Mono or OSC, your suggestions

[pete_l]

19 hours ago, smr said:

If i were to buy a Canon 600D modified for instance, would I be giving anything up in terms of quality?

My Canon 80D sensor is quite a bit newer than the 600D sensor.

Your "domestic" 80D camera will also have an anti-aliasing filter in the imaging path. This is intended to reduce the possibility of Moire patterns on images. That filter works by adding some blur - so while your camera might have an "X" megapixel sensor, your images will have much less definition than you'd expect. As part of the astro-modding process, this filter will be removed. The effect being that your astro-modded camera will produce sharper images than an unmodded one.

[newbie alert]

10 hours ago, Alien 13 said:

I cant imagine anything worse than faffing around with filter wheels and filters, besides mono cant do meteors or comets unless yo want them well in mono...

Alan

I'd rather enjoy all the designated pixels, it's not really faffing , don't take much to spin the wheel and I'm in no rush to collect data

I see your point on the comets, but meteor?? Really??

[mftoet]

On 21/08/2019 at 11:08, ollypenrice said:

He's sometimes in France (and sometimes at our place!) but his fast, portable rig means he can image after making short journeys from his home in the Netherlands without needing huge volumes of kit.

Olly

In fact, currently I am at Olly's 'Les Granges' (since yesterday). There are several reasons why I've gone back to DSLR from CCD. To be more accurate: I've gone back to a digital camera with a CFA sensor. I've been pursuing astrophotography for about 20 years. My route went from a 35mm SLR (Olympus OM-1) > 645 medium format camera (Mamiya 645) > QHY8 OSC CCD-camera > SBIG ST-8300 mono CCD-camera > Atik 11000 mono CCD-camera > Canon 5D Mark II DSLR > Nikon D810a DSLR. Here's why I prefer using a DSLR (in random order):

1) a DSLR is more versatile than a dedicated CCD or CMOS astro-camera (mono or colour). Important to note is that in all those years of astrophotography I also like to pursue landscape and macro photography and since recent years also time-lapse and nightscape photography. I can (and do) use my current DSLR for daylight photography all the time, including taking family pictures and such (like I did in my early years with a 35mm SLR). Or how about photographing a lunar eclipse? Not very convenient when you (only) have a mono camera. So I spend about € 2500 on a 'secondhand' (demo-model) D810a, giving me the pleasure of doing all sort of kind of photography.

2) My time under dark, clear skies is very limited. Maybe about 10 to 20 nights a year if the weather cooperates. I want to spend that time as productive as possible, so my equipment needs to be as reliable as possible. Do we think computers are reliable? I don't think so. Do I need to mention Windows updates and driver issues? I guess not. I dare say Olly would totally agree with me on this. In fact, he calls himself a stone-aged astrophotographer keeping things the least automated as possible (manual filters wheels, manual meridian flips, manual focuser). Being a mobile astrophotographer my experience is that matters can go worse when you need to setup and break down your equipment every session. Therefore I don't use a computer/laptop in the field, but operate my DSLR with the standalone Lacerta MGEN. Focusing is done on a bright star by inspecting the spikes of a Bahtinov mask on the LCD screen of the DSLR. When you have an observatory (in your back garden or on a remote site), you don't have to worry (as much) about connectors and such getting damaged. You can tidily permanently bundle the cables. You continually optimise your setup and therefore it becomes reliable. If I would have an observatory, I would totally opt for a mono camera for deep sky work but still would like to have a DSLR besides for travels and other kind of photography.

3) As clarified above, I started astrophotography with a 35mm SLR. Back then guiding was doing done by eye keeping a star on the cross hairs of a guiding eyepiece making slight corrections with the handbox. Declare me nuts, but I liked that way of astrophotography in the style of E.E. Barnard. It gave the satisfaction that is was you that provided that crisp image without guiding errors and aeroplane trails (you would shield the aperture during the exposure when an aeroplane was about to enter the field of view). The current way I pursue astrophotography best approaches the way I did it in the early days. By using a DSLR I see and experience more of the beautiful starry sky than I did during the period I was using a laptop to operate the camera.

4) Dew issues. Probably bad luck, but I have owned two CCD camera where dew would form on the window in front of the sensor during the course of an imaging session. Current DSLRs perform very well, so cooling is not necessary. The D810a (and other high-end DSLRs) doesn't need dark frames (low dark current, no amp glow). However it is important or at least preferable to dither between subs.

5) Power consumption. A cooled CCD and a laptop drain a relatively large amount of current. I can operate my setup in the field without having to worry my battery will run flat. I don't need a large battery in the first place.                 

[Allinthehead]

Given a choice between a mono and osc, I'd go mono all day long. It's just so much more versatile.

Having said that I'm in the fortunate position to have one of both and now i use the mono exclusively for narrowband and the osc for broadband. The results i get with the 071 are far superior to the 1600 and it's just so simple to use.

Both systems have their uses but i have to set up for each imaging session and there's less to go wrong with the 071. 

I agree with Olly that a osc works best with fast well corrected optics under dark skies. 

[ollypenrice]

16 hours ago, swanny said:

I assume you shoot RGBL?  So you are saying you only do 1.25MN subs?  Because I do 3-5MN on my camera for color frames.  Different CMOS sensors have different sensitivity as well so they are all not the same.  

No, I'm saying that, while shooting L, I'm shooting R and G and B on every pixel simultaneously. It is the fact that a mono camera can do this which makes it faster than OSC (for a given chip.)

Olly

[vlaiv]

1 hour ago, ollypenrice said:

No, I'm saying that, while shooting L, I'm shooting R and G and B on every pixel simultaneously. It is the fact that a mono camera can do this which makes it faster than OSC (for a given chip.)

Olly

Actually difference between mono and OSC is not as drastic as it may seem. Let's analyze actual differences between same sensor in OSC and Mono variant to better understand which one is "faster" and by how much.

"Speed" of course will vary depending on several factors, but we can draw some general conclusions.

Given 4 hours of imaging time, let's see what sort of signal will be gathered on same scope, same sensor, but in mono vs OSC.

Mono:

Each pixel will gather signal for 1 hour per filter for example, so we will end up with 1h of each L, R, G and B.

OSC:

For the time being, let's put aside other differences and concentrate on gathered signal, so we will have following:

Total imaging time in this case is 4h, but red signal will be recorded by 1/4 of pixels, blue signal will be recorded by 1/4 of pixels and green signal will be recorded by 1/2 of pixels.

This translates into 1h of R (4h of 1/4 pixels is the same as 1h of "all pixels"), 1h of B and 2h of G.

If we had LRGB bayer matrix on OSC in principle we would gather 1h of each LRGB same as Mono.

So what are the real differences between the two? Here is a list:

1. Filters in bayer matrix are less effective than interference filters covering whole chip. This is in part due to fact that bayer filters are absorption filters rather than interference (this is a guess on my part, I might be wrong, but it does seem reasonable to assume this as interference filters on each pixel would be hard to fabricate and reflections would probably be serious problem in such case). This relates to lower quantum efficiency of OSC sensor. Here is comparison (taken from astrojolo's website and similar article, probably worth checking out, here: https://astrojolo.com/gears/colour-camera-versus-mono-price-of-comfort/)

2. Sampling rate - with OSC you have coarser sampling rate, meaning you won't be able to record all the high frequencies, and often that will lead to under sampling (but one will be hard pressed to tell from the image that last bit if sharpness).

3. Thing that happens when we examine many short subs vs more long subs will happen here as well - impact of the noise because of "sparse" pixels and slightly lower QE. Difference between OSC and mono will depend on target brightness - this is why many have found that for really faint signal you want to go Mono + filters, and also LP impact will be somewhat higher on OSC - this is why "OSC works best with fast optics in dark skies".

Fact that we get 2h of green is not such a big problem and it just happens to be better in daytime photography, because the way our vision works. If we examine "green" (this is not strictly green response - it is close to it, but cameras need white balance to get it right) and curve of our eye sensitivity to light - we will see very good match between the two - this is for a reason.

Look at green line representing our eye's response to brightness, and for example ASI183mc QE graph (green line):

For day time photography, where we expect luminance to match what we would see with our own eyes - green bayer filter is actually luminance filter - it represents luminance of each color as our eye/brain sees it.

In this regard OSC is actually getting 2h "L", 1h R and 1h B worth of data in 4h exposure. We use "full spectrum" as L in astrophotography because every photon counts, and although our AP images differ slightly to what our eyes would see in terms of relative brightness - it does not matter much because we end up stretching our data anyway and changing brightness response to show all that dynamic range captured.

After all of this - OSC is slower, there is no doubt about it, but it might not necessarily be much slower than Mono + filters - difference will more depend on other factors than technology itself (like type of target, sky transparency, levels of LP and such).

 

[ollypenrice]

11 minutes ago, vlaiv said:

Actually difference between mono and OSC is not as drastic as it may seem. Let's analyze actual differences between same sensor in OSC and Mono variant to better understand which one is "faster" and by how much.

"Speed" of course will vary depending on several factors, but we can draw some general conclusions.

Given 4 hours of imaging time, let's see what sort of signal will be gathered on same scope, same sensor, but in mono vs OSC.

Mono:

Each pixel will gather signal for 1 hour per filter for example, so we will end up with 1h of each L, R, G and B.

OSC:

For the time being, let's put aside other differences and concentrate on gathered signal, so we will have following:

Total imaging time in this case is 4h, but red signal will be recorded by 1/4 of pixels, blue signal will be recorded by 1/4 of pixels and green signal will be recorded by 1/2 of pixels.

This translates into 1h of R (4h of 1/4 pixels is the same as 1h of "all pixels"), 1h of B and 2h of G.

If we had LRGB bayer matrix on OSC in principle we would gather 1h of each LRGB same as Mono.

So what are the real differences between the two? Here is a list:

1. Filters in bayer matrix are less effective than interference filters covering whole chip. This is in part due to fact that bayer filters are absorption filters rather than interference (this is a guess on my part, I might be wrong, but it does seem reasonable to assume this as interference filters on each pixel would be hard to fabricate and reflections would probably be serious problem in such case). This relates to lower quantum efficiency of OSC sensor. Here is comparison (taken from astrojolo's website and similar article, probably worth checking out, here: https://astrojolo.com/gears/colour-camera-versus-mono-price-of-comfort/)

2. Sampling rate - with OSC you have coarser sampling rate, meaning you won't be able to record all the high frequencies, and often that will lead to under sampling (but one will be hard pressed to tell from the image that last bit if sharpness).

3. Thing that happens when we examine many short subs vs more long subs will happen here as well - impact of the noise because of "sparse" pixels and slightly lower QE. Difference between OSC and mono will depend on target brightness - this is why many have found that for really faint signal you want to go Mono + filters, and also LP impact will be somewhat higher on OSC - this is why "OSC works best with fast optics in dark skies".

Fact that we get 2h of green is not such a big problem and it just happens to be better in daytime photography, because the way our vision works. If we examine "green" (this is not strictly green response - it is close to it, but cameras need white balance to get it right) and curve of our eye sensitivity to light - we will see very good match between the two - this is for a reason.

Look at green line representing our eye's response to brightness, and for example ASI183mc QE graph (green line):

For day time photography, where we expect luminance to match what we would see with our own eyes - green bayer filter is actually luminance filter - it represents luminance of each color as our eye/brain sees it.

In this regard OSC is actually getting 2h "L", 1h R and 1h B worth of data in 4h exposure. We use "full spectrum" as L in astrophotography because every photon counts, and although our AP images differ slightly to what our eyes would see in terms of relative brightness - it does not matter much because we end up stretching our data anyway and changing brightness response to show all that dynamic range captured.

After all of this - OSC is slower, there is no doubt about it, but it might not necessarily be much slower than Mono + filters - difference will more depend on other factors than technology itself (like type of target, sky transparency, levels of LP and such).

 

I don't disagree with this. I think that, typically, OSC might require about 20% more exposure time than LRGB but I was responding to the claim that OSC was faster. You and I agree that it isn't. Your comparison is also based on shooting equal amounts through the four LRGB filters. This makes processing easier and is a nice way to work but it is not obligatory. If trying to record very faint signal like tidal tails or Integrated Flux Nebulosity the mono imager has the option of shooting, say, 1 hour per colour and 5 hours of luminance. Whatever the speed advantage of real luminance over 'OSC luminance' has now been multiplied by 5. Yes, processing does become harder but, since the signal we are looking for is only just above the background sky level, there is no need to stretch the middle and upper brightnesses of the luminance so hard that the short colour data will be washed out.  It certainly is possible to make an image with 5 hours of luminance and only 1 per colour. On some targets it is the obvious thing to do.

By the way, I'm sure you're right that OSC filters must be simple absorption items.

Olly

[pete_l]

2 hours ago, ollypenrice said:

I don't disagree with this. I think that, typically, OSC might require about 20% more exposure time than LRGB but I was responding to the claim that OSC was faster.

The point about OSC is that you get something, quickly. And after that each additional sub adds to the overall quality of the image. You can therefore stop when you are satisfied with the result. Or when conditions dictate. And when you stop, you have usable data - you aren't missing a channel or with an unbalanced exposure on one part of your palette.
As for chasing the very faintest whiffs of nebulosity? I think we all understand that most OSC users (myself included) are not after those sorts of results. There is plenty of stuff available for DSLR / OSC imagers to capture without needing to go deeper, darker or to large + expensive imaging rigs.

It is, after all, a hobby.

[Stub Mandrel]

I think a good starting point is to work out what is limiting the quality of your images.

For me I have a relatively small section of sky I can image with reasonably controlled LP, supposedly Bortle 5 but only on very clear nights - the thinnest of cloud picks up LP.

Weather and reality mean I typically get a  couple of night's imaging a month, and 4-5 hours is a good session.

My main cause of poor results  appears to be when I have thin cloud which results in poor signal to noise.

My camera is a modded and cooled Canon 450D, and to be honest stretching my darks suggests background noise is so low they re only needed to remove hot pixels.

My view is that without access to better skies and a bigger view, it isn't worth it because I will always be limited to the few targets that are directly over our house at the start of a session, so missing a filter due to a late start or poor weather could mean waiting at least a year for another try.

I am really torn - I honestly don't know what would be the best solution for me.

One thing I would like to do is see a fair comparison between cooled OSC camera and my cooled, modded DSLR. I suspect the cost of a cooled colour cam is not worth it. Mono may be, but only if I can get the data.

I think @pete_l makes a fair point. I can sometimes get usable images for a few bright targets in one night, and then add more data from furher years.

But it would be nice to do narrowband, although the cost of a cooled mono cam plus filters, even 1 1/4", is a bit scary.

[ollypenrice]

3 hours ago, pete_l said:

As for chasing the very faintest whiffs of nebulosity? I think we all understand that most OSC users (myself included) are not after those sorts of results. There is plenty of stuff available for DSLR / OSC imagers to capture without needing to go deeper, darker or to large + expensive imaging rigs.

 

If that's understood, then fine. Looking through the posts in this thread I wouldn't say it was universally understood but, if it is, then we all agree. I also agree, without reservation, that going like a maniac after every last wisp is not compulsory! It might be more a mania than a hobby but that's hobbies for you! 

Olly

[geordie85]

I'll be completely honest and admit that I didn't really read through this thread and this post is only directed at the original post/ question. 

I starred out with a Canon 1100d but wasn't happy with the results. I then bought a modded 600d which I quite enjoyed and imaged with it for quite a while with "pleasing results". To be honest, looking back, they're rubbish. But I was happy at the time. 

I then bought an atik 383L mono and was blown away by the difference, especially since I could now image in narrowband, and that would have been that. Unfortunately due to financial reasons I had to sell my Atik. 

After a house move I bought a qhy183c and imaged as much and as often as I could. But it was missing something. I can't quite put my finger on it, but my images lacked "oomph". 

I've vert recently gone back to mono with the qhy183m and once again I'm a very happy. I'm busy using it to add luminance to my old, flat, colour images I captured with my qhy183c and its bringing the "oomph" back into them. 

This has turned out to be more of a ramble about the cameras I've owned, but I hope it makes sense and helps. 

[Whirlwind]

I've always this thought this argument depends on what you want to image and how often.

OSC cameras definitely have benefits when you only have the opportunity relatively rarely to image, need to setup and strip down everything all the time and so forth. In these cases OSC helps because you get all the data at once and if clouds appear earlier than forecast then you still have data this is useable (and aren't stuck with the R and G but not the B!  If you want to focus on galaxies then OSC can also work well (and works even better in non light polluted areas)

On the other hand if you are looking at narrowband targets as your primary dish of the day then mono is the way to go as you can be much more selective as to how much data you want from each band and that some narrowband will overlap (SII Halpha will both trigger the red pixels in an OSC etc).

[Whirlwind]

20 hours ago, ollypenrice said:

I don't disagree with this. I think that, typically, OSC might require about 20% more exposure time than LRGB but I was responding to the claim that OSC was faster. You and I agree that it isn't. Your comparison is also based on shooting equal amounts through the four LRGB filters. This makes processing easier and is a nice way to work but it is not obligatory. If trying to record very faint signal like tidal tails or Integrated Flux Nebulosity the mono imager has the option of shooting, say, 1 hour per colour and 5 hours of luminance. Whatever the speed advantage of real luminance over 'OSC luminance' has now been multiplied by 5. Yes, processing does become harder but, since the signal we are looking for is only just above the background sky level, there is no need to stretch the middle and upper brightnesses of the luminance so hard that the short colour data will be washed out.  It certainly is possible to make an image with 5 hours of luminance and only 1 per colour. On some targets it is the obvious thing to do.

By the way, I'm sure you're right that OSC filters must be simple absorption items.

Olly

I think the difference in benefit between mono and osc is more pronounced that that.  If we assume some relatively arbitrary numbers:-

Each pixel produces one photon per minute from non filtered (L) light (i.e. 60 per hour)
Each filter (including the OSC coating) reduces the photons received at a pixel by a third
The CCD is made up of 4 pixels

As such if you were shooting 4 Hrs of OSC then the number of photons you would capture would be:-

photons x number of hours x number of pixels x filter reduction factor = 60 x 4 x 4 x 1/3 = 320 photons captured

For LRGB you have to do the above for each channel each being 1 hour in length

L = 60 x 1 x 4 x 1 (no filter) = 240 photons
RGB = 60 x 3 x 4 x 1/3 = 240 photons
Total = 480 photons

Factor difference = 480/320 = 1.5

So you capture at least 1.5 times more photons in the same time.  The more you focus on L of the image the more pronounced this becomes so 2 Hrs Lum and 2 Hrs RGB would be 640 / 320 = double the photons captured.  This would hence reinforce your argument that for faint objects mono is the way to go.

This uses basic assumptions that camera sensitivity is the same in the same bands.  This is generally an incorrect assumption as the coated OSC are substantially less sensitive over the total band colour band compared to the same filters (which usually have a 90 - 98% transmission range).  However filters generally are much more specific in the wavelengths they pass (for example looking at the OSC above the colours overlap. So I'm balancing this out even though in reality you have red light in the green osc pixel.

It assumes that the cut off of the L filters is similar to that of the osc coating.  Generally for refractors this is correct as we cut at about 400nm - 700nm as lenses aren't well corrected after this point.  However for reflector system (e.g. RC) where this isn't necessary and you can happily use a clear filter then there the mono can capture a lot more flux from outside this range.

As such I'd argue that 1.5x might be the lower end of how much data you might be collecting.

 

[Danjc]

Despite dipping my toe many years ago I’m  relatively new to imaging and started with a 1100d which I had reasonable success with but soon wanted more. 

I initially purchased a asi294mc pro but actually sent it back without even opening the box as I had a big rethink and changed my mind after reading this type of thread and advice from others and went for a asi183mm-pro. 

Personally i want to catch as much data and detail as possible so figured mono was the better route for me. Yes you need filters and a wheel but what some see as a faf  I see as a challenge and something to learn. 

I picked up a used 5 position efw for £100 and a new Baader 7nm H-alpha filter for about the same price so £200 got me going and I’m happy with this for the time being as I really like mono images on there own. I will pick up some Baader LRGB filters in the near future though but if anybody has a set going cheap let me know 😁 

I’m in no doubt OSC can and does produce great images I just believe mono is the better route. I have no problems in imaging the same target over multiple sessions to get the results I want. Due to shift work, weather etc etc if I can get 1,2 or 3 good images per season I will be happy. 

I think this has turned into a personal ramble but hey ho........

 

Dan. 

[Stub Mandrel]

Just for a laugh, this data from the 20th is probably the worst I have ever got, through a 7nm Ha filter and a DSLR. Hazy cloud, moonglow and lumps of dust rolling around on the sensor as the scope was pointed almost vertically - but even these three things don'#t explain why it was just SO awful! Green is very low but there was a surprising amount of data on the blue channel. After stretching very crudely, it does show that there is overlap between all the pixels.

 

[Stub Mandrel]

Just for comparison, to prove teh DSLR can take a half-decent Ha image, this was last year:

[vlaiv]

1 hour ago, Whirlwind said:

I think the difference in benefit between mono and osc is more pronounced that that.

Your example is a bit flawed. Although we don't usually work with uniform spectrum, that is not the most important thing.

You calculated number of photons hitting "whole" sensor, and that is rather meaningless in terms of SNR, as SNR depends on intensity per pixel. Most users will use full resolution of mono sensor, so we need to calculate per pixel SNR for given time.

Actually, may I suggest a bit more realistic scenario so we can examine what happens?

Here are initial assumptions of the calculation:

- Same sensors in terms of pixel size, QE and other things, one being Mono and other being OSC

- We won't take read noise or dark current into consideration for the moment

- We are going to shoot relatively bright target in moderately dark skies - photon flux for target and for sky will be the same (for example mag21 skies and mag21 target - that is quite reasonable, but we won't calculate photon flux for each, we are going to assume same parameters as above - uniform spectral source and 60 photons per hour for full spectrum).

- 1 photon a minute in full spectrum, 4h total exposure time, filters divide spectrum 1/3 and are 100% transmission in their respective band, sky flux is the same as target 1 photon a minute over whole spectrum and is uniform (so 1/3 per each band).

- We will compare resulting SNR per R, G and B values

- all calculations will be per pixel with a difference, OSC "pixel" is twice the size and is composed out of 4 bayer matrix pixels - RGGB.

Let's do OSC first.

Total photons:

R - 80 target, 80 sky
G - 160 target, 160 sky
B - 80 target, 80 sky

SNR per channel

Red - signal 80, noise (shot + sky) = ~12.65 ( sqrt(80+80) ), SNR = 80 / ~12.56 = ~ 6.3246

Blue will be the same, so SNR in blue = ~6.3246

Green - signal 160, noise (shot+sky) = ~17.8885 (again sqrt(shot^2+sky_noise^2) = sqrt(160+160) = sqrt(320)), so green SNR will be 160/~17.8885 = ~8.9443

For OSC and 4h exposure RGB SNR per channel is (6.3246, 8.9443, 6.3246)

Let's now do the same for Mono/LRGB combination, it will require a bit more math since we need to do "color compose" of LRGB, but it's quite straight forward and we only need to do it for single color, because we have uniform light source.

Total photons (per pixel):

L = 60 target, 60 sky

R, G, B = 20 target, 20 sky

SNR per captured channel:

L - signal 60, total noise ~10.9545, SNR = ~5.477

R, G, B - signal 20, total noise ~6.3246, SNR = ~3.1623

But we need to calculate SNR per RGB and not per LRGB so we are going to apply RGB ratio method of color composing. This means that L will be multiplied with R/(R+G+B) for ratio of red for example. We are going to simplify things and assume that R+G+B is only signal and no noise (although we could do full calc but it would be a bit more complicated), so it has the value of 60.

so R channel SNR will be like this:

(60s + 10.9545n) * (20s+ 6.3246n) / 60 = 60s*20s / 60 + 60s * 6.3246n / 60 + 10.9545n*20s / 60 + 10.9545n*6.3246n/60

We have 3 components of noise and one of signal. Signal is obviously 20 - 60s*20s/60 and we need to calculate total noise by noise addition. Let's first calculate magnitude of each noise term

60s * 6.3246n / 60 = 6.3246
10.9545n*20s / 60 = 3.6515
10.9545n*6.3246n/60 = 1.1548

Now we can calculate total noise by sqrt(n1^2+n2^2+n3^2) = sqrt(40.00056516 + 13.33345225 + 1.33356304) = ~7.3938

So SNR per R = 20 / 7.3938 = ~2.705

Per pixel SNR for R, G and B after simple ratio color composing will be (2.705, 2.705, 2.705)

Even if we equalize sampling rate, and use 4 pixels of Mono+LRGB approach added together, we still have lower per RGB SNR at the end

(5.41, 5.41, 5.41) vs (6.3246, 8.9443, 6.3246)

This shows that speed of mono/LRGB approach does not come from the fact that Bayer matrix is inferior to mono. It in fact comes from those things that we left out - read noise and difference in QE between regular filters/mono and bayer filters. Point made by @ollypenrice about ability to use different ratios of LRGB than basic one is also very valid and above calculation would look different had we done so.

 

[Icesheet]

Regarding the argument of always having ‘complete’ data regardless of integration time with OSC. Why can’t you have this with Mono and filter wheel too (at least if you have a motorised one using a sequencer)?

I know there is the argument that you should focus between each filter but I’ve also read that it’s only really critical for your Lum to be in focus for the detail. So if time weather/ time is an issue why not set up a sequence of exposures that just rotate through the filters?

Sure you won’t have critically focussed RGB but then it’s a compromise you can surely accept for a ‘complete’ image? I mean OSC must be making that compromise in every exposure anyway?

[Dr_Ju_ju]

….  in the end it all comes down to "have I captured an image that I'm happy with !!"  whether that be with OSC\Mono\Mono & filters etc, its all horses for courses....

with all the other complications in this 'sport', getting something recognisable, is a triumph in itself.

[Tim]

22 hours ago, pete_l said:

The point about OSC is that you get something, quickly. And after that each additional sub adds to the overall quality of the image. You can therefore stop when you are satisfied with the result. Or when conditions dictate. And when you stop, you have usable data - you aren't missing a channel or with an unbalanced exposure on one part of your palette.
As for chasing the very faintest whiffs of nebulosity? I think we all understand that most OSC users (myself included) are not after those sorts of results. There is plenty of stuff available for DSLR / OSC imagers to capture without needing to go deeper, darker or to large + expensive imaging rigs.

It is, after all, a hobby.

^^^  This ^^^

[Tim]

2 hours ago, Icesheet said:

I know there is the argument that you should focus between each filter but I’ve also read that it’s only really critical for your Lum to be in focus for the detail.

Not sure if anyone has thrown this particular spanner in the works yet, but there are very few refractors that have such perfect colour correction that there is not a bit of star bloating from not quite precisely focused colours being captured during a Luminance sequence. Faster 'fracs are the worst offenders normally, and it isn't always a case of the more expensive, the better. Take Vixen's VSD100 as an example, Luminance subs through that lens were pretty much impossible to get tight focus on when I used one, and as was stated in the offical S@N review too.

 

 

[ollypenrice]

6 hours ago, Icesheet said:

Regarding the argument of always having ‘complete’ data regardless of integration time with OSC. Why can’t you have this with Mono and filter wheel too (at least if you have a motorised one using a sequencer)?

I know there is the argument that you should focus between each filter but I’ve also read that it’s only really critical for your Lum to be in focus for the detail. So if time weather/ time is an issue why not set up a sequence of exposures that just rotate through the filters?

Sure you won’t have critically focussed RGB but then it’s a compromise you can surely accept for a ‘complete’ image? I mean OSC must be making that compromise in every exposure anyway?

Tim picks up on this point later in the thread but I have never found any evidence that the filters themselves introduce non-parfocality in broadband imaging. I think it comes from the optics. As I said earlier, this means that the OSC user is unable to focus critically for each colour. The mono imager can focus 'per colour filter' but the luminance filter passes all colours so it is at the mercy of the colour correction of the optics just as is the the OSC camera. The luminance focus, itself a compromise based on the the optical correction, is going to define the final image because that's what luminance does. So getting excited about the focus of each colour going into the RGB layer, which will not be processed for resolution anyway, seems like a waste of energy.

In a nutshell I think that the 'focus per filter with mono cameras' argument is a red herring. Focus over time, certainly. Focus per filter - unlikely to make a blind bit of difference unless you have horribly non-parfocal colour filters. If your non parfocality comes from your optics it will hurt you in OSC or LRGB to the more or less the same extent.

Conclusion: don't reject mono because you have to refocus between filters. It's probably a waste of time because your luminance focus is an RGB compromise anyway.

Olly

Edited August 25, 2019 by ollypenrice

[Stub Mandrel]

Don't know if these two experiences are relevant:

A few weeks ago I was doing Ha with my DSLR, before it got dark enough for DSOs I had a shot at a very low down moon. I got one of my crispest moon images despite the longer exposures. Was this because the narrowband helped overcome atmospheric dispersion, or because it was beating the seeing? But it reminds me that planetary/moon imaging shows just how much the sky wobbles on a short timescale - it amazes me how we can get any sharp images with long exposures!

Tonight... it's really good seeing and really good transparency. That's so rare for me. This is actually the first time I have ever been able to collect data in Sagittarius. Despite beautiful star colours on 2-second exposure with my guide cam(!) when Ni pointed it at M24 I decided to go for M17 as it's probably my only chance ever. But i literally (as in literally, Olly!) only have less than two hours; in these circumstances I really don't think anything other than OSC is practical. I have even gone down to 2-minute exposures because experience tells me the skyglow will be too much for 5-minutes, and I haven't got a clip filter the only option for the 150PL. I've looked at two subs, and YES! I am getting swan-shaped ha so something good will happen.

For such a 'smash and grab' opportunity only a DSLR will do.

Plus I think I got my best Jupiter data for the year by a country mile, sadly saturn went behind the roof. Might have a go at it later but other targets are calling.

[smr]

This thread is brilliant, thanks for all your replies. Really interesting reading. 

I'm definitely going to upgrade to a dedicated astro camera, whether that's Mono or OSC I'm not sure. I do like the results I see from imagers using cameras like the ASI294MC Pro and with duo narrowband filters, often from skies which are Bortle 8 as well. So that is encouraging as I have Bortle 5 skies, but obviously Mono equals best data but with the caveat of added complexity. Maybe the next logical step is to buy an OSC, learn more, get better images and more experience and then progress onto Mono.

At the moment I'm still in the phase of being happy to just find and image new DSO's. Tonight I'm imaging the Eastern Veil Nebula for the first time and just to find objects like this which I have never attempted before very much has that wow factor for me !

I'm currently imaging 3 minute exposures, ISO 200 and will image as many as I can tonight. A stretch in Lightroom showing the veil (cropped in a bit).

 

 

Edited August 25, 2019 by smr

[smr]

Just a slightly OT question... 

Is it better to shoot longer subs (4 or 5 minutes as opposed to 3) ?  Or should I leave the subs at 3 minutes with the histogram data where it is?