To Gain or not to overGain

Discussion of using SharpCap for Deep Sky Imaging
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oopfan
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Re: To Gain or not to overGain

Post by oopfan » Tue Oct 16, 2018 2:49 pm

Mitch,

I just remembered that Chuck made a YouTube video of how he uses SGP with screenshots with explanation. I forwarded that link to Dave some time ago. Let me see if I can dredge it up.

Brian

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Re: To Gain or not to overGain

Post by oopfan » Tue Oct 16, 2018 3:33 pm

Mitch,

I found this very interesting:

https://www.youtube.com/watch?v=l3o8c10TCZQ

It's Chuck's entire settings in Sequence Generator Pro (SGP). At the 1:29 mark you will see his Auto Focus settings. He refocuses on either: 3.5 degree movement, 45 minutes, or filter change!

Brian

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Re: To Gain or not to overGain

Post by turfpit » Wed Oct 17, 2018 8:45 am

In my experience, good imagers don't lie but they never tell you the full story. Your questions have to be cunningly phrased in such a way that the only possible answers are 'Yes and 'No'. The problem is, it takes years to find out what the real questions are.

Dave

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Re: To Gain or not to overGain

Post by turfpit » Wed Oct 17, 2018 8:58 am

Mitch
If your using an ATIK and im using a 183M hypercam then sharing gains wont work ( i think) plus maybe you have a C11 and i have a 4 inch refractor..
I own an Atik 314E and currently have use of an Altair 183M, so I can replicate your work. It is a C8 but I also have a Lightwave 66ED-R refractor. The 66 is being used for all the wide field work.

You mentioned sharing gains won't work, well here is the Atik in action - take a careful note of the Camera Controls Group and see what you notice..
M42-Atik314E.jpg
M42-Atik314E.jpg (52.54 KiB) Viewed 853 times
Dave

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Re: To Gain or not to overGain

Post by turfpit » Wed Oct 17, 2018 10:30 am

Mitch

When I hit this 183M and Ha filter anomaly (first ever use of a narrowband filter) I did some more systematic digging. I had an insipid looking NGC7000 which I only found out about at processing time the next day. On 23rd Sept I carried out some tests at 03:00 centred on Alnitak (which was high and due south, sky clear), which I thought was a good target for an Ha region.

I used an Altair 183M + 7nm HA filter on a 66ED refractor. 4 test images were taken, all gain=100 & black level=30. The 4 single exposures were 3m, 4m, 5m and 10m (only change one thing at a time). The FITS files are shown stretched with ArcSinh(x). The screen shots are summarised in the attached pdf. I often take screen dumps into a Wordpad file when imaging, it helps for subsequent troubleshooting.

The 10m exposure histogram is just about getting to the RHS. This has left me thinking about 10m exposures with the Ha filter in place. Now I was only using gain=100 so I have a lot to play with that parameter and starting to think that the normal 'guidelines' for imaging without filters are out of the window here. In a discussion with Nick @ Altair he mentioned using gain=1000 (or 20%). That I have to try when the weather suits.

Then I got to thinking about the Atik. It has no gain or offset control so the only way forward is to increase the exposure. Of course increasing the exposure started to make me concerned that I would run into over-saturation issues but early tests show this is not the case with a CCD camera. In a discussion with a lady who is a FRAS, she suggested that for M42 (using an Atik 314L with no filters) she would shoot a set of 60s and a set of 120s and merge the 2 stacks. Try that with a CMOS camera and I imagine the core of M42 would look like a nuclear explosion.

That led me to researching 'what do experienced imagers use for settings?' Part of the answer to this lies in the questions I asked above. I know the answers but doing the research yourself will have far more impact than just reading what I write. You will miss out so much understanding if you don't do the research.

I am going to try 10m exposures with a higher gain (maybe 10%, then 20%). I will also experiment with the Atik 314E. That will be grim as I am not geared up for guiding, although image 4 in the doc doesn't look too bad. Now Mitch you have PHD2 going so perhaps you could try that. You will need to have your offset right (as per offset setting when histogram is centralised in a bias frame) to have a chance of removing the starburst. We don't need your usual 300 exposures, I think 3x10m will be enough for testing purposes.

The other thing is you put the identical post in the Altair Forum. There has been no response yet but in fairness Altair have been at the International Astronomy Show over the weekend and are just about surfacing from that. Maybe you should add to your post with a more direct question along the lines of "I am imaging object X using a 183M + Ha with SharpCap. What are some suggested settings for offset, gain and exposure?" and see what turns up. Don't use APT or SGP as that just muddies the water.

We will get somewhere with this - remember 2 Brains are better than one (or is that 2 Brians?) .. LOL

In Babylon 5, the Vorlon said "the truth is a 3 edged sword - your side, their side and the truth"

Dave
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Re: To Gain or not to overGain

Post by turfpit » Wed Oct 17, 2018 6:41 pm

Mitch

I found this https://www.flickr.com/photos/terryhancock/8140278709 . The settings for this M31 with Ha filter are relevant to your original question.

Dave

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Re: To Gain or not to overGain

Post by turfpit » Sat Oct 20, 2018 5:29 pm

Mitch

You need to look at viewtopic.php?f=16&t=1228&p=6191#p6191. 4m exposures with 1h 40m integration time.

Now if my flats exposure goes from 150ms to 15s with an Ha filter in place, what increase in exposure is needed for a light with Ha?

See also my viewtopic.php?f=16&t=1217 taken using 10m exposure with Ha filter.

Have you found time to answer the questions I posed above.? No problem if you have been too busy, I will supply the answers on Monday.

Dave

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Re: To Gain or not to overGain

Post by turfpit » Mon Oct 22, 2018 11:10 am

Mitch,

As promised, answers below.

Going here https://www.swagastro.com/ic434-b33.html and looking at the Sara Wager's Wide Field shot, keeping it simple with the mono image taken using an Ha filter.
here are some questions to consider:
What type of camera is being used?
How is the camera cooled?
What is the cost of the camera?
Pixel full well depth?
What is the exposure time for a frame?
What is the total integration time?
Camera used QSI690, Sony ICX814, CCD
Cooling TEC to -40C
Cost ~£4,000
Full well 18,000 e-
Frame exposure 1800s (30 minutes)
Total integration 20 hours

Moving on to Terry Hancock's M31 with Ha at https://www.flickr.com/photos/terryhancock/8140278709

Camera used QHY9M, Kodak KAF8300, CCD
Cooling TEC to -30C
Cost ~£2,000
Full well 15,500 e-
Frame exposure 1200s (20 minutes)
Total integration 7.5 hours

The above are facts, conclusions are up to the reader.

I started down this track after a disappointing imaging of NGC700 with Ha (my first brush with Ha). I knew at the end of the session when I took the flats - normally 150ms had become 15s - that here was something different.

I started researching the web to find Ha settings used by well respected imagers, to help formulate a strategy going forward. That led me to believe that quality narrowband images are going to require long exposures and long integration times.

My second attempt using Ha at viewtopic.php?f=16&t=1217 with 10m exposures using my Atik 314E CCD camera (full well 12,000 e-, cost £300 off eBay) was more satisfying.

Dave

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