Filter order? LRGB & NB & Sequence of Imaging - lots of questions!

[jiberjaber]

I'm eagerly awaiting my delivery tomorrow of my new toy from FLO, an ASI1600 and filter wheel. 

I've a load of questions I have been working through and I am sure there will be more, but one I haven't been able to google as such is if there is a convention on which filters go in which wheel slots ?  I've got the 8 position wheel coming and the ZWO filter set for LRGB and Ha, SII, OIII

Also interested to hear thoughts on sequencing imaging, seems most might do double on L and the run RGB (i.e. 2min L, then 1M each of R,G & B ) there also seems to be various views on should it be all on each filter or run them in sequence eg 

A: 60 min of Lx2min subs followed by 30 mins of Rx1m, 30 mins of Gx1m, 30 mins of Bx1m

OR

B: 2xLx2m then 1xRx1m then 1xGx1m then 1xBx1m, then loop around back to L to start teh sequence run again x 30 runs.

I can see some pros and cons of both above, but B seems like it might be less efficient (focus on each lens change?)

And finally, it's a bit of a step change from DSLR to mono I get the new calibration requirements but still a couple of queries...  at the moment the main one is, so I need to take flats for each filter used? (I am assuming dark-flats can be for a single filter, say L?)

 

Looks forward to some hints, tips and answers

 

Cheers

 

 

[MarkAR]

If you are running a computer controlled filter wheel the order doesn't matter as you have to tell it which filter is in what position. It will then select filter by position.

If it's a manual wheel stick to LRGBHOS.

Sequence wise for LRGB I have been running equal length LRGB, same quantity, binned 1x1. Others bin RGB 2x2. BTW I use a mono CCD not CMOS.

You should focus on each different filter and a few times per filter to take into account atmospheric and temperature shift. 

There is a YouTube video of Dr Robin Glover speaking about imaging times, one thing I gathered is that he seems to be saying  RGB should be 3 x longer than L. And NB maybe 10 x.

You will need to take flats for each filter. Also if you change binning you will need to take separate darks, flats and bias at the new binning rate. Mind you the 1600 is a CMOS so not sure if you can do binning.

[jiberjaber]

Thanks Mark. Useful info.

[DaveS]

ATM I've been running 10 min RGB and 5 min L on galaxies with my 1600 set to Max DR. The globs I've been doing I ran 5 min RGB (No L) to avoid burnt out cores. NB I tend to do at 10 min.

[Skipper Billy]

Your suggestion B will make sure you get an image albeit it could be limited data but if you refocus on every filter change you will use a lot of time focusing instead of imaging! Don't believe the hype about filters being parfocal.

Personally, I decide how long my chosen target will be visible and how long I can image for - is it a 'school day' or weekend etc. Then lets say I have 6 hours and the target is visible throughout. I will take 3 hours of Lum binned 1x1 and an hour each of RGB binned 2x2.  If I am imaging in narrowband then I usually split the time available into thirds and bin 1x1. Occasionally I will just take RGB binned 1x1 ie no Lum in which case the time is split into equal thirds.   If a session is clouded over you can always carry on where you left off.

The only calibration frames that are dependant on the optical train are flats - darks and bias not dependant on imaging train. I have always built a library of flats for every exp time I am likely to use and for each filter and each binning. I have recently learned of one world class imager that takes flats with Lum and uses them for all subs irrespective of the filter used working on the basis that dust is most likely to be on the sensor or sensor window ie not on the filters. I am going to try this next season. If it's good enough for him it's certainly good enough for me !!! Just need to make sure the filters are clean.

My filter wheel is loaded L R G B HA Sii Oiii - purely so I can remember the order although it is computer controlled.

Welcome to mono imaging - it's great when you get your head round it!!

 

[jiberjaber]

 

 

Dave, My initial plan from what I've read is to start with unity gain, a set bias and -10 degC then see how it all goes. Those exposures seem a lot longer than I was expecting for RGB, is that because of the bias offset you I guess?

Thanks Skipper - sounds like plan A is the way to go then, adapted to the amount of time available to image

Edited May 19, 2020 by jiberjaber

[Skipper Billy]

Cant help you with that - I am strictly a CCD kind of guy (old school!). Never used a CMOS camera sorry.

 

[tomato]

Hi,

I also would not support running the filters in sequence but cycling through each filter after a single exposure. Although this will allow you to build up data on all channels in a single session, this will not be very good use of your imaging time as you will spend a disproportionate amount of time moving the filter wheel and refocusing. 
It is also considered good practice to take Lum and Blue frames when the target is higher in the sky as the atmosphere has the greatest adverse impact on these channels.
 

+1 for the Robin Glover video, my CMOS imaging improved a lot after following his guidance.

[jiberjaber]

1 minute ago, Skipper Billy said:

Cant help you with that - I am strictly a CCD kind of guy (old school!). Never used a CMOS camera sorry.

 

No you have helped - thanks

Plan A just refers to taking as much as can on each filter in turn i.e. say 60 mins on L and then 120mins on each of RGB possibly split over a couple of nights.

There seems to be a table on CN which implies a lot shorter exposure for Bortle 5 skies - I think trying it will be the best approach and then see how that pans out  

 

[jiberjaber]

4 minutes ago, tomato said:

Hi,

I also would not support running the filters in sequence but cycling through each filter after a single exposure. Although this will allow you to build up data on all channels in a single session, this will not be very good use of your imaging time as you will spend a disproportionate amount of time moving the filter wheel and refocusing. 
It is also considered good practice to take Lum and Blue frames when the target is higher in the sky as the atmosphere has the greatest adverse impact on these channels.
 

+1 for the Robin Glover video, my CMOS imaging improved a lot after following his guidance.

Ta, Dr G seems to be on quite a few videos on youtube when I google "Dr Robin Glover speaking about imaging times" is there a specific one ?

[Laurin Dave]

On my ASI1600 I use Unity gain at -10C all year, 1 min for Lum, 2min for RGB and 5 min for Ha, Oiii and Sii.  any longer on LRGB and bright stars are blown out and have no colour in the core.  For LRGB images I would allocate time 1/2 Lum   1/6 each RGB..  for NB 1/3, 1/3, 1/3..  I'd do a minimum 30 minutes each channel before changing filters to avoid wasting time focusing..  Shoot Red when the target is low G and B when its higher and Lum when its highest as this will give the best resolution, if the seeing is poor don't bother with Lum - do it another time when seeing is better.   In the absence of any Lum data you can make a Synthetic lum from the RGB data

For the ASI1600 calibration requires matched darks and flat darks ie same gain offset, temp and exposure time to remove the Amp glow..  I find its best to do these with the camera off the scope with its cap on facing into a desk, that way you get no light leaks, before you do this though you'll need to figure out what exposures you need for your flats..  so you can get the appropriate flat darks, simple enough if you use a light panel or Ipad..  The Darks and Flat Darks will work well for a long time..  (I renew them at least once a year ) ..  When you calibrate your Lights and Flats make sure that in whatever App you are using that any Dark Scaling or Optimisation is turned off

This is the link to Robin Glovers talk -   I'd better watch it!

 

 

[tomato]

4 hours ago, jiberjaber said:

 

There seems to be a table on CN which implies a lot shorter exposure for Bortle 5 skies - I think trying it will be the best approach and then see how that pans out  

 

@Laurin Dave has kindly provided the link to Robin’s talk, I think  the table you found on CN is from the presentation, well worth a look.

[jiberjaber]

3 hours ago, tomato said:

@Laurin Dave has kindly provided the link to Robin’s talk, I think  the table you found on CN is from the presentation, well worth a look.

Wonderful, thanks both - I'll give that a watch through.

I did assume flat-darks were just a case of replacing my EL panel for the scope cap and shooting at the same gain, bias, temperature and exposure as the flats?