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NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 4:55 pm
by oopfan
I really do like my uncooled Altair 290M camera but one thing is missing: a sensor temperature readout. This has stood in my way of creating a Darks Library. Having a Darks Library would free up my time to concentrate on capturing Light frames.

There is nothing worse than having to waste an hour collecting darks under the stars. Furthermore, I committed the sin of collecting darks after my lights when the temperature was usually much cooler (if anything, you should collect them before.) So I was getting sub-optimal noisy images with the hint of raining noise when the temperature of my darks were very different than my lights.

I decided to do something about it. I built a Temperature Logger with an Arduino, a high-precision temperature sensor having an accuracy of 0.25C, an SD card reader/writer, and a LIPO battery. I position the unit near my telescope. It records ambient temperature once per minute and writes it with a timestamp to the SD card. I've had it running for 11 hours straight on a single charge. It could probably go longer.

On those cloudy nights I collect darks. Then the following morning I download the image files and the temperature log, and segregate the image files by temperature. These past couple nights the weather cleared to try it all out. I like the results. Smoother, less noisy prints with no hint of raining noise.

Technical Details:

William Optics 71mm f/5.9
Altair 290M camera (uncooled)
Optolong luminance filter
Unitron Model 142 GEM
Passive tracking with PEC
No active guiding

Gain 100 (FWD: 15ke-, 3.66 e-/ADU)
Offset: 20 ADU
Exposure: 50s
Camera rotation: 3.8 deg E of N

Captured over 2 nights:
70.5 degF: 1 frame
71.0 degF: 22 frames
71.5 degF: 48 frames
72.0 degF: 4 frames
72.5 degF: 69 frames
70.5 degF: 113 frames
71.0 degF: 69 frames
71.5 degF: 94 frames
72.0 degF: 96 frames
72.5 degF: 96 frames
Flats: 100
Bias: 100
Total integration time: 83 minutes

SharpCap 3.1.5219
PIPP 2.5.9
Deep Sky Stacker 3.3.2


Re: NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 6:05 pm
by mAnKiNd
That's a cool (no-pun intended) solution! Results look very nice :)

Re: NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 6:19 pm
by turfpit
Nice Bubble Brian. Useful technical detail.

Can you post a FITS Liberator snip of both the histogram and image statistics for a single light?

Re: NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 6:41 pm
by oopfan

The first attachment is a single frame of NGC 7635 in FITS Liberator. I stretched it with "ArcSinh(x)" to reveal the shape of the histogram.

The second attachment shows a bias frame. You've probably noticed that I always use Offset 20 with Gain 100 no matter what target I shoot. Offset 20 is the minimum needed to prevent clipping of the LHS of the histogram. Months ago I experimented with higher values but I did not notice a significant improvement in the quality of the final stack. I keep the Offset at a minimum to get the greatest range so I don't saturate stars.

I added a dark frame. You can see that there is quite a lot of dark current at 72 degrees Fahrenheit ambient temperature.


PS: Sorry the order of attachments and the text are off.

Re: NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 7:03 pm
by turfpit
Thanks Brian. Very useful.

I use "ArcSinh(x)" as well, much better behaved around zero than "log(x)".

I see Offset 20 still leaves you with the Image Statistics 'Min' at 448, so no LHS clipping. I have been using Offset 100 but will try 20 and see how the saturation goes. Nice shape on the histogram for a DSO.


Re: NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 9:03 pm
by admin
Excellent results and a great idea!

It's also good to see that your experience confirms the theory that raining noise is primarily due to mismatched dark frames.



Re: NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 9:32 pm
by mAnKiNd
I like how you have gain and offset fixed and then adjust only your exposure to shape the histogram.

Re: NGC 7635, The Bubble Nebula

Posted: Fri Sep 07, 2018 11:05 pm
by oopfan
Temperature-mismatch + poor guiding = raining noise but I still contend that it can be somewhat mitigated by increasing exposure but I am not too proud to be proven wrong on that last part!

Thanks everyone for your support.


Re: NGC 7635, The Bubble Nebula

Posted: Sat Sep 08, 2018 1:42 pm
by mAnKiNd
Hey Brian,

I advocate that indeed a longer exposure would in a sense "swamp" the raining noise, but I haven't done the month, just an anecdotal observation from my sessions. Also, better matched darks, as your suggested workflow above has demonstrated help mitigate the raining noise.

Why is your guiding poor? Maybe we can help?


Re: NGC 7635, The Bubble Nebula

Posted: Sat Sep 08, 2018 2:18 pm
by oopfan
Hi Minos,

I am using a 50-year old GEM. Here is a photo of my kit. By the way, since that photo was taken I replaced that Orion manual FW with a ZWO mini-EFW. Much better!

The original telescope was gifted to me by my father for 8th grade graduation. The original kit had a 60Hz synchronous motor on RA. Uncorrected I have about 50 arcsec p-p of periodic error. I replaced the synchronous motor with a stepper motor last year. It is controlled by a Raspberry Pi with software I designed. I wrote IronPython script that runs in SharpCap to snap a photo once per second for a full turn of the worm gear in order to calculate the periodic error correction. The periodic error is now down to about 10 arcsec but since I don't use active guiding I do suffer from some drift due to small polar alignment errors and other errors in the mount. All in all, I get round stars using a 50-second exposure but over the course of one to two hours I get that drift. That is what gives me the raining noise if I am not careful with the darks but I've got that solved now. One of these days I'll get a go-to mount but I do enjoy watching you guys spend lots of money on your kits while I get more with less :twisted:

BTW, that 200mm mini-scope with ASI120 is just used for polar alignment.