Some advice on Light Frames please

[Nigel C]

Hi All  - last night I tried to image M101 taking 18 subs of 3 mins at ISO 400.  After processing them there doesn't seem to be enough data above the noise.  Admittedly I only took 5 Darks and 11 Bias.  The first shot I took was at ISO 800 for 131 secs but there appears too much light pollution in this.  I have attached the 2 images and would welcome any comments on whether or not I should take longer subs say 6 mins at ISO 400 or shorter subs at ISO 800 - or am I missing something?

Thanks

Nigel

[silios]

Can you share a single unprocessed light frame ?

It looks like the camera is not getting enough signal.

I would try 240s subframe at ISO800.

At first light pollution will make the image look "awfull" (maybe burned even) but that can be processed out eventually.

[Nigel C]

Hi - These are the raw Canon frames just condensed in Lightroom to JPEG's to fit the 1MBit limit.  I haven't made any other changes to it.

[silios]

Hi Nigel

oh, then i would say its not getting the galaxy and try to bump up the ISO depending to which Canon you are using up to 1600.

Just for reference have a look at the following images (they are from an old canon 350D with the original filter removed)

Single frame 240seconds exposure at 1600ISO.

The first one is to illustrate the amount of LP i got , and the second one is with the gradient - background removed.

[ecuador]

Don't play with the ISO like that to adjust the "brightness", this is not daytime imaging. Each camera has an optimal ISO, which is usually ISO 800 for older Canons and ISO 1600 for newer ones (say last 5 years). The light frames should not look good. With light pollution they can look really washed-out, however post-processing will help and that should not stop you from taking longer exposures - it will take much more exposure time for light pollution to actually saturate your images, the ones you posted are not nearing saturation at all.

[Nigel C]

Thanks Silios I will try a few subs at ISO1600 for 4 mins - perhaps 20 shots and see what happens...

[Nigel C]

Thanks ecuador - will try iso1600 tonight - with 4 min subs

[Scorpius]

Thanks ecuador - will try iso1600 tonight - with 4 min subs

Not sure 4 min subs are enough for this target

[Scorpius]

Didn't intend such a short answer but must have hit post by mistake (darn smartphone!) I was going to say up to 10 mins if tracking will allow and LP doesn't become an issue.

[Adamar98]

Have you stretched the image? You should have some data there to use.

[wxsatuser]

Use your histogram to get the right exposure for your sky conditions and camera settings.

The histogram peak wants to be in the area 20>40%, 20% should easily clear the noise floor of your camera.

I normally aim for 25% but have gone to 40% on some nights.

A ten minute exposure of M31 f/4.5 ISO 800 with a clip filter, histogram approx 25%

[Astrosurf]

I would never attempt this faint object without a light pollution filter. I use an Astronomik EOS Clip-in Filter. It's brilliant, though a tad pricey. I got mine second hand. I've imaged this object but at ISO 1600 and 10 x 7 min subs. But the more subs the better! 

Good luck!

Alexxx

[Nigel C]

thanks Alexxx - I did buy a Skywatcher LPF (£26) - I assume the Clip-in filter is a lot better?

[wxsatuser]

Don't know the characteristics of the SW filter but the Astronomik is about as good as it gets.

[Astrosurf]

thanks Alexxx - I did buy a Skywatcher LPF (£26) - I assume the Clip-in filter is a lot better?

Ditto wxsatuser!

[ecuador]

thanks Alexxx - I did buy a Skywatcher LPF (£26) - I assume the Clip-in filter is a lot better?

The Skywatcher LPF is the same category as the neodymium filters. They cut less light than other filters, so some people like them for small apertures, but they don't help much with light pollution. I would say my skywatcher LPF is a little worse at cutting pollution than a cheaper Chinese "neodymium" clone that I have.

Don't know the characteristics of the SW filter but the Astronomik is about as good as it gets.

I wouldn't say that. It is only as good as it gets at cutting pollution, however it also cuts more of the useful light, especially if you are trying to shoot something like a galaxy or a cluster instead of a nebula. There is one more category above it where filters like the IDAS LPS and Orion Imaging put back some of the coarsely-cut spectrum and allow more useful light gathering.

[ecuador]

Also note that if you are using a modded DSLR or a CCD, you also need a UV/IR block filter in addition to most light pollution filters, except some expensive ones like CLS-CCD, IDAS LPS, Orion Imaging.