1 reply to this topic

[Vincent456888]

Hi everyone,
finally got me a C8 after years of using Newtonians. Im liking it, but i want bigger! That being said, im a bit concerned with the ridiculous focal lenghts on C11s and C14s, and especially the size of their image circle. Now I am well aware of the existance of those .7 reducers, but looking at pictures, it looks like they may not take full advantage of the light cone and worse yet, the output dosent even seem to cover a full stop 2" eyepiece! Can someone chime in on this? Is my assessment wrong?

[Eddgie]

Hi everyone,
finally got me a C8 after years of using Newtonians. Im liking it, but i want bigger! That being said, im a bit concerned with the ridiculous focal lenghts on C11s and C14s, and especially the size of their image circle. Now I am well aware of the existance of those .7 reducers, but looking at pictures, it looks like they may not take full advantage of the light cone and worse yet, the output dosent even seem to cover a full stop 2" eyepiece! Can someone chime in on this? Is my assessment wrong?

The C8 does have a rather small fully illuminated field (about 9mm or so), but most reflectors intended for visual use don't have a fully illuminated field more than 10mm, so I am not sure what your concern would be. 

 

The fully illuminated circle size is not critical for visual use and most observers cannot see the 20% illumination falloff of a C8, even when using a 2" eyepiece. 

 

The focal reducers do make the fully illuminated field smaller, but they do it for refractors and reflectors too. It is the nature of a reducer. Reducers reduce things. In simple terms, if you have a 10mm fully illuminated field, and you reduce that field by .63x, then the field will become a 6.3mm fully illuminated field. Now when using this combination with 2" eyepieces, you may indeed see some vignetting in the SCT, or simple field illumination falloff in the reflector, but refractors typically have much larger fully illuminated fields, and this is why they are popular for imaging. Still, if you use a reducer on a refractor, it makes the fully illuminated field smaller.  If the fully illuminated field is 10mm, and you use an 8x reducer, then the fully illuminated field becomes no larger than 8mm. If you used a .63 reducer, it would become no larger than 6.3mm. That is what reducers do. They make everything smaller. 

 

Anyway, unless one is using an imaging Newtonian, the Newtonian will usually have more field illumination falloff with an f/5 reflector than with an f/10 SCT. If you want to image using a full size sensor, the C8 is not the right scope. 

 

As for the focal length, it is just inherent in the design. A lot of people don't like the physical length of reflectors.  It is all compromise. You can't make the reflector really short, and you can't make the SCT really fast (well, technically not true, but the instrument would have to have a particularly large central obstruction and use some rater expensive to make correctors). 

Edited by Eddgie, 26 April 2025 - 07:16 AM.