1.25" vs 2" Question

[shredder]

Hi All,

I was just looking over a few web sites and noticed that for some eyepieces you can get a 1.25" version and a 2" version for the same model for the same focal length, ie a 1.25" 30mm SP and a 2" 30mm SP. Can anyone tell me if you have a 2" focuser is there any visual difference between the two eyepieces (apart from the obvious of a 2" has a wider barrel)? Does one look differnt from the other to look through?

Just wondering.....

M

[davidpretorius]

i have series 500 6.5mm,12.5mm, 25mm, 40mm, pretty cheap but not too nasty. i have bought a 2" ultrawide 80 degree 30mm and i would have to say that i am blown away with it. i now only use the ultra wide and the 12 and 6.5. the ultra wide gives lovely wide shots, very easy on you eyes, no squinting and you can move your head form side to side to "look around corners" it feels like.

the 2" to 1.25" adapter that goes in my 2" crayford is eassy to take out and out in as i swap between 1.25 and 2". it is worth the effort!

[rmcpb]

From what I understand the 2" diameter comes into its own when it is combined with wide angle eyepieces. As the field stop can be larger you can get a wider angle of view. I wouldn't expect there to be too much difference between two lenses of the same focal length and type as you quoted above.

Generally 2" eyepieces are for getting heaps of sky into your eye at a time. The smaller 1.25" eyepieces are for getting more detail via magnification.

Does that help or confuse?

Cheers

[asimov]

I too have thought about this shredder ie: a 30mm 2" compared to a 30mm 1.25" I assume, a wider view in the 2" would be the only advantage? I must admit, I was not overly impressed with my 30mm SV 2" in a F5 newt. My 24mm celestron ultima 1.25" gave a much better view. Not quite as wide a view as the 2"....but no 'seagulls' either.

[shredder]

Ahh I seem to have miss represented my question some what.....

I realise that with a 2" you can get eps with a wider field of view, what I was wondering was that if you had a 2" 30mm with say a 67 FOV and a 1.25" 30mm with a 67 FOV would you see any difference?

Why, I hear you ask? Well I am thinking of getting a longer focal length ep, but for the most part they use a 2" barel, and to fit it I need to get a 2" visual back and angle, making it quite an expensive investment for only a single ep. However I did notice that in some models the ep comes in both the 2" and 1.25" variety, hence if there is no visual difference then I wouldnt spend the extra on the 2" (if that makes sense).

M

[MiG]

"2" 30mm with say a 67 FOV and a 1.25" 30mm with a 67 FOV"

Is this a real life case? If so, then it appears that 2" isn't necessary for those specs.

[shredder]

Well I am not sure if they are exactly the same FOV as it isnt stated but Bintel sell 2 40mm Bintel Plossles one a 1.25" and the other a 2", the 2" is $20 more and I was wondering if there was any real difference. This isnt the only case of it but it is the one that prompted the question.

Maybe its as simple as old / new stock and a slight change of model?

M

[GrampianStars]

The 1.25" eyepieces are a standard now for over 200 + years
NOTE the 2" eyepieces do not have any benifit on 8" or smaller SCT's
because the visual backs are too small only 1.50" openings
s therefore just a heavy weight on the back end :cold:

[janoskiss]

The geometric limit on the FOV is increased with the larger body. Typically, you cannot have a 30mm 1.25" EP that has a 67 FOV. (You can if the entry lens on the focuser side is unusually close to the bottom of the EP body, but then it might not focus properly, esp. with a Barlow).

http://members.optusnet.com.au/~janosk/misc/ep-fov-sml.png

Typical FOV geometric limits are:

1.25":
FL(mm) FOV(deg)
15 93
20 76
30 56
40 43

2":
FL(mm) FOV(deg)
15 119
20 104
30 81
40 65

... bit more with vignetting

[davidpretorius]

right off the cuff, i would suggest 15mm and above would be more preferable in 2" and 1.25" for less. want to look at planets planets and finer detail, then 6mm with barlows in 1.25" for the cost factor?

[ballaratdragons]

Shredder, I have found by my own 2" and by comments by others on the net doing EP reviews that the biggest difference is better 'Eye Relief'.

If you wear glasses a 2" or a Barlow is much easier for comfortable viewing unless to can shove your glasses right up against the EP which will scratch it!

Plossls of 1.25" however do have a reasonable eye relief until you get to the shorter f/ratios.

[Starkler]

Janoskiss is right on the money.

The widest widefield available in a 1.25" format is the 24mm panoptic, which happens to have the same true field as a 32mm plossl.

A 40mm 1.25" plossl is a white elephant, in that you dont see any greater field than you do with a 32mm plossl, but suffer from a smaller AFOV and bigger exit pupil meaning the sky background is brighter.

The rule of thumb is if you want a greater true field than that provided by a 32mm plossl at 50 degrees afov, you need to go to 2 inch format.
For smaller fields, some people still buy in 2 inch format just for the convenience of having a collection with the same barrel sizes and not having to mess with swapping adapters in and out.

[janoskiss]

What do you mean by that? What sort of scope do you have? If you mean that have a 1.25" focuser and you were going to get a 1.25"-to-2" adapter, that may not be a good idea...

[shredder]

Hi All, back again.

Thanks for the update, the calculation given by Janoskiss explains it well. So the reality is that in a 1.25" going over 32mm is a bit of a waste of effort, and in going to a 2" I really need to get a SWA or similar....

To answer some of the other comments, I guess for me I already have a good range of EPs (6.7 UWA, 8.8 UWA, 13.8 SWA, and 24.5 SWA all Meade 4000s) so I was not so much looking for info on the lower focal length ep but wondering if I was to go to the 32 or 40mm focal length would it be worth the effort considering I wasnt planing to replace what I had but add to it? Hence was a 30mm 1.25" similar to a 30mm 2" (or 40mm) or if I was going to get something much different in terms of performance. So all in all I suspect getting a new visual back, and angle, and a 30/40mm SWA (or similar) is probably just too expensive for the use. Maybe I should have stated all this up front but I didnt think it added to the question at the time.

Out of interest what are peoples thoughts on using a focal reducer to get a wider FOV? Again I already have one (6.3) but am not 100% sold on it, though in saying that I have only recently gotten it and only used it in suburbia so far (so still reservice judgement)... Oh and all this is on a C8 to complete the picture, a nice old one, really old, ok handed down from granny sort of old....

Anyway thanks again for all the advice.

M

[shredder]

Oh and I almost forgot,

Thanks ballaratdragons

While I am not yet at the stage of wearing glasses at night I am sure it wont be far away, and either way I have often found in the past that the eye relief really makes / breaks the ep as it is really what I find gives you comfort and therefor ease of use (and is all too often forgotten). However with the UWAs, / SWAs this is not really much of an issue as they all have good eye relief.

And its an Orange C8....

M

[elusiver]

the 2" 's look more impressive in u'r eyepiece case :D :P ;)


el :)

[davidpretorius]

when i got my 2" ultra wide, i could not believe how big it was and still is!!!

[Stu]

The Meade 5000 2 inch 30 and 40mm ep's are almost bigger than my head.

[ving]

i have compared somewhere on the forum a 30mm SV in 2 inch and a celestron 32mm at 1.25 inch... I'll see if i can dig it up

[ving]

http://www.iceinspace.com.au/forum/showthread.php?t=639&highlight=30mm+celestron

there ya go!

[shredder]

Thanks Ving,

Good link and nice review

M