Kevin_A The bigger tha clear aperture the fainter the detail the sensor will see.
Agree, Kevin.
Here is an appendix (mostly completed) that I just pulled from the white paper which I am writing on strain wave guiding. Because they need short exposure times, it is crucial that we derive centroid accuracy from as many stars as possible. As such, I decided to add this Appendix to the main document.
http://www.w7ay.net/site/Technical/Centroids/index.html
In this appendix, you will find what happens when there is worse SNR (e.g., smaller guide scope, poor Quantum Efficiency of sensor, etc)
As it is, when you get a 12 star fix, because the centroids are weighted sums, where SNR is the weight (in today's implementations), most of the time, it is equivalent to only using the centroid 3 or 4 stars of identical SNR. And if you reduce the SNR (as in using a smaller guide scope), it gets even worse.
The stars came from the Hipparcos catalog, by the way, with an FOV that matches what I use today (250mm focal length, ASI178MM).
There is a trick that I mentioned in the Appendix, which I have been using for a couple of years now, and that is to force the centroid star detector to ignore the brighest one or two stars. By doing that, the distribution of the SNR evens out, and the weights are more evenly distributed instead of concentrating on one star (if it is 100% concentrated on 1 star, it would be like guiding with one star). However, to use this trick, you are reliant on dimmer stars and... of course, a smaller guide scope loose out again.
By the way, to study guide scopes, I had at one time even bought the 30 mm ones from ZWO and QHYCCD (slightly better optical quality than ZWO's) to compare with some Borg objectives. The Borg 36 mm objective (ED glass) and the Borg 55 mm objective (Fluorite glass) are my favorites. The Borg 36ED was actually quite usable guiding an RST-135, but there is a big difference between it and the Borg 55FL, However, a Borg 50mm (no ED glass) was actually a disappointment.
Anyhow, the quantitaive analysis bears out your gut feelings.
Anyway, even the Appendix has not yet been proof read, so be forgiving -- I am planning on sending it to Al (@Byrdsfan1948) to proof read only when the entire document is mostly finished.
Chen
