Mono imaging - a new adventure.

[Ouroboros]

After 12 years or so of OSC imaging by various means I have finally taken the big plunge and bought a mono camera (ZWO ASI2600MM), filter wheel, LRGB and sulphur, hydrogen and oxygen (7nm) filters.  I am running with an ASIair. 

I shall be doing some wide field imaging initially with my ASKAR FRA 300mm focal length telescope equipped with electronic focuser and OAG  guiding. 

OK, I’m hoping that some of you hoary old hands at the mono game will give me some pointers.

The age old questions that occur to me in my new venture are:

Exposure times, gain etc

I have pretty much stuck to 120s  and a gain of 100 with my ASI2600 OSC camera. 

What’s the rough rule of thumb for exposure times for LRGB and mono camera. 

Are much longer times used for narrow band!

Are there any good online calculators for exposure times? or recommended  video tutorials etc. 

I realise I will have to do flats and darks for each filter. I’m getting a Deep Sky Dad light panel to help with that.

I am assuming it’s a good idea to re focus at each filter change.

I think that’s about it. Have I missed anything? 
 

 

 

[Vroobel]

I have the same 2600MM camera and filters with the same width but I work with NINA.

The ASIair sets an offset at 50, so there is no way to change it. The gain at 100 is OK, while some imagers (including me) use a 101 value to ensure the High Conversion Gain is reached. I started with 300s SHO, but I live under Bortle 9+ sky, so recently moved to 180s. I feel that the moderately narrow SHO filter may pass some LP, so the 180s frames seem optimal. The LRGB frames depend on condition, Moon phase etc. I use 15s luminance and 30s RGB which gives quite good results. The shorter frames result in a bigger number which may affect processing time, but there is another benefit: it's less painful if you have to remove bad frames. BTW, take into account a rule of LRGB exposure time: a total integration of the Luminance should be equal/comparable to a total integration time of the RGB.

I did 60 bias frames (once) and 60 dark frames for each exposure time I assumed would use (15s, 30s, 60s, 120s, 180s, 300s). I operate with -10*C. You can prepare the bias and dark frames on any cloudy night. As the filters are closed in the sealed filter wheel, there is no risk of dust appearing on them. You can prepare generic flat frames for each filter, but it's highly recommended to collect the flats every time after the session. I do 30 flat frames of each filter I used at the time. 

The focusing was the main reason why I moved to NINA, even if I still have ASIair Plus to work with my 2600MC. I should mention that I use ZWO EAFs. NINA offers a great feature called filter offset. It performs a multiple procedure of determining the focus for each filter and 'remembers' the offset (number of the stepper motor steps) between each filter, so the system refocuses (if needed) using the Luminance filter while the offsets stay constant. This way you can easily perform the same number of SHO or LRGB frames each night instead of imaging with one filter per session. Unfortunately, the ASIair doesn't offer this feature. 

I hope it may help.

Good luck  

[Ouroboros]

@Vroobel Thanks. That’s very helpful. As short as 15s and 30s for LGB? I hadn’t expected that. I know from watching Robin Glover’s video that shorter times are in order, particularly for higher sky brightnesses.  Processing is not so bad as I have a new MacBook, but even so I can expect longer processing times (x3 or x4) than even now with my 120s frames. Phew! 

I’m used to doing flat frames each time - just not up to 7 sets of them! I can’t imagine one uses all filters in a session anyway. I don’t take biases because it’s included in the dark frames. I’ll build  a library of master dark frames.

Interesting what you say about focusing. I know that the ASIair can be set to change focus on each filter change. I use the  feature now to check focus every hour and on meridian flip. Seems to work well. Focusing on each change wastes a couple of minutes of imaging time of course, which is the advantage of your NINA system. 

[Vroobel]

I didn't mention that the filter offset shows its power when you do short sets of the frames, e.g. 6 x 15s L (90s), 1 x 30s x RGB (90s) or  3 x 180s x SHO, NINA allowes for it. It gives you roughly a constant proportion of frames per filter per session. You can add data session by session and compare them with previous results to know if it's enough or you should add something, like OIII or SII. 

I love the ASIair for its simplicity and it's perfect for a mobile setup with OSC camera, but it stays far behind the NINA in terms of the mono imaging.

 

Edited yesterday at 12:59 by Vroobel

[Elp]

It all depends on what your local LP is like and the target you're imaging, there's no set value that can determine this for you as locations vary around the world. Best thing to do is try a few previews and determine for yourself, the stability of your setup, where the setup is pointed in the sky, also has a bearing.

Whether im doing OSC or mono NB, I very rarely diverge away from 60-120s. Special cases are when I'm doing fast (F3 or quicker) mono RGB, due to my LP I'm limited to less than 30s if pointed high, sometimes less than 10-15s lower in altitude. Mono NB normally works fine at 60s, if I want to take less images I go up to 180s, beyond that mount stability and guiding becomes an issue depending on the FL I'm imaging at.

So there isn't really a set rule (well there is) but as long as your signal comes though the LP noise level you should be fine. I've even imaged very faint signal which you can't see on a single sub, but in the sessions stack it shows.

 

[Budgie1]

I go the other way when imaging SHO, I have the ASI1600MM Pro and 3-4nm SHO filters, so I stick to 300s subs.

My location is Bortle 2-3, so little to no LP to worry about. I very rarely use more than one filter per session, preferring to capture a night's worth of Ha, then Oiii and finally Sii. This way I have enough for a good image, even if it takes longer to get the subs from all three filters. 

I use APT which, like NINA, has the option to record filter offsets for focusing.

If you're taking LRGB data, as @Vroobel has already said, spend a lot more time on the Lum data as that's where the detail is. The RGB data can be much less per channel because you're only using it for the colour, not the detail.

[Vroobel]

Some imagers suggest that the RGB can be captured with Bin2 while the L with Bin1, as it's the detail carrier which @Budgie1 mentioned about. Similarly to the L, also Ha seems good for Bin1 while the OIII and SII can be captured with Bin2. I tried once the LRGB and once the SHO, but I wasn't happy with the results, maybe I did something wrong. Bin2 allows you to grab more data in a shorter time, so it should be beneficial. It's up to you. 

Edit, 
Bin2 requires another set of all calibration frames, I was surprised when I faced it first time.  

 

Edited yesterday at 15:25 by Vroobel

[Ouroboros]

38 minutes ago, Vroobel said:

Some imagers suggest that the RGB can be captured with Bin2 while the L with Bin1, as it's the detail carrier. Similarly to the L, also Ha seems good for Bin1 while the OIII and SII can be captured with Bin2. I tried once the LRGB and once the SHO, but I wasn't happy with the results, maybe I did something wrong. Bin2 allows you to grab more data in a shorter time, so it should be beneficial. It's up to you. 

Edit, 
Bin2 requires another set of all calibration frames, I was surprised when I faced it first time.  

 

OK. I was going ask about binning but forgot so thanks for that. I think I might keep it simple at first. 

[ONIKKINEN]

Not a mono imager myself (yet), but will say that binning does not have to be done at the time of capture. You can bin the subs after calibration, or do it easier still by binning the stack. With either of these methods you can use your existing bin 1 dark library for all the data you take and dont need a second set of darks. With CMOS sensors the only benefit from binning on camera would be smaller file sizes, so if you are low on hard drive space it is a decent option. Otherwise i would capture, calibrate, and stack everything at native resolution and then figure out the appropriate bin factor in post processing.

[Skipper Billy]

Exp times are all about the target and your location - I cant comment on Gain as I am a Luddite and use a CCD camera.

For Narrowband I usually use 900 second exp. and for LRGB it is heavily target dependant for something with a really high dynamic range it could be as little as 60 seconds but for very faint targets it could be 600 seconds - all binned 1x1. I have tried capturing RGB binned 2x2 and the Lum 1x1 and it worked well for me.

I have Bortle 2 skies though!

Take a test exposure and check that nothing is blown out - the other limiting factor is how well your mount tracks and how good your guiding is.

I refocus on every filter change - even with filters that are supposedly parfocal - I also refocus whenever the temperature changes by more than 2 degrees - my scope is very sensitive to this. My programme is also set to refocus if the HFR value changes by more than 5%

 

 

 

[Ouroboros]

2 hours ago, Skipper Billy said:

Exp times are all about the target and your location - I cant comment on Gain as I am a Luddite and use a CCD camera.

For Narrowband I usually use 900 second exp. and for LRGB it is heavily target dependant for something with a really high dynamic range it could be as little as 60 seconds but for very faint targets it could be 600 seconds - all binned 1x1. I have tried capturing RGB binned 2x2 and the Lum 1x1 and it worked well for me.

I have Bortle 2 skies though!

Take a test exposure and check that nothing is blown out - the other limiting factor is how well your mount tracks and how good your guiding is.

I refocus on every filter change - even with filters that are supposedly parfocal - I also refocus whenever the temperature changes by more than 2 degrees - my scope is very sensitive to this. My programme is also set to refocus if the HFR value changes by more than 5%

Wow! 900 seconds. 15 minutes. Is that with 7nm bandwidth or narrower? My guiding is pretty typical ie around 0.75”.  But with the occasional hiccup I think I’d be concerned to run for that long.  I used to run 300 seconds with my old DSLR and quite often had to throw frames away with elongated stars - usually caused by wind gusts. 

I’m imaging at Bortle 4 or 5. So almost certainly shorter times with LRGB I’d have thought.

Thanks for your comments.  

[Clarkey]

FWIW, when using my IMX571 camera or 1600mm pro I generally use 120 - 180 seconds for RGB (30 seconds for stars only) and 180 - 300 seconds for NB. Technically I could go shorter whilst still swamping noise, but this is a good compromise. I have just moved from 7nm to 3nm filters and 300 seconds works well. I live in B6 (despite the clear outside claim I live in B4!).

At the end of the day, you will not notice a huge difference between OSC and mono in terms of sub length. I would use the same as you were and adjust if needed.

[Skipper Billy]

23 hours ago, Ouroboros said:

Wow! 900 seconds. 15 minutes. Is that with 7nm bandwidth or narrower? My guiding is pretty typical ie around 0.75”.  But with the occasional hiccup I think I’d be concerned to run for that long.  I used to run 300 seconds with my old DSLR and quite often had to throw frames away with elongated stars - usually caused by wind gusts. 

3nm filters.