Exposures and Light Pollution?

[PRejto]

This is probably a pretty obvious question, but I have yet to see a clear answer in print. The weather is lousy again so rather than fret about my equipment rotting I shall ask:

If one is imaging in a light polluted environment the idea would be, I assume, to take the longest sub exposure possible before LP starts to ruin the background and image. Fine. Is there some magic number for sky background (ADU) where you would say that's the limit? Would that same ADU background limit value remain the same if a light pollution filter is used? I've been experimenting with a Hutec LPF that so far seems to give me pretty good results, though at the expense of longer exposure. I'm just wondering how far I can push the subs (forgetting about guiding, flexture, etc) before knowing I've gone too far?

Thank!
Peter

[leon]

Hi peter, the Hutec range of light pollution filters are probably top of the range, and when i used them the exposures could be pushed to 4 minutes each with minimal deterioration to the sub.

And my place was pretty polluted

Leon

[LewisM]

I have been doing 5 min subs with an Orion Skyglow. Gives a little aqua tinge to some stars, but fixable. Found it REALLY brought out the reds and killed the LP from mercury vapor lamps .

[cometcatcher]

Generally yes. However sometimes odd things happen with LP filters if the sky has a colour cast they are not designed to handle, blue moonlight for instance.

A case in point, I was trying to shoot the Orion nebula in moonlight with a CLS filter. After colour balancing, the image ended up noisy in spite of being "well" exposed, especially in the red channel. While the white light histogram showed good separation from the left edge, I discovered that when splitting the histogram into red, green and blue that the red channel was clipped. It had insufficient exposure. Double stacking a Baader semi-apo filter which reduced the blue channel helped get the red channel off the edge.

After that experience, I would suggest when using LP filters to keep an eye on each colour channel to make sure they get enough exposure.

[PRejto]

Thanks for all the quick answers!

I looked at some older data on the Helix Nebula. A 4 min lum image through the LPF gave me an average backgroud of 1339 and the nebula average of 1545. (measured using CCDStack). I guess my initial question of this thread is, restated now differently, by looking at these two numbers for a 4 min exposure, can I make a guess as to how much longer I could expose before the background would become too bright, or when the background would begin to interfer with the nebula?

Peter