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david_taurus83

M81 M82 NGC3077 underexposed?

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I spent a long time capturing this. Needless to say I was a bit disappointed with the master stacks. They were very noisy despite the integration time and number of subs. Also, the background sky levels were very very low after calibration. Somewhere in the region of 100 ADU! I've started thinking I'm doing something wrong at the calibration stage. All subs were 30s. The same as when I done M42 and that came out ok, good in fact, in my standards. What kind of background sky levels do other folks typically see? I really had to stretch this and push up the saturation to get a bit of colour out of it. Not entirely happy with it but it is what it is. It's still quite noisy even after a couple of NR iteration. Is it just a simple case of not exposing for long enough?

 

Taken over 5 nights in March and April. Cloud ruined a lot of it. I even had to throw away 5 hours of data after I started 3 sessions with a new laptop and had the camera set at 0 gain, doh!!!

 

William Optics GT71 at F5.9 (420mm)

ASI1600MM at -25°C, 139 Gain, 50 offset

Baader LRGB filters

 

Approx:

480 x 30s Lum

400 x 30s Red

120 x 30s Green (cloud ruined every night on green!)

350 x 30s Blue

Total integration around 11 hours.

Comments and criticism welcome.

 

M81-M82 Apr 2019 SCNR.jpg

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The image is looking good though.

Taking 30 second exposures at f6 it's probably worth increasing the gain to minimise read noise. I got this on M31 using 30 second unguided exposures at f4.7, even with the gain at 300 I don't think it was overexposing (I don't really know what settings I should be using with 1600MM cool, 300 gain might not be a good idea). However, that was from a dark site, what are your skies like please? Pulling out faint detail such as the IFN will take a great deal more exposure time when dealing with light pollution, whereas for brighter objects the difference is not so great.

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I dont know much about the ASI1600 but that looks pretty good indeed.  The red filaments are really showing in the Cigar.  If I may say, the red channel doesnt seem to be lined up quite right.

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I think it's very nice indeed but The Dr may well have point with the red channel.

Alan

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1 hour ago, Knight of Clear Skies said:

The image is looking good though.

Taking 30 second exposures at f6 it's probably worth increasing the gain to minimise read noise. I got this on M31 using 30 second unguided exposures at f4.7, even with the gain at 300 I don't think it was overexposing (I don't really know what settings I should be using with 1600MM cool, 300 gain might not be a good idea). However, that was from a dark site, what are your skies like please? Pulling out faint detail such as the IFN will take a great deal more exposure time when dealing with light pollution, whereas for brighter objects the difference is not so great.

Light pollution, sky isn't bad really for where I am. Obviously low down in the north is the Birminhgam glow but zenith and south are ok. My main gripe is the orange streetlights overlooking the garden! Gradients aren't difficult to remove though. The IDAS D2 seems to be doing a good job. Regards gain etc, I think I'm going to give Sharpcap a go. I ran its sensor analysis tool at the weekend and had a play with its smart histogram as well. It takes a few images of your sky conditions and recommends exposure and gain settings.

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1 hour ago, tooth_dr said:

I dont know much about the ASI1600 but that looks pretty good indeed.  The red filaments are really showing in the Cigar.  If I may say, the red channel doesnt seem to be lined up quite right.

Thanks Adam. Do you mean the histogram doesn't line up or star alignment? I'm struggling to see where!

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I think he means in the alignment as the red channel is slightly out.

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28 minutes ago, david_taurus83 said:

Thanks Adam. Do you mean the histogram doesn't line up or star alignment? I'm struggling to see where!

What Mark said above.  Looks like the red channel could be nudged up 2 or three pxiels.

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Ah I see what you mean. Are you looking at the stars in front of the galaxies? It could be because I created a range mask and went around and clone stamped every star out of it so only the 3 galaxies remained. Boosting the colour of the galaxies only has shown this aberration I'm guessing as I cant see it in the main star field. I use an inverted star mask for boosting star colour. I'll have a play later and see if i can split the 3 channels and manually align within the work space. The Dynamic alignment process is pretty good.

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Come to think of it, if I create a star mask should I be able to subtract it from the range mask with Pixel Math? I've never used it though so will swot up on the old YouTube this evening!

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I've not gone back on the image but its niggled me about the exposure time used. I sacrificed last nights clear sky for some testing. Initially a flattener spacing experiment which still remains unsolved...

I ran Sharpcaps smart histogram on a relatively dark target, NGC7023, as I imagine a similar exposure would do for galaxies. Some interesting results! It recommends :

Lum: 96s at 59 gain 5 offset

20190512_023626-1470x3024.thumb.jpg.361af0b8a099f2c35f34a5567e814136.jpg

 

Red: 10 minutes!!!! 59 gain and 5 offset

20190512_021931-1470x3024.thumb.jpg.c156e87048567ded969e0703f4b64dc1.jpg

 

Green: 3 minutes at 59 gain 5 offset

 

20190512_022501-1470x3024.thumb.jpg.5d11a15ef7689138745a99b67ffa788e.jpg

 

Blue: 4 minutes at 59 gain 5 offset

 

20190512_023337-1470x3024.thumb.jpg.e563f681dcaa954056736a34676c7a63.jpg

 

I also tested Ha. No screenshot but this resulted in 10 minutes recommended at 200 gain and 15 offset. This seems about right and I've seen plenty of comments where people shoot all their narrowband at 200 gain. The red result really surprised me though. Certainly goes against the thinking of shorter subs with this camera.

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