Benefits of 2 inch EP's

[Knighty2112]

Hi all. I've just received my new Sktwatcher Startravel 120mm refractor telesscope last night. Sadly, no clear skies last night to test it with, so hoping for some tonight. After looking through it today at a dull, wet chimney stack in the distance (best view I can get from my living room window as it is drizzly outside) to test the optics and EP's etc, I am pondering the following things with my new scope;

As it comes with the option to hold not only 1.25 inch EP's, but also 2 inch EP's my questions are what are the benefits of using 2 inch EP's over just 1.25 inch EP's? Is the only difference that it is easier to view through a 2 inch EP over a 1.25 inch EP, or are there other benefits? Is the FOV slightly greater in 2 inch Ep's? Are the images brighter? Is the view crisper in any larger EP etc?

As all my EP's, filters, diagonals are all currently 1.25 inch size, and as all 2 inch equipment is more expensive too, will I get any benefit over buying any new 2 inch equipment to go with the new scope, or is there no real benefit to justify any further ependiture on any 2 inch EP's, filters, diagonals etc?

Thanks!

Gus

[souls33k3r]

the only visual advantage of a 2" eyepiece is when you are veiwing at low power are you able to use less magnification and a wider field of view. the maximum field of view (fov) you can get with a 1.25" is 32mm 52 degree (or 24mm 68 degrees, Same fov at higher mag) with a 2" eyepeice you can use 41mm 68 degree's or 55mm 52 degree's so you can take in more sky at the same or lower magnifications, anything higher power (mag) than that can be done using a 1.25" eyepiece and will have no effect as the light path is less than 1.25" wide 

[Knighty2112]

Thanks souls33k3r. So no real big advantage then unless I want a slightly wider FOV with a lower power eyepiece? Not sure if splashing out probably around £50 to 60 on the lowest price 2 inch diagonal, and a further £100 plus for a 41 or 55mm EP is really worth the money then. Cheers!

[souls33k3r]

Entirely up to you mate. 

Minus all the technical jargon, see if 1.25" EP gives you the satisfaction. If it does then no need splashing out money. If it doesn't by all means open up your wallet

[John]

As above, the 2" barrel diameter allows a larger field stop to be used and therefore a larger apparent field of view - around 40% more is possible. Useful for fitting large objects objects into the field of view. As you say, you need to change the diagonal to a 2" (although those can then also accomodate your 1.25" eyepieces as well) and you need 2" filters for 2" eyepieces.

If you look through this very long running thread you will see that many folks have at least one 2" eyepiece in their set:

http://stargazerslounge.com/topic/57027-show-me-your-eyepiece-cases/

They are not essential to observing though, just rather "nice to have"

[John]

The image below illustrates the difference in the apparent field of view for approx a 30mm eyepiece in a scope of around 500mm focal length. The 82 degree apparent field just isn't possible in the 1.25" fitting, the max is 52 degrees:

[Knighty2112]

Entirely up to you mate. 

Minus all the technical jargon, see if 1.25" EP gives you the satisfaction. If it does then no need splashing out money. If it doesn't by all means open up your wallet

I'm half Scottish, and born and live in Yorkshire also, so cracking open my wallet is sometimes not easy for me to do! Hehe! ; )

[Knighty2112]

The image below illustrates the difference in the apparent field of view for approx a 30mm eyepiece in a scope of around 500mm focal length. The 82 degree apparent field just isn't possible in the 1.25" fitting, the max is 52 degrees:

ES-Eyepiece-Review-FOV-1-of-1.jpg

Thanks John. Maybe something I might get eventually then, so may add to the list of new equipment etc, but place it a lot lower down. Get good views through my Celestron Omni EP's (32mm, 12mm, 6mm) and other stock Celestron EP's currently on my other two 70mm refractors already, so should be just as good on the new 120mm refractor, so no real hankering at the moment for the extra wide FOV.

[John]

I got seduced by ultra wide fields of view from an early stage so 2" eyepieces were irresistable

[Peter Drew]

You also have to consider the magnification. The ST120 is a short focus telescope so eyepieces 44mm and longer would give too low a magnification and result in too large an exit pupil which would reduce your effective aperture.  

[Knighty2112]

You also have to consider the magnification. The ST120 is a short focus telescope so eyepieces 44mm and longer would give too low a magnification and result in too large an exit pupil which would reduce your effective aperture.  

Sorry Peter, but by this do you mean that the image will duller here when using a 2 inch 44mm EP over an 1.25 inch 32mm EP with the shorter focal length scope?

[nicoscy]

The 120mm has a focal ratio of 5 and a focal length of 600mm.

The magnification of a 2" 44mm is 600 / 44 = 13.64. The exit pupil is 120 / 13.64 = 8.80mm

The magnification of a 1.25" 32mm is 600 / 32 = 18.75. The exit pupil is 120 / 18.75 = 6.40mm

Your pupils can dilate to maximum around 7mm. Therefore if you use the first eyepiece, you are effectively cutting the 8.80mm beam of light trying to enter your retina down to whatever your dark adapted pupil will dilate. The effect is that your 120mm will appear to operate at a lower diameter and a longer focal ratio. You basically lose precious light which you cannot in any case accomodate.

The 1.25" 32mm now allows (you seem fairly young so I assume your pupils can dilate to the max) to get the full 6.40mm beam of light through the eye with no loss. Therefore there will be more resolution available since you will be working at "full aperture" and the slight increase in magnification does have a tangible effect on seeing objects.

To cut a long story short, stay to 32mm as the maximum for an eyepiece, be it 1.25" or 2".

On another note, 1.25" eyepieces are great, but your scope begs for one 2" eyepiece to allow you the maximum field of view possible. I had the same scope and I could get 4 degrees true field of view (8 moons side by side) with my Nagler 31mm. I spent the entire summer last year just panning along the Milky Way   Food for thought as they say.

[John]

Good explanation Nicos

[Knighty2112]

Thanks Nicoscy. Great explanation. Cheers!

BTW: the photo for my avatar was taken when I was 47 five years ago, so I am now into the downhill slope of maximum pupil dilation! Hehe!

[BritAngler]

Knighty, where in East Yorkshire do you live? If anywhere near Hull, there's three good Astro groups you could visit (there is a 4th, but that one rarely goes out viewing AFAIK) - East Riding Astronomers who meet in Beverley; North Lincs Astro who meet at Far Ings on the south bank, and Brough Astronomers who meet in.... erm..... Brough. If you go along, you will be able to compare the views through a 1.25" and a 2", assuming the weather is kind to us.

[Knighty2112]

Thanks BritAngler. I live in north Hull, just east of the river Hull. Beverley would be the easiest to go too rather than the others from where I live, but all are relatively easy to get to for me. I was aware of all the groups, and did have a quick look where the Beverley meet was held, so may pop along to that one at some stage to have a look if possible.

Cheers!

[BritAngler]

All three are very friendly, with the emphasis on going out observing if the clouds allow, or talks inside if they don't

If you don't already know, the North Lincs group meet on the 1st Monday of the month, East Riding on the 3rd Monday, and Brough on the 4th.