NGC772 - 450 minutes total exposure - limiting magnitude 22!

bdeclerc

So, I've increased total exposure time to 7.5 hours (in 30 second bits at gain 300) with an ASI 174MM and a 30cm Newton.

The result is the attached image.

NGC772_450min.jpg

I've also determined that the limiting magnitude is somewhere between 21.0 and 22.0, with galaxies visible on the image at 22nd magnitude in Green light, but they are typically a bit brighter in Red light (approx. 21.0) - but since the ASI174 is significantly more sensitive in green, I'd say that 22nd magnitude limit is probably more accurate (besides, it sounds more impressive!)

I've also added a labelled cutout based on some data from http://skyserver.sdss.org/dr9/en/tools/chart/navi.asp?ra=29.8&dec=19&opt=P

NGC772_450min-id.jpg

If I process the image to show the faint background details more clearly, several "bridges" between the galaxies start to become visible, including some very faint splotches of starlight near the lower edge of the field of view, left and right of the bright star at the bottom - these can be matched to this deep image taken with a 3.6 meter telescope : http://ohstarstuff.com/post/67961377754/this-is-a-stunning-look-at-ngc-772-aka-arp-78-as

NGC772_450min-achtergrond.jpg

Sam@ZWO

nice, you should try our cooled 174 camera

bdeclerc

Yeah, that's going to happen some time in the near future - unless you've got other surprises in store in the near future?

johnk

That's a simply amazing image!! Congrats!!

Really interested to know how you are determining your exposure and gain levels?

I have just ordered the ASI174mm cooled camera, and wanting to do some LRGB imaging with my 12.5" f/5 scope from a Dark Sky here in Australia.

Generally, due to a portable set up, I would like to shoot one object per night with about 4-5 hours to spare at most for the entire run.

Clear skies,

John K.

bdeclerc

JohnK wrote:
That's a simply amazing image!! Congrats!!

Really interested to know how you are determining your exposure and gain levels?

I have just ordered the ASI174mm cooled camera, and wanting to do some LRGB imaging with my 12.5" f/5 scope from a Dark Sky here in Australia.

Generally, due to a portable set up, I would like to shoot one object per night with about 4-5 hours to spare at most for the entire run.

Clear skies,

John K.

300 gain gives me a close to optimal readout noise level without losing all the dynamic range - for really bright objects (or objects with both very bright and fainter detail) it might make sense to go lower - also if you want to take very long exposures.

In my case, I'm picking 30 seconds as some tests I did showed this to be roughly the point were increasing individual exposures didn't make for significant gains in Signal-to-Noise ratio on my final stack - going shorter makes things worse (1800 1sec exposures gives a significantly worse result than 60 30 second exposures, but 3 10 minte exposures don't give me a lot better result than the 60 30 second ones - and I'm close to an airport approach lane, with 30s/frame I only lose a small amount of data compared to 10 minute frames, so it's a good compromise) - with darker skies it might be worthwhile going longer per frame.