M51 - 6 hrs mono CCD RGB + 10 Hrs DSLR

[AndyUK]

Just less than a year ago I spent 10 hrs on M51 (40D, Clip filter, MN190) and got a reasonable result -The noise wasn't great but at the time I was pleased with it...

Due to the increase in noise, I bought a mono 314L+ and noting recently that M51 has been coming within reach at about 2am, I had a few nights to capture some frames on it again with an Equinox 80. The original aim was to compare 10hrs DSLR with 10 hrs CCD (4hrs L, 2hrs each RGB, all using 600s subs, as per DSLR image), but the resultant CCD RGB image was very washed out (maybe the RGB exposures were too long?) so I resigned all the frames to the archive.

But being stuck in a hotel in Newcastle for the last couple of nights, I thought I'd see what happened if I combined the 40D frames with the RGB frames using Registar. It's certainly has less noise and more detail than the 40D image alone (although due to a lack of flats on the CCD frames there's a telling dust mote right above M51!), but the addition of the 40D data has certainly added the missing colour.

I guess you could call this a sort of salvage image as I wouldn't want to make a habit of spending 2 nights DSLR imaging and then another night on the RGB, but it has made me wonder about the possibility of taking DSLR frames for colour (with perhaps some heavy denoising) followed by a night of Luminance... Of course in a perfect world I'd have a second OSC CCD, but there's a slight problem of funding!

[Deneb]

I think it's come out rather well, it certainly deals the DSLR back into the game, when everyone is going mad on the CCD Bandwagon....

You will find that certain scopes are quite receptive to DSLR imaging, the MN190 being one of them.

Nadeem.

[AndyUK]

Cheers Nadeem - It certainly does open up some possibilities, especially at this time of year... As I noted, it is a bit of a salvage job but a 50% crop probably shows the additional detail the CCD captured (although I do wish I'd taken flats for the CCD now!)

Unfortunately I don't really have too many "stock" DSLR images to try this with, so I guess I'll need to dig it out again and have another go.

[Zul]

Amazing!

[Quatermass]

That looks great lots of detail in there, the back ground looks like it has a fair bit of colour noise but that would be easy to get out in PS using inverted layer masks. This is one I really want to go back to when I get a better mount sorted out such a cool galaxy. Have you got the tiff files so we can have tinker?

[MartinB]

Plenty of detail Andy. Not sure why the luminence made things washed out. Have I got this right, you haven't used a luminence for this image? Certainly having a lot of colour data shouldn't be detrimental to an LRGB. The thing that can hit the contrast when adding luminence is having the black point well over to the L of the histogram but you will have sorted that so can't think what the problem was.

A DSLR would work well for colour and the noise wouldn't be an issue since, provided you protect the stars, it's amazing how much you can blur the colour...so long as you have a decent luminence! However, the big time saving is if you can be collecting the colour with a DSLR at the same time as you are getting lum on another set up, otherwise, at 1000mm f/l and beyond I would prefer to bin the colour and spend most time on the luminence. Some people aren't happy with binning (concerned about bloated stars) but you get loads of extra luminence signal. Unbinned, DSLR and OSC imaging takes a lot longer to grab the same amount colour data, time that could be spent on the L.

[angusb1]

Lovely star colour Andy, nice and subtle. I've thought about trying CCD for luminance and DSLR for colour in the past since I have a mono 16HR and a 20D but wasn't sure how it would turn out. At the time Olly P and Psychobilly suggested Registar so I might have to buy a copy and see. I wasn't sure how it would work out if the pixels on the two cameras were different sizes but I think you've answered that question.

[angusb1]

Just out of interest, have you considered using the 4 hours luminance from the 314 with the 10 hours from the 40D?

[AndyUK]

Quatermass - I'm not sure if this is what you wanted, but here's the two registered/cropped/padded tiff files (You'll notice that I might have been a little heavy handed painting in the detail using a high pass layer mask ).

http://dl.dropbox.com/u/4210964/M51%2020110420%20P3%20DBE_reg2_cp.tif

http://dl.dropbox.com/u/4210964/M51%20RGB%20P1_cp2.tif

(I've never come across "inverted layer masks" before though so have no idea what they do or how to use them, but if they get rid of digital noise then I'm certainly all for it!)

Angus - Registar's excellent - It's REALLY simple to use and it's well worth the cash for combining RGB frames, frames of different Focal lengths or when trying to do co-ops with people (either with different scopes / FL and/or different framing). I must admit, I hadn't considered seeing how the CCD L works with the 40D RGB. (If the forecast is wrong tonight (allegedly due to be clear, but can't see it myself at the moment!) then I might have a go at it...

Martin - Oh it would be SO nice to have two rigs imaging at the same time, but it would completely fill my back garden... and totally melt my credit card

I had quite a struggle processing the CCD LRGB version - I took all frames unbinned, 10 mins each (24L, 12 each RGB). The L frames all had a pretty light background (combination of LP & Moonglow - I've since bought an IDAS LP filter to try and get rid of the former hoping it'll help me get better contrast data). I found it really difficult to get much colour saturation and the colour I did get wasn't going the right way at all! Moonglow may have had an impact, and I also considered that possibly the RGB frames @ 10 mins may have oversaturated everything (maybe 24x5 mins ea would have been better?).

I'm still not confident binning colour though - As you mentioned, the stars appear to get a lot bigger, and I haven't learnt how to create/use star layers either (which I'm assuming I'd need to get to grips with?). Also the idea of blurring the RGB channels causes my brain to reboot as at first it doesn't make sense to blur the detail away (although I do appreciate that the detail is supposed to come from the L... It's just a little hard for me to swallow at the moment, but I'm sure it'll come!)

I know you're absolutely right in saying that it takes AGES to get sufficient frames unbinned to put an image together reasonably quickly and if I can get away with keeping the resolution under 2 or so, then binning does make sense.... Yet another thing I'm going to have to add to the list of things to learn and try out

[MartinB]

Obviously it depends on your sampling rate but I've found that binning at focal lengths above 1000mm focal length with a 6.5 micron pixel camera causes minimal star bloat and this responds very well to deconvolution. A much bigger cause of star bloat is taking longer subs as stars approach saturation. I think the biggest cause of bloated stars with binned subs is that people use subs which are too long. To avoid this it is important to have some idea of your optimal minimum sub time. When binned I generally like to keep my background sky glow to around 300 - 500 ADUs (this figure is mainly dependent on read noise). This typically means exposures well under 1 minute. When people pride themselves on the length of subs they can take without trailing this seems counter intuitive. I recently gathered some colour data for a leo triplet image using 2 minute binned subs and really regretted it although, even then the stars were only minimally bloated compared with the unbinned luminence.

Have you ever tried blurring an RGB layer when there is a luminence sat on top. It is amazing, you can blur it to kingdom come without it impacting on the overall RGB image.

[SJJ]

Really nice image Andy. Can't wait to image this one myself..

[Uranium235]

Martin has got it exactly right. If you have a good luminece frame you can use NR as much as you like on the RGB.

This CCD/DSLR method is something that would work really well on galaxies, as youve just proven. Im not sure on nebulas.... give it a try! It would be interesting to see a Ha layer with DSLR colour.

Hmmm... maybe I wont be selling my 1000d after all

[AndyUK]

(Cheers all! )

Martin - I seriously like the idea of taking 1 min binned RGB...! 3 hours should easily get all 3 filters in one night, and if this was coupled with 20x10 mins L, at this time of the year (with a flip) that's a complete image in one night (if only I got a complete night without cloud ). I'm guessing that binning with 500mm FL with the same camera isn't quite so forgiving though, but even so, it's certainly given me some very interesting food for thought for closeups with the MN190 (thanks!). Deconvolution is something else I've toyed with but ended up with slight donut stars . I'll keep trying though...

I've obviously never properly processed a true LRGB image if blurring the RGB to hell and back will still produce a reasonable image - It still sounds counter-intuitive to me, but with so many people using just this process, I'd be an idiot not to try it out... Who knows, I might even begin to start appreciating this mono CCD imaging lark!

Ha with the Equinox 80/314L on top of RGB with the MN190/40D sounds intriguing - Unfortunately the HH/Flame nebula are skimming my roof tiles at the moment, but that would make a very interesting target for this experiment... (don't sell the good 'ol modded DSLR just yet!)

[ollypenrice]

I'm going to put in my two penno'rth for CCD. Fast, accurate, sensitive, low noise. The fastest single camera you can use for astrophotography is a monchrome CCD.

The trouble with using DSLR for colour is the lack of star colour seen in DSLR images. Exceptionally you do see some decent star colour but it is rare, maybe because of the lack of well depth causing the stars to over expose. I don't know. And while I do combine OSC and mono CCD images, obtaining the colour with the OSC is not faster than with the mono, it is slower. However, I do it to avoid running around like a blue whatsit-ed fly whirling filters around left right and centre all around the garden.

I know I'm a 'dark time millionaire' living where I do but I agree with Martin that short, binned colour subs heavily saturated and de-noised can still look good under a declent luminance layer. I don't bin myself other than in Yves' monster but it is a good compromise.

If you have a thin RGB layer that looks washed out by the Lum, apply the lum at about 30%, lift the saturation above the noise limit, and de noise by using noise reduction, despeckle and Gaussian blur. Then apply the Lum again, nearer 60%, say. Again denoise and repeat.

Meanwhile back at M51 ... it looks nice and strong. You could lasso the bunny, feather and lift it out in Curves.

Olly

[swag72]

That's a nice looking image Andy, the colour from the DSLR has worked well. This is an interesting thread, only 1 min RGB subs? While I do have more clear skies than you guy in UK, that still sounds quite tempting. So the Luminance is really the most important of all and needs time and care - So I keep learning!

[AndyUK]

Thanks for that Olly - You've given me yet more pointers for another salvage attempt using just the CCD frames.... I guess I'm trying to "cheat" by using the DSLR in this fashion (I know the arguments for CCD cameras ) - In a perfect world (where cash wasn't necessary), I'd also have an OSC CCD (and also another mount!). I just did a quick check and I only really have 2 DSLR images I could combine like this, so I don't think I really have that much opportunity to "save time" by re-using them...

I'll try out your suggestion and see how I get on... (and also try and lasso that bunny out just for posterity!). This mono processing lark just got yet a bit more complicated... but I guess that's half the fun / challenge of it (I guess ).

... So I keep learning!

Cheers Sara - You and me both...!

[ollypenrice]

When I see people using DSLRs - or when I try to help guests use them in my scopes - it strikes me as a nightmare of complexity. LRGB is so easy by comparison - it seems to me. Focus and shoot red, green, blue, Lum. Bin the colour if you like. Put the lights darks and flats into AstroArt. Choose your settings. Click. Align your RGB (Registar is best) and click again. Then into Ps or PI and have fun.

Olly

[AndyUK]

When I see people using DSLRs - or when I try to help guests use them in my scopes - it strikes me as a nightmare of complexity

I guess it depends where you're coming from... and what you consider "complex"! If coming from DSLR imaging in the first place, mono imaging so far has seemed to be a completely different prospect with all the additional post-processing requirements, but then again I'm only at the foot of this ladder at the moment... I'm sure it'll all click into place one day (hopefully ).

I'm very much looking forward to having a crack at taking reduced exposures / binned / denoised / blurred RGB's with mega-Luminance as that may get rid of the noise that I seem to be creating during post-processing!

[Deneb]

Personally, I think it's the other way round Olly, I certainly would not thrash the advice down someones throat of doing ccd if you have not done dso imaging before... & actually being honest about it - DSLR imaging is much easier & cheaper then CCD imaging... in my perspective anyway...

Nadeem.

[Looking Up]

As an owner of a DSLR I think its easier to capture the frames but more difficult to get a final image anywhere near in quality to a mono ccd imager, you end up tinkering in software trying to squeeze out the detail! But as a first step I think they can't be beat, especially if you own one already! Oh and if I had a "salvage" image of this quality I would be very pleased!!!

Spencer

[AndyUK]

Thanks Spencer... I certainly found capturing data with a DSLR a fair bit easier (especially when I didn't need a laptop ), but now that I have the mono CDD I still end up tinkering with loads of bits of software and functions to try and get the best of the data out...!