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Which eyepiece should I buy for my new telescope?

Posted by Tom Hole   05/27/2004 12:00AM

Choosing an eyepiece for your new telescope

I have found eyepiece selection can be a very personal and subjective experience. Just go and post one of those ubiquitous "what is the best eyepiece" question on any of the groups, then standby for 100 different answers. This can be frustrating for rookies, as it was, and still is for me. The Nagler vs Meade UWA vs Radian vs Panoptic vs Pentax argument could rage on for weeks with no clear winner identified. Cloudy Nights (http://www.cloudynights.com/) has a good set of articles and reviews that you should read through before buying any more ep's. This article in particular (http://www.cloudynights.com/beginners/eyepieces.htm) is a good read.

The best way to buy eyepieces is to try them first. Your local club can be a great source of try before you buy. Star parties are a good source as well, but I've found that strangers aren't keen on loaning out their $300 oculars.

Another great source for all things astronomical is Astromart. I bought my Meade 14mm UWA for $200 off of Astromart (it retails for $300). It was in perfect condition. Used it for 6 months and it was a fantastic eyepiece. But a pair of them was just too huge for the binoviewers. So, I sold it for $200 on Astromart. I was out $8 for shipping. Not a bad price for renting a $300 eyepiece for 6 months. I bought my Pentax 10.5mm XL for $205, shipped. Sold it 5 months later for $210 ($5 to cover shipping), so that one was free. I had the use of 2 premium eyepieces that retail for over $500 and it cost me $8. In every transaction, the item I purchased was in great condition. That is key to the rental scheme. It must be in great shape when I buy it and then I keep it that way. Although this may not be for everyone, it has allowed me to try out many premium ep's that I would otherwise not have been willing to buy. The key is to sell them if they are replaced. I'm not very good at that.

So, how does one choose the right focal length for a new eyepiece? Well, the best article I have read on the subject was written by Televue (http://televue.com/engine/page.asp?ID=131). In addition, Televue has recently added an article specifically for small dobs (http://televue.com/engine/page.asp?ID=221). For the XT10 (1200mm focal length), there are 3 sweet spots that should keep you going for a long time:

1. Max True Field Of View (TFOV) vs magnification

2. 100x

3. 200x

These 3 sweet spots are all that I need for a complete night of viewing. Is that all I have in my eyepiece case? No. If seeing is favorable, I am equipped to go all the way to ludicrous power on the Moon and planets, and I sometimes do. That's where the barlows and powermates come in handy. But, don't get sucked into the "I have a gap" syndrome. Just because you have nothing between 25mm and 14mm does not mean you have a gap.

What the heck does #1 mean? "Let me splain. No, is too much. Let me sum up." Televue has a pretty good write-up here (http://www.televue.com/engine/page.asp?ID=141) and here (http://www.televue.com/engine/page.asp?ID=140). I won't begin to try to explain it all in my own words. But an example may help out. Let's compare 3 eyepieces:

EP 1

Focal Length 32 mm
Apparent FOV 50 deg
Field Stop 27 mm
True FOV in XT10 1.36 deg
Mag in XT10 38x

EP 2

Focal Length 24 mm
Apparent FOV 68 deg
Field Stop 27mm
True FOV in XT10 1.36 deg
Mag in XT10 50x

EP3

Focal Length 20 mm
Apparent FOV 82 deg
Field Stop 27.4 mm
True FOV in XT10 1.37 deg
Mag in XT10 60x


Take a look at the TFOV vs magnification. All 3 ep's provide a 1.36ish deg true FOV. But as you can see, it is at different magnifications. Who gives a hill of beans about that? Well, when magnification is increased, everything gets dimmer, that's the nature of magnification. This makes the object you are looking at dimmer, but it also makes the background dimmer. This can be remarkable at lower powers, and not so important at high powers. Take a look at Andromeda (M31) or the Orion Nebula (M42) through each of the above ep's in a moderately light polluted area. It will look much more distinct in EP 3 than the other two ep's. Now, if you are lucky enough to always view from a dark site, then this may not be an issue. I am not that lucky, so I have EP 2 (24mm Panoptic). The catch is that EP 3 costs $440 (20mm Nagler type 5), EP 2 $280 (24 Panoptic) and EP 1 $110 or less (32mm Televue plossl). As a general rule, the higher the mag for a given field stop, the higher the price.

What would I buy for my new telescope? Well, that depends. Don't you just love that answer? But it really does depend on a lot of things that vary from person to person (thus the personal nature of the decision). Here are some questions, the answers to which help determine which ep might be suitable for you:

1. What is your budget?
2. Telescope type, aperture and focal length
3. Will you be wearing eyeglasses while viewing?
4. What do you like to look at?
5. What are your current eyepieces?
6. What is your local seeing and light pollution like?

Here are some considerations for each point:

1. What is your budget?

There are ep's out there from $25 to $750. You would think that the $750 eyepiece would be some uber ep that has a perfectly sharp, flat and color free 90 deg apparent field of view. Nope. It's an orthoscopic design with a paltry 45 deg AFOV made by Carl Zeiss and no longer in production. But that's what people are paying for one used off of Astromart. If you have a full set of these puppies sitting in your attic, you could sell them and afford to get on the waiting list for an Astro Physics APO. What do you get for all that hard earned cash? You get that last 2%, baby. The nirvana of optical perfection. So, we need to know your budget before we can recommend ep's. Post something like this:

"I ordered an Orion XT10 from a local astronomy shop and I was wondering if some knowledgable folks could recommend some quality eyepieces that would allow this dob to get the most out of DSO's, planets, etc?"

And you won't get many answers. Actually a good answer might be buy all the Naglers, Panoptics, Radians and Pentax lines and pick the ones you like. This may not be compatible with a lot of folk's pocket books. Many folks bought the Guen Sheng and Synta dobs because they are frugal and I would expect as much when choosing an ep.

Premium ep's seem to start at $200 or so and go up from there. The thing to consider is that ep's can be used in any telescope. Buy a good one now and you'll keep it even as you buy new telescopes (and you will, believe me).

2. Telescope type, aperture and focal length

This information is useful because it helps determine several things. First, what focal length ep might be useful. A 3mm ep might not be very useful in an XT10 (400x) but it would be a great ep for the Televue NP101 (180x). The type of focuser will help determine whether you can use 2" ep's. Also, balance might be an issue. Throw a 31 Nagler on an XT8 and watch it take a nose dive. Aperture size helps determine what the maximum useful magnification is for your telescope. 50x per inch is an oft quoted limit, but this is not an absolute. My XT 4.5 (4.5" dob) does not like anything above 180x and it's really sweet spot is about 160x. Folks who own a scope similar to yours will be able to share this kind of experience with you and help determine a good ep. So, share your equipment type when asking for advise.

3. Will you be wearing eyeglasses while viewing?

Folks who wear eyeglasses generally prefer eyepieces with long eye relief. What the heck is eye relief? I'm not sure what the official definition is, but I think it's the distance your eye needs to be from the top of the top lens in order to see the entire FOV of the eyepiece. This can vary from zero to 20mm plus. If you wear glasses, you'll want to buy ep's with longer eye relief. How much longer? I dunno, depends. Some eyeglass wearers say 15mm, some 12, some say 20mm is their minimum. It's one of those things that varies from person to person. Geesh, that's starting to sound like a broken record, isn't it?

I'm not sure if long eye relief causes this, but it looks like a good spot to talk about black outs. Some ep's exhibit a tendency to just black out when you move your eye the slightest bit. This can be very annoying as I cannot hold my head perfectly still while viewing. My 9mm Expanse has this problem and it is going away because of it. Just something to be aware of when reading about ep's.

Another thing to be aware of is the effect of a barlow on eye relief. If you put a 40mm plossl that has 28mm of eye relief into a 2x barlow, the eye relief will probably increase to 32mm or more. That will make it very hard to put your eye in the right place. Too much eye relief is a bad thing as well. This effect is diminished as the focal length of the ep is reduced. I put my 16mm Nagler into my 2x barlow and it only increased the eye relief about 1.5mm. On my Pentax 10.5mm, which starts at 20mm, I noticed no measurable increase in eye relief. The Powermate will not have this effect. They are a different design and will not extend eye relief.

Keep in mind that long eye relief is not just for eyeglass wearers. I don't wear glasses and I love generous eye relief. That is one of the reasons I liked the Pentax XL line so much.

4. What do you like to look at?

Besides telescope catalogues and magazines. This may be a difficult question for a beginner to answer. Or not, I guess. I didn't know what I wanted to look at when I got my new telescope. I wanted to look at everything. But, some folks are specialists and know it. So, their ep requirements might be different from a guy who is a deep sky specialist with an occasional tryst with the planets. This is important because a DSO ep might have different design requirements than a planetary ep. For example, the orthoscopic design is a marvelous planetary ep but might not be great for large galaxies or nebulae due to its 45 deg AFOV. On the other hand, the Orion Expanse ep is adequate for DSO observing (66 deg AFOV) but not so good on the planets. So, let us know what you like to look at.

5. What are your current eyepieces?

This helps determine where you might have some holes in TFOV and/or magnification that need to be filled. It may also help determine what sort of ep preferences you have already established. For the first time owner, this might only be the stock ep's, But since some retailers include different ep's, let us know what those are. Also, let us know what barlows you have.

6. What are your local seeing and light pollution like?

You may not know how to answer these right away. Light pollution is fairly easy. How many stars can you see at night? How much of the Little Dipper (Ursa Minor) can you see? Can you see the Milky Way or just the skyglow from a nearby city? This is important when choosing a low power eyepiece.

For seeing, the local prevailing seeing conditions are what is important. What is seeing? Some more light reading:

http://www.cloudynights.com/howtos2/seeing.htm
http://user.mc.net/arf/seeing.htm
http://www.astronomynotes.com/telescop/s11.htm
http://www.cmc.ec.gc.ca/cmc/htmls/seeing_e.html(what the Clear Sky Clocks are based on)

If you live somewhere under the jet stream all the time with terrible seeing, buyig an ep that goes to 400x on an XT10 might be a lesson in frustration. I live in southern MD and the jet stream spends most of the late fall, winter and early spring right over my house. The nights that I can get above 200x on the planets are rare. So, I certainly wouldn't recommend an entire set of short focal length Radians to anyone living nearby. But, someone living in Key West or the high deserts of Arizona might be well served by ep's with more power.

So, that's what helps us help you. A good post asking about ep's might read something like this:

"Hello,

I have a 10" dob from Hands on Optics and so far I only have the 32mm and 9mm ep's that came with the scope. I live in Detroit, MI and like to look mostly at planets and brighter DSO's due to the light pollution from the nearby motor city. I don't wear eyeglasses to observe. I am looking at adding an ep to my collection and my budget is $150. Also, would a barlow be useful and, if so, which type?

Thank you,

Billy Bob"

I know that there will be recommendations to search the archives of the many groups out there. This is good advise, but my experience as a newbie searching the archives is information overload. Do a search on "favorite eyepiece" on the Talking Telescopes Yahoo group and you'll be reading posts until a 6" APO cost $500. So, if you get tired of trying to sort through all the posts, go ahead and ask away. Just try to be specific with your request for information. The 17th "which eyepiece should I buy" post in a week might get a deadpan response.