What has caused this?

[gnomus]

I went out on 8 March to have a second attempt at the Horsehead nebula.  I went for 20 x 7.5 minute exposures @ 800 ISO.  (I tried a 10 minute exposure but this appeared over-exposed on the APT screen).  At first the exposures seemed OK.  I got this result:

However, from exposure 10 onwards the image looked progressively more over-exposed and washed out.  This is image 17 (after which I stopped):

These are both RAW images taken into Photoshop and converted to JPGs without any adjustments.  

Certainly Orion was lower in the sky by the time I stopped.  My first exposure was at 19:48 hours.  Image 17 was taken at 21:59 hours.  The sky did not seem significantly more light polluted (to the naked eye that is).  I should add that I was using my Canon 600D and I had an IDAS LP filter in the imaging chain.  I checked the objective lens after image 17 and it did not seem to be fogged (I was using a Kendrick Heater band).

It would be helpful to know what has happened here so that I can avoid it in the future.

Thanks for your thoughts.

[gnomus]

I should probably point out that I went ahead and processed the data anyway, using all 17 of the images captured (barring one, which had slight trailing).

I got this:

For comparison here is my first attempt at Horsehead: http://stargazerslounge.com/topic/236531-horsehead-first-attempt/

[red dwalf]

looks like your rgb colours are not aligned properly, easy fix using the histogram in photoshop or simular program.

nice end result though.

[lensman57]

There are two possible causes if your subs are getting washed out and both relate to background sky luminance. Either Mr Moon was about or your target was dipping low into the heavy LP zone. It may even be a combination of both. The HH is very low now at about 10pm at least from my location and M42 is a no..no as both are buried in the LP zone towards Manchester Airport.

Very nice capture BTW as you have managed to differentiate the Flame colour  from the Ha zone of the HH without overcooking it.

A.G

[wxsatuser]

I have seen mine get brighter if light cloud, contrails or mist happen along, as they often do.

[gnomus]

It was a pretty clear night - I don't remember any cloud. It is certainly true that HH was getting low in the sky. Also, I recall that the moon was low in the East as I was packing up. It probably is a combination of those two factors.

[fondofchips]

You are showing off really, nice picture though and always welcome to us observers on here.

[Carina Lass]

nice pic.

good result

[Astrosurf]

That's a gorgeous image. It's amazing what can be pulled out even when the conditions aren't ideal.

Alexxx

[toxic]

great image and the pink redish color cast looks like high thin cloud and at night it looks clear but it aint  

well done

[gnomus]

Thanks everyone for their kind comments, and their thoughts about the raw image deterioration over time.