Kevin,
On paper, the Borg 55FL (mine's focal length measured to be 204mm) with a 2.4 µm sensor has a plate scale of 2.43" per pixel and a Dawes limit of 2.11". I.e., at best, star size of 0.87 pixel.
The Askar 180p (mine measure to be 178mm focal length) with the same sized sensor has a plate scale of 2.78" per pixel, with a Dawes limit of 2.9". So, at best a star size of about 1.04 pixel.
In real life, however, the star size is much larger, and is limited by, among other things, the spot diagram. Instead of depending on advertized values (caveat emptor), I just decided to measure it.
I mounted my "reference" Borg 55FL guide scope, with a ASI183MM camera side by side with the Askar FMA180p, with my "standard" ASI178MM guide camera. I do not have a second ASI178MM, but these two cameras have the same pixel size. I can't directly compare sensitivity, but it is good enough to compare centroid accuracy.
Waited for the night to leave astronomical twilight, focused both scopes really well, and measured some actual star sizes. Some clouds are coming in, so the night is wasted anyway. My new all-sky camera (8mm Samyang Sony Fisheye with a ASI294MC and INDIGO Sky processor, and my own capture program, natch) shows this:
South is on the right, and yes, my southern skies are that bright. Towards the zenith, I had measured the SQM to be 19.3. [One should always own an SQM meter :-)] The bright star near the bottom center is Vega and crosses the meridian near zenith at my latitude (NGC7000 is wonderful from Portland :-). Polaris is just behind that clump of trees at the bottom left (you can see Merak and Dubhe pointing to its position).
OK, the actual reference 55FL star HFD is about 2.11 pixels on the plate, and corresponds to 5.11" in the sky. The bloat is a combination of the spot diagram and the "seeing" (0.5 second exposure, measured over dozens of stars),
and how anally one focuses their guide scope.
The actual Askar 180p star HFD is about 2.13 pixels (notice how close it is to the size on the 55FL!) but because of the plate scale comes out to be a larger 5.93" in the sky.
Now, assuming that we can estimate the centroid to 1/20 of a pixel on a guide plate, this would correspond to the ability of computing centroids with the 55FL of ( 5.11/2.11 )/20, in the sky, or 0.12 arc seconds.
Correspondingly, the 180p's centroid could probably be estimate to an accuracy of about (5.92/2.13)/20 = 0.14".
As you can see, the numbers are close, with the 55FL winning by just a little buit (the 55FL is a bigger light bucket, of couse, but the FOV of the 180p is larger(FOV area of 1.78x, so can find many more guide stars, if they are not in the noise), The sensitivity vs FOV probably comes out to be a wash, and that is probably why the 180p has so far been working out for me.
Now given the 0.14" type centroid accuracies, I don't expect it is possible to guide my mount to better than about 0.1" RMS per axis. But that is fine for my OTAs.
I didn't use any special instrument to measure the above, so anyone can do that for the guide train that they are using.
BTW, I finally ditched all of the EAF plates for the EAF mounting on my 55FL. I now mount the EAF directly on the 55FL's tube (again, using the brass standoffs that I used with the 180p EAF mouting). I had taped off the two holes that I had drilled earlier to mount using EAF plates :-)
(That "M57 rotating ring" is the camera angle adjuster. I always try to make my camera angles alignn with the equatorial coordinates. The Askar 180p has a fantastic camera ange adjuster, by the way. You'll see.)
Chen
