Astrophotography

Hi Prasad,
I typically always take 300 second exposures unless there is a reason not to, and there are very few reasons not to. With shorter exposures you have more latitude for accommodating tracking errors and if you have a problem you lose less potential imaging time. You do not collect less signal with two 5 minute exposures than one 10 minute exposure. Some people like to take very long exposures, maybe 15 or 30 minutes. If they have to throw away a sub, that is a lot to throw away. The only benefit they have is using less disk space. On my equipment I can easily track without guiding with a 5 minute exposure. If I went to 10 minutes or more I think I would need guiding, but I never checked that. I have no reason to. Simple is always better.
The only tracking problem I have is wind. The mount cannot react that quickly. In this case I decided to go with 120 second subs for the RGB. This is one of the exceptions mentioned above. Wind can give me football shaped stars at minimum or dramatic streaks. The 120 second subs gives me more assurance that I'll have better stars. The primary use of the RGB in this case is just the stars. I take those stars and put them with the narrowband data. The 3nm slice of the spectrum for a narrowband filter gives no color to the stars. It looks unnatural. I took only one hour of data for each broadband filter. That is all that is needed to get good star data. In regard to deciding how much data to gather the answer is that you can never have too much, especially with a target like this. Much of the Ha data is very faint so with just a few hours you wouldn't capture it. Even with 18 hours the picture would be much better if I had 30, 50 or even 100 hours. If you look on astrobin you'll see people that had those kinds of hours. The point of diminishing returns is different for everybody. Everything else being equal I try to get equal amounts of narrowband data in each channel. When getting data on a broadband target, LRGB, I try to get equal amounts of RBG, and luminance that is the total of the RGB. So if I have 10 hours each of red, green and blue, I'll try to get 30 hours of luminance.

I hope this helps.

Dan

Thanks Prasad. This picture used five filters, red, green and blue, to get good star color, and Ha and Oiii where most of the signal comes from. I also collected some data in Sii but didn't use it for this image. I would have needed the Sii if I processed it in the Hubble palette (SHO) but as it turned out I did it in the HOO palette.

Red 30 x 120 sec
Green 30 x 120 sec
Blue 30 x 120 sec
Ha 91 x 300 sec
Oiii 76 x 300 sec
Sii 14 x 300 sec

Total subs 271
Total hours about 18

This was a real challenge to process. I literally processed it nine times until I got a result I was happy with. Attached is the first try which, in my opinion, was best of the first eight iterations. Narrowband is false color (like every picture that comes from the James Webb telescope) so there are many aesthetic choices to make along the way. The attached picture actually shows more detail of the helix than the final result.

Dan

Nice work Dan!

-Lou

This is the Helix Nebula, NGC 7293, AKA The Eye of God. It’s a planetary nebula formed by the pinpoint star at the center of the blue field in the middle of the image. The picture offers a view down a trillion-mile-long tunnel of glowing gases that were expelled by that star as it neared the end of its life. Unlike much larger stars that die suddenly in powerful explosions, known as supernovas, this star, similar in size to our Sun, became a white dwarf with a hot stellar core. The core is so hot that it emits intense ultraviolet radiation causing the ejected layers of gas to glow, resulting in the pictured nebula.