[xiongz (Zonghou Xiong)]

Despite recent cloudy weather here in Sydney in the past 4 months or so since last October I managed to take a NB images of Crab Nebula M1. I thought it might worthwhile to share some tricks and thoughts here.

1. Scope. I saw a Skywatcher Quattro 12 from Daniel (Bluknghtv) and thought I might be able to convert it into a truss tube in order to save a bit weight and to reduce wind blows. I tried to find carbon tubes but those were hard to put together without special joints. I just had some scrap aluminium tubes left after installing a swimming pool. It took me a weekend to put all together and fine tune the scope. In doing so I saved some 5 kg, cutting weight from 23 to 18 kg which made it possible for my CEM60. Temperature effects on focusing seemed to be quite similar to Quattro 10 (carbon version) which indicated focusing was mainly affected by thermal expansions of mirrors rather than rube structures.

2. Mount and guiding. I shipped a CEM60 from China myself around 2016 when iOptron wasn’t available in Australia. Guiding errors were usually around 0.5 RMS at home in Sydney with my 10 inch Quattro, but much higher at dark sites (usually around 0.7). Sometimes guiding can be as good as 0.3 RMS at home, and very rarely at dark sites. With help of iOptron supports I was able to fine tune the mount from time to time, including changing a RA worm. Worms must be tightened as much as possible, contrary to what was suggested on their manuals. Miracles happened with the availability of multi star guiding since early 2023, which allowed me routinely achieving guiding errors of 0.25-0.5 RMS, with 10 inch Quattro or GSO RC10, as well as with the transformed Quattro 12. I had been thinking of upgrading to a sturdier mount for a while but now it seemed that CEM60 and modified Quattro 12 would work out very well.

3. Cameras. I probably got one of the first few of ASI1600mm in Australia which worked very well for Quattro 10 at 1000mm, with an image scale of about 0.8 arc-sec/pixel, which seemed to be optimal for average seeing. With an ASI294mm for scopes around 1200 mm (300/f4) and 1300 mm (RC10 with reducer), image scales would be roughly the same. I have a couple of observations regarding ASI294mm. As I have a Canon 400/2.8 I thought it might be useful to un-bin the camera for higher resolutions. Higher res can indeed be achieved but at the costs of much longer imaging times, and yet more details were visible only at highlights. When testing on M8 I found that bin1 mode of ASI294mm showed quite a bit details in areas surrounding M8 but default bin2 mode would cut out these details. I guess camera firmware might have treated these fainted data as noises and clipped them out. I tried stretching original single shots in Photoshop before stacking and all details were retained. In doing so strong amp glow of ASI294 were also eliminated. I found this was very useful to fully recover weak data and totally remove amp glows. To stretch single shots a curve like exponential growth curve would need be used. This was much better than raising gain of the camera as gain increase would be linear. Do the same curve on darks as well. I strongly suggest all ASI294mm users to pre-process data this way.

4. Quattro mirrors. Shywatcher F4 Quattros do produce quite good images as proven by many, particularly compared to their low costs, but I found these mirrors were a bit too thin perhaps. Extra clippers would be needed to mount the mirror firmly so that collimation would stay unchanged for long times. It took me a couple of years before I fully collimated my Quattro 10, mostly due to inexperience. Donut shapes of defused stars would become distorted if mirror mounting were over-tightened. The carbon version of earlier Quattro 10 came with 6 mounting points, but newer Quattro 12 used only 3. I also have a Skywatcher 300/F5 that used 3 mounting points and that mirror didn’t seem to be affected much by mounting tightness. I made extra long clippers for it anyway. Once longer mounting clippers were added these scopes stayed collimated pretty much forever, even with numerous trips to dark sites.

I had hope to test the modified Quattro 12 for a galaxy but Sydney weather hadn’t been helpful in the past few months and it doesn’t seem to be any better in the coming months perhaps. Here is a M1 with 3 nm Antlia 36mm filter, SOH, with 10, 10, and 9 10min exposures. ASI294mm, gain at 200, cooled to -20 degrees. Stars weren’t as tight as it was a bit windy with guiding errors about 0.5-0.6 RMS. Scope needed be fully covered under city skie.

Clear skies
John