One filter per night or split them up?

Hi Sean,

in my case, I decide what filter to use on the moon phase. If the moon is not visible or only very small, I go for OIII, because it is effected by moonlight the most. The signal is usually very faint and you lose detail if the sky is illuminated too much. If the moon comes up, I go for Ha. SII is used somewhere in between. I usually need enough nights to get my data, that the seeing differences are not too much of a problem. This method is also no law and I break it now and then, depending on how much total exposure time I already collected per filter.

CS
Christian

Sam Badcock:
I've just started shooting NB and Christian's comment is quite helpful!!

I've been trying to get "equal" amounts of frames for each filter as currently the moon is low and in early phase but generally I'm playing it by ear depending how much I've already collected for each filter so far

CS
- Sam

it is helpful to research the target. Unlike in broadband, there can be rather large differences in the strength of signal between various bands. You’ll find you have to devote a lot more time to the weaker bands (usually OIII or SII) to get a good image. Otherwise, Christian’s advice is very good. Use the phase of the moon to determine which signal to gather.

I always try to get a full set of RGB, or NB per night. Reasons:

1. If weather gets worse and I cannot observe the next day(s) I have at least a full set to combine - even with less data
2. When I shoot at one night RGB the seeing condition are similar to each filter and therefore HFR is quite identical. This avoids weird color halos when stacking
3. If I would use only one filter and a technical problem occurs, I cannot get a result since RGB is incomplete
4. Identical environmental conditions for each filter (moisture, seeing, wind, temperature, …)
5. Since automated AF is no problem anymore and you have to refocus anyway regularly over the night, switching filter is no pain anymore

If I do a BB+NB the I try to shoot the BB when moon is less disturbing.
I am choosing the RGB filter sequence based on altitude of the target over the night (red low, blue high).

Yep, moon phase makes sense. 

what i’m questioning is when i have a string of moonless nights.  Is it better to spread seeing/transparency (all filters each night) or do a single filter to keep seeing/transparency the same for that filter?

if i do a single filter, and conditions end up being excellent (it’s virtually impossible to predict), the next filter might not have as nice a night. 

or… will multiple different seeing/transparency nights screw with stacking more for the same filter?

The problem at least for me is, that you never know how many clear nights you have. Even if the forcast looks positive, it rarely stays the same at least where I live. There were times when I got lucky, but that's rare. Especially moonless nights are rare here. It feels like good wheather only happens when the moon is really bright up in the sky. I do as much faint stuff (usually OIII) as I can during those nights. Otherwise it will take too long for my projects to finish. But that's just me and my location. It all depends on your situation and I am not saying this works for others. Ah yes… maybe that's the reason I do much more NB photography than broadband.

EDIT: In the winter, the nights are much longer. Then it is easier to work. In summer, it's much harder.

But you all get the point. There are some things to think about when doing astro photography. We all know this is not an easy hobby we've chosen

I tend to use each filter each night. That way, as my subject rises to the best location I will get the benefits in all filters. As the clouds roll in, I will feel the effect in all filters equally. I tend to do something like 30 minutes per filter, then rotate to the next filter so ratios never get far out of whack if I have some bad seeing or clouds or the like. I tend not to image (except H-alpha) if the moon is up at all. OIII signal is simply too weak to handle much moonlight, even through a 3nm narrowband. As others have mentioned, I often gather a lot more subs for OIII than for HA. I try to equalize the SNR to be about the same in each stack. I usually start with 2x as many OIII exposures, then adjust as required for the particular subject to keep the SNR about equal.  I don't happen to have an SII filter, so I am limited in narrowband to HA and OIII.

For broadband, I actually use different subexposure durations for each filter so that my SNR is equal if I just do the same number of images with each. This ensures that all filters are yielding 5x RN^2 exposures (my chosen preference). As with narrowband, I shoot all filters throughout the night so they all experience essentially the same "good" and "bad" skies.  The one exception is that if I'm shooting luminance frames, I tend to do that when the subject is at its maximum altitude in order to maximize my chances of sharp luminance frames.

- Jared

BTW, if you trust your equipment, it may be worth investing the time to calculate the focus offsets for each filter. In NINA, you can do an automatic run for this. I assume, other software offers similar routines. If you do so, you don't need to refocus between the filters and can switch them in a filter wheel on a "per subframe basis". The focuser adjusts slightly after each filter change, which takes only seconds. This is really helpful and I use this sometimes with broadband images. With narrowband, I keep my moon phase logic and image in blocks.

I guess, we all use filter wheels for switching filters. I remember times when I have to change them manually. In this case, I would image in blocks anyway, because I'm lazy and I have to go out a lot otherwise.

I shoot one filter per night with a ratio of 1:2:2 for HSO. On my faster rigs I can get away with only one night of SII and OIII. The main reason is that shooting three flats and flat darks each morning is cumbersome, especially when I am running 3 rigs!