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Low power wide FOV EP


jambouk
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What is the lowest power, widest FOV eyepiece one could use effectively in a 127mm skywatcher mak cassegrain? At some stage i suspect adding more FOV on the eyepiece wouldn't add any more observable FOV.

Thanks

James

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There is a way to ascertain the maximum potential true field of view of a telescope.

For you the maximum eyepiece size you can fit in your scope is 1.25" as they dont, i believe, accept 2" eyepieces.

The maximum field stop size for a 1.25" eyepiece is 27mm I think.

To calculate the maximum potential true field of a scope you use the following equation

( maximum field stop of eye piece / focal length of telescope ) X 57.3

For you then, this is 1.03 degrees ((27/1500) * 57.3)

In buying eyepieces you can work out roughly how much true field it will give by dividing the apparent field of view by the magnification in your scope.

take for instance the 20mm Willliam Optics SWAN. 72 degrees of AFoV. eyepiece gives 75x in your scope. This is 0.96 degrees of true field. very close to your limit. I picked this eyepiece because my father owns this scope and I bought him this eyepiece as a gift and to give him something with a wider field.

with the information I've provided here you should be able to ascertain from a number of other eyepieces what the true field will be for a given eyepiece. so long as you dont exceed your scopes potential then you'll be fine.

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Can't adjust rhe values on my iphone, but probably can on the computer, thank you.

So it would look as though the 20mm (50^ apparent foc) eyepiece which came with my scope gives me about 0.68 degree true field of view. A "cheapy" 32mm EP with a 46^ fov would give me a trie field of view of nealry 1 degree.

James

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I think the options are

16mm 82 degree - Nirvana WO UWAN 0.87 degrees - this is the lowest 1.25 an 82 degree comes in

20mm 72 degree - WO SWAN 0.96 degrees (these dont work well in fast scopes)

25mm 60 degree - BST 1 degree

32mm 52 degree - TV Plossl or other cheaper plossl 1.04 degrees

Edited by Stargazing00
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Would it matter if i got an EP which resulted in a greater fov than the acope could achieve? Presumably there would just be a black rim around the edge of the fov whwn looking down the eyepiece, or would it actually generate some unsavoury artefact?

James

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great review there. wish they did one of those for 82 degrees to that level :) thanks

:cool:

Would it matter if i got an EP which resulted in a greater fov than the acope could achieve? Presumably there would just be a black rim around the edge of the fov whwn looking down the eyepiece, or would it actually generate some unsavoury artefact?

James

You might experience some vignetting at the edges... fading/darkening of the starlight of stars. Beyond a 24mm/68deg or 32mm/50deg EP you'd need to move up to 2" EPs.

However, the next practical bottle neck of the little Mak is the baffle tube which at 27mm just happens to coincide (more or less) with the max field stop for a 1.25" eyepiece. There's little benefit to going 2", and the accessories and EPs tend to be more expensive. Despite that logic, I use a 2" diagonal on my smallest SCT (C6 has same focal length and baffle tube diameter as the 127 Mak) as much because it feels a more sturdy and secure mechanism than the stock 1.25" as it has compression rings instead of the fiddly little screws. Using my 20mm Nagler (82 degree aFOV) is about its limit, although you could go further to see whether or not the vignetting is noticeable/objectionable. At which point it's already too late :D

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