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Budget/Quality Wide FOV eyepiece

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#26 Starman1

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Posted 13 July 2020 - 02:18 PM

Hey there.. I have a Celestron 130EQ.. Got it for free. But im buying a Skywatcher 10” Goto Dobsonian. Im looking to get a 2” eyepiece that will work for both telescopes (I know ill need an adapter for the 130eq).. I dont want to spend much more that $125.. I want a nice wide fov and low power for searching the night sky and galaxies/nebulae. I know its not a ton to spend but id like a quality piece that doesnt have a bunch of distortion on the outside.. Please let me know what you think. Thanks a bunch

The Celestron 130EQ is a 650mm Focal length.

You do not need a 2" eyepiece to get a wide field in that scope.

A 24mm 68° eyepiece will give you 2.4°, which is a huge field of view.

The magnification of 27x is going to be a bit low for most deep sky objects except the large star clusters.

Most deep sky objects will be better seen with nearly double that magnification, with a 12-13mm eyepiece.

 

The SkyWatcher 10" is a different beast.  It can use 2" eyepieces and you can use a good widefield as long as 30mm, like the aforementioned APM Ultra Flat Field.

Yes, there are less expensive 2" widefield eyepieces, but they will suffer from more astigmatism in the outer field due to their inability to handle the short f/ratio of the scope.

If you are looking for an eyepiece to yield star images sharp to the edge at that f/ratio, you will have a problem.  Short reflectors suffer from coma, and even the most expensive eyepieces made

will not deliver star images that are points to the edge due to coma from the scope.

So the best you can do is to eliminate astigmatism in the eyepiece, and that will not be as inexpensive as your budget.  The APM is a good possibility.

 

Now, do you really NEED an eyepiece at low power to be sharp to the edge of the field?  Especially if it is being used as a "finder" eyepiece to locate the object and see the context around it,

and is then followed by a higher power eyepiece for inspection of the object.  A good 10-12mm eyepiece will get a LOT more use than the low power eyepiece.

 

Since you have a Baader Zoom, the magnifications most useful are already covered in both scopes.

I agree with an earlier poster that a decent alternative might be a 32mm Plössl for both scopes.  Coma will be less visible than in a wider field, the field of view in the 130mm will be the largest the instrument can deliver,

and the apparent field of the 32mm will be about the same as the Baader Zoom at 24mm in both scopes, but the magnification will be lower, the field will be brighter, and you stand a good chance of seeing sharp star images over nearly all of the field.

And, it can be 1.25", saving you a lot of $.  


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#27 25585

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Posted 13 July 2020 - 03:13 PM

ES Maxvision 28, 34 & 40mm eyepieces are affordable https://www.rotherva...-eyepieces.html


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#28 SeattleScott

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Posted 13 July 2020 - 03:31 PM

ES Maxvision 28, 34 & 40mm eyepieces are affordable https://www.rotherva...-eyepieces.html

Good catch. These do fairly well in fast scopes. To me the 28 doesn’t make a lot of sense since it isn’t that much wider than a 24/68, and it will only work in the 10” and might require swapping 1.25” adapter in and out for other eyepieces at that. The exit pupil of the 40 is too big. But the 34mm could work and cheaper than the APM even with international shipping. The shorter FL of the APM would be better for keeping exit pupil in check, and the Maxvision is heavy if not decloaked. But a consideration nonetheless.

Scott

#29 DanielG8686

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Posted 13 July 2020 - 03:37 PM

Good catch. These do fairly well in fast scopes. To me the 28 doesn’t make a lot of sense since it isn’t that much wider than a 24/68, and it will only work in the 10” and might require swapping 1.25” adapter in and out for other eyepieces at that. The exit pupil of the 40 is too big. But the 34mm could work and cheaper than the APM even with international shipping. The shorter FL of the APM would be better for keeping exit pupil in check, and the Maxvision is heavy if not decloaked. But a consideration nonetheless.

Scott


Thanks a bunch scott!! What exactly is “exit pupil”? Im a beginner

#30 DSOGabe

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Posted 13 July 2020 - 03:47 PM

Why have you only used it a few times? I want a wide field for searching and nebula/galaxies

Just recently got it. And it brought several clouds with it



#31 SeattleScott

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Posted 13 July 2020 - 03:55 PM

Exit pupil is the size of the light cone in mm coming from the eyepiece. Generally the useful range is considered about 6-7mm down to 0.5mm. The bigger the exit pupil, the brighter things look. You can calculate exit pupil by dividing the focal length of the eyepiece with the F ratio of the scope. Different levels of magnification and brightness can be helpful for teasing out faint details.

The problem comes in when the exit pupil starts getting larger than your pupil can dilate. While the detrimental effects are often not a big deal at low power, you might see a dark spot in the middle of the view if the exit pupil gets too large. And you are losing aperture and contrast, regardless of whether or not it is an obvious difference at low power. Plus the background sky can look bright and washed out if you are observing in light pollution (which makes the dark spot in the middle more obvious too).

Edited by SeattleScott, 13 July 2020 - 04:08 PM.


#32 Starman1

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Posted 13 July 2020 - 04:02 PM

Thanks a bunch scott!! What exactly is “exit pupil”? Im a beginner

Behind the eyepiece is a small circle of light that comprises the view of the primary mirror through the eyepiece.

The distance the exit pupil is from the glass is known as the eye relief of the eyepiece, and the size of the exit pupil is related to the f/ratio of the scope:

Exit pupil = Focal length of eyepiece ÷ the telescope's f/ratio.

You hold the pupil of your eye at the exit pupil to see the whole field of the scope.

Here is a better explanation:

https://skyandtelesc...the daytime sky.

and a little deeper:

https://www.handprin....html#exitpupil

 

You want the exit pupil to always be smaller than the dilated pupil of your dark-adapted eye.



#33 Second Time Around

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Posted 14 July 2020 - 06:17 AM

ES Maxvision 28, 34 & 40mm eyepieces are affordable https://www.rotherva...-eyepieces.html

I contacted a lot of UK and European dealers last year to find a 28mm Maxvision and nobody had it in stock despite their website saying it was.mad.gif   It appears it sold out some time ago.

 

In the end I bought one off eBay and can certainly recommend it at f/6.  I haven't tried it in my f4.8 yet though.  I've also just bought, but not yet used, the very similar if not optically identical 28mm Explore Scientific 68deg as a Dioptrx astigmatism corrector easily fits it.


Edited by Second Time Around, 14 July 2020 - 06:19 AM.



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