Eyepeices and 'scope focal length - is there a rule?

[hozepipe]

Hi

I was wondering what the rule might be for choosing the best eyepeices for your scope? Is there some technical reason for using one design over another depending on the f ratio or something? I'm asking because I don't really have an intuitive understanding of the science of optics to be able to work it out for myself. I've noticed that my Mead 26mm plossl works brilliantly in my ETX 90 f12.5 scope, while a new Baader Hyperion 21mm wide angle (68 deg AFOV) does not seem to work well at all in terms of eye position and comfortable viewing (though I have only tested it indoors - xmas bauble test! - not on the night sky, it's been cloudy for three days!)

I would like no more than say 4 or 5 EPs that work as well in both my ETX90 and an Orion ED80 and not 8 or 10 - is this possible and what's the science I need to understand why (or why not) this is possible?

Look forward to your replies.

[GordonCopestake]

I always use the exit pupil as a guide line rather than the magnification. The smallest exit pupil i find comfortable is 0.5mm and you can then work out the corresponding focal length of the eyepiece. The "work horse" eyepiece should have an exit pupil of about 2mm. The largest exit pupil my (poor) eyes can take advantage of is 5mm. etc

[FLO]

Short/fat scopes with fast f-ratios of about f5 or below have steeper light cones that require more correction at the eyepiece.

Simply put, the faster your telescope, the more you should spend on eyepieces.

If on a budget, it is best to avoid 'affordable' wide-field eyepieces and buy a good quality simple design like a Kellner, Plossl or orthoscopic.

[FLO]

This might help:

http://tinyurl.com/coryv

[hozepipe]

Thanks that helps! Short n fat = spend more cash

OK, so my criteria for an EP are:

Highest detail and resolving power

No colour (if possible to the edge of the field - the Mead 4000 26mm has some colour which I'm not fussed on at all)

Good transmission and brightness

After my experiance with the Hyperion I've gone off the wide-field-on-the-cheap route - I doubt it can be done without sacrificing the more important attributes above.

I'd prefer to spend more on the best EP I can afford than amass a load of medeocre EPs.

What brands should I look for, Televue?

[GordonCopestake]

Televue and Pentax are regarded as the "best" eyepieces at the moment. Naglers are considered the standard to compete against. They can be picked up for less than £200 on astro buy/sell second hand and once you have them you never have to buy another eyepiece again (assuming your 'scopes dont change)

See the thread "Whats in your eyepiece case?" for what other people are using

[Andrew*]

In light of what Steve said about fast scopes, I hope once after I get an 8" f/5, I won't need to spend another fortune on new EPs. I have these three EPs:

Orion Sirius plossl, 25mm and 10mm

Celestron 15mm plossl.

I find them very decent in my 4.5", but once I upgrade to the faster scope, will I start to find faults in them?

What are your general comments on these EPs?

Andrew

[GordonCopestake]

Theres nothing stopping you using your plossl eyepieces in fast scopes. Although that being said f/5 isn't vastly fast and you should be fine. It's only when you get to fast large aperture dobs or s/n's at f/4 etc that you NEED something better than the plossl design. Although obviously better eyepieces always help

[FLO]

Andrew,

A good Plossl will work well in an f5 Newt'.

I have no direct experience of the ones you mention - try them and let us know

[hozepipe]

Televue and Pentax are regarded as the "best" eyepieces at the moment. Naglers are considered the standard to compete against. They can be picked up for less than £200 on astro buy/sell second hand and once you have them you never have to buy another eyepiece again (assuming your 'scopes dont change)

See the thread "Whats in your eyepiece case?" for what other people are using

I'll check that thread, thanks.

So are Televue Plossls better than other makes or should I only buy TV Naglers?

[GordonCopestake]

TV Plossls are better than other plossls in my experience, TV Naglers are better than TV plossls but are much more expensive. It comes down to a cost/performance thing and where on the curve you want to be

[Andrew*]

Thanks guys. I'll tell you how they perform in a more demanding scope.

Andrew

[hozepipe]

I'd prefer to have the best possible views from my scope - makes sense to spend on the EPs because you can keep them when you upgrade the scope. I want the best I can afford.

I've got three Seibert Optics EPs on order (two Starsplitters and a premium 1.25" 15mm) I don't know what they're going to be like but I've read very good things about them. Seem like a lot of bang for the buck, but ordered them before I started asking questions. I'll post a review when they arrive.

[GordonCopestake]

Seibert have a good reputation and I doubt you'll be disappointed. Personally though I prefer the naglers, especially in faster 'scopes. Pentax is on par at the shorter focal lengths and Televue dont make naglers shorter than 7mm. The 3-6 Zoom is great at those short focal lengths but the Pentax are better prime lenses than the Radians. If you need eye relief at longer focal lengths go for the panoptics over the naglers as some naglers can be short on ER if that bothers you.

[hozepipe]

Are the Naglers and Panoptics 'fussy' about eye position? I want an EP that feels naturual to use, so that you can really look at the view and concentrate on it - I often spend long periods on the same view extraciting as much as I possibly can from it. I can see the Hyperion being a pain because of how fussy it seems to be about eye position.

[FLO]

The new Meade Series 5000 Plossls are excellent

http://www.firstlightoptics.com/products.php?cat=43

[Jeff RV]

I always thought its better to say use a 9mm with a 2x Barlow rather than a 4 or 5mm ep for better eye relief,but im not very experienced but these guys wil know? Jeff.

[GazOC]

Jeff, I prefer not to use a Barlow unless I can help it, but its purely a personal 'thing'.

[GordonCopestake]

I'm with Gaz on barlows, if I dont have to use them i'de rather not.

The Meade 5k UWA's are pretty good, but i believe they are clones of the early nagler designs. Not 100% sure though

[FLO]

I always thought its better to say use a 9mm with a 2x Barlow rather than a 4 or 5mm ep for better eye relief...

If you need extra eye-relief, that is an excellent idea. Also, a good barlow can overcome some of the shortcomings of 'affordable' eyepieces by removing some of the abberations in the light cone before it reaches the eyepiece. However, when using a barlow, you are putting extra elements into the light-path which can affect colour/contrast.

Worth mentioning that it is every bit as important when choosing a barlow, as when choosing an eyepiece, to select one with good glass and construction. (The barlows normally supplied with scopes are usually of poor quality).

[Jeff RV]

Points taken. I use a 2" Revelation Barlow,good or bad i don't know? but it seems quite good, There's only one element inside i think but its not used often.

[FLO]

If its the Revelation 2 Inch ED Barlow Lens then yes, it is remarkably good - I have been using one for a while now

[Jeff RV]

It has *APM083* ...APM Barlow 2" on box

[FLO]

Mine says different but I think the packaging has changed recently. It is the same as the one offered by a number of other quality brands sourced from the GSO factory in Taiwan. It is supplied with 2" to 1.25" adaptor with compression ring, fully multi-coated optics with blackened lens edges and one ED element. It is also threaded for 48mm filters.

[Jeff RV]

That sounds like it Steve. Ask my daughter to order it from T/House as its a nightmare out here to get anything like that and expensive. purchased a 2"Dielectric mirror diagonal, and a 50mm Rev ep with it,Im happy...Jeff