What is the Mono Imaging workflow

[smr]

Hi,

I'd like to upgrade to a dedicated Astronomy Camera but I'm undecided as to whether to go for a Mono or OSC Camera. 

I don't mind imaging over several nights to achieve a final image, and indeed that is the way I go about imaging with my DSLR. 

At the moment my setup and workflow is very basic, if imaging a nebula I'll just use my Canon 600D with the Optolong L-eNhance Filter and shoot flats at the end of the night. I dither with APT/PHD2 and so I don't use dark frames, I just use flats and bias. 

For Flat frames I just take 20 images at the end each session by using an LED panel. 

With an OSC Camera I understand that I'd benefit from being able to take dark frames by using set point cooling on the Camera and don't think I'd have much problem adapting. 

But for Mono I'm still not sure of what's involved in the work flow exactly.

If I had a Mono Camera and Filter wheel, would I have to take flats for each filter, if I shot in LRGB? 

Would taking flats with an LED panel work for a Mono Camera or is there another way that this has to be done?

It would be great if there was a detailed youtube video / tutorial on all of this but I can't find anything which goes into detail in exactly what is involved in Mono imaging.

One thing I've never done is refocus throughout the night with my DSLR, I'll focus with my bahitnov mask before imaging and then just lock the focus down and leave it, I've seen quite a few posts about having to refocus when using a dedicated camera because the temperature can change over the course of an imaging session, yet I don't understand how this works and is put into practice. If I'm imaging a nebula with my DSLR and the temperature drops a few hours into the session I don't understand how I'd be able to refocus the camera if there aren't any bright stars to focus on with the bahitnov mask. 

If anyone could help with advice on these points I'd really appreciate it.

 

Edited January 14, 2021 by smr

[tooth_dr]

5 minutes ago, smr said:

If I had a Mono Camera and Filter wheel, would I have to take flats for each filter, if I shot in LRGB? 

Would taking flats with an LED panel work for a Mono Camera or is there another way that this has to be done?

 

You can take in luminance only and the flat should be ok for all four filters

The process of taking flats is the same with mono and osc, using the same flat panel.

[tooth_dr]

7 minutes ago, smr said:

I'm imaging a nebula with my DSLR and the temperature drops a few hours into the session I don't understand how I'd be able to refocus the camera if there aren't any bright stars to focus on with the bahitnov mask. 

 

You just accept the compromise of not refocusing.  Or slew to a bright star and refocus, or use a motorised focuser/software which measures all the stars in the field and doesn't require a bright star to work so you don't have to move the telescope.

[smr]

12 minutes ago, tooth_dr said:

You can take in luminance only and the flat should be ok for all four filters

The process of taking flats is the same with mono and osc, using the same flat panel.

ok thanks, so it's just the same as what I do at the moment, if I took 2 hours imaging with each filter, and end up finishing on the green filter for instance I just take 20 flats at the end and that would be ok ?

Edited January 14, 2021 by smr

[smr]

9 minutes ago, tooth_dr said:

You just accept the compromise of not refocusing.  Or slew to a bright star and refocus, or use a motorised focuser/software which measures all the stars in the field and doesn't require a bright star to work so you don't have to move the telescope.

Ah ok, as I use plate solving I could do that, temperature drops a certain amount, slew over to a bright star, refocus and then plate solve back to the target and carry on imaging ? What change in temperature would mean I'd have to refocus? 

[DaveS]

If you're not stripping the imaging train down each time then there's no need to take flats after each session. Just take one set when it's convenient.

[smr]

32 minutes ago, DaveS said:

If you're not stripping the imaging train down each time then there's no need to take flats after each session. Just take one set when it's convenient.

/facepalm 

So you're saying I need not take flats after every imaging session I do if I leave my scope and DSLR on my mount all the time and don't alter anything?

[DaveS]

Nope, you're just wasting imaging time. If everything is left as it is, then just do the one set. Only do another if you notice new dust bunnies appearing, or if you've had to strip the rig down for any reason.

Do take Dark Flats though, this will help your calibration.

[smr]

7 minutes ago, DaveS said:

Nope, you're just wasting imaging time. If everything is left as it is, then just do the one set. Only do another if you notice new dust bunnies appearing, or if you've had to strip the rig down for any reason.

Do take Dark Flats though, this will help your calibration.

ah ok, makes sense! 

Are we talking about mono or CMOS with regard to dark flats? 

With my DSLR I use flats and bias only, should I be using dark flats in conjunction with them or to replace bias ?

[DaveS]

You may need to experiment. With my ASI 1600 I absolutely *must* take Darks, and Dark Flats, but Bias just stuffs everything up. In the case of my old Trius 694 I don't use Darks, Bias does instead.

TBH I'm in the dark myself as far as the new breed of CMOS without amp-glow, so cannot really advise, though I suspect it will be a case of taking Dark frames but not Bias, but that's just a guess.

[rickwayne]

Flats  correct not just  for vignetting in your OTA, but also flaws or dust on the filters themselves.  I certainly  couldn't get away with just doing one set of flats. Fortunately  with an electronic wheel it's super-easy, I just have one sequence  that steps through  all the filters.

The advice you've already  gotten on refocusing is correct. Unless the temperature is changing rapidly, once your gear has cooled to ambient around once an hour should suffice. But it definitely does change, e.g. the critical focus zone on my f/5.5 scope is 74 microns deep, so a contraction of 1/20th of  a millimeter (assuming I  nailed the middle of the zone) will put me out.

The added wrinkle for mono+filters is that unless your filters are perfectly parfocal -- mine sure aren't --  you have  to refocus with  every filter change. Frequently folk with  motorized focusers get around this by recording and averaging how big an offset is required filter-to-filter, then focusing once with the base and blindly  applying that office when the filter changes.

As for dark flats, the idea is to get a base signal from the camera with no light and subtract that. This can be done either with bias frames (shortest possible exposure, no light) but for some cameras, short exposures aren't repeatable, so those people use dark flats -- same gain, offset, and exposure as for the flats, but with the tube covered as for darks.

Edited January 14, 2021 by rickwayne

[david_taurus83]

Regarding focus, I find that if I leave everything out for an hour or 2 before imaging to cool properly, focus doesn't change at all. I use NINA and have the HFR values embedded into the image name so I can see in numbers if focus shifts.

For flats, I shoot for each session. It only takes for 1 spec of dust to shift to potentially waste your hard won data.

Edited January 14, 2021 by david_taurus83