Adding light polluted data to dark sky data

[CalvinKlein (Kelvin)]

I have some Andromeda galaxy images I have acquired from a couple of different dark-ish sites SQM overhead of 21.7 to 21.9 (though Andromeda is only just above the light pollution domes and have gradients, but overall I consider very good data)

Is it worth me trying to add to them at my comparatively light-polluted farm (SQM 21.3 overhead) where I would be imaging at 20 degrees through the light dome over Coolangatta ? I understand I'd have to endure roughly 6.3 x the amount of imaging time to match the dark sites (assuming 2 SQM magnitudes of sky lightness difference - maybe worse).

In other words is it worth mixing small amounts of good data with lots of "bad" ?

(I use a 1600MM-Pro, 36mm EFW filters and a 300mm F2.8 lens.)

thanks.

[glend (Glen)]

I would not bother trying to image Andromeda down at 20 degrees over Coolangatta.
My advice is to save your good Andromeda data until you can get back to good dark conditions. Your data isn't going anywhere, so why rush?

[multiweb (Marc)]

If you use a good sub with contrast to normalise and reject all the LP affected subs you could get some more signal. I used to do that from home from one night to the other. Seemed to work. Or maybe it's not advisable. Rick's probably done the maths on that.

[CalvinKlein (Kelvin)]

I guess there's no real rush - it's just that Andromeda isn't going to be around with us much longer and my really dark sites with a good view north are over 2 hours away.

[RickS (Rick)]

Hi Kelvin,

It's hard to say whether it would be worthwhile or not. As you say, it's going to take a lot more integration time at the less dark site to beat down the shot noise from the sky glow so you're at a disadvantage to start with. You also have existing data so you'll need to add a significant amount of additional signal to get an appreciable improvement in SNR. The part that's difficult to predict is how the gradients will interact. As Marc suggested, you definitely want to use one of the dark sky subs as the reference for normalisation. LocalNormalization may also help to to reduce the effect of the sky glow.

If you're really keen you could try collecting a night of new data and adding it to the integration to see if it makes an improvement to SNR. A combination of careful blink comparisons and some measurements might help decide if it is worth continuing.

My experience with adding data from home with data from dark sites is that it hasn't been worth it for LRGB data. But I suffer from worse light pollution than you and I haven't tried collecting very large amounts of additional data...

Cheers,
Rick.

[RyanJones]

There's no doubt that dark site data is worth considerably more that LP data but what I've found of my continuing data collection of Tarantula is that you can use your LP data to reduce core blow out. If you reduce your sub length or ISO on your LP subs, the LP has less effect and this data also tames the core. Just my thoughts ...

[CalvinKlein (Kelvin)]

Thanks for your valuable advice everyone. Based on what you have said I've decided to just be happy with the Andromeda data I have and move on to the Orion region.

Unlike most other targets this year I wont leave my run too late !!

Cheers, Kelvin

[xelasnave]

It probably depends on what you image.
I combined Sydney captures with Tabby captures for the Lagoon and it worked well but I doubt if a combination would work for M31.
Alex