Telescope Reviews: Why a refractor over a larger SCT at less cost?

In fact, I agree that an 8" APO has contrast transfer that is about the same as a C14 at the mid and low frequencies, but at the high frequencies (smallest detai) the C14 will prevail.

This is what MTF is all about. It tells you how much contrast will be preserved from the target.

But this fits exactly into what the OP asked, thought in this case, the answer is glaringly apparent.

Why a refractor over a larger SCT at less cost?

Well, when an 8" APO costs far far far more than most of us could ever consider spending on a telescope.

In that case, the question could be reversed to ask "why spend $20,000 on a large refractor that needs a 250 pound mount or permanant pier when you can get about the same planetary performacne from a $4500 SCT???

Included is an MTF plot. The red line shows the contrast transfer of an unobstructed 14" aperture.

The red dashed line shows an MTF plot for a C14 with 32% obstruction and I even threw in 1/8th wave of spherical abberation (about the amount mine has).

The green line represents a perfect 200mm refractor.

Notice that for very large detail, the C14 looses only a couple of percent of contrast as compared to the 8" scope.

For example, at the about .15 fs/max, the 8" APO has lost 30% of the contrast from the target. The C14 is loosing about 33% at the target. A difference of 3% is detectable by the scotopic eye (and the camera), but not by the mesoptic or scotopic eye. For these modes, the contrast has to differ by about 5% to 15% depending on the observer. For visual use, these two instruments would appear quite similar in the low frequency part of the size spectrum.

When the detail size gets past about .3 of the fs/max, the C14 pulls smoothy ahead.

So, while it may not have exactly as much contrast tranfer as the 8" APO does for very large details, it will be superrior for smaller details, and indeed will show the finest details with more contrast than even the perfect 8" APO.

The eye often cannot reallly see these fine details though, but the camera can, and that is why you see such amazing planetary images taken with C14s.

But the bottom line here is that most of us simply can't afford large refractors, and once again, back to the OPs post, for many, the answew is that depending on what you want to look at, a big SCT may indeed be not only a cheaper choice, but a better choice.

I have a super high quality 6" APO, but it is no match for a C14 on planets.

An 8" APO might be, but I lack the $30,000 to buy the scope and build the observatory that it would require.

Anyway, here is the MTF plot. I believe you friend. An 8" APO will give about the same performacne as a good C14 on planets.

Rather than ask which one you would prefer, let me ask you which one you could afford.

Edited by Eddgie (05/18/12 03:46 PM)