5 replies to this topic

[Cielo_nocturno]

How can I properly measure the field stop of an eyepiece with and without a Barlow lens?

 

I want to test the differente spacers between the eyepieces and the Barlow so I can know accurately the result field stop.

 

Visually I can see how the field stop changes of size but I don't know how to properly measure it.

 

I remember that one time I saw a post of someone who did it with a ruler and a projection of light, but I don't find it.

Edited by Cielo_nocturno, 11 April 2025 - 01:14 PM.

[areyoukiddingme]

Stick a measuring tape on your wall, measure the width of the field with vs. without the barlow. This gives your barlow factor, and will be the proportionate change in field stop.

[CrazyPanda]

How can I properly measure the field stop of an eyepiece with and without a Barlow lens?

 

I want to test the differente spacers between the eyepieces and the Barlow so I can know accurately the result field stop.

 

Visually I can see how the field stop changes of size but I don't know how to properly measure it.

 

I remember that one time I saw a post of someone who did it with a ruler and a projection of light, but I don't find it.

Field stop is not going to change with or without a barlow. The field stop is a physical ring somewhere inside the eyepiece.

 

The *true field* is what will change.

 

You can either do a star drift timing test with and without the barlow in place, or take a picture of a ruler through the eyepiece. You just have to be careful because a phone's autofocus changes the distance of the lens to the sensor and can actually change the image scale. I have to use a manual camera app on my phone and set the focus point to a fixed point to ensure auto focus won't create differences in image scale.

Edited by CrazyPanda, 11 April 2025 - 10:49 PM.

[CeleNoptic]

...or take a picture of a ruler through the eyepiece.

 

Taking a picture isn't absolutely necessary. You can just write down the FOV width in mm visible through the eyepiece without the Barlow and with the Barlow and find the ratio.

Edited by CeleNoptic, 11 April 2025 - 11:04 PM.

[Ernest_SPB]

Lets see influence of a Barlow to FOV

 

 

AFOV usually not influenced. TFOV is changed becoming Barlow magnification less. So just measure how much changed TFOV (star drift method or similar) and you could realize Barlow magnification or how much it reduces image of eyepiece FS.

[Starman1]

1.find a star on or near the celestial equator.
2.time the passage of the star from edge to edge.
Measure twice and take the larger figure as it is more likely a diameter of the field.
3. insert Barlow and do a timing again. the first timing divided by the Barlow timing is the power of the Barlow.
4. Convert the eyepiece-only time into decimal minutes by dividing by 60. Divide that figure by 3.99 and you have the eyepiece field in degrees.
5. plug the field in degrees into this formula to get the field stop (FS) of the eyepiece:
FS=(TFxTFL)/57.3
TF is true field in degrees
TFL is telescope focal length in mm