Washed out subs - Please help

[richyrich_one]

Hi

After 9 months building my observatory there's no doubt I'm rusty, I was hardly experienced before. But we get a clear night and got guiding working nicely hoping to get some nice long exposures but I'm getting this...

I thought I'd have a go at M33. All taken at ISO 800.

A 60 sec sub...

Reasonable I thought so decided to crank it up to 600 secs and I get this...

And at 300 secs...

And for reference a 300 sec sub of the horsehead I took back in February 2015...

Any ideas what is causing this? I must be doing something wrong.

Many thanks

[wxsatuser]

Moon!

Was Moon up?

Just wondering, why do you have LP in the earlier image but don't now.

I would think 300secs is about right but check the histogram to see where the peak is.

You cannot just expose as long as you like, there is a limit with a dslr.

[Pompey Monkey]

Was this on new year's eve/ new year's day? I live near Portsmouth and the sky got progressively more washed out as the night went on. Add a rising 60% moon to the mix and that could easily explain the lack of contrast in your subs.

As Mike says, it definitely looks like light pollution.

[richyrich_one]

It was new years eve/day...Have we had another clear night that I missed?!

Prob was just the moon then...doh! Like I said, inexperienced.

Didn't realise there was a limit with a DSLR...Noted.

Thanks for the help, clear nights are so rare I was really hoping to get something useable. Turns out it wasn't as clear as I was hoping.

[Pompey Monkey]

It was new years eve/day...Have we had another clear night that I missed?!

There was a clear spell for about an hour the night before!

[wxsatuser]

It was new years eve/day...Have we had another clear night that I missed?!

Prob was just the moon then...doh! Like I said, inexperienced.

Didn't realise there was a limit with a DSLR...Noted.

Thanks for the help, clear nights are so rare I was really hoping to get something useable. Turns out it wasn't as clear as I was hoping.

The limit is your sky brigtness.

As you found out, when the sky is brighter the detail is washed out.

To expose correctly for your sky brightness the best way with a dslr is to check the histogram

on the back of the camera. A good exposure is one that has the peak away from the lefthandside

but no more than 40% away. The general area is 20>40%, this will give you the best exposure.

If you have LP and don't use an LP filter, you can get longer exposures by getting a decent filter..

Does'nt matter what scope, camera lens, ISO or aperture you use, just expose so the histogram peak

at least reachs the first line on the histogram.

I normally expose to around 25% but have gone as far as 40%, you can experiment with your camera.

Like this 10minute ISO 800 f/4.5 exposure, the white luminance peak is around 25%.

[richyrich_one]

Thanks Mike, that's clarified things a lot and very helpful.

[ChrisLX200]

The atmosphere on that night - at least where I am and suspect UK-wide - was murky, thin high cirrus which robbed detail and added a wash of background brightness. It wasn't really visible to the eye but my sensitive AllSky cam picked it up easilly enough.  I was imaging with narrow-band filters and they offered little protection. Resultant image quality was poor and is probably also responsible for what you see in your own images.

ChrisH

[Jonk]

New year's eve night was very clear here - i considered setting up around 9pm, but there were already fireworks going off so I didn't think it was worth it.

I agree the washed out images are likely to do with the moon.

[D4N]

Unfortunately there isn't much you can do about the moon if you want to image galaxies, the moon and the galaxy are both full spectrum so any filter you use to cut out the moon glow will also remove light from the galaxy.

On emission nebulae you can use a UHC or narrowband filter as this will cut out a significant amount of moonlight without blocking light from the nebulae.

/Dan

Sent from my iPad using Tapatalk

[Tinker1947]

I can get 20 minute plus exposures when there's no Moon, full Moon it's washed out at 3 minutes, Narrowband filter will help with the Moon, but it can get costly......

[richyrich_one]

It's reassuring to know, at least this time any way, that I was a victim of the conditions and not my incompetence.