eyepiueces

I have just got into this telescope thing, Bought an 8"sct celestron adv. I am looking for an eyepiece for wide view not so much power. I have bought a 32 celsrton ultima(pretty nice). a Meade 18mm uwa(okay), just purchased a 20 mm type5 nagler and a 16mm same type 5 and also have a 13 mm nagler. What larger eyepiece would be best for this scope>>>>Thanks for all the help...Terry

Terry:

Hello and welcome to this fine hobby and to Astromart too...

Now to your question: The maximum possible field of view is determined by two factors, the focal length of the telescope and the maximum possible field stop diameter of the eyepiece.

A short explanation: when you look into the eyepiece, the outer edge of the view you see is determined by a ring which is at the focal plane of the eyepiece. The observer sees this ring as the black edge. If you turn the eyepiece around and look down it backwards, you will see that ring in the barrel.

That ring can only be as big as the inner barrel diameter, with the 1.25 inch eyepieces, this happens to be 28mm. A 32mm Plossl type eyepiece like the Ultima will have a field stop very nearly 28mm.

There are two ways to increase the total field of view of the C-8: A F/6.3 focal reducer which reduces the effective focal length of your C-8 from ~2032mm to ~1280mm or a 2 inch diagonal which allows the use of 2 inch eyepieces.

- F/6.3 focal reducer. This is a nice tool to have and will be useful with your 32mm Ultima. The formula for calculating the TFoV is:

TFoV = 57.3 degrees/radian x (field stop diameter)/(focal length telescope)

With your C-8 it looks like this:

TFoV = 57.3 x 27.5mm/2032mm = 0.775 degrees

TFoV = 57.3 x 27.5mm/1280mm = 1.23 degrees...

More on that later...

- However you have chosen some 2 inch eyepieces, the 32mm LX: A larger diameter field stop is possible because the barrel diameter of the eyepiece is larger, this allows for a larger TFoV. In the case of the 2 inch eyepiece, the largest possible field stop is 46 mm, with the C-8, this results in a TFoV of:

TFoV = 57.3 x 46mm/2032 = 1.29 degrees.


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So, the question is: Why not use both a 2 inch eyepiece and the focal reducer??

This brings up another issue, the telescope itself. The hole in the back of the Celestron 8 inch SCT is only so big, (this is known as the "rear baffle")and acts as a stop to limit the field of view. Since it is not at the focal plane it does not provide a hard edge like the field stop but it still limits the possible angular field of view. The diameter of the rear port on an C-8 is 1.5 inches or 38.1mm.

The TFoV calcuation for the C-8 looks like this:

TFoV = 57.3 x 38.1mm/2032mm = 1.074 degrees.

What this means is that any eyepiece or combination (focal reducer +eyepiece) that offers a True Field of view greater than 1.074 degrees will Vignette, the edge of the field of view will not be fully bright. One can cheat a bit here but not a lot, one is already cheating with the focal reducer and a 28mm field stop or 2 inch widefield with a large field stop.

With the 32mm Ultima, a 2 inch eyepiece, one can estimate the True Field of View using another formula, not as accurate as the first but still reasonable:

TFoV = AFoV /Magnification

TFoV = 70 degrees /(2032/32) = 1.1 degrees at 63 X.

One can go wider than this but not a lot... the combination of the 32mm Ultima and a F/6.3 focal reducer would show obvious vignetting and a restricted field of view.

Uncle Rod Mollise, the guru of the SCT can advise on exact eyepieces that will maximize the field of view without Vignetting in the SCT...

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Since the limit on true field of view is the focal length of the telescope and not the aperture of the telescope, shorter focal lengths are desirable for those who enjoy the widest possible view of views. Often this takes the form of a relatively small refractor with a 2 inch focuser and a relative short focal length, this would be called a "relatively fast scope."

For example the William Optics Megrez 110mm has a focal length of about 660 mm, with an eyepiece with a 41mm field stop this provides a TFoV of:

TFoV = 57.3 x 41/660 = 3.55 degrees... This is very generous and such scopes are wonderful for scanning the night sky at low powers.

But if one wants to enjoy the benefits of aperture and maintain the capability to achieve the widest fields of view, the Newtonian offers the shortest possible focal lengths in the large apertures. For example, the counter part to the C-8 SCT is the C-8N, an 8inch F/5 Newtonian (1000mm Focal Length) and a 2 inch focuser, this scope is capable of a 2 degree Plus field of view.

And up the scale, a 16 inch F/5 Newtonian has the same focal length as the C-8 but with a 2 inch focuser that can allow full illumination of the field of view, such a scope can offer a 1.3 degree TFoV but with an image that is 4 times brighter than the C-8...

But of course, such scopes are not without their own set of issues and compromises...

BOttom line: There is no perfect scope but if one is looking for the widest, brightest fields of view, the SCT is not the best choice... Jack of all trades, master of none...

Best to you and enjoy your telescope and the wonders it can show...

Jon Isaacs

so for powering up to the planets of moon which would be best for the sct8"????