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Eyepiece Question

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#1 mjb1966

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Posted 26 July 2013 - 06:30 PM

I currently have the Celestron set which includes 32, 15, 9, 6, 4 plus I have a 2" 30mm. I haven't used these EP in a long time and don't remember much about them. I like to step up from the 32 to get to higher magnification would it make it easier if I had a 20 to get between the 30 and the 15 because 30 to 15 is a pretty big jump.

#2 astrophile

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Posted 26 July 2013 - 07:08 PM

Assuming this to be used in an 8" dob or so?

#3 mjb1966

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Posted 26 July 2013 - 07:13 PM

yes 8" dob

#4 Ultron

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Posted 26 July 2013 - 07:32 PM

Here's a good article by Don Pensack, a regular poster here on the forums.

1X/2X/3X eyepieces - Click on the "Extended Information" tab.

You normally want a low, medium and high power eyepiece.

Some of those eyepieces that you got in the kit might be uncomfortable to use due to short eye relief. If they are plossls, the 6 and the 4 will be tough.

With my 8" dob, I find my 24mm gives good low power views, my 10 gives good medium power, and the 6 gives good high power. Sure there's a big gap between the 24 and the 10, I'm thinking of getting a 16 to fill that.

I would say a 20 would be a good one for you. I would also look into getting a more comfortable to use eyepiece in the 12-10 and 6-5 range. You should look for ones with a wider apparent field of view so the object you are looking at stays in view longer.

The astrotech paradigms are supposed to be good eyepieces. You should check them out. Link

#5 astrophile

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Posted 26 July 2013 - 07:39 PM

Sorry--I should have also asked at the same time--are you looking at adding another plossl like the others in the set, or a nicer/wider FOV eyepiece? It matters because you want to consider field of view as well as, and sometimes more than, magnification.

Assuming 1200mm FL, the 30mm gives you 40x and depending on what its Apparent FOV is, ~1.7 to 2deg true FOV.
The Cel 15, likely a Plossl, will do 80x and ~0.65deg TFOV.

So, if you're looking for another Plossl, go for something that gives close to 1deg TFOV, yes that means about a 20mm ep. An alternative would be to replace the 15mm with a 16-17mm 82deg ep, that also gives just over 1deg TFOV with a bit more magnification.
[edited to correct TFOV of 15mm Plossl, sorry]

#6 StrangeDejavu

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Posted 26 July 2013 - 07:40 PM

If you go 20mm, and you're on a budget, then I recommend checking out the Sterling Plossl 20mm. I own the 17mm version, and it is a great eyepiece for the price. It still sees constant use right next to my ES 82's.

#7 mjb1966

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Posted 26 July 2013 - 07:47 PM

I was looking at the 20mm Meade 4000 series EP

#8 StrangeDejavu

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Posted 26 July 2013 - 07:58 PM

Just to expand on what astrophile was saying about wide-field and FOV. The Meade 4000 20mm offers 52° FOV, the Sterling Plossl offers 55° FOV. Less FOV means you'll be moving your Dob more often to chase the object back into your eyepiece again. Wider FOV eyepieces allow the object to stick around longer, so just keep that in mind when shopping. :waytogo:

#9 mjb1966

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Posted 26 July 2013 - 08:19 PM

Cool thanks for all the replies. It sounds like the sterling would be a better choice.

#10 BDS316

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Posted 27 July 2013 - 08:36 AM

30 to 15mm is NOT a big jump. In my scope I go right from a low power 30-32mm eyepiece to an 11mm Nagler and almost never use anything in between.

#11 wcstarguy

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Posted 27 July 2013 - 09:40 AM

I agree on the Sterling plossls. I have the 12.5, 17 and 20 and they are all very good. Some have suggested their field of view to be closer to 58 degrees. The eye relief gets shorter with the 12.5mm but still good for me as I don't wear glasses. For ranges in 6-9 you will get differing views. I like the Agena EWA eyepieces (same as the Orion Expanse but about 1/2 the price) with nice eye relief and 66 degree fov but some folks do not care for these, I also have some shorter focal length Celestron x-cel lx eyepieces with 60 degree fov and they are very good for about $15 to $20 more than the Agena EWA. wc

#12 Fuzzyguy

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Posted 27 July 2013 - 06:16 PM

I think the lack of an EP between 32 and 15 is because most scopes are sold with a 25mm or 26mm EP. Probably, most of those kits are sold to people who either are buying or just bought a new scope and they don't need a 25mm EP.

I have a 20mm EP for my 8" scope and I like the 2mm exit pupil it delivers. For me it provides a nice balance of darker sky, but large enough exit pupil to make a lot of DSOs stand out a little better than other focal length EPs.

#13 Tony Flanders

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Posted 28 July 2013 - 05:23 AM

30 to 15mm is NOT a big jump. In my scope I go right from a low power 30-32mm eyepiece to an 11mm Nagler and almost never use anything in between.


Hmm, I sort of both agree and disagree.

On the one hand, I do think it's a pretty big jump. On the other hand, I find this particular gap less of a problem than most.

I'm assuming this is a typical 8-inch f/6 Dob. In that case, the 32-mm eyepiece gives a 5-mm exit pupil -- a nice, low magnification, great for wide-field viewing. The 15-mm eyepiece gives a 2.5-mm exit pupil, which is about where I start when I want to view deep-sky objects in detail.

From 2 or 2.5 mm on to 0.5 mm exit puils, it's really useful to have a good range of eyepieces, generally in increments of around 1.4X. So one might have a 15-mm, 10-mm, 7-mm, 5-mm, and 3-mm. Or more likely a 2X Barlow.

Intermediate powers, exit pupils between 3 and 4.5 mm, are somewhat less useful because they hardly ever offer the best detailed views, nor do they offer the widest possible field of view. However, you do occasionally find objects where these exit pupils turn out to be the ideal compromise. More to the point, they're often the best choice in conjunction with aggressive filters like the O III or H Beta.

On an f/10 scope, the considerations are totally different.






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