M51 Whirlpool - First light for Sony A7S Camera

[MarkS]

This is the first light for my new Sony A7S full-frame mirrorless camera on a Celestron C11 with 0.8 reducer/flattener i.e. imaging at F8.  A total of 2 hours of data in 90sec subs at ISO 2000 was taken in SQM 21.0 quality sky at 6C ambient temperature.  The subs were short to overcome a flexure or mirror shift problem I was experiencing but this has not affected the image quality because of the remarkably low read noise at ISO 2000.

This is a crop of the image because I had very squiffy stars further away from the centre, probably due to incorrect spacing and/or alignment of the reducer.



The full size (i.e. 1:1 scaling) of the crop can be found here:  http://www.markshelley.co.uk/Astronomy/2015/m51_a7s_c11_20150420.jpg

I've deliberately left the image as "raw" as possible. Dithering was done during acquisition and darks, flats, bias have been applied followed by a straight sum of the 80 frames after aligning the frames.  This was followed by an arcsinh stretch of the luminance channel using IRIS.  The RGB balance is the camera's own daylight balance and no colour saturation or sharpening has been applied.  This makes it easier to judge the overall image quality and noise characteristics.

Analysis showed the following statistics for each sub:
Read noise:  1.4e/pixel
Thermal noise:  2e/pixel
Skyglow noise: 4.5e/pixel
Combining these quadratically gives a total noise of only 5.1e/pixel

It is notable that even at such a slow F8 focal ratio the skyglow (i.e. light pollution) is still the main source of image noise, though this is partly because of the low ambient temperature of 6C.  However even on a warm Summer evening in the UK i.e. 20C the thermal noise would only rise to 4e and thus be similar to the skyglow.

Note that this image was taken with the camera in its unmodded state.  I'm hoping to DIY mod it next week

Mark

[The Thing]

That's a really nice image. 90s subs at f8! I wish. There are some nice background galaxies in there, with my 1000d they would be lost in the noise. I would love to be able to do so little processing as well I'll have to start 'convincing' Fiona that I can't live without one.

[JohnP]

Hi Mark,

I'm guessing you are pretty pleased with that for a first light?

Is that with the standard non-modded camera...?

Like Dunc said 90secs at f8 - amazing..

John

[Kenny]

I agree with the others, that is a stunning image. Well done Mark. Seems like the camera was a good investment!   

[julian]

Very nice Mark.  Not only does it take nice images, I like the live view feature for the night sky.
Now where did I put my credit card?

Julian

[Mac]

Very nice image.

Now where did I put my credit card?

You can have mine, im sure you will spend less then the wife

Mac.

[Carole]

Amazing data Mark from such short subs and an unmodified camera.

Having seen through the live view of this camera at Kelling, it was absolutely breathtaking.  I look forward to watching your progress with it.

Carole

[Fay]

Outstanding Mark, and you have not even modded it yet!!!!! also such short subs!!!
I could go on..................

[MarkS]

It's not all good news though - I have a number of niggles:

1) The narrow diameter of the E-mount opening means that very severe vignetting can occur at the corners of the full-frame sensor depending on what correctors and adapters are in the optical train.  I had hoped that a mirrorless camera would put an end to vignetting caused by the DSLR mirror box.  Alas no.

Here's the warts and all version, also showing the nasty artefacts in the flat caused either by the flattener or my cheap and reflective Sony to Canon adaptor:



2) Sony have stayed with their 12 bit image format for this camera and they also continue to compress the raw file.  To be fair though, this appears to have little (if any) impact on final image quality - at least when deep sky imaging at ISO 2000-4000.

3) There is very little software support in the astro-community as yet.  For accurate focusing I wrote a PixInsight script that auto-opened images that that the Sony software dumped in  a chosen monitored folder, at 1:1 scaling - this allowed me to use Bahtinov Grabber.

4) Sony appears to apply some kind of hot pixel and dark pixel suppression algorithm - the evidence for this is almost everywhere in the raw files where pairs of identical warm pixels and pairs of identical cool pixels appear all over the place.  I'm quite concerned that this algorithm could have nefarious effects similar to the infamous Nikon "star eater" algorithm on undersampled stars.  I'm still investigating this issue.   It could certainly rule out photometry at shorter focal lengths.

5) The camera has a price premium over other full-frame cameras, apparently caused by it's ability to do 4K video - something I don't really care about

Mark

[Fay]

well blaze on Mark. good job you are knowledgeable enough to be able to investigate all the problems.