Notes

Reviewer #1 (Comments to authors):

“The decametre sky at sub-arcsecond resolution” by Groeneveld et al. presents a pilot study demonstrating the ability of LOFAR to make deep observations at < 30MHz. These observations are the deepest and highest resolution at such low frequencies — which are traditionally difficult to observe at — and consequently the scope of the paper is potentially suitable for Nature Astronomy. The details of the observations themselves are well presented in the paper. However, I have found a number of issues which I would require to be addressed before accepting for publication and I list these below.


Major comments:

1) While the observations are pioneering in terms of frequency and resolution, the actual science content of the paper is extremely light, consisting of only a cursory level comment on a few very-steep spectrum radio sources. The title of the paper is “The Decametre sky at sub-arcminute resolution”. I was quite surprised that, given this title, no substantial characterisation of the radio sky based on the observations undertaken is provided. Rather the paper describes some proof of concept observations which are the first of their kind at <30MHz with 45 arcsec resolution, this would be fine if the authors then characterise their observations well. At present this is lacking. At the bare minimum I would expect a paper of this nature to present some source statistics that future works can use as a benchmark. For instance:
- Flux distribution
- Size distribution (or at least fraction of resolved vs compact)
- Spectral index distributions for those with LoTSS matches
- If possible it would be good to see the Euclidean normalised source counts presented here (even with fairly large errors). Although it might be that the relatively limited number of sources (~3000) makes this impractical to effectively show, if so then please comment to that effect in the response to the review.

2) My next major comment concerns the flux measurements used in this work. The authors mention that the Gaussians fit by the source-finding are inappropriate for extended morphology radio sources. This in itself is a reasonable statement. However, they merely state that ‘circular apertures with a radius of 2 arcminutes are used to measure the flux instead’, without stating exactly how this measurement is performed, and as such it would be impossible for another scientist to repeat the measurements on the same images. I assume the authors are taking the source positions from the PyBDSF catalog, but they should explicitly state this. For the measurements themselves, are the authors just summing the pixel values within the aperture or is a more complex calculation being performed? If the former, how are the authors converting a sum of Jy/beam pixels to a Jy (or equivalent) flux density. Furthermore, while the Gaussians fit by the source-finder may not be appropriate for extended sources, they are the expected model for point sources. Therefore, the authors should compare their aperture fluxes to the PyBDSF measurements for point sources in their catalog (for which the two measurements should be approximately the same). This will a) reassure the reader that their method for measuring flux densities is appropriate, and b) allow the authors to identify and characterise any systematics that may be present in their flux measurements.


Other Comments:

3) 1st paragraph after abstract: the authors state that radio studies are usually carried out at higher frequencies due to “greater observational and technical advantages”. The authors should list a few examples of these advantages here.

4) Towards the end of the first paragraph on page 4 the authors state that only a few ground based studies have detected more than a few dozen sources (e.g., Kassim 1988). Here, and whenever the authors are listing example citations, they should list more than a single example. In cases where there are only a few papers I would suggest listing them all, but for cases where there are many possible references I suggest listing at least 2-3 to show the range of literature.

5) Paragraph 2 on page 5: the authors state that their sensitive of 12mJy/beam is an improvement of three orders of magnitude over previous work. This statement seems inconsistent with their previous mention of the 0.3 Jy/beam sensitivity of the 8C survey — approx 1.5 orders of mag fainter, albeit at slightly higher frequency. The authors should clarify this statement (or correct if needed).

6) The final sentence of the main body (page 7) states that “such a telescope could perhaps operate at 5-10MHz around solar minima, when ionospheric disruption is smallest”. This statement comes out of the blue, and requires (at least) a few sentences of discussion and justification.

7) End of the very first paragraph on page 6: the authors state there are no systematic flux density scale offsets relative to previous source catalogs and refer the reader to the methods section. However, the methods section appears to contain no comparisons with other source catalogs. These comparisons should be included.

8) Are the authors planning to release the catalog of sources they create? This is entirely their decision of course, but I think it could be a useful resource, and may increase the impact of the work — as the authors themselves state, these are the first observations at <30MHz with sub arcmin resolution! If they do wish to make the catalog public they should include some comment on how they plan to make it available. Additionally, a suggestion (again, entirely at the authors discretion) would be to include the 5 < S/N < 7 sources, perhaps with some kind of flag or warning, as well as just the S/N > 7 sources. Many of these will be real, albeit with a higher false-positive rate. Speaking for myself, if I were to make use of a catalog, I would certainly find that additional information useful.