Seeking your thoughts

[AXE]

Hi all

I'm considering purchasing another camera to take over from the 294MC Pro. So at this stage i'm wondering whether to keep at it with a OSC or move to Mono.

The ZWO 1600MM is on sale at the moment with a filter wheel and filters. But I also see that ZWO is releasing a new OSC next year that looks like a pretty big upgrade on the 294, the ASI2600. It's got a price tag to match but i'd be interested to know what people think. Should I save up for the better OSC or go to mono?

Thanks,
Alex

[Imme]

Hi Alex,

I had (still have) an asi1600 OSC and I loved it. Easy to use and suited my imaging style.

After seeing all the narrowband shots doing the rounds I thought I'd take the plunge and go mono to allow me to do the narrowband and LRGB imaging. I purchased an asi 1600 pro. The ability to image with the moon with NB was an attractive option too!

To be honest I regret the decision and miss the ease of use with the OSC. Life becomes a lot more complicated balancing image captures and making sure you get enough exposure on each filter. The issue I've found is that I may plan an image run...get great exposures on LR&G....but clouds or some other issue comes up and I miss out on B! Yes, I can wait until the next clear night to grab that elusive B filter but with the OSC it didn't matter....I just processed a little less exposure time.

I've also not seen the image quality i was getting with the OSC. I'm sure it's my abilities and in time I will see an improvement but hey....is the improvement in images worth the extra effort? (I'm not sure on this yet, the jury is still out).

I have evn tossed up removing the mono and replacing it with the OSC that is still sitting in the cupboard.......the only reason I haven't is I like a challenge and from everything I read mono imaging is supposed to be the only was to go

My 2 cents from personal experience

[AXE]

Is there any real difference in resolution between the two? I keep reading that because the OSC uses bayer filters you only get an effective 1/3 resolution and then the software averages the pixels.

[Camelopardalis]

Yes, _any_ OSC camera interpolates the data to give a full resolution colour image (actually, the interpolation is part of the debayering process that runs on your computer, not the camera itself, which just captures the data). In most cases it goes unnoticed. However, if you were to do a comparison of the same camera in both mono and OSC, you'd likely see a small difference, depending on the conditions and your focal length.

It really depends on whether you intend to go narrowband...this is most effective with mono cameras, and with good narrowband (i.e. narrow) filters you can image certain types of target for a little more of the month (so long as the Moon isn't too offensive).

[Merlin66]

AXE,
The results from OSC is definitely not has bad as that!
The De-Bayering process actually sets up "virtual" pixels (to form the image) with the same size and distribution as the original chip.
(This is why you don't see the individual RGB pixels at the end...)
The overall impact IMHO is at least a 80-90% resolution of a similar Mono.
(It depends on wavelength)
http://www.stark-labs.com/craig/articles/assets/Debayering_API.pdf

[Imme]

I'd back Ken on that figure......I haven't seen any significant difference

[AXE]

Ok, so OSC isn't really that much worse qualitywise. I've got a fairly dark sky so light pollution isn't too much of a worry for me.



My issue with the 294MC is that even with all the calibration frames it still never seems to provide a processed image free from weird colour artifacts (some vignetting and an odd glow that that I find really hard to get rid of in stacking and post). Is this something that OSCs are known for? Is it easier to clean up a mono image?

[The_bluester]

Do you mean a square'ish halo not quite centered? The 294 is more or less known for that but I Have found that with lots of subs you need to stretch the data pretty hard to see it.


Are you using flat frames? That should sort vignetting, but on the 294 the flats need to be taken at an exposure time of around 5 seconds or more of they will cause problems. The sensor in the 294 is a SoC (System on a Chip) design and below something like 4 seconds (I have not found out the exact time) the exposure is controlled by the SoC, and above the threshold, it is done by the external electronics made by ZWO. That means different electronics are active and the noise profile changes.


It is like the difference between a timed exposure and bulb on a DSLR.

[Stefan Buda]

Indeed, although I love the quick results that OSC cameras can provide, there is another drawback, apart from the slightly lower resolution. The tiny RGB filters that make up the Bayer mask are not as good at colour separation as proper full size filters, so it is harder to produce a nice colour balance.

[AXE]

Didn't know that about the flats, i'll try it next time I get out. I've been doing <1 sec exposures.


There is a big round glow that takes up about 1/3 of the image in the bottom right corner. It's not an issue when I only do minor stretching but it quickly gets out of hand when I try to bring out detail.

[Paul Haese]

If you are doing less that one second exposures I don't think you are getting above the noise floor and gathering enough signal. Your sub lengths should be at least 30" to 2 minute. This will give a reasonable signal length for this type of sensor.





The big round glow could well be a light leak somewhere. Or amp glow perhaps, though I don't know that sensor. Perhaps could you post an image of one of the affected frames?

[AXE]

My lights are usually between 2 and 5 minutes. Flat frames are <1 sec.


This image shows what i'm talking about. It's been stacked in DSS and then moved to StarTools and developed. I flip both vertically and horizontally when capturing so the glow seems to be right side. Only happens when the image is stretched a fair bit.

[The_bluester]

This is a long way from a finished image yet but you should be able to get a result more like this out of the 294, this was shot with mine, this is from 20 X 300 second subs. But you will probably benefit from longer duration flats. I aim for around 8 seconds and for about 30K average ADU on the flats.


SO far I have found Astro Pixel processor to work best for me in stacking.

[Paul Haese]

Ah that makes sense, regarding exposures.


Have you queried this with ZWO at all? Is this a common problem with that sensor? It might well be that you have an issue with that sensor.


Personally, I have been using a monochrome camera for years and would never go back to OSC unless I was doing a nightscape. I don't think I have seen this sort of problem before though.

[The_bluester]

It is a known issue with that sensor, more or less incurable it seems but with good flats you need to have loads of subs to give a really smooth background to be able to profitably stretch hard enough for it to be intrusive.

[AXE]

I wonder if the new ZWOs will have the same issue then? The ad for the 2600 says it has the Sony ExMor IMX571. Anyone have any idea what this one is like?

[The_bluester]

From what I have read, the difficulty getting flats to work right is more or less peculiar to the IMX294, like the microlens diffraction issue is known in basically all cams that share the sensor with the ASI1600.

I plan to change cameras in the next year or so but I am going to sit tight until I see what issues the next generation has as there is nothing in the generation of the ASI294 that I would change to as for me it would be just swapping one issue for another. The ASI2600 looks tempting if I was to stick with OSC but I would want to see some subs form them before I would lay down my money. Seems the best bet usually is to wait for the second revision so they can iron out the wrinkles that are just never seen until a few hundred users are out knocking the paint off the corners.

[AXE]

The only downside with the new 2600 that I can see is the smaller pixel size (3.76um) , so i'm assuming sensitivity will be less. I was thinking about getting the 071MC but thought i'd wait to see what the new ZWOs are like before deciding.

[The_bluester]

3.7 or so micron pixels seem to be about the most common size in a lot of the CMOS sensors so it is going to be hard to get away from. That said, the (Not Sony) sensor in the ASI1600 seems to be a pretty sensitive thing and that has the same pixel size.

3.8 ish seems to be a reasonably flexible size, on the likes of an SCT you would be a little oversampled but binned 2X2 you would be reasonably well sampled. Pair it up with around 400mm focal length and it is in the typical "well" sampled range where my ASI294 os unedrsampled and I use heaps of subs and drizzle processing to try to recover more detail.

Binning a CMOS does not provide the read noise advantage that it does on a CCD (Four pixels read as a single pixel in a CCD, so one sample of read noise, versus four pixels read once each and combined in software for a CMOS) but it does not really seem to have great drawbacks apart from paying for sensor resolution and not using it.

[MoTab]

Binning on the mono IMX455-based cameras like the QHY600 (the bigger mono cousin on the IMX571), does provide most of the benefits of CCD binning. The reason it works is fairly technical and involves removal of correlated noise -- the read noise in a 2x2 block is exactly that. The reason it doesn't work on an OSC camera is that you have a Bayer matrix there so a proper 2x2 block isn't meaningful. It has to be something that takes into account the Bayer pattern and by then, the photosites are no longer nearby each other to take advantage of that correlated structure.

The whole OSC versus mono debate is pretty huge. While almost any tool will recover most of the lost resolution, it still doesn't address a few of OSC weakness: you have twice the green sensitivity as red and blue, you cannot easily do narrowband imaging without expensive dual and triband filters, your overall camera sensitivity is lower with and without these special filters. Especially in light-polluted areas, being able to image in narrowband is a huge advantage. OSC is generally more convenient, however.