2x Barlow FOV Question

[refractordude]

Attaching a 2x barlow to a 42mm 65 degree fov eyepiece, will give me a 21mm eyepiece. What will be the fov of the 21mm? Will it be 65 degrees or 32.5 degrees? 

[Ben the Ignorant]

65. A barlow doesn't change the eyepiece's field.

[Stu]

4 hours ago, refractordude said:

Attaching a 2x barlow to a 42mm 65 degree fov eyepiece, will give me a 21mm eyepiece. What will be the fov of the 21mm? Will it be 65 degrees or 32.5 degrees? 

It is the apparent field of view you're referring to there and that does not change when used with a barlow. The actual or true field of view does halve because the x2 barlow effectively doubles the focal length of your scope and doubles the magnification, halving the field of view.

[refractordude]

9 minutes ago, Stu said:

It is the apparent field of view you're referring to there and that does not change when used with a barlow. The actual or true field of view does halve because the x2 barlow effectively doubles the focal length of your scope and doubles the magnification, halving the field of view.

so the field of view will be 32.5 degrees for the 21mm?

[Stu]

16 minutes ago, refractordude said:

so the field of view will be 32.5 degrees for the 21mm?

No, the apparent field of view is 65 degrees and stays that way.

Let’s assume your scope has a focal length of 1000mm

The 42mm eyepiece gives a magnification of 1000/42 which gives a rounded x24

The actual field of view through the eyepiece when out in the scope is 65/24 = 2.7 degrees of sky.

With the barlow, the focal length doubles to 2000mm so

2000/42 = x48 magnification

65/48 = 1.35 degrees of sky seen which is the actual field of view.

Apparent field of view is a characteristic of the eyepiece and does not change (unless using a zoom where this can change over the range of zoom)

True field of view is the amount of sky you see when using the eyepiece with a particular scope and barlow/reducer combination. Hope that makes sense?

[John]

Actually, in practice barlow lenses can sometimes restrict (vignette) the apparent field of view of eyepieces with very larget field stops. I've seen this on a number of occasons.

If the 42mm eyepiece has close on the maximum sized field stop that can be accomodated within the 2" format (which is likely) you may seem some loss at the edges of the field of view.

 

[wookie1965]

1 hour ago, Stu said:

No, the apparent field of view is 65 degrees and stays that way.

Let’s assume your scope has a focal length of 1000mm

The 42mm eyepiece gives a magnification of 1000/42 which gives a rounded x24

The actual field of view through the eyepiece when out in the scope is 65/24 = 2.7 degrees of sky.

With the barlow, the focal length doubles to 2000mm so

2000/42 = x48 magnification

65/48 = 1.35 degrees of sky seen which is the actual field of view.

Apparent field of view is a characteristic of the eyepiece and does not change (unless using a zoom where this can change over the range of zoom)

True field of view is the amount of sky you see when using the eyepiece with a particular scope and barlow/reducer combination. Hope that makes sense?

Can you tell me if this is right then Stu a 15mm 60º FOV on the Tal (1000) without the Barlow my TFOV would be 0.9º. (1000/15 =66.6 round that to 66 then 60/66 =0.9 is that correct.)

[Stu]

3 minutes ago, John said:

Actually, in practice barlow lenses can sometimes restrict (vignette) the apparent field of view of eyepieces with very larget field stops. I've seen this on a number of occasons.

If the 42mm eyepiece has close on the maximum sized field stop that can be accomodated within the 2" format (which is likely) you may seem some loss at the edges of the field of view.

 

I don’t think I’ve ever bothered barlowing a long focal length eyepiece so haven’t seen that John, although it makes total sense.

The fact remains that the afov of the eyepiece remains the same and doesn’t halve as was suggested by the OP. The effective afov when used in the barlow could probably be calculated by using the field stop method ie 57.3 x eyepiece fieldstop/Telescope focal length

[Stu]

9 minutes ago, wookie1965 said:

Can you tell me if this is right then Stu a 15mm 60º FOV on the Tal (1000) without the Barlow my TFOV would be 0.9º. (1000/15 =66.6 round that to 66 then 60/66 =0.9 is that correct.)

Hi Paul, yes that’s correct

Stu

[John]

1 hour ago, Stu said:

I don’t think I’ve ever bothered barlowing a long focal length eyepiece so haven’t seen that John, although it makes total sense.

The fact remains that the afov of the eyepiece remains the same and doesn’t halve as was suggested by the OP. The effective afov when used in the barlow could probably be calculated by using the field stop method ie 57.3 x eyepiece fieldstop/Telescope focal length

It's not a thing that I've done often - the extension to the already long eye relief, apart from vignetting, makes it less than desireable for me.

I agree that the vignetting is much less than 50%.  

[Cornelius Varley]

8 hours ago, refractordude said:

Attaching a 2x barlow to a 42mm 65 degree fov eyepiece, will give me a 21mm eyepiece. What will be the fov of the 21mm? Will it be 65 degrees or 32.5 degrees? 

The barlow lens acts on the focal length of the telescope, so doesn't affect the focal length or fov of the eyepiece.

[Louis D]

Speaking from experience, I can assure you all that barlowing near widest field 2" eyepieces will yield full-on field cut-off, not subtle vignetting.  It looks as if a new field stop has been placed 5% to 10% inboard of the physical field stop.  If you move your head around while viewing a bright object like the moon, you can sometimes glimpse a thin line of light between the newly blackened area and the physical field stop.  I don't know what that indicates at all.  I'm always surprised it's not a subtle vignetting like you get with 32mm plossls in 22mm clear aperture binoviewers.

I get this false field stop effect with both my 40mm Meade 5000 SWA and 30mm ES-82, so it affects both positive-only and negative-positive eyepiece designs.  It's probably there with some of my other widest field eyepieces, but I don't generally barlow them.

If you then put a Televue Panoptic Barlow Interface into the barlow (assuming the barlow has a similar focal length to the TV Big Barlow, which the GSO 2" ED 2x barlow does), the false field stop disappears and exit pupil fussiness disappears as well because the barlow is now a telecentric magnifier.  In fact, it becomes difficult to tell there's anything extra in the light path other than the increased magnification.  Since I have to remove my coma corrector to reach focus, I've noticed it also helps to lessen the impact of Newtonian coma on the image by extending the focal ratio.

Back to the original OP's question, the apparent field of view remains the same for a 42mm 65 degree eyepiece if  you use a telecentric magnifier rather than a simple barlow.  Whether or not you get a 21mm eyepiece depends on how accurately the manufacturer designed the 2x barlow, and if the eyepiece's field stop happens to be placed at exactly the design distance to yield 2x.  In reality, it's more of a ballpark estimate.  You'll get somewhere around 2x, give or take a few tenths of an X.

[25585]

On 07/06/2018 at 08:15, John said:

Actually, in practice barlow lenses can sometimes restrict (vignette) the apparent field of view of eyepieces with very larget field stops. I've seen this on a number of occasons.

If the 42mm eyepiece has close on the maximum sized field stop that can be accomodated within the 2" format (which is likely) you may seem some loss at the edges of the field of view.

 

Was that the case with TV 35 & 41 Panoptics, which before the Powermates were introduced, required an interface optic for the Big Barlow etc?