1.25" vs 2.0"

[neil groves]

I just read some reviews on this subject and someone describes the 2.0" ep's as giving a more 'dreamy' view than the 1.25"s. what would you make of that? what might he mean by 'dreamy'? I kinda like the idea since when I am viewing I tend to 'dream' lol, I go to my own little place in my mind if you know what I mean.

[nicks90]

i have no idea - but fingers crossed, weather gods appeased, cloud god mollyfied - I will tell you early next week when i try out my new 2" panaview against my simillar focal length 1.25" ep!

I hope the difference will make me go "ooooooooooooh, eeeeeeeeee, wowwwwwww"

[damnut]

The 2" eyepieces give a wider field of view. The benefit of that is that you can fit larger objects like galaxies or clusters in the view at the eyepiece. Limited by mag and type of scope and your age!(pupil diameter)

Have a look at :-- http://www.12dstring.me.uk/fov.htm remember to click on visual view!!

And :-- http://www.stargazing.net/naa/scopemath.htm

[NGC 1502]

Dunno about 'dreamy'

I suppose what may be meant is that a low power wide view of the night sky can be stunning. There is something that grabs many of us about observing that way. My favourite low power wide field view is through my short focus refractor at 18x and 3.8 degree field that shows the double cluster together with Stock 2 open cluster, or the whole of Kembles Cascade, or the Pleiades with plenty of surrounding sky or........you get the picture........sort of well erm er......dreamy I suppose

Regards, Ed.

[ronin]

"Dreamy" likely refers to the possibility that in some you look through them and are not aware of the edges, they are outside your field of view. Has been described as appearing to float in space.

Now some like this and others dislike it. Recent post on CN where one person really dislikes the effect, so not necessarily for everyone. As he finds all he does is search around for the edge and doesn't observe.

A 2" eyepiece is big, surprisingly so. I have a 20mm 2" eyepiece and the box that is in will hold 6 TV plossl boxes.

[RikM]

I am one of those who doesn't like ultra-wide fields. I like to be able to see the edge of the view otherwise I get sort of vertigo feeling. However, I don't like to feel too restricted, like looking through a drinking straw. 70° aFOV is just right for me.

A 2" eyepiece barrel can accommodate a larger fieldstop than 1.25" and this is what sets the field of view, not the eyepiece barrel itself. Manufacturers can choose to use a larger fieldstop or not, so it is not in every case that a 2" will show more than a 1.25". It depends on the design. In practical terms though, most designers would only use a 2" barrel when they want to use a larger fieldstop, so in the majority of eyepieces, a 2" will be wider.

When people say 2" eyepieces being 'dreamy', they probably really mean ultrawideangle eyepieces (which will likely be 2") are dreamy. It is easy to misunderstand.

[Stargazer_00]

2" eyepieces are only a way of extending beyond the limitations of a 1.25" barrel. the mechanics are identical.

As a thought exercise, consider a 25mm focal length eyepiece. If you make the apprarent field of view variable, the field stop (diameter of the glass on the telescope side of an eyepiece) will be different. The wider the field stop the wider the apparent field of view (assuming that the focal length is constant);

If the field stop is constant, lets say the maximum field stop size of the 1.25" barrel which is somewhere around 26mm I think, and the focal length of the eyepiece is different.

A 32mm focal length eyepiece can show 50 degrees AFOV using the maximum field stop of a 1.25" barrel

A 25mm focal length eyepiece can show 60 degrees AFOV using the maximum field stop of 1.25" barrel

A 24mm focal length eyepiece can show 68 degrees AFOV using the maximum field stop of 1.25" barrel

A 17.3mm focal length eyepiece can show 72 degrees AFOV using the maximum field stop of 1.25" barrel

A 17mm focal length eyepiece can show 82 degrees AFOV using the maximum field stop of 1.25" barrel

A 13mm focal length eyepiece can show 100 degrees AFOV using the maximum field stop of 1.25" barrel

Does that make sense?

Most sets of eyepieces keep their AFOV constant across the range. So in a range of 82 degree eyepieces the point you need to start buying 2" eyepieces is maintain 82 degrees AFOV is 18mm I think.

I hope that makes some sort of sense. Back to my previous point, the point of 2" eyepieces is to extend past the limitations of the 1.25" size. You could equally build all 1.25" eyepieces specifications into a 2" barrel but it'd be a waste of maetrials and they'd be pointlessly heavy.

Televue are well known for squeezing the maximum size out of a given AFOV in terms of focal length often having 1 or 2 more mm than other makes who simply dont try as hard