EPs and fast scopes.

[Liz]

I know I should know this, and am sure Suzy does () but can I have a few explainations re why I shouldnt use some EPs in my scope? I have a 10" Dob f/4.7, focal length 1200.

Recently I was going to buy a 40mm EP of Tony (Casstony) and he was very kind to tell me that it wouldnt suit my scope.Thank you Tony.

..... If the exit pupil from the eyepiece is bigger than your dark adapted pupil, you're effectively reducing the aperture of your scope, with the extra light hitting your eyeball outside of the pupil. For an f/5 scope a 35mm eyepiece is ok from a dark site, 30mm in town, or 35mm plus paracorr in town since the paracorr increases magnification by 15%.

Alex also commented - If only it was a 30mm I would have not hessitated, . Just too long for my f/4.5 scope, .

Does this mean that I shouldnt use 40mm and over in my scope, or does it depend on the other measurements - focal length, eye relief, exit pupil etc.?

[Pi]

My understanding is it depends on the size of your pupil when viewing vs the exit pupil of the eye piece. If the exit pupil of the eye piece is greater than your pupils diameter, your eye won't receive all the transmitted light and your scope will become "inefficient" as a percentage of the transmitted light is not reaching your pupil.

An analogy would be like trying to poor water from a bucket into a bottle. Most of the water wouldn't even hit the opening of the bottle, it would just splash around and be wasted.

So with that in mind, the exit pupil of an eyepiece is the focal length of the EP divided by the focal ratio of the scope. In your case, 40mm/4.7 = 8.51mm meaning for your pupil to receive all the transmitted light, it would need to dilate to 8.51mm or greater, which is up there in the higher octaves of standard pupil diameter.

Going by Tony's advice, with an approximate pupil diameter of 7mm at a dark site (35mm/5 = 7mm), the largest focal length EP you should use is 32.9mm

My question to the pros is how can I increase my field of view once I've hit this EP focal length barrier?

Hope that helps some Liz

[astro744]

A large field of view is invaluable when star hopping and if this is all you are doing then does it really matter that you are losing some light? Don't worry too much about excessive exit pupils if general scanning of the sky is all you are doing. Yes the sky will be a ltlle more grey but stars are still there in high contrast.

A more important issue with low power eyepieces and fast 'scopes is that unless you get a highly corrected (more $) eyepiece you will get significant abberations off axis; a common one being astigmatism but also field curvature. If the edge of field is not sharp then having a wide field eyepiece is of no benefit. If stars are out of focus, imagine what a galaxy looks like (invisible).

Note too if you use a Paracorr your exit pupil is reduced by 15% and many abberations are corrected especially coma which is from the primary optics (paraboloidal mirror).

[mental4astro (Alexander)]

Originally Posted by Liz

Does this mean that I shouldnt use 40mm and over in my scope, or does it depend on the other measurements - focal length, eye relief, exit pupil etc.?

Hi Liz,

There are a few reasons, some of which you answered.

Eye relief isn't a problem. The longer the better though.

Exit pupil, if this exceeds 7mm, light is being wasted as it all doesn't get though our pupil. And as we age, this exit pupil we should look at reducing to 6.5mm, or even 6mm. A good rule to follow is this:

longest focal length EP = 6 X focal ratio of scope

Then, because EPs vary in focal length, the absolute max limit will be 7 X the focal ratio.

The longer the focal length too, you may start seeing the shadow of the secondary mirror if your scope is a reflector of some type.

The last consideration I can think about, especially for a fast scope, is the quality of the EP and how coma and astigmatism affects the image quality. The faster the scope, the more obvious the coma is. It isn't a manufacturing defect, its physics of light and glass. The better quality EPs will be able to correct for this much better than cheap ones, but also, if the field of view gets into the extreme, even high quality EPs need the extra help of coma correctors, but these won't help with astigmatism.
Astigmatism IS a quality issue, that is why cheaper EPs are cheap, the quality of glass and lens design doesn't work well enough.

Your scope is f/4.7, the max f.l. would be 28.2mm, with a margin of upto 33mm. Any longer and you a cheating yourself of light. It'll still work, but the effect would be like reducing the apeture of the scope- the image won't get any brighter.

I really like the Astro-Tech Titan Type II's range as they are the exact same EP as the TMB Paragons, and a little cheaper. Their quality is fantastic for their price, and the level of coma correction is brilliant, with only a hint of it along the very edge of the FOV. Their FOV is also 68deg., which matches the angular movement of our HUMAN EYES! Not 82deg, or 100deg. But, that is my preference.

As an even cheaper alternative, the GSO Superviews (other than the 20mm), are very good. They do exhibit more coma, and some astigmatism, but if you are prepared to accept these short comings, they are excellent 68deg FOV EPs for their price.

Mental.

[Liz]

Thankyou for that interesting info guys.
I will read and reread to take it in, alas, off to work now.

[Liz]

I have a couple of EPs of 40mm, and I use them, well, try and use them to find the area of comets and galaxies, then would pop in a higher mag Ep. this has never worked too well, and this is why. Star hopping would be Ok though, and the view would be just dimmer due to less light coming in. Never realised that before.

Would a 2" 40mm let in much more light than a 1 1/2"?

Have no plans for a Paracorr (yet) .... do many people use them?

I do have plenty of good quality EPs in my collection, so less problems by the sounds, in regard to coma/astigmatism.

Will keep on reading about it, as interesting stuff.

[AG Hybrid (Adrian)]

Originally Posted by mental4astro

HUMAN EYES!

Man... human eyes are over rated!

I'm still waiting/hoping a TV fanboy will suggest you buy a 31T5 or a 21E.

[mental4astro (Alexander)]

Originally Posted by Liz

Would a 2" 40mm let in much more light than a 1 1/2"?

Yes and no. But, the answer is a little more complicated. What a 2" EP will do in the longer focal lengths, say 25mm and above, is give a wider field of view than a 1.25". This is because of what is called the "field stop". In a nutshell, the wider the field stop, the wider the possible field of view. The kicker is that the longer focal length EPs require a wider cone of light to enter the EP.

Get two EPs, both the same focal length of 40mm, but one 2" and the other 1.25". What you will see is that the aparent FOV of the 2" will typically be greater than 65 deg., while the 1.25" will not exceed 45 deg. This is an enormous difference in field size, but both allow roughtly the same amount of light to pass into the eye. A 2" EP should give a brighter image, but I doubt weather our eyes are sensitive enough to notice the percentage difference.

This is due to the size of the "cone of light" that is able to enter through the 2" EP being wider.

Eyepieces upto and less than 25mm in the 1.25" size require a smaller cone size to enter the EP to give wider FOV.

I've got a 30mm 2" GSO superview. It shows both coma and astigmatism aroung the edge of the FOV. When I get around to it, one-day, I'll be happy to upgrade to a mid range EP like the Paragon or Titan Type II 30mm. 68deg. FOV, no astigmatism, tiny, tiny amount of coma, and better contrast in the image. I don't need anything wider, nor bigger price tag. I just don't see the need. You really don't do serious observing at the very edge of the FOV which only shows more coma, and then more money to correct for. Again, that's me.

The rest of the money I then spend on flowers for brownie points to be allowed to go out into the dark night and play with strangers in remote and lonely places, .

Adrian, I'm just waiting to the time that animal-to-human transplants becomes safe so I can have leopard eyes transplanted to replace my current whimppie human eyes. Leopard eyes are about 5 times as sensitive to light as ours, .

[AG Hybrid (Adrian)]

Originally Posted by mental4astro

Adrian, I'm just waiting to the time that animal-to-human transplants becomes safe so I can have leopard eyes transplanted to replace my current whimppie human eyes. Leopard eyes are about 5 times as sensitive to light as ours, .

Leopards eyes aye. After you've taken its eyes you can use its pelt as a fancy telescope cover or lounge room rug.

Cybernetics one day maybe the way to go.

What about owls eyes? You'll look weird but the max exit pupil would be enormous. We will be able to use 50mm eyepieces in a fast dob.

[casstony]

Looking at the eyepieces in your signature Liz I don't think you need to buy any more - maybe even thin the herd a little .

A paracorr would noticably improve the edge performance of your eyepieces, if the edge of field is bothering you.

Here's some info about eyepieces you might enjoy reading:
http://www.astrosurf.com/luxorion/re...ons.htm#barrel size

[Liz]

Originally Posted by casstony

Looking at the eyepieces in your signature Liz I don't think you need to buy any more - maybe even thin the herd a little .

cant help myself, but you are right Tony, I will try and keep on hte straight and narrow!!

[casstony]

Originally Posted by Liz

cant help myself, but you are right Tony, I will try and keep on hte straight and narrow!!

It is a bit tough with sales all over the place and the threads about shiny new mounts, scopes and eyepieces. I've got a problem fitting the new purchases into my eyepiece case. Though I figure it won't be long before a falling dollar and end of sales rescues us from our shopping therapy.