Sh2-170 in Ha

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oopfan
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Sh2-170 in Ha

Post by oopfan » Mon Oct 22, 2018 3:44 am

Tonight was washed out with clouds shortly after I began collecting data. I was able to eke out 10 frames of Sh2-170 for a total integration time of 33 minutes. It really needed 2 hours in order to get the noise under control but alas it was not meant to be.

Normally I would not publish a feeble image like this but it is an interesting emission nebula not commonly known. It is in northern Cassiopeia at 65 degrees declination. It currently crosses the meridian at 11pm. If it is not in your catalog, no worries, I will give the coordinates.

Technical Details:

Emission nebula: Sh2-170 (Sharpless Nebulae Catalog)
J2000 coordinates:
RA: 00h 01m 42s
DE: +64d 37m 24s
Size: 20 arcmin

William Optics 71mm f/5.9
Altair 290M camera (uncooled)
Orion 6nm Hydrogen Alpha filter
Unitron Model 142 GEM
Passive tracking with PEC
No active guiding

Gain 389 (1.0 e-/ADU, FWD: 4ke-, Read Noise: 1.38e-)
Offset: 30 ADU
Exposure: 200s
Camera rotation: 5 deg E of N

Ha: 10 frames
Flats: 50 frames
Darks: 45 frames
Bias: 100 frames

Total integration time: 33 minutes

92% waxing gibbous Moon approximately 30 degrees altitude southeast azimuth.

SharpCap 3.1.5219
PIPP 2.5.9
Deep Sky Stacker 3.3.2
StarTools 1.3.5.289

Sorry the image is so small. I applied 4x4 binning in post-processing to help reduce noise.

By the way, the mottled texture of the nebula is real. DuckDuckGo it!

Brian
Attachments
Sh2-170_Processed_Bin4_Ha_10x200s_G389_BL30.jpg
Sh2-170_Processed_Bin4_Ha_10x200s_G389_BL30.jpg (28.51 KiB) Viewed 1858 times
Last edited by oopfan on Mon Oct 22, 2018 1:38 pm, edited 1 time in total.

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oopfan
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Re: Sh2-170 in Ha

Post by oopfan » Mon Oct 22, 2018 3:51 am

Here is a screenshot of SharpCap's stretched preview of one 200s Ha frame. The nebula is the "big donut" roughly in the center of the screen. Amp glow is seen in the upper-right corner.

Brian
Attachments
Sh2-170 Ha SC screenshot.jpg
Sh2-170 Ha SC screenshot.jpg (525.56 KiB) Viewed 1856 times

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oopfan
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Re: Sh2-170 in Ha

Post by oopfan » Mon Oct 22, 2018 3:54 am

And here is the histogram of that one uncalibrated Ha frame.

Brian
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Sh2-170 Ha FITS screenshot.jpg
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turfpit
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Re: Sh2-170 in Ha

Post by turfpit » Mon Oct 22, 2018 11:55 am

An informative technical write-up Brian. Love the fat histogram - plenty of data in there.

Dave

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Re: Sh2-170 in Ha

Post by oopfan » Mon Oct 22, 2018 1:32 pm

Dave,

You would think that this image was difficult to process but it wasn't. I found that as long as I've got well-exposed Lights matched with Darks of a similar temperature then post-processing is not tedious.

Lessons learned (the hard way):
1. Underexposed Lights -- BAD
2. Temperature-mismatched Darks -- BAD

How do you tell if you've got a well-exposed Light? There are two methods:
1. Turn on stretching in the SharpCap preview window. If you can't see the target then increase the exposure (not the gain!)
2. Look at the histogram in FITS Liberator. The "leading edge" must be convex-shaped not concave.
3. Do both for confirmation.

Also:
Exposure calculators should be used as a starting point, a good initial guess. Use your brain from that point forward.

Finally:
"Total Integration Time" can be misleading. You are mistaken to believe that 200x10s equals 10x200s. You can try 200x10s but you will be bitterly disappointed. You might need 2000x10s to get close to my 10x200s. It is not a linear relationship!

Given a choice between a short exposure and a long exposure, take the long exposure unless there is some compelling reason not to.

Brian

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