Mono Vs OSC considerations and focus shift

[mos]

Hi everyone,

Soon enough I will make the leap from my DSLR to a cooled CMOS camera - either ZWO's  294MC or 1600MM mini kit. And to put it short I'm struggling. 

I've done a lot of research on here about the pros and cons of color vs mono with the underlying sentiment of wanting to have the potential for great images but these don't necessarily have to be the best I could achieve . So going mono over OSC because mono could produce great images and is much more versatile as opposed to OSC producing very good images and being less versatile (by way of a simple comparison) is not the biggest factor I'm struggling with here.  (For the record I'd really love to go mono for these reasons).

The biggest concern for me with this is in the 'overheads' of getting good mono images and specifically the need to potentially refocus between different filters. Simply put...between balancing family and other commitments I don't know if i'd be able to give mono imaging the attention it requires. I often remote connect from inside the house to be able to keep an eye on things but something like an auto-focuser to keep the current and relatively 'hands off' ability I have with my DSLR is a bit too far for my current budget. 

I use a 150PDS reflector (with an MKiii MPCC) which means filter focus shift may not even be an issue? But I haven't been able to find any info on whether it is as the majority of information around parfocal topics usually involve refactors. 

Apologies if this seems like I'm making a mountain out of a molehill for such a little detail but this is probably the last point at this stage of my life where i'll be able to sink some money into this amazing hobby and I'd love to make sure I'm doing the best I can to maximize my actual time capturing quality images, as this new camera will be in my setup for the foreseeable future.

Any help and guidance very much appreciated and if you just want to say "Go mono, you won't regret it" that helps too :')

Thanks,

Matt

 

[Pompey Monkey]

I don't really think the focusing interval would be any different between osc and mono. You'd want to be checking the focus every couple of hours minimum regardless. 

I used to focus at the start of the night, then again a couple of hours later and leave it going (subject to weather!) for the rest  of the night. I almost invariably wished that I'd refocused when i checked my subs the next day ?

Have you considered a diy auto focus?

[alacant]

5 hours ago, mos said:

I don't know if i'd be able to give mono imaging the attention it requires

I thought I'd get in before the mono brigade take hold...

If you've ever setup your dslr alongside someone with a monochrome camera, you'll get an idea of how much more involvement there is. Then the filter changes with a refocus for each and then finally 4 sets of data to stack and process... But I'm not going for perfection, just enjoyment and simplicity and as you also mention my dream upgrade, the zwo294, you can probably guess what my choice would be;)

HTH

[Ouroboros]

I sometimes  wonder whether OSC versus mono is like comparing chalk and cheese. DSLR versus cooled OSC might be a better comparison. DSLRs have several good points. Readily available, fairly cheap, reliable, uncomplicated set up, stand alone (if required), live view for focusing/framing at the scope and give reasonable results for someone with limited opportunities for imaging. What's not to like? Well, it would be nicer if DSLRs were also cooled. OK cooled DSLRs are available, but it's a bit of a "DIY" solution, even if you buy one from a bloke in a shed. Sorry ?. So, those of us who enjoy one shot colour astrophotography using DSLRs might well consider the question of whether to buy a cooled OSC CCD/CMOS is the more interesing and relevant comparison than whether to go for mono. 

[RayD]

The 'mono vs OSC' debate has been done to death so I won't go in to detail on this as a simple forum search will present a whole raft of useful information on this.  It comes down to what suits your style of imaging and your expectations.  Both will work just fine, and both will produce excellent images but, of course, both are subject to your limitations in processing, which at our level would typically be the main limiting factor.

As noted by @Pompey Monkey you will probably need to refocus no matter which one you use.  The need to do so isn't only down to filters, it's down to changes in temperature also.  To achieve the absolute best images with your kit, as you appear to want to do, then you should look at your focus fairly regularly (every couple of hours).  If you use par focal filters such as Baader, they can limit the need to refocus between changes, but in my experience they do still need a little 'tweak'.

Don't be scared off by mono processing, it isn't as onerous as it sounds, and as the colours are already separated and contain higher resolution data, some may argue it is actually easier.

Either way you will need to process the data, whichever method of data collection you choose.

The most important thing is to enjoy the hobby, so choose the one which you feel will allow you to do this best.

[Adreneline]

Hi!

Well for my sins I have a 428ex mono and a 414ex osc. My favourite operating mode at the moment is using both with my Samyang 135mm lens and 2" Baader filters - supposedly parafocal and my experience supports that claim. I used to recheck the focus each time I manually change a filter but the FWHM figures are always essentially the same, circa 1.3 or less, irrespective of whether I am using Nb with the 428 or UV/IR with the 414. A minor adjustment to the focus position is required for the 414 compared with the 428 but again my experience is that it does not change during an imaging session.

I also have a set of 1.25" Baader parafocal filters I use with the 428 when attached to either my SW-ED80DS-Pro or WO-ZS71 and my experience is they are not as parafocal as they claim, especially the Blue filter which has been changed by FLO but I still find I have to refocus and still fail to get FWHM figures as low as the other filters. I am beginning to suspect the failing is with the ED80 and ZS71 as I rarely achieve an FWHM below 2 and quite often it is more like 2.5 or higher. I occasionally use my Canon 70D with these OTA's and find focussing with BYEoS FWHM very easy but again I rarely achieve an FWHM below 2.5.

I have a manual fine-focusser fitted to the Samyang (from www.AstroKraken.fr). I use a SW motorised unit fitted on both the ZS71 and ED80; I gave up long ago using the autofocus feature in SGPro - it drove me nuts with the lack of repeatability and contributed more than anything else to lost imaging time. Even when using the SW units I still manually 'jog' the focus unit from the software interface on the PC.

A final comment I would make is I live near one of the worst polluted areas in the UK and the biggest obstacle for me trying to achieve a consistently low FWHM is pollution and atmospheric noise - there is nothing particularly clean or stable about the atmosphere around here! Looking to the East I have to contend with the atmospheric disturbance caused by a 2000MW coal fired power station and I know - having worked there - what effect that has on the local atmospherics. With the right temperature inversion conditions the atmospheric disturbance can reach 25,000 feet or more and be seen from over 40 miles away.

Good luck with your deliberations!

Adrian

[ollypenrice]

I favour mono but don't consider myself to be part of any 'brigade.'

One thing needs debunking straight away: focus. The only difference between mono and OSC is that you can refocus per colour with mono but with OSC you can't.  If you have decent filters they will be parfocal. Any shift in focus comes not from the filters but from the optics. With OSC the best you can do is take an average focal point. With mono you can, if you wish, fine tune focus per colour. You don't have to. You could focus through your luminance filter and run your sequencer at the focal point, so getting the same compromise as you'd get with OSC. Indeed on our widefield rig (Tak FSQ106N/Atik 11000 working at 3.5"PP) I do exactly that: focus only in L and run the sequencer. In the high res rig 'TEC140/Atik 460/0.9"PP I do refocus between filters but that is largely because I don't run the sequencer. Instead I exploit yet another of monochrome's advantages: I shoot Lum and blue when the target is at its highest and red and green at lower elevations. With OSC you can't do this. You're stuck with another compromise. (You can't shoot only colour on nights of bad seeing, either, and wait for good seeing for the L. Yet another OSC compromise - though these only matter much in high resolution.)

In an observatory the difference in setup time between OSC and mono is no more than 30 seconds. (You need to write four lots of file names instead of one.) With a mobile setup you have either one or two more cables to connect depending on whether or not the wheel is USB powered.

As you've probably read elsewhere, the expert OSC imagers feel that the inability to shoot dedicated luminance adds about 20% to the integration time. I think that's about right but if the target is Ha dominated then the mono camera streaks away. I did this 2 hour Heart to prove the point. 20 minutes per colour and an hour in Ha. It isn't the best image in the world but it only took 2 hours.

Olly

 

[glowingturnip]

was going to chime in and say what Olly said about focussing for lum and then not worrying too much about the focus on the R, G and B in the same session.

But also if you went OSC, you're going to find it very difficult/impossible to do any narrowband with it in the future.  I never thought I'd get into narrowband, but I was definitely wrong there...

[gorann]

2 hours ago, glowingturnip said:

was going to chime in and say what Olly said about focussing for lum and then not worrying too much about the focus on the R, G and B in the same session.

But also if you went OSC, you're going to find it very difficult/impossible to do any narrowband with it in the future.  I never thought I'd get into narrowband, but I was definitely wrong there...

I do not fully agree that NB imaging, at least Ha, is very difficult/impossible with a OSC. Below is an image I took with my ASI071 (cooled OSC) on an Esprit 150 and a Baader 7nm Ha filter. Having said that, I have now also bought an ASI1600mono to collect NB with another scope side by side with the ASI071.

 

[RayD]

3 hours ago, ollypenrice said:

One thing needs debunking straight away: focus. The only difference between mono and OSC is that you can refocus per colour with mono but with OSC you can't.  If you have decent filters they will be parfocal. Any shift in focus comes not from the filters but from the optics. With OSC the best you can do is take an average focal point. With mono you can, if you wish, fine tune focus per colour. You don't have to. You could focus through your luminance filter and run your sequencer at the focal point, so getting the same compromise as you'd get with OSC. Indeed on our widefield rig (Tak FSQ106N/Atik 11000 working at 3.5"PP) I do exactly that: focus only in L and run the sequencer. In the high res rig 'TEC140/Atik 460/0.9"PP I do refocus between filters but that is largely because I don't run the sequencer. Instead I exploit yet another of monochrome's advantages: I shoot Lum and blue when the target is at its highest and red and green at lower elevations. With OSC you can't do this. You're stuck with another compromise. (You can't shoot only colour on nights of bad seeing, either, and wait for good seeing for the L. Yet another OSC compromise - though these only matter much in high resolution.)

That's very interesting as you don't seem to make any focusing allowance for change in temperature.  

Just as a sanity check for me, are you saying you don't believe temperature change has any effect on focus, that it does but not enough to worry about, or that it does and you deal with it in processing?

I find with my FSQ106 that from the beginning of a night's imaging to the end, if the temperature has changed significantly (more than a few degrees), the difference in counts following auto-focus is considerable, and I would have thought more than enough to have a noticeable impact, albeit I haven't tried by not refocusing when the temperature changes.

[RayD]

48 minutes ago, gorann said:

I do not fully agree that NB imaging, at least Ha, is very difficult/impossible with a OSC. Below is an image I took with my ASI071 (cooled OSC) on an Esprit 150 and a Baader 7nm Ha filter. Having said that, I have now also bought an ASI1600mono to collect NB with another scope side by side with the ASI071.

Nice Ha image, Gorann.  I've not used a OSC camera, although I have used DSLR, and wonder how it fares with other NB filters such as OIII and SII?

[gorann]

5 minutes ago, RayD said:

Nice Ha image, Gorann.  I've not used a OSC camera, although I have used DSLR, and wonder how it fares with other NB filters such as OIII and SII?

Thanks! I have only used the ASI071 for NB with a Ha filter and it may be less satisfactory with the usually quite low intensity signals from Oiii and Sii, but much of the Oiii signal will be collected by the green channel (which means twize the number of pixels of the red channel) so Oiii may work.

[ollypenrice]

23 minutes ago, RayD said:

That's very interesting as you don't seem to make any focusing allowance for change in temperature.  

Just as a sanity check for me, are you saying you don't believe temperature change has any effect on focus, that it does but not enough to worry about, or that it does and you deal with it in processing?

I find with my FSQ106 that from the beginning of a night's imaging to the end, if the temperature has changed significantly (more than a few degrees), the difference in counts following auto-focus is considerable, and I would have thought more than enough to have a noticeable impact, albeit I haven't tried by not refocusing when the temperature changes.

No, the need to refocus due to temperature change is a constant whether using OSC or mono, so I didn't include it my comments. The telescope is the deciding factor on this. The ED versions of the FSQ106 are amongst the most sensitive to temperature change along with the TeleVue 101. I rather like our older fluorite FSQs because they are less temp. dependent.

The other thing is that the pixel scale affects the depth of focus, a coarse pixel scale remaining effectively focused for longer. I normally refocus once an hour in the early evening and then stretch that to 90 minutes as the night settles. If the seeing is improving, as it often does, I'll bear that in mind as well. For me the number of times I focus is no different between mono and OSC.

Olly

[gorann]

2 minutes ago, ollypenrice said:

No, the need to refocus due to temperature change is a constant whether using OSC or mono, so I didn't include it my comments. The telescope is the deciding factor on this. The ED versions of the FSQ106 are amongst the most sensitive to temperature change along with the TeleVue 101. I rather like our older fluorite FSQs because they are less temp. dependent.

The other thing is that the pixel scale affects the depth of focus, a coarse pixel scale remaining effectively focused for longer. I normally refocus once an hour in the early evening and then stretch that to 90 minutes as the night settles. If the seeing is improving, as it often does, I'll bear that in mind as well.

Olly

So you never go to sleep during your imagung nights Olly? Very impressed! I usuall go to bed after the meridean flip, turn on my rain alarm, and hope for the best.

[tooth_dr]

20 minutes ago, RayD said:

Nice Ha image, Gorann.  I've not used a OSC camera, although I have used DSLR, and wonder how it fares with other NB filters such as OIII and SII?

Hi Ray.  Apolgies that this is off topic from the original post, but I wanted to answer your comment.

I've shot Oiii with a normal DSLR, and its faint, but can produce reasonable images.  This is my bicolour Pelican with a 40d, using 12nm Ha and Oiii filters in 900s exposures.  I've also shot the Tadpoles too but the seeing was poor and my processing bad.   Xiga produces some of the best narrowband images with a DSLR I have seen, using Ha and Oiii filters.

 

 

 

[ollypenrice]

Just now, gorann said:

So you never go to sleep during your imagung nights Olly? Very impressed! I usuall go to bed after the meridean flip, turn on my rain alarm, and hope for the best.

I sleep in episodes, Goran! I've a bed in the warm room or the caravan in the field and set a timer to give me 90 minutes between checks. The thing is that other things can go wrong as well and, as a provider, I try to catch them before they do. Besides I'll often be doing some visual observing with guests or helping with problems arising with their own kit. I'm in a different position from most imagers since this is a large part of my job.

Olly

[gorann]

1 minute ago, ollypenrice said:

I sleep in episodes, Goran! I've a bed in the warm room or the caravan in the field and set a timer to give me 90 minutes between checks. The thing is that other things can go wrong as well and, as a provider, I try to catch them before they do. Besides I'll often be doing some visual observing with guests or helping with problems arising with their own kit. I'm in a different position from most imagers since this is a large part of my job.

Olly

Impressed! So when do you usualy go for breakfast?

[RayD]

1 hour ago, tooth_dr said:

Hi Ray.  Apolgies that this is off topic from the original post, but I wanted to answer your comment.

I've shot Oiii with a normal DSLR, and its faint, but can produce reasonable images.  This is my bicolour Pelican with a 40d, using 12nm Ha and Oiii filters in 900s exposures.  I've also shot the Tadpoles too but the seeing was poor and my processing bad.   Xiga produces some of the best narrowband images with a DSLR I have seen, using Ha and Oiii filters.

Wow that's impressive Adam.  I had no idea any OIII would be possible.

Thanks for that.

[RayD]

1 hour ago, ollypenrice said:

No, the need to refocus due to temperature change is a constant whether using OSC or mono, so I didn't include it my comments. The telescope is the deciding factor on this. The ED versions of the FSQ106 are amongst the most sensitive to temperature change along with the TeleVue 101. I rather like our older fluorite FSQs because they are less temp. dependent.

The other thing is that the pixel scale affects the depth of focus, a coarse pixel scale remaining effectively focused for longer. I normally refocus once an hour in the early evening and then stretch that to 90 minutes as the night settles. If the seeing is improving, as it often does, I'll bear that in mind as well. For me the number of times I focus is no different between mono and OSC.

Olly

Ah ok, thanks.  Makes sense also with the ED as that's what mine is.

[gorann]

With my Esprit 150 and ASI071 (=0.94"/pixel) the focus is steadfast all night but then it is also at f/7 which is quite forgiving. Not sure yet how my Esprit 100 with f/5.5 will behave since I just got it before the clouds rolled in.

[Allinthehead]

I have both a mono asi 1600 and Osc asi 071 and i use the 071 primarily and I'm considering selling the 1600. 

I set up the osc and leave it do it's thing all night. I'm lucky as far as focusing goes because my scope doesn't require re focusing during the night. 

What i love about the osc is the fact that i always get an image to play with the following day. To be honest i found lrgb imaging quite difficult and i really enjoy how simple osc is. 

Here's an example of what the 071 is capable of and it's probably capable of more in the hands of someone more experienced. 

My opinion for what it's worth is go for whatever suits your circumstances, sky conditions etc. Mono cuts through light pollution better than a osc.

Richard 

[RayD]

14 minutes ago, Allinthehead said:

Here's an example of what the 071 is capable of and it's probably capable of more in the hands of someone more experienced. 

Well if ever you wanted a testimony for what's achievable with OSC this has to be it.  Great images, Richard.

[mos]

Thanks so much for all the replies and info! I've learned a lot more and seen some spectacular images! And it's great to hear so many different circumstances and scenarios on what people find work for them. I guess that's the main thing...what will work for me. I'm getting the impression that mono might not have that much more involved, especially once I get some more experience. But it's also great to know and see that OSC can produce what I think are spectacular images if I go down that road. 

Richard, Adam and Goran - The OSC images both NB and RGB are wonderful! And Olly - thanks for the 'quick' mono composition! If I ended up producing an image like that I would me more than happy  

Thanks again for all the insights and knowledge provided. I think now that whatever I buy i'll be happy with...just need to commit and go!

- Matt

P.s. I'm actually a bit blown away with the responses and the time people have taken. I truly appreciate it
 

[RayD]

12 minutes ago, mos said:

P.s. I'm actually a bit blown away with the responses and the time people have taken. I truly appreciate it

Welcome to SGL, it's like that all the time here.............often...........sometimes.............occasionally ?

[gorann]

You are most welcome Matt!

If you go for a colour CMOS, you may think about going for an ASI071 rather than ASI294. A significantly bigger chip so more of the photons your scope collects will end up on you chip rather than around it, so more "real estate" as some here call it here. Will allow you in many cases to catch the whole object rather than a part of it.