Lerager But I have never try 0.5s exp. Normaly 2 sec. sometimes 3 sec.
In your case, you should not have a problem with longer exposures, since you don't expect the guide rate to exceed 1 FPS anyway. The problem of the 678 on ASIAIR (not a problem with INDIGO) is not being able to achieve 2 FPS when it is set to 0.5 sec exposures.
The first derivative of the error curve of my mount is large enough that I need 0.5 second exposures to freeze the guide stars so they don't trail within that 0.5 second, and at the same time, I also need to immediately issue (thus need for 2 FPS) immediate correction pulses, otherwise the error would build up to more than 0.5" after a second of time.
The 0.5 second/2FPS guiding allows me to make rapid small corrections, instead of larger corrections.
Since you are using Bin2, the 2 µm pixels should help, since after binning, you will get a more reasonable 4 µm superpixel, instead of the 7.5 µm (!!) superpixels for your current camera in Bin2.
Bin1 will not work well obviously, with a color camera.
If a star is centered inside a pixel (or superpixel, in the case of Bin2) then the centroid detection is not sensitive to small star movements if you are highly undersampled. I.e., the centroid of the star would move from inside a pixel to also inside the pixel. The only way you can detect any movement is from the tails of the point spread function, and those may be very contaminated by noise.
1s. exp. gives me to many false correction.
There are two possible (at least :-) reasons for that.
One is that you are using a single star to guide and another is simply the camera has insufficient sensitivity to pick more than one star to guide.
If all else were equal, using two stars is equivalent to using twice the exposure time with one star (if those two stars have the same signal to noise ratio). So, with multi-centroid guiding, it is quite easy to get good measurement accuracy with shorter exposures. But if your mount does not need fast, short guide frames, then you don't need to use shorter exposure times and faster frame rates.
Interestingly, the FOV of the 678 should be huge compared to your current camera. I am already using an ASI178MM, which has the same pixel count as the ASI678, but larger 2.4 µm pixels, so the FOV area is 1.44 larger, and for multi-centroid guiding, has about 50% chance of finding more stars.
By the way, if you plan on doing near-IR guiding, you will need to replace the camera's protect window (which says IR cut in the spec sheets) with an AR (anti-relection) window. (The way to open the camera chamber is now different -- i the past the front flange is threaded; with the new pancake cameras, the front flange is held by 4 screws from the back.)
In any case, unless your guide scope can handle 450nm all the way to 900nm (large range than apochromatic scopes), you will probably need to choose either visible light guiding (using an IR cut window) or near-IR (NIR) guiding by using a high pass (often called an IR-pass) filter. I.e., limit the wavelength span the camera sees.
Anyhow, for me, I have gone back to my ASI178MM for guiding (good session again last night, especially after just a day earlier installing an electronic focuser to my guide scope to make it easier to get good focus with a helical focuser). In my case, I decided to forgo 2 µm pixel and NIR guiding just so I can get 2 FPS rate that I need more.
Chen
