I am looking to buy the ZWO AISI 1600 cooled camera and jump into CMOS photometry.
Can anyone advise what size of filters is correct for this camera. The detector is 21.9mm diagonal so in
my ignorance I am guessing they have to be greater than this.
The answer obviously determines what filter wheel I get.
The filters seem to be very difficult to get hold of and so far I have only found these 2":
You can only buy the full set. Very expensive but If I have to, I will bite the bullet.
I'd say there really is no "correct" size. I think this would depend on just too many factors to calculate, especially for users of typical SCT's or other reflectors. (For users of exotic, specially designed astrographic reflectors and multiple lens group wide field refractors, this may not apply.)
Although intermediate sizes are available from some retailers, you really don't know if the 31mm or 36mm filters would do just as well for your system as the 2 inch filters. What you do know is that sensors are getting larger and cheaper, and that photometric filters are getting more expensive.
Fifteen years ago the standard size for amateur photometric filters was 1.25 inches. This was large enough for the chips in cameras available to amateurs at the time. Lots of those cameras are still in use and producing good data. If you are a beginner and not sure if you will be doing photometry for the long haul you might consider buying an older used CCD camera and 1.25 filters. If you are planning to be doing photometry for a long time, say another 10 years or more, then you should consider what filters to buy for a new, current model camera and the cameras you may own in the future.
I think of 2 inch filters as the new 1.25 inch filters of yore, the standard filter size for today's sensors that will be useful for a long time.
Thanks for the link to the filter sizing tool! The tool says "Find the optimum filter size to avoid vignetting". The problem is that in many amateur systems vignetting is already baked in with the optical design and the physical structure of the telescope before the light even gets to the filter. If you use this tool to size your filter you may well still have vignetting no matter the size filter you use. However, I think this tool would be very useful to show the minimum filter size you could use to prevent the filter from making the vignetting any worse. Yes, this would be the "correct" size.
That said, I still think the "optimum" filter size, considering the high initial cost and the difference in cost between a set of 36mm filters and a 2 inch filters (relatively small) would be the filter you could use longer with larger chips in the future.