The Antennae - FINAL RESULTS NOW UPLOADED

rmuhlack (Richard) · #1 03-05-2013, 12:47 PM

This is a DSLR "Work-in-Progress", and also an exercise in theory meets practice.

I have been experimenting with different ISO settings on my uncooled 400D in an attempt to find an optimum, and (being the nerdy type that I am) thought the best way to settle it would be to conduct an experiment. Firstly run a series of darks at different temps and plot SNR vs exposure time for different ISOs and temperatures (as per Roger Clark's methods here). Graphs are relative to the noise of my 400D at 300s ISO800 and 20 degrees C (the plot for 20C is not shown here)

To my surprise ISO1600 came up trumps, so it was time to test with something challenging. I decided NGC4038 - The Antennae.

The data I have so far was taken over two nights using my VC200L with focal reducer - this is a HDR combine in Pixinsight of 67 x 180s ISO1600 subs (ambient temps of about 10-14C) and 53 x 240s ISO1600 subs (ambient temps of about 8-13C). Hi res version here: http://www.astrobin.com/full/40743/?mod=none

Am hoping to add to this over the next few imaging sessions, but I thought the process of determining exposure time might be of interest to some. And of course any advice or comments on my image capture and processing is also very welcome. The balance between the background and extended galaxy arms in particular I found quite challenging. I plan to post some updated images when I have collected some more data.

Thanks for looking

gregbradley · #2 03-05-2013, 12:57 PM

Great image Richard. Way better than I would normally expect to see from a DSLR.

So you took various images at various image lengths and plotted the resulting SNR (measured at the same spot on each image?).

I wonder if this result would vary with ambient temperature as sensor noise varies with temp.

Great work. I'll have to do the same with my Nikon D800e to see what the ideal ISO is for it.

Greg.

tilbrook@rbe.ne (Justin Tilbrook) · #3 03-05-2013, 12:57 PM

That's impressive Richard!

I tried the Antennae last year, got nothing like your image.
you've inspired me to try again.

Cheers,

Justin.

rmuhlack (Richard) · #4 03-05-2013, 01:07 PM

Thanks Greg/Justin.

Greg - it does vary with temp as you would expect. The graphs show the SNR for two data sets - one for an ambient of 14C (this was basically constant throughout this test) and then another where ambient varied from 5.5 down to 3 C during the test (and this variation should be taken into account when interpreting the results. ideally I should do it again when temps are expected to be more constant). Its done by taking a series of dark frames only, and then measuring the standard deviation of each dark frame (across the entire frame). Roger Clark's website gives the formulae for then calculating relative SNR.

rmuhlack (Richard) · #5 03-05-2013, 01:14 PM

One clear insight from those plots is that my earlier idea of using ISO400 which has a lower gain and therefore greater dynamic range wasn't doing me any favours with respect to noise (even though I did see much better star colour). eg this image of Corona Australis from two weeks ago: http://www.iceinspace.com.au/forum/a...e.php?a=137559

I think these results suggest that a better approach for me to enhance star colour would be to take a separate set of short subs at ISO1600 (say 60s) and then do a HDR combine in Pixinsight.

graham.hobart (Graham stevens) · #6 03-05-2013, 01:35 PM

That's pretty cool Richard.
What do you think about a cooled DSLR?
If this keeps temp relatively constant (my CentralDS modded Canon will stay at 1'c EXIF) then shoot at 1600 but also with some shorter subs for the star colour?
Great thinking!
I like the cut of your jib my man!
PS Antennae are excellent
Keep up the good work
Graham.

rmuhlack (Richard) · #7 03-05-2013, 01:42 PM

Thanks Graham. By cooling the DSLR you're going to get a similar sort of boost to SNR (when compared with an uncooled equivalent). However unless you have a very old CentralDS camera (like a 350D) the ISO you should use may be different as your camera will have different gain settings as its 14bit instead of the 12bits that im using.

But yes, i would imagine the same principle would still apply: Determine what ISO maximises your SNR, and then vary the sub length and use HDR combine to increase the effective dynamic range of your image

ps as a side point, i'm curious to know what sort of condensation problems are seen with a cooled DSLR, when the cooling takes the camera below the ambient dew point..?

RickS (Rick) · #8 03-05-2013, 03:16 PM

Very nice work, Richard! Some good research and a nice image.

Larryp (Laurie) · #9 03-05-2013, 03:52 PM

Very nice, Richard

multiweb (Marc) · #10 03-05-2013, 04:46 PM

Impressive.

Great write up. Thanks for the links.

Ross G · #11 04-05-2013, 09:37 AM

Great looking photo Richard.

When I was shooting with a Canon 350d and 1000d I also found that 1600asa seemed to be the "sweet" spot.

Good luck.

Ross.

David Fitz-Henr · #12 04-05-2013, 09:57 PM

Excellent image Richard!

rmuhlack (Richard) · #13 05-05-2013, 07:04 PM

Well I now have almost twice the amount of image data on the Antennae that I did before. As far as a practical test of using ISO1600 to maximise SNR, I think the results here speak for themselves. I am very happy with this result

The final tally comes in at:

Day 1: 67 x 180s ISO1600 subs (ambient temps of about 10-14C)
Day 2: 53 x 240s ISO1600 subs (ambient temps of about 8-13C)
Day 3: 47 x 180s ISO1600 subs (ambient temps of about 10-13C)
Day 3 continued: 49 x 240s ISO1600 subs (ambient temps of about 7-10C)

I have uploaded a "widefield", a close-in crop, and just for laughs a comparison with Hubble's famous image of this galaxy pair

Processed with Pixinsight. Hi res image here: http://www.astrobin.com/full/40910/?mod=none

As always, feedback welcome