Field Stop

[jflowers]

Does anyone know where I can get the 'Field Stop' values from for my eyepieces? I am trying to set up Stellarium as best as possible to my actual scope and eyepieces. I just cant find the correct values for this part though. I have tried using different values and it changes the view significantly, so its obviously an important part of it. As far as I understand, it is the narrowest part that light has to travel through to get to your eye.

My eyepieces are

Meade 4000 plossl 32, 26, 15, 9.7mm

Celestron x-cel LX 5mm

Celestron and Meade websites are no help (well i couldn't find the info)

Many Thanks

Jon

[John]

I thought the 1.25" Meade 4000 plossls all (apart from the 40mm) had an apparent field of view of 52 degrees. Tele Vue publish their field stop sizes but most other brands don't so you may have to resort to physically measuring them if the state AFoV is not accurate enough.

[jflowers]

Yes the Meads are 52 degrees and the Celestron is 60, is this related to Field Stop? Measuring sounds like the way to go! Thanks

Tele Vue are a little out of my league

[John]

The field stop is the aperture ring within an eyepiece that defines the physical size of the apparent field of view - so, yes, they are directly related.

With plossls the field stop is found in the eyepiece barrel, below the bottom or field lens of the eyepiece. With more complex designs the field stop is hidden within the body of the eyepiece so won't be easy to measure without dismantling the eyepiece - not recommended !.

From the apparent field of view (AFoV) you can work out the true field of view (TFoV) or how much sky will be shown for a particular eyepiece used in a particular scope. The formula is: AFoV divided by magnification = TFoV in degrees.

Example:

My 13mm Tele Vue Ethos has an AFoV of 100 degrees. In my 120mm refractor, which has a focal length of 900mm, this eyepiece gives a magnification of 69x. 100 / 69 = 1.45 degrees so thats how much sky I can see with that eyepiece in that scope.

Hope that helps.

PS: You can get used Tele Vue plossls for between £40 and £50 but the Meade 4000's are not at all bad in themselves.

[jflowers]

Very helpful, thanks.

Jon