After moving on from my previous ATIK 383L+ to my new QSI 660i, I thought I would compliment the move with new filters. Everyone who has looked into them will know Astrodon filters tend to be very high-end, but also very highly-priced (depending on size). Thankfully, the move of CCD camera took me from a 17.6 x 13.52 mm Kodak KAF-8300 CCD sensor to a 12.48 x 9.98 mm Sony ICX694 CCD sensor. The diagonals here are very different, from 22.19 mm down to 15.98 mm. Additionally, my ATIK 383L+ had a backfocus distance of 17.5 mm whereas the QSI 660i has a backfocus distance of 14.6 mm. This means that not only is the CCD sensor on the QSI 660i significantly smaller, but the distance from it to the filters in the filter wheel is also smaller (a grand total of 27.1 mm in this case). What this does is imply that I can afford to use smaller diameter filters without introducing vignetting. However, I was not 100% sure until I tested them out.
The only thing I was worried about is that I specifically bought the QSI 660i (slim model) CCD camera because I had recently bought the Starlight Xpress USB Filter Wheel with the Slimline OAG attachment. Not wanting to lose out on using it, I did not bother with the QSI 660wsg-8, which comes fully-equipped with the internal filter wheel and OAG. A great thing about the internal field wheel is that the filters are about 20 mm from the sensor, just over 7 mm closer than mine are. This had me a little worried as I waited to receive my order of the 1.25" filter set of the above. Thankfully after receiving them today, testing with flats showed there was no extra vignetting by using 1.25" filters over 2" filters in my optical system.
The above two screenshots show Luminance flats with both filters. The left is with the Astrodon 1.25" Luminance and the right is with the Baader 2" Luminance. The top screenshot shows the raw flats non-stretched and the bottom screenshots shows the raw flats stretched with PixInsight's STF AutoStretch function (to demonstrate the vignetting). Despite the dust on both flats (present on the CCD camera's optical window, now cleaned off), we observe very, very similar vignetting patterns on both. Still, nothing is proved without a bit of mathematics. I created preview boxes in the centres and corners of both flats and used PixInsight's Statistics tool to measure the Mean of the preview boxes. By comparing the Mean of the centre with the Mean of the corners, I noticed the Baader 2" Luminance filter flat had 93% brightness on the corners compared to the centre. Much to my excitement, the Astrodon 1.25" Luminance filter flat had the exact same 93% brightness on the corners compared to the centre. The same measurements were performed to compare Red, Green and Blue filters and achieved the same result of approximately 93% brightness.
This means that using the 1.25" filter over a 2" has introduced no further vignetting into the optical system. What is there at present is therefore due to every other optical element, as is naturally present. This has given me extra confidence in not needing an upgrade to the QSI 660wsg-8 and allowing me to keep the QSI 660i. I now await the arrival of Astrodon Tru-Balance Hydrogen-Alpha and Oxygen-III 3nm 1.25" narrowband filters. These will mostly fill my filter wheel's 7-filter carousel, leaving one space blank in case in the future I decide to get another narrowband filter (though I feel this is unlikely as I have not had good experiences imaging in Sulphur-II and do prefer the Bicolour Palette when it comes to pure narrowband imaging).
This means that using the 1.25" filter over a 2" has introduced no further vignetting into the optical system. What is there at present is therefore due to every other optical element, as is naturally present. This has given me extra confidence in not needing an upgrade to the QSI 660wsg-8 and allowing me to keep the QSI 660i. I now await the arrival of Astrodon Tru-Balance Hydrogen-Alpha and Oxygen-III 3nm 1.25" narrowband filters. These will mostly fill my filter wheel's 7-filter carousel, leaving one space blank in case in the future I decide to get another narrowband filter (though I feel this is unlikely as I have not had good experiences imaging in Sulphur-II and do prefer the Bicolour Palette when it comes to pure narrowband imaging).