M1 and equipment thoughts

[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

[Dave882 (David)]

Nice image John and well done on the modifications. I love your ingenuity!!
I recently did a version of m1 myself and find it to be a really intriguing object, although very low in the sky for us. I imagine you’d need a full shroud to eliminate dew and stray light in the suburbs?

I’m not sure about the pre-stretching methodology, perhaps there are some other settings you could employ to avoid that extra step?

[xiongz (Zonghou Xiong)]

Hi David,

Thanks very much. I took the scope to a dark site only once in the past few months. It needed partial shroud at least as shown in the picture. At home with street and house lights it must have full shroud. There were heaters installed for both primary and secondary mirrors by original owner Daniel, which made this scope especially worthwhile to be modified. Little did I know that I could do it with scrap pool fence bars. As for M1 it was low but real problem was my 2-storey house allowed only a couple of hours of imaging every night, and yet clear nights were so few in the past 4 months here in Sydney. For pre-stretching of ASI294mm shots, I found it was best and perhaps necessary for extreme processing. I noticed this problem when I first tested the camera with my Canon 400mm/f2.8 II. Back in October weather was good. I tried bin1 first as the Canon lens was capable of resolving fine pixels and I got lots of details around M8, as seen from nebulae images by many latest cameras. However I was totally surprised that all details were clipped out after DSS stacking for default bin2 shots. Here are some samples. Bin1 image had to be downsized due to data restrictions. Maybe other stacking software would behave differently.

John

Originally Posted by Dave882

Nice image John and well done on the modifications. I love your ingenuity!!
I recently did a version of m1 myself and find it to be a really intriguing object, although very low in the sky for us. I imagine you’d need a full shroud to eliminate dew and stray light in the suburbs?

I’m not sure about the pre-stretching methodology, perhaps there are some other settings you could employ to avoid that extra step?

[gb44 (Glenn)]

Great work John.

With regard to carbon fibre joints I have a suggestion. I used this method for aluminium tube. Get dowel to fit inside your choice of tube. Slit the dowel to accept a piece of flat bar to protrude from the end (aluminium or steel). I presume you aligned the rings of your truss tube with a laser - once the tube mounting holes are in the rings the truss tubes can be made and glued up in exactly the right place. Glue the bar into the dowel and into the tube with epoxy glue.

Cheers
GlennB

[Ryderscope (Rodney)]

Great project John and very innovative

[xiongz (Zonghou Xiong)]

Thanks Rodney

Originally Posted by Ryderscope

Great project John and very innovative

[xiongz (Zonghou Xiong)]

Hi Glenn,

Thansk very much. I actually used very primitive methods putting the tube togeter. Having learnt how to collimate these mass production newts like Quattro 10, I knew the only thing need be pricise would be optical axis of mirrors which must be aligned. I used a ruler to find out rough distances of seocndary mirror and re-used the mounting rings and plates of the scope. These were pretty stable. I could add another dove plate for more precisions but to save weight I just used fence bars. Once these three piece were put together I could easily put more bars around. I only used a hammer and a drill, and no joints were needed at all. These can only be done with metal bars but not for cabon tubes. One problem was to work out angles to fit flattened bar ends. After a few trials it would be easy. Once the bottom part for primary mirror were done, I used flat tops of pool fence for focusing unit for better stability. Some trial and errors would be needed to adjust levelness of finished top ring. It's important to measure all distances between focus plates etc. For better mounting of primary mirror I used some metal sheets around the base (I could have cut off some bottom parts of the original tube) which also added a bit weight. To be honest, in the end I made the scope so much more accurate in every aspect than original factory setup. All mass production scopes would need precise tuning. I find literally nothing were precisely put together with Skywatcher tubes. Centre dots on primary mirrors were definitely off on all three scope I had. Secondary mirror positions were never accurate. However, optical qualities of mirrors seemed to be ok, which means there scopes can be turned into very cost effective equipment.

John

Originally Posted by gb44

Great work John.

With regard to carbon fibre joints I have a suggestion. I used this method for aluminium tube. Get dowel to fit inside your choice of tube. Slit the dowel to accept a piece of flat bar to protrude from the end (aluminium or steel). I presume you aligned the rings of your truss tube with a laser - once the tube mounting holes are in the rings the truss tubes can be made and glued up in exactly the right place. Glue the bar into the dowel and into the tube with epoxy glue.

Cheers
GlennB