First go with Flats

[broadsword]

After many months of putting it off I've finally got round to trying flats, involving a method using a white t-shirt, a 37 inch plasma TV screen and a white Powerpoint slide. Whilst the results have made post-processing far easier by not having to fight vignetting there do seem to be some new challenges along the way and I wanted to get some feedback before progressing too far. My first target was M81 and the result after stacking and some processing in DSS (colour balancing, increased saturation, fiirst stretch) is shown. The exposure details were 35x120secs at ISO 1600,darks, bias and flats were used.

Although the corners are much better there is a light gradient bottom left and some streaking in places. There's also a strange artefact on to the lower right of the galaxy, plus a lot of pixel trails, although the latter may be due to the pixel removal option in DSS for some reason having turned off. The light gradient proved impossible to move, even using DBE in Pixinsight.

Overalll, the final result after processing in GIMP (also attached) was way better than attempts without flats, but any comments appreciated...is there anything obvious I could do to get better, as flats were the last big step to try.

[Jessun]

Nice work Neil!

The streaking is your drift - and this can sometimes affect the whole image. The cure comes in three steps: Do a better polar alignment, use medium to large dither for the guiding, and finally allow the camera to cool between subs. These steps will work wonders for streaking.

Overall I think it's a great capture there! Brilliant result after all the trouble with the mount. The background is a notch too dark for most though. Your pixel value for the black is in the 2-5 range. I can't give you a direct figure, but maybe 15-18 is something to start with, and see what you think. By 'clipping' the black as a general approach, you risk losing the very faint information in the darkness. There's stuff hiding pretty much everywhere if you have enough data...

/Jesper

[broadsword]

Hi Jesper, thanks for the kind words. Glad you remembered the saga of the GOTO alignment, it was quite a thread. Don't think I'll be guiding for some time yet, but I'll look at the other ideas.

On the topic of the dark area pixel values, I thought they were around 10 to 15 on the original file! Not sure why they're so low now....I needed to work hard on that gradient, so I think that led to the background going dark and some loss of faint detail in the galaxy...

[ollypenrice]

I go for a background sky value of about 23 these days. A dark site helps because there are few gradients to deal with, though they do appear sometimes.

It looks as if your flats have worked well. If the gradient is genuinely present in the sky, as it might well be, the flats won't remove it. In PI did you try raising the 'tolerance' value?

This first step into using flats is really important. Keep at it!

Olly

[broadsword]

Hi Olly, yes I did as far as I can remember- I was using the Astroshed tutorial and needed to turn the crosses white. Unfortunately my free PI trial period has ended is I can't go back and tinker, unless I commit to buying. In the end, I had to resort to cropping off part of the bottom LH corner and cloning some dark background!

I've done f few more since. The flat method I'm using (as above, generally exposing 10-20 frame at ISO 200 for 1/5 sec) works well but the colour often looks a bit green. I also find the image dark and detail low, even at 1600 (I was hoping to drop to 800). Do you think it is just down to a slow scope and being limited to 120 sec exposures? Hence will just have to do more frames?

Thanks again.

[ollypenrice]

Slow scope, long FL, small pixels... It's never going to be a photon hoover. Still you have a result.

If you have Ps then Gradient Xterminator is a good and affordable plug in. It's by Russ Croman.

Olly

[broadsword]

Yes, thought so Olly. I've spent the last 9 months trying to improve to the point where I know I'm getting about the best I can expect from my set up. If someone of your experience thinks so I'll move on to focus more on the objects themselves than the technicalities.

Still, going to invest in an ED80 in the near future although need a way to sneak it past the CFO....