V Curve from NINA & Moonlight Nightcrawler

Is this repeatable?

If you are using hocus focus you can enable saving the focus images so you can inspect strange outliers visually.

If you get an hfr of 0 something obviously went wrong.

What settings are you using for the focus curve interpolation?

Maybe N.I.NA. log would reveal what caused it?

Very likely a small passing cloud. If it doesn’t happen regularly, I’d ignore it.

John Hayes · Nov 2, 2025, 04:19 PM

Jerry,

I’ve found that that’s not uncommon with NB filters if the exposures are too short. Try increasing the exposure time by maybe 50% just to see if that fixes the problem. If it does, then dial the exposure down a little to keep it above the threshold where you get this problem.

John

Hi John,

Yep, and on top of using an Ha filter when this occurred, the moon is getting full. Between the filter and the moon it’s no wonder this happened. I increased the exposure time to 10” and this helped. I’m still getting over 10 points on the curve and the HFR is normal, so I think it’s OK.

Thanks,

Jerry

Jerry Gerber · Nov 2, 2025, 05:13 PM

Hi John,

Yep, and on top of using an Ha filter when this occurred, the moon is getting full. Between the filter and the moon it’s no wonder this happened. I increased the exposure time to 10” and this helped. I’m still getting over 10 points on the curve and the HFR is normal, so I think it’s OK.

Thanks,

Jerry

I’m relatively new to NINA and maybe there’s a way to solve it but I haven’t figured out how to designate which filter I’d like to use for focusing when shooting narrow band. In SGP, I can tell the system to use the red filter for focusing when shooting Ha. In NINA, it appears to force me to use the Ha filter for focusing which requires super long exposures—and that’s just crazy. Maybe someone can jump in to tell us how to fix that problem, which would fix the kind of data dropout that you originally asked about.

John

John Hayes · Nov 2, 2025, 09:04 PM

I’m relatively new to NINA and maybe there’s a way to solve it but I haven’t figured out how to designate which filter I’d like to use for focusing when shooting narrow band. In SGP, I can tell the system to use the red filter for focusing when shooting Ha. In NINA, it appears to force me to use the Ha filter for focusing which requires super long exposures—and that’s just crazy. Maybe someone can jump in to tell us how to fix that problem, which would fix the kind of data dropout that you originally asked about.

John

I’m still using legacy NINA and in the autofocus section, you can choose the filter used for AF. I have a backup computer with the latest NINA, though, and like you said, I don’t see where you can choose an AF filter…..wonder why not. In any case, you’d want to have offsets, or parfocal filters, if not focusing with the filter you’re imaging with.

Cheers,
Scott

Scott Badger · Nov 2, 2025, 09:17 PM

John Hayes · Nov 2, 2025, 09:04 PM

I’m relatively new to NINA and maybe there’s a way to solve it but I haven’t figured out how to designate which filter I’d like to use for focusing when shooting narrow band. In SGP, I can tell the system to use the red filter for focusing when shooting Ha. In NINA, it appears to force me to use the Ha filter for focusing which requires super long exposures—and that’s just crazy. Maybe someone can jump in to tell us how to fix that problem, which would fix the kind of data dropout that you originally asked about.

John

I’m still using legacy NINA and in the autofocus section, you can choose the filter used for AF. I have a backup computer with the latest NINA, though, and like you said, I don’t see where you can choose an AF filter…..wonder why not. In any case, you’d want to have offsets, or parfocal filters, if not focusing with the filter you’re imaging with.

Cheers,
Scott

If you select Filter Offsets button at the top of the Options→Autofocus page, it’ll show the option to set the AF filter on the bottom.

Thanks for all of the answers about AF in NINA. Yes, I know where to set the AF filter, but that’s not what I’m looking for. NINA appears to assume that you will always have offsets set up and that’s certainly the best way to operate. However, it is also possible to operate without offsets if you can choose to focus for NB imaging using a broadband filter. The difference in focus position for a red filter and a Ha filter will be zero. So, in this case, I simply want to tell NINA to use the red filter for focus whenever the sequence is calling for the Ha filter to image. That is easy to do in SGP so I’m wondering if the same capability exists in NINA.

John

John Hayes · Nov 3, 2025, 12:47 AM

Thanks for all of the answers about AF in NINA. Yes, I know where to set the AF filter, but that’s not what I’m looking for. NINA appears to assume that you will always have offsets set up and that’s certainly the best way to operate. However, it is also possible to operate without offsets if you can choose to focus for NB imaging using a broadband filter. The difference in focus position for a red filter and a Ha filter will be zero. So, in this case, I simply want to tell NINA to use the red filter for focus whenever the sequence is calling for the Ha filter to image. That is easy to do in SGP so I’m wondering if the same capability exists in NINA.

John

Then just set the offset to 0.

Ok, it sounds like what I’m asking for is possible so I’ll just have to go play with it a bit more to figure it out. Thanks everyone!! - John

One thing that I would note about offsets in NINA is that a lot of folks use the “filter offset calculator” to obtain the offsets. I use this as well but have found that it can yield often wildly different offsets from one run to the next, despite having a high quality focuser (Nightcrawler, Optec, etc.) set up properly and a well set up focus routine that yields nearly perfect curves and high confidence levels. I suspect it is not the routine but the variable conditions between runs that are the cause. The short version is never depend on just one or two runs.

To the “but that is not possible” folks, I would suggest that you do a few runs. You will find the results vary and what is worse, the variance is often significantly greater than the depth of focus for your system. Use those results and you all but guarantee bad focus.

My procedure is to figure my DOF and then do many runs of the calculator, keeping track of the results for each. When you have at least 6-10 runs, you should be able to reject the outliers and converge on an offset that is within the DOF for your system.

One of the things about a good set of offsets is, of course, speed, since you do not need long exposures for the narrowband filters but it also lets you (in NINA) specify multiple exposures per focuser position in the curve, which statistically improves the results. That does take a bit more time but nothing like what focus per narrowband filter does.

Results may vary but I always focus with the green filter. It is not much more time consuming than the luminance but yields smaller stars and is mid-wavelength so works well for both red and blue.

I tried that test you suggested Bill, and I got a max variance of 4 focuser steps which is 0.25 degrees of rotation and a median variance of 1 step or 0.0625 degrees over 10 runs. My maximum filter offset on my Antlia set is also 4 steps, which may well just be variance on a basically parfocal set since 5 of the 7 from a 3 run average have offsets of 0 steps, but on my little scope this doesn’t make any meaningful difference. Might be enough to upset something with a longer focal length though.