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M81, M82 and a long night of learning

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Hello All,

A few nights ago the weather forecast looked good; I managed to open the obsy up and start everything up at dusk, and get on-target as the last vestiges of twilight slipped away. Up until now I've been playing around with imaging - mainly M42 and the moon - and I'm still at the low end of the learning curve. This time I thought I'd give M81/M82 a go. It's the longest imaging run I've done; the weather forecast was good and I risked leaving everything running as I went to bed at 2 am. The clouds rolled in around 4 am, mercifully without the rain.

I feel like I learned more on this one night than I've done in the last 12 months:

  1. Focusing is part art, part skill, part science and if you have a less than perfect focus mechanism, part luck.
  2. A bhatinov mask is probably a good investment - I didn't have one, but the order was placed next day...
  3. It's not enough to focus once and lock down - it can drift through the night so re-checking is necessary.
  4. It's not necessary to get hung-up on matching camera rotation angles after meridian flips - Deep Sky Stacker does a great job of co-adding with rotation.
  5. Omitting flats from the run is a big, bad idea.
  6. There's a reason people list multiple software packages in their post-processing workflow - it's not easy to find everything you need in one title.

I hesitated to post the results because I know there a heap of issues which let down the resulting image. But I have to admit to being pleasantly surprised at what came out after some work (attached) so here it is, warts and all.

This is the result of 5 hours (made up of 10 and 20 minute subs) with a TOA-150 on a Mesu-200 mount, guided with a Lodestar on an OAG. The CCD is the first generation QHY8 one shot colour. One bias frame and (unfortunately) no flat - so I used the gradient remove tool in MaximDL, with poor results. A fairly gentle high pass filtering in MaximDL, then some RGB histogram equalisation and stretch in GIMP.

Criticism very welcome. I can see I need much more signal, and the absence of flats probably explains the bright left/right sides. I think my polar alignment is off somewhat, and I know the focus drifted between frames - this might explain the odd star shapes. (Either that, or there's a collimation issue, and I don't want to go there...) Is my processing heading along the right lines?

I bought a One Shot Colour CCD (years ago - I've only just started using it properly this year) because of its apparent convenience and absence of the need for filters. But I'm wondering how much different (better?) this might have been, if I'd been using a more recent mono camera - e.g. an Atik 460ex?






M81 & M82
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Not an imager, so can't comment on that side, but from a purely observation side it's a beautiful image. Well done! :) 

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Don't worry about the image you have posted- it is very good & you should feel proud that your efforts have paid off. A quick tweak with GradientXterminator in PS or DBE in PixInsight will quickly fix the gradients. The image is slightly noisy but not much & that is easily fixed with the noise reduction tools found in either PixInsight or Startools. I think the detail you are showing is great and as already commented the colours are very natural.

Focusing is a royal pain in the bum. I have automated focusing which checks the focus on every guide exposure & automatically refocuses. I measured the change over even an hour and it can be pretty dramatic as the temperature drops so I agree it is really important to keep well focused or your stars will bloat particularly on the Celestron cats.

Flats are so important. I'm having to reshoot a UVCUT flat tonight as a dust bunny moved on the filter rendering my existing UV flat useless so unless everything is kept super clean you will have to redo them many times. I suggest getting a panel as sky flats can be a pain as you should really stack 20+ to make your master.

Good luck with your imaging & hope to see more of your work in the future.

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Hi Nigel,

I hope you don't mind but I just tweaked your image to knock down the gradient and neutralise the background - it shows the galaxies off a little better I think. Keep up the good work.




Edited by derrickf
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Hi Everyone

Thanks for the positive comments and likes - which are very encouraging!  Pyrasanth, thanks for those points; I'm kicking myself for not taking flats. The focus issue is obviously really critical, and having just seen your other post on your M51 image and the focus shift you saw in your C11 image, it's clear that you really need to monitor this through the night.

Derrick - thank you for working on the image! That looks significantly better; I'm going to go back to the original and try to replicate your improvement. Much appreciated!

Thanks all,


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