Cone Nebula - a comparison

[centroid]

I wouldn't have bothered 'posting' this image, but I thought it would make a good comparison between the sensitivities of a colour CCD camera, and a mono one.

Last night, both Roger (Celescope) and I, imaged the Cone Nebula (NGC2237), using ED80 scopes. I used my SXVF-H9C 'one shot colour' (OSC)camera, and Roger his Atik 16HR mono. In my case, with just an IR blocking filter, and Roger an Ha filter.

We both used 15 mins subs. I managed one Hour 45 mins, before the cloud/seeing called a halt to the proceedings, and Roger just one hour 15 mins.

With 30 mins less exposure, the mono camera captured far more detail than did the colour camera. I estimate, that on faint objects like this one, the OSC camera would require twice the exposure time of the mono camera, to capture the same level of detail.

BTW, yes I do realise that I should have angled the camera differently to encompass more of the object in the 'frame', but I've now learnt from my mistake.

I've included Roger's image here too, so as to have the two images alongside each other. I'll pay you later Rog.

Dave

[Helen]

Hmmm... interesting comparison there Dave! I suppose that double the exposure time would still be shorter than doing a LRGB though.

Helen

[jgs001]

Dave, that's a very interesting comparison thanks.

[barkis]

Well it was a good exercise for the purpose of comparing the two cameras. It is plain that the OSC camera does require longer exposure to match the Mono's sensitivity. The image, and your explanation, make it quite evident Dave.

Whilst I quite like mono images, especially ones covering objects like this, there is the choice of tricolour with the mono, which obviously requires the capture of more data using the necessary filters. That there is the increase in time, together with the processing, to get the final colour image. Is it therefore more advantageous to use OSC for the required time to capture an image of x quality, or the mono route, with filters to get an image of the same quality.

I hope the question is not too confusing, perhaps I could have worded it better, but I think, and hope you can decipher what I am trying to say.

Cheers.

Ron.

[steppenwolf]

Dave, an interesting comparison although not empirical because of the different filters used and their effect on drawing out the detail against the background 'haze' of colour. I have often wondered what the difference is but as Ron and Helen have said this is complicated by the need to take 4 sets of images for LRGB and then combine and process them!

What I have learnt is that certain objects fare very well as grayscales or even false colour renditions instead of 'true' colour images and although the OSC handles these very well, the images have to be binned 2 x 2 to get the quality and this reduces the resolution, of course - a mono camera, for obvious reasons, is on 'home territory' here!

[centroid]

As you say Steve, a better comparison would have been possible if Roger hadn't used an Ha filter, but our imaging sessions weren't planned as a comparison exercise.

It was only on seeing Roger's image, and comparing it with mine, that the idea of 'posting' them alongside each other might be useful, came to mind.

While Ha will of course bring out or emphasise some detail, it won't necessarily add much in the way of captured photons.

Having previously used another OSC camera (MX7C), and then converting it to mono, I am well aware of the increase in sensitivity that was achieved. The 'bottom line' is that you can't put colour filters (Bayer Matrix for OSC cameras) in front of the CCD, without losing some light throughput.

I've been down the LRGB route, and yes, it is a more scientific way of achieving a final colour image, but I wouldn't say that the colour rendition was any better or worse, assuming that the correct balance between the separate RGB frames is achieved.

The main advantage with LRGB, is that the Luminance frames are captured as mono images, using the full sensitivity of the mono CCD, and of coarse Ha filtering can also be employed.

The RGB frames can be captured at 2x2 binning, which offsets the loss of sensitivity through the filters.

I should point out here, that with a OSC camera, binning at 2x2 destroys the colour data, as the Bayer Matrix is set for full resolution only.

You can of course image at 2x2 binning with an OSC camera, which although the Bayer Matrix is still physically there, the binning will effectively increase the sensitivity, in mono and at lower resolution.

For me, its a time thing, in that given the limited number of clear night we get, I found that separate LRGB imaging was taking too much time, and hence I went back to OSC.

If I lived in a location where clear skies were the norm, I'd probably use LRGB, but as I live in the UK, for me it has to be OSC.

The 'bottom line' is, the choice of OSC or LRGB, is really down to personal preference, and "each to his own" as the saying goes.

Dave

.

[CELESCOPE]

No chance imaging in my location without the HA filter Dave , all i would get is a white image eheheheheheh , i thought the HA response was pretty good with the OSC, seems i get plenty from my beast HRC,i see Steve L managed a pretty good Cone with his QHY8 , he did 2x2 binning , dont know much about that ,

As for me i think the HRC would benefit mainly for the brighter objects, faint stuff will prove very hard for me , although i have,nt pushed it yet to 30 min subs ,

will try this at some point , but the weather is holding things up big time for any experimental work.

Rog

[SteveL]

...he did 2x2 binning , dont know much about that

My cone images were captured at 1x bin (normal), were stacked with 2x drizzle, then 2x2 binned to bring the image size down and SNR up....

[steppenwolf]

The main advantage with LRGB, is that the Luminance frames are captured as mono images, using the full sensitivity of the mono CCD, and of coarse Ha filtering can also be employed.

I agree completely - this makes absolute sense.

I found that I could take Ha images perfectly well with the OSC binned 1 X 1 and then convert them to mono but I got a much more nicely defined image detail with 2 x binning, deliberately 'destroying' the Bayer Matrix coding but the downside was less resolution and ironically, it is better to have a higher resolution luminance channel (1 X 1 binning) and a lower resolution set of RGB channels (by whichever means you prefer).

I too went down the OSC route for reasons of time in our fickle climate!

[MartinB]

Very interesting comparison Dave. I don't know whether an OSC saves time or not. The key to LRGB imaging is a motorised filter wheel and a set up sequence so that everything runs through automatically. Maxim will keep doing sequences of LRGB. Capturing the RGB takes 3/4 of the time of the luminence so LRGB takes 1.75 times as long as mono. But that might be similar to the exposure time required by OSC. As far as processings concerned, combining the colour subs is very straightforward so I don't think there is much of an advantage with a OSC, you still have to work on the colour.

I will have a much better idea when I finally get my QHY8 to work