exit pupil'....   When trying to view pairs at and even a little below your scope's theoretical limit* you'll want to be able to clearly examine the Airy disc/s of the star/s as well as to be able to discern 'oval', 'peanut', 'snowman' or 'dark sky' splits.  150x  or even a bit more is entirely reasonable for this sort of effort...

 

:waytogo:

 

In my 80mm F/7, I will use the 3.5 mm Nagler with a 2x Barlow.  That is over 300x and is about 100x/inch.  The seeing here is generally quite good and on a good night, I will use that same combination in my 10 inch F/5 for 821x.  

 

Jon



#13 Ernest_SPB

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Posted 18 April 2019 - 05:34 AM

Here is a list of Test pairs for your 80mm....  From Stella Doppie, the best friend website a double star enthusiast has...

 

...

 

The pairs were chosen to have separations from 1.1" to 2.0", have the primary be brighter than mag 7.0 and to have a magnitude difference of no more than 1.0 magnitude and be in the spring and summer sky via the excellent search function on the website...

 

Very interesting site!

 

Could you specify format for "Separation" text box in case selected "between"? I am failed to search from .. to ..


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#14 Cotts

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Posted 18 April 2019 - 01:37 PM

Very interesting site!

 

Could you specify format for "Separation" text box in case selected "between"? I am failed to search from .. to ..

If you select 'between' then type in the box the two values separated by a comma, no spaces.

 

e.g. for 'between' 1.1 and 2.0 arc sec type 1.1,2.0

 

Dave



#15 Cotts

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Posted 18 April 2019 - 01:49 PM

I do not believe the Rayleigh criterion yields a true "dark sky split", a Rayleigh split is when the first minimum of one star matches the center of the second star.  

 

http://hyperphysics....opt/Raylei.html

 

 

 

Hi, Jon.  Whether or not the Rayleigh split is true dark sky depends on the magnitude of the stars.  If the stars are brighter the central discs of the diffraction pattern each fill more of the space out to the first minimum causing overlap.  For a pair with two fainter stars, the central discs are smaller (even though the radius to the first minimum is the same) and there is a better chance for a dark sky split. 

 

In a given telescope for a pair of 4.0 mag stars at Rayleigh separation there will not be a dark space but for a pair of 7.0 mag stars (e.g.) at the same separation there will be..

 

Since there are far more faint pairs than bright ones my generalization is about 90% correct...my observing experience matches this....

 

Dave




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