Meade ACF vs Celestron XLT Schmidt Cassegrain

 

 multi coating:

Meade: ACF 

Celestron: Starbright XLT 

 

secondary:

According to Telescopes, Eyepieces, and Astrographs by Smith et al, page 250, aspheric secondary coma-free SCT's like the Meade ACF have prolate ellipsoidal secondaries.  In other words, a stretched sphere.  The primary is still spherical.  The schmidt corrector still addresses spherical aberration.  The secondary addresses coma but not field curvature.  Theoretical on-axis performance is not quite as good as in a standard Schmidt Cassegrain, but off axis spot sizes are, of course, much better due to the suppression of coma.  An 8" f/10 version (per Smith et al) should be diffraction limited over a 20 arc minute diameter vs. 7 arc minutes for a standard SCT.  That means a diffraction-limited field of roughly 12mm for that same 8" f/10 scope.  They don't happen to have theoretical values for an f/8 version.  

For comparison, the aplanatic flat-field design that they believe is essentially that of the Edge HD has slightly worse on-axis performance (though easily diffraction limited), a touch of longitudinal chromatic from the field lens, and is diffraction limited across 47 arc minutes, more than twice the size of the Meade ACF.

Obviously, all of this represents the theoretical maximums for perfectly executed examples of either scopes.  Your mileage may vary.  However, I expect they are at least correct with regard to the shape of the secondary on the ACF.  They say it's a prolate ellipse.

 Dave Bush:

This is not fair to compare EdgeHD with a nomalt SCT. It is included here just for reference

Daniel here's what I'm understanding from this thread...

    The EdgeHD is so well corrected that it's revealing the faults with the eyepieces used.  If you use an eyepiece in an EdgeHD scope and you see less than a nearly perfectly corrected image, it's the fault of the eyepiece.

    Since the EdgeHD produces or presents such a flat, well corrected image, you need to use eyepieces that are essentially free themselves of any aberrations. 

    Lessor eyepieces, those that are not free from aberrations, will very likely perform quite well (better in fact) with non-EdgeHD SCTs

And I believe the conclusion that your evaluation may come to is that for those folks who already have and use eyepieces that are not highly corrected, an EdgeHD would not be a good choice. The standard SCT would be best.  Unless of course they were willing to replace their eyepieces.  

 

Starman1:

So, I tried the standard f/6.3 focal reducer/field flattener on my SCT.  I noted that the star images at the edge, AT THE SAME POWER, shrank because they were in better focus.

I did not see a reduction in coma.  In fact, coma became more noticeable because the FC was seriously reduced.  I saw a slight increase in lateral CA, but it was still minor.

OTA weight :

Meade 8" ACF 7 KG vs Celestron C8 XLT 5.7 KG

mirror lock:

Meade LX200 series ( 8" to 14") has mirror locks

From Uncle Rod:
Which should you choose? The ACF or the Edge? I own an Edge and am quite content with it. However, I find the field edge of the ACF to, frankly, look every bit as good as that of the Edge to my aged eyes. The ACFs I’ve used have been impressive, and if I were to buy a new SCT, which doesn’t seem that likely at this juncture, it might well be a 10-inch f/8 ACF, ie better cost for performance

However Meade is 10% more expensive (1277 Eur) than Celestron with a similar spec (1155 Eur).

cloudynights 

Uncle Rod

shootout did not finish however. For on axis or central field, the differences are minor. There may be slight difference on the edges. And discussion start to drift to comparing eyepiece performance rather that the scope itself.

cloudynights on Meade F/8 SCT RCX400