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azrabella

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Hello folks. I currently own just one scope - namely a 150mm f/1200 Dobsonian with a very good set of mirrors, and use it in less than pristine skies. My main interest is double stars but have not really used eyepieces shorter than 9mm. I did try a BST Starguider 8mm but found it far too soft to offer any advantage. Perhaps my most often used eyepice is the 16mm Nirvana ( barlowed) and that seems to work ok. I can comfortably discern a roughly equal pair of stars down to about 1.0" which is probably approaching the scope's theoretical limit. I have a hankering for the 7mm version of the Nirvana (there is currently nothing in between) but am on a tight budget and worry that the 7mm might be a step too far, given the scope being used. If anyone has any advice or opinions on this, I would appreciate it very much.

Thanks in advance.

 

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Dear azrabella,

Your scope is F8 so it should be fairly tolerant of even cheapo eyepieces. On the face of it you could probably go down even to 4mm for x300 but on a Dob this can make it hard to track the target. Also at magnification more then your aperture in mm, you start to see the diffraction rings very prominently especially in obstructed scopes.

A cheap eyepiece I regularly use is the 6mm Svbony with 68 degree Fov (with the red stripe not the yellow). It suffers from kidney beaning on bright targets especially the Moon but has good eye relief and I found that on double stars it works well with my old Skymax 127. I bought mine from ebay new for about £20 and at this price I'm very happy with it. 

So far I've not been able to resolve anything below 2'' with my old Skymax 127 though. I tried Zeta Herculis, no luck at all.

Nikolay

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If the 7mm Nirvana does not give good sharp images of double stars at 171x with your 150mm F/8 scope, I would be suspecting the collimation or conditions rather than the eyepiece.

The Nirvana is close to Tele Vue Nagler in optical quality.

If you are resolving equal brightness pairs down to a 1 arc second split with a 9mm eyepiece you are already doing very well indeed. Getting better than that, a whole lot of things need to come together in my experience.

 

 

 

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I agree with John regarding the 7mm Nirvana. That would give you c x171, which should easily be comfortable for your Dob.

My scope is also F8 (1040mm, 128mm refractor), and I could not separate two equal stars at 1" separation at 133x (which is what your 9mm is giving you at 1200 FL), so if you can separate 2 similarly bright stars at x133 there can't be much wrong with either your eyes or your optics!

That said,my eyes are not what they used to be. Even so, and allowing for your secondary obstruction (quite small at F8), your reflector should comfortably go up to x200 plus on steady nights.

The big issue, if collimation is spot on, might be the speed with which objects move through your field of view, necessitating frequent nudging of your Dob.

That's why I use a tracking equatorial to be honest - for doubles, I personally don't think you can beat an accurate tracking mount. Mine has just a simple RA drive, but even with rough visual alignment with Polaris I can keep an object centred for 20-30 minutes, even at 150-200x. That really gives me time to watch an object and wait for the steadiest moments of seeing to "get" those elusive tight splits.

Even an undriven equatorial with manual slow motions is, I find, easier than nudging, but of course you may find that not to be the case at all.

And a Nirvana at least has a wide field of view, which should minimise the nudgings a bit..😊

Dave

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Well, it seems that my optical choice is perhaps so far the best I could have made, thus far. I think I'll go for the 7mm Nirvana 82 degree (x150) as that will give me the widest field/longest drift time, I guess. It surely must yield a somewhat brighter image than barlowing the 16mm as presumably less glass in the optical path?

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It depends on the quality of the Barlow you use.

I use a Baader Hyperion Zoom 2.25x barlow.which is excellent and I cannot usually see any difference between an eyepiece barlowed versus a shorter one at native focal length (assuming decent quality of all components).

The cheaper Baader Q Turret Barlow is also excellent, but has a set screw rather than a compression ring. The Hyperion zoom Barlow also has a T2 thread.

Dave

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1 hour ago, azrabella said:

.... It surely must yield a somewhat brighter image than barlowing the 16mm as presumably less glass in the optical path?

Probably won't be noticeable. Modern glass and coatings reduce lost light from additional elements to very small levels which would not be noticeable visually.

Light scatter might be slightly increased though.

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I have an Orion Shorty plus 2x APO Barlow. I've had it for near 18 years, its so good with all my EP's. More recently purchased a GSO 2.5x APO Barlow, now considering it was half the money its impressive to.

Rob

Edited by Rob
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The Dawes Limit for a 6" is 0.75", but to reach that, you have to remember the 3 "C"s:

Collimation.  It can be so-so for low power, but at high power (150-300x in a 6" scope), it has to be perfect.  What collimation tools are you using?

Cooling.  Even a 6" will yield blurry images until the mirror has cooled to the ambient temperature.  Put the scope outside at sunset, and when it's dark 90 minutes later, the mirror should be well on its way to thermal equilbrium.

https://garyseronik.com/beat-the-heat-conquering-newtonian-reflector-thermals-part-1/

Conditions.  Yes, the seeing conditions of the atmosphere determine what you can or cannot do at high power.  Even the best scope made is powerless against this.  Sometimes, you just have to keep looking night after night to get an hour or two of steady skies.

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I have no trouble with  collimation, and have my scope outside at all times. So no issues with thermal equilibrium. Of course I cannot do anything about the state of the atmosphere, but we are drifting from my original query. I'll be more precise - would there be any practical benefit to me buying a 7mm Nirvana eyepiece over using a 16mm Nirvana with a telecentric amplifier on a 150mm f/1200 Dob?

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18 minutes ago, Louis D said:

My concern would be, will the Nirvana 7mm be any sharper on axis than the BST Starguider 8mm that disappointed you?  Has anyone done a comparison between these two?

I've used them both but not at the same time !

On axis I doubt there was much, if any, difference. Most eyepieces today are pretty good on axis.

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