Pinwheel noise question

[Marky1973]

Took a chance on the pinwheel last night with my signature kit.

45x 120sec subs at ISO1600. Total stacked was 39 of the subs + DARKS/BIAS/FLATS

There as some trailing and I appreciate this is a faint subject at 21million light years away so I was never going to get great subs on an EQ3-2. It also appears very small on the 1100d, so a major crop is needed, reducing the resolution and hammering home the trailing.

Anyway, I had a quick fiddle and knew the results were not going to be great, but was fun catching something so far away. I spent nearly an hour on Monday trying to find that bad boy and then found it first time last night. (When I realised that was probably because of the increased ISO, I could have screamed, but it was midnight and thought better of it.)

Slightly over-processed, but I noticed that there is a diagonal pattern running from bottom left to top right, almost like the weave in fabric. It maybe due to over-processing and zealous cropping, but I just wondered if this could be the result of something specific in the process, or it is just the natural noise present in the 1100d?

It is present in the JPEG crop below

and I kind of expect the lower quality with the JPEG, but it was also in the TIF stack which is attached below.

pinwheel crop.TIF

Any thought, or is just natural noise, and just another reason why I should get myself a CCD camera..... 

Thanks!

Mark

[frugal]

I think that the mottling is an artifact of both the higher noise of a DSLR compared to a CCD, and also the bayer matrix so that the software has to guess the other two colours for any given pixel.

The only thing that will help is LOTS of subs as far as I can tell.

[wxsatuser]

Last year I did 4 hours on it and it was'nt enough.

With my filthy skies it almost begs to be left alone.

[Marky1973]

Cheers both, it's kind of what I thought/hoped - noise, rather than anything I was doing that was hopelessly wrong. It was incredibly faint on the subs, and I reckon that must have been the reason I missed it - but it more than I would have seen of it visually, and it is still exciting to catch something even if the images aren't that great.....perhaps it is time to sell some stuff and get a mono ZWOASI120! :-)

[nigelbeaumont]

I had a crack at M101 last night as well - 50 x 120 sec lights, 50 x 120 sec flats, 50 x 120 sec darks at ISO1600 on an astro-modded 1000D on a 200PDS and NEQ6, with a Baader UHC filter because of the light pollution. Reasonably happy for my first go at this DSO, but there is a strong red gradient from the lights of Teesside to the east

[dph1nm]

I would try stacking without the darks.  The streaks are typical of the pattern you get if there has been some drift between exposures and the darks aren't perfectly matched.

NigelM

[lensman57]

If you had 45 of  600s subs then you'd be singing here. The galaxy really does need  long subs as it is very faint.

A.G

[Astrosurf]

perhaps it is time to sell some stuff and get a mono ZWOASI120! :-)

Oooh, I wouldn't! The ZWO is really for planatary (nice camera though!). Your 1100D is more than capable. You just need darker skies and, as A.G. says, longer subs on that object, but I know you're limited by your mount. It's tricky at the best of times as it's so faint. You've done a good job though! Get more wow factor by trying brighter objects. 

Alexxx

[Marky1973]

Thanks Nigel, I'll play around with stacking and see what happens! 

Lensman, yeah, I realised I would need more, but just like tracking down different targets! I've played around a bit with the subs and got some slightly better images than the one above, which I did push a bit to show up the noise - still lacking a lot though! 

Definitely Alexxx, an HEQ5 is on the list and I am furiously saving for one! Is the ZWO really no good for DSOs then? I thought they were marketed for that purpose as well as planetary - kind of a happy medium? Obviously the price is an attraction and thought the mono version would be a good way to get into CCD imaging and later use for guiding? Maybe not - time for an SGL forum search for alternatives! 

[frostynixon]

Hello Marky,

i think that noise pattern you see is what Tony Hallas might call "color mottle". You can get completely rid of it by dithering your mount between exposures. This is typically done by software like "BackyardEos". 

Keep gathering data on M101, it takes quite some exposure time

br

[Astrosurf]

Definitely Alexxx, an HEQ5 is on the list and I am furiously saving for one! Is the ZWO really no good for DSOs then? I thought they were marketed for that purpose as well as planetary - kind of a happy medium? Obviously the price is an attraction and thought the mono version would be a good way to get into CCD imaging and later use for guiding? Maybe not - time for an SGL forum search for alternatives! 

I'm not technical but ZWOs are like webcams and take lots of short shots that you stack. Great for bright objects like planets. DSOs need long subs, preferably of several minutes.

[Marky1973]

Hello Marky,

i think that noise pattern you see is what Tony Hallas might call "color mottle". You can get completely rid of it by dithering your mount between exposures. This is typically done by software like "BackyardEos". 

Keep gathering data on M101, it takes quite some exposure time

br

Thanks for that - I am using APT and the box titled "Don't dither this plan" is unchecked if I remember correctly - I don't have it open in front of me but will check later. However, is this something that can be done with an unguided mount? My EQ3-2 is simply motorised and I have no control of it via the software....or is this something that is done "in camera" by the software. I have to admit that I was aware of the term but hadn't spent much time thinking about it as i didn't think it was something I was able to do with my set-up, so I'm off to do some more reading! :-)

[Marky1973]

Ah, oh well, I thought I had found a cheap starter route.....or are you just trying to tempt me to buy an ATIK camera Alexxx? 

[Astrosurf]

Ah, oh well, I thought I had found a cheap starter route.....or are you just trying to tempt me to buy an ATIK camera Alexxx? 

Ha ha, no, your Canon is fine! Stick with it! Maybe consider having it modded to include emission nebulae you can image.

[Marky1973]

That was my other thought, or pick up a second hand modded model. Think it is an HEQ5 first though, then guiding.....so the ZWO might be a good option for that later down the line as well as planetary....

[Astrosurf]

Certainly you need a good mount and the HEQ5 is the minimum. It would only need to be the HEQ6, or equivalent, if you wanted to mount large or heavy scopes. Good luck!

[Marky1973]

Once I've paid for an HEQ5, I don't think I'll be buying any large scopes for a loooong time!