Wide angle

[Legin (Nigel)]

Hello,

I was wondering what is the actual limit on how wide you can go with a scope. I currently have a nice 28mm, 2 inch eyepiece and love the nice wide field views it gives. The scope itself is a 12 inch Newtonian that has a focal length of 1500mm, f/4.9.

I have heard that the widest you should go is approximately 1/5th your focal length. Then you may get interference, I am not sure what you call it but basically the reflection of the primary mirror begins to appear in the eyepiece and can really ruin to wide a view.

I am still relatively new to the eyepiece game so any advice will be great. I get the basics eye relief, focal length, magnification etc. Just checking out my limitations

[brian nordstrom (As avatar)]

Hi Legin , I don't know the science behind this but when I use my 31mm Celestron Axiom LX 82*in my Takahashi SKY90 ( f 5.5) the views are awesome , 16x , flat field and in perfect focus edge to edge .
4.5 degrees easy and on the southern milky way it is perfect .
Brian.

[Allan]

G'day Nigel. The lowest power eyepiece is mostly determined by exit pupil. Fully dilated your eye pupil opens to about 7mm. This reduces with age though. So you want to pick an eyepiece for low power work that gives an exit pupil no more than 7mm. Another thing to consider, if viewing from a light polluted site your pupil will not dilate to 7mm, so you will need to use an eyepiece with a smaller exit pupil, ie higher power.

To work out exit pupil for your eyepieces, divide the aperture by the magnification of the eyepiece.

So in your scope, a 35mm eyepiece will provide 43x. 305mm divide by 43 gives you a 7mm exit pupil. So 35mm is the longest useful focal length for your telescope. Though practically it would only be useful at a dark site where your eye can fully dilate. A more useful low power eyepiece might be closer to 6mm exit pupil. My 31mm eyepiece is close to that and works well in the same size telescope that you have.

Another factor affecting the size of the view is the apparent field of view of the eyepiece. Comparing a 110 degree eyepiece to a 30 degree one reveals a huge difference in how much you can see in the field of view.

[Wavytone]

Hi Nigel,

The usual question is what is the longest eyepiece focal length to suit a given telescope. The answer is Fe = p x f, where p is your eye pupil (normally assumed to be 5-6mm) and f is the focal ratio of the scope.

Alternatively, given an eyepiece focal length Fe = 28mm, and eye pupil 5mm, this suggests the telescope needs to be no faster than about f/5.5.

As for the widest field of view, that's determined by the diameter of the field stop in the eyepiece, and the telescope focal length. If your field stop diameter was d and the scope focal length is F then the angular field of view is (to a very good approximation):
A = arctan(d/F)

You can go as small as you like for the objective. For example, if you cannibalised a 50mm finder - typically a cheap achromatic doublet f/4 to f/6, it will have a focal length around 200 - 300 mm, giving a magnification around 10x with your eyepiece. If the eyepiece field stop was 28mm across (just a guess, you can measure it for yourself with calipers) you'll have a field of view over 6 degrees wide.

To go wider, you could use an even smaller objective - say a 30mm binocular objective (often around f/5) and an ultrawide eyepiece with a larger field stop - a field of 10 degrees or more is feasible, though the image quality might be pretty awful near the edge.

To deal with that you will need to find a better wide-field objective and, for low magnification, the old-school telephoto lenses from film SLR's from the 1970s or 1980s work well enough, and you can still find a few in the junk bins in camera shops. For many years I used a finderscope consisting of a 135mm f/2.8 telephoto coupled to a 25mm eyepiece, giving about 6X magnification and a nice bright, wide field. Camera lenses in the range 135-200 mm work quite well as wide field low power scopes. To put an eyepiece on the back the simplest way is to cannibalise a rear lens cap.

[cometcatcher (Kevin)]

Originally Posted by Legin

I have heard that the widest you should go is approximately 1/5th your focal length. Then you may get interference, I am not sure what you call it but basically the reflection of the primary mirror begins to appear in the eyepiece and can really ruin to wide a view.

I'd say you're near optimum with the 28mm. It is likely with a 40 or 50mm eyepiece you will get a dark shadow in the middle caused by the secondary obstruction. It becomes more apparent with lower power eyepieces.

[Legin (Nigel)]

Originally Posted by Legin

Hello,

I was wondering what is the actual limit on how wide you can go with a scope. I currently have a nice 28mm, 2 inch eyepiece and love the nice wide field views it gives. The scope itself is a 12 inch Newtonian that has a focal length of 1500mm, f/4.9.

I have heard that the widest you should go is approximately 1/5th your focal length. Then you may get interference, I am not sure what you call it but basically the reflection of the primary mirror begins to appear in the eyepiece and can really ruin to wide a view.

I am still relatively new to the eyepiece game so any advice will be great. I get the basics eye relief, focal length, magnification etc. Just checking out my limitations

I should have mentioned 5 time the F/stop not the focal length. So my scope being f/4.9 would be about 25 mm.

It should be noted I have tried a 42mm in this scope and it works great for visual work. I have no luck for eyepiece projection though. I suspect it is just luck that I see anything other than the primary mirror, as nice and shiny as it looks.