xiongz
02-03-2025, 12:30 PM
I tried to image N70, a dim super bubble in LMC near NGC2114, while fine tuning my rebulit Skywatcher Quattro 300F4, ending up with some 42 hours of data. About half had to be trashed due to spiky stars. I thought my experiences might shed some lights on various aspects of this scope.
I worked on 3 Skywatcher Newtonian astrographs, Quattro 10, 300F4 and 300F5. My Quattro 10 was bought from Bintel with a carbon fibre tube. 300F4 was from a member of this forum, and 300F5 was also from a member of this forum some 10 years ago and it was brand new. The two 12 inch scopes were rebuilt into truss structures using aluminium bars from pool fence leftovers. I had to rebuild the two 12 inch scopes for two reasons. They were too heavy (23+ kg) for me to move around with a CEM60 mount. By rebuilding them into truss structure I not only saved some 4-5 kg of weight, I also made them more suitable for dark sites when winds were always problems for guiding. While 300F5 was spot on with everything the two Quattro scopes, 250F4 and 300F4 gave me tremendous headaches.
I think real issues with Skywatcher F4 scopes might be mirror thickness and/or stiffness of mirror cell mounts. Mirror deformations and holding collimations were conflicting problems. I had to add a metal plate underneath the 250F4 primary mirror to stop all 6 screws changing collimations. There were 3 sets of screws for collimation, with an adjustment scew and a locking screw each. On all 3 scopes these 6 screws all affected collimation, and on Quattro 10 it was the worst. After adding a stiff metal plate (ring) collimation was manageable and stayed for years unchanged, despite numerous trips to dark sites. After I rebuilding the 300F4 is seemed to worked ok when I was using ASI294MM. Despite I meticulously centred everything on Quattro 10 before I didn't do so for the two 12 inches when I first rebuilt them. I thought as long as optical paths were aligned properly minor positioning errors might be ok as there would always be error in collimation. These worked fine with ASI294mm, but stars started to show spikes on ASI2600mm with smaller pixels, which became more serious after I replaced my RA gear, worm, and bearings of the mount with much improved guiding accuracies. I had to re-centre the secondaries and collimate the scopes to the best I could. After fine tuning the 300F4 worked fine, and it was also less sensitive to temperature changes, but there were still lots of spiky stars, and sometimes collimation were lost during imaging sessions. I trimmed top rim of a unused 12 inch fry pan and put it under the clippers of the mirror mount. These worked very well for a few nights but collimation was changed again a couple of hours into imaging one day. I found the mirror mount screws were a bit loose and tightened them further last night. However, stars were spiky and out-focus donuts weren't round and all data from last night were useless. There could be other factors which I had yet to identify but even with a stiff ring spreading clumping forces on the F4 primary mirror there still need balances between mirror deformations and collimation.
Anyway here are pictures of the past month for N70, 05h 43m 17.5s, −67° 50′ 48″, with some 26 hours of 10 min subs. 3nm Antlia filters, 128, 18 and 12 subs for Ha, Oii, and Sii, in SOH colours. I took a total of 226 subs for Ha and actually used 194 subs for B chennels of RGB (SOH). I aslo added some 8 hours of colour data from 250F4/ASI1600mm taken years ago to enhance the colours a bit. 128 subs of Ha with tighter stars were used for luminance which really matters for resolutions. Total imaging time including 250F4 data would be around 50 hours, with some discarded.
Clear skies,
John
I worked on 3 Skywatcher Newtonian astrographs, Quattro 10, 300F4 and 300F5. My Quattro 10 was bought from Bintel with a carbon fibre tube. 300F4 was from a member of this forum, and 300F5 was also from a member of this forum some 10 years ago and it was brand new. The two 12 inch scopes were rebuilt into truss structures using aluminium bars from pool fence leftovers. I had to rebuild the two 12 inch scopes for two reasons. They were too heavy (23+ kg) for me to move around with a CEM60 mount. By rebuilding them into truss structure I not only saved some 4-5 kg of weight, I also made them more suitable for dark sites when winds were always problems for guiding. While 300F5 was spot on with everything the two Quattro scopes, 250F4 and 300F4 gave me tremendous headaches.
I think real issues with Skywatcher F4 scopes might be mirror thickness and/or stiffness of mirror cell mounts. Mirror deformations and holding collimations were conflicting problems. I had to add a metal plate underneath the 250F4 primary mirror to stop all 6 screws changing collimations. There were 3 sets of screws for collimation, with an adjustment scew and a locking screw each. On all 3 scopes these 6 screws all affected collimation, and on Quattro 10 it was the worst. After adding a stiff metal plate (ring) collimation was manageable and stayed for years unchanged, despite numerous trips to dark sites. After I rebuilding the 300F4 is seemed to worked ok when I was using ASI294MM. Despite I meticulously centred everything on Quattro 10 before I didn't do so for the two 12 inches when I first rebuilt them. I thought as long as optical paths were aligned properly minor positioning errors might be ok as there would always be error in collimation. These worked fine with ASI294mm, but stars started to show spikes on ASI2600mm with smaller pixels, which became more serious after I replaced my RA gear, worm, and bearings of the mount with much improved guiding accuracies. I had to re-centre the secondaries and collimate the scopes to the best I could. After fine tuning the 300F4 worked fine, and it was also less sensitive to temperature changes, but there were still lots of spiky stars, and sometimes collimation were lost during imaging sessions. I trimmed top rim of a unused 12 inch fry pan and put it under the clippers of the mirror mount. These worked very well for a few nights but collimation was changed again a couple of hours into imaging one day. I found the mirror mount screws were a bit loose and tightened them further last night. However, stars were spiky and out-focus donuts weren't round and all data from last night were useless. There could be other factors which I had yet to identify but even with a stiff ring spreading clumping forces on the F4 primary mirror there still need balances between mirror deformations and collimation.
Anyway here are pictures of the past month for N70, 05h 43m 17.5s, −67° 50′ 48″, with some 26 hours of 10 min subs. 3nm Antlia filters, 128, 18 and 12 subs for Ha, Oii, and Sii, in SOH colours. I took a total of 226 subs for Ha and actually used 194 subs for B chennels of RGB (SOH). I aslo added some 8 hours of colour data from 250F4/ASI1600mm taken years ago to enhance the colours a bit. 128 subs of Ha with tighter stars were used for luminance which really matters for resolutions. Total imaging time including 250F4 data would be around 50 hours, with some discarded.
Clear skies,
John