Couple from last night...

[JohnP]

Managed to grab a few quickies last night before clouds arrived at 11.00pm I added about 1hr of L data to my m81 from a few weeks back & also did a few frames centred on m86 (bottom of Makarian Chain) - I wanted to do this as I love this area of sky & always fancied doing a mosaic of some kind. Hopefully go back to it later. Normal setup - ZS66 + Atik 16ic.

Cheers,  John

[Ian]

I particularly like that second one John, very nice both of them.

[Fay]

You did well, John!! Really well focused, as usual, & really good images, you know I love the bottom subject. What length subs & how many?

[JohnP]

Thks Ian & Fay.

Details on bottom was total 60mins - combination of 3 & 5min subs - CLS filter. This is actually quite hard to process as SNR is low (galaxies are small with lots of space in between) so keeping noise down was quite tricky.. It probably needed longer subs to bring out some of the structure in galaxies but it is tough to see at my image scale anyway..

Thks again,  John

[Mike]

Really nice John. It would be great to see a mosaic of this area.

[MarkS]

Those are good "quicky" images John.   There's some good detail appearing in Makarian's Chain.  At that scale, a mosaic would look really cool!

Mark

[JohnP]

Hi Mike, Mark - Appreciate your comments thanks. Hopefully, I will get a chance to add more to the mosaic.

Cheers, John

[RobertM]

Excellent results John.  You did really well to keep the noise down, conditions were none too great that night, especially later on.  Not sure which I prefer, they are both good but I'm certainly looking forward to seeing the finished mosaic of the second

btw. There's no noticable coma on the ZS66 - could have been the small chip but did you use a FR/Flattener or was that taken at native F/L ?

[JohnP]

I think the small chip helps a lot - I am also using a WO 0.8X field flattener but to be honest it's just to give me a slighter larger FOV not because of coma issues. I definitely need the flattener though when I image with my EOS through the ZS66.

Noise - Yep that's interesting. I made some interesting discoveries yesterday which has changed the way I now process. I couldn't understand why my images always seem noisey compared to Chris's & yours for e.g. so I have done some experimenting & found that if I process with Flats & flat darks only I get a much cleaner noise free stack then if I use bias frames for my darks (I have never used proper darks as 16ic doesn't seem to need them & they take too long to do) I normally use a stack of 16 bias frames but seems these introduce quite a bit of noise. I even tried taking 100 bias frames yesterday & using those but there was still more noise then if I didn't use at all...

So for now I think the only preprocess I will be doing is flats & flat-darks...

John

[RobertM]

Yes, the noise issue is always interesting.  It seems that with the Sony chipped cameras subtracting bias can have little benefit or even a negative impact.  I must admit I usually only subtract Bias (when the filters are clean) and the difference is minimal but it should result in cleaner images using flat darks as well as flats.  I need to experiment too but will certainly give your method a go next session.  Thanks for sharing your experience.

[RobertM]

John, Excuse my ignorance ... Do you take flats as normal (i.e. 1/2 well capacity) then place the cover on the OTA to do the Dark Flats with the same exposure times.  If you do it that way then do you take them at night (I assume) or doesn't it matter ?

[Fay]

Robert, don't go there. Can't believe you have got away with it all this time! No dust bunnies!!!

[JohnP]

Hi Robert - I built myself a very simple lightbox. What I do is when I've finished my lights I put the box on the end of the scope & then take flats to fill well depth between about 1/3 to 1/2 (not critical) - this is normally around 17-20,000 ADU. I then put the cap over the scope & take the same amount of darks using the exact same exposure. That's it... Obviously, the length of time to take the flats depends on how bright your light source is & if you have any filters in place (like Ha) but typically they take between 2 to 12 secs. I normally take 20 flats & 20 flat darks (but I may try taking twice as many next time to see if it effects noise). Below is image of the flat I took when I took the M86, m81 images recently - It always amazes me how disgusting it looks... For some reason I always get the dark bit across the corner so I'm guessing my camera or something must be not central (strange cause it's in a refractor so should be symetrical...)

HTH, John