The phrase "are hard to observe" is correct and usable in written English.
It can be used when discussing things that are difficult to see, notice, or detect in various contexts, such as scientific observations or behavioral patterns.
Example: "The subtle changes in the ecosystem are hard to observe without careful monitoring."
Alternatives: "are difficult to notice" or "are challenging to detect".
Simply stated, owing to their distance and therefore their dimness, galaxies are hard to observe and the observations even harder to interpret.
"Additionally, children can seldom see what parents do when the parents commute to work or have jobs that are hard to observe (such as the paperwork jobs of many white-collar workers) or off-limits to children (such as industrial plants)." Also, he adds, the decision-making process has been complicated by the increasing desire for high-prestige jobs: doctor, lawyer, professor.
It is also noticed that silica nanostructures are hard to observe at an annealing temperature below 900°C, while abundant silica nanowires are found all over the substrate when the temperature is above 1,050°C, which reminds us that temperature is another critical parameter to determine the growth behavior of silica nanostructures.
This latter point highlights the difficulty in assessing wireless broadcast protocols, and more generally any distributed wireless mechanisms: simulation campaigns tend to focus on large networks to demonstrate scalability, and to benefit from statistical effects (such as the diversity of propagation paths) which are hard to observe at smaller scales.
As a rule, FCCs exist only "fleetingly" and are hard to observe.
They classify net social welfare as total welfare excluding hospital profits because those are hard to observe or verify.
As with rank, differences in RIC are harder to observe in the 5 ×512 networks due to their limited capacity.
This conveniently lets them get a feel of the layout of an urban area and helps them to collect environmental data on places that are harder to observe using street-level imagery alone.
Finally, tissue-specific (and often lowly expressed) genes are harder to observe experimentally, adding a bias towards broadly expressed proteins to the phylogenetic bias.
Check all fins, as some fins are harder to observe than others (i.g. anal fins).
It is hard to observe this production intensity directly.
