Review of “A novel infrared imager for studies of hydroxyl and oxygen nightglow emissions in the mesopause above northern Scandinavia - Peter Dalin et al. 2023”

The authors present a new imaging system that measures the OH and O2 airglow emission originating from two different altitude layers in the middle atmosphere. The imager is equipped with overall five filters, four interference filters and one dark filter. Two of the interference filters are for the observation of OH(3-1) P1(2) and P1(4) to derive the rotational temperature in about 87km altitude, one for to observation of the O2 emission layer in about 94km, and one for the background. An additional dark filter is for corrections of the dark current. One complete cycle through all filters is done every 2.75 minutes. The instrument is thoroughly calibrated with an absolute calibration considering the imager and all the optical components. A geometrical calibration is used for georeferencing the all-sky observations.

The OH rotational temperature is derived from the two observed OH(3-1) emission lines following a procedure based to the one from Pautet et al. (2014), but slightly modified with new Einstein A-coefficients. An error analysis for the temperature derivation is done and shown in detail. The derived temperatures are compared with the temperature observations of a nearby LIDAR instrument at Esrange in about 40km distance of the new imager’s site for the imager’s pixel location nearest to the LIDAR and the altitude of the OH layer in the LIDAR data. The author’s find a good agreement of both measurements within about 3 K for a particular night and only -0.2K±1.6K on average. Additionally, they compare the temperatures derived from the imager with Aura/MLS satellite measurements and find an average temperature difference of 2.8K±7.8K. Considering the different field-of-views and spatial distances between ground-based instrument and satellite this seems reasonable.

In a case study the authors highlight the observation of a ripple structure visible in the OH temperature map with about 4-6 K temperature amplitude and with a horizontal wavelength of 4-8 km. This shows nicely the potential of the imager to resolve small-scale structures in the temperature maps. Also, they show a medium-scale gravity wave which is visible in the OH and O2 layer simultaneously. Analyzing daily mean OH rotational temperatures from January to April 2023 the authors see seasonal temperature variations typical for the site’s location. They also see peaks in the data and speculate whether these could be due to the sudden stratospheric warming event starting in February 2023.

Overall the author’s present a valuable new imager instrument for atmospheric observations in the middle atmosphere utilizing two airglow emissions. Especially the combination of OH temperature maps and observing the O2 layer quasi simultaneously offers great opportunities for future investigations of gravity waves and many other phenomena. The manuscript is well written and the analyses described concisely. I recommend it to be published in AMT after addressing the following few minor points.

General remarks:

1. I suggest a short paragraph about currently used imagers to emphasize the novel aspects of the presented imager instrument. E.g. many imagers operate in the VIS-NIR range (e..g Li et al. 2005), but also many in the SWIR range. When integrating over larger parts of the SWIR range the integration time is often of the order of a second (Hannawald et al. (2016)). There are also other instruments utilizing a filter wheel to observe different airglow emissions simultaneously, but often integrating over larger parts of e.g. the OH bands (e.g. Mukherjee et al. 2010), so that no temperature derivation is possible. On the other hand, Pautet et al. (2014) uses the AMTM to derive temperatures (which was shortly mentioned in the manuscript). Other imager types focus on very small scales (e.g. Sedlak et al. (2016), Hecht et al. 2023 (https://doi.org/10.1029/2023JD038754)). However, none of these instruments - as far as I am aware of - derives temperature and uses multiple emission layers. It would be valuable to work out the differences of the new imager to current imagers.
2. The shown images and the temperature map look very nice and highlight the capabilities of the instrument. However, I imagine that one or a few video sequences of the shown night as supplement data might by quite more impressive and useful to show the potential of the instrument.

Specific remarks:

L44-45: “hot topic of current atmospheric research.” It would be nice to have more up to date articles . Wüst et al. (2023, https://doi.org/10.5194/acp-23-1599-2023) might be a good fit.

L136: The re-imaging lens was not described in the setup before while the primary lens and telecentric lens were. Could you also describe it shortly?

L146-147: “…implying that the instrument can readily resolve the highest frequency range in the gravity wave spectrum.” Waves with frequencies near the Brunt-Väisälä-Frequency (2*pi/(5 * 60) = 0.021 assuming a BV-period of 5min at the mesopause) might be aliased as the Nyquist frequency of the instrument is f_ny  = (0.5 * 2*pi / (2.75 * 60) = 0.019. Especially small-scale gravity waves tend to have periods near the BV-period (see e.g. Tang et al. 2014). I suggest to formulate the sentence a little more cautiously. In this context is it even nicer to see the ripple structure proposed later in the manuscript to show the potential of the instrument even for small-scale variations.

L184-185: How was the flat-field correction done? Was a black-body source used or an average of measurements or something different? I can imagine it wasn’t easy to perform a flat-field correction with a 120° field-of-view.

L193-197, L204: Can you give some additional information about the geometrical calibration? Unfortunately, in Dalin et  al. (2015) I couldn’t find more details on the geometrical calibration with reference stars (also not in the publication Dubietis et al. (2011) therein) which is directly related to the geometrical calibration done here. E.g. have you used an infrared star catalogue to find reference stars as you observe in the SWIR range rather then the VIS-NIR? What form has the 3rd order polynomial?

L209-210: For me it is not clear where this formula is coming from, especially the meaning of k_1 to k_7. Is k_4 e.g. the standard deviation of the noise n_dc? Why is there a different parameter for n_dc in denominator and numerator? What is meaning of the scaling factors k_1 and k_5? I think that formula would be much clearer with a few lines about what the coefficients mean and how they are “determined in a laboratory”.

L222: I may be wrong, but I would calculate the angular side length of a single pixel with 120° / 512 pixels = 0.23 °/pixel.

L333: “due to small-scale gravity waves (ripples)…”. Following Li et al. (2017) and references therein, ripples are not gravity waves rather than instability structures and are often referred to as "wavelike structures" or simple "ripples". It might be difficult to distinguish between small-scale gravity waves and ripples. I suggest rephrasing to make clear that these are different phenomena.

L476: The doi of Dalin eta al 2015 seems to be wrong, I think it should be https://doi.org/10.1002/2014GL062776

Figure 6: I wonder about the same timestamp in the title of all 3 emission plots (6a-c). I would expect 30-32s gaps between P1(2), P1(4) and O2. Additionally, I can't recognize the small wave structure of 4-8km in P1(2) or P1(4) which could be an issue of contrast/color range (I can see it in the rotational temperature map). I suggest to add another figure with a small crop/zoom of the whole images focusing on the small-scale structure and adjusted color range.