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Old 22-09-2019, 11:11 AM
AnakChan (Sean)
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Newbie: Dedicated Astro mono camera questions

Hi folks,

Well ok, not entirely newbie as I have a CentralDS Astro A7S which has served me well for the past few years however Ií,m considering ďgraduatingĒ from it.

I have the following scopes which I intend to do astrophotography :-
1) Takahashi Mewlon 250CRS (2500mm/F10) - this has so far been my current scope with my cooled A7S
2) Pentax 125SDP (616mm/F4.9 - with reducer) - large image circle akin to the FSQ106 88mm image circle
3) Takahashi FSQ85ED (450mm/F5.3)

Iíve been thinking of getting a dedicated astro camera and am thinking of dipping my toes into monochrome as I think a OSC isnít going to be massive difference/improvement over my Astroís A7S. Please do correct me if my thinking is wrong.

Having said that Iíve been thinking of the QHY600M or the ASI6200M as they seem to have the pixel pitches suitable for the aforementioned scopes above. I could go for a smaller sensor for the 250CRS/FSQ85ED with their smaller image circles but would be nice to get a full frame for the 125SDP and thatís where I think the IMX455 sensor is suitable. For either the QHY or ZWO, Iím likely to pick up a filterwheel and seeing the large sensor size, Iím guessing Iíd have to go with ~50mm sized filters. Iím estimating at minimum, a USD$7000 investment there for either brands incl the LRGB and NB filters. As it is a substantial investment, it naturally has made me re-think of whether to just go for the OSC version of the QHY600/ASI6200; as such the initial question in the former paragraph.

Am I right or wrong in thinking that a dedicated OSC is going to be similar to my current Astro A7S, and that if I want setup up to the next level I should consider monochrome? If so, then if the QHY600/ASI6200M package are deemed expensive, I should look into a smaller monochrome sensor within budget? With my range of scopes I do have a preference of a 3.5-4 micron pixel pitched sensor.

Any advice youíre able to offer is truly appreciated.
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Old 22-09-2019, 08:13 PM
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Camelopardalis (Dunk)
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Sean, it’s important to understand what you’re looking to gain/achieve from the upgrade.

Performance wise, it might not be as clear cut as you might think, especially with the new tri-band filters for OSC cameras...
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Old 22-09-2019, 10:53 PM
AnakChan (Sean)
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Cheers for your reply. Itís a fair point about the duo/tribands. I have the STC Duo Narrowband myself and have been seeing the results of the Radian Triad Quad Narrowband (quite pricey) on the CN forum.

In no particular order, I am looking at :-

- improved resolution & detail
- higher sensitivity (bypassing the Bayer filter as such the preference of monochrome)
- better control of different wavelengths - even with the Duo/Tri/Quad NarrowBands with OSC, exposure time is equal for each as opposed to mono.
- something different from just red Ha - i.e. getting into false colour. Even with the aforementioned narrowbands, colours are still mostly red with OSC. It seems remapping OSC colours using some kinda Hubble Palatte Synthetisers arenít so easy too.

Here is a sample Iíve taken with my STC DuoNarrowband with my cooled Astro A7S with my Pentax 125SDP. Pardon the poor tracking in this particular case.

P.S. seems in typing this I feel Iíve talked myself more into the mono path.
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Old 23-09-2019, 11:28 AM
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ChrisV (Chris)
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One of the things you are after is better resolution. In that case mono might be better. I've been using the STC duo for a while now, and to really get full bang I've found that you have to superpixel debayer to get good colour separation
- see
https://www.cloudynights.com/topic/6...4#entry9649949
That really reduces resolution.

Regarding your image, it's also best to split the channels and process them separately to stop the red Ha dominating.

Also, given what I get with the duo/OSC my 'impression' is that real NB with mono will 'look' better. I know this sounds a bit vague but you are looking at a big investment
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Old 23-09-2019, 04:38 PM
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While mono cameras mean more work, they definitely provide higher resolution and sensitivity.

Tri and Quad band filters are marketing gimmicks at best, as if you use them with a colour chip, 2/3rds of your pixels will be blind to the any particular emission band. NB with colour cameras is simply not optimal.

Mono filters allow you to use all of the pixels in whatever band you want, plus unfiltered give you appreciably more signal for an L channel.

As an aside, if you calculate sensor resolution in a mono camera in the same way as colour sensors are, you triple the sensor's effective number of pixels. Hence a 16mp camera gives you 48mp of RGB data, add in a luminance channel and you have 64 megapixels ! ( you are taking four exposures, rather than one bayered RGGB set...there is no free lunch, as colour cameras simply interpolate the data back to the mono resolution of the sensor)
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Old 23-09-2019, 07:13 PM
AnakChan (Sean)
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Cheers. The comments posted here so far does seem to lean towards little gain in going from my cooled Astro A7S to a dedicated OSC, and mono is the way to go. Yes the IMX455 mono based iterations irrespective of QHY or ZWO will be huge investment. I don't have the luxury of going small first (183/8300/1600) just to try, then move up to something larger later - so I'm going all in large and deal with the steeper learning curve upfront.

My understanding about effort required with mono is that it's not necessarily more time consuming since it's more sensitive, however more "interaction" since it's changing the LRGB (and/or NB filters) more frequently throughout the imaging session.

Now the concern is going to be more brand specific issues - e.g. ZWO's CMOS microlensing, QHY driver issues, etc.

P.S. Separate note, I've read that dedicated astro cameras have more "worries" or more "maintenance", such as dew or frosting during imaging, and regular desiccant drying, or argon re-gassing. These are things I did not have to worry about with my cooled Astro A7S nor D810A.
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Old 23-09-2019, 07:47 PM
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FWIW, the learning curve going from OSC to mono is the same regardless of sensor size or cost. The larger sensor just means more cost, and possibly more grief dealing with spacing or tilt issues that would be less noticeable at a smaller imaging diameter.

Just to clear a few things up (and just my opinion)...

For sensors where there is both a mono and OSC version available, the mono version is not more sensitive...they’re both the same, it’s just that the OSC permanently has a colour filter over the pixels. In practice, the mono sensor does too, since you need to use some kind of filter to produce a meaningful image.

There’s not a lot of direct interaction with the filters themselves...you tell the software when to change and it does it. Job done, in seconds.

ZWO’s microlensing is specific to the 1600 only...and it affects all cameras from other brands using the same Panasonic sensor. This has been found to be caused by a sheet of glass covering the sensor that isn’t anti-reflective coated at manufacture. In practice, it only becomes an issue if you like taking long exposures of bright stars. Personally, the only shot I’ve taken where it has been noticeable was of the Horsehead Nebula, with Alnitak making its presence known.

Regarding worries...personally I’ve not experienced any of these issues. I’ve had my newt dew up on me several times on what seems like the perfect night. If it’s not one thing, it can easily be another
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Old 23-09-2019, 08:51 PM
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Quote:
Originally Posted by Camelopardalis View Post
.....

For sensors where there is both a mono and OSC version available, the mono version is not more sensitive...they’re both the same, it’s just that the OSC permanently has a colour filter over the pixels. In practice, the mono sensor does too, since you need to use some kind of filter to produce a meaningful image.....
Sorry. This is just wrong.

Because mono cameras don't have a bayer matrix, ALL of the pixels produce signal in R, G , B or from whatever, from the filter you have (or not) in place.

That is not the case with a OSC. Only 1/3rd or so of the pixels are producing signal in R, G or B.

The signal in a mono camera is higher as a result of all of the pixels being used and producing a spectrum specific read-up charge.

In short....mono sensors are more sensitive.
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Old 23-09-2019, 09:04 PM
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Quote:
Originally Posted by Peter Ward View Post
Sorry. This is just wrong.

Because mono cameras don't have a bayer matrix, ALL of the pixels produce signal in R, G , B or from whatever, from the filter you have (or not) in place.

That is not the case with a OSC. Only 1/3rd or so of the pixels are producing signal in R, G or B.

The signal in a mono camera is higher as a result of all of the pixels being used and producing a spectrum specific read-up charge.

In short....mono sensors are more sensitive.
So...what youíre saying is that a mono sensor, with a colour filter over the pixels, is more sensitive than the same sensor with a colour filter array over it? At the pixel level? Are you sure about that?
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Old 23-09-2019, 10:11 PM
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billdan (Bill)
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My understanding is the QE of a camera sensor is the same whether it is colour or mono.
A single pixel doesn't care whether it has a bayer red filter or a filter wheel red in front of it.

The only thing that changes is the resolution where Mono plus filters is better than OSC.
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Old 23-09-2019, 10:36 PM
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billdan (Bill)
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This is taken from the Starizona website - Note there is no distinction made between Mono or OSC.


Quantum Efficiency
As photons of light hit a CCD chip they are converted into electrons which are stored and then read out at the end of the exposure. But not every photon that hits the chip is converted into an electron. How many photons are converted depends on the camera's quantum efficiency, or QE. QE is expressed as a percentage of the number of photons converted.
If all the photons produced electrons, the QE would be 100%. Most amateur CCD cameras have QEs in the range of 25-50%. More advanced "back-illuminated" CCDs have QEs around 85%. Supercooled professional CCDs used at observatories have QEs closer to 98%. Compare this to film, which has a typical QE of around 2%.
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Old 23-09-2019, 10:49 PM
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Quote:
Originally Posted by Camelopardalis View Post
So...what you’re saying is that a mono sensor, with a colour filter over the pixels, is more sensitive than the same sensor with a colour filter array over it? At the pixel level? Are you sure about that?
Of course at the individual pixel level, when filtered they are virtually the same.

But that misrepresents the reality of what goes on at the focal plane.

With a mono sensor you have more pixels collecting signal in the wavelength you are interested in.

Instead of one pixel in a Bayer cluster (4 pixels) picking up say red photons, you have four. i.e. 4x more detected signal for the same incoming flux.

P.S. There is no free lunch here. With a mono sensor you get more detected signal in R,G,B and even more so in Narrow band
(as you are not uselessly pre-filtering a filtered image) But you have to make 3 exposures to make a colour image...and that takes time.
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Old 24-09-2019, 05:55 AM
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One way to put it would be that per pixel, per unit of exposure time (Call it per minute) you will get more signal on average out of a mono and filters due to collecting the wavelengths of interest on all pixels instead of one half or one quarter of them as is the case with a bayer matrix (Depending on the wavelength, half for green and a quarter of the others, with some crossover as a recent experiment I did showed)

Last edited by The_bluester; 24-09-2019 at 06:09 PM.
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Old 24-09-2019, 05:35 PM
AnakChan (Sean)
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Cheers for all the responses. I have to say I understood sensitivity to how Peter and Bluester/Paul have described it. Maybe the definition "sensitivity" that was in question, however when I initially wrote that I was seeking "higher sensitivity", I meant that within a certain period of time I could collect more photons per pixel well without a bayer matrix than with (barring thermal, residual or any other non-photon-generated noise, etc.).

Anyhow, I'm more convinced now mono seems to be more the preferred route for my next "upgrade" for my goals. It's merely a QHY vs ZWO iteration of the IMX455 now.
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Old 24-09-2019, 10:13 PM
ericwbenson (Eric)
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Hi,

The difference between OSC and mono is a recurrent subject on just about every astro msg board! So some simple math to demonstrate some of the differences:

Imagine 4 adjacent pixels that fit inside the star spot diameter (or nebular feature diameter) all getting the same flux because the object being imaged doesn't vary over such a small angular scale. Let's also imagine the object is white and keep the total imaging time for mono and OSC equal.

The mono imager with a color filter collects 100 photons in each pixel in 1 minute, i.e.
100 100
100 100
therefore a total of 400 photons for R, G and B over three minutes, ignoring time for filter changes, downloading etc.

The OSC unit cell is:
RG
GB
Each pixel collects 100 photons in 1 minute, and 300 photons in 3 minutes:
300 300
300 300

Comparing the total number of photons collected in 3 minutes for both cases:
Mono OSC
R 400 300
G 400 600
B 400 300

So OSC gives you a better green channel result but worse red and blue channels. While this might be fine in daylight scenes, this is generally not what one wants in astrophotography. The sensor QE is weaker in the blue and the red for almost all CCD/CMOS sensors, so really the opposite (more red/blue compared to green) is desired, and you can't do anything about it, the OSC filters are permanent, you'll just keep collected more green than you want!

It gets worse, the fixed color filter in the bayer array work by absorption rather than reflection like the dieletric color filters in the filter wheel. The dielectric filters have a flatter/sharper passband resulting in greater color differentiation and letting more light thru where you want it (so higher effective QE for the equivalent spectral passband).

Another advantage to mono is a smaller impact from read noise since you can make your pixel four times the area as compared to OSC with the same digital resolution limit. Or conversely for the same read noise impact you have 2 times the resolution.

The penalty for mono are 2 extra filter changes and downloads for every OSC exposure, so that means the individual exposures need to be long enough to swamp this extra time cost.

Of course putting a narrowband filter over a OSC camera is a disaster for the collected signal since you are now looking thru two filters simultaneously all of the time. You now have to do everything a mono imager does (multiple filter changes and downloads) but with <1/4 of the Halpha and SII signals, and about 1/2 of the OIII. (3/4 of the pixels but non-optimized Bayer response).

Another problem with OSC is stacking. Before stacking multiple frames they must be aligned, which involves interpolating from a group of pixels in the source frame to a slightly shifted pixel in the destination frame. Of course OSC raw frames need to be converted to normal images (i.e. debayered) before this can happen, and during the interpolation phase noise can leak from the pixel of one color to the neighbouring pixel of another color (e.g. the weak blue pixels leak their noise into the brighter green pixels). Now that the noise has been mixed adjacent pixels the stacking cannot recover the true signal as easily as for mono stacking.

The final major advatange of mono is the ability to take luminance frames. Full resolution with at least 3x the signal of an individual RGB frame, the LRGB technique can substantially reduce the time for a nice color image of a deep sky object. My guess here is about a x2 time reduction for faint objects. NB There is usually no benefit to LRGB for brighter objects, like star clusters, since your sensor is not signal starved per subexposure.

Regards,
EB
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Old 25-09-2019, 01:05 PM
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Quote:
Originally Posted by ericwbenson View Post
The difference between OSC and mono is a recurrent subject on just about every astro msg board! So some simple math to demonstrate some of the differences...
...Regards,
EB
Thankyou Eric, that was a smashing good read
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