IC 4685 - a quick attempt before Sagittarius departs for another year

[Fegato]

I was planning on imaging the Iris Nebula on Tuesday, but it seemed particularly clear, so I had a quick go at IC 4685 before I started on it. I've been wanting to have a shot at this, but it's so low in the sky that any slight haze or cloud makes it impossible, and this was last chance saloon for this year. Although it was already past the meridian and heading for the trees when I started, I managed to get a series of 5 second exposures off, and was pleased to be able to use all of them to get this end result. The RASA maybe can't produce the absolute best of images, but it really comes into it's own when time is at a premium.

RASA 11 on CEM120, ASI2400MC Pro, 105 x 5s. Processed in Pixinsight.

[Avocette]

Lovely image (and especially from such a short nine minutes of total integration time)! Just curious about the 5 second exposures. I have been shooting 15 and 30 seconds recently because of blustery conditions, but otherwise tend to go for 120 to 300 seconds range, guided, with my f4.5 and f6.5 refractor/reducer combinations. 5 seconds at f2 equates to 11.25 secs and 16.25 secs at f4.5 and f6.5 respectively, and 8.75 minutes at f2 to 19.7 minutes at f4.5 or 28.5 mins at f6.5, all of which seems quite unexceptional.

[Fegato]

6 hours ago, Avocette said:

Lovely image (and especially from such a short nine minutes of total integration time)! Just curious about the 5 second exposures. I have been shooting 15 and 30 seconds recently because of blustery conditions, but otherwise tend to go for 120 to 300 seconds range, guided, with my f4.5 and f6.5 refractor/reducer combinations. 5 seconds at f2 equates to 11.25 secs and 16.25 secs at f4.5 and f6.5 respectively, and 8.75 minutes at f2 to 19.7 minutes at f4.5 or 28.5 mins at f6.5, all of which seems quite unexceptional.

Thanks!

I tend to alter exposures a bit depending on conditions and target, and it does depend on the camera as well obviously. My baseline is that I've calculated a range of median ADU values for my camera (ZWO ASI2400MC Pro, normally at Gain 140 and Offset 15) based on swamping read noise x3 up to x10 (which gives me ADU values of 510 - 560).  Having said that, the RASA is so fast, that I often exceed this. 

With no filter, I'm generally shooting 5s up to 30s. With 11 inch aperture, the stars can saturate very quickly. So sometimes I'll take a bunch of 5 second exposures for the stars, and then go longer for the target, using star removal in processing. A bit of a faff, but shooting everything at 5 seconds can become a bit of an issue with disk space and a slog in pre-processing. Anyway, this target was very low, it wasn't astronomical night, and I wasn't going to be pointing at it very long, so I just used 5s. Even just 5s took me beyond the x10 read noise swamp figure. I should add, it's just about Bortle 3 here when it's properly dark.

With the NBZ dual narrowband filter, I tend to use the range 60s - 180s.

By the way, my calculator gives me: F/2.2 = 5s, F/4.5 = 20.9s, F/6.5 = 43.6s ?   That's based on T = 5 / (2.2/4.5)^2 etc.

 

[Avocette]

4 hours ago, Fegato said:

By the way, my calculator gives me: F/2.2 = 5s, F/4.5 = 20.9s, F/6.5 = 43.6s ?   That's based on T = 5 / (2.2/4.5)^2 etc.

I think your calculator got it right! My own thinking was a bit loose and very optimist!

[Varavall]

On 28/07/2022 at 10:10, Fegato said:

I had a quick go at IC 4685

Hi

That inspired me to have a go while waiting for my main target of the night. SW 200pds, 0.9x coma corrector, ASI1600MM-cool, Astrodon LRGB, Astrodon 5nm Ha. Optolong 7.5nm Oiii and Sii.. First night, 5x300s each Ha, Sii and Oiii and second night, 5x300s L and 3x300s each RGB.  Aligned and stacked in ASTAP and processed in GIMP. This is my humble effort. As always any constructive comments welcome 😀