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Ultra Wide 82 degree fov


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#1 RickScofield

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Posted 09 October 2014 - 07:17 PM

I am looking for advice, I want a wide angle lens for looking at planets and deep space objects. I am using an ETX 90 and limited to 1.25, what would be a good all purpose eyepiece for this?
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#2 MistrBadgr

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Posted 10 October 2014 - 05:40 AM

Hi Rick,

 

It depends on what wide angle you are talking about.  If you are talking about the Apparent Field of View in the eyepiece, I have been having good experiences with my 5000 series Ultra Wide Angle Eyepieces.  The longest focal length is 14 mm, which gives you a good magnification and gets the field of view by spreading the scene around in your eyeball.

 

If you are talking about True Field of View, or how wide of a chunk of the sky you are looking at, then longer focal lengths with narrower fields of view actually give a wider view in that sense.  There is just so much that can be packed into the 1.25 inch format.  As the field of view inside the eyepiece widens, geometry enters the picture, and a narrower and narrower piece of the sky is allowed.

 

A regular 40 mm Plossl gives about the widest chunck of the sky, but the magnification and field of view inside the eyepiece are so small that I normally do not like that.  I use a 32 mm, which takes the field back to 50%, has almost the same area of sky, and has better contrast.  Something like the HD 60 25 mm is next with a little less sky, but a wider field in the eyepiece, then comes 70 degree eyepieces in normally around 20 mm as the max focal length, then comes  the ultra-wides with around 82 Apparent Field of View, with a maximum focal length of around 14 mm in the 1.25 inch format.

 

In order to calculate how much sky you have in the view, or True Field of View (TFOV), you first calculate the magnification (scope focal length divided by eyepiece focal length).  Then divide the Apparent Field of View of the Eyepiece (AFOV) by the magnification.  The other thing to watch is the pupil size.  If ti gets too large, which I do not think you will run into with your ETX 90, you divide the diameter of your scope by the magnification.  About the widest pupil size anyone has is around 7 mm.  Typically, young people will have that.  As you get older, the diameter shrinks.  Normally 6 mm is a good number to use for most people.  Mine has shrunk to about 4.5 mm at age 63.

 

Hope this helps,

 

Bill


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Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#3 Meade

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Posted 10 October 2014 - 10:46 AM

Hi Rick,

 

Depending on what eyepieces you have now the first thing many people pick up is a set like the Series 4000. Although not of wide field design a full set is very reasonably priced and using the lower magnifications will of course increase field and image brightness on fainter targets. Very low powers will work better if your sky is fully dark where you routinely observe otherwise you just start picking up more sky glow. 

 

I will also throw out there that while wide field lenses can be used for planetary observing this is not usually a factor for most planetary guys, since the planets will never remotely approach filling the field of view of even a "standard" Plossl eyepieces 52 degree apparent field. Wide field eyepieces really come into their own for deep sky and in particular dark sky deep sky observing IMO. 

 

Street price on the Meade Series 4000 Eyepiece Set is $199 which gets you six eyepieces, filters for the Moon and planets, a Barlow, and aluminum case. Our 1.25" Series 5000 Ultra Wide Angle eyepieces go for $129 a lens.

 

Hope the thoughts help and best regards,

 

John Piper

Customer Service Manager

Meade Instruments Corp.  


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#4 Mark Sibole

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Posted 10 October 2014 - 12:23 PM

As John stated you dont see to many people use ultra wide angle or wide angle eyepieces for lunar or planetart viewing.Untrawide and widefield EPs are generally used for large dimmer objects,They work great for things like the Veil area  the Anromada galaxy and fitting into the FOV of M81 and M82 for example.

On my LX200 I generally just use a 32 mm EP It gives good magnification and a good field of fiew for the gig targets.When viewing the moon or planets I like the 16 mm wide angle EP and then use a barlow for more magnification.The Meade series 5000 set is a nice set to cover a ton of objects.I dont own the set persay but have picked up individual eyepieces to make a complete set along with the series 500 2x 3x and 5x barlow.


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Mark Sibole
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#5 RickScofield

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Posted 11 October 2014 - 08:09 PM

Thank you Bill,John,and Mark.
Totally understand what all of you are saying. I probably should have listed the eye pieces I have when I asked my original question, I have the original eye pieces that came with the telescope series 4000 26 MM and 9.6 MM I also have the entire set of 6 ,1.25 series 5000 that range from 26 mm to 4.5 mm. I recently purchased the set of 3 ultra wide angle set of 1.25 series 5000, with 14 mm 8.8 mm and 5.5 mm Ultra Wide Angle with 82 % afov. I must admit that these last 3 eyepieces really are impressive and was looking for possibly a 30 or 32 mm UWA for deep space objects. I have been spending a lot of time exploring Sagittarius with all the Miessier clusters and nebula and found I was using my 5000 series 26 mm, 18 mm, and 14 mm UWA all the time. I had not tried Mark's idea of using barlows with these to increase magnification but will give those a try since I have a 2X and 3X of those. Anyway like I said I am really impressed with the view I get with the UWA eyepieces and would like to find an UWA between 26mm and 32mm in the 1.25 format but then maybe I don't need it. Thank you all again.

#6 Mark Sibole

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Posted 12 October 2014 - 11:39 AM

With what you have stated in eyepieces and barlows you should have all of your bases covered.The only thing I do now see listed is a focal reducer.Other than that you should be good to go.

 

Mark


Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com

#7 MistrBadgr

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Posted 12 October 2014 - 01:55 PM

Hi Rick,

 

The problem with anything much bigger than the 14 mm UWA cannot be supported in the 1.25 inch format.  The image is simply cut off by the side of the eyepiece...just geometry.  You have to go to a 2 inch eyepiece, with a 2 inch diagonal.  I could be wrong, but I do not think the ETX 90 can support that.  You have all the eyepieces that will fit your scope, unless there is a focal reducer that will.  A focal reducer is the opposite of a Barlow and reduces magnification.  Accordingly, the field of view goes up, but an image can be still limited a bit as to how wide a chunk of sky can be viewed.

 

Bill Steen


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma




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