Imaging before astro twilight?

[BrendanC]

I'm getting frustrated with capturing M101 the Pinwheel Galaxy. I got some subs a few weeks ago and want to get some more, but my lowly AZ mount isn't at all happy tracking it at the alt it reaches after astro twilight (around 22:30 where I am). So I'd like to start a bit earlier and get more subs, say, 21:30, while it's lower in the sky and my mount can handle it.

Is this feasible? If I reduce the exposure time or dial down the ISO setting (I use a modded EOS1000D) would this counteract the fact that it's not quite fully dark? Or is it a total no-no?

Also, I guess the wider question is around imaging generally during the summer months where, for a lot of us, there is no actual astro twilight at all. What's the way around this? Or do we just give up, twiddle our thumbs, or actually have to look through our eyepieces and - shlock horror - observe?

Edited April 21, 2020 by BrendanC

[tomato]

You won’t get optimum results, but you can still image, the attached was captured in July in Nautical twilight, as I recall.

[BrendanC]

Nice one, thanks! I think I'm going to go for it.

[wimvb]

Imaging when you don't have proper astro darkness, is like imaging with light pollution. As long as it's moderate, it's possible. The sky brightness will be removed during processing, but you'll be left with the associated noise, and you need a much longer integration time to bring the noise down to acceptable/ordinary levels. Also, the blue channel will be affected most, and usually this is the part of the light spectrum where cameras are least sensitive. In short, double trouble. For me, imaging season has ended already, and I will be cleaning my scope and drooling over suppliers' catalogues until end August.

Edited April 24, 2020 by wimvb

[gorann]

Like Wim I have given up for the season, but then we are both at 60°N, I am sure I could still get 1-2 hours of data in a night but it would be light pulluted and I do not want that kind of data if I can get much better data in late August. So I will start digging for a new pier and obsy instead. Drewling over new equipment, which I usually do a lot in the summer, is also difficult in the present situation since delivery dates are very uncertain, especially from China (like ZWO and Sky Watcher), see for example this message https://www.pierro-astro.com/. I am still waiting for an ASI camera that I paied for in February.

[BrendanC]

That's a pain. I mean, we all like a good drool, but I also like to be out there!

I'm at 51°N so I think I get a bit more time than you, but that's not for long.

I think I'll spend the time doing more observing, getting to know the sky better. With an eyepiece in one hand and a nice cool beer in the other!

[iapa]

55.8N

no astro dark after the weekend.

So, lots of setting up, aligning, tweak this, hammer that.

Probably end up with lots of data across only a few targets - may be able to fool myself that the processed results looks good.

Something to aim to improve on tho', 

[andrew s]

Just remember astronomical twilight is an arbitrary choice, albeit informed by experience  from pre CCD/CMOS days, you can stretch it a bit on either side.

Regards Andrew 

[iapa]

From a practical point of vie, if your a viewing it shouldn't make too much difference,

Although as astronomical darkness is the period that the sun is 18deg below the Western horizon until it reaches 18degrees below Eastern horizon, during this time is is not illuminating the sky. Outside those times there is (if you will pardon the language ) "pollution" from the sun.

For imagers, narrowband filters should help offset that to some extent, but it's still additional noise we don't need

I suppose it's how much you want to process it out.

[BrendanC]

I have been experimenting with an hour either way and it seems ok so far (for imaging). But that's with at least some astro darkness in between. I think I'm resigned to not being able to do anything from mid-May to mid-July.

[wimvb]

"Up here", the nights get so short so fast, that it just isn't practical to do any imaging when there's no longer any astro darkness. I noticed the difference when I tried to stretch my luck by a few nights. Absence of darkness means that the noise increases fast. So you have to compensate by taking more exposures. Which are more affected by the increased light. Which in turn means you need to take more exposures still. Which ... (I think you get it by now).

[Rodd]

On 28/04/2020 at 18:45, wimvb said:

"Up here", the nights get so short so fast, that it just isn't practical to do any imaging when there's no longer any astro darkness. I noticed the difference when I tried to stretch my luck by a few nights. Absence of darkness means that the noise increases fast. So you have to compensate by taking more exposures. Which are more affected by the increased light. Which in turn means you need to take more exposures still. Which ... (I think you get it by now).

But Planetary, Lunar, Solar, are still very much dooable--and these are are very rewarding (I've not done solar, but I assume).  And Ha can be shot (especially for inserting into galaxies).  It depends on teh sky brightness though.  My astrodark meridian has a mag of 18.52, whicjh I bet is brighter than your nautical dark Meridian in July.  That's just a guess.  My point is the time where the sky is 18+ can still yeild data.  So maybe one does not have to miss teh entire summer?

Rodd

[wimvb]

I haven't measured it yet, but the nights are definitely brighter than mag 18. Only 7 degrees further north, one can enjoy the midnight sun. Except for the summer triangle, there are basically no stars visible, and my scopes lack the focal length for any serious planetary imaging. I tried on Jupiter with my 150pds a few years ago. Never tried solar imaging, though.

Up here, summer belongs to mosquitoes and barbeque.