M27 - the dumbbell nebula

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Re: M27 - the dumbbell nebula

Postby The Man with the Corrugated Iron Roof » Thu Aug 17, 2017 8:22 pm

I think it's a great image but I would try to make it even better. One thing I've learnt is to take loads of dark frames (at least 8 but 12 is even better). Also I've been taking lots of "light" frames. For my full disc lunar frames I often take 150 or more.
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The Man with the Corrugated Iron Roof
 
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Re: M27 - the dumbbell nebula

Postby Aratus » Fri Aug 18, 2017 11:27 am

Thank you. There are 2 point of improvement that I've identified. There first is working on the focus. The battery on my focuser chose that moment to run out, which meant I couldn't get a fine focus at the computer screen. It isn't bad, but I know it could be better. The obvious improvement is that this is only a single frame. Once I get the mount alignment sorted out it will be a lot easier to get dozens of good frames (without trails) which can be stacked. That will smooth out the grain, and add more detail, especially to the red areas. I think I used 5 averaged dark frames on that shot which was the default that came up on the software. I too use about 100 frames for full lunar disk, since it is such a large target, and it is not affected as much by seeing conditions. I take 3000 or 4000 frames for lunar detail or planets since at higher magnifications the seeing spoils a lot of frames. I do know others who take 8000 or 10000 frames! I expect they have a more powerful computer than me! :D

As soon as I get clear skies, I'll have another go.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, with an 80mm refractor as a guidescope. They are housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC for imaging.
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Re: M27 - the dumbbell nebula

Postby Aratus » Sat Aug 19, 2017 2:36 pm

Much to my surprise the cloud cleared late of Friday evening, and stayed clear for about 2 hours. Although it took time I adjusted the mount in azimuth (side to side) so that the telescope no longer drifted downwards. At least not noticeably in 45 seconds. Having sorted that out I took 30 images, out of which 12 showed no significant trailing. I stacked these to give a noticebly better result than the single shot I took a few days ago.

Image

The reason why 18 of the 30 images still showed trailing is because of 'Periodic Error' which refers to imperfections in the gears. During the 8 minutes it takes for the gears to go around once, the telescope may at certain points not track accurately. That's when you get the trailing. This can be overcome with 'Periodic Error Correction' which can be recorded by the electronics in the mount, and compensated for in future exposures. According to the instructions, it requires accurately manually guiding on a much magnified star for 8 minutes. I'm told by those who have done it, that it is a 'long 8 minutes'!
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, with an 80mm refractor as a guidescope. They are housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC for imaging.
Aratus
 
Posts: 567
Joined: Tue Jan 12, 2016 2:17 pm
Location: East Lincolnshire

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