Field stop ratios

[morls (Stephen)]

Hi,

Reading the documentation that came with my new Nagler, I came across this:

"Tele Vue recommends choosing low and medium power eyepieces in ratios of field stop diameters. For example, factors of 1.4 or 2.0."

also:

"a decreasing factor of 2 (which results in a 4x decrease in area size)"

I'd be really interested to know, if field stop ratio has been a significant factor in eyepiece choice amongst members here, why a particular ratio has been chosen....it's one thing to read about it, but real-world experience is king, and if anyone could offer their thoughts I'd be most appreciative.

Cheers

Stephen

[rmcconachy]

I don't pay much attention to field stop ratios. My first thought is that I want a `low power/wide FOV' type eyepiece with an exit pupil of about 5-6mm. On a 6"+ scope I'll probably want another eyepiece with an exit pupil of ~2mm which seems to work nicely for many deep sky objects. What eyepieces I select after that depends on the scope in question (aperture and focal length) and my observing conditions (I'm often constrained to using <200x from my backyard due to lousy local seeing but from a decent site I can often use well in excess of that).

As an example, when observing from home with my 10" f/5 Dob I most commonly use eyepieces with focal lengths of 30mm (6mm exit pupil), 10mm (2mm exit pupil) and which ever of 8.5mm, 7mm or 6mm gives the best `higher magnification' view that night. From a better observing site I'd be reaching for my 5.2mm focal length eyepiece sometimes.

[Steffen]

Originally Posted by morls

I'd be really interested to know, if field stop ratio has been a significant factor in eyepiece choice amongst members here, why a particular ratio has been chosen....it's one thing to read about it, but real-world experience is king, and if anyone could offer their thoughts I'd be most appreciative.

For any given scope there is a direct proportional relationship between the eyepiece field stop diameter and the true field of view. Therefore, it's easiest to go by field stop diameter if one wants to build an eyepiece line-up in decreasing TFOV order. The eyepiece focal length does not give this information.

If you don't watch the field stops and buy a mix of Plössls, Panoptics, Naglers etc in different focal lengths for your wide-field kit you might end up with a set of eyepieces that all have a similar TFOV.

For higher magnifications the actual magnification ratio is often more important than the TFOV, so those eyepieces are selected by focal length (which has an inverse proportional relationship with magnification).

I believe this is what's behind Tele Vue's suggestion.

Cheers
Steffen.

[morls (Stephen)]

Thanks for the replies...

I'm especially interested to know why a certain ratio is chosen, for example a factor of 1.4 rather than 2.0. Are there any "rules of thumb" involved, such as doubling the TFOV with each increase in f/l? (for low to medium power)

[koputai (Jason)]

Hi Stephen,

If you go in a ratio of 1.4x then the viewable area doubles with each step. If you go with a ratio of 2x, then the viewable area increases by a factor of 4 each step, so it pretty much comes down to your personal ratio of gap size versus dollars spent.

What Steffen says about choosing wide eyepieces based of field stop, and high power eyepieces based on focal length is wise.

So, how did the new Nagler go? Did you like the view? Was it comfortable to use?

Cheers,
Jason.

[morls (Stephen)]

Hi Jason,

It's been too cloudy in Sydney to get the scope out....maybe tonight.....must be the curse of buying new gear

[koputai (Jason)]

Ah, I got out for about two hours last night, 10pm - Midnight was clear, but yeah, first time for a while.

I was comparing a number of eyepieces on Jupiter. I must say, the Nagler 9mm Type 6 must be one of
the best eyepieces ever built, by anyone, it just never fails to impress.

The big suprise of the night though was an old Edscorp 12.5mm Orthoscopic, in tatty condition, that I
picked up for something like $10. It's been sitting in the eyepiece box, but when I finally put it in the
scope last night on Jupiter, WOW! Definition of clouds, Great Red Spot, and belts were amazing.

I usually find ortho's too hard to use, but the old Edscorp really impressed.

Cheers,
Jason.

[morls (Stephen)]

sounds like a nice find....

[morls (Stephen)]

Well, I got the 13mm Nag out for the first time this evening, albeit very briefly around midnight....

Loved it....it was my first view of M42 as well, and it looked great in the ep, as did Jupiter and 47 Tuc.

It's got great contrast, and the wide field is pretty impressive....I think I'll have to get used to the eye relief - it's quite tight, but not uncomfortably so....overall I'm very happy, and I think as my experience grows I'm going to like this eyepiece more and more...

[PN PM]

If you don't watch the field stops and buy a mix of Plössls, Panoptics, Naglers etc in different focal lengths for your wide-field kit you might end up with a set of eyepieces that all have a similar TFOV. (Posted by Steffen.)

Well-said, sir.
It`s to avoid redundancy. (Much like this post?)