tbhausen Is exact tilt really that critical for a guide camera? (I do see it's separately focusable from the main sensor)...
No, not tilt, but the distance of the two sensors from the OTA's metal back (assuming the OTA has no field curvature to start with).
Take a tiny M2 screw, it has a pitch of 0.4mm (i.e.,400 microns per turn of the screw). A 4 µm adjustment would require just 1/100 of a turn to get it just right. Now try to do this in mass production on an optical bench, with assemblers who are not skilled in optics.
The big headache is going to come from people using cheap optics whose field curvature is not properly corrected to the image circle that the second sensor requires. How are they going to open up the camera chamber and refocus the sensors to 1/100 of a turn of an M2 screw to match the field curvature of their OTA?
Depth of focus (the sensor end of the "depth of field") is much more critical too with a wide camera lens. A f/2.8 lens will require the focus to be twice as precise as an f/5.6 optics.
Unless this is addressed, I think you might see a lot of disgruntled customers.
Like @hyiger, albeit with the meager data that I have on hand, I do expect the narrowband filter to be a worse problem.
So far, my experiments in the past months have shown that you need a camera gain that is 20 dB (and extra gain of 200 in ZWO's gain units) beyond after getting 12 stars in multistar guiding in ASIAIR (this is caused by ZWO using signal-to-noise ratio as the centroid weighing function; a mistake to start with).
I need a couple more clear nights to be able to gather data that I can rely on (almost got it last night just before clouds came in).
A 3nm filter by itself cripples the guide camera by some 20 dB to 23 dB to start with (assuming unfiltered starlight goes from 450nm to 800nm, and accounting for filter losses).
This will not affect people who don't use strain wave gear mounts that needs short exposures to make multi-star guiding mandatory, or those who don't care if their strain wave geared mounts can only guide to 0.7". So far, my measurements show that if you only have barely 11 to 12 stars for multi-star guiding, just the centroid measurement error alone (not including any mount error) will contribute a total RMS error of some 0.5", thus making it impossible to consistently guide to better than that.
So, if you are using a narrowband filter and using short guide exposures on ASIAIR, and can barely get 12 stars in multi-star guiding, you will be stuck with not being able to guide at better than 0.5" total RMS, and probbaly in the region of 0.7" to 1" RMS.
I only have raw data to look at casually right now. Once I gather enough data, I will tune my software to filter out the pertinent data and get more reliable numbers.
With proper engineering procedure, ZWO should have done these measurements before manufacturing the camera. Perhaps they can shed some light [sic] on it.
Chen
