Early work in progress, M81

[Shibby]

Hi,

M81 is my second deep sky imaging target, I want to see how good I can get this image using what I think is a modest setup: 130P Alt-Az and friend's unmodified D70.

I plan to add many many more subs when the clouds clear. It's currently 19 ISO800 subs stacked together, most 30s but managed to get 2 half-decent (maybe quarter-decent) 60s subs in there too.

I didn't notice any "outer fuzziness" until I clipped the bottom of the red channel off, does this mean light pollution is getting in the way? I have ordered the £20 skywatcher LP filter and will see if that improves matters...

Any tips & advice much appreciated! Will I get rid of the noise, and if so will more detail pop out? I took darks but should I be taking 'flats'?

[jgs001]

That's a very good start. There's hints of outer fuzziness there. You really need many more subs, the more the merrier. As that's tracking in AltAz, you've got the correct exposure settings to avoid trailing, you could try 40-45 seconds you may find you can get away with that, and that's what you're aiming for, the longest exposure you can get away with, without throwing too many away. I'd also suggest keeping all the subs to the same exposure length (it's just simpler to process). The LP filter will certainly help, the SW LPR isn't as good as the CLS, but for the money it works well. Darks and flats will certainly help, take them at the time of imaging. But really... get it's sub numbers that will bring this out in the end. This is the sort of target, you want to be hitting 150 or more (quite possibly a lot more)... (do the stacking overnight, it's going to take a very long time ).

[Shibby]

Thanks, I will try to take a load at 45s. I have to use a (very unresponsive) TV remote control to open and close the shutter, will it matter that the subs vary by a few seconds?

How does it work with darks and flats, when imaging over different evenings? Do I combine darks from different evenings, or take them all at the end...? I don't quite get how it'd work.

[jgs001]

I've not noticed an issue with slight variations... even with my timer remote I get the occasional sub that thinks it's a second or 2 different..

Over multiple nights, when you put the files into DSS, add one file to the main group, this will add group 1. Remove the file from the main group. Then add the files from night 1, lights, darks and flats, to group 1. The next night, add all the lights, darks and flats to group 2 etc. That way, the calibration frames are only applied to the lights from the same session, then once calibrated, all the frames will be stacked by DSS.

Any files in the main group will be applied to all the other groups data... so if you had bias frames for instance, you could put them in the main group and leave them there. Make sure you save the file list after each update.

This is how I did my multiple nights stacking for my M31, M45 and other images.

[Shibby]

Thankyou for the advice, it will prove very useful (if those clouds ever part!)

The tracking isn't perfect, in fact some of the frames that DSS stacked did have a bit of trailing - it seem like the stacking process deals with this. Can I take it that its always better to add more subs, even if they are of lower quality? Or is it best to manually select the best ones?

Sorry for the persistent questions!

[jgs001]

I'm not 100% on that... you could always just let DSS decide on the % basis. I tend to throw everything in and see what happens, and if there's something iffy I then look to see if I've got some rubbish frames in there. I stacked all 36 of my frames on the Orion image from Sunday, but there was a real mess across the bottom, checking the last 3 were horrid thanks to Orion sinking into the murk, so I threw them out.

I never had a problem with very minor trailing when I was using my AltAz mount, the stacking process seemed to do a very good job on it, the high number of frames helps, as with the correct stacking mechanism the odd pixels in the trail are thrown out anyway as they are not in the majority of frames.

[gary1968]

Thats a great start. The SW filter should be OK, its what I use and it really makes a differance.

As for the noise, more subs = less noise.

Thanks for sharing.

Gary

[jnp]

I concur thats a good start, especially as M81 is not the easiest of targets to learn on.

As you are limited to, say 45 seconds due to the Az mount, you could experiment with ISO 1600. You will get more signal and more noise, but DSS will hopefully cancel much of the noise. It works for me on a Canon 1000d, but I don't know how the D70 will behave.

cheers

John

[Sparrow]

Hi Shibby,

Yes - definately take flats - if you get them right you'll illiminate most of the vignetting which will allow you to bring out more of the detail. And as others have said - more data will help. If you can push for 1 minute subs it will really make a difference as 1 minute exposures will collect more detail.

Regards

John