I've heard that the Nexstar 8 SE behaves like a 5.5" APO refractor. Its secondary mirror is 2.5" wide, so 8"-2.5"=5.5"
There are quite a few misconceptions in this thread. I'll try and clear some up.
If one is concerned about light gathering, subtract the area of the secondary, not the diameter. sqrt(8^2-2.5^2) = 7.6 inches. So the 8-inch is equivalent to an unobstructed 7.6 inch telescope.
Trollmannx wrote:
Small good refractors can deliver very contrasty images. Even if resolution is missing what is seen is seen better than in a rather low contrast reflecting telescope. But if looking carefully the less contrasty image from the C8 should usually show more detail than the higher contrast but lower resolution refractor - that is: if the atmosphere is rather calm and without too lively turbulence...
First about contrast larger than the diffraction disk. It really depends on how well each instrument is made and the coatings, not what kind of instrument. For example, refractors have 6 to 6 surfaces, SCT 4. Lens surfaces much be well polished and have excellent anti reflective coatings. Mirror surfaces must be polished very well. Small pits left from inadequate polishing result in flare and loss of contrast. Refractors, with multiple strongly curved surfaces are more prone to reflections between surfaces reducing contrast, especially if the best anti reflection coatings are not present.
Where contrast typically differs between reflectors and refractors is near the diffraction limit. The secondary obstruction in SCTs and Newtonian telescopes throws light from the central diffraction disk to the outer diffraction rings. Thus, for planetary detail, a slightly smaller refractor will resolve finer planetary detail than a reflector with a central obstruction. This is where the idea that about a 6-inch refractor shows similar planetary detail to an 8-inch reflector. But in this day with stacking, the problem is more about light gathering to build S/N and stacked videos to produce the best planetary image is probably done better by the larger aperture, not whether refractor or reflector. And this effect is totally irrelevant to astrophotography where seeing limits resolution and fine contrast, not diffraction. Thus, this refractor/reflector effect really only applies to visual observing of planets and the moon and sun.
Michael S. wrote:
If you want to photograph - you are in need of a low aperture - something around f5, lower aperture, less exposure time you do need to collect photons.
The above confuses camera exposure with subject exposure. For example, an 8-inch f/10 SCT (7.6 inch unobstructed aperture, 193 mm) collects (193/81)^2 = 5.6 times more light from the subject. For example, if the 81 mm were f/5 (400 mm focal length), compared to the 8-inch f/10 (2000 mm focal length), make 2 images with each telescope with the same total exposure time. The ratio of the focal lengths is 5, so bin the image from the SCT 5x5 pixels and you would have the same resolution on the subject with 5.6 times more light per pixel. And if binning is a goal, use a focal reducer to get a head start.
Aperture area is key in astrophotography, both in resolution and light collection. The only question is will the intended subject fit in the field of view with a given focal length? Contrast depends more on build quality than a particular design.
Am 8-inch f/10 SCT versus an 81 mm lens (f/5?) is really for different targets. Although if you are spending that much money on an 8-inch telescope, also consider a larger aperture lens.
Roger
