4 replies to this topic

[denzel200219]

I currently have a Nexstar 8se. I have learned that for 1.25 inch focusers, the maximum field of view is achieved with either a 32mm plossl or a 24mm 68 degree eyepiece. What about using an 82 degree 10mm eyepiece with a 0.5 focal reducer that you can thread on the eyepiece? Would this allow me to also reach the same field of view of a 24mm 68 while also achieving higher magnification? Is this a good idea? Also I have heard that the 10mm Luminos is good in slower f/10+ scopes, but would adding a 0.5 focal reducer cause the eyepiece to have the same performance as if it were on a f/5 eyepiece? 

[Starman1]

I currently have a Nexstar 8se. I have learned that for 1.25 inch focusers, the maximum field of view is achieved with either a 32mm plossl or a 24mm 68 degree eyepiece. What about using an 82 degree 10mm eyepiece with a 0.5 focal reducer that you can thread on the eyepiece? Would this allow me to also reach the same field of view of a 24mm 68 while also achieving higher magnification? Is this a good idea? Also I have heard that the 10mm Luminos is good in slower f/10+ scopes, but would adding a 0.5 focal reducer cause the eyepiece to have the same performance as if it were on a f/5 eyepiece scope? 

Yes.

A 10mm eyepiece with a 0.5x focal reducer is, effectively, a 20mm eyepiece.

The field stop of the eyepiece is also doubled in effective size, so the very approximate field stop of the 10mm, ~13.5mm, becomes 27mm, so you would have the same true field as the 24mm 68° or the 32mm 50°.

 

Focal reducers aren't panaceas, though.  They add glass, result in field curvature, add chromatic aberration.  And they cause the eyepiece to see a faster light cone, which can induce aberrations.

So yes, the eyepiece would see, effectively, an f/5 scope.  The Luminos doesn't handle an f/5 scope well, so I could predict a wider true field at 1/2 the power, but with more outer field aberrations.

In this case, it likely would also vignette, since a 20mm 82° eyepiece should have a somewhat wider field stop than 27mm.  The 20mm 80-86° eyepieces out there are all 2" eyepieces for that reason.

[SeattleScott]

The other issue is the 0.5x reducers are generic. They aren’t optimized for any scope. So if you try to use the full clear aperture, the edges are gonna get ugly. Or they will vignette. One or the other, or some combination of the two.

What you won’t get is the equivalent of a 20mm Nagler. You might get something with close to the same field size as a 20 Nagler, but with messy edges. Or something a bit smaller field than a 20 Nagler, with more tolerable edges.

If you want to use a reducer on a SCT, you should generally get an SCT reducer designed for the scope. Like 0.63x for the standard Nexstar. Watch your clearance with the base though.

Even then, a reducer isn’t magic. It will give you a wider view with 1.25” eyepieces. About as wide as you can get with 2” eyepieces and no reducer. But if you try to use a reducer with 2” eyepieces, now you are back to vignetting. (And you would seriously have to watch your clearance!) So it is really use a SCT reducer with a 32 Plossl or something, or go 2” format with a 40mm 68 AFOV. That’s the practical maximum. The generic 0.5x reducer is tempting because it is dirt cheap, but remember, you usually get what you pay for. They are really meant for EAA. I have never met anyone who uses them visually. In certain circumstances, they can work, particularly for high power eyepieces in slow scopes. But using them visually is kind of uncharted territory and you just don’t know how well it will work. It’s a bit like buying a raffle ticket. It’s cheap, and who knows, you might get lucky and get some utility out of it.

[Jon Isaacs]

The AgenaAstro 1.25 inch O.5X GSO focal reducer has a clear aperture of 22 mm... 

 

A 20 mm 82° eyepiece has a field stop of 27 mm or more.. you won't get the full field of view.

 

I agree with Scott.. these are meant for cameras with a small chip.

 

An SCT F/6.3 reducer-corrector is what you want. Its designed for a SCT and has enough clear aperture to provide the widest TFoV possible with a 1.25 inch eyepiece.

 

Jon

[denzel200219]

I see. Thank you all for the responses. Ill try and get a dedicated focal reducer.