M45 - Pleiades

[OzDave]

Hi,

Here's my first attempt at the Pleiades. It was composed from 37 subs of 2'30" each at ISO400. I used the generic Skywatcher Field Flattener (0x reduction) on my Equinox 80. Subs were calibrated with darks and flats.

Conditions were excellent with no wind at all. I'm not really sure how to judge seeing.

I actually had more subs, which presumably would have been perfectly ok, except that my auto-guider decided to lose the guide star for about an hour, right after I checked everything was ok. Grrr!

I processed the subs with Nebulosity on my Mac. The image posted here has been 2x2 binned twice to arrive at this size.

This has to be one of my favourite night sky objects and I'm quite pleased with this outcome. But as with my previous post of M33, I'm hoping some people with much more experience than me can provide some constructive comments on what I can do better.

Regards,

David

[johnh]

Wow that is excellent with Merope nebulosity extending right out and nebulosity round every bright star.

You just have a gradient issue to resolve as stretching so much has caused. GradientXterminator is great software to remove this.

You could always try for longer subs but low ISO is the way to go, the more you stack the smoother the image.

John.

[OzDave]

Hmmm, yeah the gradient issue again. Seems like the same cause (probably) as in my M33 image. Do you think there is a root cause I should look for to get rid of gradients, or whether I should just accept that they sometimes occur and use software to get rid of them?

I haven't tried longer subs than 150 secs, mostly because I'm afraid my mount and guiding setup won't cope. Also, I don't do a particularly accurate polar alignment, so after say 30 mins of imaging, I will notice my guide star has drifted in DEC by a small amount (say 2-3 star diameters). I don't guide in DEC because it didn't seem to work too well when I tried, so longer exposures would maybe begin to start suffer from DEC trailing.

But, having said the above, those are clearly things I could do to improve. Do a better alignment, maybe use drift alignment. Sort out DEC guiding. Do longer subs at a lower ISO.

I have heard it said that the best images contain many hours of data, but that once you get past a certain amount of data, the benefits of having more data are less. That it, there are diminishing returns. So I guess one question I have would be, how much data should I be aiming to capture if I want to improve?

David

[Beyond_Vision]

Nice image Dave . There is quite a bit of reddy brown dust extending around M45 which can be mistaken as a gradient.

Regards

Kevin

[johnh]

You have lots of nebulosity by overstretching which has produced bad gradients so you need better control using curves or better quality flats. Making your own light box will give excellent flats if your into DIY. I would aim for 2-3 hours worth of data as the dark dust Kevin mentioned will start to come out after 3 hours.

If you overstretch even a smooth image you will cause problems but Noels Action's can remove even bad gradients although your image is pushing it a bit.

Regards, John.