Advice on duo-band filter selection for astrophotography setup

I have been shooting with the ALP-T for several years and the D2 for over a year in Bortle 7 suburban skies with f /l ranging from 250mm to 1550mm and find that combination works well. I have been using 2” filters with a filter wheel and the ASI2600MC for the past 2.5 years, and before that the ASI294 MC PRO, and had similar good experience. With your much better skies your experience might be quite different, but I think you will find the investment worthwhile. You can see many examples in my gallery if it would help.

I also have the narrower Ultimate HA/Oiii filter I find myself using occasionally, but most of the time I stick with the ALP-T.

Being in Bortle 9, I use dualbands with OSC (a) Altair 1.25” 6nm bandpass dualband filters (HaOiii and SiiOiii) w a 294MCPro on a Meade ETX125 running at f15, and (b) Antlia 2” ALP-T 5nm bandpass (HaOiii, HbSii) on an f5 full frame setup.

Both combinations work wonderfully, and I haven’t felt the need to go for the tighter bandpass in any of those.

Tbh, the main thing that strikes me is that if you’re imaging from Bortle 1-2, these filters are going to cut down the signal coming through so much that to swamp your read noise you may need very long images -how good is your mount?

I looked into this a little while back, and the conclusion I came to was that in Bortle 1-2, the sky noise is so low that the read noise on the sensor becomes the big factor, and that to swamp that you need either much wider bandpass, or very long exposures? But other astrobinners may be more experienced directly with this.

Leela.Astro.Imaging · Aug 31, 2025, 11:40 AM

Tbh, the main thing that strikes me is that if you’re imaging from Bortle 1-2, these filters are going to cut down the signal coming through so much that to swamp your read noise you may need very long images -how good is your mount?

Pretty good, I think. It’s an AM5 and the only mount I’ve ever had.

I shoot 300 second subs for most targets, except bright stars and really bright things like M16 or M42. (I need to re-do M42 because the core completely saturates the sensor after a minute or so :-/ )

Before I got the guiding gear (and the RedCat, an the Meade de-forked) I used to shoot 300s subs un-guided with my 200mm zoom lens and that used to be fine.

I think that mount should be able to handle it (I’ve never used one, but lots of people do) - as @Tony Gondola points out above at gain 120 you can get shorter exposures, although the full well drops so you will have lots of files to process. Hope your imaging PC is fast.

I think in SQM 21 skies you’d still need pretty long exposures with an Antlia dualband (18 minutes at gain 120 by my calcs if you have 4nm bandpass filters?) so you may want to only use those filters when the moon is out and SQM is around 18 or so.

Tony Gondola · Aug 31, 2025, 12:34 PM

The reason standard filters won’t work with very fast optics is because the bandpass of the filter will actually shift because of the steep angle of the incoming light. The basic effect is this moves the filter’s bandpass away from the desired frequency, reducing signal.

Considering the fact that you already have Ha/Oiii I’m not sure you need anything else. You have very dark skies so I would think that going UV/IR would be the best solution on most nights. With OSC, Sii data isn’t going to add very much as it’s very close in color to Ha which is going to dominate.

On read noise, as long as you’re at gain 120 with the 294, read noise is so low that you really don’t have to worry about it. Exposure times as low as 48 sec, will give you a swamping factor of 12. Even 15 sec. exposures will be usable at a swamping factor of 3.

I’m not sure I understand why Sii wouldn’t be viable with OSC? You can simply extract the individual narrowband channels, after which it’s pretty much the same as processing mono images (other than the decreased efficiency of course).

I’m not sure what you mean here by ‘swamping factor’, but with OP’s setup 15s subs is FAR from usable.

Assuming OP’s skies are mag 21.8, with the 294mc, 7nm filters, all at f6.3, they’d need to expose for ~158s before the background noise would equal read noise, still far from swamping in my opinion. I’m guessing you assumed a UV/IR cut filter was used instead of a narrowband filter.

Read noise in modern cameras seems low until you do narrowband from a dark site ;)

It gets really ugly once you start going deep on a target. Then all kind of banding, mottling, walking noise ect. start appearing. It’s not something that can be easily fixed by noise reduction either. In Bortle 8 I feel like you might struggle to reach the integration time needed to see these artifacts, since like you said, LP noise would dominate instead.