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Suggestions for next eyepiece

accessories eyepieces dob Meade
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#1 pzanon

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Posted 01 June 2020 - 09:33 AM

Hi everyone,

my first post here. I hope I won't break any rule by starting a new topic... :)

My question for you guys is the following: I'm completely overwhelmed by the number and types of eyepieces available on the market. I have a Meade Lightbridge 16" (FL 1829mm) with the following eyepieces, most of which came together with the scope (bought used): 

 

Meade QX Wide Angle 26mm
Meade Eyepiece Series 5000 UWA 20mm 2"
Baader Okular Morpheus 76° 17,5mm
Baader Hyperion Clickstop Zoom 8-24
Omegon LE Planetary 12.5mm
Omegon LE Planetary 5mm

 

Which one should I get next? I was thinking to the Explore Scientific 2", 68° 40mm N2-filled eyepiece....but before handing out 300 dollars, I wanted to hear opinions on possible alternatives. I'm trying to look for something that would give me a significantly larger FOV compared to the ones I have now.....

 

Thanks a lot in advance for your help!!



#2 sanbai

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Posted 01 June 2020 - 10:16 AM

Hi Pzanon,

 

This eyepiece will NOT work well for you. The resulting exit pupil (scope is f/4.5; 40 mm / 4.5 = 8.9 mm) is too big. You will waste light (it won't pass your iris) and you will see the shadow of the secondary mirror. Maximum is a 7mm exit pupil as a rule of thumb, this is a 31 mm eyepiece for your scope (a bit less possible even better).

If you want larger True Field of View (TFOV) you have to look for eyepieces the largest Field Stop possible; that works for any scope. In your case, being f/4.5, the eyepiece must have a maximum of 31 mm of focal length. These will be with Aparent Field of View (AFoV) of 82 ° (Naglers, ES82...) a 31mm; with expensive 100 ° AFOV you have 25-21 mm options. With lower AFoV you will have to increase the focal length above the recommended value of 31mm or cut the maximum TFOV, but 68 ° AFoV and above may still work (e.g. APM UFF 30 mm).

 

You may want to trade your 26mm eyepiece if you go that way, but won't get much for it.

 

If you are in a heavily polluted area, a 7 mm exit pupil may be too much sometimes, but it will be useful with narrow band filters.

 

Planetary season is starting now, so you may also want something with short focal length, like a 3.5 mm (if the seeing in your area allow that).

 

You may also consider a coma corrector to get most of  your scope. At f/4.5 it is very recommended. Some will extend the focal length of your scope a bit, so the maximum eyepiece focal length can be a bit longer than 31. For example, with TeleVue Paracorr, you can go up to 35 mm eyepieces.


Edited by sanbai, 01 June 2020 - 10:28 AM.

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#3 Pentax Syntax

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Posted 01 June 2020 - 10:20 AM

A Dob at F 4.5 is going to test any wide field eyepiece.  To get the wide field you desire, the 40, 68 degree you mention would be good albeit with a large exit pupil.  Others to consider are the Pentax XW40, Televue Panoptic 35, and WO 2" Swan 40. You can also go to large Plossl (55 or 56) to avoid the large exit pupil.  This will work if you don't mind the loss of magnification.  These are all great eyepieces and can be used on any of your future scopes.


Edited by Pentax Syntax, 01 June 2020 - 10:25 AM.

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#4 sanbai

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Posted 01 June 2020 - 10:24 AM

I would advise you to read more about eyepiece selection. The best is to plan your eyepiece collection. You can always change it, it will likely happen, but it's a good exercise to see where you want to go, and what you need as a whole.

 

To help you understanding the variables playing around and how to plan, look in these websites (aside from looking and asking here in CN):

https://www.eyepiece...OPES_s/1828.htm

http://televue.com/e...page.asp?id=154

https://agenaastro.com/articles/guides


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#5 havasman

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Posted 01 June 2020 - 10:52 AM

Hi pzanon and welcome to the forums!

 

MANY people would love to have the set you already have. The Morpheus in particular is a very high performance eyepiece. My advice for you is to use that set in your 16" for a while, at least a few months. By then you will have a better idea which you like and which may deserve replacing. Eyepieces, within the limits of optical physics, are very individual preferences.

 

The advice above that the ES68 40mm is a bad option for your scope is good advice. I have one and it's a fine eyepiece but not for your 16" Lightbridge.


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#6 SeattleScott

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Posted 01 June 2020 - 01:45 PM

Think more like 30mm APM UFF, or 30mm ES 82. Televue has some more expensive competitors.

Scott
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#7 pzanon

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Posted 01 June 2020 - 03:51 PM

Hi everyone!

Thanks soooo much for all the suggestions and for the links to more information on eyepiece selection. You saved me from getting something that, albeit not bad in itself, would have not worked well with my setup. I clearly have some reading to do :)
Thanks also for the warm wellcome to the forum and cheers from Switzerland!


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