Narrow Band Imaging in Bortle 1 Skies

If you can’t do 5 min subs then definitely think about replacing the mount. I mean even my Meade LXD55 virtually recovered from a skip can do that. In B1 sky I cannot find many reasons if any at all to go tight NB, especially with small, slow refractors and, yes, IMX571 can show some noticeable banding after some time when the BG level is virtually nil and the calibration isn’t absolutely perfect. Same here after 2 year of using it. Point is the is very little difference, as NB bandpass get smaller and smaller, from a B8 site and a B1 site. That’s the whole point of having them. Unless you have a faulty unit you read-out noise should be in the range of 1.5e- but you best check with SharpCap the actual results from your preferred mode of operation.

It is strange that you can’t do 5min subs with an harmonic mount. Theoretically if you are guiding you could make exposure as long as you want and get pinpoint stars discounting transient events like clouds or gusts of wind. If you are not guiding then this could be the answer since an harmonic mount need to be guided, otherwise this technology’s large periodic error will mess up your subs even at relatively short exposure.

Now for the second part, while theoretically as you computed the optimal sub exposure length can be very long under dark sky, remember that this is only a computation of the time required so that the read noise contribution vs the sky glow noise contribution is minimized relatively to your LP level. By that I mean that a 5min sub in a Bortle 1 sky will have far, far, far less noise than a 5min subs under Bortle 9, even if the former is ten times less than the “optimum” for a B1 sky, while the latter is more than enough to minimize the relative contribution of the read noise vs LP noise in B9. Its just that any remaining noise in the B1 sub will be mostly read noise, but the total amount of noise itself won’t be much (especially using an IMX 571). I’m suprised that you mention very noisy stacks with artifacts under such conditions to be honest.

I quick look at IOTD winners on the bin will show that 300 to 600 sec. is the most common for narrowband under b1-2 skies.

I think showing us pictures of your problem would be beneficial to helping you here…. but one thing I keyed in on is using ‘HCG’ and minimum gain. Would this imply you are using gain zero?

I run my OGMA at gain 200. I chose this because the ADU counts most closely matched up with my QHY-268M that I was running in cooperation, dual imaging on separate mounts.

Could this be your issue?

P.S. Yes I am fretting too much about what the calculated sub exposure times should be versus reality.

Steven Zuhoski · May 12, 2026, 09:15 PM

Thanks folks. I think I got my answer: I should run narrow band sub exposures between 300 and 600 seconds and not worry about swamping out the read noise.

I run my OGMA AP26MC most of the time in HCG mode at the minimum gain for that mode: Gain 100. With these cameras I can run it in HCG mode (or LCG) without having to set the gain to a particular level. I have used higher gain settings with narrow band filters, but you pretty quickly start getting ADU counts at 65K with modest sub exposures (~5 minutes).

I run all my IMX571’s at 20 minute narrowband exposures standard, save for certain targets which are really bright to start with. Those I’ll run 10 minute since it helps build rejection quicker as well. I’ve done a couple runs at 40 minute narrowband but the gain is not worth the risk in rejection.

As for broadband I typically run 5 minutes standard, with a fewer dimmer targets in the 10 minute range.

You speak about ADU’s reaching 65k, but what you’re probably seeing is just the stars saturating here and there. Depending on the mag of the star, this can happen on even the shortest exposures and you should not be too concerned. There’s alot of misinformation running around the internet, I urge you to experiment for yourself.

I have done alot of experimenting myself. Had a rough night of bad seeing and thin clouds, so I decided to attempt 1 hour subs, just to test the ability of the camera. Now, let me preface by saying I am in no way recommending you run 1 hour exposures, the cores of two of the galaxies are in fact blown out, however… this image is still very recognizable. Here is a single 1 hour luminance exposure taken with the same exact camera you talk about above, the AP26MC. Gain 200. The flaws in the background are from thin clouds, however you can easily see I’ve picked up the entire tidal tail in a single sub, from a bortle 4 sky.